VHB carb slide refurbishment?

arancia

arancia

Senior member
Location
Arizona, USA
I'm working on VHB carb refurb for my 750S, I've a couple of pairs of carbs, both have well-worn slides and bodies, it looks like the original slides had a layer of paint or some other friction-reduction layer to make them open and close more easily against strong spring pressure, this is now well worn and I'm looking at refurb or replacement of the square carb slides.
Any experience or suggestions on how to refurb these slides to reduce sloppy clearances and maintain easy twist-grip movement?
 
Not sure about squareys, but my round slide PHBs had worn bodies - I line bored them and made hardchrome bronze slides for them.

Can't you just buy new square slides? Or have they become like snowflakes in hell?
 
piranha-bro2 said:
Not sure about squareys, but my round slide PHBs had worn bodies - I line bored them and made hardchrome bronze slides for them.

Can't you just buy new square slides? Or have they become like snowflakes in hell?
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No, you can't get new ones.; Parts for them are scarce but they were used on a lot of bikes of the period. Ducati singles, Moto Guzzi twins and singles, Laverda, probably most Italian bikes from the mid 60s to early 70s.

Paul
 
Just for fun... I recently worked on a set of VHBs where a DPO probably though the 'black stuff' was dirt and tried to clean it off. Derrrrr. FFS.


They had to be reused, but I checked them out with a mic and the working areas were dimensionally close enough to unworn, but its a shame the hard anodising was gone unnecessarily.
 
Even once the anodising/coating has gone, the carbs will give suitable service for several more decades, probably until the last drop of petrol is dispensed.

Far more important are needle and needle jet wear. Both V3/6 needles and AN265 jets are getting scarce.

piet
 
The square slides appear to wear a lot less than the round ones. Probably to do with a larger surface on the thrust face of the slide.

Paul
 
I did that twice, the first time worked very well on the Delordo Carb off my TE 350 Husky. That bike had issues with smooth application of power, it had a horrible jerky effect just off idle and that teflon coating worked great so I tried it on the slides in my 3C. I used Hot Jet Coatings in Melbourne, not good that time. The slides came back with lumpy blisters all over them, I tried removing this with an oilstone. Nope, so I bought 3 new slides at eye watering cost.Not much of a sample size, but make some inquires as the people that do this might have improved. The coaters reckoned they did this a lot with Off Road touring companies who hire bikes out, apparently carbs on there bikes cop a lot of wear from sand
 
I did find a pair of NOS slides in my stockpile. I had thought that the number related to the cutaway similar to the round slides but found that where the originals have stamped code 7454.50 and my NOS slides have 7454.60 the new slides do not easily drop in, sticking about 2/3 way in, and measurement shows them about 0.1 mm wider.
There do seem to be various slides available new at high price but with different last digits. I've been hunting for my copy of the Dellorto tuning guide, misplaced as I haven't needed it for about 20 years! I don't remember if it talks about VHB slides. Does anybody have any info on the square slide numbers, does the .xx number mean something other than cutaway?
The 7454.60 casting is different in the side section, it was new in sealed bag, opened to check, measure and photo, the scratches on the side show how it's too tight in the body with only gentle finger persuasion to fit, the surface coating seems very fragile.

Thoughts on sanding or wet-stoning the slide width to make them fit easily or is there something else that the .60 means?IMG_1075 - Copy.JPGIMG_1076 - Copy.JPGIMG_1077 - Copy.JPG

I have several new V2 (750GT according to green book) needles but no V3 (750S), anybody interested in a trade?
 
Paul Marx said:
The square slides appear to wear a lot less than the round ones. Probably to do with a larger surface on the thrust face of the slide.

Paul
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The roundies come in certain basic sizes that cover (I think) two venturi sizes each; so a 32mm and a 36mm use the same body size, 38mm and 40mm use the same body size, and the larger venturi items of each body size will wear quicker than the smaller, due to less metal supporting them (particularly noticeable when lifting off idle with high intake vacuums).

