Right/Left EGT Imbalance with Airflow Straightener

·         John Austin

·         Topic Author

·         Offline

·         Posts: 7

·         

·         

·         

When still fairly low-time, my Jab 3300 (#2427) had an EGT imbalance. Rotating the bottom of the carb towards the right cylinder bank (the one with the higher EGT's) did not help much. Pete Krotje at Jabiru USA gave me a cross vane airflow straightener with 4 internal vanes 90 degrees apart to install between the 90 degree air duct (smooth bore to minimize turbulence) and the carb's intake. He emphasized to align one set of internal vanes exactly vertical relative to the carb intake.

This worked pretty well for nearly 300 hrs. I'm now pushing 400 hrs and for whatever reason, seem to have more of the imbalance again with higher EGT's on the right cylinder bank. At full throttle/3190 rpm, #1, 3, and 5 show from 50 or 60 degrees F to 100-125 degrees F higher EGT's, usually with progressively more difference from front (#1) to back (#5). If I throttle back, however, to 2750, the EGT's are almost perfectly balanced.

I once again tried rotating the bottom of the carb towards the hotter side as in Pete Krotje's article, "Tuning the Bing." I loosened the carb/engine clamp and the straightener/duct clamp and rotated the carb and airflow straightener as one unit. It really didn't seem to make much difference except maybe the imbalance was slightly worse!!!

My question is, does the airflow straightener negate the effects of rotating the carb? Would it help to rotate the airflow straightener and not the carb?

Thanks,
John
N750A
Zenith STOL CH750

Please Log in or Create an account to join the conversation.

John
N750A
Zenith STOL CH750, Jabiru 3300

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Hi John,

With a few hours on the clock it might be something other than a carby issue... It'd be worth giving your engine a look over, checking for cracked sealant on the intake manifold, damaged seals in the intake pipes, cracking or other leakage on the carburettor connection rubber duct. On rare occasions you can get internal issues in the carby too - the diaphragm can leak or crack or you can get blockages or varnish build up on the needles and jets. Occasionally you can get similar effects from leaks or damage on the exhaust side too.

If that all checks out then get back to me - it might be a flow issue in intake area upstream of the carby - maybe leakage or excess turbulence from the air filter and/or intake duct. But to my mind it'd be unusual for that end of things to go squiffy after 300 hours of trouble-free operation, which is why I'd check the engine over first.

Regards,
Doug.

Please Log in or Create an account to join the conversation.

·         John Austin

·         Topic Author

·         Offline

·         Posts: 7

·         

·         

·         

I'm coming up on the annual condition inspection and I'll give everything a thorough inspection as you suggest! However, I know the rubber carburetor/engine connection duct is difficult to evaluate since it is not easily accessible and the clamps cover most of it! Since the engine is 5 years old, 4+ years of time in-service, and nearly 400 hrs, would it be just as well to go ahead and replace the rubber duct even if it looks OK? I didn't know if there was a recommended replacement interval or time in service interval, but I'd hate to button everything back up and then have it fail in 6 months! Ha!

Thanks for the quick response!

Please Log in or Create an account to join the conversation.

John
N750A
Zenith STOL CH750, Jabiru 3300

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

No problem John 🙂

As a rule that rubber connection ages pretty well so I wouldn't usually recommend a pre-emptive replacement, but it depends on the installation: how hot it gets, how much stress it's under (in some applications there's some permanent load on it due to forces from the intake pipes), etc. Even without deterioration though it's worth checking that it's on straight and secure... and it's one of those parts where it's easier to remove it totally and give it a proper going-over than to try and get a good inspection done in-situ.

The more likely suspects are usually the black sealant used where the intake pipes go into the intake plenum and the tightness / security of the attachment of the intake pipes to the cylinder heads (especially on older engines with intake pipe gaskets)... Dirty or torn air filters can also cause issues, as can tears or bunching in the SCAT hose leading to the carby or cracks in the rubber connectors for the intake pipes... the list goes on!

As you say, sounds like a good idea to give the whole installation a good going-over and proceed from there!

D.

Please Log in or Create an account to join the conversation.

·         John Austin

·         Topic Author

·         Offline

·         Posts: 7

·         

·         

·         

Doug,

One more question and I'll give you a break for a while! Haha!

I know there were superficial cracks on the rubber intake runner connectors that go between the engine and the manifold and I will carefully check those again! Is there a sufficient gap between the metal intake runner tubes that the rubber connector can be slipped on or off, OR, do you have to loosen the intake runners at the engine? I couldn't find anything about it in the maintenance manual.

John

Please Log in or Create an account to join the conversation.

John
N750A
Zenith STOL CH750, Jabiru 3300

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

All good John, that's what I'm here for!
There's not enough room to get those rubber joiners off without removing the pipes.. And the lower ones are sensitive to being wriggled around which makes it harder. The rubber couplings are generally pretty good - as you say, the exterior cracks are usually superficial - so unless you see something pretty striking on them I'd be inclined to leave them alone.
Definitely worth checking the security of the screws holding the intakes to the heads though, and the gaskets there (if your engine has them).
Enjoy!
Doug

Please Log in or Create an account to join the conversation.

·         Ian McClelland

·         

·         Offline

·         Posts: 2

·         

Hi Doug and John.

Did you manage to resolve this issue? I have had the same issue on my Jab 3300 powered 601 for some time! EGT's are reasonably even except for full power where No 5 spikes up to 720 Deg C or higher if I let it. All the other cylinders stay below 680 Deg C.
I have tried a carb vertical intake vane and installed a smooth carb intake pipe. Also checked for manifold leaks etc.

I have an old engine (S/N 007) but it has had the carb Jet mods done. It also has 6 new heads installed and the latest sump and intake pipes.
I have noticed improved performance with the new swept intake but still the High EGT on No 5 persists - except worse!

I have checked all the things you list below.
Any ideas?

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Hi Ian,
Unfortunately this is another of those items where, once you'd eliminated the usual suspects, it can be quite a hunt to find the culprit. That said, a spread of 680 - 720 deg C is actually pretty good; you have a little breathing space on the upper temp limit yet. How much higher than that will it go if you let it?
Regards,
Doug.