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

[Chuck78]

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?

Thanks,

Chuck

[Chuck78]

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 with going larger than 72mm 489cc / 72.5mm 495cc / 73mm 502cc, JE Pistons 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(????)

[Chuck78]

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.

[John Park]

It's not clear here exactly what you are trying to do. It seems like you are trying to put 450/500 cylinders on a 400/425 bottom end, which has different stud spacing as I recall. Much as I can sympathize with running the roller crank, and the stroke is a bit longer than the plain bearing one, the only plan as I remember was to bore the original jugs and install larger sleeves, with all the dangers therein.

As to the counterbalance shaft, it's there to quell the rocking couple which is a combination of all the unbalanced forces. While the plot, as i see it, is to balance each single to give an even horizontal/vertical imbalance and then counter it with a counter rotating imbalance which will also be inherently balanced in both planes. The motion won't be perfect as no crank/piston motion is ever a perfect sine curve, but you can get close. I don't think that a heavier or lighter piston would alter the equality of the planes of vibration enough to be worth considering as long as you stayed within 10% of the original. Greater than that and you may end up with a motion thats too elliptical for a counterweight to work as well with, but nothing near an uncounterbalanced engine. Each piston is balanced by its counterpart going the opposite way anyway, so it's just a matter of how well the counterweighting is done to be similar to the balance shaft motion.

I've put 450 pistons on a 400 block [67 to 71mm as I recall] and the weight went up by about 20+ grams [8%?] and the resulting motor is a tiny bit smoother than an original 450, but that could be sample variation and tuning. I didn't notice any increase in vibration after the mod other than that caused by more power. I can weigh the pistons again for an exact figure if you are interested.

While I'm here, if you are planning on using the original frame, be aware that the top tube to the head tube is lacking the gussets that you see on the later versions. Of course the coils are right where you would put the bracing, but that's easy enough to fix. There's a reason all the other frames had braces there.

[Chuck78]

It's not clear here exactly what you are trying to do. It seems like you are trying to put 450/500 cylinders on a 400/425 bottom end, which has different stud spacing as I recall. Much as I can sympathize with running the roller crank, and the stroke is a bit longer than the plain bearing one, the only plan as I remember was to bore the original jugs and install larger sleeves, with all the dangers therein.

Yes, I will be re-sleeving the GS425 block. I am aware of the different stud spacing.

[Chuck78]

While I'm here, if you are planning on using the original frame, be aware that the top tube to the head tube is lacking the gussets that you see on the later versions. Of course the coils are right where you would put the bracing, but that's easy enough to fix. There's a reason all the other frames had braces there.

Yup I have taken pictures of junkyard 450's & 650's frames to know exactly how the factory did the bracing that I need to replicate and improve upon on 400/425 & 550 frames. Plus some of the OSS bracing that the bigger bikes get will be going in at my discretion

[Chuck78]

I talked to Rich Graver a while back, a semi-retired prominent WERA Formula 500 racer of GS450's, & he said he was running CR37 smoothbores. I suspect he is right at 550cc on his however.

Thank you for the 10% figure and comments on piston weight and balance shaft operation/fundamental theories.

[GregT]

While I disagree with your fundamental aims - as the plain bearing bottom end is a much better starting place - on my 450 I ran Dellortos bored to 34.5mm.
In conjunction with .418in lift cams this gave a wide power band. Suitable for our tight tracks and street circuits here in NZ.

[Chuck78]

I was thinking .395" intake and .365" or .380" lift exhaust cams.
I'm pretty dead set on the (early) roller crank engines vs a plain bearing 450 engine, although a friend has a plain bearing (450) model that I may have a hand in.

[rapidray]

Chuck, there's no replacement for displacement! Go as big as the rules allow. I would build it 12 to 1 compression, .420/.400 lift on the intake & exhaust, bigger valves, porting and the 36 RSs. You will be amazed at how the thing pulls & how mellow it will seem! We raced a 93 GS 500 out of my shop in Hawaii with a 565 kit, 37 FCRs, 2 over titanium intake valves & a head I built. It would pull 600s on the front straight!!
Ray.

[Chuck78]

Here are my sleeve / bore limitations on the GS400/425 stud spacing. The oil passage outer studs have tons of clearance, & the closer inner studs and cam chain tunnel areas are less critical and larger than they need to be, therefore the bigger sleeves could encroach upon them slightly. On the GS500 head gaskets for 74mm stock pistons (same spacing as the 450 but 74 mm vs 71 mm pistons) the oil pressure outer stud passages are the ones that are closer as the bores are shifted outward slightly.



The 425 head gasket pictured here is for a 67 mm piston, therefore it was designed to take a factory over bore of up to 68, so the bore diameter is probably 68.5 or so. it should be no problem to fit stock bore 71 mm gs450 sleeves into here, which I have a set of already. it may be possible to fit stock 74 mm bore GS500 sleeves as well if I really wanted to push it. in my amateur seat of the pants opinion just from this visual comparison, I would think that 73mm up to 75 mm would be about the limit that I would want to push. As I said earlier, that would be 502cc or 530cc


I'm really pulling for the 73mm 502cc due to the smaller 16mm wrist pins on the stock rods (GS450 @ 69mm was largest OEM bore on 16mm wrist pins, 70mm GS1000 went to 18 as well as 71mm GS450), as well as an 11:1 73mm piston would also double as a 10.7:1 944cc GS750 piston...
and a compression ratio that could run 93 octane (10.75:1 roughly however perhaps bigger cams would allow me to run 11:1?). I'm certain that I'll want to take this on the back roads as well, & I have already made provisions to have a street wheelset (3.50 / 2.50 rims) & a road race / race compound rubber wheel set (3.00 rule limit of Formula 500 rim width - can run Avon 130VB65-18 cantilever bead race tire).

Ray - I'll leave it to you and CP/Carrilo to tell me how big I can reasonably/reliably resleeve, & how big of piston I can run on 16mm wrist pins and stock rods, as well as the budget damage on Carillo rods... Thanks! hopefully that will be happening over the winter.
FYI Ray I decided against the ported gs750 head for now because I may as well do 944cc and welded crank at the same time, and figured I should keep that as a reliable street bike and just go all out on the 1105cc GS1000 and this gs425 track bike and keep the GS750/920cc as a good street bike for now.

[Chuck78]

I will go ahead and pick up the RS36's I suppose, although I am pretty certain they will not fly in AHRMA and I would need to get some CR 35 or CR 37 carbs if AHRMA racing

[rapidray]

Chuck, just use round slide dirt bike carbs. Cheap to buy & have all the circuits for nice idle & tunability.

[Chuck78]

I'm working a package deal on a GS550 & GS850G parts bikes, 3 tray bike hauler trailer, & RS36 twin rack carbs for not a ton of cash and a set of KZ1000 carbs. I think I can get the RS36 2cyl rack for maybe the equivalent of KZ1000B4 VM26's + $75-ish... pretty appealing.
Dirt bike carbs will require a split Y-cable setup. Any idea how to make that easily work / source from?

[GregT]

Chuck, just use round slide dirt bike carbs. Cheap to buy & have all the circuits for nice idle & tunability.

Agree. And twin cable twistgrips are available too.