Discussion
When on neutral throttle [in any gear] the engine surges power as if you are on/off the throttle every 1- 2 seconds.This only happens between 1200 to 2000 rpm.I have tuned the idle to perfection but the screws are a long way in compared with the recommended 1.5 turns out which gives a far too rich idle mixture.All other settings are as from Am Sp.
The engine is Am Sp '500 bhp' and carb Holley 80528 race,750 cfm.Ultima exhaust.
Do they all do this or is there a cure ?I d'ont want to be chasing a loser.
Any help would be much appreciated.
Frank Rabbets
The engine is Am Sp '500 bhp' and carb Holley 80528 race,750 cfm.Ultima exhaust.
Do they all do this or is there a cure ?I d'ont want to be chasing a loser.
Any help would be much appreciated.
Frank Rabbets
It sounds like the throttle closed position is not closed to the spec Holley specify (don't know off hand what this should be).
What happens is the closed throttle covers some very small holes or slots in the side of the venturi. If the blade is not covering the slot then extra fuel is getting drawn in.
If you go to the Holley web site you should be able to download a set of instructions.
Steve
What happens is the closed throttle covers some very small holes or slots in the side of the venturi. If the blade is not covering the slot then extra fuel is getting drawn in.
If you go to the Holley web site you should be able to download a set of instructions.
Steve
Sounds like it could be 'tramping'.
From what I know, this is a symptom of agressive cams used (valve overlap).
Best way to sort this as much as possible is to use a rolling road to duplicate the problem and tune the carb accordingly (I have same engine, carb etc as you and there is always a hint of it there at low revs but it is not bad at all).
Other causes may be running too lean, too much play in tranmission/cv joints.
Have you had the flywheel lightened?
Hope this helps?
Martin.
From what I know, this is a symptom of agressive cams used (valve overlap).
Best way to sort this as much as possible is to use a rolling road to duplicate the problem and tune the carb accordingly (I have same engine, carb etc as you and there is always a hint of it there at low revs but it is not bad at all).
Other causes may be running too lean, too much play in tranmission/cv joints.
Have you had the flywheel lightened?
Hope this helps?
Martin.
Thanks Martin
Do you know how to alter the mixture between 1200 to 2000 rpm ? In neutral with the car stationery you can feel the problem when you hold the throttle at this rev range so I may not need a rolling road to improve things .Flywheel is standard from Ultima.No play anywhere.If I need a tuner can you recommend someone?
Frank
Do you know how to alter the mixture between 1200 to 2000 rpm ? In neutral with the car stationery you can feel the problem when you hold the throttle at this rev range so I may not need a rolling road to improve things .Flywheel is standard from Ultima.No play anywhere.If I need a tuner can you recommend someone?
Frank
Steve_D said:
It sounds like the throttle closed position is not closed to the spec Holley specify (don't know off hand what this should be).
What happens is the closed throttle covers some very small holes or slots in the side of the venturi. If the blade is not covering the slot then extra fuel is getting drawn in.
If you go to the Holley web site you should be able to download a set of instructions.
Steve
Those are transition slots to admit air when going from off idle onto the main jets. The blades should not be opened past the slot at idle as they won't be working in that situation.What happens is the closed throttle covers some very small holes or slots in the side of the venturi. If the blade is not covering the slot then extra fuel is getting drawn in.
If you go to the Holley web site you should be able to download a set of instructions.
Steve
OP could also have to much timing advance. That can cause snatching.
Sometimes this can happen just due to the installation.......in any car!
The only way to be sure is systematically work through everything on your motor..even then you may not find it. Start with air leaks, if you can remove the carb and pushrods...keeping things in order...gently pressurize the inlet side to check for air leaks. Check the carb halves for flatness and assembly errors. Check the dissy for loose/ broken bits. Coil and ignition system for function/short circuits. Fuel pressure and needle valve..float failure..idle jet/circuit full or partial blockage (very common as alloy castings fur-up after a dry period) etc etc etc. Might take a couple of weekends/few nights but make notes as you go.... one step at a time is the only sure way of elimination. Even then you may have to admit defeat...sometimes a very minor installation problem can cause serious issues with the fuelling of a performance motor. Especially when you are dealing with a big lump of Holley. Big,thirsty carb's/float chamber's can turn minor issue's (resonance etc) into a major headache. (larger volumes of un-pressurized, warm fuel turn to foam quite easily)
Once did a sidraught installation whos thackery washers were temperamental to about eight thou.........like tearing your hair out trying to suss it though!
Good luck.
The only way to be sure is systematically work through everything on your motor..even then you may not find it. Start with air leaks, if you can remove the carb and pushrods...keeping things in order...gently pressurize the inlet side to check for air leaks. Check the carb halves for flatness and assembly errors. Check the dissy for loose/ broken bits. Coil and ignition system for function/short circuits. Fuel pressure and needle valve..float failure..idle jet/circuit full or partial blockage (very common as alloy castings fur-up after a dry period) etc etc etc. Might take a couple of weekends/few nights but make notes as you go.... one step at a time is the only sure way of elimination. Even then you may have to admit defeat...sometimes a very minor installation problem can cause serious issues with the fuelling of a performance motor. Especially when you are dealing with a big lump of Holley. Big,thirsty carb's/float chamber's can turn minor issue's (resonance etc) into a major headache. (larger volumes of un-pressurized, warm fuel turn to foam quite easily)
Once did a sidraught installation whos thackery washers were temperamental to about eight thou.........like tearing your hair out trying to suss it though!
Good luck.
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