What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Sticking Exhaust Valve - Diagnosis and Fix!

Oil Pressure

My understanding is that the 115 limit was primarily to keep people from panicking over above red line readings following a cold start.
The new Cessnas reading of 80 psi is really 90 psi. That is Cessna's version of the mushroom theory regarding dealing with pilots. Politely stated the less information we provide to the pilot the less the pilot will worry.
 
Add me to the list

Of those with a stuck valve. I have a new YIO-360-M1B with 312 hrs. Twice in the last week I have had the symptoms of a stuck valve. Today with the help of a friend we found the #2 exhaust valve to be very stuck. We basically followed the well documented instructions provided by Mike. I use avgas only and run LOP most of the time. Philips XC. Has anyone found a reason for this or a way to prevent it?
 
Me too!

I too have had a problem with #2 exhaust valve sticking on my TMX360. Fortunately, the first time it occurred I was on the ground at initial start up. After shut down and restart, the problem was gone. I too saw the CHT on #2 staying at ambient temp. The second time was during a condition inspection when it wouldn't hold pressure during the compression test. My A&P recommended 4 oz of Marvel Mystery Oil in each tank at fill up and I haven't had a problem since after more than 150 hrs.

The high oil pressure discussion is interesting. I typically run 85psi at operating temp and felt that this was very high (based on my automotive background). I use Exxon 20w-50 oil.

When it first happened, I thought that it could be my LOP operation. I typically run slightly lean of peak. I continue to run slightly LOP and haven't had problems.
 
I have a Superior IO 360 with mags on both sides in my RV with 325 hours on it. I have have had several incidents of my number 2 cylinder EGT dropping very low and the engine running rough as if only running on three cylinders. It only lasts for a few seconds, so its hard to note other changes. My response has been to come back on power and richen mixture. But its usually over by then.
My oil pressure has always run high, I use 100 LL only, change oil every 25 hours and oil filter every 50 hours. I usually run ROP ,but occasionally LOP if at low power settings. My only real sin is I warm up the engine at low RPM's, usually about 700 because it runs smooth and vibration free at that RPM. I have recently read a Lycoming article about idleing the engine at 1100 to 1200 RPM's so as to run the the engine hot enough to activate the the lead scavageing chemical in the fuel. I have changed my warm up accordingly.
I am in the mdst of checking the number two exhaust valve and reaming if necessary. My spring compressor and ream are ordered and should be here tomorrow.
More to follow.
 
i payed my mechanic $80 to do a complete wobble test of all 4 cylinders on my rv. i have a mid time engine with no indicated problems. a new engine is set up with .010 " clearance on the exhaust valve. at tbo it will wear to .035". all of my exhaust valves were worn to about .020"-.024". a log entry with the SB and values entered = done. be safe out there. turbo out.;)

IMG_0723.jpg
 
IO-360 Cylinder Ring Replacement

I had a similar EGT dropping issue on #3 cylinder, usually about half-way through climb-out.

Turned out to be a corroded cylinder ring on #3 which apparently allowed oil into the combustion cycle once temperatures increased sufficiently, causing EGT cooling on #3 and a little rough running.

Took a couple of takeoff cycles to put the complete picture together, then a bore scope inspection on #3 and subsequent disassembly to see where the problem arose. New rings, no problem afterward.

Hope this was helpful.
 
I would look at the injector for visible obstruction and if cleaning it does not get resolution, I would look for trash floating around in the fuel distributor clogging the line going to the injector.
 
Thanks again to Mike Bullock, the OP, for his detailed post.
My #2 exhaust valve was indeed stuck so that I could not turn or push by hand. I'm amazed it ran so well with only an occasional complaint.
I followed closely what the OP did including especially his great tip on compressing the lifter to get the rocker reinstalled. It took me a lot longer than several posters seemed to indicate. I'm sure it will go much more quickly next time. It was an 8 hour project for me. I'll button it all up tomorrow after taking some time to make sure its all back where it should be, tools accounted for etc.
I'm looking forward to hear it run.
 
New Tool!

