Light Aircraft Association

The C90 in our Jodel is suffering from rough running / vibration. We replaced two spark plugs and cleaned and gapped all remaining ones. After a successful ground run we flew in the circuit. The take off and climb at max rpm was OK but when throttling back to around 2300 rpm in the cruise the rough running and engine vibration started. Applying carburetor heat the engine ran OK. Carburetor heat back to cold and it started running rough and vibrating again. Reducing or increasing rpm from 2300 with carburetor heat cold it ran OK. A tight circuit with carburetor heat on and we landed safely!

We are a little puzzled at to why the rough running / vibration occurs only around 2300 rpm and why carburetor heat seems to alleviate the problem. We suspect the carburetor and our local LAE suggests that we strip and check it. Has anyone had any similar experience with a C90 or any ideas to the cause of the rough running?

Peter Curwen

Check the intake rubbers and also fit a "Pepper-pot jet". This is the correct jet for the one-piece choke tube mod.

Clean the airfilter hot air bypasses it.????

Thanks for the replies.

We will have a look at the intake rubbers this weekend. The aircraft has not flown many hours since the air filter was cleaned at the 100 hour inspection, but we will check it again.

Peter

Check the engine mounting rubber cones.

Had similar symtpoms a few weeks ago.
Cleaned plugs and Cleaned carb, checked fuel filters, nothing looked amiss.
Eventually found that one of the studs had pulled it's thread. When I stripped the inlet gasket was split and I could see dark colouration wher it had been sucking air.
Good advice earlier to also check / replace the intake rubbers.

Fitted helicoil, refitted and running well again

My rough runnnig was however at lower revsm about 1000 to 1200rpm, it seemed fine above 2000rpm.


Good luck

Gavin

P.S. does it run the same on each mag, or is one mag worse than the other?

We checked all the intake rubbers and intake pipes today and found our problem. We had lost a nut on the inlet pipe to cylinder 4 and the whole inlet pipe was loose and leaking in air. I will order a new inlet gasket, nuts and washers. I suspect that should solve the problem.

Thanks Jim and Gavin for the advice!

Peter Curwen

Nordic Flyer wrote:The aircraft has not flown many hours since the air filter was cleaned at the 100 hour inspection, but we will check it again.

My understanding is the 'normal' small Continental air filter should not be cleaned, but replaced when grubby. Sounds an expensive option, but they're only around a fiver, if memory serves.

Peter, did you ever resolve?

I've had an ongoing saga with my O-200. To cut a very very long and expensive story short, my carb had been fitted with a single piece ventuuri, but not the pepperpot nozzle. Replaced this at the weekend and it's like a new aircraft.
Nearly +100rpm static and +100fpm in the climb. Smooth as silk.

Cheers

Gavin

We fitted a new inlet manifold gasket and tighten up the nuts and it been fine every since.

Peter

My C90 ran like a pregnant hippo it turned out to be an inlet manifold gasket to the head, loose and gasket torn. Good as gold after that was replaced and tightened. I would cut out completely at low rev on that cylinder.

Never knpck a pregnant hippo...they are VERY fast....faster than you or I might run....and the hippo is responsible for more human deaths than any other animal in Africa, humans apart

Yes but they wobble and shake all over the place just like my engine did.