VHB setup

Paul Marx

Paul Marx

Hero member
Location
France
Done to death question, but...

My 750 S is at last useable but carbs would need fine tuning.
I have the seat of the pants impression that it's a bit rich. Very discreet stuttering when on the road. All the brass bits in the carbs are new and to standard specs.

Turning the mixture screws has practically no effect on the engine at idle. I can screw them right in, the engine won't stall. It will if I screw them out quite a bit.

Am I right in assuming that it's too rich, the idle jets passing enough fuel not to need setting of mixture screws?
The engine would respond to mixture screw on rh carb but not the lh. So I changed lh carb assuming the idle circuit was obstructed, now it won't respond either side.

The carbs have been cleaned to death, I've been at it over a year now!

Tappets and timing are spot on.

Paul
 
Has someone riding behind you observed black smoke gusts when you accelerate?
How do you sync the carbs?

Gerald
 
Which slides are in it Paul? Cutaway 40 / 50 / 60? Should be 50 as std, but I reckon they are a bit too rich and require adjustment - that depends on the bike though. Maybe you have 40s in there?

VHB suffer from needle/jet wear - letting juice slop all over the place, but you say the brass is new....

Also, look for air leaks - they can cause all sorts of issues, not always towards constant lean.

 
They have the same choke system as VHF etc don't they, you are positive the plungers seal perfectly?
If it doesn't get too lean with the mix screw all the way in, I would be trying smaller pilot jets. Air leak would make it leaner would it not, requiring more fuel mix to make it happy? It isn't a tooth out on the valve timing (causes idle problems too)? Stuttering at smallest throttle when riding (pickup from closed throttle) is definitely slide cutaway, but from my experience wrong cutaway hasn't affected idle itself.
 
Haggis said:
Which slides are in it Paul? Cutaway 40 / 50 / 60? Should be 50 as std, but I reckon they are a bit too rich and require adjustment - that depends on the bike though. Maybe you have 40s in there?

VHB suffer from needle/jet wear - letting juice slop all over the place, but you say the brass is new....

Also, look for air leaks - they can cause all sorts of issues, not always towards constant lean.
Click to expand...

Every thing is bog standard and new apart from the slides,50s, which aren't unduly worn.

Every thing has been checked, including cam timing.

Paul
 
Tippie said:
They have the same choke system as VHF etc don't they, you are positive the plungers seal perfectly?
If it doesn't get too lean with the mix screw all the way in, I would be trying smaller pilot jets. Air leak would make it leaner would it not, requiring more fuel mix to make it happy? It isn't a tooth out on the valve timing (causes idle problems too)? Stuttering at smallest throttle when riding (pickup from closed throttle) is definitely slide cutaway, but from my experience wrong cutaway hasn't affected idle itself.
Click to expand...

Yep, same choke system.
Plungers are new, choke cables loose.

Cam timing, tappets, ignition are spot on.

Trying it yesterday to tank up for Saturday, I had the impression it was running better cold which would confirm richness.

Standard idle jets are 55, will chuck in some 50s and try later on.

Paul
 
Tippie said:
Air leak would make it leaner would it not, requiring more fuel mix to make it happy?
Click to expand...

Yep, of course that makes perfect sense, but I've had occasions where richness is the result of small air leaks (stress small). I think the problem with small air leaks is that at low rpm (high draw on the front of the carb) they can cause the user to chase the carb settings towards rich. As the rpm/load rises (even a little bit) the small air leak cannot pass significantly more air, but the rich setting still delivers more fuel. I agree, it doesn't follow theory, but it sure unbalances everything.
 
Gerald said:
Has someone riding behind you observed black smoke gusts when you accelerate?
How do you sync the carbs?

Gerald
Click to expand...

No black smoke and exhausts not excessively sooty.
I synch my carbs by equalising slide lift with my finger.

Paul
 
Most carb issues are electrics related and vice versa...

Check the electrics. Especially the headlight shell, the ignition switch but also the ground wires etc., Ignition and so on. If carb settings are correct (including float levels), everything is sealing and air-tight, the valve adjustment is righ etc., it only can be something electrical...
 
Laverdalothar said:
Most carb issues are electrics related and vice versa...

Check the electrics. Especially the headlight shell, the ignition switch but also the ground wires etc., Ignition and so on. If carb settings are correct (including float levels), everything is sealing and air-tight, the valve adjustment is righ etc., it only can be something electrical...
Click to expand...

There was a dubious electrical connection in the headlight Lothar, things were much better after dealing with that, but I do need to clean up that loom.

