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

2601 Views 10 Replies 5 Participants Last post by  Brucekirk89
I've been compiling parts to build a WERA Formula 500 / Vintage 3 etc 489cc or 502cc 79 GS425 track day bike, & potentially (hopefully) race some WERA F500 races on it, or possibly AHRMA. AHRMA rules seem a lot more strict about parts used, & despite similar flatslide Lectron carbs being available in the early 1970's, the RS carbs are considered a more modern version of those and more likely to get turned down if racing AHRMA.

I have a chance to buy a set of 4 cylinder Mikuni RS36 accelerator pump flatslide carbs that were chopped down to fit an EX500 (we assume). Not the best job converting a 4 cyl rack to a 2cyl version, but I can get them for a reasonable price.

Will these be too much for a 502cc GS425 with custom big bore pistons, Megacycle or Web custom camshafts in the .380" to .395" lift range? I'll probably be doing a street / tight course road race ported head and +1mm oversized valves through Rapid Ray whenever I can stop putzing around on 8 bikes and work on just 1 at a time...

I have 2 very nice racks of Keihin CR31 carbs which would be ideal for my 1105cc GS1000 engine for the Rickman CR road racer as well as a conversion to a 2cyl rack to fit the GS425 and still be without a doubt AHRMA & WERA compliant. Even in WERA, if I were to run the RS36's, it is possible that if I started beating people, they could protest the race results due to my perceived parts advantage with parts from after the correct period.

I can buy GS500 specific CR33 carbs in a 2 cyl rack which would be a much bettter choice, but for $749.99!!!
I also could trade 1 set of these CR31's for a set of VM29 smoothies which would go very well on my GS550-740cc Wiseco project.

Opinions on RS36 being too much for a 502cc? It'd be the same as fitting them to a GS1000 with high compression stock bore pistons and moderate performance cams basically. My intuition says 36 is too big, but they do have an accelerator pump and my buddy ran RS34 pumper on an 810cc GPZ and said it never gave him symptoms of being too large, although perhaps if he ran cr29 carbs, he would notice a big Improvement?


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WERA rules state that a GS450 up to 550cc can run in Formula 500... should I try to punch this thing out THAT huge? Anything over 475cc requires resleeving, and I already planned on 71mm stock bore GS450 sleeves. I think over 72mm 489cc / 72.5mm 495cc / 73mm 502cc, JE or Carillo pistons will tell me I need larger wrist pins, therefore I will need custom Carillo rods. Getting the crank welded was already on my mind, although not sure a 320lb 2cyl bike would need a welded crank even if being raced. I'd also like opinions on that. It WILL BE one heck of a powerful GS400/425/450... BUT still only guessing mid 50's or 60-ish horsepower(????)

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75mm pistons would equate to 530cc, & that would be the absolute limit of GS450 sleeves at +4mm overbore.
my bigger concerns lie with the piston weight vs the GS twins' counterbalance shaft. I believe they are 180 degree twins, so would the engine vibrations increase more with bigger pistons? I'm guessing that an aftermarket JE Piston @ 71mm + lightened up wouldn't weigh much more than a cast OEM/ART 67mm piston. So 73mm doesn't seem too far of a stretch. BUT going from 502cc/73mm up to 516cc/74mm up to 530cc/75mm does seem a stretch. Perhaps Carillo could build me a really strong lightweight piston and rod set that wouldn't be too far off in weight from OEM? 8mm additional forged aluminum piston diameter would be pushing the weight limits vs OEM cast, not to mention the larger pin diameter that may be required.

GS500 sleeves could also be used to give more wall thickness at 75mm/530cc, as they are 74mm stock bore.

Thoughts on bore/displacement, or balancer shaft issues?
Reliability is a concern so I don't want to push the weight limits of the pistons vs balance shaft, & the thing really needs to be able to rev to the moon and do it rapidly, so lighter pistons would be best. 72mm 489cc with lightened JE Pistons or custom CP (Carillo Pistons) may be the ticket, but breaking the 500cc mark at 73mm/502cc seems like a good target goal if I don't need to bump up from 16mm wrist pins to 18mm wrist pins.
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In my opinion :| and it is only an opinion; counterbalance devices on a motor kill its performance and contribute to additional stresses on an engine, they don't negate stress and forces that exist as the result of other things like pistons and chains flying about inside your engine, the best they can ever hope to do is shake things in the opposite direction with an opposing shake and once they get slightly out of sync as the result of a stretched chain they simply destroy everything. It's the shape of the crank that counterbalances the weight of the piston and connecting rod and you need only look at the difference between a modern piston and an obsolete piston design to see where they are finding ways to make pistons lighter, modern pistons look more like ring holders:

