Opposed twin surging w/ odd findings

[drumrkane]

I have a 98 MTD 800-series garden tractor with the 18.5 opposed I/C twin. (4-screw pump, non-adjustable carb).

Here's what it does:

1) Takes a LOT of cranking to start when cold. When warm, fires up with barely a touch of the key.

2) Surges at full throttle. Give it a slight amount of choke and it smooths out.

3) Needs more choke to idle without stalling.

4) When under load (mower or snowblower engaged or driving) it runs fine with no choke until it warms up (see #5.)

5) After about 20-30 minutes of use, it begins to stall and lacks a severe amount of power. No amount of choke seems to make a difference one way or another. Once it's had a chance to cool down a little, it will run a little better, but not for long.

6) Here's the weird one. Whether hot or cold, it runs fine with no choke if the air filter is removed (new filter). Placing the filter over the carb opening immediately causes the surging again until I choke it a little. I have not tried this under load as mowing kicks up more dust and debris than I care to suck into the intake.

What I've done:

1) New plugs and ignition coil.

2) New air filter.

3) New fuel lines (all) and filter. Have confirmed that no air is entering the fuel system ahead of the pump.

4) Rebuilt/cleaned carb several times. Can't get any cleaner than it is. Pump is filling the bowl as it should and float seems to be functioning properly.

5) Tried a completely different carb off a good-running engine.

6) Checked flywheel key... No issues there.

7) Both cylinders have great compression.

8) Eliminated tractor kill switch wiring... No change.

9) Adjusted governor...seemed to be properly adjusted from the get go and linkage is intact.

All of the above have made either very little or no difference in the performance. In general, the problem has gotten worse throughout the last 20-30 hours of use. Been chasing the for about a year now and I'm out of stuff to try. Any help would be greatly appreciated.

 

[Scrubcadet10]

Surging is caused by a engine that's running lean,so you either have an air leak into the intake past the carb, or a crack in the intake manifold allowing too much air in, if i remember right there are 2 gaskets and a space where the carb mounts to the manifold and a gasket at each cylinder head for the intake, I use WD40 in a trigger bottle, and with the engine running, spray down the intake manifold, at the head and where the carb bolts to the manifold, if it blows white smoke or there is a change in engine speed you have an air leak.

 

[Hammermechanicman]

Have you doubble checked compression when hot? Possible head gasket leak. Just reaching here as you have already check a lot of things. The air ceaner thing confuses me.

 

[drumrkane]

Surging is caused by a engine that's running lean,so you either have an air leak into the intake past the carb, or a crack in the intake manifold allowing too much air in, if i remember right there are 2 gaskets and a space where the carb mounts to the manifold and a gasket at each cylinder head for the intake, I use WD40 in a trigger bottle, and with the engine running, spray down the intake manifold, at the head and where the carb bolts to the manifold, if it blows white smoke or there is a change in engine speed you have an air leak.

The gaskets between the carb/spacer/manifold are all new and seem to be sealing properly.

I'll check the manifold itself more thoroughly and report back.

Thanks!

 

[drumrkane]

Have you doubble checked compression when hot? Possible head gasket leak. Just reaching here as you have already check a lot of things. The air ceaner thing confuses me.

I have checked the compression when hot and both cylinders test OK. Don't remember the PSI off hand, but both were nearly identical.

 

[Hammermechanicman]

I have checked the compression when hot and both cylinders test OK. Don't remember the PSI off hand, but both were nearly identical.

Gremlins, has to be gremlins.

 

[Fish]

Check the valve clearances.

 

[Tinkerer200]

Sounds to me like you have more than one problem.
Walt Conner

 

[drumrkane]

Alright...I've thoroughly checked for intake leaks and have found none. I took everything apart (manifold off) towards the end of the mowing season just to have a better look at the intake, check for cracks, etc...found nothing and put it back together. It ran a little better (maybe?) but it was kind of hard to tell as I never really got the chance to load it down with heavy grass again for the rest of the year.

Fast forward to last week...I put the snowthrower on it and started to clear the several inches we got. It was running fine for about 10 minutes, then began hunting and lacking power. It got to the point that it wouldn't even run with full-choke. It starts right back up as if nothing was wrong and runs OK for about a minute, then stalls again. Upon messing around, I notice that if I put my finger over the two little holes next to the carb opening (the two away from the engine) it begins to run better. Not good, but better. Whatever that means. I just noticed that it seemed to be pulling more fuel down into the intake when I did that. Even so, it would still only run like that for so long before it would barely run even with full choke.

The snowthrower is arguably the largest continuous load that this thing sees...so all the issues were much more pronounced than while mowing.

The air filter thing still exists and I've noticed that it doesn't even need the top cap placed over the filter to seal it up...just laying the filter where it goes changes how it runs.

Good compression when hot, so I'm not thinking anything with the valves or head gaskets.

Is it possible that there's something goofy with the governor? The linkage is intact and it seems to respond to loads as it should (if it's not starving for fuel).

Hoping someone here has some ideas...I was ready to set the thing on fire while clearing the driveway last weekend.

 

[cpurvis]

Is a '4-srew pump' a fuel pump? If so, have you checked it?

When an engine winds down under load, it is usually because it isn't getting enough fuel. When it does that, remove the load and reduce the speed to idle before it dies. Let it idle for a couple minutes, then apply the load again. If the cycle repeats itself, there is a fuel supply problem somewhere between the carburetor and fuel tank, such as bad fuel pump or debris in fuel tank.