GS550 to 650 cylinders questions, compression, heads, vm vs bs

[cowboyup3371]

I've been reading all I can find on the swap lately. Can you use the coils and ignitor from the 550 after converting to 650?

Is it possible to save the original 650 head gasket? I know the top end slides down on the studs from the block and I'm hoping it's not necessary to disassemble it too much. The head gasket is over a $100!

edit: I dug through the parts fiche, the 550's and 650's use the same ignition stuff except for the 650G. The coils are different on the G model, maybe for more tank clearance.

Once a gasket is compressed, it's done. Just fork out the money for a good new one or you'll be cursing the leaks you'll face in the future.

Remember too that if you remove the head gasket you might as well take care of the base since that will get disturbed.

[Suzuki_Don]

You also need to keep the GS550 cams. The 650 cams aren't clocked the same, and the crank sprocket is larger on the 650.

You can fit the 650 cams and it is best to do so. They give the motor much more torque low down and when you get to 5K you'd better hang on. You must use the 30 tooth cam sprockets from the GS650E motor - not the sprockets from the 650G motor which are 34 teeth. If you are going to dial the camshaft in (degree them) you will need to lop the end of the camshafts off so the dial gauge pointer can sit on the edge of the bucket to gain the correct opening for the degree wheel readings.

On my motor I used a 0.8mm home made base gasket to obtain the correct squish band clearance. Someone quoted .125", don't know where that figure came from but each motor has to have the head and cylinders removed a few times to find the correct size of the base gasket needed. If you don't know how to measure the squish band clearance get back to me and I will let you know. If the squish band is more than 0.060" it's effect will be lost, it's best to have the clearance at about .020" to .040".

It all depends how much trouble you want to go to and how many horses you want to produce. Mine is putting out around 80hp at present and when you consider a Honda 750/4 only put out 67hp in 1969 it means these 550/673 motors go quite well and they handle.

The ign systems between the 550 and 650 are totally different, so I must disagree with who ever stated they are the same. Just check the ign advance figures and again if you want to know how to modify the advance unit get in touch with me. If you don't do this mod the motor will ping continually.

That's all I can think of now. Oh, forget about using VM carbs, and 550 head on 650 cylinders. These motors get their power from the design of the combustion chamber.

[Toasty]

What parts from the 650 ignition system do I need? Will the ignition advance from a 650G work (I have the complete motor)? For the cams, what are the benefits of using the 650 cams over the 550 ones? From what I've read already the 550's are more agressive on top. Are the 650E and G cams the same?

[Suzuki_Don]

What parts from the 650 ignition system do I need? Will the ignition advance from a 650G work (I have the complete motor)? For the cams, what are the benefits of using the 650 cams over the 550 ones? From what I've read already the 550's are more agressive on top. Are the 650E and G cams the same?

You need the advance/retard unit from either model GS650 (they are the same). But the unit has to be modified to fit on the end of the crank (at least it did on mine which is a '77 model 550) I assume other 550 crankcases are the same. The 650 A/R unit is longer and when the bolt is fitted to hold it in place the head contacts the points cover. I have used a Dyna-S on mine with the advance/retard unit suitably modified.

If you use the 550 cams you will retain the same engine characteristics as the 550, not as much low down power and definite power bands as you hit 5K, 7k, etc. I've found the 650 cams to work best with plenty of power everywhere.

And on another point I've found the 6th gear to be superfluous with the extra 30hp. My bike will pull 120mph in 5th and 6th is like an overdrive. If you look at the teeth numbers on 5th and 6th gears there is only one tooth difference. The drive and driven gears on 5th are 22 and 23 respectively. On 6th the gears are the other way around 23 and 22 respectively.

EDIT: Sorry, forgot to say both the 650G and 650E are the same camshafts, it's only the sprockets that are different. GS650E is 30 tooth and GS650G is 34 tooth. You need the 30 tooth cam sprockets as the 550 crankshaft sprocket is 15 tooth.

[Toasty]

Hmm well it's something to consider, torque is always good but 650E stuff is kind of hard to come by. If I can use the G cams with E sprockets I may go with that. What other mods have you done to your engine?

Btw, my 550 has CV carbs (same body as the 650) do you think it's worth getting the dynojet kit (plus whatever it doesn't come with) or just get the 650 spec jetting individually? I appreciate the detailed responses btw, this is gonna be my first big engine build

[Suzuki_Don]

Hmm well it's something to consider, torque is always good but 650E stuff is kind of hard to come by. If I can use the G cams with E sprockets I may go with that. What other mods have you done to your engine?

