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O320 rough idle

hcccs

Well Known Member
I'm test running my O320 and it runs rough at idle and when I throttle back all the way it stops. I tried to adjust the idle rpm with the screw on the carb and I could barely get it to run at 650 RPM and then the rpm fluctuates between 600-700 rpm and it misfires. As for the adjustment screw I think it is screwed in almost as much as it can be (see photo). Does this seem normal? I can't even decide if it is running too rich or too lean. The magnetos are correctly set, the leads and all weather spark plugs are new. Any ideas?


 
A rag in the carb? You gasket between carb and filter? Don?t ask why I know. I chase that exact symptoms for a few hours on the ground.

Confirm you have nothing in the airways first. Then it should be proper carb setting configuration.
 
What sort of prop do you have? I have a wood prop and I can't get mine to idle smooth down that low. It is only happy at 800+ RPM. I think it has to do with the inertia of the prop (or lack of it in my case). My carb is set up to idle at 600 RPM and it will run, but it's not happy. No backfiring though.
 
What sort of prop do you have? I have a wood prop and I can't get mine to idle smooth down that low. It is only happy at 800+ RPM. I think it has to do with the inertia of the prop (or lack of it in my case). My carb is set up to idle at 600 RPM and it will run, but it's not happy. No backfiring though.


Yes, I have a wood prop. My previous plane had a Continental C-90 with wood prop and it idled happily at 500 rpm when cold. I think I have to check magneto timing and spark plug leads.
 
The blueish plate between the carb, gaskets and sump- is that leakfree? Can you check for induction leaks with egts on each cylinder? The lighter prop is more revealing of any issues at idle.

Which part number carb? Is it new or newly overhauled?
 
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The blueish plate between the carb, gaskets and sump- is that leakfree? Can you check for induction leaks with egts on each cylinder? The lighter prop is more revealing of any issues at idle.

Which part number carb? Is it new or newly overhauled?


Marvel-Schebler MA4-SPA No BL-22-14429. I could check the EGT on Skyview.
 
It is likely too lean. If it were too rich, increasing the RPM would likely help it, as opposed to resisting it. Give the idle mixture screw (It has a large screw head with knurling on it and faces aft at the base of the carb - not the idle speed screw highlighted in your picture) a good half turn out (CCW) and see if it improves. Then follow the procedure for properly adjusting your idle mixture. If it were rich enough to resist additional RPM, you should be able to see a black or grey color in your exhaust output.

Larry
 
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It is likely too lean. If it were too rich, increasing the RPM would likely help it, as opposed to resisting it. Give the idle mixture screw (It has a large screw head with knurling on it and faces aft at the base of the carb - not the idle speed screw highlighted in your picture) a good half turn out (CCW) and see if it improves. Then follow the procedure for properly adjusting your idle mixture. If it were rich enough to resist additional RPM, you should be able to see a black or grey color in your exhaust output.

Larry


Larry,
I'll give it a try.
 
It is likely too lean. If it were too rich, increasing the RPM would likely help it, as opposed to resisting it. Give the idle mixture screw (It has a large screw head with knurling on it and faces aft at the base of the carb - not the idle speed screw highlighted in your picture) a good half turn out (CCW) and see if it improves. Then follow the procedure for properly adjusting your idle mixture. If it were rich enough to resist additional RPM, you should be able to see a black or grey color in your exhaust output.

Larry


If it were too lean would it run well at higher revs. Mine does so I wonder.
I have removed the carb now to check it and upgrade with new floats. When unscrewing the throttle adjusting bolt all the way back the throttle valve closes before the stop on the throttle shaft hits the stop on the carb body. When the valve is closed only the upper primer delivery hole is exposed (see picture) but no air could go past the valve so I can't see how it could possibly idle in this position. How big would the valve opening be at idle?

 
Normal for mine..

I am running an O-320H2AD/1600 hp with a Sterba wood prop, and idle at 600-650 is rough. I have it set for that only to allow me to throttle way back, but I keep my idle around 800 RPM on ground to be happy. There is little similarity between an O-320 and a C65-C90 which will purr at low idle with a wood prop. There just isn't enough inertia/weight in a wood prop for the low idle on the O-320
 
If it were too lean would it run well at higher revs. Mine does so I wonder.

