GS750 Rebuild options?

[Guest]

So I've got my GS750 pretty well dialed in now. Did the brake upgrade and a couple other things and I think it's time to start thinking about a rebuild. It has always smoked a little and I have noticed it getting worse lately. So what are the best options for a top-end? I don't really NEED more power, but it would be cool to have. Meaning I am content with the current power, but if i'm already in there... It looks like the 844 big bore is the best option if I need to bore the cylinders. Here's some questions I have:

What is the likelihood that my stock bores are okay for a hone and re-ring at 36000 miles?
Does anyone make a stock size high compression piston for the 750?
How about milling the head for a little more compression?
Are the original valve guides usually good for another 30,000?
How much weight does the 1100 swap add?

I'm not really sure what I want to do so I was hoping someone could offer some suggestions on what the best course of action is. I was thinking about an 1100 swap, but not sure if I want to go that far (plus the 8v looks cooler imo). I get nearly 50 mpg now and don't want to lower that too much. But I figure if I'm going to do a top end I might as well "blueprint" the engine with higher compression, degreed cams, etc. Any and all suggestions are welcome?

 

[Sam 78 GS750]

Be careful of "while I'm in there..."

Your stock bores and valve guides should be ok. As far as the rest, you've got plenty of options. You can deglaze the cylinders and put in new rings, then maybe deck the head a bit and call it a day. At the other end is the 7/11 swap.
One thing you didn't mention which is a popular one is keeping the 750 bottom end, and swapping the top end in from a gs850. From what I understand it's pretty straight forward.

 

[Guest]

It depends on your budget. As Sam mentions it is relatively cheap & easy to swap on stock cylinders and pistons from an 850; I did it and i'm not much good with a wrench. If you have some more $ to spend you could use the 850 cylinders and Wiseco's high comp 844 pistons that they make for a bored out 2 valve 750. It's probably quite a bit cheaper to find a nice used set of 850 cylinders than pay for the bore and hone job, but you can do that math for your local prices. Generally the valve guides are okay, but it is best to replace the oil seals while you're in there. You should also measure all you cams, valves, etc per the service manual while you have it apart to check for issues.
7/11 swap is for the later 4 valve bikes I think, your best swap would probably be a 2 valve GS1000 engine; it is actually lighter than the 750 engine. From what I understand it is requires a little bit of fitting but nothing too crazy.

 

[Guest]

Be careful of "while I'm in there..."

That's how this whole charade started but I ended up with a mostly restored GS, so i'm okay with it.

As for the rebuild, if the stock bores are okay I think I would lean towards new rings and a decked head to raise the compression ratio to maybe 10:25 : 1. What is the limit for 87 octane? How about 93?
What other issues are there with decking the head? I know the cams would need to be degreed but are there any valve-piston clearance issues? I would guess that a combo like that would be good for maybe 5 extra hp over the stock engine?

The more I read about the 7/11 swap the more do-able it seems. If I am going to spend the time and money to find 850 cylinders I think it would be more worthwhile to find an 1100.

 

[Nessism]

I wouldn't bother milling the head unless it's needs a clean up pass. Pretty sure you won't get anywhere near 10.5, not to mention, you will mess up the cam timing. Not worth the hassle.

 

[Steve]

What is the limit for 87 octane? How about 93?

Compression ratio is not the only thing that determines the need for higher octane. :-k

I think the Twin Cam Harley engine has a ratio of about 9.0:1 and requires high-octane fuel. My 13-year-old Toyota van has a 10.8:1 ratio and does just fine with 87 octane. The new 2019 Toyota van has 11.90:1 ratio and does just fine on 87 octane. :-k My 850's ratio is 8.8:1. Actually might be just a bit higher, since I had the head and block milled a bit to get them straight. Does just fine on 87 octane.

Besides compression ratio, other factors that might require higher-octane fuel include the diameter of the piston and operating temperature. Cam timing and fuel mixture are also major players.

Back to your question: I don't know what the maximum would be for a GS engine, just letting you know that it's not all about the ratio.

.

 

[portdave]

I have a used 78 1000 engine that needs to be rebuilt (low compression) but would be a nice candidate for a swap. I even have the brackets for the swap. I just never got around to doing it and have sold the bike. PM me for more info.

 

[Guest]

I have a used 78 1000 engine that needs to be rebuilt (low compression) but would be a nice candidate for a swap. I even have the brackets for the swap. I just never got around to doing it and have sold the bike. PM me for more info.