[Update to my home-made valve compressor tool]

So my #1 issue with my home-made valve compression tool is it is a royal pain in the *** to put the rocker arms back on as I have to compress the hydraulic lifters. I decided to make an alternative pivot point for my valve compressor tool. Previously, I put a socket extension through the rocket shaft hole, and use that as a pivot point to place my tool on. I thought I could make an alternative pivot point on the bottom of the cylinder using some leftover aluminum angle. I started off with a leftover piece of AA6-125-1.2x2 angle, cut to 3" in length.
DSCN5789.JPG


On the 2" side of the angle, I cut out a centered 2" wide section.
DSCN5790.JPG


This would leave 1/2" on either side.
DSCN5791.JPG


The finished product.
DSCN5792.JPG


Next I needed (4) 1/4" diameter holes. They would be separated by 2 7/16" horizontally, and 1.5" vertically. Additionally, they would be offset on the vertical axis by 7/16".
DSCN5793.JPG


And here's the result - this bracket screws into the existing valve cover holes.
DSCN5794.JPG


The next step was to "mill" out a hole for the valve compressor to insert into and pivot around. The hold would be offset from the bottom by 1/8", and it would be 7/16" x 1".
DSCN5795.JPG


Not my best work, but a 1/8" drill and a unibit with some filing at the end gave me a good enough hole.
DSCN5796.JPG


The finished product with my home-made valve compressor next to it.
DSCN5797.JPG


This is how the two parts will interface to each other. The "tab" on the back of the valve compressor tool gets inserted into the "slot" I made in the angle.
DSCN5798.JPG
 
First step is to install the new pivot point angle.
DSCN5799.JPG


Step two is to insert the tab on the back of the valve compressor tool. Lift the handle up and the spring gets compressed!
DSCN5800.JPG


Worked like magic. I was able to install the rocker shaft nearly instantaneously! This additional tool took about an hour to make...and the best part is it was FREE!
DSCN5801.JPG
 
To heck with the fix...Man...where does a guy lay in a supply of that pink shag carpet?

Were you Austin Powers in a previous life??

:D
 
Hangar towels...direct out of my grandparents collection! :D I'm too cheap to throw perfectly good ugly towels away!
 
Why is it often # 2 cylinder concerned by stuck valve? In my O320 lycoming egt# 2 is often low , sometimes 150 ?F below the hottest? I think I ?ll inspect my cyl #2 very soon...At start up it runs rough during a minute , next time I look closely at the EIS # 2 Egt .
 
Another Stuck Valve

A year ago I had a sticky valve on #2. I was going to do a precautionary check on all the cylinders as soon as the weather turned bad for a few days. Sunday was perfect weather in NE Ohio so I planned a breakfast run of about an hour over some rough terrain to W Va. My buddy canceled so I stayed close to home. After 30 minutes of local flying #4 cyl went dead and I was able to land at a nearby airport. Borscope revealed a perfect impression of the exhaust valve in the top of the piston so I pulled the cyl and the valve is very stuck along with bent pushrod and tube. The piston feels ok but I am having a qualified shop inspect everything. My understanding is that this problem is often caused be a poor hydraulic actuator so that will need to be inspected also. 40 Year?s of flying Lycoming Engines and never had a cyl removed.
 
A year ago I had a sticky valve on #2. I was going to do a precautionary check on all the cylinders as soon as the weather turned bad for a few days. Sunday was perfect weather in NE Ohio so I planned a breakfast run of about an hour over some rough terrain to W Va. My buddy canceled so I stayed close to home. After 30 minutes of local flying #4 cyl went dead and I was able to land at a nearby airport. Borscope revealed a perfect impression of the exhaust valve in the top of the piston so I pulled the cyl and the valve is very stuck along with bent pushrod and tube. The piston feels ok but I am having a qualified shop inspect everything. My understanding is that this problem is often caused be a poor hydraulic actuator so that will need to be inspected also. 40 Year?s of flying Lycoming Engines and never had a cyl removed.

Is the valve bent or stuck in the guide? If stuck then the problem is guide to valve clearance. Unless the lifter stuck in the case do not see how it would affect anything. Its there to maintain proper clearance for the valve.
What I have run into with lifters is they go flat as in wont pump up with oil pressure for some reason. I have seen problems when the clearance between the stem and valve was just a little too tight and after running for several hours stuck on a warmer day than usual.
 
I have very little knowledge in this area but the Lycoming tech rep who has been very responsive and helpful said it is often the the lifter that starts the problem. This was also the opinion of the rep from one of the well known engine shops.
 
A question for Malon and other experts

My YIO 360-M1B with 440hrs had a very stuck valve that may have ruined the valve seat and put a perfect imprint of the valve on the piston which calls into question the condition of the piston, bearings, etc.

