90 Marathon 2 Stroke no power in bottom end

seahunter

This thing is starting to drive me crazy. I've tried just about everything I can think off. I must be missing something obvious.

Symptoms:
Slow, no power on the bottom of the rpm range. Once it gains RPM it runs pretty well until the RPM drops back in the lower ranges again.

You will be running along and out of the blue you will get a couple of dead misses then it will run OK again.

Existing Conditions:
Compression 95 PSI. Adding oil to the cylinder does not increase the compression.

It starts on the turn of the key, even in very cold conditions

New seals installed last Spring

Almost acts like the timing is retarded. Checked, in the middle of the adjustment slot

Has HEI ignition upgrade on it.

New carb and the plug’s insulator is a nice light brown color.

Motor sounds good while running. No pre-ignition, bangs or rattle

Pulsar fails ohm tests. Still outputs ½ volt when cranked

A new spark plug seems to help the miss for a while. No real affect on the no power in the lower RPM issue

Removing muffler has no affect

BGW

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hms11

95psi is about 35-45 psi short of what you need. Believe it or not these little sewing machines need 130-140psi comp to run correctly.

Actually, judging by most of the comments you see on here, your lucky the thing runs at all being below 100.

Sounds like a rebuild is in store for your little 3pg, luckily their cheap kits, especially if you only end up needing a top end rebuild.

seahunter

The compression maybe on the lower side, at least when measured with this compression gauge, but I don’t see that as being the root cause of the problem. The fact that a "wet" compression test yielded the same results as a "dry" compression test pretty much tells me it is not a ring problem.

How would low compression cause an intermittent miss?

hms11

Well the intermittent miss could be a whole different ball of wax, but the fact is that 95psi is just not gonna cut it. Low compression is definately going to kill your power, especially in the bottom end when the cart really needs the power to get moving. Like I said earlier... I'm actually amazed the thing runs at all with below 100psi, let alone runs decent once it starts winding out.

If Dave or any of the other Marathon 2-stroke experts come along I'm sure they'll agree. Before you spend to much time hunting down other problems your going to have to fix the compression problem.

Since the plug seems to be nice and golden brown I'm going to go ahead and say I doubt its a fuel or spark issue.

Dave Box

hms11 is on the money. Your engine is not going to be that great with that compression. The wet compression is not a valid test on a horizontal cylinder as the oil is not going to be able to seal the rings as it does on a vertical cylinder.
As you will be aware the 2 stroke engine needs to produce crankcase pressure oscillations in order to drive the fuel pump and transfer fuel from the crankcase to the cylinder. If there is sufficient blow by you may well be interfering with the rhythm of the oscillations causing a momentary lapse in fuel delivery.
If everything else checks out (including the driven clutch not being jammed open) then I too see a top end rebuild in your future.

Dave

seahunter

I think I found the problem. The cylinder is mounted incorrectly. It's supposed to be mounted horizontally, not vertically as it is now.

Guys, I'm pretty sure it’s not a compression issue. I have enough experiance on two strokes to know a compression issue when I see one. Yes, the compression might be a bit low but it’s not the root cause of this problem.

How does the ignition advance work on these things? I like the clutch idea as well. That could explain the perceived loss of power

simple man

Just a thought, I'd try another compression gauge. I say this because, as others have said, the engine shouldn't even run with the reading you obtained!

hms11

There is no advance mechanism, mechanical or electrical. The timing is set at 2degrees (ish) BTDC and doesn't move. Its part of the reason that this motor makes 11HP and the same motor in a polaris 4-wheeler makes 20HP, it has an advance curve which I'm 90% sure is built into the flywheel somehow.
If your sure the compression isn't the problem (which I don't understand. Again, these engine NEED 130psi+) then I'm not sure what else to tell you. Like Dave and I and others have said, there's no point in even looking anywhere else until that compression number is 40 psi higher. All your doing is tuning up a sinking ship, and no matter how much you polish it, its still going to be a turd.

Like Simple Man said, try another compression gauge, if it still comes up 95psi then no matter how much you replace otherwise, its not getting better without a top end rebuild. And remember, just because a wet test doesn't change the reading, doesn't mean you haven't melted a piston, no amount of oil will fix that.