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99 rm125 won't start and backfires

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Hello all,

I have recently bought a 99 rm125 last week, it needed a new crank. The guy gave me the engine in 2 boxes the rest of the bike was mint and I checked and all the parts for the engine were there. 

I ordered a rebuilt crank from fleabay and I got new hotrod main bearings and seals. The cylinder and piston seemed to be in good shape. I also cleaned out the entire engine, took everything apart, transmission apart and cleaned thoroughly, power valve governer sprayed and blown out all the bearings new clutch heavy duty springs. Everything. I always like to know everything was done right and is clean to my spec. so every part of the bike that had to do with the engine had to come apart and get cleaned. I do this because sometimes a part of the engine could've been fiddled with by someone who doesn't really know what they're doing. On this particular bike the power valve governer rod clip was on backwards causing the rod to hit against the case. Things like that is why I take every piece of the engine apart and clean and inspect it. Okay so long story short.

(Also I pre lubed all the parts in the engine with the appropriate oils and put two stroke oil on the crank and piston bearings and the pistonand cylinder.)

I put it all back together. 

I mixed a gallon of 15:1 gas poured it in and this time I was stupid. I pumped the kick start lightly to prime the engine and it started second kick. I was thinking about running it through heat cycles but I didnt think it really needed to. But anyways we got it running, and it started pouring gas out the lowest vent on the carb. (Keihin). it would leak gas sometimes and then it would stop randomly. To be honest it was a different style carb it had some electric wire coming off of it like I said its new to me but I missed that part and didnt clean it. Tomorrow I'm going to take the carb off again and blow some compressed air through the top half and take the entire carb apart and clean and rebuild it. But anyways again, I was tuning it with my brother on the hill where I live, and I was testing the jetting. So it was running really shitty about mid range of throttle. And it wouldn't idle.

So the thing I'm upset about is that I was dumb and took it for a joyride up around the neighborhood to test out the jetting and got carried away. I took it up this street to test the upper level of the jetting and the power valve adjustment. brought it WFO and the powervalve didnt feel like it even kicked in until you gave it a quick blip of the throttle WFO and the entire noise the bike made changed. But as I got to the end of that street I went to press the rear brake, and it felt like the bike completely died (I had the clutch pulled in too) and when I released it it started going again. I took a left turn to go back to my house and I stopped at the crossroad and the bike stalled and I couldn't get it started back up again. I had to walk it back to my house. I bought a new spark plug for it thinking that was the problem but it wasn't. Brand new plug and wouldnt start. Every kick now and then it would backfire  and sometimes really loud. And this was about 5-6 hours ago. I tried starting it an hour later when I put the new plug in and still nothing. 

Hope someone can help. I wanted to fit as much info as possible for food for thought. 

Problems: 

mid range of throttle is boggy.

Doesn't feel like it has power when you try to take off.

Bike was running but then eventually died and now it won't start.

Edited by motocrossmike 759
Corrections

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Anyone? I took the carb apart this morning and cleaned all the passageways. I took out the 55 pilot and put in a 50 and lowered the float height. I was kicking the bike over and it just kept backfiring and sounding like it wants to start but still nothing. When I went to check the spark plug the exhaust was hot?!? Does anybody have an idea what's Wrong?

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See the piston I think the piston melted to the ring a little bit. but the jets are both richer than the manual says I cleaned the carb out a little more I dont know how many hours are on the piston but the bike sat for at least a year before the po sold it to me if that matters..

20170815_170132.jpg

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Did you center the crank after installing? 15:1 seriously no need for that. Probably blown up again take cylinder off check it out. Or broke a reef off but doubt it

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15:1 is what the manual recommends when you did a full rebuild or put a new topend in for the break in procedure. And I have the tusk crank puller. What do you mean by centering the crank? I installed it correctly and stuff.. But here's a pic of the piston this is what the problem is. Can someone explain why it did this? Its not melted but it looks bent on top where the exhaust ports are. And hammered on it till the piston ring won't come off.

20170815_175301.jpg

20170815_175309.jpg

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It may be 15:1 when using oil from that era. But today's oil should be 32:1 for break in and 40:1 for normal operation. I think it heat seized from not enough fuel in the mix and/or air in the radiators and it overheated on a fresh top end seizing it. 

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On 8/15/2017 at 3:26 PM, motocrossmike 759 said:

15:1 is what the manual recommends when you did a full rebuild or put a new topend in for the break in procedure. And I have the tusk crank puller. What do you mean by centering the crank? I installed it correctly and stuff.. But here's a pic of the piston this is what the problem is. Can someone explain why it did this? Its not melted but it looks bent on top where the exhaust ports are. And hammered on it till the piston ring won't come off.

20170815_175301.jpg

20170815_175309.jpg

Piston is hitting the power valves, they are somehow sticking too far past the exhaust port and into the cylinder. The 99 had an issue with the power valve linkage arm breaking, I suggest fully disassembling the entire power valve assembly and finding out why it's doing that. Then inspect the cylinder itself, make sure it wasn't damaged in the process. As far as jetting goes, I'd just set it to factory for the time being until you have the rest figured out. 180 main 55 pilot.

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On 8/18/2017 at 1:11 AM, criggs88 said:

Piston is hitting the power valves, they are somehow sticking too far past the exhaust port and into the cylinder. The 99 had an issue with the power valve linkage arm breaking, I suggest fully disassembling the entire power valve assembly and finding out why it's doing that. Then inspect the cylinder itself, make sure it wasn't damaged in the process. As far as jetting goes, I'd just set it to factory for the time being until you have the rest figured out. 180 main 55 pilot.

Hello I had figured that out before I read this that bike is already gone to a new home

Edited by motocrossmike 759

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Ok so i have had every problem you mention

What solved the carb overflow?

What solved the ratttle?

What fixed the starting?

What fixed the no low end power?

Help please ????

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