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Mopar crazy
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Discussion Starter #1
I have a old Trail 90 that I've been trying to ride every now and then. Every time I ride it, it runs great for a little bit then runs like crap. Sometimes I can shut it off for awhile and let it sit, it'll start right back up and run fine for a little bit, then right back to running like crap. It sat for about 10 years before I got it so I rebuilt the carburetor, put a new set of points and condenser on it, and of course new fuel lines and cleaned out the fuel tank. A few years ago I took it hunting. The ride was about 10 miles or so and never had any trouble with it until I got really high in elevation. It kinda bogged down so I parked it and walked the rest of the way. I'm not sure what to think. Any thoughts
 

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Mopar crazy
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Discussion Starter #3
I haven't checked compression yet. I guess I don't understand what you mean by fuel pickup
 

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Mopar crazy
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Discussion Starter #5
Oh. It doesn't have a pickup. The tank is above the carb and the outlets are on the bottom. Would low compression cause it to run like crap occasionally
 

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Compression problems will cause cold starting issues and loss of performance all around, all of the time. To me, this doesn't sound compression related.

Ignition problems do come to mind... Coil, stator, plug boots, even bad plugs - all things to definitely check.

It also sounds like there could be a problem with the carbs where excess fuel is making its way into the crank case. Sitting for a while might allow some of that to evaporate out and go back to running ok again. If you have the drain plugs on the bottom of the motor, under each cylinder, I would see what size wrench fits and throw it in your pocket. The next time it acts up, shut her down and pull one of those plugs. If a bunch of fuel pours out, you have either a bad pulse line for your fuel pump, a bad pump, or issues in the carbs.

Make sure you have some mechanics gloves with you so you don't burn your hands on the drain plugs.
 

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Depending on how much higher the elevation is it can mess it up slightly. Higher elevation=less air= engine running rich. I don't know that your trail is that huge of a difference,

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What sort of difference in elevation from starting point to where it bogged out are we talking about? 10,000 feet difference?
 

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And meburdick, I assume he doesn't have a fuel pump being the tank is above the carb, gravity fed, and it is a fairly small engine.
It's certainly possible that it is completely gravity-fed... Dunno... Was just throwing out general troubleshooting ideas. Whatever doesn't apply could obviously be skipped.
 

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Mopar crazy
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Discussion Starter #13
The difference in elevation was roughly 500 to 1000 ft from unloading to where it bogged. I guess I should have added to that also. I was wondering if it would have been possible that it would have done damage to it. I didn't immediately stop when it bogged, I tried to ride it a little further before parking it. I just assumed that it fouled the plug. I would not restart when I got back so I just coasted off the mountain back to the truck.
Joshkidner, you are right, it doesn't have a fuel pump where it is gravity fed. I have talked to an old Honda motorcycle mechanic about tuning the carb. He told me about where he sets them for this elevation but it still does the same thing. Is there a way I can test the coil while riding it to see if that is my problem?
 

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Meburdick, I'm not positive but I think those Honda bikes had a knob or lever of some sort for elevation and it changed the "jetting" I know someone who has 2 ST90s and a trail 90, if I remember right, when I worked on them for him I saw the setting for it. If it were the coil, once the engine was hot, it would start bogging. It would be very hard to start back up until the engine cooled.

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Mopar crazy
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Discussion Starter #16
It has a needle jet that I have tried in all 5 positions. It seemed to run better occasionally but I can't remember which was the best. I tried putting it where the Honda mechanic recommended but it was the same result. The main jet is right. I made sure of that when I rebuilt the carb.
 

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Dumb question, but is it the right plug? If the plug isn't firing hot enough, it could be leaving unburnt fuel and/or fouling.
 

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Mopar crazy
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Discussion Starter #19
I've been using the plug it says to use on the frame. Should I try using a hotter plug and see what happens. I've been using NGKs
 

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Next time it does the bog thing, verify if the spark is still bright blue or if it is yellow or white.
This will tell you if it is a coil/electrical problem or not.
 
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