Why is #1 leaner than others in climb but not cruise?

[lr172]

RV-6A / O-320 / Carb / Catto FP

At WOT cruise power (8K DA), I have a spread of about 1+ GPH (#4 is the richest, but the others are pretty close) and is manageable, however, in analyzing my EMS data, it appears that my #1 cylinder isn't getting enough fuel in cruise climb (140 MPH). I have an FP and most of my climb is around 2500 RPM. #'s 2-4 are all in the 200 degree ROP range. However, #1 is around 100 ROP. Not surprising, the CHT is around 20-30+ degrees higher than the others. I am well under 400, but ambient was only 30.

I don't want to drill the jet out any more, as 2-4 seem to be where they belong. Any ideas here? It doesn't seem like an intake leak, as it is in range at lower MP's and at 29.5 MP, there is very little vacuum, so a leak wouldn't seem to account for this.

Any thoughts or ideas here?

EDIT: I should clarify my request here. I am wondering 1) is 100 ROP a problem I should be concerned with and 2) is there anything that I should be checking or can do to resolve this, short of drilling out the jet larger still (at .039) now

Larry

[alpinelakespilot2000]

Your issues sounds a lot like mine. Your carb (+ sump combination) probably does not distribute fuel very evenly. This is a common feature of carbed engines that we didn't know so much about until we had 4 cylinder CHT/EGT monitoring. For me, WOT (in climb) #1 and #2 run leaner. Pulled back a bit (in cruise) #3 and #4 run leaner. Some have found a bit of carb heat helps even out the flow, though I have not yet been successful at this.

[jrs14855]

I have seen claims that wrapping the lean cylinder intake tubes with exhaust wrap will richen the mixture slightly. I have never tried it.

[videobobk]

I expect you will get a number of people who live with this. I do. To me, it is just the result of having a carb vs. nicely balanced fuel injection. As said before, we didn't even know this was happening until all the nice numbers were right there on our panel. There seems to be several settings where the numbers come closest, and I can even things out tremendously at cruise. I just stay our of the "red" and don't spend much time in the yellow, and don't worry about it. I have over 1000 hours on as set up like yours and haven't had a compression check of less than 76 on any cylinder, ever. Usually 78 or better.

Bob

[Jesse]

If that cylinder is staying under 400 CHT in the climb I wouldn't worry about it at all.

[BillL]

Larry, Are you running at WOT in both conditions? Have you tried reducing the throttle just a hair to use the throttle valve to tumble the mixture? It has been known to help with equalization, w/o significantly affecting power.

It is an easy thing to try on your next flight.

[lr172]

Originally Posted by BillL

Larry, Are you running at WOT in both conditions? Have you tried reducing the throttle just a hair to use the throttle valve to tumble the mixture? It has been known to help with equalization, w/o significantly affecting power.

It is an easy thing to try on your next flight.

Yes, these were both WOT. I noticed during experimentation up high that my EGT spread comes together a bit better at WOT. It will try pulling the throttle back a bit during climb and see if that helps.

Thanks for the idea.

Larry

[lr172]

Originally Posted by Jesse

If that cylinder is staying under 400 CHT in the climb I wouldn't worry about it at all.

It was 380 at 30 OAT. Not sure it will stay under 400 in the warmer weather. I seem to remember seeing one of the CHT's peaking over 400 during climb in Sept (early in break-in). I am not concerned too much about 400-410 in climb. I was more worried about detonation. I suppose I need to live with it and check for signs of detonation as time goes on. I could drill the jet out some more if I see any detonation evidence.

Larry