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Sudden onset of misfire between cruise power & WOT

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Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Posted by: Another Roger on 18:43:58 02/05/15 Thu

On my last flight the 2200A engine ( s/n 995) of my UL450 developed misfire between approx 2600 - 2900rpm. This has subsequently been replicated on several ground runs on one/both mags, with the electric pump on/off, carb heat on/off. The fuel filter has been changed, air cleaner checked, carb stripped all to no avail. All t&ps normal, the engine accelerates from idle as smoothly as ever until the top range is reached. Has anyone experienced similar or suggestions to offer?
Anonymous
 

Re: Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Posted by: DL on 18:53:55 01/18/15 Sun

Interesting symptoms. although unlikely as you seem to be experiencing problems with both mags on and you seem to have carried out the normal service checks (tappets, filters etc, I would check that:

1. coils - ensure they are good and wired up correctly (earth, HT leads distributor and rotors as moisture is a right pig especially in cold weather).

2. do a leak down and ensure that compression is good. use a stethoscope to check that you do not have a problem with rings (oil filler hole), exhaust (exhaust outlet) or inlet valves (carburetor mouth) not sealing. You may also want to check valve springs (are they within spec)?

3. Probably worth checking fuel level in carburetor bowl and running a fuel flow test (check manual)

4. check for leaks in inlet manifold (check manifold rubber for cracks)

5. check exhaust manifold is properly secured and makes a good seal (a visual inspection around exhaust port may reveal carbon deposits indicating that there is a leak)
Anonymous
 

Re: Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Posted by: Gary on 19:30:10 01/18/15 Sun

Unless you have 2 plugs in the same pot that are faulty and by what you have checked I think it rules out electrical issue.
Have you/did you remove the inlet rubber and check it internally as well as externally for splits
Cheers
Gary
Anonymous
 

Re: Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Posted by: BobP on 11:00:12 01/19/15 Mon

In addition to the above it might be worth checking:

Plug colours (and EGT temps if you have an EGT meter) ie correct mixture strength. Check all plugs in case one cylinder is different (plug is a different colour). Replace with new plugs while you are at it (you can always save and reuse the old plugs if they are OK and the plug swap makes no difference).
Prop pull-through gives even forces on all cylinders?
Check the carb needle/piston assembly move smoothly (lift it with your finger through the carb air inlet).
The inside of the exhaust pipes (not too sooty)
Ignition coil to magnet clearances. Also colour the magnet poles with a marker pen so you can see if there is any scraping.
Check the flywheel is not coming loose.
clogged/dirty air filter.
Balance tube from carb to air filter missing.
Make sure your fuel is not stale.

If the above does not find the problem try things that are more messy/difficult to do:

fuel level in the float chamber and gunge in the float bowl/jets
Fuel pressure(engine running) at the carb inlet (Tee off a cheapo mechanical fuel pressure gauge)
Sticking exhaust/inlet valves (I don't know an easy check for this)

A friend's plane that developed "hiccups" was sorted by blasting an aerosol of carb/inlet cleaner into the engine whist running. At the time the (then) UK agent suggested to check that a valve seat had not come loose.
Anonymous
 

Re: Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Posted by: Another Roger on 21:05:45 01/19/15 Mon

DL, Gary, Bob P,
Many thanks for all of your input & thoughts - much appreciated.
I have carried out most of the checks suggested, so far without result but will pursue the others as necessary. ' Dynamic compression test' all 150 +\- 5 psi, plugs all good colour. Tomorrow hope to carry out fuel pressure test and further checks for inlet/exhaust air leaks. Will inform when/if positive result found for general info.
Thanks again
Anonymous
 

Re: Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Posted by: Vic on 09:04:59 01/20/15 Tue

Had a 3300 once that was fine in all respects until about 300ft off the ground at full power when it would cough. Cut a long story short but it turned out to be a tiny sliver of rubber on the fuel inlet side of the carb blocking the float needle. Only did it when at climb AOA at full power!!
Anonymous
 

Re: Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Posted by: Another Roger on 17:04:45 01/21/15 Wed

Vic, Many thanks for your reply. Have checked this area, but will look further for similar as your experience seems identical to mine.
Anonymous
 

Re: Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Posted by: Vic on 07:50:13 01/22/15 Thu

The only way I found it was when I took the float bowl needle valve out and blew back with a bit of tube on the float bowl side and a bit of tissue to catch anything on the inlet pipe to the carb.
This was only after several hours of head scratching, fuel flow checks with the tail held down, ground runs and a couple of buttock clenching test flights!!!!
Anonymous
 

Re: Sudden onset of misfire between cruise power & WOT

Postby Anonymous » Tue Feb 10, 2015 10:17 pm

Did you ever find what it was Roger?

Posted by: Clive on 18:43:58 02/05/15 Thu
Anonymous
 


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