carb size for a 750 ??

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marinatlas

marinatlas

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Hi, my engine is hotstreet one, with JS pistons, and stage one cam, maney cylinder but a Std RH5 head, which size do you advise 32mm or 34 mm , I could order the JS flat side one as a lot of people praise them, though I have a pair of 34 mm vm mik under hand, but lack of space (Seeley frame) will give some trouble to fit them ( I had used them in a featherbed commando engine bike , but without the Sudco splayed manifolds, just fettling with some angled adapters.....)???
 
Acotrel [a forum member] races a Seeley Commando, he`d be approachable on the subject..
 
I might think that JS's 34's would be right.
I have the same cam, combat head, 1 1/2" exhaust, but not the pistons. They are hepolite 20+.

Anyhow, I did try the JS 32 carbs and they were very good but I think the 34's might be a little better. When that cam hits 3750rpm, it just wnats more.
 
comnoz said:
VM 34s work very well on a racer. Jim
Click to expand...

Jim,

I just bought a set of hardly used VM34s with bellmouths (velocity stacks)....the PO had trouble using them on his road bike....evidence is a bunch of jets and a couple of pairs of slides come with them...my thinking has been that his problem was mainly getting them to run smooth low down with filters...

I am looking to use them on the 750 short stroke I am building as a race bike, and may use them on a long stroke 750 racer initially....assuming 2 into 1 decent bore exhaust, 4S or Norris Magnum 70 cam and just over 10:1 compression, can you give me a head start on a workable base setting?

Steve
 
SteveA said:
comnoz said:
VM 34s work very well on a racer. Jim
Click to expand...

Jim,

I just bought a set of hardly used VM34s with bellmouths (velocity stacks)....the PO had trouble using them on his road bike....evidence is a bunch of jets and a couple of pairs of slides come with them...my thinking has been that his problem was mainly getting them to run smooth low down with filters...

I am looking to use them on the 750 short stroke I am building as a race bike, and may use them on a long stroke 750 racer initially....assuming 2 into 1 decent bore exhaust, 4S or Norris Magnum 70 cam and just over 10:1 compression, can you give me a head start on a workable base setting?

Steve
Click to expand...

Steve,
I can't help much with jetting numbers-it has been some time since I have run VMs on a bike of my own. I would just start with what is in there and go from there. Suggested jetting usually only works on a stock application anyway. Too many variables.

For the shortstroke motor I would want to go to larger carbs. 36 or even 38. Jim
 
I've asked about using a single 34 Mike carb to break in Peel's 10.5 CR drag cam 920 w/o the blower and told mainly need to up pilot jet size to #43 as the base line tune should work ok to monitor and plug chop even better. The main issue will be low down throttle response not pulling enough fuel up spray tube till rpms rise, so if Miki's has race needles similar to Amal, ie: sharper taper for sooner richness, that may help better response. These race needles are the same OD as regular #'d needles beyond the tapper zone so only alter the partial throttle zones. Could also try your hand on needle profiling.
The other end that really makes a difference to how the carbs and hot cam inhale is the exhaust system tuning. Header length is pretty correct as factory supplied so only the header diam. and the type of silencer need considering. Y'd two into one or cross over tube definitely help the low rpm response to hi-fast throttle opening yet don't take away from WOT red line power.

Mukuni VM race needling
http://www.google.com/#hl=en&tbo=d&scli ... 31&bih=568
 
SteveA said:
comnoz said:
VM 34s work very well on a racer. Jim
Click to expand...

Jim,

I just bought a set of hardly used VM34s with bellmouths (velocity stacks)....the PO had trouble using them on his road bike....evidence is a bunch of jets and a couple of pairs of slides come with them...my thinking has been that his problem was mainly getting them to run smooth low down with filters...

I am looking to use them on the 750 short stroke I am building as a race bike, and may use them on a long stroke 750 racer initially....assuming 2 into 1 decent bore exhaust, 4S or Norris Magnum 70 cam and just over 10:1 compression, can you give me a head start on a workable base setting?

Steve
Click to expand...
FWIW, There is a lot of frustration associated with getting twin Miks to run on the street. They're are too darn fussy to run down low. I am sure some have done it though. Race, yes. Street, no.
 
this is a good place to start,

http://victorylibrary.com/MIK-BT.htm

i've setup Miks VM for a number of british twins in various states of tune (including a new set for upcoming combat build that i'm sure will be great)- most people get frustrated as there are almost to many options of jets/needles combo's etc - once dialed in they are great carbs
 
I dont know for sure, but as a guess, Yamaha XS/TX forums might well have some ball-park settings for VMs useful for similar sized 4-stroke twins too.
 
Steve,
I can't help much with jetting numbers-it has been some time since I have run VMs on a bike of my own. I would just start with what is in there and go from there. Suggested jetting usually only works on a stock application anyway. Too many variables.

For the shortstroke motor I would want to go to larger carbs. 36 or even 38. Jim[/quote]

Thanks Jim, I also figured that what was in there would at least start and run the bike.....I will do some checking through the box of bits I have with them as well...

I ran Amal Mk1 36s on my race 850 in 76, after some initial teething problems which actually had nothing to do with the carb bore they seemed to run fine to me, but I listened to those running bored Mk1 33mms and saying...'they are too big'...so I changed them to Mk2 34s, and very quickly took those off and had them bored to 36mmwith the velocity stacks on them! Good choice actually, smaller carb body and easier push starting from cold....still not sure with the 750ss I di dthink 36s but others had suggested they might be too much....experience should tell me not to listen right!...but....

My logic with the Mikunis was, they were available at a decent price, so I will start with 34mm and when developing the bike further go to 36s, I am inclined to think that 38s might just be too much on our tight UK circuits without any particularly long straights, some guys are using them sucessfully on 900 Weslakes....both VM Mikuni and Amal...
 
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