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Author Topic: engine breaking up bad under load  (Read 1613 times)

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engine breaking up bad under load
« on: January 01, 2016, 07:27:29 pm »
Just picked up my first lt250r, it's an '89 and I'm trying to work out the bugs and could use some 2 stroke help. 

Problem- The motor runs absolutely horrible under load.  Breaks up bad.  It starts and idles pretty good and if you ease through the throttle (no quick movements) it will go up through the rpm's.

I haven't been inside the motor so I'm not 100% sure of what, if anything, has been done to it.  Appears to be stock setup with a stock pipe and FMF silencer.

So far I've cleaned the carb, it has a 250 main jet, 30 pilot, needle is in 3rd clip, but won't idle at 1.5 turns on the idle/mix likes to be around 2.5 to 3.

Also carb was leaking out of the overflow when I got it, and did briefly when I put it back together, but seems to have stopped now.

I also took a look at the reeds and they look good, and the PV is opening and closing in the window.  Just wondering if I should be focusing on carb, or looking else where, like stator or coil for weak spark.  Or a possible air leak, bad seal, plugged exhaust, low compression, etc...

I'm a very mechanical person, but this is my first 2 stroke so any help would be great.  Thanks
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Re: engine breaking up bad under load
« Reply #1 on: January 01, 2016, 08:01:06 pm »
Is it a keihin or a mikuni carb?  Those jets just don't sound right. 
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Re: engine breaking up bad under load
« Reply #2 on: January 01, 2016, 09:04:48 pm »
It's a mikuni, by the measurements I took it appears to be a TM32-1.  FMF's site says 240, 30, and 1.5 turns for there silencer.
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Re: engine breaking up bad under load
« Reply #3 on: January 01, 2016, 09:30:27 pm »
I just checked FMF website and they want a 240 in it for a main jet.  One of my friends has a 250 and he has a 300 main in his.  I would bump up the main jet and try that.  Easy and cheap enough to do.
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Re: engine breaking up bad under load
« Reply #4 on: January 01, 2016, 09:42:23 pm »
My mistake, it has a 250 in it.  I'm at work and was going off of my questionable memory.  I'll fix it in the original post
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Re: engine breaking up bad under load
« Reply #5 on: January 01, 2016, 11:44:48 pm »
it could be the spark plug amongst other problems.
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Re: engine breaking up bad under load
« Reply #6 on: January 01, 2016, 11:48:57 pm »
i have had a few bad sparkplugss that spark ok but "break up"  and miss when  given gas. kinda like the timing is off.  heard others with the same problem.
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Re: engine breaking up bad under load
« Reply #7 on: January 02, 2016, 02:03:27 am »
I tried 2 plugs now, even pulled the gap down to see if it would help if I did have a weak spark.  Didn't make a difference though.  I just ordered a carb rebuild kit to replace everything instead of just cleaning it.  I'm going to check the timing also
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Re: engine breaking up bad under load
« Reply #8 on: January 02, 2016, 09:36:13 am »
Breaking up is usually lean bogging out is rich I'd fatten up your main and see if that helps
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Re: engine breaking up bad under load
« Reply #9 on: January 02, 2016, 04:27:13 pm »
I agree LT500Kid it does seem like a lean condition.  When I get it back together I will start playing the jetting game.  Today I checked the pick-up coil gap and it was .048 so I moved it in to .020.  Spark looks better now, not that this was the only/main problem, but I'm sure it wasn't helping things.
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Re: engine breaking up bad under load
« Reply #10 on: January 02, 2016, 04:41:37 pm »
Also, I was checking my reeds, and it appears to be a stock cage. Not exactly sure about the reeds though they look stock, but there are no travel limiters?? Is this a problem, or should that not really affect anything.
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Re: engine breaking up bad under load
« Reply #11 on: January 03, 2016, 08:44:08 pm »
if the reed tips are seated tight/ it is the carb or perhaps the compression.
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Re: engine breaking up bad under load
« Reply #12 on: January 03, 2016, 10:10:59 pm »
Quote from: twostrokewhip on January 02, 2016, 04:41:37 pm
Also, I was checking my reeds, and it appears to be a stock cage. Not exactly sure about the reeds though they look stock, but there are no travel limiters?? Is this a problem, or should that not really affect anything.

The big stock reeds will float and can cause a misfire around 7500 RPM.  Removing the reed backstops will exaggerate the problem
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Re: engine breaking up bad under load
« Reply #13 on: January 04, 2016, 02:13:43 am »
Sublunacy, The reeds look to be sealing well, I did loosen the screws and try to square one side up a little better.  There's not much play there, but it made me feel better for trying to micro manage.  :D    As for the compression, with throttle wide open and about 6 to 8 kicks I had 150psi.  Not sure if that's too many kicks, or if kick count isin't as important as the final pressure reading though.

Motorgeek, thanks for the reply that makes a lot of sense.  I did order some boyesen reeds yesterday (cheap insurance to rule out reed problem), do you think I should try to get some backstops, or with the boyesens' it shouldn't be an issue?

Also I'm wondering if the bike is fairly stock and seems lean with a 250 main jet if I don't have a crank seal leak, does that sound like a possibility?

Thanks everyone so far for the replies and help +k2

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Re: engine breaking up bad under load
« Reply #14 on: February 16, 2016, 09:29:31 am »
Exhaust smell funny? Leak down test pass? I had a similar problem on my KTM and waiting for seals for my Zilla. Its was pulling in a very small amount of transmission fluid. It was doing the same thing you described.
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