My bored-out 36es with heavy hardchrome bronze slides worked really well, but I should have made the cutaway 50 or 60 instead of the 70 that replicated what I had been running in them when they were worn out. Had started making removable cutaway sections so I had cutaway options but once I got the FCRs I parked the job.
 
arancia said:
I did find a pair of NOS slides in my stockpile. I had thought that the number related to the cutaway similar to the round slides but found that where the originals have stamped code 7454.50 and my NOS slides have 7454.60 the new slides do not easily drop in, sticking about 2/3 way in, and measurement shows them about 0.1 mm wider.
There do seem to be various slides available new at high price but with different last digits. I've been hunting for my copy of the Dellorto tuning guide, misplaced as I haven't needed it for about 20 years! I don't remember if it talks about VHB slides. Does anybody have any info on the square slide numbers, does the .xx number mean something other than cutaway?
The 7454.60 casting is different in the side section, it was new in sealed bag, opened to check, measure and photo, the scratches on the side show how it's too tight in the body with only gentle finger persuasion to fit, the surface coating seems very fragile.

Thoughts on sanding or wet-stoning the slide width to make them fit easily or is there something else that the .60 means?
Click to expand...
If it is just the narrow side section I would think it should be fine to sand it down a tenth of a mm as it is not load bearing at the side.
Check the height of the cutaway to see if it is 5mm, 6mm etc. I would have thought that the last digits referred to the cutaway.
 
The pictured used slides look quite OK. The new ones will look much the same in a very short time! The flat slides don't have anywhere near the sealing problems the round slides have. Once the engine creates a bit of vacuum, the slides seal perfectly, worn round slides flop about and let loads of air simply go around them.

piet
 
Wolfgang sent us a square slide from a Laverda and a Guzzi recently with the intention of us machining a run of square slides from billet and having them hard anodized
My CNC guru mate had a look at them last week and he said there is no major issue in machining them but we need to work out where we can have a radius and where we need to have a sharp edge and I do not have a square body carb at all to allow me to work that out

With subtractive machining there has to be a radius at some point on the body, does not have to be large as we can run the final pass with a 2mm end mill so radius will only be 1mm
Just need to find a carb body we can borrow to work it out

Two slides supplied have the following numbers on them
Laverda 7454-40
Guzzi 7454-50

Anyone on here from Aussie have a spare square body carb they can lend us for a short period ?
 
Here's a Laverda forum thread from 13 years ago that explains the cutaway numbering on VHB slides. https://laverdaforum.com/xf/threads/313/

The last part of the number is the cutaway in tenths of a millimeter. So a 7454-50 will have a 5mm cutaway. To fabricate them by CNC machining, I assume you'd need a series of programs to make the different cutaways. I dunno what the range is, but I've seen numbers ending in 40, 50 and 60.
 
Tippie said:
Check the height of the cutaway to see if it is 5mm, 6mm etc. I would have thought that the last digits referred to the cutaway.
Click to expand...
I'd originally assumed too that the .50 and .60 were a cutaway specification and had thought that my NOS .60s were not suitable. Using a couple of straight edges and feeler gauges show about 1mm difference so perhaps it really is the cutaway notation and the width difference is just a lack of precision in manufacturing.
From Red from Wolfgang, "Laverda 7454-40" I do now wonder what were the originals. Both sets I'm working on may not have original slides, one pair came via eBay "from Laverda SF" the other on the chopper I bought for the motor and it had certainly had some modifications. The chopper ran but I was never brave enough to exercise it aggressively and check the full carburation range before tearing the motor out. Parts guides for the S/GT MacCormack era M numbers and SF/GT Laverda numbers don't help much with slide specifications.
With Piet's suggestion that they don't look too bad, I think I'll be reassembling with existing used slides for now and wait and see where things go with Red's proposed new production.
Thanks to all for your inputs.
 
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