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Unread 04-15-2017, 04:37 PM   #1
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Default '85 Marathon Gas Runs Erratic - full bore or nada

Just recently acquired a project cart - 1985 EZGO Marathon Gas - with the reversible 2-stroke engine.

I am a pretty good mechanic, but I have little experience on 2-stroke engines (other than chain saws, trimmers, etc). And I have never seen an engine that ran backwards to make the cart go backwards - kinda messed my head up for a while.

For the following descriptions, the wheels are blocked up off the ground to keep cart stationary:

Engine cranks OK, but doesn't start/run/fire, at least at first. If you hold the throttle wide open, it will fire and run wide open for a few seconds, then die out for a few seconds, then run wide open for a few seconds, then die out, and so forth and so on. It will not idle or run at any lo-mid throttle, only WOT. Those few seconds that it does run seem fairly smooth (except for loud and fast).

If I keep squirting starting fluid in the carb, it'l keep running, for the most part.
Choke doesn't seem to have much effect.

I replaced the carburetor with a brand new one. Still has EXACT same problem. I have not made any adjustments on the carburetor; just installed it out of the box. What adjustments do these need?

I bought a kit for the fuel pump (thinking maybe it is starved for fuel). I read several posts about the two diaphrams in the fuel pump. I disassembled my pump and I could not see any discernable rupture in either of the diaphrams (not that it wasn't, I just didn't see any). The replacement diaphrams were a brown 'fabric' looking material, while the old/existing diaphrams were clear plastic. I have not yet reassembled/replaced anything, I left the parts soaking in gasoline overnight to get them good and clean. -- There was a 1/8" thick (approx) plasitc block that was installed between the pump and the engine block. It looks kinda raggedy; can I pitch this?, or must it be used (my guess is it is mainly a temperature isolator or something) (I see blocks like this under carburetors a lot).

I felt the hole under the fuel pump mounting while cranking and it felt like strong pulses of air there that should activate the fuel pump. The only thing that seemed even remotely irregular is that everything did not seem very tight when I removed the fuel pump.

Does it seem like I am headed down the right path? I have not been too thorough just yet, still trying to develop an understanding of how these 2-cycle engines work. I have not questioned the ignition since it does run sometimes - I haven't even removed the spark plug yet - just haven't gotten that far.

There's a lot I don't understand, like the role the muffler plays; can I just remove the muffler and run a straight pipe, or maybe configure an old automotive glasspack or tractor muffler? Does there have to be a precise amount of back-pressure for it to run right?

How does the ignition timing work? Is it always right at TDC so it can run in either direction, or is there something that modifies the timing whether it is in reverse or forward to keep timing at ?-degrees BTDC in either direction?

Is there anything else that is a usual/typical culprit with symptoms like mine?
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Unread 04-22-2017, 03:59 PM   #2
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

Haven't done much on this since I removed the fuel pump. I have disassembled the fuel pump and compared the innerds to the parts in the kit. All the kit parts fit just fine, but the stackup of gaskets and diaphrams is slightly different. The existing fuel pump has a thin black gasket between EVERY metal surface and diaphram surface. The kit parts look like pictures I have seen elsewhere on this forum. So when I reassemble, I am guessing that the new diaphrams go directly against the valve block section (middle section) with the new gaskets between each new diaphram and the top/bottom caps (that matches parts in the kit). -- Does this sound right?
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Unread 04-25-2017, 11:31 AM   #3
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

If you have the fuel pump off, get you a vacuum guage and check the vacuum. this fuel pump uses vacuum from the motor to move the diaphrams. I don`t recall what the min. vacuum is needed, but i am thinking around 5 to 7. I you have low vacuum, it could be the main crank seals need replaced. I think there is a thread on this site that can walk you thru the deal.
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Unread 04-25-2017, 04:39 PM   #4
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

Crankshaft seals are always a major concern, but since we don't know the history of this engine, it's worth looking to see if somebody put the piston in backwards.

Remove the exhaust pipe.
Look in the exhaust port while moving the piston up, and down.
If you see a square port on the piston from the exhaust side, it's in backwards.
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Unread 04-25-2017, 04:42 PM   #5
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

You could probably use this manual too.
Attached Files
File Type: pdf 85-EZGO(2) (1).pdf (6.71 MB, 0 views)
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Unread 05-02-2017, 09:22 PM   #6
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

Thanks guys.

I have no idea about the pedigree on this engine either. It may have been rebuilt for all I know, or not.

To make sure I understand piston orientation, the side of the piston with the hole (port) goes on the intake/carburetor side?

I did get my hands on a compression tester - compression is in excess of 130psi in about 4-5 revolutions of crank. I think that is good on just about any engine.

I have not measured amount of pressure/vacuum on the fuel pump port; I have only felt it while cranking witth fuel pump removed - there is some fairly pronounced puffs of pressure during downstroke, I can't really tell how much it sucks during upstroke.

Fuel pump still doesn't do much even after rebuilding it. With rebuilt pump installed, fuel barely trickles out. Maybe pushes up about 1/2" beyond hose disconnected and straight up. But I still don't trust the rebuild since the parts in the kit didn't match the stackup of diaphrams/gaskets in the pump I have.

I think I have determined it had the wrong spark plug. Had a NGK B6HS, should be BP4HS.
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Unread 05-02-2017, 09:24 PM   #7
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

Quote:
Originally Posted by cart around View Post
You could probably use this manual too.
Thanks for the manual - VERY helpful.
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Unread 05-03-2017, 07:55 AM   #8
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

You definitely want to check the piston to see if it's in backwards. 130 psi is way too good for an old engine. That sounds like a fresh rebuild that never ran.
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Unread 05-03-2017, 08:27 AM   #9
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

Quote:
Originally Posted by cart around View Post
You definitely want to check the piston to see if it's in backwards. 130 psi is way too good for an old engine. That sounds like a fresh rebuild that never ran.
That has become a suspicion of mine as well. I'll be checking that out for sure.
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Unread 05-07-2017, 04:23 PM   #10
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Default Re: '85 Marathon Gas Runs Erratic - full bore or nada

I checked it out - the piston is DEFINITELY in RIGHT,

I decided to go ahead and remove the engine to clean it. Everything underneath is covered and caked with mud and sand and dust, so I was afraid the cooling fins on the cylinder might be clogged with mud/sand. Sure enough, it was; much cleaning to do. In doing so, I saw nothing else that looked suspicious as far as crankcase leaks are concerned, except that there was quite a bit of dried baked caked grease on the output shaft and the crankcase around it (clutch end). I don't know how old this mess is, but I have to assume that the engine has been out and maybe overhauled before (hence the high compression); and I discovered that several of the motor mount bracket bolts were missing or broken off. What I could see of the piston looking through the exhaust port looked very good, clean and shiny and smooth.

So I have decided since I have gone to the trouble to remove the engine, I am going to go ahead and replace the crankshaft seals anyway, needed or not - only a little extra work at this point and maybe maybe might fix something (or at least lengthen its life). Less than $20 for the parts.

Will let you know it turns out - probably be a week or more, depending on how long it takes to get the seals.
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