It was suggested that I have all 4 cylinders sent to one of the custom engine shops to have them "fix" the problem. My question is whether there is a fix for this issue rather than just trying to stay ahead of it with inspections.
 
As several other posters have mentioned, Marvel Mystery Oil has worked well for me for years. I bought an Apache years back with a couple of high time O-320's, that would pop and snort on a cold start until warmed up. I added a quart of MMO to each engine, and the problem disappeared almost immediately. This stuff is an excellent upper cylinder lubricant for valves and rings. I plan on adding a pint to my oil changes once my new YIO-360-M1B is broken in. They say the "Mystery" additive is banana oil, whatever, I am a believer in this stuff and the Old Timers swear by it.
 
Marvel Mystery Oil

For years I used it in Turbo Charged Continental engines to help with sticking valves. I don't think it was approved by Continental but many if not most operators swore by it. Anyone know what Lycoming has to say about it?
 
The only "Lycoming" approved additive is LW-16702 Anti scuffing agent. It adds about $35 to the cost of an oil change. However there are several "FAA" approved additives such as Avblend,
 
Aeroshell Plus

The only "Lycoming" approved additive is LW-16702 Anti scuffing agent. It adds about $35 to the cost of an oil change. However there are several "FAA" approved additives such as Avblend,

The Aeroshell Plus grades (i.e. W100+) contain LW-16702 so you don't have to buy a bottle of additive separately.

Skylor
RV-8
 
Wish I had read ths sooner.

I should have looked at this sooner. Yesterday the #2 exhaust valve on my Lycoming IO360 stuck completely and bent the pushrod. Fortunately I was in the pattern at a nearby airport and immediately landed. It's now at the FBO at the same field getting a rebuilt cylinder. It had been running rough at idle and low power settings but smoothed out at runup power and higher. The engine had only 380 hours on it. I'll be doing a wobble test on the other cylinders.
 
I should have looked at this sooner. Yesterday the #2 exhaust valve on my Lycoming IO360 stuck completely and bent the pushrod. Fortunately I was in the pattern at a nearby airport and immediately landed. It's now at the FBO at the same field getting a rebuilt cylinder. It had been running rough at idle and low power settings but smoothed out at runup power and higher. The engine had only 380 hours on it. I'll be doing a wobble test on the other cylinders.

Frank,
I'm like you. I never paid attention to this post until your write-up today, because it happened to me this week.

My factory-new-plus-400 tach hours IO-360-M1B suddenly got rough under 1350 RPM along with an associated total drop of EGT on cylinder #4. I immediately showed it to Randy Richmond (aka Monk, our local 52F totally awesome expert of everything RV). He ran a compression check (there was none....0 inches, with a loudly audible hiss from the exhaust pipe). Diagnosis: Stuck exhaust valve.

Apparently, many (most?) mechanics will simply tell you the jug must be removed and sent to some repair house somewhere. Also, apparently, few will take the trouble to work the problem on-engine, if it can be properly worked on-engine. Randy actually conducted something akin to endoscopic surgery to clean the valve and inspect the pushrod.

All is Code 1, no bends, compression is good, and a cleaner valve. Of course, if the bits and pieces showed anything beyond mere carbon (or other gunk) buildup causing the stuck valve, he'd have elevated the level of repair.

I can't thank Randy "Monk" Richmond and Richmond Aviation enough. KELLI GIRL flies again!
 
Over the last few flights I noticed the #1 exhaust valve might be sticking. When the engine was warming up, it ran rough under 1,000 RPM and the CHT and EGT were low on #1 (CHT was 150 where others were 200+, EGT was 700 where others were in the 900's). As soon as I increased the throttle past 1,000 RPM, CHT and EGT increased to their normal places.

The timing of this was fantastic, as I was just starting the conditional inspection.

