Dellorto question,

  • Thread starter Thread starter bob
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nobody mentions the differing amounts of "lift" due to the main jet height,required to get the fuel from the float chamber into the venturi until now,

yes of course the fuel flows through the main jet, but until three quarters throttle it is "metered" by the needle, above (aprox) that it is controlled by the main jet orifice.I still say the dimension of the jet  away from the venturi, controls the point at which fuel will go through it or not.Some fuel will go through the pilot jet, and does not stop as the slide is lifted, the rest goes through the main jet, needle jet/emulsion tube, all of them in the same line, the cutaway, creates lift at low openings where there would otherwise not be enough, that is why some racing two strokes have the cutaway facing the engine, so that fuel goes through even if the slide is shut (as when slowing hard for a corner) but (for those types) fuel will still lube the cylinder, but with conventional slides, when they are shut, they are shut, as they open they draw fuel through the main jet (unmetered) and controlled to a lower level by the metering affect of the tapered needle, and later through the main jet orifice,now unrestricted by the needle, but the height of the bit that is at the bottom of this tube is the main jet, and it will vary the starting point for the fuel flow, but it is minute.

dont take any notice of me though, I am just an amateur with no formal traing or qualification, at least as far as automotive engineering is concerned.

Somone can prove me wrong if they like,no problem with that

CLEM

 
I think we need to separate the discussion into 3 different possible solutions:

1) Difference between short nut with short mainjet-seat and large nut with large mainjet seat. Here Clem has a point potentially - don't know.

2) Modern Version with lots of drillings against one of the above (Point 1): As Marnix points out, the fuel-flow-rate is way improved by that

3) Difference between short nut with short mainjet-seat and large nut with SHORT mainjet seat: I assume that the point I wrote before regarding stalling is the point to use that combination to cure the known problem without having to exchange the complete float-bowl (which would be the better way I'd say, but more expensive).


HTH
 
on the factory SFC's and (I believe) the original space frame racers there were no acelerator pumps fitted.On the SFC's tthe carbs were of a PHF type that did not have that component, not sure about the original space frames.This was done , because if a racer was charging hard into a slow corner following a flat out straight, (as he should be if has got big nuts) then the float bowl can drain, due to the flat out WOT blast and several hard throttle blips to get into the slower corner, maybe this nut modification was connected to that, perhaps could give a little longer without hesitation on existing the corner, which is why the racers did not have the pumps.
CLEM
 
I doubt if the various evolutions of the float bowl nut has anything to do with Moto Laverda, it will almost certainly be a development by Dellorto. It's a standard Dellorto part with no reference to Laverda on the Dellorto website.  Presumably contemporary Guzzis and Ducatis had the same kit fitted.

I imagine that the disconnection of the accelerator pumps on the works bikes had as much to with obtaining a lighter throttle action for a 24 hour race as it did for fuel economy. I think Piero has disconnected the pumpers on his race bike whereas I prefer to run my bike with them fitted. My bike has 300cc per cylinder so the fuel requirement per cylinder will be similar to a 1000cc triple. I've never noticed any fuel starvation issues but of course I've got the advantage of having one fuel tap per carb instead of two taps feeding three. Also, Messrs Brettoni and Gallina  et al would also have the throttles open far wider for much longer :D :D
 
There was a differant fuel bowl fitted with Dellortos fitted to post 2000 year model Husky with an overflow moulded in so fuel would drain out of the motor when they fell over,they probably did redeigns to Dellortos to suit big cliets needs over the years
 
The disc brake SFCs had PHB carbs, very similar to PHF, but no provision for pumps. The Chott had a PHB too by the way.
I run my SF2 without fuel pump. When raced ? never experienced emptying of the float bowl, but it did colour the plugs black. Never used the pumps since (on any of my 750 twins).

I do experience fuel starvation problems with my Egli though. It happens when raced in most long fast corners both left and right. The Egli has VHB square slide carbs of 30 mm). These carbs have a different construction, because the 'nut' that closes the float bowl holds the main jet. I have tried everything from better breathing tank cap, better flowing petcocks, linked through fuel lines etc.
Only thing I have not tried is raising the float height, which is supposed to be 24 mm instead of 18 mm of the PHB and PHF carbs..

Marnix
 
All these theories make perfect sense. I suspect the real reason has more to do with manufacturing economy than fuel flow and supply.
Its been a long time since I've had an old square body carb apart but I think it uses the same main jet/emulsion tube holder and some of them had an accel. pump in there too.So it would make sense to use a proven part on a new carb to reduce costs and development time.As the carbs got bigger and the surrounding areas they live in got smaller,saving 14mm or so off the bottom of the carb would allow Dellorto to sell more carbs for more applications with a mininum of retooling.
Just a thought.
 
If you look at the design of the various float bowl nuts they won't be easy or economical to produce so I doubt if it was a cost expedient measure. Holding the main jet immersed in a bath of [relatively] still fuel makes perfect sense to me. I suspect the clue is in Greg's comment about the increasing use by manufacturers of Dellortos [and other carbs] in a downdraught position exacerbated fuel starvation issues and prompted Dellorto to look at solutions to overcome fuel surge.
 
The stock setting for PHF accellerator pumps on 1000 triples is 0.5cc fuel/20 full throttle strokes.

Don't see any danger of fuel starvation from the bowls at that rate!!  Reckon that can be put down as a myth...  The amount of fuel the pumps deliver would more likely bugger the carburation on re-opening the throttle.

Pumps are not necesarilly needed on either twins or triples, with attention to set-up, they can be done away with completely.  Just a quick-n-simple factory method of ensuring the beasts recieved enough fuel when needed without worrying over consumption figures or clean carburation.

The distance between the main jet and mixing chamber has a very slight influence on float height/required mixing chamber vacuum/jet size, so Clem is, at least in part, correct on this point!  Don't think the slight difference will be felt in practise, though.

piet
 
I agree with you Pete but I think you'll find the pumpers use a bit more than half a cc per 20 strokes, the setting for the 500 is 5.5cc per 20. I suppose you could work out whether the bowls would empty in race conditions- let's assume the rider exits a 2nd gear corner  and gives it four big handfuls getting up into 5th plus a couple of blips to change down for the next corner. That would activate the pumps 7 times. It's going to use a couple of ccs per carb...
Added to that you've got the combined effect of three jets at WOT. I'd be very surprised if the flow rate from the taps wasn't adequate. The main restrictor would be the needle valves.
 
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