Vintage Dirt Bike Q & A

Hey guys i have just totally restored a 79 cr250 and can not get it to run right . When you try to ride or rev the bike out on the stand anything past quarter throttle opening creates a popping from the exhaust it is quite a fierce surge down the pipe as it looks like it is trying to blow the exhaust off the stub at the barrel if you hold it in the problem zone . My first thought was reeds as this is what it sounds like from past experience of having reeds crack on other bikes so i had a look at the reed block this seems ok only the rubber is a little hard so i got some boyesen reeds and tried these the problem was still there ,i have gone back to the standard set i had as these seem to seal better even though the mating surface is a bit hard with a torch in the dark you cant see light coming through any gaps . I am going round in circles now i have replaced ign. coil with a new one the stator re wound tried two diferent heads pistons and barrels two different carbs one a flat slide ,I have had a strobe connected to it to see if i can see a break down in the ign. system pressure tested the crank cases and barrel it holds ten psi for as long as you leave it ,i have tried it even with no exhaut on at all in desperation to cure this gremlin it still had the same sort of popping. the set up is pretty much standard apart from a pro circuit tail pipe and a wiseco piston on a nickasil bore with the correct wiseco rings for this barrel finish . any help would be great thanks cheers James .

Hi James, This might be frustrating, but look on the bright side: at least you got to use a whole slew of your gadgets trying to solve this one!

I have two ideas for you:
Idea 1) Having read everything else you tried, it seems that huge popping sounds like it could be the ignition sparking way too late - as in when the exhaust port is coming open already so almost all of the combusion energy is routed right out the pipe rather than pushing the piston down. There are two causes I could think of: a) the timing is set after TDC, rather than before TDC. That'd be so weird because your timing marks would be way off and you'd catch it, besides that probably wouldn't be enough to cause the huge popping, it would just be way down on power. b) the CDI unit that controls the ignition mapping is screwed up. When you hooked up the timing light to check for missing spark, did you aim it at the rotor to see if timing advances as the engine speeds up? It would be a dead giveaway if the ignition timing retarded as the RPM's increased.

Idea 2) Sell the bike on Ebay listed as completely restored with hundreds of detail hours spent, and only ran at 1/4 throttle for break in.

Good luck,
Jon

Yes all of the tools have been in and out several times except the sledge hammer that may come soon..... I did check the ignition advance with the timing light in the inspection hole and it seems to advance and retard with the rpm fine , the timing is set in position with a dowel on the outer case so the timing of it should be bang on although i did try it without the dowel in and moved it back and forth with it running as you said this just made it lack power (even more) so the dowel is back in at the minute anyway cheers Jon for the input much appreciated James

James,

To follow up on Jon's reply - at one point my 175's points ignition was double-sparking (before both TDC and BDC). It wouldn't do it all the time, but every now and then it would blast from the crankcase out both the intake (piston-port, no reeds) and exhaust, and scare the crap out of me. After I put in a CDI ignition it never happened again. Also, if the exhaust pipe is loading up with unburnt mixture for some reason (incomplete or intermittent ignition, clogged silencer, rich jetting, etc.), it can sound like a bazooka when a good combustion cycle comes along and fires up that fuel-filled pipe.

Another thing that can cause this are the holes that some piston manufacturers (like Wiseco) recommend that you drill in some of their pistons for exhaust bridge lubrication. If the holes are drilled in the exactly correct locations there shouldn't be a problem, but if not you can have exhaust flame shooting into the crankcase, definitely not a good thing.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Are the carbs you used jetted properly for your bike? Maybe your a little rich 1/4-mid throttle. Check the needle jet and jet needle size, maybe swap with a little smaller set.

Good luck