Btw, my 550 has CV carbs (same body as the 650) do you think it's worth getting the dynojet kit (plus whatever it doesn't come with) or just get the 650 spec jetting individually? I appreciate the detailed responses btw, this is gonna be my first big engine build

Glad to help, if you do a search on my user name there are many threads on my engine build: degreeing cams, adapting 550 4/1 exhaust to 650 head, fitting new fuel tap as old fuel tap fouled the new CV carbs (you will not have that issue), clutch plate issues, etc. A lot of stuff. I have fitted the 32mm BS carbs from a GS650G and rejetted to 650E specs, pushed mains up from 97.5 to 100 is the only difference. It's only if you use pods that you will need a kit from dynojet. I have a 4/1 exhaust and no jetting changes required. My bike looks standard with original air filter, etc, but goes like a bat out of hell.

Use the 650G cams and look out for a set of GS650E cam sprockets on Ebay.

There are a number of threads on my engine build, I split the crankcases and did everything from the bottom up.

Here is one thread worth reading - a lot of camshaft info, mainly installation:

http://www.thegsresources.com/_forum/showthread.php?t=155072&page=7

.

[Chuck78]

Great looking bike, Don, & I really appreciate all your advice & experience! Found a wrecked gs650, hope to swap dual discs to my 77 gs750 & engine to my wife's 77 gs550 if the cylinders check out okay or if I can track down some factory oversized pistons (may be real tough to find, discontinued I suspect).

Thanks!

[Suzuki_Don]

Great looking bike, Don, & I really appreciate all your advice & experience! Found a wrecked gs650, hope to swap dual discs to my 77 gs750 & engine to my wife's 77 gs550 if the cylinders check out okay or if I can track down some factory oversized pistons (may be real tough to find, discontinued I suspect).

Thanks!

No problem, glad to help. A lot of people helped me with advice when I was doing my rebuild. If I don't chime in as this thread progresses then sent me a PM as I am involved in a Guzzi project at the moment and don't look at the GS site every day.

BTW Don't worry too much about O/S pistons, the extra capacity they provide doesn't affect performance much. You can hardly feel the difference.

[Chuck78]

I was referring to the odds that the cylinder sleeve walls will be intact and useable. My buddy at the junkyard said if we can tear the engine apart and see if the cylinders and pistons are in good useable condition with rings and a hone before I commit to buy. Score.

I am really excited at the thought of building this spare GS550 engine up this winter as a 674cc. Seems that the piston/comb.chamber design is definitely the best of any of the old GS's, and that the compression ratio and extra cc's with K&N filters, glasspacks, and rebuilt/jetted carbs will surely scream & give a mostly stock GS750 a serious run for it's money! So I HAVE TO up my 750 to a 920cc now! (550 is my wife's)

[Suzuki_Don]

I was referring to the odds that the cylinder sleeve walls will be intact and useable. My buddy at the junkyard said if we can tear the engine apart and see if the cylinders and pistons are in good useable condition with rings and a hone before I commit to buy. Score.

I am really excited at the thought of building this spare GS550 engine up this winter as a 674cc. Seems that the piston/comb.chamber design is definitely the best of any of the old GS's, and that the compression ratio and extra cc's with K&N filters, glasspacks, and rebuilt/jetted carbs will surely scream & give a mostly stock GS750 a serious run for it's money! So I HAVE TO up my 750 to a 920cc now! (550 is my wife's)

Chuck the 650 puts out 1hp more than the 750 and weighs 10kg less. So it is comparable, but certainly quicker than the Honda CB750/4 with only 67hp as against 73hp of the GS650 and is a heavier bike anyway.

So you will have to watch out your wife doesn't flash by you while you're not paying attention.

[tkent02]

Chuck the 650 puts out 1hp more than the 750 and weighs 10kg less. So it is comparable, but certainly quicker than the Honda CB750/4 with only 67hp as against 73hp of the GS650 and is a heavier bike anyway.

So you will have to watch out your wife doesn't flash by you while you're not paying attention.

Especially with that close ratio six speed and her lighter weight. You may not be able to hang with her anymore..... Do you really want that?

[Chuck78]

I can barely hang with her as it is even though my bike is faster! Her leading a ride means 80-100mph & insane speed through turns, & she has a frowned-upon 130 rear tire even! Crap with some avon roadriders in the proper size, lookout!
... and to think she worries about me when I go out "riding with the boys" in Hocking Hills State Forest... sheesh!

[Chuck78]

She complained after I got my gs750 that her bike just wasnt running strong enough, "fix it" she sez... so I had my eyes peeled for a 650. I test drove it for her, thinking somethong was wrong, & it definitely ran better before I raised the jet needle one more notch, but holy smokes I practically melted the back tire off of it launching several times real hard on my "test ride!" The 550's are GRRREEEAT bikes for sure, especially with better air filters, exhaust, & rejetting!

[Toasty]

I've been looking around for 650E cam sprockets but so far it's all G/GL/GX ones. I know it needs a 30 tooth sprocket, so is there a reason why the stock 550 (30 tooth) ones won't work? I'm undecided on which cam I should go with, but if I were to use the 550 cams why wouldn't the stock sprockets work with them?

[tkent02]

I think the 550 sprockets should work on either cam, shouldn't they?