I said your "idle mixture" was too lean. Your idle mixture screw only sets the mixture for low RPM operation, up to about 1000-1200 RPM. After this, your mixture is controlled by the main circuit (no adjustment other than main jet size). One has nothing to do with the other and are not related, except in the RPM range where control is transitioning from the idle circuit to the main circuit.

Larry
 
The blueish plate between the carb, gaskets and sump- is that leakfree? Can you check for induction leaks with egts on each cylinder? The lighter prop is more revealing of any issues at idle.

Which part number carb? Is it new or newly overhauled?


The carb number is 10-5009 which is the one to use for O320-E3D. I took it apart and cleaned it before installing it. I don't know yet what jet is installed.
 
Back again

Back again with my rough idling O320. I took the carb apart and tested the idle passages and fuel passed through. A mechanic told me to turn the mixture screw out 4 turns and start testing from there. I did so and the rpm went up 50 when leaning, just as it should, but it ran as bad as before at 600-650 rpm. I even tried 4? turns. Now I read something in a document from Kelly Aerospace: "An idle adjustment screw that requires more than four turns out to achieve a proper mixture rise at ICO is a good indication of contamination in the idle circuit." Maybe there is some contamination in the idles mixture passages after all. How do I clean them, ultrasonically or with some other means?
 
If the cylinders/rings are new and the engine is not broken in, it might take some hours on the jugs for the idle to smooth out when idling as low as 700-
 
I am running an O-320H2AD/1600 hp with a Sterba wood prop, and idle at 600-650 is rough. I have it set for that only to allow me to throttle way back, but I keep my idle around 800 RPM on ground to be happy. There is little similarity between an O-320 and a C65-C90 which will purr at low idle with a wood prop. There just isn't enough inertia/weight in a wood prop for the low idle on the O-320


Removed the magnetos, changed one faulty coil and the engine runs a little better but not much. What difference could there be between a C90 and a O320? O-320 has 50% more cylinder volume but both have a compression ratio of of 7:1 so where else is it different?
 
You just said it..50% more volume..Its just plane old math. Prop size/weight isn't much different,compression ratio is just the math multiplication factor for volume, but the amount of air compressed is much more.
 
Hey Hans, Long time since we talked. Hope you are well.
Some basic things to check.
Make sure the carb air temp hole in the carb is plugged.
Make sure all of the 1/8 pipe plugs, where a primmer fitting or FI nozzle would go in the cylinders, are all there.
If you have any sort of primmer system installed disconnect it as close to the engine side as possible, block it off on the engine side, and try running with the primmer system removed.
Screw the isle mixture screw adjustment all the way in and back it out 11/2 to 2 turns.
Good Luck,
Mahlon
 
I can't even decide if it is running too rich or too lean.

I presume you checked for correct idle mixture adjustment at a higher idle rpm where it was running OK. If so try pulling the mixture full out to idle cutoff when it is “misfiring” at low rpm. If the misfire ceases and it revs up considerably before the engine stops then it is likely too rich.

If it does seem too rich then the next step would be to check the actual fuel level in the carb bowl. Remove the carb bowl drain plug and make a temporary replacement plug such as a barbed fitting with a clear plastic tube attached and route it vertically to above the carb. Directly observe the fuel level which should be somewhere below the level of the main gasket. (reinstall the plug before flight!!)

Fin
9A.
 
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I have an O-290-D2 which is pretty much like an O-320 only with an MA-3SPA carb. Mine will idle down to 500 RPM with a wood prop. Looking at the Marvel Schebler troubleshooting guide for rough idle, induction leaks and primer leaks are two of the top suspects. Also, that hole in the carb barrel above the Throttle Fly is (I think) the "Primary Idle Delivery". This is the one controlled by the big Idle Adjusting knob mentioned earlier. Just under the Fly are the Secondary and Third Idle Delivery holes. There are lots of good diagrams here: http://www.insightavionics.com/pdf files/MA-4 Carb Manual.pdf
 
