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Post a reply to the thread: Holley carb tuning
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After a lot more messing around and getting a second opinion I asked on the Com Cams tech forum and got a very helpful response from one of their guys. It seems like it is a characteristic of the cam so I was recommended another one (XE274) which I have now fitted and wow what a difference. I've lost all the judder at low revs and it's got a stack more torque at the bottom end although it hasn't got quite as much punch when the power comes in (which may not be a bad thing!) I think it still needs the timing fine tuning but I'm a happy bunny at last.
Oh and a bit thanks for everyone for the help and advice
Thanks for the explanation, I think I'm getting it slowly.
I'll try a couple of small holes in the front butterflies and see if this helps.
no problem, dyno conditions are simulated, actual setting when the engine has a different load to lug, ie the vehicles weight from a standing start and the fluid resistance of a torque converter mean the timing setting may have to change to suit the application. Total timing should be near enough the same as the dyno but the initial timing will almost certainly need to be upped as the dyno doesn't know if you're engines in a t-bucket or serveral tonnes of winniebago
1500rpm would indicate, you're off the idle circuit in the carb, have the blades cranked open slightly and will be be seeing reduced vacuum as a result. Drilling the hole, effectivley adds an extra venturi just for the idle circuit, without affecting any other circuits in the carb. Thats not strictly true as the hole is still present in the blades for all other circuits but the air drawn through it versus the amount down the carb is minimal. The purpose of a venturi is to speed the air entering the carb to aid fuel atomisation, so when vacuum is weak, an air bleed / hole placed in the path of the idle feed will provide an increased signal across the idle feed. The benefit of this is you can close the blades almost fully, restoring the transition circuit to it's intended purpose. If I recall on a 4150 carb, they have lead shot hammered into the air bleed holes in the metering block. On a race carb, these holes are air bleeds which are tapped to take small jets. Smaller bleeds increase signal and will therefore richen, bigger slow the signal and lean. Start the holes in the blades as small as you can and work up to the desired size by testing each hole size. Drain the float bowl by removing only a bottom screw and drain it off into a container.
Thank you very much for your very comprehensive reply.
The engine has been set up on a dyno so I assume total timing and mixture are correct (it seems to go ok apart from the jerking at no load cruise speed 1400-1500rpm) I don't really want to go messing it all up.
I think your statement "you can get there, just need to understand what to adjust in relation to the engine signal" sums my problem up, I don't understand what is going on.
With my cam am I going to get a poor engine signal at 1500rpm with no load (vacuum is about 11") and will drilling holes in the butterflies help this or are the holes only to help set the idle? (which is fine at the moment) I was hoping that the getting it to run ok with 10 degrees of timing might be a clue (ie too rich or too lean)
with that cam you need to accecpt it got low vacuum, yourtiming and carb set up are dependant on the presence of vacuum. You'd be better with a double pump jetted square but you can tune a VS. Bin the vac advance, plug the ports and bump the static timing up to compensate.
Run the motor up to temp, stick a dial back timing gun on it. Rev and hold the motor to 4000rom, point the gun on TDC and turn the dial until your TDC marks line up. Read the marks on the dial, this is your total timing, back the revs off to 1000rpm, do the same and take a reading. Total timing minus static timing is the range of centrifugal advance you have. The curve plotted on a graphs would be RPM across the bottom, degrees of timing on the left. the curve statrs at say 14 degrees static @ 1000rpm and rises to 36 at XXXXRPM (whatever the total amount of advance is set to peak at, ie "all in by XXXXRPM") Now you know what you have in your dizzy currently. Pull the cap off, and work the rotor arm manually to move the baseplate. This is the movement made when advancing, the rate and total amunt of movement is normally controlled by small springs retaining weights. As centrifual force builds with rpm, the weights centrifugal mass increases and theythrow out pulling agaisnt the spring, moving the baseplate and advancing the timing. Light spring give lot of timing early, heavier restrict timing advance until higher rpm. The full extent of their movement will be governed by a stop of some kind, bending or modifying this will allow more or less total timing. So.....set the intial to you're desired amount, 12-14 should be OK assuming the motor can crank it over. Take a reading for total timing, then modify the baseplate to get your desired total timing, assuming it needs it.
