dellorto 36 carbs on small valves 750SF head

E

enricozago

New member
Location
Bologna, Italy
Dear all
I was wondering if I could expect any significant benefit in engine performance by fitting 36 carbs instead of original 30 on a small valves '72 750SF head.
I found these carbs out from a ducati of more or less same period and I thought I could replace them on my laverda...
Thanks for sharing your experience.
 
Let's put it this way: if done properly, you will not lose to much power/torque...

:D

Honestly: if you simply stick 36mm carbs on a 30mm head, the head can not make use of the larger volume of the carbs. Worse than that, with every twist of the throttle, a larger displacement will be opened to the engine, which might be to much easily in lower revs, as the engine does not produce enough underpressure to suck enough air in and with the air, the gas itself. Vaporization of the mixture will be not as efficient as with the smaller carbs in lower to midrange revs. Once the engine has picked up some revs, this is balanced a bit more, but basically what might happen is simply that you will use more gas to have the same power and torque - or less. Adding - in a negative way... - to that is that due to the design of the PHF's, the largest diameter is roughly 38mm on PHF's around the needle. This causes the slowest air-movement at the point where the fastest would be necessary to pull more gas and get optimal vaporization.

You can prevent that by tapering the intake, but that is not easy. it would require either working on the head or a tapered manifold to make that work.

My conclusion would be that it is a lot of work that provides no real benefit. I might be proofed wrong thoug by the next one who posts...  ;)
 
"Significant benefit" I doubt, as you will likely lose some bottom end and gain some top end, as Lothar says. Unless your 30s are stuffed and you need new carbs it is pretty pointless in my opinion. It will require a lot of trial and error jetting on what are old possibly worn carbs. Are you sure they are 36mm, my memory was Ducati used 32mm, same carb body was used from 30mm (?) up to 36mm.
 
Enrico,

Just fitting them will give you a headache, inlet stubs are totally different, the original 30mm stubs don't lend themselves to opening out to 36mm.  Ports are on the small side, as are the valves originally used in the small-valve heads.  Not worth the trouble imho... why do you want to do it anyway?

If you want a significant boost, fit the later head.  Somewhat of a sacrilege on an original bike, most wouldn't notice if it's a bitsa.  Could even keep the original small-valve exhaust system, works extremely well with the later head and cams. :D  Just doesn't look as flash as the later system.

If the orignal VHBs are knackered, why not go to a pair of 30 or 32mm PHFs?  Or a pair of Mikuni VM?  That way you could retain that lovely torquey engine characteristic.

piet
 
I'll be using 36's on mine and mine was a big valve head. Red still punched the ports out.
Said if I had a small valve head I'd need to find a big valve jobbie ............. so if I'm using 36's with ports you dock a ship in, doubt it's going to be of benefit to you.
But then; What would I know, I'm not a mechanic, just a salesman.
 
  Did a head job on a small valve head along time ago - used 40mm inlet valves and fitted triple inlet stubs and 32mm PHF's - was an extremely nice motor after that - really improved acceleration. I like to use the small valve heads for my tuning work as I can get up to an extra 12? of "downdraught" with some machining - will also take up to a 44mm inlet valve (for overbores). - Regards - Phil
 
Used to see lots of ex-GSXR 29mm Mikuni flatslides on Fleabay for not a lot of pennies.

Might they be worth a try?

Would better fuel vapourisation from higher inlet air speeds give a better burn and more torque.

 
Thank you all guys,
plenty of experience and good ideas through your posts.
I don't want to swap the carbs at any cost I was just wondering since I have a couple of 36 at hands.
I agree on all the line and I think I will keep them until maybe I'll find at least one good big valves head.

Thanks again for your support and cooperation.

Enrico
 
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