C90 uneven mixture issue

ThePipster · Post by **ThePipster** » Tue May 01, 2012 10:00 am

Hello All,

I have a slightly puzzling problem with my recently rebuilt C90 engine. The front cylinders are running lean and consequently the EGT is about 150-200F higher than the rears. The front plugs are very pale and the rears are sooted up. The engine has less than an hour on it since a complete rebuild which included new induction tube rubbers and all new gaskets and these all seem to be secure.

The engine originally had a Stromberg carb which was in a sorry state and so we have fitted a Marvel-Schebler unit instead. It has been suggested that the problem is related to turbulent or uneven air flow through the carb causing an effective difference in mixtures between the front and rear cylinders.

Can you think what may be causing the issue?

Thanks

Phil

Gerry Holland · Post by **Gerry Holland** » Tue May 01, 2012 11:33 am

Phil
Take a look at these comments from Harry Fenton. They may give a clue.

http://www.bowersflybaby.com/tech/fento ... i_cyl_temp

I know it sounds a pain but would it be possible to refit Stromberg Carb. just to test fuel distribution results for diagnostic reasoning.

What about the Marvel Jetting. Is it matched to C90?

Regards
Gerry

ThePipster · Post by **ThePipster** » Tue May 01, 2012 12:16 pm

Hi Gerry,

I have trawled through the Harry Fenton site and even sent him an e-mail.

The carb is the one listed on the type certificate for the C90, but one difference is that I have a carb with a one piece venturi which is also supposed to change the carb characteristics.

Phil

G.Dawes · Post by **G.Dawes** » Tue May 01, 2012 9:26 pm

I had this problem, and the new one piece venturi ran awfully so I had to invest another hundred and so for a pepperpot main jet aerating tube, don't know if it works yet as I put the original stromberg on for the moment and that runs beautifully. Simple seems best, but also check for leaking gaskets at the head or rubber tube connectors, it doesn't take much of a leak to upset everything.
Graham

ThePipster · Post by **ThePipster** » Tue May 01, 2012 9:51 pm

Thanks Graham,

Was that a brand new carb or a refurb? I have bought a brand new factory built carb, it would be pretty poor show if I needed to spend more money to get the thing to work well.

Phil

G.Dawes · Post by **G.Dawes** » Wed May 02, 2012 5:41 pm

The carb was refurbisheded with all the new parts required by the makers ie one piece, (£70) venturi, then it needed the pepperpot tube and jet (£120) and then the new metal float (£65 required by the makers) was to be replaced by the new?? blue polyurethane float another £100 or so.
Bit miffed really but it should be alright now but the stromberg is still on and running very well, until I get it changed sometime in the future.

ThePipster · Post by **ThePipster** » Sat May 05, 2012 7:33 am

Spent a chilly spring evening switching carbs, the problem is still the same. Stromberg or Marvel the front cylinders are still EGT 150-175F higher that the rears.

I am going to pressure test the spider tomorrow, but I have another thought regarding the exhausts.....

Unlike virtually every other C90 or O200 installation my exhaust stacks are individual rather than 2 into 1. My rear stacks are about 25% shorter that the front stacks. Is it is possible I have a mixture issue due to different exhaust back pressures?

Pipster

Ian Melville · Post by **Ian Melville** » Sat May 05, 2012 8:16 am

Hi Phil,
I am no expert on engines, but applying a bit of logic would indicate that unless the exhaust length changed during or after rebuild, then it is unlikely that the length is the culprit. Otherwise you would have had the same issue before. Are these temperatures measured on ground runs or in the air?

However the length of the exhaust does effect scavenging efficiency. 2 into 1(and 4-2-1) also assist evacuation of burnt gas at the designed RPM. Exhaust are far more than drains to get the hot stuff out of the engine bay

G.Dawes · Post by **G.Dawes** » Sat May 05, 2012 8:23 am

You almost certainly have a air leak in the system between the spider and the cylinder. if the engine has been rebuilt then the likely culprit will be the castings to the cylinder and the gasket between them, the castings bow and should have been faced on a flat surface to check.. The gaskets will also leak if the the nuts are slightly loose, Check them first. The rubber joints are also a source. check the obvious first and don't dismiss something because you think it is done up tight.

