VHB carb slides

N

Normski

Senior member
Location
Devon, England
Can someone tell me the correct slide for a 1970 'S' please.
The bike hasn't run for many years and I fitted the carbs with standard jets etc ( it had a very open 2 into 1 exhaust and was running big main jets) but could find no data on the slides so assumed the ones fitted were correct. However the bike wouldn't run right at about 3,000 rpm 1/3 throttle and after checking the ignition system several times I was put on the right track by Stevio. Having changed the needle jets from V3's to V2's the problem running is improved a lot but not yet right. I've looked at the slides in a pair of VHB 29 carbs and the slides are 7454-60 the VHB 30's fitted to the bike have slides 7454-50. I'm thinking that if the 50's are incorrect (richer?) then I've used 2 wrongs to make a right by fitting the leaner V2 needles!
Thanks Norm
 
Hi Norm

the "50" slides have a 50 mm cutaway, pass less air at smaller throttle openings, so are "richer"

the "60" slides have a 60 mm cutaway, pass a bit more air at smaller throttle openings, so are a bit leaner.

at 1/3 throttle, the slides are part way up the carb bores, I doubt the slide cutaway is doing all that much, the cutaway has much more of an effect at initial throttle opening.

at 1/3 throttle, the slides have lifted the needle a fair bit, the carbs will be mainly on the needle taper at that point, affected by the size of the needle jet (which come in a number of diameteres).

the very ealy bikes, 1969/1970 would have originally issued from teh factory, at least in 1969, with a V2 needle, and a55 pilot jet. There was a factory service bulletin and dealer recall sometime in 1969, instructing a change to a V3 needle combined with a 50 pilot jet.

sorry, don' t have any of the documentation on the original slide, or the original needle jet. I have rebuilt a few sets of these early square body carbs, and am about to do another for a friend when I fit larger J & E pistons in a 72 750 sometime in the near future. That bike is running great, I'll pop the carbs apart and make note of slides, needle, needle jet, pilot jet, and main jet for you. May be a bit, though, still waiting for the pistons.

Paul LeClair
 
Well this is very enlightening. I have 50/3 slides in my RGS and it bogs down low. I will have to return the 60/3 back in there and see if it makes a difference. My RGS carb set up is pretty close to US spec. I think the only change is K1 needles and maybe pilots - can't recall what they are. The mains are 108, guessing the emulsion thing is AB262 and it has Bub exhaust which are probably marginally less restrictive than stock. The 60/3 were stock - I think... ::)
 
while we are on the subject; what does the /2 or /3 mean on the slides? My Dells have 60/2 slides.

Gerald
 
Hi Gerald the /2 or other numbers denote the slope characteristics for the operation of the pumper jet arm.
        Paul - the 50 and 60 slides are with 5mm and 6mm cutaways - I expect that's what you meant anyway!  - Regards - Phil
 
Hello Everyone,
My two-cents here:
I just looked at my slides from my 1973 750 GT #15517 7K miles and I am the second owner so I beleive them to be original issue-
VHB30A slide number 7454.50
Larry
 
Hi Norman,
Here is my current set-up (engine is S type, 4S cams, high comp pistons, etc...)
- Slides: 7454/50 (originals)
- Main jets: 128 (orig: 125)
- Needle: V3 (orig. V2)
- Needle jets: 265N (originals)
- Pilot jets: 58 (orig: 55)
Depending where you live and the quality of the fuel you can find, especilly if you have the 4S cams, be extremely careful to put smaller pilot jets than 55.

jlo
 
Thanks for the replies everyone. I've spoken to Stevio again today and he advises V6 needles so I've ordered a couple. Meanwhile I have a rally to go to this weekend, so tomorrow I'll try the V2's on bottom notch and the V3's on top notch and see which appears best. In fact I may try one in each carb and see if I can hole a piston before sooting up a plug!
I'm surprised that a bike with basically standard engine and exhaust won't run better on standard settings though.
jlo -you seem to have richened everything up from standard whilst I'm trying to lean off the lower running ( haven't really done enough miles to get onto the main jet yet) are you running any airbox and filter? I'm happy with the running at tickover and low throttle openings. I am running 4S cams - are they problematic?
 
Hi Norman,
I run no air filter, just the original open stack (picture). Lean everywhere with the original setting, white plugs, just like the 1972 SF (same engine except the exhausts and the SF had K/N air filters) I own some years ago. Everything is ok now, maybe still a bit lean at transition (just when I open the throttle) but very very close to perfect.
Just like my old SF, I find that the engine is very sensitive to the carb set-up, you can be frankly rich or frankly lean just by changing some points on the jets (or the needles), example I'm rich with 60 pilot jets and lean with 55, one full turn of the mixture screw between them!
I think that the 4S cams are one of the causes of this sensitivity, however they are not problematic, all the contrary, one of the best cams for the 750cc!
jlo
 

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Hi Phil

I continue to demonstrate my difficulties with the metric system.... ???

Although Canada has been a "Metric" country for eons now, I am old enough I was raised on the Imperial system. Thanks for the clarification.

Paul LeClair
 
Just to update everyone who has helped with this thread,-thank you. I've now tried V6 needles which were better than the V3's but at the moment I'm running V2's on the middle notch - these seem to be the best so far.
I can only guess that by using an airfilter everything is made richer. My pilot screws are set 1 turn out with 55 jets. I'll soon need to look at how the bike runs when on the main jets, I've a feeling it will also be rich.
 
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