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dt rev problems


BenP
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Hi all sorry about yet another post.

I have a 1991 dtr 125 and just put a 1998 engine in it as mines went pop

Anyway I got it all up and running but after letting it wear in blah blah blah I thought I would cane it just to check it was all running as it should however after 8500 rpm the bike acts as if it's starved from fuel or something it keeps slowing down then picking up speed again but won't go any faster so I thought this could be the reeds so I changed them to boysen carbon reeds as the ones I took out were very worn now after putting the carbon reeds in it hasn't made any difference apart from throttle responce so I'm wondering if it could be an electronic problem? But I can't see how has its still running on the 1991 electrics and the other engine was fine but ever since putting this 1998 engine it just won't rev high

If anyone has any ideas I would highly appreciate it

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  • Moderator

Did you investigate why the other engine went pop? Chances are that the bike was running lean if "pop" means the piston holed. Jetting and the correct amount of air are key to 2 stroke reliability and running characteristics.

Before you do anything you need to check that the bike is running within the air/fuel/oil range. checking the plug colour after a spell of 3/4 throttle riding is the way to start. Next would be to check the jets and clean and re set the carb and make sure the air filter is still doing its job. Assuming you have no other air leaks that is.

High throttle position problems normally are: the main jet, air flow or petrol flow.

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Yes it is the original carb and when I say pop the big end bearing went causing the debris to go up into the barrel destroying the piston the crank was mullered etc so I just decided to buy another engine

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  • Moderator

High throttle position problems normally are: the main jet, air flow or petrol flow.

This still applies. If the plug is the right colour I would check the fuel tap filter, the fuel tank breather, check the air filter and clean and reset the carb.

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Thanks very much for the help I checked the plug and it was a nice Brown so I'll do the following of which you have stated and I'll let you know how I get on thanks again

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  • Moderator

It could be loads of things but I would suggest you check the most common faults and cheapest to fix first. If you haven't messed with the stator then they rarely move. Coils breakdown and bad earths appear but a big percentage of issues like this on a 2 stroke are air and/or fuel related. You need to check through in a systematic way so you can eliminate things.

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I have now eliminated all the things you have said about turns out there was dirt in the carb however it didn't sort the problem I also decided to change the stator plate so I put my original one in from my last engine and still no difference so I'm really running out of options so my last guess is. Could it be the crank seal letting in air disturbing the fuel/air mixture ratios at high rpm?

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  • Moderator

How did you clean the carb?

Did you check the size of the main jet? - possibly go up one size.

Did you reset the float height?

Did you check and clean the fuel tap and check the tank vent for a blockage?

What size is the tank to carb fuel line -sometimes they restrict when you need the most fuel -is there an in-line fuel filter?

Normally the crank seal would mean that the bike would not idle, especially when hot.

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