Project 2-4: Installing a 2-Stroke Motor in a 4-Stroke Frame, Part 7

Jul. 12, 2011 By Rick Sieman

So far, our project bike has been a lot of work, but rather straightforward work. Sure, we took a lot of time fabricating motor mounts and a whole a lot more time making a pipe that made sense and cleared everything properly. But here is a bit of warning for you should you choose to undertake such a project: it's the details that will drive you nuts!

Previous Project 2-4 Stories:

Part 6

Part 5

Part 4

We got the bike altogether and got ready to start it up and hear it run for the first time.  About the fifth kick, the RM motor coughed once and then came to life. It really came to life as the RPM was on the high side, probably about 4000 RPM or so.

Rather quickly, the choke was shut off in the hopes that the motor would calm down to a more sensible RPM. But no, instead the RPM went higher and higher, until we were forced to shut the motor down to avoid damage.

A basic quick inspection showed that the way the throttle cable came into the top of the carb left an air leak that no doubt contributed to the problem. After rummaging around in our carb box, we were able to come up with enough of the right bits and pieces to shut down the air leak.

The project bike look great, but refused to start for us.

Then it was kick, kick, kick, kick and kick some more. A fresh plug was put in and checked; we had plenty of decent spark. More kicks followed by more kicks, until drops of sweat dripped off our sizable nose.

Three more days of frustration followed, trying every jet combination imaginable, starter fluid into the airbox, and more and more of the irritating kicking. Every once in a while we’d get the bike to give a little pop, but that was it.

Here is the man who got us running in front of his shop.

My old friend Matt Cuddy suggested that I seek help from a savvy dirt biker, because I was too close to the problem to see what probably was the real problem. He suggested that I pay a visit to Stephen Gautreau (owner of SG Cycles, 10402 E. Apache Trail, Apache Junction, AZ 85120, 602-705-5876) and see if he could find out why the motor wouldn't talk to me. Located about a half-hour from Phoenix, Stephen’s shop is one of those rare places that does a lot of service and repair but doesn't sell any major brands. We worked with him in the past on various project bikes, and he was very savvy.

Stephen is one of those rare individuals left in the country nowadays who have an affinity for mechanical things. He knows how they work, why they work and how to fix them. As a matter of fact, he takes it as a personal challenge to fix things that have eluded the all too ordinary basic repairmen.

I explained to him all that I had done, and not done, with the project bike and why I threw my hands up in the air in complete surrender.

Stephen got the bike running on the second kick.

The bike was unloaded and rolled into his shop. He gave the bike a few introductory kicks and the damn thing started on the second kick, which caused my eyes roll back in my head and my jaw to bounce off the cement floor. As with the first time that the bike had started, the RPM were way too high and everything was incorrect.

The bike called for a 36mm carburetor, but I didn't have a complete 36 in my collection and decided to use a 38mm unit instead. The jetting for the 36 calls for:

Main jet – 290
Needle jet – R6
Jet needle – 6FJ6
Pilot jet – 60
Slide number – 3.0

The carb was our first problem.

I had a 38mm Mikuni on the bike, as this was supposed to be the hot ticket back in the day to let the bike breathe better and to get more revs. The main jet was increased in size to a 320 and I had a wildly different needle jet and needle in there. They came with the carb, so I figured it was in the ball park. Wrong!

Stephen thought we ought to start out at Ground Zero, that is, with his closest we could come to the correct jetting in a 36mm carb. Between the enormous collection of jets that he had and all the carb bodies that I brought with me, we were able to come up with a 36 mm Mikuni with just about the perfect jetting. The only difference was that he had a 2.5 slide in instead of the 3.0 slide. He was convinced that he could make adjustments around that should he need to do so.

With the combination of my carb bodies and Stephen's jets, we were able to get the bike jetted to very close to stock specs.

After much experimenting, we settled on a 45 pilot jet and the 2.5 slide. Stephen took the bike outside and after a few kicks, it fired up and ran okay. He spent a lot of time adjusting the idle, in playing with the air correction screw. Eventually, he got the bike idling perfectly and responding nicely to the throttle. That's when we got the big thumbs up and an ear to ear grin.

After a few kicks, Project 2-4 came to life.

He took the bike off for a ride and was gone about 20 minutes.  Then we saw him pushing the bike back to the shop, shaking his head from side to side. Apparently he rode the bike for about 10 minutes when it stopped running and it refused to start again.

We were rewarded with a big thumbs up.The bike was put into the air and given a serious examination. Stephen then turned to me and pointed out that one of the rear cylinder hold-down nuts was very loose to the point of backing off all the way. Further checking revealed that all of the cylinder nuts were horribly loose and the base gasket was sticking out.

Well, there was the answer as to why the bike quit running. All of the pressure from the lower end was not going up through the transfer ports, but instead a huge amount was leaking out through the base gasket. I assumed that the previous owner had the cylinder tightened and I had only taken the head off to check the piston.
Lesson learned. Don't take anything for granted on a part you buy from someone else.

Lesson number two. When you run into a mind-boggling problem on your project bike, go to a savvy craftsman like Stephen Gautreau to help you through the seemingly impossible things.

Previous Project 2-4 Stories:

Part 6

Part 5

Part 4 Newsletter
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