Yup, definitely a stuck #1 exhaust valve. If you're wondering about the bracket I have installed on the cylinder, it's what I used to pivot my home-made valve compressor on (see http://www.vansairforce.com/community/showpost.php?p=1167192&postcount=59).
DSCN6270.JPG


1/2 way through reaming out the guide, I took a pic with my borescope of what the inside of a guide looks like that's gunked up.
DSCN6272.JPG


And after! Like night and day.
DSCN6273.JPG



The only thing I changed from my original write-up (besides my new and improved valve compressor) was my method of keeping the exhaust valve seating for installation. Originally I used the "Rope Trick". One thing pointed out to me was there is a small chance of the rope forming a knot inside the jug, and then you have to yank it to get it out. So this time I tried using compressed air, delivered through the differential compression tester. I rotated the prop until the #1 piston was at the bottom of its stroke and the intake valve was closed. 60 psi of pressure seemed to be the right amount to keep the exhaust valve forced tight enough against the seat to install the springs.
DSCN6274.JPG



All back together and ready for the next adventure! The whole process took me a little over an hour, including dropping and reinstalling the exhaust.
DSCN6275.JPG
 
Valve Sticky

I think this is a great reference thread on something that seems to come up regularly. Any chance of the moderator making it a Sticky in this category or maybe in the Traditional Engines section?
Al
 
So Mike, inquiring minds want to know......

How come your guides are getting gunked up?? This is the second jug to have the problem for you, correct?
 
As DOM for a small fleet, we see this issue from time to time.

A couple of notes:

After reaming the guide and cleaning the valve's stem, it's possible to (very sparingly) use valve grinding compound along with a rubber hose on the valve stem hooked to a battery drill to very slowly, carefully and intermittently (I know you guys understand this) clean off the valve face and seat via an "in place lapping" . Apply a tiny quantity of valve grinding compound (half a drop) through the exhaust port with a Q-Tip. Cleaning up afterwards takes some time, but it's possible to correctly ensure no valve grinding compound remains.

The reason to do this has to do with heat transfer. The valve will remain slightly cooler if it has a good heat path at the valve seat area. Leading to fewer problems with valve sticking.

Aeroshell 15W-50 use seems to result in fewer events. In fact, the only time we've experienced it (not so for others) is with an oil other than AS 15W-50.

Also, a very lean taxi seems to help.
 
Last edited:
Reamer

Does anyone know if that reamer size mike ordered is the same for all lycomings? Angle valve and parallel valve?
 
Been there, done that!

I had this same experience at the 600 hour mark on my Lyc O-360-A1A. That was 4 years and 800 hours ago. I was at 13,000 feet above Columbia, SC on an IFR to Charlotte when JAX Center gave me a descent to 11,000. I looked at the panel when the altimeter crossed 12,000 and the engine got the shakes. Instinctively pushed the mixture, hit the fuel pump, switched the tank, rev?ed up and down and no change. Noticed #4 EGT & CHT plummeting. I told ATC that I needed to land in Columbia as I lost #4 cylinder. JAX Center told me he had declared an emergency for me and gave me to Columbia Approach as they were expecting me.

Although the engine was running rough I still was easily flying at 142 KTAS at 2250 RPM which was the most comfortable setting. Crossed the threshold at 100 knots as wanted to keep the engine running if possible at low RPM and flew it down to the ground. Thankfully I think there is 10,000 feet of runway at Columbia International. Taxied to FBO and shut down with a loud backfire and some black smoke. Pulled back the canopy and the good ole boy at the FBO said, ?That don?t sound too good.? The emergency vehicles were all on the runway when I landed and drove up to make sure I was OK. I was but I can?t type what I said in the next few minutes.

Based on what was going on Eagle Aviation (GREAT PEOPLE!!) got me a hangar as it was clear I would be here for a while. I was introduced to a number of great locals that were more than willing to help. There were quite a few Team RV members in the area, some EAA national board members at KCUB, and a ton of RV builders there too. After describing what happened they all shrugged their shoulders and said, ?Stuck exhaust valve. Here?s what you need...? I learned this entire method from the very beginning of this post on the fly in real time. Thankfully airplane engines are essentially highly glorified lawn mower engines so I pulled it off after driving to Charlotte for 3 days of business meetings.

The gang at KCUB leant me everything I needed with the exception of a reamer. I had valve compression tools of two types and a wobble tester etc. Everything else I had was in my flying tool kit. Yes...I now have everything I need in my flying tool kit to do this job easily in the field, but I don?t carry the wobble tester. The old timers all told me to use a piece of bent welding rod or coat hangar wire and a piece of red 3M pad in a drill since nobody had the right reamer. In a pinch it worked.

In my case the valve seized and bent the push rod and tube. Thankfully there was a parts shop on field and in two days I had push rod, push tube, exhaust gaskets, and tube seal O-rings. Grand total parts was $136. During the entire descent all I could imagine was how much this was going to cost because I was not sure what happened until I pulled the cowl. I was stunned how cheap I got away with on this one.

