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Old 07-13-2020, 01:58 PM   #1
GetchMech
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Join Date: Jul 2020
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Post G21A Only runs at extremely low throttle

I'm working on a 2001 G21A Yamahauler. It has the JR7 357cc Engine. Been doing small engine repair for 20 years but never really worked on these carts, so I'm hoping I've overlooked something simple because I'm about ready to give up on this one. The problem is that it will only fire with the throttle just barely applied. Once you advance it any further it stops firing and just cranks and begins backfiring through the carb like its too rich or timing is off or something. I have thoroughly cleaned the carb multiple times and even tried one of the cheap after market carbs with the same result which makes me think it is something else. Here is what else i have done/checked.

-TCI ignition is new last year (cart had been sitting and mice had destroyed original spark plug lead wire.) checked and gapped correctly.
-In-line spark tester shows good spark even once it stops running and is just cranking with new BPR4ES plug. Haven't tried a BPR2ES yet.
-Great fuel flow from pump when disconnected form carb and engine cranking(Fresh ethanol free fuel with no water. Tried different fuel source just to be sure)
-Thought Head gasket, so removed Head and de carboned, Lapped valves and re-installed with new head gasket.
-Compression 150 PSI
-Valves adjusted to .004 in cold
-Exhaust blows air through so I don't believe its clogged
-New intake gaskets and spacer appears to be in perfect shape.
-Throttle is opening and closing the carb butterfly as it should

My thoughts: Cart was running fine this spring then degraded to point described above where it barely fires, so it was working prior. I started with the carb clean and fuel check to ensure fuel was good along with spark plug check/change. Thought maybe the flywheel had sheared the key causing timing to be off so pulled engine and removed flywheel nut and washer to check. Did not remove flywheel but it seems to not have moved looking in at the key-way. (I see this problem often with push mowers that hit something but figured id cover all my bases) being belt driven I didn't think it was likely. I then moved on to the head and figured at worst I would be freshening up the valves and cleaning her up. So as far as I can tell at this point I have spark, fuel, air and good enough compression. And unless something else affects timing that I'm not aware of on these engines i think im good there. I do not know what else it could be. Any one more familiar with these guys im hoping will have encountered something like this before. Any help here is greatly appreciated!!
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Old 07-16-2020, 05:29 PM   #2
GetchMech
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Join Date: Jul 2020
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Default Re: G21A Only runs at extremely low throttle

Have some progress but not sure if I need a new thread for this but ill put it here for now. I think I have it narrowed down to the spark plug cap in a strange way. I was doing more testing for spark and now with the in-line spark tester hooked up between the cap and plug she fires normal and revs up fine. Connect back direct to plug without tester and problem is back, she barely fires. I don't know if its changing the resistance somehow but either way here we are. I have ordered a new NGK cap assembly (this is the one that screws on to the wire from the TCI ignition). Any input of course is appreciated. Maybe someone else has had this part go bad or had problems with it. I hope its not the ignition since its only a year old.
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Old 07-17-2020, 06:53 AM   #3
slonomo
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Default Re: G21A Only runs at extremely low throttle

I'm guessing it's a poor connection between the plug wire and plug? Maybe the tester has a tighter grip on the plug than the plug wire does.
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