Vintage Dirt Bike Q & A

Hey guys, I was having trouble with my 74 CR 250 pinging last season. I've played around richening the jets on the carb but never could get it dialed in, it would be too rich and four stroke or run good but ping. All the seals are new and I've checked for leaks. I pulled apart my cylinder and the only thing I can think of is that there is some pitting in the cylinder head from a ring breaking and this is causing my problem, the pits are too deep to polish or sand out. Also I was wondering, I have a new Wisco piston that is 2 over, does the head need to be modified because of the overbore? One more question, I have a 75 CR250 head, with the cut down fins for the exhaust, and the dome is in perfect condition so will this fit on my 74 CR250 with no problems?

Thanks for the help,
Matt

Sounds like its out of time? I'd check that first. What octane are you using? Pump gas or racing fuel? I wont burn anything less than 92octane. Do you buy from the same place or from diffrent stations.

Did the piston you have change your compression ratio? If the distence from the piston pin to the top of the piston is longer it will increace that ratio. That could cause the combustion temp to rise which in turn creates pinging when lean and dieseling when rich. More so if you are using bad or low octane gas.

I have a .75 over kit that I use with a stock head, carb it runs great. No mod needed to the head with the overbore.

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W.F.O. All the way!

I was running 104 Octane then toward the end of the season I was using Turbo Blue 110 Octane, Timing I set at very slightly retarted from what it should be.

Matt

I know this might sound like a redneck fix. But try using two head gaskets at the same time. Or a thicker one once to see if increasing the compression will help clear it up.

Also what oil to fuel ratio do you use? It sounds like the bike is getting to hot.

_________________
W.F.O. All the way!

The compression was about 150~160psi when I ran a compression test which I've heard is about normal for a rebuilt engine. Does anyone know if I can throw on that 75 CR 250 Head on my 74 without damaging anything. The mounting holes are the same and it apears it will work, this would allow me see if the pitting in the dome on my 74 head is the problem. I think the pitting is causing hot spots which in return would cause it to pre-ignite the fuel.

Matt

As long as the pits dont pertrude into the combustion area it should be fine. And im not positive but the 75 head should work.

_________________
W.F.O. All the way!

Is it pinging undae full throttle or at half or so throttle?
Mine pinged really bad after a rebuild and I threw in the next size main jet. Now it seems lean at part throttle so I think I will need to richen up the needle next.