Took the better part of a day to do the job as I was learning on the fly. Proper spring compression tools are a must but are small and simple when you get the right ones. The bar that Spruce sells is my favorite. Simply put the valve guide was gunked up bad. I had a rough start a few weeks before this event and looking back on it I should have known. I also wish I had been up to speed with the Lyc SB that outlines this whole condition.

I finished the repair and got the engine buttoned up and did a short test flight. All good. Flew home 600 nmi the next day. I bought the proper reamer and then did all 4 exhaust valves. I found #1 a little dirty, #2 was tight, #3 was nothing (funny that one is always the hottest), and #4 got a proper cleaning when I did the reamer. I did all 4 in a morning. The rope trick is fine but 60 psi of compressed air is better.

After talking to a lot of old timers and a Lyc employee at Sun N Fun who would not admit he told me so, all suggested that using Marvel Mystery Oil at 4 oz per 10 gallons of gas and occasionally substituting one quart in the engine oil 1-2 times per year would eliminate this from ever happening again. So that?s what I have been doing. Based on the Lyc SB I thought another check at just over 1,000 hours would be a good idea. I did. Everything clean as a whistle and wobble test was great. I?m nearly at 1,500 hours now and no sign of any issues at all. Since my incident I have flown nothing but AeroShell 15W-50 with Camguard.

I would have rather not had this experience but am a lot smarter for it. Hope my story might help somebody else one day.

To the chap that wrote this original post: Bravo! I wish I had found it when I was in need. I just read it today for the first time. Excellent post and pics. Well done my friend.
 
To the chap that wrote this original post: Bravo! I wish I had found it when I was in need. I just read it today for the first time. Excellent post and pics. Well done my friend.

Hi Steve,

Thanks! I have since moved on from the "rope trick" to using compressed air to keep the valve closed. As part of my annual inspection, I check each exhaust/intake valve for how freely they move int he guides. It adds less than an hours to the total inspection time and gives me great confidence in the fact I won't get a stuck valve at an inopportune time in the next year.
 
Does anyone know or remember the post a year or so ago that discussed using a flex hone to clean up guides? There were two sizes listed that would work. I thought I had that info saved, but can?t locate it. A search hasn?t turned up any info..
 
Marvel mystery oil

We had a Commanche 250 who's O-540 was notorious for stuck valves. So, we ran marvel mystery oil in the gas AND in the oil. It's old school I guess, but we had no issues with stuck valves, and the minty scent was very pleasant.
 
We had a Commanche 250 who's O-540 was notorious for stuck valves. So, we ran marvel mystery oil in the gas AND in the oil. It's old school I guess, but we had no issues with stuck valves, and the minty scent was very pleasant.

What quantities did you run? Haven't had issues with stuck valves to date but have often wondered about using the MMO as a preventative measure.
 
Marvel Mystery Oil

You know, it was so many years ago I can't remember, but I think there was a recommendation on the dang MMO bottle. But, I think I'd just pour a half quart in each side upon gas fillup (?)... Something like that. Our A&P actually recommended this to us at some point, so we went with it. He said it acts as a light oil, which is better able to penetrate and lubricate the top end. ( Your mileage may vary...) I'd snag a bottle and read about the stuff. We liked it, and it's really cheap insurance against the sticky valve thing, plus it does smell nice.

:)
 
The early O-540A's in the 250 Comanche had 7/16" valve stems -- later models [and the fix] was to replace with 1/2" heads, which has long been standard. The issue was heat transfer - the smaller diameter stem heated up more, and resulted in 'coking' more than the 1/2". And that is still what causes this - 'cooking' gunk into the guides/ onto the stems. Full rich ground ops, cold cylinders, etc.: you get gunk. Can't be absolutely avoided, but can be reduced by operational methods.
 
The early O-540A's in the 250 Comanche had 7/16" valve stems -- later models [and the fix] was to replace with 1/2" heads, which has long been standard. The issue was heat transfer - the smaller diameter stem heated up more, and resulted in 'coking' more than the 1/2". And that is still what causes this - 'cooking' gunk into the guides/ onto the stems. Full rich ground ops, cold cylinders, etc.: you get gunk. Can't be absolutely avoided, but can be reduced by operational methods.

