compression test

[rmrider45]

just did a compression test on my 2003 rm 125 and for being the same top end in the bike for 2 years the bike has 180 psi:) kicked it with the throttle wide open and clutch pulled in and kicked it till the gauge stoped. i have one ? when i put a new spark plug in the bike it gets real tight

but after riding it for a while the spark plug comes loose?it like backs out :( is it because i run regular gas in it (87)?

 

[Uchytil]

is it because i run regular gas in it?

I doubt it.

It sounds like you have other problems.

 

[jsantapau]

I tend to believe it's possible, if you are detonating because you are running cheap gas that could loosen up the plug

 

[whenfoxforks-ruled]

If the spark plug is backing out, you are destroying the piston, with detonation, from running regular gas. It will keep running until a significant piece takes the cylinder with it, then the crank. Or it blows a hole through the piston. You are abusing the pressure readings, they mean nothing the way you are using it. Check it when its broke in, and keep checking it until there is a slight drop, then re-ring/top end. If the book says 7 hours, check it at 10, if you are not wfo much. A few chunks of debris or sand and all bets are off. The rings or piston was gone some time ago. Running long on re-ring/top ends will oval the cylinder bore. If you want to run regular gas, lower the compression and altar the port timing. Or just buy a kdx. Vintage Bob

 

[rmrider45]

ok what gas should i run 92? and how to lower the compression?if i wanted to run 87

 

[whenfoxforks-ruled]

Premium, the most expensive stuff. Send the top end to Eric at Forward Motion, or Pete Payne from this site. Tell them what you want. It should not be too much?

 

[rmrider45]

how about running vp or cam 2 fuel what Octane are they 98?

 

[whenfoxforks-ruled]

Some of those fuels are NOT for 2 strokes, and can cause issues. Check the companies web site, call and ask. Most mx bikes run fine on clean premium pump fuel.

 

[1998RM250]

Will running lean also cause detonation? a weird "clacking" type "hiccup" noise almost?
It's hard to explain...

 

[ellandoh]

Will running lean also cause detonation? a weird "clacking" type "hiccup" noise almost?
It's hard to explain...

get some decent fuel and tighten your chain properly THEN let us know your remaining issues

 

[2strokerfun]

The "clacking" is a fairly good description of what detonation sounds like. And running lean can cause this when engine is under a load. As others have said, dump all your 87 fuel and get the best premium you can locally find. Try to avoid ethanol in your gas if possible and get real gas (if you can find it). Then see what it does and report back. Don't try to save 30 cents a gallon by using 87 if something better is available.

 

[whenfoxforks-ruled]

2 different posters 2strokerfun. 98rm250 guy, a lean engine does make some odd noises, it can melt parts, calling it detonation is really pushing it. Could be the chain, could be a few other things. Verify by checking the plug. Ethanol is not an issue directly, its the water that comes with it in pump applications.

 

[rmrider45]

get some decent fuel and tighten your chain properly THEN let us know your remaining issues

tighten your chain properly?

what does that have to do with anything?

even tho its tight

 

[RM_guy]

Dumb question...is your plug installed tight enough? I knew a guy that was so worried about stripping the plug threads (he'd done it several times) that he never torqued it down tight enough and it backed out a lot.

Low octane gas causing detonation could be the real cause though.

On the chain, a chain that is loose and slaps around can sounds like engine trouble. Even a loose pipe gaurd or missing exhaust srping can be misleading.

 

[rmrider45]

Dumb question...is your plug installed tight enough? I knew a guy that was so worried about stripping the plug threads (he'd done it several times) that he never torqued it down tight enough and it backed out a lot.

Low octane gas causing detonation could be the real cause though.

On the chain, a chain that is loose and slaps around can sounds like engine trouble. Even a loose pipe gaurd or missing exhaust srping can be misleading.

its real tight when i install it

 

[1998RM250]

I didn't want to post an entire new thread, but I put my new jetting specs in my last thread a few posts below this one compared to the old, maybe you guys can help me. Don't ever use 87 in your bike, at least run 93! I run 93 with yamalube 2R @ 32:1. My chain is tight, and I know its noise coming from the engine, I have a good ear for that stuff.

 

[2strokerfun]

its real tight when i install it

Your chain should have some up and down play in it. Too tight can be almost as bad as too loose.

 

[rmrider45]

Your chain should have some up and down play in it. Too tight can be almost as bad as too loose.

i meant the plug is real tight

 

[Uchytil]

i meant the plug is real tight

This thread is getting "real tight"....Just run some premium gas - what's that now,,,92? Most of the stock bikes I own recommend 95 or higher but that's not happening at the local pumps, or is it?

Eric Gorr sells a real nice book(s) that cover bike set-up, maintenance, and repair. I recommend you get that and the owners manual, maybe a torque wrench too. :cool: