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Rough engine on takeoff roll

9

9Driver

Active Member
RV-12, 18 hrs, OAT 85. Flew the pattern three times, with a full stop, taxi back and takeoff. On fourth attempt, just prior to liftoff, engine became extremely rough. Aborted takeoff, taxied back to ramp. Did ignition check at 4000 - OK. Made three full power ground runs all resulting in a smooth running engine. Taxied back and started takeoff roll. Again just prior to liftoff engine became extremely rough. Returned to hangar, removed upper cowl and checked engine. No fuel smell, no apparent issues. Fuel pump and hoses very hot. Possible vapor lock? Apparently only occurs above static rpm range. This is first time to have just flown pattern.

Any help appreciated.

Thanks in advance
 
Since the mag drop was good (most likely not electrical) and the run ups are good then the next step is to pop the carb bowls off and take a look. These are common symptoms for a reduced fuel flow. Carb debris needs to be ruled in or out and the easiest place to look and is a common cause. Debris usually won't show up on a run up many times. Too long to explain, but will get in the main jet fuel flow at full throttle during a climb or flying.Don't look for exotic problems, I'll bet money it's an easy fix.
 
9Driver said:
RV-12, 18 hrs, OAT 85. Flew the pattern three times, with a full stop, taxi back and takeoff. On fourth attempt, just prior to liftoff, engine became extremely rough. Aborted takeoff, taxied back to ramp. Did ignition check at 4000 - OK. Made three full power ground runs all resulting in a smooth running engine. Taxied back and started takeoff roll. Again just prior to liftoff engine became extremely rough. Returned to hangar, removed upper cowl and checked engine. No fuel smell, no apparent issues. Fuel pump and hoses very hot. Possible vapor lock? Apparently only occurs above static rpm range. This is first time to have just flown pattern.

Any help appreciated.

Thanks in advance
Click to expand...

Get a fresh start in the morning. See if the problem has corrected itself. If the plane runs fine then it's heat related. How long was your taxi back between the last normal takeoff?
 
The same has happened to me in another 912 powered aircraft. It is most likely fuel contamination. When you remove the bowls look very closely for ANY debris in the fuel bowl. Then remove the jets and carefully clean them and inspect them for blockage. It does not take much to stop up the main jet on a Bing carb.
 
Rough Engine on take off

I had a fuel filter that started to plug and would only act up when at full throttle long enough to make the engine go lean....
 
In addition to the previous excellent suggestions...

Download flight data from your Dynon. You may see fluctuations in fuel pressure and fuel flow. Earlier RV-12's had the fuel pumps which were superceded by an improved one by Rotax. BTW, the fuel pump does not have to show signs of leakage to be partially inoperative.

One other thing to check with the top cowl off: Make sure your ignition modules are tightly grounded to those bolts on the intake manifold. If not, spark could be intermittent.

The above is not speculation; I experienced both in the first 50 hours.

Good Luck,

Jim
#264 flying 245 hours
 
My Recent Experience

9Driver,

Been fighting this same/repeatable problem for a week now on a 100hr engine. Every indication (to me) was ignition breaking down at high temps... run-up and full power ok once, maybe twice, after reaching 122* oil temp, then 3800 max and missing at full power. Let it cool and repeated numerous times. Changed plugs, checked all grounds & plug wires etc., and no change. Based on low CHT on #2 and EGT on #4, and advice from Lockwood, I went ahead and pulled the left carb. Bowl was dishwasher clean and not a speck of debris. Removed floats needle/seat, blew out all jets, and re-installed. Never positively identified the problem, but it's GONE! Not a real confidence builder.

I believe Roger is spot-on in his diagnosis of your problem. Follow his lead and advice.

Have a great day!


9Driver said:
RV-12, 18 hrs, OAT 85. Flew the pattern three times, with a full stop, taxi back and takeoff. On fourth attempt, just prior to liftoff, engine became extremely rough. Aborted takeoff, taxied back to ramp. Did ignition check at 4000 - OK. Made three full power ground runs all resulting in a smooth running engine. Taxied back and started takeoff roll. Again just prior to liftoff engine became extremely rough. Returned to hangar, removed upper cowl and checked engine. No fuel smell, no apparent issues. Fuel pump and hoses very hot. Possible vapor lock? Apparently only occurs above static rpm range. This is first time to have just flown pattern.

Any help appreciated.

Thanks in advance
Click to expand...
 
Rotax problem

We've experienced that also and it was the float adjustment . It was set to low and one carb was actually running out of fuel, and it always happened at take off power, not sure why those carbs do that but they do. Saw it on an older 80hp and the newer 912S.
 