Paul
 
Do yourself a favour and beg/borrow/steal a vacuum guage set to synch the carbs.  Your finger method does not take into account any unequal wear/leakage issues between the carbs.  You need to get the vacuum on both cylinders equal.  The slide heights will be identical only under ideal circumstances, rarely the case with 40-year-old instruments

I've always advised using V3 needles in the VHBs, but have since found V6 to be more suitable in some cases.  They're a fair bit leaner low down, resulting in a crisper engine response.

piet
 
Laverdalothar said:
Most carb issues are electrics related and vice versa...

Check the electrics. Especially the headlight shell, the ignition switch but also the ground wires etc., Ignition and so on. If carb settings are correct (including float levels), everything is sealing and air-tight, the valve adjustment is right etc., it only can be something electrical...
Click to expand...

Sometimes true, Lothar, but doesn't take into account that the pilot mixture screw can be wound all the way in with no effect. I like Piet's idea of a leaner needle profile - also, if fuel is managing to bypass the pilot system on an almost closed throttle I'd suspect it's getting in around the needle/needle jet area. I remember being very surprised when I first noticed an ovalised needle jet and corresponding wear around the needle on my SF1 and realised how much wear can be inflicted on that round brass tube ... probably by sloppy slides.
 
I'm just surprised that your 750S didn't come Amal's - Let me think about this. Granted your 30mm VHB's 30A's are Open Mouth - What exhaust system you using ?

Suggest you contact Wolfgang in Napsuk B.C. Canada http://www.laverda.ca/  re: the 69/70 Breganzie 750S. That's his main ride.

The Breganzie 750S was actually a pre-build for the SFC and SF0.

Air Velocity Sync tubes are really a waste of time and expense - I'll get back to once I know more about your 750s.

 
Piranha Brother 2 said:
Sometimes true, Lothar, but doesn't take into account that the pilot mixture screw can be wound all the way in with no effect. I like Piet's idea of a leaner needle profile - also, if fuel is managing to bypass the pilot system on an almost closed throttle I'd suspect it's getting in around the needle/needle jet area. I remember being very surprised when I first noticed an ovalised needle jet and corresponding wear around the needle on my SF1 and realised how much wear can be inflicted on that round brass tube ... probably by sloppy slides.
Click to expand...

OK - I assumed the carbs have been fitted with new components (all jets, needles and gaskets). If that is not the case, that would be my starting point. Also to be checked: Float valve is closing properly, no blocked passages, idle jet is the right size (measured, not just looking at the number stamped on it; who knows what the DPO has done to it...). Not to forget other adjustments and checks for air-leakages (exhaust and intake).

 
If you get on the phone with Wolfgang Hearter; he will talk your ears off about the Breganzie 750S - Guaranteed.

As far as servicing, jetting and maintaining a Dellortto VHB 30A = No different then a Mikuni , Amal, Solex or Khinen that have no Accelerator pumps.
 
Piranha Brother 2 said:
Sometimes true, Lothar, but doesn't take into account that the pilot mixture screw can be wound all the way in with no effect. I like Piet's idea of a leaner needle profile - also, if fuel is managing to bypass the pilot system on an almost closed throttle I'd suspect it's getting in around the needle/needle jet area. I remember being very surprised when I first noticed an ovalised needle jet and corresponding wear around the needle on my SF1 and realised how much wear can be inflicted on that round brass tube ... probably by sloppy slides.
Click to expand...

The needles are usually quite loose in the slides and slop around happily by themselves.

I replace both needles and jets as a matter of course when rebuilding a carb, irrespective of assumed mileage.  Wear is almost impossible to measure, only remedy is to fit new original components (at least as long as they are available!).

If a carb does not react to the mixture screw, there is bound to be a blocked passage somewhere in that system.  Ultra-sonic cleaning often fails due to the path these passages follow, solids can be trapped despite being loosened by the ultra-sonic.  Ideally, these solids should be dissolved in the bath, but often, whatever fluid dissolves them, will dissolve the carb at the same time.  :o  Need to be quite carefull!

Amals were never fitted to any production Laverdas other than the early SFCs, Ron!

piet
 
Stand corrected no Amal's on the 750S - Personally I hate Amal's from my experience with BSA's and Triumphs.

If they are VHB 30A's s after a through clean and setting floats, equal set the slid height resting on a simple 8mm octagon screw driver bit. Change out your main jet accordingly so it does not run rich or lean out on the top end - Anywhere between a 25 to a34 jet will work - I had to settle for a 32 main jet for my ride with a 2 into 1 Slater Bros Exhaust on my 70 SF0.

--------------

Shit! I forget what needles I was using - I'm sure a narrow profile ! 

Every Engine and Bike is different. You have test ride and make adjustments.

Piet: I sure Wolfgang told me his early model 750S came with Amals !
 
You must log in or register to reply here.
Back
Top