Trouble Shooting PA28 140

[Cadismack]

Was on a cross country flight, switched tanks and continued to my destination. Shortly after shutting off the fuel pump the airplane started to surge as if there was a fuel starvation issue. In flight I switched tanks, turned on the fuel pump, tried to operate with different mixture and throttle settings, leaned out to peak and tried to clean the plugs on each magnito, carb heat, all to no avail. The problem was most pronounced at cruise to full throttle settings. After landing I check all three low points for water/contamination and it came up clean. Normal runup, static RPM and another 'burn off cleaning' for each mag and i was unable to develop the same type of surging issue on the ground.

Took off to return to base and the issue started happening again in cruise flight. At first I thought that it could be something in one tank over the other. Ran a few circuits on one tank, no problem. Switched tanks and again then the problem occured. while on the suspect contaminated tank I simulated a cross country (4500' asl) and five minutes later the surge came back. Still couldn't correct it by switching tanks. This time i monitored the fuel pressure which didn't fluctuate.

Thinking it could be a poorly ventilating cap, switch around the caps. Draining the gas and replaced with new. Did the same simulated cross country and after about a .1 in cruise the same issue happened again. The issue never seems to occur while in climb or on the ground. Had the surge happen and returned to land, taxied back and took off for an uneventful circuit.

Problem only seems to occur in cruise flight at about 65%+ power.

The carb was replaced about 15 hrs ago with no issues until recently. I'm assuming that this is the root of the problem but was wondering how a carb would develop this problem, and a reason why it would only happen in cruise flight but work fine after sitting on the ground for a while and then continuing the flight.

Fuel lines needing purging possibly? But again why only occuring in cruise flight? Vapour lock in cold temps? Mechanical fuel pump while the electric boost is working?

Any suggestions? Thanks

[ruddersup?]

Mags, mags, mags. Try (borrow) a different set. They can drive you insain sometime. Even bench checking is not a absolute way of verfying them. You must eliminate the mag possibility then you can move on. I don't have any confidence in the Slick mag. Were new Slick mags installed recently?

[Cadismack]

Not recently but they were bench check less than a month ago. I was just wondering how mags would cause this issue only in Cruise flight?

Thanks for the idea!

[warner]

any hoses changed when carb was done .

[black hole]

Check the runnout on the cam. recently had a cam with a flat lobe. Only showed up a higher power settings.

BH

[ian254]

happen to check your carb heat?????

[SeptRepair]

Not recently but they were bench check less than a month ago. I was just wondering how mags would cause this issue only in Cruise flight?

Thanks for the idea!

Defective magneto coils usually begin to break down when hot. So when doing ground runs you may not be getting the magneto hot enough. You also mention the carb was replaced recently. Make sure the proper model/part number was installed. If the correct carb is installed, a sticky float can also cause problems but more so when banking then in level flight. Further more was the replacment carb current with all AD's c/w? Has the metal float and one piece venturi mods been carried out? Food for thought.

[Cadismack]

Thanks for all the ideas. Still not quite working yet, but heading out tomorrow to check the screens on the carb.

Yes the carb is up to date with ad's an s/n. I found out it was sitting for a little while before it was put on so we are going to check for contamination in the screens to see if that was causing the problems.

As for the mags, is there a way of trouble shooting mags without having to replace/borrow another set? I'd hate to just replace parts to trouble shoot a problem but if that is what needs to happen....

Carb heat slightly smoothes the engine but it quickly makes no difference in the performance. It just continues to serge and progressively gets worse. Running on one mag instead of two does cause the engine to quit though. No difference if its the right or left.

Question about hoses changed... yes, all of the oil hoses where changed in the last annual. Fire guard was also wrapped around just about every hose.

Last attempt at a fix we suspected that the fuel hose from both the mech and elec pump was bending and restricting flow causing fuel starvation. We secured the fuel hose that was attached to a 'spring like' to the torsion bars ensuring that the bend in the hose wouldn't change with additional pressure in the cowl. It was a stretch we know. During the flight, .2 hours of cruise and the issue happened again. This time we quickly increased and decreased both the throttle and mixture. I think it was the mixture ICO but after we did that, the problem went away for again another .2.

For those in the know, does this sound like a sticky float in the carb? Would this cause this problem?

Thanks again for all you help!

[tiny]

Usually if there is an internal mag problem there will be a larger mag drop at higher rpm. Compare the mag drop at idle, 1200, 1700 and 2000.

Tiny

[Cadismack]

FIXED IT!

Fuel filter in the carb. The thumble looking one that attaches to the inlet fuel line. It appears that it wasn't properly inspected before the carb was installed on the a/c. It was clogged to the point that only a small amount of fuel was able to enter the carb; enough to sustain cruise obviously for 12 mins before starvation started to occur.

The filter came with the carb after it had been sitting for a few years. The carb had been green tagged before its last install some 5 years ago and only had 15 hours on it before it had gone in (controlled flight into terrain).

Before we installed this carb we had gone through 3-4 rebuilts that just weren't working (all the correct mods and ad's). We had this one cleaned but obviously not thoroughly enough and it ended up work great, until this problem developed.

Thanks for the ideas and help. Learning alot and appreciate the input into the situation. Glad that it was an easy fix!

[Dr.Piper]

I would strongly suggest checking and cleaning your gascolator filter and fuelpump filter as well. In all my time working on cherokees, I have never found stuff in that thimble filter. I would suggest checking the screen and filters after a few runs to make sure there isnt any more.
Just a suggestion.