I, reckon that I've read most of the Holley threads on the forum but I haven't come across anything that relates to my problem. Most concern stumbles at some point in the rev range, I'm looking to refine my settings. 390 Cadillac with a 262 degree cam (at 0.050 lift) 750 double pumper 4 corner idle etc etc. I am jetted almost square, 74 front 75 rear. 2300 lb car, stick shift I have a lambda sensor on the exhaust and the mixture readings are showing the following... Cruise she runs a little rich If I depress the throttle pedal gently the mixture runs almost out of the range weak. Mash the pedal from almost any revs and and she'll pick up and go like a scalded cat with the mixture-light spot-on the maximum power figure. At idle the figures jump all over the place but this is to be expected she's pretty lumpy even at 1000 rpm (I have 3 to 3.5 percent on the exhaust gas meter) This is all happening around 3000 rpm where the mains are just coming in. So it's inadequate signal (not enough flow with very light acceleration) and a little too much signal at cruise. What do I do to refine this please... Or do leave it alone and blame the cam? Jerry
First make sure you have a .018" gap between the acc. pump arm and the cam lever... Try that out first if any adjustments were made... If nothing happens, you might have to play with the shooter size.. Go up or down on the shooter size till it clears up .... It helps to have accellerator pump tuning kit from Holley for when your making those adjustments... Visit: www.holley.com or Summit has those in their catalog...
You need to change the power valve. What vacum setting is it? The gap under the accel pump arm is measured at full throttle not at idle. All that does is ensure that the linkage does not run out of travel before full throttle. If you have a gap at idle it will make the hesitation worse.
Useful stuff guys. I have refined the 'squirter' cam's a little, going for those with the most initial. The 'absolute' dose is probably enough as if you mash the throttle she goes like a demon, no hesitation no black smoke. So it's probably enough fuel but at the wrong time. I'll put a bigger squirter in the front to start with. This makes the most sense given the symptom are for just a little tickle of pedal. Failing that I'll drop the power valve down one at a time. To answer one of the questions... No vacuum advance. I'm using a 'vintage' Mallory twin point(can't have modern in vintage racing over here) with the drag vacuum 'brake' It's hopeless so I have locked it off and gone for just mechanical. I have refined this using my distributor tester machine. Although it still needs a couple of degrees of total knocking off as the motor seems to like 12 degrees initial, which puts the total curve a little high (32 degrees) for the 99 octane unleaded here. On 102 race fuel it's fine but that's a bit expensive for the road (£4.00 that's about $6.00 litre!) at 10mpg Thanks to you all.
maybe you need to raise the power valve...it's opening too late, right? (as the vacuum drops as you ease the throttle open)
jerry, I'm the guy always saying how the carb is incorrectly blaimed 99% of the time, so I have one question. Why the talk of such high octane fuel? You mention 99 unleaded, and even race fuel, but why? These fuels burn very slow, do you have a effective compression ratio fiqure, or even a calculated/static one? Thank you, TR
I have no doubt that you are aware that our 99 octane(pump) equates roughly to your 94. The Race fuel is Sunoco 102 which being a US brand... It occurs to me could be as high as 107 when rated like UK pump fuel.(I need to look at the can.. RON, MON etc!) My static CR has been calculated at almost spot on 10:1 The settings are very nearly there, the pinking isn't present at all on the 99 when the motor is cool(ish) it's only once she's really up to temperature on a 25 degree day... 190f on the temp gauge that she'll pink at full throttle at the revs where the advance is full in . Agreed that a slower burn = more advance needed which may be another reason why the pinking obediently goes away when the 102 is put in. Theoretically therefore it needs more advance to really take advantage of the race stuff. I'm not blaming the carb for the pinking, that's down to refining the advance curve. According to the sensor the mixture is spot on when the pink occurs, so I'm convinced knocking a couple of degrees total off will sort this out. I'm only considering the larger squirter as #1 possible cure, as the 'weak-spot' is only when the throttle is opened the smallest amount. Again... it is agreed that the vacuum will drop when the throttle is opened by any amount, but do I want a gush of fuel then? I suppose what I need to ask is... How sensitive is the metering on the power valve feed circuit? Will the motor take merely what it want's / needs or will she go rich on me?
The power valve is almost an all or nothing proposition. It may pretend to have a halfway point as it flutters at the point of breaking open, but it certainly isn't graceful in it's opening. You are really playing around on the transition slot that get's you off the idle circuit. I've been learning recently how to tune this transition by adjusting the secondary idle gap. This may not apply if you have a 1:1 linkage to the secondaries, but it may be something to look at. On a Vacuum secondary carb, the idle air comes from two places. Part comes from the primary butterfly gap set by the idle screw, part from the secondary butterfly gap set by that tiny set screw under the right side of the secondary shaft. If you close the secondaries too tight, then you are forced to add air via the idle screw, advancing your position on the transition slot. Too much and you won't even be able to get down on the idle circuit. Now go back and open the secondary screw a bit and you will need to take air out on the primaries, retarding your position on the transition slot. There is actually a bit of room to tune your transition timing versus rpm in this area. Again, I haven't set down and looked at a double pumper with four corner idle, so it may not apply, but food for thought.
Scottybaccus... Yup, that applies. Makes great sense, easy to do and with no cost. Perhaps this first then. I'm starting to get a really clear picture here, I reckon between you all she'll soon be spot on. Again my thanks, very useful stuff. JT Spiderdeville. More fuel... OK, where, when, how?
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