Potential Group Purchase - Wiseco GS650 741cc pistons

[Tooheys]

Count me in for a set if I'm not too late.

[tkent02]

You're not, we're still waiting.

[Ace07]

Ok sweet, with tookey and borish that puts us up to 24 now. One more and the price drops to $95 a piston

[Chuck78]

Give it 3-4 weeks and my buddy David (Drey6, new GS owner, not yet a regular poster) will have hopefully gotten his first few paychecks at his new job and is hoping to get a set as well for his 82 (?) GS 550. He got a kicker 78 bottom end with a rusty Yoshimura pipe (LUCKY GUY) & Dyna S, GS1100E swingarm, etc, & is hoping to be getting underway with getting the gs on the road & building up the spare with his 650 jugs and head and hopefully these slugs.

I got the wife's 550 on the road today & the gearbox has yet to act up on it on my test drives around the neighborhood (it was last year, I think some fresh oil helped a little bit with the gearbox/shift fork wear), so I think we are good to let her ride it across the country Ohio to Texas and back before I have to build the 550/650 spare engine up.

[frankenwabbit]

ok....count me in as well.

[Ace07]

Cool that makes us up to 28 now. My bike has been put on the back burner the last couple of weeks. I have had to do alot of traveling for work. The bike is completely apart right now, and I have started taking the engine apart. I am hoping in a week or two I will have the frame powder coated.

[Old Colt]

Count me in for a set.
I was active on this forum many years back but got dropped at some point. I expect my threads are gone as well.
I am revitalizing my 740 project and was just getting ready to rework the squish area in the chambers in the NOS 650 head I am using. These pistons should greatly reduce some of my time in this build. My only fear is my 650 cylinders have been bored already for the 750 pistons so I sure hope these new pistons are the same size or a touch larger than what I have.

[Ace07]

Hey colt, glad to hear you are in for a set. The piston we are planning on ordering should be the same size as the stock 750 Pistons. I already have bored my cylinder to fit the 750 piston as well. The previous owner was bored it out and was running some orignal 750 pistons. He finished the project, but was having trouble with the engine over heating, so he sold me the top end.

[Chuck78]

Does anyone have a GS650 head handy? Mine is packed away to move from my old place currently.
I was wondering if someone could measure the i.d. of the intake port where the carb boots bolt on.

I was looking at re-racking a set of 78 kz1000 vm28 carbs onto a VM 22 rack to run on the 650 head, but I'm afraid the port is probably closer to the 32mm of the BS32 carbs.
Don't want the air fuel muxture to expand much, as that, so I have heard, will cause more raw fuel to condensate on the port walls.

I'm trying to find an economical & solid solution for carbs. GS650 carbs + boots + rebuild parts will run me $300, and the Dynojet needle and hopefully smaller DJ jets than usual from them will put this at $400.

I still have a set of BST33 Slingshot carbs that seem like they'd be a real performer, but still not sure what intake boots Boonton Mike used to mix & match and get the rack spacing to work on his 650 head (I think he maybe had BST34's).
I also have some lunchbox filters for them as the late model katana crowd calls them, 2 pods to fit 4 carbs, but I'm not sure the gs550 frame has room to clear them as they stick out a good bit. The flatslide slingshots aren't as long front to rear though so maybe I have a chance in hell of them fitting. The wife is in Florida on her GS550 now though so I don't have one to measure!

[Old Colt]

I run a set of 29mm smoothbore pumpers on my '79 550. The ports on the early head are small and benefit greatly from porting. To mount the carbs I made adapters and cut & welded a KZ rack to suit the narrower spacing. My carbs are not spaced as the 550s were but rather spaced to equalize the port flow. This work was all done way back when the bike was new.

I do have a factory new 650 head on the bench and somewhere have an early 550 head on a shelf as well as 750 bits. Once I get my hands on the 550 head I can measure and get pics of it.

[Old Colt]

Here is what my 650 head has,
650port.jpg

[Chuck78]

Very clever there on your 29 S/B 550 rack!