Is like the difference between yer grandma in a long dress and yer daughter in a mini skirt and I'm guessing the pistons you are thinking of fitting in there for high performance are not wearing mini skirts.
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well its is a 180 crank i asked the fellas at dime city for ya chuck ,i know you cant be botherd with knowing the shit you cant change ,well not easily anyway

hah !you know i am just takin the piss out of you chuck !
personally i think you will have plenty to gain and tune on without going maxi cc's you will be probably have a completely different squish to tend to and adjusting comb chamb volume fak it if it was me i would stick with the mild cc increase what 475 ? and tune the shit out of it learn to make it run purdy before going too beserk
i dont know why you cant disconnect the balancer i think it is probably to tune out the tinglys
try it road test it it wont gernade for that
you want quick revs ? take the maggot flywheel off that turd and go bat ignition
that is proly 6 lbs of more revability
carbs i dunno its just me thinking but a pair off a 250f mxer would fit the bill
I can't speak to the balancer issue but the carbs sound too big to me. I raced a Norton 850 in WERA V4 and V5 and it had 34mm Mikunis off an old dirt bike. I ran 1 AHRMA race years ago and they bitched about the carbs even though they were off a '70s dirt bike. The Norton never starved for fuel with 34s so I can't imagine a GS 450 needing 36s. WERA has a 'like design' stipulation but I don't know if pumper Mikunis were available in that time period.

I switched from a big bore, high comp piston setup when I was racing CCS Twins classes to a stock piston and cylinder for vintage racing. The high comp engine needed a top end overhaul after every couple of races where the stock top end would run all season. For Vintage I would go closer to stock and see where you run. There aren't that many races in a season so a more stock type motor is more likely to get you to the finish and through the season.

In V3 and F500 you will be up against some strong RD Yamahas at times and I don't know if there is any way to compete with those guys on a GS 450. But a good rider can compete and have a lot of fun in these classes.

If you are planning on running AHRMA, I would build the bike to adhere to their rules as they are much more likely to boot you over some small item that isn't correct. The WERA racers are not going to protest you unless you are winning every race.

Good luck with the bikes.
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I might be able to get the rack of RS36's for $50-70 plus a rack of KZ1000B4 VM26 carbs in trade (in a package deal for 2 GS550/850G parts bikes and a 3 tray bike hauler trailer). Running 2 dirt bike carbs would require a special Y-cable as well, & I'd prefer a fixed linkage to keep them in perfect synch and not complicate things up top.

As for the counterbalance shaft, a 30yr veteran WERA F500 GS450/GS500 racer warned me not to remove it, as he once tried that and it ended up cracking his frame at the engine mounts!!! Another account I read they guy said he removed his and the engine blew up shortly thereafter from vibrations.
This bike should compete fine against GS450's as most of them aren't running anything over 487cc as that is GS500 displacement and the valve pockets aren't quite right for the 450 head so you can't run big cams without machining the valve pockets offset a bit.
A GS400/425 can run Wiseco 10.25:1 GS750-844cc pistons for 449cc and 10.55:1, this bores straight in with no other mods. Shave the head and block to increase CR more and optimize quench band height, run bigger cams...great budget screamer. I was tempted to do that although with the 425 cylinders I could re-order a friend downunder's 894cc GS850 custom pistons from JE for 475cc with no custom calculations at all and just bore them in. However having a GS550-740cc project, 920cc GS750 daily rider / sport tourer / street racer, & an 1105cc GS1000 project engine for a Rickman CR road race chassis, I had better step up my game on the featherweight GS twin so that it is still very fast and an absolute blast to ride at 320lbs fueled...
I think 502cc 11:1 will be the ticket, or if Carillo okays 74mm on the 16mm wrist pins, then I may have to go 74mm 516cc 11:1 and .395" lift cams.
I'll probably get the RS36 carbs but may have to swap them for CR33's if I do ever race AHRMA. AHRMA would mean bigger vintage racing contingency - bigger grids, but way more travelling and time off work to get to races as very few are an evening drive from Ohio. WERA has a couple of races that I could get to on a Thursday night after work.
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Not sure what size RD's I'd be running against but if RD400, I'd have to really be stepping up my game!

This bike is mostly just going to be a fun canyon carver / Appalachian hills twisties / track day bike, but racing is definitely part of the dream. Better do it before I get much over 40 yrs old!!! That's next May... time flies, I still "think" that I'm not a day over 30... and very free and unsettled for that even!
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.
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old post from GS forum said:
I have a big port GS1000 CV carb 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...
I may seal the deal tonight
Clean them up inside out clean jet ports turn jet to 2 turns out get bike fuiling and idling then take it out on the hyway and trying to give it hard throttle threw every gear and listen for misses flat spots or extra air being sucked in,then go on or Honda jet kits online and use thier jet chart and buy that and one size bigger jet kit and try them out.
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