Considering 750SF Carb Change

Laverda SF

Laverda SF

Hero member
There is noway I can run Air Filters, let alone Velocity stacks, on my 72 750 SF with Delardo VBH 30A's and 2 into 1 Slater Headers. I have the mill working great hitting near 8000rpm after changing out the main Jets from 125 to 130's, but noway will it run over 5000rpm if I approach the entry of these small hi-velocity carbs with any type of restriction, even if I jet bigger or smaller; so I'm thinking of going to a bigger carb if I want to run Air Filters.

I have a set of Mikuni CV 34's off a Yamaha XS400R an a set of Mikuni VM 36's which possibly have Primary Type Fuel System for 2 Strokes (if that makes any difference). The VM's would be a simple install compared to the CV's.

I do like the way the engine runs with the open VBH 30A's but then again I would prefer the protection of running air filters - What do you think?

Note: This bike came from the factory with 2" Velocity Stacks and Dual DGM Silencers which I unfortunately no longer have.
 
Humm - 91 views and no comments.

I believe I attempted to discuss this issue about as year ago with about the same results.

The question is it even worth the effort and if so should I experiment with the Mikuni VM 36's as they would present lest problems in the conversion.

Any comment please :o
 
I have to do a Mikuni conversion on a SF2 in a couple of months time
am planning on using 38mm RS Flatslides which I have here in stock

I have no comment on the VM or CV carbs, never used them

HTH
Red
 
If you like how the bike runs with VHBs and velocity stacks and would only be switching to get the protection of air cleaners, then I would first determine how much of a risk the open stacks pose. I had a Bevel Ducati 900SS that had stacks, and it ran much better with them. I tried K&Ns, but it liked the open stacks better. I decided to keep the stacks, but if their was going to be a longish trip, or there was a lot of dust in the air, I would switch to the K&Ns. After riding the bike with the stacks, I would remove the slides and wipe them off, to keep the dirt to a minium. With the fragile bottom end of the bevel Ducs I was changing the oil around 1K anyway, so ingested dirt fouling the oil was less of a concern; I was mainly concerned with premature wear of the slides. Most of the wear will be on the edge of the slide facing the engine, where it is sucked towards the intake. You can monitor this and see if it is acceptable wear if you decide to keep the stacks. VHBs are simple carbs to maintain; I like the square slides, though they are hard to find if you wear them out, and the needles, at least in the Morini sizes I needed them, were unobtanium.
 
I'm somewhat of a Carb enthusiast and familiar with anything from SU/Stromberg, Delortos, Mikuni's, Holley, Carter, Rochesters to Webers ...etc.

Other then for the Flat Slide, the Delorto VBH is very similar in simplicity to the Mikuni VM's.

It appears I've about reached the limit of the VBH 30A air flow capacity with engine & exhaust modifications. No matter how much I love them VBH's they no longer respond above 4500 to 5000rpm, except when OPEN yet hit 7800rom without any restriction PERIOD, no mater the Jetting to either Velocity Cones or any restriction such asAir Breathers. I've been running this bike since 72 without an air filter, changing Oil and keeping the Carbs religiously clean and I'm still with in STD Bore Spec's at the cost of one set of Pistons which was the result of Seal Failure after time in storage.

I gotta admit the Mikuni CS 34's are built nicely but they are Vacuum with a throttle plate compared to the Mechanical slides of the VBH and VM's, which I prefer (Flat or Round) when it comes to tweaking the throttle on the upper end.

I, in my Old Age - LOL, will probably never go above 7500rpm and do more cruising now then ever, so looking at more time On the Road compared to Short Blasts - I'm thinking now either separate Oil Bathed Filters or design a Paper Element Box using a larger Carb which the VBH 30A can't handle.

I will go ahead with the VM 36 adaptation with Air Filters and keep enthusiasts informed of the results, ONLY IF, my peers think it is worth it, to me & to them; in other words, I will not bother and ride the Open VBH 30A's.

On the other hand it will keep me occupied were trouble can't find me :o

Sure like the looks and Quality of them Mikuni CV34's though.

Perhaps I should just leave her alone with the character she has. It's not as I'm after, other then Air Filtration, more performance, but her fuel stand off is dirty and a pain to clean.
 
SF i confess to reading and not replying sooner

the fitment of larger carbs won't do much good unless you work the ports a bit
a straight step from 34 to 30, or even a steep taper (as in a manifold only) may do worse than what you have
more so with 36
the good news is there is a lot of material to reshape in the head
the bad news is it is best done on a bench
the tricky part is removing material in such a way to improve airflow without killing port velocity.
ever so often i roll a small port head around in my hands looking at what there is to work with ...
been leaning toward an offset port where one side goes straight to the side of the valve
and the other left alone. the theory is the bend will impart a swirl to the mixture going into the cylinder
allowing better filling and hopefully, a more homogenous mixture leading to improved combustion efficiency

worth looking into vizard's work on BMC A engines
here's something i was doing some years back http://www.oldtunnel.com/
both have the offset port

consider looking for someone with a flow bench, pull a silicone plug from the inlet, make a plaster cast of the port
shape and reshape the plaster port on the flow bench until you find what works best

you may also find the std inlet won't work and one from a triple could be the solution but you will likely need to
modify some fins near the port to get the inlet to fit.