What brackets need to be changed to fit the GS1000? Just the front lower supports? I may be interested. What other differences? I know there is no kickstart which I don't really NEED but is kinda cool to have

 

[portdave]

Here is a thread that shows the details. You have to use the brackets and move a tab on the frame. A few guys here have done it.

https://www.thegsresources.com/_for...Tarbash27-s-1979-7-11-build&highlight=Tarbash

 

[portdave]

You can send me an email. Just click on my name. I am probable a few hours from you.

 

[Guest]

Okay so I think I figured out a plan... I found an 82 GS850 for sale near me, it runs but not well and is more or less a restoration project/parts bike. I may go buy it and steal the top end for myself. My questions is what else should be changed? I know the 850 has different cams, are the 750 or 850 cams better? How about the carbs, this 850 has CV's?

How about bolting the 750 top end back onto the 850? That way I could at least sell a running bike and recoup some of my money

 

[Guest]

Did you ever get answers to these questions? I'm kind of looking at doing a cylinder/piston swap to the 850 as well because of bad pitting and too small a budget for wiseco and rebore.

 

[Chuck78]

MTC has a few +2mm overbore 67mm high compression pistons for the 750 8v for $250/set, 798cc into stock 750 cylinders!
They're old school NOS, so a bit more weight than if a modern piston were made in a comparable size. Have to email them about those, not on their website. They can mill the domes to bring them down to 9.5 or 10.5 compression from 11.5:1 or so.

 

[Chuck78]

Another point on the 8v 7/11 16v swap is that the 1100 16v engine will add 35lbs to the weight of the bike... puts it at a bit more portly disposition if wanting a very nimble handling vintage machine...
AND... GS1100E 16V engines generally sell for big bucks, not much less than a neglected 1100E project bike.

 

[Chuck78]

Also, Cruizin_Image_Co sells 870cc GS850 +1.0mm oversized stock compression stock style cast piston sets for around $189 shipped from Japan... another great option if you find a cheap 850 cylinder that could use overbored slightly to clean it up. Buying new OEM rings will run you perhaps $140 shipped, so you're almost to the cost of those pistons, just have to find a good machine shop to bore, hone, & possibly deck the cylinder.

most 850's need the deck surfaces milled however, due to the earlier original OEM 850 head gaskets decaying and corroding themselves to the block & head deck surfaces. Bonus for running stock type pistons, as you will get a slight compression boost from the milling then, which also reduces your quench height or squish band height, which also reduces chances of pre-ignition/detonation while boosting the compression & allowing your engine to run more efficiently.

If you can find a shop nearby that has a Serdi valve cutting machine with the right size cutters for the GS valve seat and stem diameters, this will give you improved flow and more power as well as efficiency. Or send head to Rapid Ray on this forum, he has the Serdi setup for our heads & is a GS engine builder genius.

798cc or 844cc high compression, or 870cc standard compression, all three will really wake up the 750...

750 cams have slightly longer valve opening duration vs 850/1000 cams which have slightly higher lift. 750 cams scream slight bit more 6500-10000rpm, 850/1000 cams have more street grunt at 4100-5400 rpm.

The Delkevic stainless megaphone exhaust and K&N RC2222 dual air filters are also killer additions to a performance 750 build.
Add some great Bridgestone, Continental, Avon, or Shinko tires, Mike's XS cartridge emulators & chopped/shimmed OEM springs, + a set of YSS E-302 shocks in 345mm or so, or some old vintage Fox Shox in 13-3/8", & proper carb jetting, & you'll have one helluva machine.

Have you upgraded your charging system to a Shindingen SH775 (Polaris OEM #4012941, buy used on ebay $44.99) yet, & test your stator? Rewire for an ignition coil relay?

All of these things will really pay off in the long run in terms of reliability, power, and miles and miles worth of smiles...


Also, the 1000 or 1100 engine swap has one big drawback... Those engines are really really fast, and are almost too much for a stock crank. The cranks on the 1000 & early 1100 need to be sent to John Pearson of Pearson Racing in Ohio and have them indexed and welded. Otherwise, a hard ridden stock 1000 or 1100 with good tires, or especially a modified engine with even just pods and pipes & a sticky back tire, is plenty enough power to twist the crank if the tire will grip good enough on launches or 1-2 shifts...

You just reminded me I dropped off some parts to Pearson months ago and need to call him back and confirm things with him! Same wirh Rapid Ray...
I'm even getting a 750 crank welded by Pearson ( I tend to have an addiction to twisting throttle and abusing things to the extreme, extra insurance...).

If the crank is in really good shape and has not been abused, it's not very much, maybe a little over $200 shipped back to you. But if the crank does not check out perfect, it will need a full dismantling, inspection, reindexing, and welding, which will put you close to $600