The black stuff that fills the clearance between the stem and the guide is coaked oil. Coak deposits are black and very hard and are the result of heating oil past it's oxidation point. fuel mixture during ground ops has nothing to do with this, nor does running your engine cold. The only real preventative measure is to increase oil flow to the rocker box, as increased oil flow in that area helps to cool the exhaust valve stem. I suspect that running LOP in most high CHT environments would also help, but brings detonation risk with it. Ensuring good seating between the valve and valve seat also helps to insure a lower stem heat.

MMO in fuel can help reduce the combustion temp at which lead in the fuel is atomized (exits via exhaust versus deposited). However, coak is not a lead deposit. Have no idea if MMO in the oil helps, but doubt it.

Larry
 
Last edited:
Guys. Need help with securing the valve through either the plug hole or the exhaust hole. My 4 finger pick will not grab the valve enough or right. Can you post a link of what you are using?

Also, the fishing line is too thick to pass between the valve and the guide. I was thinking of using the magnet. I am scared that if I drop the valve, I won?t be able to pick it up.

This is for a parallel valve 360.

Thanks
Erik
 
Guys. Need help with securing the valve through either the plug hole or the exhaust hole. My 4 finger pick will not grab the valve enough or right. Can you post a link of what you are using?

Also, the fishing line is too thick to pass between the valve and the guide. I was thinking of using the magnet. I am scared that if I drop the valve, I won?t be able to pick it up.

This is for a parallel valve 360.

Thanks
Erik

Hi Erik,

I used a popsicle stick with a notch carved out of it to lift the valve, and help align it with the guide:

DSCN4501.JPG
 
Ahh. I read the post many times, clearly forgot the popsicle stick. Will post back tomorrow with how it went, which me luck.
Erik
 
I used a piece of small rubber hose with a loop of safety wire through the tubing. I loop the safety wire around the valve stem and use it to hold the valve and reinsert it after the guide has been reamed. I do this through the exhaust port
 
And you CAN pretty easily fish a valve out of the bottom of the cylinder (pickup fingers, safety wire, magnet pickup, etc). For your own piece of mind, find a local shop that has a trash cylinder lying around and prove it to yourself. Once you've done it a few times on the bench, that particular fear melts away.
 
Maybe my engine has a sticky valve as well

So I was ground testing a new propeller today and the engine is having some trouble:

O360-A1A

Starts up fine.

At idle the engine has a lot of afterfires - some loud.

Run the engine at 1500 rpm and all 4 CHT's and EGT's some up together. Engine runs smoothly.

Between 1200 and 1500 you start to get engine stumbles. CHT-3 and EGT-3 start to come downbelow the others

Between IDLE and 1200 more stumbling and more afterfires.

Worst at IDLE - there is more stumbling and more afterfires. Some loud. EGT-3 drops rapidly to a point 300 degrees below the other three. CHT-3 follows suit only more slowly. It ends up about 50 degrees cooler than the other three.

Come back up on the throttle to 1500 rpm and it all smooths out. CHT-3 and EGT-3 temps come back up even with the rest.

Behavior is the same whether on LEFT mag, RIGHT mag or BOTH.

Things tried:

Compression on #3 cylinder was measured at 77

#3 Intake gasket was brittle so it was replaced. The other 3 intake gaskets will be replaced.

#3 Intake line inspected for cracks - none. Made sure it was tight.

#3 Plugs were swapped with another cylinder.

All Rocker covers removed and valves inspected while turning the prop - all worked fine. All rocker gaskets replaced.

None of these ideas worked.

I'm getting the impression from reading this thread that this might be a sticky valve.

Any other possibilities?
 
How many hours on the engine? This usually starts occurring north of 500 hours. To verify your assumptions you will have to do either a wobble test which requires a special fixture, or remove the springs and check with a ream.
Does this happen with the engine cold or at temperature?
 
How many hours on the engine? This usually starts occurring north of 500 hours. To verify your assumptions you will have to do either a wobble test which requires a special fixture, or remove the springs and check with a ream.
Does this happen with the engine cold or at temperature?

About 400 hours.

The stumbles and backfires started at once with the engine cold but through the day we ran it several times so it was well warmed up. No change in behavior.
 
Either you have bad intake leak in number 3 or your ex valve is sticking. I have seen on two occasions, engines with 400 hours have a sticking valve. I am leaning towards the valve. It is not that hard to check the valve. I usually pull the exhaust stack and hold the valve with a short piece of tubing with a length of safety wire looped though it and around the valve stem to hold the valve. If you find one that’s in need of cleaning, I would do them all .
 
Back
Top