i've thought about doing a DCOE conversion but then i'd need to cast an inlet or make one from sheet metal
i've also thought about a large plenum and using a single carb. obviously i spent more time thinking than doing  :o
so i've yet to port a small valve head

one could, i suppose, stuff a bit of shop towel down into the throat area open the port to 32 with a gentle taper to the throat,
vacuum out the swarf, pull the towels and vacuum again and put it together. trick here is enough room to run the die grinder.
before anyone gets up in arms i recall doing just this to fit a 45DCOE inlet on a 907 lotus engine and it worked just fine. should point out i was incredibly anal about how it was done, each cyl in turn at TDC, well packed single piece of paper towel, etc. and there was ample space to work. going to 36 is a pretty radical change and you'll want to work on the throat from the valve side so the head must come off

food for thought, and no doubt more opinion  ;)
scott

 
best done on a bench - worth looking into wizard's work on BMC A engines
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GeeZ that's what I want to hear - Man I'm a head porter, Rev'd Stk Chevy SBC Mills over 8 grand.

Is there a Dimension Chart or do I just do what I do best in comparison with Grumpy Jenkins Specs for a Laverda ?

Your implying that my relative Stk Heads and Carbs have reached MAX velocity for fuel dispersion?

SF heads are easy to remove with the availability various cam profiles - Your choice - Want to continue  where an SF goes beyond 30 VBH's & the 8G realm?

Basically all I want to do is hit 7500rpm with an Air Filter and not have to clean the dirty Fuel stand off where I can't do over 4500rpm with an air filter with 30A VBH's.

She's a 750 8G "Complaining Pig" with them 30A VBH 's and probably will, after 40 years, scummed to  leave her be.

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I've scene change out them fuel lines. Stk Cam, Oversize Manley Ultra Lite Valves, Fresh Valve Springs and Minor port flow Porting. Truly Basic SF Mill with Headers.

If you want me to get into port surgery and bigger carbs in order to use an Air filter, I'm game, lets hear it.

NOTE: The 8 Fin Cylinder Head Casting. This mill can do 7800 and can stretch to 8000 rpm with Open VHB 30A's.

I don't need any more power, it's just that shes a "Dirty Breather" which about boils down to Laverda Complaints.

So what are your suggestions - Leave her be or continue to modify ?
 
Question to the learnered few,

What's the deal with the early 17 fin heads and the later 16 fin heads?? I have an early SF1 motor here with 17 fins and spare SF3 head with 16 fins and my SFC has 16.
 
What's a Fin or two between Mic's ?

How many Fins does the FSC have?

You see my engine and can count the Fins.

Other then I report to have 8 Fins with 500 to 800 more rpm then most 750's, which I believe is what the SF along with them God Damn VBH 30A's DeLordos is all about and I'm considering changing them out, not because of Performance but because they are DIRTY.

I would appreciate some input into whether experimenting with other carbs is worth it. I've no problems with Larger Carbs, Profile Cams or porting as long as I have a proven template.

Then again who the F*&K cares about a Breganze - Save us.
 
motoddrob said:
Question to the learnered few,

What's the deal with the early 17 fin heads and the later 16 fin heads?? I have an early SF1 motor here with 17 fins and spare SF3 head with 16 fins and my SFC has 16.
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No difference internally, Rob.  At least none that I've noticed.  Perhaps some issues cropped up concerning the casting process so the factory changed it somewhere along the line.  There's a slight difference in the cam end covers to match the pitch of the fins, looks a bit strange when non-matching items are fitted!

piet
 
Never saw an official explanation for the fin less on cylinder and head, but I'm pretty confident it was done to reduce 'singing' of the fins.

Marnix
 
reduce 'singing' of the fins.
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Guess I would have 8 + 9 = 17 Fins and can play Mary had a Little Lamb on them.

Notice I tapped the Main Case Oil Gallery that feeds the top end - Not much pressure there perhaps 3 to 5 PSI when oil is hot. The tap does provide a source to ensure the Oil Pump is working and Oil is getting to the Top End.

I eventually plan on mounting an 8 or 10 Psi 1.5" Pressure Gauge in the Cockpit or just screw it into the Block Tap.

igs-ng3150_w.jpg


or this one, both available from Summit Racing basically used for Fuel Pressure readings.

IGS-NG1124.jpg


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I mounted a Cylinder Head Temp and an AMP Gauge years ago and they work great.
At a glance you can pretty well tell what's happening.
An Oil Pressure Gauge should give you more confidence.

You're reading the mileage correct, only 22000 Klic's on her and every Klic a Joy.
 
Hi,
The way I see it is as follows.

You can get the desired RPM from the engine as is,
You can get the desired RPM from the carbs as is,
So this means that neither is causing a restriction.
Your only problem is when you add a filter or "velocity stacks"
Which would indicate that either your mixture is going out when these are added. OR and most probably (as you say you have tried altering the mixture settings appropriately) the filters or "stacks" used are causing a restriction.

Through various and numerous tests on my flow bench I have found that "velocity stacks" are often "flow reduction stacks" and that the shape, length and exact profile of the "bell" makes all the difference between working or not.
This is where I would be looking first.  Carbs off and on a flow bench with different bell mouths and filters!!

Julian.
 
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