Carb Tuning with a 'big' cam

Roscobbc

Moderator
There have been a few threads relating to carburation recently and perhaps this explanation on potential issues relating to use of a 'big' cam and how it can significantly effect carb tuning. The method of drilling the 'butterflys' is soemthing we use to do 40 or 50 years ago. Higher spec' Holley and generic equivalent will now have the adjustabilty needed to do this without drilling the carb.
 

stealthyflatfish

CCCUK regional rep
Interesting article Ross, exactly the issue I have with mine, always seems to run rich, at any point in the rev range, I've fiddled and altered the settings on my old carb, better but not perfect, no stumble just rich I've never wanted to drill the butterfly's as this seems to be a last resort. I'll digest this article again and have another go, I'm hoping by closing the transfer slot a smidge I can entrain slightly more air.
 

Roscobbc

Moderator
Perhaps 'crank' both primaries AND secondaries open a 'knats cock'?. Even though mine runs acceptably there is sometimes a 'bucking' on transition from idle to main jets at 1000/1200 rpm. Idle is quite rich (race carb) which, again is ok and adjustable but the adjustment in this case is very delicate (half a 'knats cock') and the plugs can carbon-up in town and need replacing far more frequently.
 

stealthyflatfish

CCCUK regional rep
By cranking the primary and secondary throttle plates open a knats to let more air in, is that not opening the transfer slot slightly thereby letting more fuel in ?
 

Roscobbc

Moderator
Possibly if you overdo things. My logic is less on primaries, less on secondaries to minimise 'pulling' too much fuel through idle transfer slot before the slot is exposed too much. Should also help to put more a more balanced supply of air in whilst at idle and transitioning through to the idle transfer slot. This is presuming you have '4 corner' idle mixture screws?
 

stealthyflatfish

CCCUK regional rep
Yep 4 corner idle Ross, Quick Fuel, brawler 850 double pumper to go on, recently flown in from Kentucky via Texas, shame i hadn't been sent a Ultra QX might have made it even easier to dial in.
 

stealthyflatfish

CCCUK regional rep
No I think they're the press fit type, think you have to go Ultra and above to get the removable bleed screws. Assume you can put bigger bleeds in to get more air in! But at the end of the day you need a fuel ratio meter to measure any changes, otherwise it's just guess work. I'll work on that one.
 

Roscobbc

Moderator
No I think they're the press fit type, think you have to go Ultra and above to get the removable bleed screws. Assume you can put bigger bleeds in to get more air in! But at the end of the day you need a fuel ratio meter to measure any changes, otherwise it's just guess work. I'll work on that one.
Yes you are correct - replacing edle bleed jets is very much a guess and doesn't necessarily do what you expect and there tends to be trade-offs in other parts of the rev range.
 

stealthyflatfish

CCCUK regional rep
Does it have removable idle bleed screws in the top of the carb?
Having now checked again, the Quick Fuel Brawler does have removable air bleed screws, so should make it easier to tune, and as a bonus it's probably a third lighter than the original Holley.
 
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