Diagnosis Rough Idle/Surging ... Carb ?

[Dave O]

Hi I hope someone can help with this. I've reviewed the archives and there seem to be a lot of possible solutions ... some of which are very time consuming. If I could isolate this down to the top 2 or 3 possibilities that would be great.

Late model A4. At fall haul out I closed the fuel valve and ran the engine till fuel was exhausted. Along with normal winterizing I filled the fuel tank.

Spring launch new points, plugs, dist. cap, condensor, Racor filter and Moyer micro filter. At launch she started right away but ran rough. After 1 hour operation she continues to run rough at idle and under load. When throttle is set for 1,200 rpm she hunts from 1,200 rpm to 2,500 rpm + and back.

I pulled the plugs and all 4 are identical ... very light deposits. I reset the mixture as best I could under the circumstances.

Any thoughts as to the top 2 or 3 things to check ?

Thank you

Dave

[Dromo]

Bad gas

Dave It sounds like you might have water in the gas tank.
Rick

[Mo]

Hi Dave,

My suggestion is to drain your water separator / filter and check for water. You might also try a gas-line antifreeze or two in the tank. If it is water it may resolve your problem.

Mine did the same thing the other day then ran well after. I was going to dump a gasline antifreeze in the tank but forgot...it resolved on it's own after a few minutes and hasn't happened since. ( I use the boat daily).

That's what I'd do first. Let us know what happens.

[Dave Neptune]

Surging

Dave, either it is a bit of water dribbling in or the fuel flow is comprimised. The fuel could be flowing slow enough that until the engine slows down and the float bowl fills again and the RPM's go back up and suck it almost dry again~repeat~repeat.
Contaminated fuel should be consistant as far as RPM's but very little power. The water could cause the cyclic behavior as well. Look for water and if you find none check the fuel preassure at the carb. If the preassure is good at the carb check the needle and seat for a chunk of something.
I doubt it has anything to do with the ignition side.

Dave Neptune

[Dave O]

Hello everyone:

Thank you for your suggestions.

Today I removed the main passage plug and pumped fuel through the carb.
It was a bit cloudly with one larger particle. I repeated the process until the fuel was clear. I then drained the Racor filter and also replaced the downstream Moyer micro-filter with a new filter with no improvement.

With throttle set between 700 to 1,200 rpm the A4 continues to surge to 2,500 rpm + at random.

This evening I ordered a new carb from Moyer and will install as soon as it's received.

In the meantime if anyone has any suggestions that would be appreciated.

Thanks for your advice.

[hanleyclifford]

If you filled the fuel tank at the end of last season the fuel, with ethanol, had all winter to separate and/or clean up the crud in your fuel tank. When you started up this season junk could have made it's way downstream to the carb which may be fouled inside by now.

[Dave O]

Still Not Working

Hello everyone

I received my new carb from Moyer Marine and installed it today. Installation went fine but no improvement in results at all.

As a quick summary, following spring tune-up (points, plugs, rotor, dist. cap, new Racor and Moyer micro filter) the engine will not run smoothly.

When set at 800 or 1,100 rpm she idles smoothly for a few moments and then rpm's surge back and forth to 2,000 rpm plus. Sometimes as high as 3,000 rpm. While this is happening the engine is rough and "staggering" to find a smooth idle. Continues for as long as I run the engine
(30 minutes plus.)

First fix was to drain fuel through the main passage plug. Fuel was a bit cloudly and a few dirt particles. Flushed until clear, drained the Racor filter and replaced the micro filter. No improvement.

Today replaced the carb with a brand new one. Exactly the same symptoms.

Any suggestions or ideas ? Tomorrow I will replace each of the tune-up components I installed during tune-up with the old parts to see if there is an improvement.

At this point I do not see it as a fuel problem.

[Dave O]

I've solved the problem my retracing each of my spring tune-up steps and reversing the process.

Removed the new plugs and replaced with old ones. No improvement but I checked the compression just in case. Readings were fantastic.

121
120
120
120

Then removed distributor cap and rotor and changed them back to the old ones. No improvement.

Then removed points and condensor and changed them back to the old ones and bang that did it. She now runs as smooth as my Honda Accord. The face of the points are in perfect shape and correctly aligned, so I can only presume the new condensor is faulty.

Glad the problem is solved ... not so pleased about the expense of a new carb. Please see my post under "for sale" as I now have a lightly used carb I no longer need.

[David Masury]

Keep the old carb, it is always good to have a spare, but next time you put the engine to bed for the winter, after you shut the fuel off and run the engine dry, drain the remaining fuel from the carb bowl. That little bit of fuel can create a nice mess that will make you spring somewhat annoying.

Point do have a way of oxidizing over the winter, glad the old set worked out foryou.

David