No power when hot

I raced this weekend for the first time this year. I put 6 hours of practicing on a newly plated cylinder and a new piston and rings. The bike seemed to be fine. This weekend I had problems almost every race with the bike losing power towards the end of the moto. It was so bad that I had to pull off because I was barely making jumps. I have a temp strip on the cylinder right below the head. The hottest that it read was 185f but for the most part it was around 170f. I didnt lose coolant and the plug looked great. I even went up to a 170 on the main to see if that would help and it did nothing. I made sure the exhaust valves moved freely and they did. I am running 100 octane mixed at 32:1. I am thinking that a ring is collapsing when it gets hot but the bike isnt running that hot? Also, I did notice my bike does not smoke at all. Even when cold... if you rev it it will let out a little puff but thats it. Oh... and I have used a whole can of brake clean and even tried some propane looking for an air leak and couldnt find one. There is some blow by but there was blow by after the first hour i put it in... its black up to about a 1/4inch below the 2nd ring.

Is it still revving freely, or is it struggling to rev, and running like it's bogging?

Is it still revving freely, or is it struggling to rev, and running like it's bogging?

Also, do a compression test had see what your getting.

struggling to rev and makes a bogg sound. The bike has no power at all.

my guess is that cylinder is not the correct bore size. so when the cylinder heats up and expands you are loosing compresesion.

my guess is that cylinder is not the correct bore size. so when the cylinder heats up and expands you are loosing compresesion.

+1. Check the diameter of both and see how much space their is between the piston and cylinder wall with a feeler gauge.

Could very well be. How can I check? Just pull the piston and cylinder out, put the rings on the piston and stuff it in and measure with a feeler gauge? My ring end gap was right in the middle of the specs listed in the manual you so you would think if the cylinder was too big I would have had a ring end gap that was out of spec right?

Pull the rings and then put the piston in the cylinder and check the spacing inbetween the cylinder wall and piston ring lands and/or skirt.

Don't rip your motor apart just yet. Check your exhaust for obstructions. Pull the silencer off. Is your power valve working correctly? It may be moving just fine, but be backwards. Valve is wide open at idle, but closing, and restricting the exhaust port under throttle. I doubt it's your piston being too small. You would have noticed that one when you checked your ring end gaps, and really would have noticed something was wrong when you installed you cylinder! The piston, and rings would have practically fell in the bore.

Yeah thats what I was thinking... if the bore was too big I would have noticed that when checking my ring end gap. I pulled the exhaist valve cover and opened it, and let it spring back to verify it worked. How could it be backwards? And if that was the case wouldnt it do it all the time? Im just completely lost right now I cant for the life of me firgure this thing out. Would the fact that the bike does not smoke have anything to do with it?

with the plug out the crank spins easily? bike starts easy, correct? :thumbsup:

Might be an electrical issue...

Yeah the bike starts easy (one or two kicks) ill go through and visually check connectors and wiring... The only thing electrical that I can think of that woud cause this is a coil but its a pretty low hour bike so I dont see how that could be a problem. I almost just want to go to the dealer and buy an new rmz im so frustrated with this bike...

I still think the bike is jetted on the lean side. You said the plug look good, but can you post a photo of it so we can see on what scale of coco brown its burning. Are we talking about a very dry light coco brown or a more darker brown. You have a new plated cylinder and piston with 6 hours of break in time. What you describe is a lean condition in the mid to top range. What groove is your slide needle c-clip in? If its in the 1st groove, then I can understand the flat running. You said the main was up to #170. Did you also try to increase the pilot?

I don't know about you guys up north, but we have been in a 22 day over 100 degrees every day with an average of 108-109. This weekend we busted to 113 on Saturday afternoon. It is playing hell on everyone's jetting from two strokes to four. What I am getting at is we are all in the mid summer months and if the bike is jetted on the lean side to start with then it should be tweaked for the temp and humidity. Our bikes are just not getting any air. Hell it hard to breath for a human.

i think you're losing spark, coil or CDI box, would be my guess.

Yeah coil might be getting too hot on these 95* days and cutting out...

Its always so hard to diagnose these problems over the internet

...

Yeah thats what I was thinking... if the bore was too big I would have noticed that when checking my ring end gap. I pulled the exhaist valve cover and opened it, and let it spring back to verify it worked. How could it be backwards? And if that was the case wouldnt it do it all the time? Im just completely lost right now I cant for the life of me firgure this thing out. Would the fact that the bike does not smoke have anything to do with it?

YEP! Pull the exhaust off. Check the exhaust port to make sure the PV is closed, and moves the proper way. Check the pipe for any blockage. Check the can for plugage. Make sure the air filter is clean, and the ports in the box aren't blocked. Pull the filter, and make sure noting is plugging the air box boot.

Well the silencer is pretty new along with the pipe. The exhaust is flowing out just fine its just not smoking. Padgett, I was running a 48 pilot and a stock needle in the stock position with a 168 main. If anything, the bike would have been running rich with the high temps and humidity. Is there anything I can do to test the coil? Like a stress test? Even though I doubt it will show up because the problem only occurs when the bike gets very hot. I have an 03 RM125 that is torn apart... can I swap coils?

put the new rings into the cylinder and push the ring into the cylinder using the piston

then measure the gap that the ring has

Well the silencer is pretty new along with the pipe. The exhaust is flowing out just fine its just not smoking. Padgett, I was running a 48 pilot and a stock needle in the stock position with a 168 main. If anything, the bike would have been running rich with the high temps and humidity. Is there anything I can do to test the coil? Like a stress test? Even though I doubt it will show up because the problem only occurs when the bike gets very hot. I have an 03 RM125 that is torn apart... can I swap coils?

Well if you tried that jetting then that is not the problem. That is what I run on the sand tracks and that is rich enough. I drop to a #45 pilot most of the time.

I have no ideal on the coil swap. I have not tried to swap our 125 coil over to the 250. Someone else will need to advise on the coil subject.

I know the feeling your talking about. Comes on the pipe, but flat with no power. The only other thing I would suggest is checking the stator timing. One of my RM250s I got used and the stator had been jacked with and it had a very soft hit. Set the stator back to stock and she come alive.

I beat you to the stator timing lol. I checked it when i had the cover off checking for an air leak.... There is practice tomorrow at the track I race. I will just go and ride it hard and see if I can get it to do it and mess with it some more. The no smoke thing really has me scratching my head.... I am mixing 32:1 and am jetted on the "fat" side and still dont get smoke.. good exhaust flow but no smoke... does super techniplate really burn that well?

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