Light Aircraft Association

Hello
Any advice welcome on a problem our C90 has just developed. The engine starts fine from cold and runs very nicely except:
after a short period at idle (on a glide approach) the engine runs down after landing and will not restart in the short term. On a go-around from idle on the approach the engine will not accelerate to full power and sounds very rough - obviously a dangerous situation.
The ignition system seems fine with minimal mag drops and the engine has not is not long from a top overhaul. No sign of fuel contamination on draining the fuel filter, examining the filter and draining the float chamber. I understand that the engine has been run on Mogas rather than Avgas recently although yesterdays instance occurred after partly filling the tank with Avgas.
On an earlier test circuit where some power had been used on the approach the engine did not run down after landing.
Any suggestions on where to look?

My friends who fly behind C90's tell me they are pigs for carb ice at idle and they're usually pretty sharp at getting the carb heat on after starting. Don't know if that's what you're getting and if your's has just begun exhibiting the effect, probably not.

I'm not familiar with the carb on these engines but if it has an idle jet it might be a speck of dirt partially blocking that. Mine isn't a C90, not even close, but I had a similar problem many years ago eventually traced to that very thing.

John, my first thoughts would be carb ice. You don't mention if carb heat is being used (In sufficient quantity and at the correct time)

Another thing that can happen on all this type engine is that there is an air leak in the induction side look for loose blots on the manifold to carb, the rubber tubing clips and the prime candidate the gaskets and bolt on the elbows to the cylinders, also make sure that the tickover is not too low especially with a light wooden prop. I does sound like carb icing though plenty of heat on the way down and only put it off at the last moment for full power if needed for a go around.

Thanks for the replies. We did think about carburetor icing. But in the case of the go-around and the engine not picking up, the throttle was only at idle for perhaps 30 sec - would it strike that quickly? Also the pilot who has regularly flown the aircraft is in the habit of doing power off approaches from the circuit and this behavior of the engine running down after landing has only just manifested itself.
John

I have unfortunate experience of the vicious suddenness of carb ice onset on a small Continental. Sounds like classic symptoms to me.

You say it has only recently started and you've recently started using mogas. Why won't you see the connection? Our experience of mogas vs avgas in a Volksplane was mogas = icing, avgas = no icing.
Also, ask the old car fraternity about modern fuels vaporising more readily in fuel lines and carb float bowls than the fuel did when the cars were built. Fuel vaporisation (or more specifically, the latent heat of evaporation) is half of the cause of carb ice, the other half being water vapour in the air.

You say the ice forms 30 seconds after closing the throttle on landing: OK, but the throttle has been well reduced right through the approach for the previous 3 or 4 minutes, so heat from the exhausts to ease icing is much reduced too. The carb is very cold at this point and ice is probably already building up around the throttle butterfly. Closing the throttle will bring the layers of ice together, filling the small opening that is the idle position of the carb so the engine stops. The ice will be quite likely to cover the main jet at this point so there will be no fuel entering the air passing through the carb until the ice melts.

The above may or may not happen on a particular day, depending on atmospheric conditions, particularly temperature and humidity. The CAA Safety Sense leaflet on Carb Icing covers this better than I can here.

It would be prudent to check the seals, gaskets etc in the induction system as others have said, but go back to straight avgas and see what happens. If you have 2 independent tanks (and a nice long runway) try avgas in one and mogas in the other and do back to back tests.

Sorry to be so blunt, but risking life, limb and aircraft just to save a few quid annoys the heck out of me.

Carb ice is a possibility but no two engines react the same way. I have a C90, I use mogas and I have never had a carb ice issue with it. Others' experiences have been very different I agree but it is difficult to make direct comparisons.
If the problems started with the use of mogas then the obvious starting point is to first check that your carb heat system is OK, no leaks in the hoses, no splits in the exhaust sytem inside the heat muff, full and correct operation of the flap valve etc, and then try on both fuels to see if you they behave differently.
If the problem persists I'd go for the carburettor first. Maybe there is some dirt partially blocking the slow running jet or airway. It may even be the float needle not fully seating, allowing the carb to flood. At higher throttle openings the excess fuel can be coped with but at tickover it runs very rich and cuts of takes a while to clear when you throttle up again. It won't do it on initial start up because a cold engine can handle a little extra fuel. When the engine is right up to temperature, preferably after it has flown, allow it to tick over for a while and then open the throttle, is there excessive black smoke from the exhausts?
Is an electric fuel pump fitted? Could this be overpowering the float needle? Could you safely fly the circuit without the electric pump on, if so give that a try.
Air leaks in the spider/hose system has been mentioned, an easy thing to check.
Is the baffling OK? You haven't had it off or fiddled with it recently. That can make the engine run rather hot due to air leaks and hot engines can decide they do not want to tick-over.
Fuel starvation, the opposite from the above suggestions. Do a fuel flow check to make sure all is well.
Problems like this are a pain and you have to have confidence in your engine to enjoy flying behind it. Be methodical and try one thing at a time.
Caveat of course, get your inspector on board to help and to OK anything you or anybody else has fiddled with. If you are not mechanically proficient, seek the help of those who are to help you.
Good luck.

I would go for the easy things first. Drain all the fuel out and run only on avgas, the alcohol in mogas will hold water in the molecule and this will increase the risk of carb ice by releasing it in the carbs.

If this does not solve it try examining the hot air box that ducts hot air into the carbs. A small crack or hole can cause a venture effect nullifying the hot air.

I would then go for fuel pressure from any aux pump deflecting the float.

Lastly I would look at vapour locking in the carbs caused by alcohol in mogas, You can end up with more volatile fuel about this time of year prior to the summer stocks coming through.

John.

Looking at the post above reminded me of another cause, an exhaust leak into the hot air system putting exhaust gas back into the intake and causing a cutout at low revs.

All of above worth checking.
Of particular value is to check the carburettor air passages for dirt or blockages. The 3 holes that enter the Venturi at the point where the throttle closes are particularly important. Any blockage here will effect the mixture through the whole range.
When the engine is leaned in the cruise, will it run rough earlier than normal?
If so, it is worth to check the position of the mixture screw. 1\2 a turn will make all the difference.

Also, worth to check the engine log books to see if a one piece Venturi was fitted. If so, it is best to also fit the pepper pot nozzle which cured the issues caused by single pièce Venturi.I appreciate this is a new problem, but this is worth a check anyway

Let us know how you get on

Gavin

Had very similar symptoms with an A65. Fly for more than 30 mins and the engine quit on the landing roll. Would not re-start for over an hour (hand swing). Leave it for two hours and it would start first time.

The engine had Bendix mags and the insulation on the coils had broken down. It behaved perfectly when cold - mag drop was OK etc. But once the mags had heated the spark deteriorated. The coils should have been replaced years ago. Obviously the deterioration was more evident at low revs when the spark would be weaker anyway. Possibly this explains why your engine will not pick up from low revs but is OK when the revs do not drop down to tick over.

I hate to say this, but regarding Mogas. I have a C90 in my Emeraude, I have run this on Mogas for the last six years. It has a Marvel Schebler carb which I fitted six years ago. I have never had a problem with carb ice except once on wet grass taxiing, quickly resolved. I am well aware of the reputation as ice making machines and I apply carb heat at the appropriate times but not excessively. The engine starts easily after a few prime squirts and has never missed a beat. Please tell me why. I should point out that the engine is not "just out of maintenance".