From what I can tell, vm26's & vm28's (& 29 smoothbores maybe?) Will swap directly onto the 550 rack using the 550 fuel tees, throttle shaft, choke linkage/shaft, and throttle linkage, with the only hangup being the need to fabricate a lower throttle return spring bracket, as the 550 has holes for.

If I were doing a gs750 piston'd gs550 build with 650 cylinders, I think this is definitely what I would be doing.
Unfortunately I'm afraid the port is too big for direct linkage carbs' smaller venturi. We'll see shortly. Running VM33 smoothbores on a BS32 head for a 740cc bike would be absolutely ludicrous, and I don't know about the swapping of the 33's onto the vm22 rack. Although with big cams and these high compression pistons, they may just work. They are more finicky to tune for idle & low speeds on engines that aren't big cc monsters.

Keihin CR31 smoothbores may actually be a pretty good option for these bikes, and they have adjustable spacing racks, and a multitude of intake boot adapter spigot options available. They are the same price as a set of the rare & fully rebuilt set of vm29 smoothbores however, around $700-750 new from Z1 or Sudco. I almost was going to get a set or 2 eventually, but I'm trying to keep with all VM type tuning (jets/needles) & rebuild parts.
I am even looking to try and build a set of 2 cylinder VM29 smoothbores for the GS425-475cc so that I don't have to have a sole set of GS500 spec Keihin CR33 smoothbores.

[Chuck78]

Here is what my 650 head has,

(eBay photo, I was told vm28's actually measure 28.7mm or 28.8mm, but this guy has the calipers jammed all the way in and is not measuring on the fine point blade portion, but rather the flat stock at the base, so his numbers are slightly lower @ 28.54mm)

Bingo! Looks like I will be digging out my calipers to see if the mounting flange screw locations are the same on the 22's vs the 28's! I'll have to consult Rapid Ray and other race tuners, but I read one member talking about vm33 and other carb options saying that for road racing you want the carb venturi to be 1mm larger than the head port i.d., and for drag racing, you want the carb venturi to be 3mm larger than the head port. I think a lot of this is in relation to porting the heads as well, and should not be a rule of thumb used to concretely size carb swaps onto stock heads, but that is a great find there! THANKS COLT!

[Chuck78]

The only other assumption I am making here is that since the 550 and 650 bore spacings are the same, that the carb spacing is the same as well. This would likely require vm26 or vm29 GS intake boots, and assuming that they are the same offset etc as well. The 26 boots may need hogged out a bit on the head side, but that could give an opportunity to blend the contours offset more if there was any misalignment.

Fuel tee/couplers are the same diameter between the vm22/26/28, but on the VM29 smoothbores they are much larger (2mm-ish?), & custom machined tee/couplers would be required for that application to work on this.
The bolt flange spacing was about 50mm on the 28's but I left the house before I ran upstairs to check the vm22 but I think they are the same. All use the same fuel bowl gaskets, so the carbs are all the same basic width.

The only real kicker here would be the height from the mounting flange centers to the throttle shaft linkage... I did not consider that. Might make for a fun/challenging fab job here still...

Once I get my 750-920 smoothbores reassembled, I'll go move another load of shop stuff including the crate with the 650 head/cylinders etc so I can look I to this further

Best thing is, I already have a set of these kz1000 non-pumper vm28's! The center fuel tee was so worn out I can wobble it, needle and seats so shot that bike was flooding, previous owner caught the bike on fire! I was going to rebuild them but realized quickly they were not original to this gs750, & shelved them. Only have to clean melted uni pod foam off them. Was still running when the guy put out the fire... sucked melting foam into the #2 slide, slightly jammed now

[Old Colt]

As I am just getting my head back to this project I have been looking the the valve seats in the 650 head. The stock seats have allot of room to go bigger, like 4 or 5mm more intake which would help immensely with the major increase in displacement we will have. The 650 head has generous material to carry the larger throat size all the way up to the carb face. It would not surprise me if 33s might not be wrong. I am quite sure I will run the 29s I have but newer carbs are an option as well for me especially if I have larger valves made.