Vintage Dirt Bike Q & A

Hi all,
I just joined up here. It looks like a good place for Elsinore fans, and maybe someone here can lend some insight to a problem I'm having with my '79CR250. I've been working on this bike on and off for almost 2 years now. All was fine and running well until I bolted up a DG pipe to it. It seemed to run fine off the bottom thru the mid, but would break up horribly when I tried to open her up.
I was told to go up on the main jet one or two sizes, so I did, and the problem is worse. The main jet that was originally in there was a 185 which is stock. I tried the 192. Is it possible I should be going down on the main jet instead? I'm also not sure of the needle setting that is currently there. I didn't look yet, but I would like to get this thing to run as clean as it did when I had the stock pipe on there. It was great with the stock pipe.
Any help would be appreciated. Thanks.

-Joe

Put a new plug in it and ride for a few minutes, then pull and check it. That'll at least tell you which way you need to go.

I did run it and check the plug, but not with a brand new one. It looked ok, but just a tad rich.
Problem is that I don't have a lot of yard space to wring it out much. I'm going to try a size down, and see what happens.
May just have to take it to the track to open it up a bit I guess. Down the road would wreck the knobs. Thanks.

You would be surprised how much asphalt a knobby can take before it start throwing knobs. I run mine up the street for testing fairly often with no real harm.

Ok. Well this does not seem to be a jetting issue. The rest of the carb seems good internally, although I did not check the float height. And I did not do an air leak test, but I'm thinking, maybe it's an electrical issue. The bike feels like it's hitting a rev limiter. The marking on the stator cover indicates that it is an 'E' ignition, and saw someone mention somewhere that this ignition does have a limiter. If it does, and this is the problem, it comes on way too soon. Are ignitions 'A', 'B' the ones with no limiter? Or could have someone swapped a non-matching CDI, stator, or rotor to mess it up?

By "break up horribly", I assume you mean four-stroking/blubbering/smoking?

Before you start going leaner on the jetting, I would check for some kind of obstruction in the pipe or silencer (even if you're sure there is none) just to rule it out. It would also be a good idea to try the old pipe again just to double-check.

A higher-performance pipe should increase airflow through the motor, usually requiring richer jetting. This is an oversimpllification, but still valid as a rule of thumb.

Since your bike ran fine before the pipe change, I wouldn't suspect the ignition until you've put the old pipe back on and still have the break-up.

Re the knobbies... my RM has been ridden mostly on pavement its entire lfe, and the OEM Bridgestone M22/M23 tires still have square knobs with plenty of life left. It amazes me.

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

Actually...no. It's not blubbering or smoking. It actually sounds like it's hitting a rev limiter.
Just high revving break-up once I hit a certain rpm. It idles perfect, and throttle response is fine. If you just rode this in the low-mid all the time, you'd never know there was anything wrong until you cracked the throttle open wide.

I will look for a pipe obstruction today, but I'm starting to question my decision on how well it ran with the stock pipe. I'm not so sure I tried to open it up as much with the stock pipe at the time I had it on. I just plain don't remember at this point, as it hasn't been ridden much at all. And when it was, it was in a small back yard which is why I may not have noticed it at the time the stock pipe was on. Part of the reason I changed it was because the stock pipe has a small hole in the seam.

I tried richer jetting first as I figured I would need too. I went up two sizes, with no change. And then a leaner jet two sizes, and the outcome in performance was still the same. And trying to check plug color after revving with all the break-up wasn't going to give anywhere near an accurate color or deposit check.

Yes, some knobbies last a while. These are the old 756's. Kind of soft, and I don't think they would hold up well on hard surface. Edges would be gone in short notice.

Here's a thought. I started thinking when you said to look for an obstruction in the pipe. I didn't look yet, but was wondering if the silencer that I have on there could be the culprit. It's a older S&A Answer with the turbine core spark arrestor. It's in fantastic shape with fresh packing, and clear, but could the turbine core explain the problem by having a little too much back pressure to breathe when I open it up? It may explain why it's running strong in the low to mid, but I don't know if that would cause the top end issue I'm having. Any thoughts on that?

This happened to my MR for a while. I would change the plug, the bike would run perfectly for one or two rides, then the problem would gradually return. It turned out to be lead fouling from the 112-octane leaded race gas I was using. Switched to unleaded, problem solved.

I'd try changing the plug with a brand-new one just to be certain. If that's not it, it could possibly be high-rpm coil breakdown, plug cap problem, C.D.I. box failure, etc.

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

Problem is solved, but I guess I had to learn the hard way. Thanks for all the suggestions.
Rayivers, you're thought on silencer obstruction was it, and I should have listened to my own instinct as well. Even though it was in perfect condition, it was in fact the turbine-core silencer/spark arrestor. It just caused too much back pressure for it to breath properly when opened up. I put the DG silencer back on, and now it rips all the way thru the range using the stock main jet, and the plug looks great. Something I should have tested first, and I was making it harder than necessary to determine the problem.

But it does put me back to another problem I had before. The reason I used the Answer silencer is because it gave sufficient clearance for the shock reservoir that would normally hit the DG silencer. I guess I'm gonna have to lengthen the inlet to the DG, and reposition the mounting bracket. Wish I was a welder.
Thanks again y'all.

I have a '78 CR250 with Ohlins piggybacks and had the same problem. I'm using the stock pipe and silencer though. A friend moved the silencer hanger forward about 2 inches and fabricated an arch(to clear the piggyback when the suspension was compressed) connecting the silencer to the pipe. I've had no problems.

That's an interesting way to do it. Good to hear about different ways to accomplish the same thing. I took the DG silencer apart, and reversed the body which put the mount more towards the front in the perfect place. Then I had an extension pipe of the correct length welded to the silencer. Works great with plenty on clearance. The only aesthetic drawback is that the 'DG' emblem on the silencer is now facing the wheel, so you can't see it unless you look under the fender. It's now a 'stealth' DG silencer. :~)