Your opinion -VM 36's or CR 31's?? or CR29's

[mrbridgeport]

Hi Guys,

Just about done building this AHRMA race bike and I am down to selecting carbs.

The engine is a 77 GS750. Punched out to 851 with a henry abe big bore kit (picked up from one of the guys on the list). I had the head ported and sent the cams out to mega cycle and chose the 381-70 option (.380 lift). Did the titanium bits and racing valve springs with shim under buckets.

So this thing should have some great flow. My fear is that CR 29's will be fine on this engine but if I uend up moving up to a 1000cc engine I'd be stuck w/ the CR29's

Will CR 31's be too much for this build?

What about running Mikuni VMs? and what size would you choos? HELP!

 

[49er]

Pops Yoshimura ran CR 31's on his GS 1000 engines. I think they will be too big on your 851 cc engine. CR 29's or VM 29's should both work well.

Don't even consider the VM 36's on your present spec engine.

 

[GregT]

Depends really on the circuits you run on....

I've got a customer 1075cc Z1 running 29CR's which has brilliant midrange and response so 29's wouldn't be too small if you do go out to 1000cc.

Our current roadrace GS1000 is running 33's and i have to say they only show a significant advantage over our previous 31's right up top.

The 29's came off ebay some time back from a guy who had abandoned a CR750 Honda project - so they are about.

 

[mrbridgeport]

Thanks for the insight guys. Looks like either 29's or 31's should do it. Let's see if the Ebay goods can come through for me on a set.

 

[trippivot]

My opinion is a set of vm 28's with the acclelerator pump will have your bike ripping!!
the popular carbs mentioned will make the bike go but are too big.they'll cough and stumble unless you can restrict WOT to 70 or 80% fully opening

The 31's would be best for a 1200cc engine that had a head flow of 100cfm. -- 29's are still too big for a bunch of tight corners and only 800 cc.-- What does the head flow ? unless you got 80 or more 28 mm venturis will fill the cylinders in ALL conditions at any lobe centers better.

just for reference :
vm 33mm smoothbores Wide Open Throttle flow 97CFM @10" test. and if you don't have a 40mm intake valve to let that air thru. -

 

[pontiacstogo]

CR's don't have an accelerator pump, so some 'rolling on' of the throttle is required. No traditional choke either, but they have a very effective enrichment circuit for cold starts. I wouldn't be scared to go bigger than some of the CC's being recommended here.

By way of comparison I have an 81 Honda CB900F (stock engine) running CR31's and two 83 Honda CB1100F's (stock engines) running CR33's.

My 82 GS1000SZ (stock engine) runs CR33's and it runs perfectly;

 

[mrbridgeport]

I have not checked the head flow. I am tearing down the top end again and I doo have an extra stock head around as well. Is there an easy setup you've used to check the airflow?

 

[mrbridgeport]

Thanks for the insite. Guys on my end are saying to get a set of the 31's so when I switch to a 1000 2 valve engine next year I won't be stuck w/ 29's.

 

[gs11ezrydr]

Yep 31's seem the way to go from what I have gathered here.So by the way I was noticing the heat shield between the carbs and the cylinders that you have on the pic of the Katana.Was that something you made or was it purchaced some where? Either way how do I get one or could you poss. supply me with a parts list or maybe some suppliers for the materials to make one..Looks neat and totally functional.Thanks.

 

[mrbridgeport]

That pic was posted by pontiacstogo. I don't know much about it.

 

[slayer61]

So by the way I was noticing the heat shield between the carbs and the cylinders that you have on the pic of the Katana.Was that something you made or was it purchaced some where? Either way how do I get one or could you poss. supply me with a parts list or maybe some suppliers for the materials to make one..Looks neat and totally functional.Thanks.

I saw one of the heat shields in question offered by motogpwerks on ebay a while ago. you might check with the seller.

 

[pontiacstogo]

Yep 31's seem the way to go from what I have gathered here.So by the way I was noticing the heat shield between the carbs and the cylinders that you have on the pic of the Katana.Was that something you made or was it purchaced some where? Either way how do I get one or could you poss. supply me with a parts list or maybe some suppliers for the materials to make one..Looks neat and totally functional.Thanks.

Sorry - missed that question altogether .

I purchased the shield from Japan - it was actually included with the carbs when I bought them. Not sure who (if anyone) supplies them here, but they'd be pretty easy to fabricate. The one I have is a flexible plastic material with a rubber bead glue around the outside edge. I've seen them done in aluminum too.

The idea obviously is to keep the fuel in the bowls as well as the intake charge cool, but I doubt this has any measurable effect on the street, plus there my even be a reduction in overall cooling efficiency (though this would be negated on my kat by the aftermarket oil cooler).

 

[motogpwerks]

29-33mm Really depends on what track your running and engine spec also if your riding skill can keep the bike in the power with bigger carbs.
I have keihin 29mm tickler carbs on one my katana 1000 ama bike they work fine but are a pain for tuning and parts .If your going to make any more improvements on the engine in the future then a new set off keihin CR31 would be better including parts are easy to get if in a pinch at track or any overseas events.
Why a 750 and not a gs1000,cost the same to build either bike...just wondering.
Heat shieds to keep the float bowls cooler .They were never proven but the concept was there.Carb/frame covers were install later on to prevent turbulence around the velocity stacks on the last off the 1983 Gs1000r engined endurance bikes that was the biggest improvement.
Thanks Chris

 

[mrbridgeport]

You know, I just fell into the 750's. I was at a trackday w/ my 1100 and was telling some of teh guys I'd like to build superbike heavy for AHRMA. One guy had a 750 parts bike and another guy had a one owner 750 sitting around with bad compression. So since this is my first full build and first real season I thought I'd start low key. I think I picked up both bikes for 350$. I thought that it waws a good enough starting place.
I should have picked up a 1000 and had the chance to later but was knee deep in this build. I still may pick up a 1000 2v motor for next year.