Can-Am Off-Road Dirt Bike Build, Part 4

Nov. 11, 2010 By Dan Paris, Photos by Dan Paris, Carrie Homenuck, Heather Collis

When we left the Can-AM project at the end of part three we were still in a euphoric post-victory haze. My dad’s old bike had run great in Ormstown, Quebec, and although there were many things left to finish I knew we were well on our way to building a truly competitive vintage motocross bike. Then something weird happened.

I brought the bike to our local motocross track on a whim a few weeks after Ormstown. It’s a tight, manmade little track filled with the kind of stupid double jumps that kids love and veterans loathe. My goal was simple: To beat one rider. Yup, just one rider on a modern bike. And I did. I actually beat a few of them … for about four laps.

Then, inexplicably, the 175 Rotax engine refused to rev up into the powerband. It ran fine at low rpm, but as soon as it started to pull it gagged and gurgled and would refuse to rev any higher. I changed sparkplugs. Same thing. The air was cooler than it had been in Quebec, so if anything the bike should have been running lean, but instead it was stupid-fat. I drained the gas and poured in fresh 92. Same. Puzzled, I opened up the ancient Bing carburetor and tried a few different needle positions. No change. Must be ignition, I thought, even though the bike had plenty of spark. Still, I had bought the Bosch CDI ‘black box’ used and cheap at Ormstown, so it was immediately suspect. However, nothing I did that day would make the bike run past mid-throttle.

Back home I studied, guessed and swapped parts but nothing changed. After pulling out my hair and calling everyone I could think of for advice I gave up and took the bike to Vintage Parts 4U to see if they had any ideas. A brand new Bosch CDI was installed, and in the shop the bike seemed to run just fine. I took the Can-AM back to the track. One or two laps later, blaaaaaah. Same problem. So I swapped the entire ignition system from end to end. Nothing changed.

Clutching at straws, I installed a new, pre-jetted 34mm Mikuni Carb. After all, everyone I spoke to said to throw that ancient Bing as far away as possible. But the bike ran so well at Ormstown … it didn’t make sense. Anyway, on went a Mikuni, with a 1984 Chevy S10 CV boot making a perfect-fitting adaptor between the airbox and the carb. I was getting desperate here, because the Gopher Dunes ‘Sandstorm’ (the biggest vintage race in Ontario) was only two weeks away!

At the local track the bike started and ran fine at low rpm, just as it had before, but it still refused to rev out. It absolutely refused to go beyond maybe 6000 rpm. Ready to hit a wall myself, I fiddled and farted around until the bike started showing all the classic signs of crank seal failure. They had been replaced when we first built the bike and had only about an hour of running time since then, but they were obviously gone. It’s a pain to change crank seals in Rotax engines, requiring splitting the cases…Grrrrrr….

Now with new seals, a new carb, a completely new ignition, all the top end measurements within spec with no signs of damage, the ignition timing spot on, the oil-injection pump adjustment spot on … this thing had to run! But it still didn’t … at least not past midrange. Angry and frustrated, I held the throttle wide open in neutral in an attempt to either blow it up or fix it by some act of divine intervention. Neither happened, but it did blow the crank seals again. With a tight work schedule and time running out I took the bike back to Vintage Parts 4U. Out of the frame the engine came, to be completely torn down again with one week to go before Sandstorm. They were working their hearts out to get the bike ready for me, but honestly by this time I was making plans to borrow another vintage bike for the race.

The night before the race a phone call from Vintage Parts 4U confirmed the engine was back in the frame and running great with new crank seals and gaskets. Feeling like a factory rider, they also promised to deliver the bike to the Gopher Dunes Sandstorm race for me first thing in the morning. At 730 a.m. on race day, as promised, the bike ran great. Feeling very gun shy I rode a couple laps in practice, slowly. The bike did indeed run great. So I got on the gas, made a couple turns and blaaaaah … we were right back to square one. I rode the bike off the track after finishing that lap, feeling very dejected. I must add that the vintage motocross crowd is an amazing group of people! Offers to let me race their bikes came pouring in, words of encouragement and advice were spoken and condolences were offered. Enter Oscar Gaetan and Ron Harten. 

Gaetan is a multi-time AHRMA champ and the friend who inspired this project in the first place. He’d driven up from Tennessee the night before for the race. He and Harten, the race promoter, are Can-AM gurus. After describing the problem to them they asked, “When was the last time it ran right?” After regurgitating the story I’ve just told you, they asked, “What’s your home track like?” As I described it and they said, “Take the seat off and go for a ride.” Huh??? “If you did a seat bounce to clear a double or something the steel seat base will bend enough to choke off the airflow to the filter. The engine has to get air someplace, so it will suck in the seals…” Before their explanation was finished I had the seat off and was standing with the throttle pinned down the field beside the track. It ripped, 10-zillion RPM and as smooth as a turbine! Feeling quite sheepish I rode back to the pits, thanked them and began beating in the steel seat base with a giant rubber mallet. I made it to the line just a few minutes before the gate dropped for the first ‘short travel’ moto.

The rest of the day is history. Dad’s little 175 flew in the deep beach sand, carrying me to seventh in moto one and fifth in moto two against a field of predominantly 250cc or bigger bikes. Stoked isn’t the word!

That was good for fifth overall, and during the trophy presentation I couldn’t thank Vintage Parts 4U, Gaetan or Harten enough!

Since then we’ve done a few things. A bit of proper sheet-metal banging was done to the seat base and nylon spacers were cut to ensure the seat/fender gap had adequate airflow to the filter.

The stock Can-AM chain guide was cheesy at best, so my dad made from scratch using a KTM chain guide block. even sent over some cool Can-AM retro riding gear! So I’ve been riding my dad’s old bike, bombing around our family grass track and through the woods just like he did back in 1975!

Now that the bike runs the way it should we’re ready to start making it pretty. Aside from cosmetic detailing and proper reassembly of some very hasty racetrack repairs we’ll be installing a bigger rear sprocket and lighter aluminum rims, preferably from a vintage bike that had good brakes like an old YZ. We’ve got offset steering cones to try that will let us run either 29 or 27 degrees of rake instead of 30, which will hopefully tighten the fuzzy handling Can-AMs are famous for. We’ll let you know how our family heirloom comes together later this winter! Newsletter
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