Still misfiring

After being away for more than a month I've returned to my misfiring O320. Today a friend helped me spraying start spray on the intake flanges and hoses to detect any air leaks. We couldn't find any. I ran the engine for 15 minutes and noticed lower temps on cylinder 1. If it is misfiring all the time that would explain the lower temps so what is the next step? I don't know - plugs are new but sooty, ignition wires are new so what else is there to choose from? Any ideas are most welcome!
CHT: 87/116/129/137, EGT: 359/519/475/479


screen.jpg
 
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I have an O-290-D2 which is pretty much like an O-320 only with an MA-3SPA carb. Mine will idle down to 500 RPM with a wood prop. Looking at the Marvel Schebler troubleshooting guide for rough idle, induction leaks and primer leaks are two of the top suspects. Also, that hole in the carb barrel above the Throttle Fly is (I think) the "Primary Idle Delivery". This is the one controlled by the big Idle Adjusting knob mentioned earlier. Just under the Fly are the Secondary and Third Idle Delivery holes. There are lots of good diagrams here: http://www.insightavionics.com/pdf files/MA-4 Carb Manual.pdf

Thanks for attaching that link! I am experiencing a slight, momentary hesitation during acceleration and this guide indicates that my accelerator pump linkage is misadjusted. Perfect timing!
 
New isn't always right

I'd check the new ignition wires and their end connectors. Sometimes, new electrical components that come from the factory are bad. Easy ohm check just tp be sure. Start with the ht wires on number one cylinder. Good hunting!
 
Picture is worth 1000 words.

Your engine data shows very low oil temperature and low cylinder head temps. So we submit the following from the Lycoming O-320 Operators Manual.
Idle Speed and Mixture Adjustment.
(1) Start the engine and warm up in the usual manner until oil and cylinder head temperatures are normal.
(2) Check magnetos. If the mag-drop is normal, proceed with idle adjustment.
(the following adjustments are fine adjustments of 1/4 to 1/2 turns, and may have to be repeated)
(3) Set throttle stop screw so that the engine idles at the airframe manufacturer recommended idling RPM. If the RPM changes appreciably after making idle mixture adjustment during the succeeding steps, readjust the idle speed to the desired RPM.
(4) When the idling speed has been stabilized, move the cockpit mixture control lever with a smooth, steady pull toward the Idle Cut-Off position and observe the tachometer for any change during
the leaning process. Caution must be exercised to return the mixture control to the Full Rich position before the RPM can drop to a point where the engine cuts out. An increase of more than
50 RPM while leaning out indicates an excessively rich idle mixture. An immediate decrease in
RPM (if not preceded by a momentary increase) indicates the idle mixture is too lean.
If step (4) indicates that the idle adjustment is too rich or too lean, turn the idle mixture
adjustment in direction required for correction, and check this new position by repeating the
above procedure. Make additional adjustments as necessary until a check results in a momentary
pick-up of approximately 50 RPM. Each time the adjustment is changed, the engine should be run
up to 2000 RPM to clean the engine before proceeding with the RPM check. Make final adjustment of the idle speed adjustment to obtain the desired idling RPM with closed throttle.
The above method aims at a setting that will obtain maximum RPM with minimum manifold pressure.
In case the setting does not remain stable, check the idle linkage; any looseness in this linkage
would cause erratic idling. In all cases, allowance should be made for the effect of weather
conditions and field altitude upon idling adjustment.

Let us know your results.
Steven
 
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My o-320 gives me a MAP of about 11 at 800 rpm. Yours is showing MAP of almost 15, which I suspect would indicate an intake leak. I would be inclined to check that area again, including any connections such as the MAP hose connections, carb/bracket/sump gaskets.

I had repetitive leaky intake gaskets, but solved that with an o-ring kit from one of our VAF posters/advertisers,Ross from SDS. http://www.vansairforce.com/community/showthread.php?t=161030

About 1 year and 100 hours with the SDS o-rings. Works well so far.
 
There is a chance that the exhaust valve may be sticking in Cylinder #1. I could not find any reference in this thread to a compression check being done (maybe I missed it), which would help diagnose a sticky valve.

Good luck!

(BTW, it looks like you didn't have the Alternator online during this engine run, with a "Batt Volts" indication of only 12.1V)


I'll do a compression test to make sure. As for the alternator I'll remove it and have it checked. It was given to me and I don't know the condiction.
 
My o-320 gives me a MAP of about 11 at 800 rpm. Yours is showing MAP of almost 15, which I suspect would indicate an intake leak. I would be inclined to check that area again, including any connections such as the MAP hose connections, carb/bracket/sump gaskets.

