GMS .76 engine mods?
 

The GMS .76 engine is a ringed engine with a steel sleeve. You can buy replacement sleeves from TH for $17.00

At that price--I could try a few mods to it.

Has anyone ever done any mods to this engine?

I wonder how much you could get out of it if you checked and increased the intake and exhaust duration? I wonder what polishing it and matching the ports would do?

I saw a website that stuffs the case by wrapping a very thin steel sleave around the crank counterweight. Then they fill the area around the counterweight with lightweight material--to keep from throwing the balance off. They claim it works REALLY WELL!!

If ya think about it--there's probably about 3 or 4 cc's of volume around the counterweight area where it's machined up to the rod pin. If you could fill up that space and decrease the swept volume in the case---think of the acceleration into the transfer ports!!![X(] A couple cc's of volume in such a small space would probably make a huge increase in transfer velocities.

I'm really wanting to buy one with an extra sleeve for it. Maybe a ring too. Get out the dremel and the degree wheel--and go to town!!

What say you? Anyone ever tried it. Anyone willing to try it with me?

RE: GMS .76 engine mods?
 

Chris:

Have fun.

I have gotten to the point of selecting an engine for the power I want, then leaving it stock.

Concerning the stuffing of the case. K&B did it in a similar manner, but instead of filling the ground area they made slots in the web and then shrank an aluminum ring around the outside. Picture shows the "Hot" crank for the Q40 on the left, the standard crank on the right. They can be installed interchangeably.

Bill.

RE: GMS .76 engine mods?
 

I'm hoping you will give us a complete report on your GMS 76 mods as soon as you get them accomplished?

Enjoy,

Jim

RE: GMS .76 engine mods?
 

I've got a long way to go. I need to purchase the engine and extra sleeve first. It may be a month before I get the funds for that.

I want to start out by installing the ring around the counterweight and filling the space to stuff the case.

What type of lightweight material can I use to take up that space? It needs to be fuelproof, but lightweight--so that it doesn't effect balance too much..

How will I get a ring built/welded to the right size to wrap around the counterweight?

I wonder if a very thin walled piece of pipe/copper/aluminum could be slightly undersized---then heated to expand it enough to slip over the counterweight. When it cooled off and shrunk--it would be like a press fit. But--there's not usually much room around the counterweight. It would have to be a VERY thin walled piece--to avoid rubbing on the crankcase.

Once the case is stuffed--then the fuel charges will be accelerated through the transfer ports. I can drill out the crankshaft hole to allow a bigger charge to be sucked in at those higher velocities.

Port matching is really all that I want to do to the transfer ports.

I would like to increase intake duration and raise the exhaust port to increase duration there as well.

I'm wondering if the carb will flow enough fuel?

On dremeling the clyinder sleeve:
TH states that this is a steel sleeve. Can I accomplish my grinding and port matching work with a carbide bit on my dremel? What types of tools do I need to grind on the sleeve?

On the case transfer ports:
Should I use fine emery cloth and polish the transfer ports in the case and make them shiny--or is it desireable to have leave them rough--to turbulate the fuel charges as they enter the combustion area?

Should the tops of the transfer ports in the case and sleeve be ground to a more aggresive angle to allow for smoother flow?

I bet that you can tell that I've never done this many mods to an engine before.[:-]

I've drilled a crankshaft on a 61FX once and also attacked the exhaust port on that same engine. I was pretty amazed at the boost in performance.

If You guys can help me out--and tell me what kinds of grinding tools I need. As well as where and how much to take off here and there--I'll follow directions.

I've searched around for numbers and figures on intake and exhaust durations--but not having much luck. I know that there is a point of diminishing return on the durations, but can't find the numbers.

I guess the best place to start is to buy the engine and post the stock timing specs.

RE: GMS .76 engine mods?
 

BOUGHT a GMS 76 and it vibrated like mad. I was ready to throw it in the trash but instead took it apart and found the crank counter weight ran full circle although thinner on the crak pin side.I ground away all of the thin part and even rouned the crank behind the pin.Hoever, don't get carried away here or you'll weaken the crank.Put it back together with a new ring and now it is one of my favorite engines.Try it you'll like it.