Mikuni RS36 carbs on a 502cc GS425 too big? WERA or AHRMA??? vs CR31/33

[pete]

I can't add anything of value here, but this discussion is giving me good thoughts for the future of my 450 once my Kat is done and dusted. I've been contemplating a do over of it including somewhat of a hot rod motor. I was initially thinking just the simple GS500 piston/barrel upgrade but now I see 530cc is possible with the stock barrels you're making me re-think this! Mind you I'd be aiming at keeping this as my daily rider, just a more fun version.

So just excuse me tagging along here for the ride

[Chuck78]

Well I measured the GS425's Mikuni BS34 intake boots, they will accept the 42mm o.d. RS36's perfectly so it seems, & guess what? The intake ports on the head measure 35.6mm! Not much difference there... I am thinking I will jump on those for now although I do recall a while back someone stating that for road race applications, it was best to have the sizes the other way around - port mouth being about 1mm larger than the carb spigot/venturi. I'm sure there is a lot of fuel atomization science to be discussed here.

Although Rich Graver the very dominating semi retired GS450 Formula 500 racer said he was running CR37's on his race bike. I love tight twisty roads and hope to find tight twisty tracks as well. The nearest cheap track day I'd a supermoto/go kart race course in Circleville Ohio. The other two are Nelson Ledges and Mid-Ohio Sports Car Course, which are a mix of long very high speed straights and twisty slower bits.
There's also ECTA's "The Ohio Mile," an old airplane runway where they do one mile land speed record type time trial races.
I much prefer the tighter stuff and I believe RS34's surely would be a better fit, but with the better response from a flatslide smooth bore and the accelerator pump, I think these would really be a fun carb to wind out a 502cc or 516cc built up GS twin with. Swap to CR roundslides if I ever do start AHRMA racing. It's a possibility as a friend wants to do AHRMA and would love a travel partner, but there are more WERA events closer to us.

[Chuck78]

this discussion is giving me good thoughts for the future of my 450 once my Kat is done and dusted. I've been contemplating a do over of it including somewhat of a hot rod motor. I was initially thinking just the simple GS500 piston/barrel upgrade but now I see 530cc is possible with the stock barrels you're making me re-think this! Mind you I'd be aiming at keeping this as my daily rider, just a more fun version.

no aftermarket pistons are available for the 450/500, but some guys on the GS twins forum (500's primarily) had been boring their 500's out as large as 570 or 572cc. One of the big bore options was using some Wiseco Kawasaki KZ big bore pistons, but Wiseco has long since altered the dome shape of these pistons and they no longer worked for the GS500.
It should be pointed out that the 450 can run 500 pistons but the valve pockets are not in the correct locations. Running bigger lift cam grinds will require machining of the piston domes to accommodate this. Therefore the KZ carbs in question may or may not have fit the 460 head before and/or after the redesign of the wiseco KZ piston dome.

custom pistons are in order at any rate, which has us what I am doing for mine. I'm sure if you wanted to put a good chunk of change into it, you could send your head and cylinder to RapidRay etc and have a Serdi valve job done, resurfaced, & have some custom big bore pistons made to your bore spec through RapidRay via Carillo Rods / CP or JE Pistons

Cams reground for a performance profile, performance 2 into 1 exhaust, and extensice carb rejetting or bolt on a set of very nice Keihin CR Special carbs (CR33 for GS500 apllication, $800-ish).
The pistons and carbs get very pricey!!!

On a budget, GS500 cylinders pistons carbs (the 03 or 04+ style?) with k&n filters and a dynojet kit and MAC 2 into 1 exhaust would be the best bang for your buck. And a fresh valve job from a recommended shop (or ship head to rapid ray).

[Chuck78]

I have a big port head with 36mm Mikuni RS flatslides. The power gain at the top end with these is amazing. The good news is that RS carbs are relatively efficient at small throttle openings, so you've almost got the best of both worlds. Because of their design, the flatslides have a fairly high depression in the venturi through most of their operating range, making them draw fuel well at low throttle settings. In the mid 80's when they came out, they were remarkable for this characteristic, at a time when equivalent roundslide smoothbores such as mik 33's, Keihins CR's and Amals were a bit flat unless WFO.

this is not the first time that I have read comments like that about the RS's
I am pretty sold on them although I wish they were RS34's...
May seal the deal tonight

[pete]

no aftermarket pistons are available for the 450/500, but some guys on the GS twins forum (500's primarily) had been boring their 500's out as large as 570 or 572cc. One of the big bore options was using some Wiseco Kawasaki KZ big bore pistons, but Wiseco has long since altered the dome shape of these pistons and they no longer worked for the GS500.
It should be pointed out that the 450 can run 500 pistons but the valve pockets are not in the correct locations. Running bigger lift cam grinds will require machining of the piston domes to accommodate this. Therefore the KZ carbs in question may or may not have fit the 460 head before and/or after the redesign of the wiseco KZ piston dome.

custom pistons are in order at any rate, which has us what I am doing for mine. I'm sure if you wanted to put a good chunk of change into it, you could send your head and cylinder to RapidRay etc and have a Serdi valve job done, resurfaced, & have some custom big bore pistons made to your bore spec through RapidRay via Carillo Rods / CP or JE Pistons

Cams reground for a performance profile, performance 2 into 1 exhaust, and extensice carb rejetting or bolt on a set of very nice Keihin CR Special carbs (CR33 for GS500 apllication, $800-ish).
The pistons and carbs get very pricey!!!

On a budget, GS500 cylinders pistons carbs (the 03 or 04+ style?) with k&n filters and a dynojet kit and MAC 2 into 1 exhaust would be the best bang for your buck. And a fresh valve job from a recommended shop (or ship head to rapid ray).

Good to know Chuck! My original thoughts were the GS500 pistons/cylinders... I already run '01 GS500 carbs and camshafts with velocity stacks and I have my locally made custom exhaust which flows pretty well (although there's a way to make it flow more and get louder if I want). I always thought there was the option of running aftermarket GS750/850/1000 pistons on the 450? (can't remember which model now)

[Chuck78]

750/850 2v pistons use a smaller wrist pin, later 750 use correct wrist pin but are 4v. 1000 2v uses correct wrist pin but I have no idea on dome shape and valve pockets are likely not quite correct. 1000 has a much longer stroke so 10.25:1 compression pistons might yield 9:1 compression if you're lucky. Could mill head and block for a slight bump.

I'm curious now if the 1000 pistons would work...

[TeamDar]

750/850 2v pistons use a smaller wrist pin, later 750 use correct wrist pin but are 4v. 1000 2v uses correct wrist pin but I have no idea on dome shape and valve pockets are likely not quite correct. 1000 has a much longer stroke so 10.25:1 compression pistons might yield 9:1 compression if you're lucky. Could mill head and block for a slight bump.

I'm curious now if the 1000 pistons would work...

Just an FYI. GS400/425 shares the components with the 77-79 GS750. 16mm wrist pins and valves are the same. GS400 and GS750 use the same pistons. GS1000 and GS450/500 use 18mm wrist pins. Dar

[Chuck78]

Yes. And Wiseco 844cc GS750 pistons make a 10.5:1 449cc GS400/425. A great budget build