I had repetitive leaky intake gaskets, but solved that with an o-ring kit from one of our VAF posters/advertisers,Ross from SDS. http://www.vansairforce.com/community/showthread.php?t=161030

About 1 year and 100 hours with the SDS o-rings. Works well so far.


So, it comes down to a leak after all. I'll check all intake tubes and carb/oil pan gaskets and seal with some Permatex Blue RTV to make sure. That would be OK, or what do you think?
 
So, it comes down to a leak after all. I'll check all intake tubes and carb/oil pan gaskets and seal with some Permatex Blue RTV to make sure. That would be OK, or what do you think?

Didn't catch how many hours/years on the engine.

I have not used RTV, and I don't think it is recommended for fuel systems. I have used Hylomar Universal Blue as a gasket dressing, and that really helps when it comes time to change gaskets. It helps prevent the gasket material from sticking to the parts. There may be other materials that others have had some success with.

Changing the intake gaskets is cheap and not too much time or effort.
 
Didn't catch how many hours/years on the engine.

I have not used RTV, and I don't think it is recommended for fuel systems. I have used Hylomar Universal Blue as a gasket dressing, and that really helps when it comes time to change gaskets. It helps prevent the gasket material from sticking to the parts. There may be other materials that others have had some success with.

Changing the intake gaskets is cheap and not too much time or effort.


The engine has 4000 hours and new Millennium cylinder were install 700 hours ago.
 
Hans: Did you find the problem?


No.
Lycosaurus suggested earlier that I must have a intake leak and a club member thought so too so I had to check again. I have also removed the carb and magnetos. I uppgraded the magnetos with new distributor gears according to SB15 (?). The distances between the old gear electrodes and the distributor house electrodes were 0.35mm and with the new 0.15mm and I thought that would make things better but it didn't. The engine started more willingly but nothing more. An old experienced mechanic in the club helped med and while I sat in the cockpit and kept the RPM at 850 he screwed on the idle mixture screw and he said the engine did not reacto to anything he did so he said there must be something with the carburetor. I have taken it apart twice and upgraded it with the new, blue epoxi floats and checked the idle channels. Next step is to check spark plugs and ignition wires. They are all new so I haven't paid much attention to them.
 
Hans... our local RV guru Tom Berge saw your issue and he has some ideas. He asked that I post his contact info so you could drop him an email directly if you like. His email is: [email protected]
 
Rough idle update

My o-320 gives me a MAP of about 11 at 800 rpm. Yours is showing MAP of almost 15, which I suspect would indicate an intake leak. I would be inclined to check that area again, including any connections such as the MAP hose connections, carb/bracket/sump gaskets.

I had repetitive leaky intake gaskets, but solved that with an o-ring kit from one of our VAF posters/advertisers,Ross from SDS. http://www.vansairforce.com/community/showthread.php?t=161030

About 1 year and 100 hours with the SDS o-rings. Works well so far.


I'm back again with my rough idle O320. I had the magnetos checked by a air mechanics shop and they were fine. A friend suggested I had a look at the throttle shaft and bushings. I did so and the bushings were quite worn so it is most likely leaking through there. The carb has come off now again for the Nth time. I've ordered a new shaft and I'm making new bushings and I'm confident that this has been the problem. What about lubrication? What do I use to lubricate the shaft/bushings?
 
Carb

You should buy factory bushings and they need to be reamed after installation. They are held in place by locktite.
If you have a wood prop and high compression engine, standard 160 hp or higher compression it may never idle very smoothly. Mine with 10-1 pistons requires about 900 rpm before it smooths out.
You need to get an overhaul manual for the carb and follow the procedure for the throttle shaft bushings.
 
I'm back again with my rough idle O320. I had the magnetos checked by a air mechanics shop and they were fine. A friend suggested I had a look at the throttle shaft and bushings. I did so and the bushings were quite worn so it is most likely leaking through there. The carb has come off now again for the Nth time. I've ordered a new shaft and I'm making new bushings and I'm confident that this has been the problem. What about lubrication? What do I use to lubricate the shaft/bushings?

