GT750 converted to CVT for A Modified Autocross car

[RoyalViking]

Rather than add onto the cylinder removal thread I decided post this separately. I bought a basket case years ago and assembled it to race it one time. Due to the throttle slide coming off of the throttle cable the #3 cylinder siezed while racing the car and it sat since 2010. Recently I found motivation to work on it again. I pulled the head and then the cylinders off and found that #3 is pretty bad. I'm new to 2 strokes and didn't realize the loss in power was the piston seizing.

So no worries, I thought I could replace the pistons and rings and have it going again in no time. It turns out that the cylinders were bored out to 72mm and after asking a machine shop check my measurements they confirmed that the cylinders are worn beyond their limit. Unfortunately the cylinders were ported too. I have searched and I guess 72mm is the largest oversize piston available. I contacted a couple places to see if cylinder plating could restore the bores. Their recommendation is to have the #3 cylinder sleeved but they didn't answer my question about plating.

I have another engine case, cylinder, and head that were part of the original purchase as a spare (it is a 1975). I checked the wear and I have to go to .5mm oversize if I plan to use this one.

I'd like to be able to use the original cylinders if possible. Wiseco can make a custom set of 72.5mm pistons. They have a minimum run of 12 at about 145 a piston (piston, rings, and I think the wrist pin was included). Since there are 2 left hand pistons and 1 right hand piston per engine that would leave me with 6 sets unless I had a total of 24 left hand pistons made. It all sounded great until I did the math.

At this point the easiest path would be taking the stock 75 cylinder and having it fitted for the 70.5mm pistons. I'm open for any suggestions you might have.

[Suzukidave]

Maybe Wossner , they make over bore sets for the GT750 and Kawa 750 ?

[tz375]

That's an interesting project.

At 72mm, the cast in liner is already very thin and since they were never well located in the mold, going larger could be a problem. Ron at Fahron used to reline those as did Barton when they were still in business. Try one of the race barrel plating places such as Millennium or maybe even bore-tech and see what's possible.

Getting a stock barrel bored is probably cheaper, but then it needs to be ported to get the power back - assuming that you liked the way it performed before the meltdown. Do you know how much it was ported by any chance and what pipes and carbs is it running with? That may give a good indication of which direction to take.

[RoyalViking]

I have already contacted Millenium and one other company. I did contact Millenium and they haven't gotten back to me yet. I also contacted Langcourt Performance and am still waiting to hear back from them. It may be less expensive to find a good stock barrel to work with.

As far as how it was ported I can't remember. That conversation with the guy who built the car was over 24 years ago. It may just be a mild port. I'm going to take measurements to see. I've found web pages that say to shave 3mm from the top and add a spacer on the bottom along with some changes to the piston skirt.

What I do remember is that the engine was probably putting out 80-85 hp with the changes. It was fitted with Mikuni carbs and the three-into-one chamber.

In the mean time I have a spare stock barrel from a '75 that I dropped off at a motorcycle shop to see what size overbore it will take. I haven't committed to anything at this point but I was considering going with the ebay pistons that are selling for $100 and running it with stock porting. That would have the car running again for this fall and I could then focus on changing the ports later.

[tz375]

There is a lot of old information on line and not much of it is proven.

3mm spacers (lift plates) are a common - easy way to lift the transfer ports, and exhausts still need to be raised further. Spacers, head gaskets and base gaskets are available from COMETIC in OH.

[Jimroid]

I'd be more concerned with the cobbled up jackshaft support. That looks like a grenade with the pin pulled. Sorry, just keeping it real.

[RoyalViking]

I'm not a fan of it either. The car had run like that for 10 years before the owner sold it to me. I laid it on the engine to show how things are set up. You're only seeing a third of it.

[Jimroid]

Well if it lasted 10 years it must sorta ok! How about some more pictures?