Vintage Dirt Bike Q & A

I have a '79 CR250. I did a quick clean up on it when I got it and it seemed to run alright so I went out riding. Put maybe 4 hours of run time on it and it died. Wouldn't start. Notices a little seepage around head gasket and base gasket but felt like good compression. It fouled the plug and fired up briefly after cleaning it then crapped out and wouldn't start. So I got new gaskets and rings. Put them in today. Cleaned the piston and chamber (they were rather sooty, as expected) and also did a quick hone of the cylinder. Also put a new throttle cable on while I had it apart (old one was sticky).


Put it back together and it now fires up for 5-10 seconds and craps out, usually with a backfire. If I try starting it again it does nothing or backfires. Plug is fouled. Clean the plug, try again withn same results. Runs for 5-10 seconds and dies with a backfire and fouled plug. Wet fouled actually.

I'm thinking maybe I didn't put the needle back in the carb properly. How should that be adjusted? Also, where should I set the idle mixture screw on the side of the carb? How about timing? Just want to cover the basics. I imagine it's carb related since it's wet-fouling.

It could be anything but often stop with backfire means starving for gas. [Tank isn't venting, floats wrong, kinked fuel line, plugged fuel filter, ...] That kind of stuff.

While your plug shows otherwise, it could be running rich just until it starves. That is you may have two problems, fuel starvation and wrong fuel mix/jets.

Rick

Just touching on the basics. Spark plug should be an NGK B9ES/BR9ES
or equivalent. Ignition timing should be 15 degrees BTDC at 7500 rpm.
Standard carburetor settings (good starting point): needle jet - third groove,
air screw 1-1/2 turn out, #185 main, #200 air jet, #60 slow.

Try it with the above settings. If it still is overly rich, raise the needle one
groove 3rd to 4th, or you can turn the air screw in 1/2 turn increments.

It sounds like a simple fuel or timing problem but with these vintage motors
it is hard to tell. You might also check your carburetor float level (standard
level is 19mm) and your reed valve assembly for cracks or chips.

Good luck with it.

dogger

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Adjusted carb and it ran very well. Rode around the block and it felt nice. Loaded up in the truck with my other bike and my buddy and I went out. Once again the thing crapped out and couldn't be ridden. Rode it for maybe 5 minutes, if that, and it started smoking very badly and dripping oil from the small drain/weep hole in the expansion chamber. Seems to me like it's pulling oil from the transmission. Brought it back to the truck and we beat the piss out of the little bike before heading home.

It's frustrating, but I guess now I have no choice but to rebuild the engine. Just as well. I've wasted time twice taking thsi bike out when it was fine romping around at home but after loading on the truck, drivng out to where I ride, and riding it dails in short order.

Bad news. That's what I meant about "these vintage engines".
You're talking about seals nearly thirty years old that have been
subjected to who knows what. A bottom end rebuild is a good idea
and will allow you to check everything over and replace other parts
that are worn out or about to wear out. I recommend a leak down
test after the rebuild to head off any potential hard starting/poor
runnng problems. It's always good to be proactive - right?

dogger

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Yep. definitely gonna do a full teardown now. Replace every gasket and check all hard parts and replace as necessary. Debating at this poisnt if I should just take the bike down to the frame and paint the thing and check the rest of the bike for damage. I don't care about restoration, just want a good reliable rider. Wouldn't hurt to take the time to make it look clean too.

Not made of money so cheap is always good. I got 3 other projects that need attention ('72 CB750, that will be restored this winter, and also two '60s Galaxies). I figure the dirt bike is the easiest of them all so I'll do that first and get it out of the way so I can enjoy it... Just in time for snow. Snow just makes riding more... Interesting.

With that many projects going on, I really recommend that you diagram, label, and organize everything as you work on it. You may find yourself waiting a month or two on the occasional tough to find part. When you get back to the project, you may find that your recall isn't as great as you thought it was. My '76 CR250M project was like that. Had everything laid out how I wanted, then wait to scrounge parts. In the mean time, working on other projects and then going back to the Elsinore, once parts came in, made me long to have diagrammed things, or organized them a little better.

There are some great vendors listed in the "Resources" area of this site, and a lot of parts are available from them, but there will be the odd critical part that you will find yourself searching for all over the country at every motorcycle bone yard you can find, and if all else fails, there is always the suspense of the final minutes of an e-pay auction, where a couple others desperate for the same part are all competing.

Good luck with the project.

_________________
Steve Amling

1976 CR 250M
1980 Yamaha DT100G
1980 XR 200
1981 CSR650

Careful with the vendors, I have had great luck with Honda dealers, Western Hills Honda, Vintage Iron and Badbrush.. Dealing with Vintage Elsinores has cost me about 5 months riding between two rebuilds.. First time waited about 2.5 monthths for bearings and a rod kit. Second time (shame on me) waited 3 months for about the same items, all the while being told (when they would answer the phone) that the parts "will be in in 2 weeks" . Never again!