Potential Group Purchase - Wiseco GS650 741cc pistons

[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,

 

[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.

 

[Ace07]

Hey guys,

Glad you are looking back into this project. I have been iching to get back after it as well, but I have been traveling for work alot, so the only thing that I have done so far is taking the bike apart completely and organizing the parts I have. Hopefully in the next couple days I will order some parts from bike bandit, and find a good place that does powder coating.
On the carb side of things, I am very new to tuning any carbs. I currently have my stock CV carbs. I was hoping that I could you those with different jetting. Is that an option? Is it a good option?

 

[boris.h]

Hey!
Unfortunately, you can not count me as interested anymore, I finished assembling the engine with standard gs 650 pistons and rings... It was too long for me. Maybe later, I would be interested. We are now buying a house with my wife so that's not a priority anymore for the instant

With my configuration:
full ported head
62.00mm gs 650 pistons
gs 550 camshafts
"big" exhaust (ported the head to be able to fit a bandit one or similar as kawasaki Z900 / 1000....
I will start with gs 850 carbs, BS32ss, which have the good venturi diameter for the engine... I put mikuni 112.5 main jets, which correspond to 1.25mm diameter. That was the diameter I calculated, so I think it would be ok for me, to start

 

[tkent02]

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.

550 and 650 valves are the same, I was thinking 750/850 valves or maybe 1000 valves might be a bit bigger diameter, I think the valve stems are the same diameter but they might be longer. No need to have expensive valves made if some off the shelf stuff will work just as well. This is a Suzuki after all. I'm not able to measure for a while, anyone know the diameter of the 750 or 1000 valves? How about the length? Still need to find a 650 head to do another engine.

Carburetion, I want to stay with some form of CV due to the large range in elevation I need it to work well in, using GS 650 BS32s now. I think some of the ones from 90s and 2000s year model bikes probably work a lot better. Haven't even started to figure out which ones yet. Can figure that all out later.

Anyway I'm still in, I can slap these big pistons and a freshly bored cylinder on my existing 550/675, don't need to wait until I build a whole new engine. Can do that later and switch the cylinders around again. Whenever the deal goes I'm in.

 

[boris.h]

Not need to go with bigger valves. Valves diameter is ok until 10500 / 11000 rpm... do you plan to go up than this limit? You will also find other issues above 11000... I kept the oem valves. I found aftermarket valves that are thinner than suzuki valves, above the angles where they seat.
Moreover, even if there is a lot of room on the seats, you have to know that the temperature is well driven by the seat if this one keeps a lot of volume (if you go to bigger valves, the seat will be thinner and won't keep the same physical caracteristics).

-> if you reduce the volume of the seat more that 20% (this is not so big), the seat will loose its properties (regarding the temperature of the valve)

so, for me this is a waste of money, time regarding the potential gain...

What I did (and this is free of money), I have 2 complete sets of valves, springs, everything. I measured each spring length and I measured the weight of every part moving then the engine is running. I found a lot of differences, as the spring length, and the weight of the parts (you'll need a precision instrument). I choosed the parts I found similar to make one full balanced set....

 

[Old Colt]

The 750 valves are a few mm larger than will fit on the 650 seats, they are also around 10mm longer than than our engines valves. I have not pulled the 750 valves from the head to get accurate numbers. I have not looked at other bikes to see what might be simple to fit.
For me, I will be installing larger valves. I have no issues with cutting the seats as needed. The 650 valves are undersized for me.
My 550 has always been run with a 12,500 rev limit and on some tracks in the past such as riding Daytona, the engine was at 12,500 all the way around the banking every lap. That was 9 sessions a day, 3 race groups. Always nervous of when it would fail but it never did. Other clubs I could run 4 race groups a day plus we rode an endurance race which my bike shined in.
For me now, this will be a trackday bike and not strung out to the limits as when the bike and I were young.

 

[tkent02]

Never rode it at 12,500 all day long, but certainly above 11,000 up long grades and such, ran the **** out of it all the time, very high RPM occasionally, the 550 just keeps making power as the RPM climbs. Never had a problem in many many thousands of miles, took my first 550 apart after 120,000 miles of this to fix a myriad of oil leaks, all of the metal parts looked fine and went back in. 550s are hella stout overbuilt engines.
Trying to make the bike easier to ride here at high elevations, using 8,000+ RPM just to start out from a stop gets old. The 650's top end and cam gives it some decent mid range torque, bigger valves probably won't help that but may help those looking for more top end power.

I don't really know what size valves would be the best, but if it makes a better engine it's worth trying. Couldn't a good machinist shorten a set of 750 valves and cut the slot for the keepers farther down??

 

[Old Colt]

I do not think the big valves would be right a high altitude. You need higher port velocitys for that thin air.

Yes the 750 valves can be shortened. But I am not a believer of reducing diameter and shortening. Manley sells valve blanks for builders. These would give the valve head for the desired use and then the stem is cut and grooved as needed.
But there might be a Honda or Kawi or some other valve that is right to work with.

 

[boris.h]

I calculated the valves diameter, and the results were, for max rpm 11500, were the OEM diameters. But I could understand that you want to put bigger valves... I just said that regarding what you are going to get (good power between 11000rpm and 12500rpm), it's for me a waste of money. Buying bigger valves, machining the seats, maybe cut the stem and reproducing the end of the stem....even if i'm a believer in exotic modifications, I won't use my bike everyday between 11000 and 12500 rpm, I prefer working on increasing the air flow on the carbs, and in the head... So you agree that, about increasing the valves diameter, it depends of the final use of the bike... if the bike is destinated to make races, where everything "more" counts, ok... but for a "daily / Weekend rides" bike.... that's, for me a waste of money. There are other places on the bike where to spend money better

 

[tkent02]

I do not think the big valves would be right a high altitude. You need higher port velocitys for that thin air.

Is this correct?

 

[Old Colt]

For midrange power I think so. You do need a good amount of compression, that will bring your power up.
Larger ports will make the engine lazy at low revs since the air column is lighter due to your altitude. A higher velocity port flow will pack more air in the chamber than a slow moving light load of air.