ThePipster · Post by **ThePipster** » Sat May 05, 2012 10:09 am

Ian Melville wrote:Hi Phil,
I am no expert on engines, but applying a bit of logic would indicate that unless the exhaust length changed during or after rebuild, then it is unlikely that the length is the culprit. Otherwise you would have had the same issue before.

There a few unknowns with this whole setup. The first is that my inspector and I have the grand total of 10 minutes flight experience on this aircraft as it is over 8 years since it last flew regularly so we don't actually know if there has always been a problem. The second is that the engine has new had ECI Titan cylinders fitted rather than TCM originals so that could also be a factor. Finally the new carb is a Marvel rather than a Stromberg.

Ian Melville wrote:Are these temperatures measured on ground runs or in the air?

Both on the ground and in the air, the symptoms are the same.

Phil

ThePipster · Post by **ThePipster** » Sat May 05, 2012 10:17 am

G.Dawes wrote:You almost certainly have a air leak in the system between the spider and the cylinder. if the engine has been rebuilt then the likely culprit will be the castings to the cylinder and the gasket between them, the castings bow and should have been faced on a flat surface to check.. The gaskets will also leak if the the nuts are slightly loose, Check them first. The rubber joints are also a source. check the obvious first and don't dismiss something because you think it is done up tight.

That is my next port of call. I am going to pressurise the spider and check for leaks with soapy water. I am also going to disassemble the spider for a visual inspection inside.

Phil

G.Dawes · Post by **G.Dawes** » Sat May 05, 2012 6:26 pm

Another thing is that you have these EGT senders and they may just show what you didn't know before that you do now, it might be worth changing them around

Ian Melville · Post by **Ian Melville** » Sat May 05, 2012 7:49 pm

G.Dawes wrote:Another thing is that you have these EGT senders and they may just show what you didn't know before that you do now, it might be worth changing them around

The state of the plugs back up the sender info?

Steve Brown · Post by **Steve Brown** » Sun May 06, 2012 1:23 pm

Hi - the C90 and O-200 are notorious in their poor mixture distribution. This is mainly due to the tortuous long inlet tubes, the offset nozzle and the design of the cold spider positioned so close to the throttle valve.

There is no time for the mixture to fully atomise before it is 'split' so this configuration causes the fuel droplets to be directed onto the spider walls, which re-liquifies and dribbles neat into one or two inlet tubes resulting in a richer mixture.

The one piece venturi exacerbates this further because it is a far 'clumpier' casting than the much more precision made two piece version and it doesn't flow as well. Fixing one problem can cause other problems- sometimes more serious! Hence the revised AD (below) and TCM 'fix' of the pepperpot nozzle to compensate.

http://rgl.faa.gov/Regulatory_and_Guida ... enDocument

The ideal is probably to revert back to the two piece venturi while still using the better atomising pepperpot nozzle but this requires re-activating the recurring annual/100hr inspection to ensure the two piece venturi is not loose.

I guess I am saying that provided you have taken the good advice & eliminated the aforementioned issues like air leaks etc. you may have to live with the EGT spread (which in my view is not atypical or too bad). People spend a lot of money on fuel injection and even individually tailored GAMI nozzles to get a more even spread of EGTs.

Having good instrumentation is a real benefit for sudden unexplained changes and fault finding and also precise leaning but increased information can sometimes worry one unduly. Just like dust under the carpet!

ThePipster · Post by **ThePipster** » Sun May 06, 2012 5:45 pm

Ian Melville wrote:
G.Dawes wrote:Another thing is that you have these EGT senders and they may just show what you didn't know before that you do now, it might be worth changing them around
The state of the plugs back up the sender info?

Yep, very black.....