GT750 Rebuild Questions

[dirtburner]

Hello all. New here to the forum and sincerely hoping to get some good advice on my build. Some history. I have a GT750L that I bought as a basket case about 5 years ago, I always wanted one and now was my chance. The motor was torn down because of crank seals but the bike is in pretty nice shape. stock seat in decent shape, Paint is Okay for a rider, looks like its been garage kept its whole life. so its a good specimen to work on. I purchased a 76 engine soon after just to quickly get it running, Which worked for about 2 years and then something came apart in the lower end. Didn't lock up but it sounds like the big end of the middle rod banging away and there were chunks of lower bearing stuck in the plug. So its done! Bikes has been sitting a few years now and this year I finally have enough parts collected to put a motor together. The original 74 engine cases and covers are pristine and I would like to use them but I am planning on using TM34's for carbs and keeping the SRIS so I need a way to pull the oil pump open. I bought a 73 engine off evilbay to use the cases but it looks like the chain came off of it at one time and someone did a crappy repair (not disclosed in the add). So I think I want to use the 74 cases and make a cable bracket for oil pump. Has anyone done this? Maybe some pics? Ideas? Also the bike came with an old set of JR pawer pipes they are chambers with integrated silencers They look pretty well made. tried to find info but they have been out of business for a long time. Anybody have any experience with them? Opinions? I know a little about porting. been playing with 2 strokes for 20 years but I have never built a piston port buffalo and it is a little different animal than a modern dirt or snow machine. I have looked at the port maps on this site at length. I am thinking about an 1/8 cylinder spacer from cometic and cut the top of the block accordingly and maybe put a clean up cut on the head. For a fun fast street machine what are your opinions on the porting? also what about cutting the head and running premium pump gas? How much compression is ok running pump gas without a quench band? Then there is the E-85 option which would let me run more compression? I am having a hard time deciding and would like to hear opinions from guys who have done it already. Sorry for writing a book and thanks for the great forum

[tz375]

Welcome.

If you come form a background of dirt bikes and sleds with tight powerbands, a 3mm spacer will still feel tame. If you were brought up on the soft lazy way that a GT750 rides, that may be a little edgy.

Jemco make good pipes. J&R had a mixed reputation. I have seen some people get excited but the only test I read was in Bike magazine I think where they built a Dresda framed Kettle and fitted J&R pipes and they hated the way the bottom end and top end both vanished and left a peaky mid rev performance.

34 flats don't fit without tilting the intakes because they are too tall. At least the TMX34 I have here are and they hit the clutch cover. Small body 34mm carbs fit without modification and almost anything will fit if you tilt the intakes because that raises the port considerably.

If you have a 73 cable operated pump, that would be perfect. Add a short aluminum bracket to the crankcases to act as a cable adjuster-stop. That has been done a few times.

[dirtburner]

[quote="tz375"]



Jemco make good pipes. J&R had a mixed reputation. I have seen some people get excited but the only test I read was in Bike magazine I think where they built a Dresda framed Kettle and fitted J&R pipes and they hated the way the bottom end and top end both vanished and left a peaky mid rev performance.

That is exactly how it ran before it puked. I was blaming my inability to figure out the bs carbs but maybe it was the pipes. Ok... good to know. Thanks for the info. I have a 73 oil pump, carbs, cables, Petcock, and twist grip. so maybe I will just put this stuff on there with some jemcos and 3mm spacer. I have read that I need to shorten the intake side piston skirts by 4mm is this true?

Thanks again.

I will post some pics when I get it all started.

John

[tz375]

People used to shorten the piston skirt to get more intake time-area. Raising the barrels with a spacer increases time-area for exhaust and transfers and shortens it for the intake, so people are convinced that they must take a lot off the intake with a spacer.

If you use an LMAB barrel, just clean up the intake port floor slightly to improve flow and widen it slightly and you will have a much better bike at all engine speeds. With a JKL barrel you could clean a little more off the floor, but again, only enough to improve flow and don't bother with shortening the piston unless you want to lose more low end than you add at the top end.

If you are making it into a drag race bike or road racer where it never runs at less that say 5000 revs and revs up to 8500 or so, you can afford to add more intake time.

[dirtburner]

Thanks for the advice TZ. Been getting in some last minute snowmobiling and ended up blowing up the T cat so I havent been on in a while. Anyway, I will leave the slug skirt alone. I have the L barrel that was on the bike factory, and I have a K barrel that came with the cases I bought. I also have the M jug on the motor I blew, up and an Unknown one that I bought from Evilbay. So... Which cylinder would you guys use for a strong street racer?

P.S. Its back on to the bike now. It is getting warmer up here

[tz375]

I would take either the K or L barrel and raise the exhaust ports 3mm and leave the rest alone.

[dirtburner]

Raise the exhaust port 3mm even with the 3mm spacer for a total of 6mm? leave intake and transfers alone? maybe just clean the casting up inside?

[tz375]

2.5mm takes an L exhaust up to MAB specs, so I added 0.5mm for a good time.

With a 3mm spacer, you could leave the exhaust alone and see how you like that and then you could go back and take a little more off later to sharpen the powerband up later.

With a spacer, you have the opportunity to clean up the shelf effect where a stock intake port meets the liner. There's usually a significant step there that doesn't help flow.

[bighole]

dirt,
If youre up for a road trip to Michigan I think I have everything you would need...
PM me if interested as it is time for me to have a garage sale.

TZ, do you still remember how to get here?

-John

[Craig380]

I have a set of J&Rs on my 380, which is otherwise 100% stock (standard airbox, filters, carbs, porting). The pipes work very well with this setup, with no discernible loss at the low end and quite a bit more above 5,000 ... but they do cause power to peak about 500 rpm earlier than stock. The pipes are pretty slim and with gentle cone angles, which probably explains why they work the way they do.

I've no idea how they would behave on a ported motor.

[dirtburner]

Thanks for the input guys. Yea, I figure I already have the J&R's so I will give them a shot first. More than likely it sounds like I will end up with a set of Jemco's before its over though... This is spring cleaning weekend. Put the sleds away and get the summer toys out... So I will get some pictures up soon.

P.M. sent big...

[dirtburner]

The crank from the original motor looks pretty good so this is the one I will have bearings and seals put on. I think the rods were weight matched, judging by the grinding. It is engraved that it got new seals in 1977..

[dirtburner]

Here is a pic of the old fella in its present condition... Maybe a little neglected but a good specimen to start with I think.

[Blokhead]

The crank from the original motor looks pretty good so this is the one I will have bearings and seals put on. I think the rods were weight matched, judging by the grinding. It is engraved that it got new seals in 1977..

I've had rods balanced before that were ground on both ends. That might be a nice smooth crank.

[dirtburner]

Thanks for the GS trans write up gents. I got the GS trans all fitted up. ready for cleaning then assy. I decided to use the GT output shaft because I have no plans on a wider tire in the rear any time soon.