The float arms should be 10.5mm from the edge of the bowl seat to the top of the float arm. You can set a MM ruler (or buy the expensive Rotax tool gauge) on the bowl edge and slide it over to the float arm. If it is too high or low the tab that the Viton tipped needle valve is attached to can be bent slightly to make the arms hang higher or lower. The carb needs to be off the engine and upside down to do this. Check both sides of the float arm to make sure each float arm is equal to the other.
Sometimes if you have an old needle valve the internal spring goes bad and it won't control the level well. Then you typically have a little flooding.

This is very easy to do.
 
Have ordered float bowl gaskets. Will check/clean bowls and jets and then fly or attempt to . Will post results. Thanks to all for info/suggestions.
 
Removed carb float bowls. Found a few very small black flakes? particles? in both bowls too small to identify. Cleaned bowls, removed and blew out jets. Reinstalled with new gaskets. Ground run OK. Flew this morning - smooth on takeoff and throughout the entire flight.

Many thanks for all the help.
 
small black particles are either rubber hose specs or the floats were flaking. In another 3-10 hrs. pop the bowls off the bottom and check once more to make sure it isn't going to be a repeat issue at the wrong time.
 
My Experience so far

We have around 50 hours on our -12 and have experienced this exact problem. In October we replaced the old style fuel pump with the new one (just as a pre-caution). About a month ago we did a takeoff and the engine went very rough right after adding take-off power and then could not replicate it again. Then a couple of weeks ago it was happening on the ground almost every time power was added. Did the "mag check" and found that it did not change anything. So we pulled the carbs and replaced all the suspect parts everyone has talked about here. We started it up and did lots of full power run ups and everything appeared to be great so we took off and flew around. I did notice the fuel pressure dropped a little when climbing, something I had not noticed before. So, over the airport (probably should have tried this on the ground first:roll eyes:) I pulled the fuse of the electric fuel pump. I have done this many times and typically the fuel pressure drops a few psi and then comes right up. This time it dropped to .1 psi and the engine started missing. It came back to life after I put the fuse back in and we landed. On the ground I pulled the fuse again and it did the same thing. Went to about .1psi but this time the engine kept running and didn't go rough.

We have a new mechanical pump coming and will update everyone on what we find. We may check the float adjustment as well.

Everyone may want to try this test on the ground and see if there was a bad batch of the new style fuel pumps out there.

The other thing we cannot figure out is the fuel smell on takeoff during this time. We never had it until these past few flights and can sometimes see a little fuel in the aluminum catch under the carbs. If the mechanical pump is bad, why would that cause fuel to come out of the carb. It does not look like anything has come out of the overflow tube on the mechanical pump.

I will update after the new pump is added and more checks are carried out.
 
I think this illustrates the downside of not having a fuel pump switch. If the electric is running all the time you may not find a mechanical pump failure until both are inoperative. I've posted before on the mod I did to wire the electric pump into the unused Nav light circuit that gave me a simple way to add a control switch. I conduct run ups on the mechanical pump and then turn on the electric as a backup for takeoff and landing.

I understand the argument that a continuous run pump is one less switch for a pilot to forget, but in this case I believe the switched pump is the better design.
 
RFSchaller said:
I think this illustrates the downside of not having a fuel pump switch. If the electric is running all the time you may not find a mechanical pump failure until both are inoperative. I've posted before on the mod I did to wire the electric pump into the unused Nav light circuit that gave me a simple way to add a control switch. I conduct run ups on the mechanical pump and then turn on the electric as a backup for takeoff and landing.

I understand the argument that a continuous run pump is one less switch for a pilot to forget, but in this case I believe the switched pump is the better design.
Click to expand...

I agree with the fuel pump switch but I believe that leaving it on during flight is safer to avoid vapor locks in the fuel line upstream of the mechanical pump. I do not switch the fuel pump on during ground operations or during run-up. I switch it on just before takeoff and check its impact on fuel pressure before going WOT. I believe this mode of operation allows to detect potential failures on both the electrical and mechanical pumps in a safe place i.e. on the ground on a routine basis.
 
switch

Perhaps a guarded switch for electric pump and a added item to pre take off check list to ensure mech fuel pump is operating normally
 
Good point, Jean-Pierre. I never thought much about that issue, but regardless of the cause for rough running I would turn on the electric pump and figure out the cause when I'm back on the ground.
 
Floats

I have an old BMW "airhead" motorcycle which has Bing carbs like the Rotax. The carbs floats have to be adjusted just right between the point where they overflow and where they suck the bowl dry.
 
bmw

jpowell13 said:
I have an old BMW "airhead" motorcycle which has Bing carbs like the Rotax. The carbs floats have to be adjusted just right between the point where they overflow and where they suck the bowl dry.
Click to expand...

Me too, gas on your foot when it overflows.
 
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