That is a good start. A carb that does not respond to adjustment of the idle mixture screw indicates a problem of some sort. These carbs use a needle in the fuel delivery side. You want to crank it down to force it more lean and see if it will lean out and wether the current set up is rich or lean. bad throttle bushings would make it lean and hard to get rich. If you can observe an RPM rise while leaning it out, that points away from bad throttle bushings. If you crank the screw all the way in and it is still running, you have other problems, like a leak or bad float bowl needle/seat.
 
Great find Hans. Let us know how it works out.


The new throttle shaft is installed together with new bushings. I ran the engine today and it ran slightly better but the original fault is still there. Another thing is that it does not react to the mixture screw settings and does not rev up when I lean to near cut off. There's a leak somwhere and I can't find it. Back to square one again I suppose.



screenshot-SEXJJ-SN08613-15-3-3-4688-20200319-132645-106-en-US.png
 
Quite a perplexing problem your having. Seems you've addressed every field addressable option and sometimes a field repair is not possible, and the suspect component has to be sent back to the manufacturer or their authorized representative. Since only they have the appropriate test equipment to find the defect .
I make this suggestion from experience. Had a similar head scratcher, tried absolutely everything everyone recommend with out success. Gave up on fixing it myself, sent it in for overhaul / repair . Turn out to have had a casting defect, hairline crack in the carburetor body allowing a vacuum leak that only could be detected by bench flow & vacuum test .
 
I had the symptoms of an induction leak and it took me a long time to find and even then it was by accident.
Despite there being no soap bubbles from any of the potential leak areas when a vacuum-cleaner discharge hose was gently held up to the carb inlet and after changing of all the gaskets at the cylinder head, I elected to change all of the intake ?rubbers? that seal the join between the pipes extending from the oil sump to the individual ?riser? elbows attached to the intake port on the cylinders, more as a prophylactic measure than a suspected leak source.
It was when I was twisting one of these rubbers to get it off the lower pipe that I discovered the pipe itself was rotating where it was swaged into the sump.
And that was the leak source.
 
It looks like you need a second gasket between the cable bracket and the carb.

This setup needs a gasket between the sump and the bracket and one between the bracket and the carb.
 
It looks like you need a second gasket between the cable bracket and the carb.

This setup needs a gasket between the sump and the bracket and one between the bracket and the carb.


I do have two gaskets, one on each side of the cable bracket.
 
That is a good start. A carb that does not respond to adjustment of the idle mixture screw indicates a problem of some sort. These carbs use a needle in the fuel delivery side. You want to crank it down to force it more lean and see if it will lean out and wether the current set up is rich or lean. bad throttle bushings would make it lean and hard to get rich. If you can observe an RPM rise while leaning it out, that points away from bad throttle bushings. If you crank the screw all the way in and it is still running, you have other problems, like a leak or bad float bowl needle/seat.
The throttle bushings leaked and I put in a new throttle shaft and bushings and there was not much difference. I then turned the mixture screw all the way in and started the engine. No change at all. Next 1½ turns out, then 2½ and finally 4 turns and no change. It behaves the same, MAP is 14" at 800 rpm. Leaning from 1000 rpm to nearly cut-off changes nothing, no increase. I was thinking I had turned the mixture metering valve 180 degrees wrong but it can only be assembled one way so that can be ruled out. I also cleaned the idle tube with compressed air. This is today´s situation and I'm out of ideas - again.
 
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The throttle bushings leaked and I put in a new throttle shaft and bushings and there was not much difference. I then turned the mixture screw all the way in and started the engine. No change at all. Next 1½ turns out, then 2½ and finally 4 turns and no change. It behaves the same, MAP is 14" at 800 rpm. Leaning from 1000 rpm to nearly cut-off changes nothing, no increase. I was thinking I had turned the mixture metering valve 180 degrees wrong but it can only be assembled one way so that can be ruled out. I also cleaned the idle tube with compressed air. This is today´s situation and I'm out of ideas - again.

If it runs below 1000 RPM with idle screw all the way in, the fuel is coming from somewhere it shouldn't. Could be a crack in the casting or possibly a bad float or needle/seat, allowing the fuel in the bowl to get too high and feed out of the main jet. Normally the main jet won't start flowing fuel untill around 1200.
 
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Your MAP, as suggested before, still indicates [EDIT] a possible induction leak. It should be around 10-12" at ~800 RPM. (credit: John Deakin).
 
