GT750 early model versus late and Chambers

[Vintageman]

Hello,

I see that the GT750 72-74 has different porting than 75-77. Will 3 into 3 chambers perform differently on these models? For example was Jemco's really designed for one or the other? Will any top end performance b egained on the early models? I had always thought the later model would be more performance for 3 into 3.

Thanks

[tz375]

Good question.

In theory, MAB barrels with longer duration would need a longer chamber to resonate at the same revs, or the JKL would peak at lower revs. So they should still work. Now if we remember that the softer JKL probably has lower EGT because of the state of tune, that lower temperature would generate a lower speed of sound, so the chamber would actually resonate at higher revs.

Two ways to look at it and both give different answers.

All I can tell you for sure is that on my ported A barrels they work fine thank you very much and I'd expect that they would wake up a stock JKL as well (just not by as much) and maybe at a different engine speed.

Jon Easton can tell you what they were designed to work on - brackets may be different too. But a ported motor makes them work even better than they do on a stock motor. You could port your JKL to MAB exhaust specs or go further than that and that should work.

[Vintageman]

tz375,

I called Jemco and got Dan not Jon. He was not really aware of the porting difference, but said he has sold manyfor both and no one has complained. He also said just bolt on too and no porting or any other changes needed and it works well. "It's a big ol bike.." he said. He does make a different set for the JK versus LMAB so it can be mounted.

I always thought I would like the MAB better based on its state of tune. I am sure people have different opinions on that. Well. if the 3 into 3 chambers make it hit harder mid and increase top end a little that would be awesome.

If you like top end tune, wouldn't a first step be to get the MAB cylinder and Head? Or is your custom porting much more than this?

[tz375]

There's a few issues here. MAB has a taller exhaust and lower intake floor. That makes for more power higher up and less low down.

Jemco pipes seem to tilt the power curve and give less HP below 5500 and more above that on an MAB motor and will probably do something similar on a JKL.

Use JKL barrel and port the exhaust to MAB specs and you should get the extra top end and keep the fatter bottom end.

Porting usually goes beyond just opening up the intake and exhaust to MAB specs. The thing that takes most time is opening up the intakes and shortening that long bridge in late barrels. That takes a lot of time. Then if you go to a stage 2, the transfers need to be modified and that takes more time and specialized porting tools.

Then there are big bore options and new liners and reed valve conversions and straight cut primary gears and keyed cranks and dry clutches and billet squish heads and that takes you to 10k territory unless you can do it all yourself.

A sensible build is a completely rebuilt bottom end plus pipes plus mild porting and raised compression with retarded timing and a good ignition. That's a cost effective solution for the street that won't send you mad or to the poor house. (do they still have those? )

For a really sweet mild ride get a stock JKL GT750 and don't mess with it. It's mellow.

[Vintageman]

tz375 (et al),

If I do buy this L I will leave engine alone to start. I wanted an M A or B, but not much out there. These things are getting real expensive too.

I do like chambers and would do that. If 3 into 3 helps boost power that is great. If the Jemco's boost at 5500 - 6000 maybe this is too late on the J K L.

Has anyone ready this post put Jemco's on a J KL or anothers mfg's chambers please respond.

Maybe 3 into 1 is better????

"Use JKL barrel and port the exhaust to MAB specs and you should get the extra top end and keep the fatter bottom end. "

I like this . I would also use the thinner head gasket and late model head (or have the early one milled the same amount). I would look for a spare Cyls and head, just so I always have original I suppose or i ncase I fail

I see that during 1974 a change was made to ensure water didn't leak into studs, causing rust and preventy cyl from being removed. I will check the engine number and see if thiis has it. Maybe this question is a separate post. But if this is a real issue I should be aware now

For now I am wondering how Jemco's perform on stock JKL GT750... if we know

Thanks

[tz375]

In my opinion, the 74 L model is the best for a rider.

Don't worry about the water leak/rust mod - that was basically a change to use washers with a rubber membrane on the 11 sleeve nuts. That's a quick, easy, cheap upgrade.

For compression, use whatever gasket you get and shave some off the head to raise the CR. In theory, what Suzuki did was to raise the static compression slightly to make up for the way that the taller exhaust port drops the "corrected" compression. You can replicate that by milling the head or you can mill off a whole lot more.

Alan P put chambers on a K, but he also had it ported by SCR at the same time and that bike ran really well.

Raising the exhaust port roof is pretty easy - even for a beginner to two strokes. Raise the center by the amount you want and then make the roof into a nice curve (raise less at the sides than the center). Easy.

No downside as long as you chamfer the ports and don't go mad. Then JEMCOs will be a perfect match for your bike. Even with stock JKL ports they should bring a top end improvement. Try to avoid over thinking this. The pipes will work with a wide variety of ports and the power they make is related to port time area. Think of it this way. Pipes will work just fine with JKL ports and slightly better with MAB ports and better still with a ported motor, but in all cases, they are better than stock - at least from 5.5k up.

If that's much higher than you want to rev the bike in normal riding conditions, stock pipes are best. Stock make the most power of any pipes we tested yet at <5,500. It all comes down to what you want and how you ride.

[Vintageman]

tz375,

Thanks again.

I was worried about the leaking bolts for If I did want to take it a part to do porting. I hear if it leaked it is almost impossible to remove Cyl. But I suppose it is a matter of patients and has probally been addressed on this site before.

Well, I found the Vin number at what point they made the change maybe I'll be lucky. Also I see there is a better clucth bushing upgrade to needle bearing and different clucth material among other serices things

If anything, It sounds like the JKL is great runner and feeling better about that. Thanks

[tz375]

Barrels always seem to be hard to remove and some are harder than others for sure. A puller plate is the way to go there.