Assistance Request - Interesting Engine Troubleshooting Problem

[lr172]

Originally Posted by vic syracuse

[i] It's all I can do to not delete the post so it doesn't send someone down a wrong path in the future.

Vic

I strongly encourage continued restraint from deleting posts with which you disagree. Discussions such as this, as unpleasant as it may seem, is how we collectively debate and dispell the large body of inaccurate knowledge that is based upon decades of hearsay. Imagine if all of Deakin's material was deleted because it was "known" to be wrong.

My apologies to Canadian Joy for the thread drift.

Larry

[Canadian_JOY]

Originally Posted by rocketbob

Ok. What is the fuel flow when the stumble occurs?

Fuel flow is passing north of 15GPH - haven't closely watched where it peaked as I was spending more time looking at EGT/CHT.

[Canadian_JOY]

Originally Posted by lr172

My apologies to Canadian Joy for the thread drift.

Larry

No need for apologies, Larry. All of this discussion has been really good for keeping my troubleshooting mojo flowing. It's all good!

[rocketbob]

Originally Posted by Canadian_JOY

Fuel flow is passing north of 15GPH - haven't closely watched where it peaked as I was spending more time looking at EGT/CHT.

When it stumbles lean the mixture. If it quits with very little mixture pulling then you're not getting enough fuel. Check that and report back.

Other than than not getting enough fuel then it is possible you're getting preignition at high cylinder pressures. I've seen cylinder deposits do this.

Eliminating the above I would start looking for a broken valve spring.

Eliminating that I would check dry tappet clearances to see if you have a rounded cam lobe.

Sounds like both of your ignition systems are working fine but you have a full-power issue.

[Canadian_JOY]

Last night I found that if I pulled the mixture out about 2" the engine would run perfectly with both ignitions firing. That's when I once again started to think this a fuel/air problem rather than an ignition problem. Carb is being changed again today...

[lr172]

Originally Posted by Canadian_JOY

Last night I found that if I pulled the mixture out about 2" the engine would run perfectly with both ignitions firing. That's when I once again started to think this a fuel/air problem rather than an ignition problem. Carb is being changed again today...

You can take a look at the needle/seat on the carb. That is the most likely suspect given these symptoms, as well as the report of blue staining on top of carb, IMO. Are you able to observe black smoke coming out the exhaust while running rough? That would help confirm an overly rich condition. However, I am struggling to correlate that with the ability to run well on one ignition, but not two.

A fuel delivery problem can introduce trash into the carb and that is most likely to produce problems with the needle/seat. Debris caught in the rubber part of the needle will allow the bowl to overfill and cause an overly rich condition. There is an overflow vent on the carb top where excess gas goes when bowl level can't be controlled by the needle/seat.

Larry

[rocketbob]

Originally Posted by Canadian_JOY

Last night I found that if I pulled the mixture out about 2" the engine would run perfectly with both ignitions firing. That's when I once again started to think this a fuel/air problem rather than an ignition problem. Carb is being changed again today...

Bingo. Too much fuel. Carb has wrong jet. What's the part number of the carb?

[Canadian_JOY]

The carb in question is the "loaner" carb from the overhaul shop. I have no idea what jets are in it since it is indeed a "loaner".

Aircraft owner changed out the carb today - removed the "loaner" and re-installed his original, freshly overhauled carb and performed a ground run. Results were encouraging, thus a flight test is to take place in an hour or so. Will report back with results.

[RV Jerry]

If you have a primer I would check it first

[vic syracuse]

Originally Posted by lr172

I strongly encourage continued restraint from deleting posts with which you disagree. Discussions such as this, as unpleasant as it may seem, is how we collectively debate and dispell the large body of inaccurate knowledge that is based upon decades of hearsay. Imagine if all of Deakin's material was deleted because it was "known" to be wrong.

My apologies to Canadian Joy for the thread drift.

Larry

I did not delete it, and won't. But it is flat out wrong that timing is additive to the 70 degrees that was mentioned in that post.

Vic