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Your MAP, as suggested before, still indicates an induction leak. It should be around 10-12" at ~800 RPM.

both lean AND rich conditions will cause a high MAP at idle. It is not a positive diagnosis of an induction leak. The most common pre-AFR era, idle mixture setting protocol, outside of aviation, was either max RPM or max vacuum (many protocols called for a certain amount lean of best mixture). Both will peak at optimum mixture and will fall off on either the rich or lean side. Max vacuum is synonymous with min MAP.

I do agree that an induction leak is far more common cause of high MAP. However, not when a fully closed idle mixture screw allows the engine to still run.

Larry
 
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I've enjoyed reading through this thread and the responses you've received.
I'm assuming you haven't found a solution yet so will add my $.02

-How about that fuel level, has that been checked to ensure the float needle is sealing? I know you have the new blue float, but that's no good if the needle valve leaks.

- Have you checked the accelerator pump check valve?
If that is leaking, fuel will be pulled out of the accelerator pump discharge tube while idling and basically make your mixture screw useless for adjusting the idle mixture. Pull it, take it apart and clean it.

When an engine is idling a lot of air is being drawn through the carb. This said, most troubleshooting guides suggest that no fuel is drawn from the main jet side of the carb when idling......most of the time this is true, but when things aren't right with the carb fuel can and will flow on that side of the carb.....sometimes into the engine with the airflow and sometimes into the airbox without airflow.

Something to think about.....
 
I've enjoyed reading through this thread and the responses you've received.
I'm assuming you haven't found a solution yet so will add my $.02

-How about that fuel level, has that been checked to ensure the float needle is sealing? I know you have the new blue float, but that's no good if the needle valve leaks.

- Have you checked the accelerator pump check valve?
If that is leaking, fuel will be pulled out of the accelerator pump discharge tube while idling and basically make your mixture screw useless for adjusting the idle mixture. Pull it, take it apart and clean it.

When an engine is idling a lot of air is being drawn through the carb. This said, most troubleshooting guides suggest that no fuel is drawn from the main jet side of the carb when idling......most of the time this is true, but when things aren't right with the carb fuel can and will flow on that side of the carb.....sometimes into the engine with the airflow and sometimes into the airbox without airflow.

Something to think about.....


sstocker31

Do you mean the pump inlet check valve or the pump discharge check valve? Anyway, I checked both now and I don't know in what state the discharge check valve was but it's clean now. If it were open before not much fuel can have been pulled in when idling, as I see it, since the discharge tube sits on the high pressure side and the opening is very small. I think you're right that fuel is drawn from the main jet. To draw fuel from the main jet the throttle needs to be opened a bit more and that's why my MAP at idle is higher than normal, 14-15. I've got hold of another MA4-SPA 10-5009 carb which I will test. If it works - fine, I'll use that instead but when will I ever find the problem with the first carb?
 
sstocker31

Do you mean the pump inlet check valve or the pump discharge check valve? Anyway, I checked both now and I don't know in what state the discharge check valve was but it's clean now. If it were open before not much fuel can have been pulled in when idling, as I see it, since the discharge tube sits on the high pressure side and the opening is very small. I think you're right that fuel is drawn from the main jet. To draw fuel from the main jet the throttle needs to be opened a bit more and that's why my MAP at idle is higher than normal, 14-15. I've got hold of another MA4-SPA 10-5009 carb which I will test. If it works - fine, I'll use that instead but when will I ever find the problem with the first carb?

Is it possible that no fuel is flowing through the idle jet and you are having to open the thottle enough to pull from the main jet? When it is idling, how far off the idle stop screw is the thottle linkage? Any black smoke or other signs of running rich? May want to lean it out, while idling, via the red knob to see whether you are lean or rich or possibly spot on.

I wouldn't expect you to be able to pull from the main jet with an RPM of 800 unless the fuel in the bowl is too high or something else is going on, but I suspect it may be possible. The main jet flow is determined directly by air flow volume/velocity and not MAP. It is indirectly determined by pressure on the fuel in the bowl (combination of atmo pressure and fuel height, which is set via the tang on the float) The correlation between air flow and MAP can be influenced by several factors.

Larry

Larry
 
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Airbox and filter installed?

Something to consider.. The airflow into the carb will not be smooth or accurate unless the airbox and filter is installed (ask me how I know!).
 
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