Low power during run-up

[btbell]

Hi all,

My wife and I flew up our '46 BC12B this morning for just under an hour with no problems. We shutdown for about 45 minutes and then fired her backup with no problems. She started on the first pull as usual. When my wife went to do the run-up she could not get power above 1500 RPM. When she advanced the throttle the engine would actually lose power and want to quit. We had to leave the plane stranded at the airport we flew to until we can figure out the problem. The AP is not back until Monday and I am not sure about his familiarity with T-crafts and A-65s. Anyone else ever had this happen or have any ideas?

Thanks,
Brian
NC43255
New Jersey

[Howard Wilson]

Re: Low power during run-up

Maybe an air leak in induction system or loose carb, or an exhaust system restriction, orrrrr throttle cable slipping at carb arm attach point???? Just a few guesses

[L-2 GARY]

Re: Low power during run-up

STUCK VALVE? , PULL PROP THRU TO CHECK COMPRESSION. IF THAT
IS ALL OK, KEEP LOOKING. CHECK THE NUTS ON THE CARB MOUNT,AIR TUBE
RUBBERS TIGHT? MAGS? ENGINE RUNNING ROUGH? I HAD THIS HAPPENED TO ME ONE TIME IT WAS A STUCK VALVE,I FIXED IT AND STARTED USING
ALCOR (TCP) KEEP LOOKING.

GARY

[OldBits2]

Re: Low power during run-up

The symptoms described are identical to those where the pilot pulled the carb mixture to "lean" on a Stromberg carb. If you have a Stromberg carb I would look there first.

Because the Stromberg has no Idle cut off (it is a back suction mixture), it is only effective at or above 1500-1800 RPM. If during shutdown or even descent, the mixture lever on the carb is actuated to 'lean' (voluntary or involuntary), it will allow the carb to operate and the engine to start and operate normally in the idle and mid-ranges, however as the throttle is advanced, the engine will stumble and even die while backfiring and bucking as the mixture goes lean.

Because of this phenomena, many flight schools wired the mixtures to rich and disconnected or disabled any cockpit mixture controls. If that lever on the carb moved, even inadvertantly, it would cause this kind or problem.

Later, many schools even ordered carbs with no mixture plates and a blanking cover so that the issue did not arise with an inadvertant use of the mixture. FWIW, these carbs (unlike others), are self leaning up to about 8000 feet, so disabling this feature is of little consequence to most operators.

Stromberg "leaning" is accomplished by using draft induction air to reduce the pressure on the fuel in the float chamber, effectively reducing the float / fuel flow so that less fuel is mixed with the air available. A low float level, or debris in the needle/seat that simulates a lower float level, could cause the same indications.
D

[flyguy]

Re: Low power during run-up

Strombergs have a vertical feed pipe (don't know the proper designation) that has a VERY small opening. If it is totally blocked by a bit of debris (in my case it was gasket seal) it will cause the engine to not run in mid range, but it will still idle. Whether you could get a partial blockage of this pipe I don't know, but it might cause that kind of problem. I would particularly think about it if you had done any sort of negative G maneuver such as a push-over. That brought the debris up from the float well in my case. Same thing happened to a friend of mine's Luscombe about 2 weeks later. By the way, he nursed it to the field by using the primer. Good idea to remember.
DC

[Ragwing nut]

Re: Low power during run-up

It would not have started so easy if it was an induction leak. I would check for a partially sunk or stuck float.

Mike

[48fordnut]

Re: Low power during run-up

Several yrs ago, I had a similiar situation in a luscombe. Mine was after take off, when power rolled back to 1450 rpm. quickly landed downwind and taxied in. would not go above 1450. opened cowling, pulled plugs, last plug rt fwd had metal deposites. pulled jug found metal on cyl wall. cked timing ,mag had slipped and detonation had ruined piston ,and jug. that was my power loss.

[Pearson]

Re: Low power during run-up

Brian,

Look for the simplest solution first. Any chance it was just carb ice???

Check for the basics first, fuel, air, mixture, timing. How about the butterfly plate in the carb heat box?

[Howard Wilson]

Re: Low power during run-up

Brian, Do you have a prognosis yet??I'll bet some enquiring minds want to know.

[btbell]

Re: Low power during run-up

The same thing happened two days later to the AP who is based on the field. As he tried different things, carb heat on/off, different throttle settings, gas on/off the power suddenly came back. Has been fine since he said. He looked everything over twice and can't explain it except for a possible stuck float that freed itself up. Currenlty I am in TX on vacation and the plane is stuck in PA due to severe winds.

Any other ideas anyone has before we pickup the plane? The AP does not seem too worried about it.

[barnstmr]

Re: Low power during run-up

Where in TX are you?

[Ron Babos]

Re: Low power during run-up

btbell: So, just so I have this straight, it's mechanical, probably the float is sticking in the up position or hanging somehow and it's going to fix itself. I've never seen something mechanical fix itself, only break itself. Nothing but removing the carb and inspecting the float and needle for correct operation is really acceptable. You still have the original problem, and from the original post I read it sounds like the fuel level in the bowl is too low for midrange and high speed operation. If it happens again, pull the primer out and start pumping. If it speeds up, you have your answer.
Ron

[jdoran]

Re: Low power during run-up

Drain the carb bowl. It only takes a minute, and a piece of safety wire. I had the same problem-found some water in the bowl, but strangely, none in the gascolator.

If you are careful, you can catch whatever drains out of the carb in a glass cup for scrutinizing.

Look for water or dirt.

[btbell]

Re: Low power during run-up

barnstmr - I am in Ft. Worth until Thursday and then we start our three day drive home thorugh AK, TN, VA, MD, and PA.

jdorn & Ron - Thanks for these two great suggestions. Actually we did drop the gasculator and found nothing abnormal - clean as clean can be. Actually it was dropped about 15 hrs ago in the August annual. They could not get the drain out of the carb and did not want to try to hard and break it. I am going to look at it and see if I can;t get it out to take a loot and drain the carb bowl. I suspect either we had a stuck float or water as most suspected.

All - thanks for sharing your help and experiences the past week or so.....I will be sure to keep the group updated of any further developments.

Brian

[Ron Babos]

Re: Low power during run-up

btbell: Unfortunately the plug on a Stromberg is not at the lowest part of the bowl. The dirt and water will stay in the carb even if you pull the plug.
Ron