Mixture. Forget the idle screws for the time being, these are finite adjustment for when you're close. Back the fast idle off completly to close the blades, people use this to get to where they want but invariably end up with an idle too high. By opening this, you uncover the transition slot which flows far more fuel than the idle micture screws. What you need is to promote engine signal with the blades closed. Drill a 0.8mm hole in each primary blade in line with the idle feed down the bore. This will increase the signal strength in the area of the idle feed without having to crank open the blades and increasing rpm. Start small and increase the hole size until till the idle mixture screws become responsive, ie you notice an affect by adjusting them. Your idle RPM should be reach with a combination of this, and a slight opening of the fast idle screw, quarter turn or so. If you have an AFR meter or lambda, at idle you want it fairly lean, 13.5-1 stoich or 0.95 lambda orhigher will be fine. By now you should have good control over the idle.
The response you get when stomping on the peddle will be governed by the pump shot, power valve and toan extent thetvaccum diaphragm spring. Pump shot is simple, go big till it puffs black out the exhause, then go down a size. Check you have sufficient clearnaceon the arm / mechanism, can't recall what it should be off the top of my head but 35 thou rings a bell ?
Power valves flow in the absence of vacuum, you sound like you've taken the vacuum reading and referenced the chart already. The vac secondary is essentially held shut by vacuum, as this cams got balls, you're low on vacuum,meaning they could be open too early. Wire the mechaism shut with some MIG wire and and see if the throttle is more responsive. Youcan adjust the rate of opeing by buying a spring kit with colour coded different strength springs.
Last point, if it's VS carb, the secondary jetting /metering is controlled by a plate. If you still have it, replace it with the kit they sell which allows you to add a proper metering block with screw in jets. Keep jetting up until your MPH at WOT starts slowing down, then drop down a size.
The above method of tuning has gotme and a number of mates very close of tune, you can get there, just need to understand what to adjust in relation to the engine signal.
EDIT - on idle speed, don't have it too low, remember you need splash oiling up in the cam area 1100rpm should befine
EDIT 2 - contrary to what some have written, vacuum advance, doesn't retard it advances timing. Vac advance increases the timing in the presence of vacuum to fire the plug earlier on the ignition stroke, producing a more efficient burn, thus reducing the amount of unburnt fuel exiting the cylinder when RPMs are low. A longermore efficient burn also puts the heat energy into the piston crown , bores and oil, thus reducing cylinder head temps. Try it, retard a motor whislt running, headers will glow red and temp will shoot up. By removing the vac advance, you need to bump the initial timing to compensate.
I know it's dragging on but I'm still tryng to sort this one out...
Went down the wrong road i think as I'd convinced myself there must be a vacuum leak on the inlet manifold so I've spent time trying to find it and eventually changed the gasket. No difference.
Also tried a different carb, although only a 500 cfm and I had to wind it right open to get it to idle I just wanted to check if the problem still existed. It did.
So I started supecting the timing, which a few of you guys have suggested might be a cause. After advancing the idle timing and it not getting any better I asked my brother for some help. He's got mappable engine management on a kit car (Vauxhall red top 4 cyl), looking at the timing for that it starts at 16 degrees at idle but drops down to 8 degrees at 1200 (no load) before increasing again at higher revs.
So I set my idle timing to about 6 degrees (idle now very rough) and disconnected the vacuum advance (aiming for about 10 or so degrees at 1400rpm, the problem speed) and the problem is very much improved. Obviously this has put the timing at idle and further up the rev range completely out, so have I found the problem or is changing the timing just masking the real problem?
If this is a cure I think I need 14 deg at idle, 8 at 1400 and then up to 36 at 3000. I see MSD do a digital programable ignition but it looks expensive is there other ways to get round the problem?
After a busy weekend decorating I've just found time to check the idle mixture screws. I've tried adjusting them half a turn at a time and then tried it. I went up to 2 1/2 turns out. It did make a very slight improvement at about 1 turn out but no miracle cure.
I'm getting to the stage where I think I need to take it to an expert. Is there recommendations for someone in the Beds/MK area that really knows Holleys and Chevy's?
check the idle slot on the rear as well
Very little transition slot showing (looks like a square!) so I guess I need to try richening the idle mixture. I didn't realise this was still working once cruising.
Many thanks for replies, I'm sure I'll crack it eventually.
Have you tried richening up the idle mixture screws as a trial? Will only cost you a few minutes to judge the effect.
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