2 stroke power loss

need2understand

need2understand

Member
Hi,

Been trouble shooting this power loss and learning as I go. It's been fun to learn but i'm getting frustrated and need ideas as to where to go from here.

1979 yamaha dt250.

Been losing power most of last season and it's been getting worse as I've been riding.

Carb settings don't make any difference and I have tried several different settings. I took the carb apart and found that the clip was not attached to the main jet. Placed the clip lean and rich and found no difference. Carb was clean and all parts seem to be working.

Noticed that there was a slow leak from the cylinder so this winter, I replaced all the gaskets. Bore and piston looked good (used youtube and books on restoration to know what to look for). Rings didn't show much wear using a feeler guage. Maybe i should replace them anyway. Could rings cause this issue?

Anyway, I started the bike today and found no improvment. Any ideas?
 
When was the last time the bike was rebuilt? COuld be loosing compression (piston worn, rings worn, CYLINDER Worn/no longer in spec) Might have to be rebored and have a new piston in it. You cant really eye ball a worn cylinder (other than the obvious gouges etc) I would check the compression...
 
Last edited:
piston rings for starters, check crank seals and cdi timing and clean out the exhaust port and baffel.
 
need2understand said:
Hi,

Been trouble shooting this power loss and learning as I go. It's been fun to learn but i'm getting frustrated and need ideas as to where to go from here.

1979 yamaha dt250.

Been losing power most of last season and it's been getting worse as I've been riding.

Carb settings don't make any difference and I have tried several different settings. I took the carb apart and found that the clip was not attached to the main jet. Placed the clip lean and rich and found no difference. Carb was clean and all parts seem to be working.

Noticed that there was a slow leak from the cylinder so this winter, I replaced all the gaskets. Bore and piston looked good (used youtube and books on restoration to know what to look for). Rings didn't show much wear using a feeler guage. Maybe i should replace them anyway. Could rings cause this issue?

Anyway, I started the bike today and found no improvment. Any ideas?
Click to expand...

How could you tell if the rings weren't leaking just by looking at them? You need to do a compression test....If the piston/rings haven't been replaced since 1979...Theres a very good chance you have low compression
 
JohnnyP636 said:
How could you tell if the rings weren't leaking just by looking at them? You need to do a compression test....If the piston/rings haven't been replaced since 1979...Theres a very good chance you have low compression
Click to expand...

I checked the rings with a feeler guage only. The guy I bought it from said he did a top end rebuild but who knows. It past the thumb check but I guess i'll need to break down and buy a compression tester.

I'll need to read up on timing but would a timing issue cause slow power loss over time? The shop manual says there's no points, it uses dynamic timing.

Thanks for the quick replys.
 
I would suspect low compression is the issue. However since you"ve ripped it all apart it's too late to do a compression test, Since it's all apart get the cyl deglazed, and install new rings. You should be fine.
 
the compression test should be done with the engine at running tempature not cold for a true reading. also check for vaccume leaks at the carb manifold and presssure leaks on the exhaust side.
 
Since the cylinders on most older bikes do not have a nikasil coating, they can wear faster. I would definatly get the cylinder bore messured if you have it apart.
 
I would give the exhaust a thorough cleaning before spending any money.
 
Awesome advice guys. I bought a compression tester and will be checking vacuum leaks and doing a throrough cleaning of the exhaust.

I tried googling exhaust pressure leaks but it's not really clear to me what i'd need to do to check that. Am I looking for holes?
 
Replace the spark plug while you are at it. Two strokes eat up a lot of plugs. Buy a ngk iridium plug. They last a lot longer and are well worth the money.
 
On my old Can-am I could remove the baffle from the exhaust and heat it with a torch to burn off the carbon.
 
JohnnyP636 said:
How could you tell if the rings weren't leaking just by looking at them? You need to do a compression test....If the piston/rings haven't been replaced since 1979...Theres a very good chance you have low compression
Click to expand...

I'm with J on this, even if the rings were replaced since 1979, its 24? yrs old. The guy you got it from did a top end, ok when? 9 yrs ago. I'm not sure i'd even test it, i'd just change it.
 
Seems the compression is low. Not sure how accurate my testing was as i was not able to screw the tester in all the way because of the angle but it was reading at 120 range.

Checked with my local shop and was told that it might be hard to find replacement rings for this model. He gave me a number for a parts place in NY. I'll need to mesure the bore and piston clearance and contact them.

I keep the air filter clean but I haven't done anything to the exhaust yet.

Thanks for the advice. I'm new to this sort of thing, so your comments have helped a great deal.
 
Doesn't a DT250 have a 64mm bore with a 1.1mm top ring and a 1.4mm second ring?
Same as a RZ350, RD400, RD350 and a YFZ350 (Banshee ATV).

If you can't find rings I will have something that will work. (Yamaha is USUALLY pretty good, if you can't find OEM rings Wiseco makes a ring for that bike, failing that Perfect Seal makes a ring for EVERYTHING under the sun).
How did you test for vacuum leaks? A leak down test is best. Plug up all the holes, pressurize the case to 7PSI and dump it in a tub of water. If it bubbles you gots a vacuum leak.
120PSI for a compression test on a kick start two stroke is pretty good.
 
bitzz said:
Doesn't a DT250 have a 64mm bore with a 1.1mm top ring and a 1.4mm second ring?
Same as a RZ350, RD400, RD350 and a YFZ350 (Banshee ATV).

If you can't find rings I will have something that will work. (Yamaha is USUALLY pretty good, if you can't find OEM rings Wiseco makes a ring for that bike, failing that Perfect Seal makes a ring for EVERYTHING under the sun).
How did you test for vacuum leaks? A leak down test is best. Plug up all the holes, pressurize the case to 7PSI and dump it in a tub of water. If it bubbles you gots a vacuum leak.
120PSI for a compression test on a kick start two stroke is pretty good.
Click to expand...

My book says the bore is 70mm and rings are .2-.5 mm. Guessing wear and that it might have been rebored would make me have to remeasure. If you can provide the contact info where i can get parts, that would be great.

I checked for leaks on the carb side by spraying wd40 around the carb when the motor was running and checked for a difference in rpm. Gaskets are new and the rubber boots are not cracked. Think it's ok.
 
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