Rough Running at Full Power

[Ragwing nut]

Re: Rough Running at Full Power

You have a NAS3A-1 carb.

1. Is the mixture installed? If so is it hooked up? If not is it wired rich or have a cover plate? If cover plate installed, is the reed valve plate installed still? If plate installed, is it tight and not leaking?

2. How old is the fuel hoses? Might be collapsing internally. How are they routed? Are they routed away from the exhaust?

3. Intake leak would not cause rough running at power, it causes rough idle and or hard starts.

4. What measurement didi you set the float level at?

[mohawktipi]

Re: Rough Running at Full Power

Don't forget to check the fuel tank vent system. I accidentally covered my fuel vent with tape and it prevented the fuel from flowing properly.

[Guest]

Re: Rough Running at Full Power

the vent! excellent!

I wish I would have thought of that!

that needs to be checked

[rotaxflyer]

Re: Rough Running at Full Power

Doesn't the main tank vent through the fuel gauge hole in the cap? It is always been the same as it is now and the wire is noticably loose in the hole, so I would assume that it is vented that way.

[flyguy]

Re: Rough Running at Full Power

RE vent, yep.

Sounds like you are checking all the right stuff. One additional one from me: I have heard of the mixture control plate getting debris in the air holes and causing problems. If you are checking it the main thing to remember is the largest hole must be aligned with the large base hole for the carb to run narmal rich.

Actually the plate leaking (rough plate, whatever) will only affect the operation if you are trying to produce a leaner mixture by rotating the upper disk to align a smaller hole to produce less than atmospheric pressure in the chamber.
Oh, you might try one thing: when it throttles down pop the primer and give it a series of shots, if the revs go up you do have a lean problem.

DC

[rotaxflyer]

Re: Rough Running at Full Power

I did try the primer test during the last test run and there was no change - just a bunch of extra black smoke. I'm going to disconnect the primer, plug the hole in the spider and test run. My current theory is that after a certain period of time, the engine vacuum sucks fuel through the primer circuit and the engine goes so rich that it starts to run rough/miss.

I'm running out of systems to check now, so it is only a matter of time before I get this.

My primer is the type where you pull it out against a spring and then let go to prime. Never seen that type before this plane - are they OK?

Andy

[Ragwing nut]

Re: Rough Running at Full Power

I did try the primer test during the last test run and there was no change - just a bunch of extra black smoke. I'm going to disconnect the primer, plug the hole in the spider and test run. My current theory is that after a certain period of time, the engine vacuum sucks fuel through the primer circuit and the engine goes so rich that it starts to run rough/miss.

I'm running out of systems to check now, so it is only a matter of time before I get this.

My primer is the type where you pull it out against a spring and then let go to prime. Never seen that type before this plane - are they OK?

Andy

that sounds like a very old maybe orginal primer. disconnect it and see if it cures the problem

[Len Petterson]

Re: Rough Running at Full Power

hallo andy. your coils are braking down, sa when they are warning up the spark gets weak.
in the final death troughs it may die and comes back a bit.
if you warm her up she runs rough let her cool off and she is ok for a while.
you need new coils.
I hade a fuel hose sweling internaly and choking of the fuelflow it would idle ok but as soon I OPEN the trouttle she would only go to 1900 or sobut it ran ok cleanly! just lack of fuel.
Do you have slouching compont in the tank? it can come loose and plug the outlet.
lenny. call me.

[Len Petterson]

Re: Rough Running at Full Power

Andy In my friends Thorp it was a loose baffel in the muffler what partly covered the outlet.
it cant happen in T craft system can it?
I stll think you need new coils in the mags. I remember this symtom from an outbord engine.
Len
Last your phone # call me.

[Carrieve]

Re: Rough Running at Full Power

One of the ways to check the coils is to remove the mags (yes it is a job) and put an ohmmeter on the primary side and then warm the coil with a heat gun. The resistance will go up a little normally but if the meter go to infinity you have your problem. I have used this on cars and it saves a lot of test driving and towing back to the shop.

[rotaxflyer]

Re: Rough Running at Full Power

I have a spare mag, so I may swap that out and see what happens, but really, what are the odds of both mags failing in exactly the same way? It runs equally bad on either mag or both for that matter.

I did disconnect the primer and runup with the same results.

Andy

[dennispip]

Re: Rough Running at Full Power

From what I was told by my mechanic, mags have a way of failing after about 700-800 hours after new. They may have a calendar life factor as well as TT in service. So the odds of them both acting up around the same time, if they are of the same vintage, would be pretty good. The odds of them both going totally dead at the same time, very low, as they seem to act up for a while before totally going dead. He suggested to me that it is a good idea to have them be of different TT of use or "staggered" by a few hundred hours and have one of them pretty fresh at all times.

Just a thought worth consideration.

[mohawktipi]

Re: Rough Running at Full Power

Discussing your problem with a couple of "old timers" (eighty somethings). They asked a question about oil pressure. Is there a change in oil pressure when the rough running starts? They suggest that the valves may be sticking once the engine heats up. Could be caused by lower oil pressure/poor oil circulation or weak valve springs.

[rotaxflyer]

Re: Rough Running at Full Power

Oil pressure comes up quickly (say 3-4 seconds) and stays above 30 except when idling. No change in pressure associated with the roughness/missing

Andy

[Pearson]

Re: Rough Running at Full Power

Andy,

It sounds like you are getting a lot of good advice. My guess is fuel starvation. I suspect maybe your float is sticking or something like that is keeping the bowl from refilling with fuel, or otherwise restricting the fuel.

How about trying this; run the engine up, and when it starts running rough and the rpm drops off, SLOWLY add primer fuel. If the rpm picks up, that confirms my theory of fuel starvation. You still wouldn't know exactly what to fix, but it would eliminate the mags and valves as a possibility.

On the other hand, if the rpm does not pick up, I would think that would warrant investigating the mags and then the valve train.

My thinking is that if the coils/mags or even the valves are the problem, you wouldn't be able to go to idle for a few minutes, and then be able to go to full rpm for 30 seconds before the problem appears. If it were mags or valves, the problem would be there right away because the engine is still hot.

I would try to find a known "good" carburetor to install for a test run.