I have 600 cfm DP (model no.4777) on a mild cam 302 engine, Weiand dual plane manifold, MSD 6T ignition and duraspark II distributor. I am having a problem to properly tune the carb. I can start the engine only with rich settings and choke making huge amount of white smoke filling garage for 2 cars withing a few seconds. Idle mixture screws are 1 3/4 turn out, the engine stumbles at a leaner mixture at no throttle or WOT. Fuel is set to lower 1/3 of the adjusting nut, even at this level fuel still leaks through the sight plug. But fuel level set higher than 1/2 causes that a stream of fuel appears at a venting tube on the top of the carb. Timing is set at TDC with the engine off, so far could not measure timing at idle. The distributor picks vacuum from timed spark port. What am I doing wrong? Is this carb broken or too large for the engine?
double pumper tuning with sbf
- Thread starter 67rcks
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393strokervert
Member
+1 on getting a wideband, they are expensive but more than worth it.
also, you need to advance your timing a little...start @ 10* btdc. that could be most of your problem, it is firing way too late/retarded ..IMO.
definently sounds like your carb could be messed up. how much fuel pressure are you running? needs to be around 4-5 psi.
also, you need to advance your timing a little...start @ 10* btdc. that could be most of your problem, it is firing way too late/retarded ..IMO.
definently sounds like your carb could be messed up. how much fuel pressure are you running? needs to be around 4-5 psi.
4777 isn't a 600. It's a 650. You have the idle screws out too far (start with 1-1/2 turns)Timing should be set at about 10-16* BTDC, the throttle blades should be only cracked slightly open (close the throttle screw then screw it down til it just moves the throttle) The floats should be set off the carb, turned upside down, then set to where the float top is level with the roof of the bowl. Then fine tune if necessary when the engine's, running. Remove the slack from the accellerator pump linkage. With all this it should start and run with little or no other adjustments. All that aside the 650 DP is too big for that motor based on what you've posted. It will be fine at all rpms, only at lower rpms when you nail the pedal, it'll be too much till about 5000 rpms. A smaller carb will work better at all rpms.
I tried everything (advance timing, leaner mixture), nothing worked, and decided to take carb apart and figure out the problem by myself. It turned out that new needle that I replaced as a part of rebuilding this carb (was part of holley universal kit for 4150s) had slightly different oring. The fuel seeped around the o-ring, that's why it got floody and fuel leaked all over the place. That's why I could not set the correct fuel level as tons of web sites instruct. The worst thing was that I was able to run this engine with this needle assembly last fall, so I looked for problem elsewhere - suspected power valve because this engine saw like 20 backfires. But it turned out working fine. Will see how the motor performs with this carb; IMO 650 is more than marginal too.
The DP centerhung bowls also use a different needle/seat than the side hung bowls too. Gotta watch that if you've got a box full of carb parts (like I do) This is the second time in a month where some one I was advising ended up with an assembly problem on a Holley. The other guy installed a gasket wrong. It's an easy mistake, everyone does it. Good luck, glad to see you've found the problem.
woodsnake
15 Year Member
brianj5600
Active Member
I don't think a 650 is a bad choice. Dual planes have better signal strength than a single plane and it does not have a big cam.
Marginal I meant the point that the engine starts to disbenefit from any higher cfm. i.e. carb provides more mixture than engine can suck. IMO this is around 550 cfm for this engine. Why is even 600 too much? E.g. Holley recommends 390 cfm, I know they are more on conservative side. If you have actually tested similar engine with 390 and 650 carbs I am eager to hear you experience.
woodsnake
15 Year Member
I didn't see where you listed your transmission in the car. If it's a stick, I would say the the 650 dp is a fine choice for your mild 302.
The usual formula that gets quoted "CFM = Cubic Inches x RPM / 2 / 1728"
doesn't take into account any performance mods, added gears, or lots of other factors that need to be considered when planning for fuel requirements.
However, you may be happier with a smaller carb. My 69 Cougar came with a 450 cfm from the factory. It did an adequate job moving the car around. When I stuffed a big cam in it, and changed the rear gears to a 3.89, I put a 670 Holley on it. I love it.
I think though that you are still dealing more with an over all state of tune, rather than carb problems specifically. Do you have a vacuum gauge?
A lot of folks will condemn a carb, when they actually have other ignition problems to fix.
One way to verify your problem might be to try and pick up a smaller carb from your local craigslist and see if your car likes it better. There are always lots of used carbs on my CL, but I've had to force my self to stop buying them!
The usual formula that gets quoted "CFM = Cubic Inches x RPM / 2 / 1728"
doesn't take into account any performance mods, added gears, or lots of other factors that need to be considered when planning for fuel requirements.
However, you may be happier with a smaller carb. My 69 Cougar came with a 450 cfm from the factory. It did an adequate job moving the car around. When I stuffed a big cam in it, and changed the rear gears to a 3.89, I put a 670 Holley on it. I love it.
I think though that you are still dealing more with an over all state of tune, rather than carb problems specifically. Do you have a vacuum gauge?
A lot of folks will condemn a carb, when they actually have other ignition problems to fix.
One way to verify your problem might be to try and pick up a smaller carb from your local craigslist and see if your car likes it better. There are always lots of used carbs on my CL, but I've had to force my self to stop buying them!
You went from a vacuum 450 to a vacuum 670, big difference between the 670 and a 650 DP. Your motor only takes what it needs from the 670, the DP 650 gives it all when you nail the pedal. And that "all" isn't needed till around 5-6 grand. Anything below that, it slows the car down when compared to a vacuum carb. That's due to the decreased air velocity across the venturis when all 4 bbls are fully open at an rpm where the extra flow isn't needed. An easy way to see this is to nail the pedal on a DP carb, then back off the pedal, if the volume is too much, you'll be able to feel an increase in accelleration as you back off the pedal.
Right, still could not start it although carb does not flood the engine any more, AC shoots fuel, and 4-4.5 psi pressure builds at the needle, fuel level is stable. MSD gives spark to 1st cyl. so I assume it gives to all cylinders or shouldn't I? So played with timing. Started at BTC 12, went up to BTC 6. Have no idea other than the ring on the balancer have slipped a lot and need a new one (anybody has a new one for sale? - 28oz, 4-bolt). TDC was at compression stroke when firing (both valves were closed when turning engine 20 BTC to TC).
woodsnake
15 Year Member
To repeat D. Hearne, the car did NOT START at all?
This is changing direction from a carb headache into a more involved diagnosis.
Have you ever changed out the timing gear set on the car?
This is changing direction from a carb headache into a more involved diagnosis.
Have you ever changed out the timing gear set on the car?
White smoke? If you had the choke shut I would expect black smoke... How is your coolant level? White smoke might be a leak sucking coolant into the combustion chamber, either head gasket or manifold gasket (especially since you said it was backfiring alot). Doesn't hurt to check the coolant level real quick.
Got it running; turned out 20 btc initial is the best for easy starting (w/o choke) and running. Before I ended up at 12 btc which was too late for starting engine yet too early for backfiring.
Also DP was running way too rich, now the needle assembly is all the way down, yet I would still need it leaner - any tips other than rejetting? and if rejetting what is the smallest size for this carb that would not require modification of some other carb component?
I have seen so far only white smoke on a stock, gas car with choke shut. Coolant level remains the same.
Also DP was running way too rich, now the needle assembly is all the way down, yet I would still need it leaner - any tips other than rejetting? and if rejetting what is the smallest size for this carb that would not require modification of some other carb component?
I have seen so far only white smoke on a stock, gas car with choke shut. Coolant level remains the same.
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