carb jettin

[Ron816]

Update

Race cars verses street cars: Different animals and WAY different tune-ups.

Re checked timing retarded a degree or so, tuned curb idle, much better, but still not there stumbles a bit on initial acceleration

 

[coloradodave]

Okay, so first deal with the timing. Advance the initial 2 degrees at a time until either the engine 'pings' on acceleration or won't crank when hot, then back off 2* to the last good setting. You want as much initial timing as you can get. At that point you want to check your total timing with the vacuum advance disconnected and plugged. As has been stated you want to mark the balancer or use a timing tape to check your total advance and at what rpm it occurs. Write the results down so you can always go back and reset your tune. Reconnect the vacuum advance . Try it on timed vacuum as well as full manifold vacuum and use whichever your engine likes. If you can borrow or buy a dial back timing light it is easier, but not necessary.
Then, you can start dealing with the carburetor (always timing first). The following need to be checked carefully and nothing should be 'assumed' or you could be chasing your tail for a long time.
1) Float levels and idle mixture screw settings.
2) Power valves and their gaskets (concentrically centered). Any good performance shop will have a vacuum tester for power valves. If your idle vacuum is 12", 6.5" power valves are fine.
3) Accelerator pump linkage. ANY movement of the throttle should move the arm and result in a good, strong stream of fuel being discharged into the venturis. Holley has very good information on tuning their carburetors on the net. Search it out.
4) Accelerator pump cams. Use a setting that gives you a big shot of fuel early.
5) Squirters. 31s should be fine, but try 35s just to see the difference.
6) Large enough (height and diameter)and clean air filter.
7) Clean fuel filter.
8) Blow carburetor cleaner through all the little passages on the top of the main body and the same with all the passages in the metering blocks while you have the bowls off.

If you make changes and check things systematically, I have no doubt that you'll find the problem and once again be a happy camper...

 

[Ron816]

Did everything you just recommended, has very slight hesitation much improvement have 31 nozzle 74 jets primary,76 secondaries, 6.5 pv carb is brand new as well as filter lines etc total restoration . should I try changing accelerator pump cam position?

 

[coloradodave]

Did everything you just recommended, has very slight hesitation much improvement have 31 nozzle 74 jets primary,76 secondaries, 6.5 pv carb is brand new as well as filter lines etc total restoration . should I try changing accelerator pump cam position?

Yes, try changing the location of the attaching screw and then recheck that ANY movement of the throttle moves the accelerator pump arm. Holley also makes a kit that has several different pump cam profiles for tuning. You might also SLIGHTLY raise the float level on the primary side. If you're close to being perfect, open both idle mixture screws 1/2 turn and see how it reacts. Good luck...
PS: Where did the initial and total timing end up?

 

[Ron816]

Initial was 10 degrees ended up at 12 didn't think a couple degrees would make that much difference but it did I'll try cam position change tomorrow I'll get back to you on findings. When I first ran the carb out of the box it was running way too rich after jet change and float level adjustment it made a big difference.

 

[coloradodave]

Glad to hear it...

 

[440beep]

Here's the Holley accelerator pump cam kit
https://www.holley.com/products/fue...components/accelerator_pump_parts/parts/20-12

A diagram of the pump cam profiles
http://documents.holley.com/pumpcamgraph.pdf

 

[383man]

I run the Holley 850 DP also. I also used it when I had the mild 440 in my car and I remember I used 76 and 80 jets with a 5.5 P/V in the primary side only. I had also opened up the primary squirter to 34. It worked great like that as I could drive on the same plugs for years without fouling.
Your car sounds to me like it is rich as when the plugs get to fuel fouled it will cause the eng to miss and pop like you said as I have seen it many times on cars running to fat on the street. I also ran about 24 initial timing and 38 total in my old 440. It sounds like you just about have a good handle on it now. Let us know if you need anymore help.
I do still run the 850 DP on my 440/493 eng but I had to jet this eng up to 88's and 92's as this bad boy wants a ton of fuel compared to my old 440. Ron

 

[Ron816]

Thanks for the input I was going to try a 5 pv I think that might do the trick

 

[Ron816]

Update: went with 5.5 pv no change then I went with 9.5 recommended by another member after Adjusting curb idle everything seems to be ok test ride tomorrow weather permitting

 

[coloradodave]

Ron,
You're right that Holley recommends that you use power valves that are 1/2 the value of your idle vacuum, and, after all, they are the ones that designed and manufacture them. If you use higher than a 6.5" it may cure your tip in problem (that hasn't been my experience) at the expense of losing a fair amount of MPG. Every time you take off from a light, even a gradual take off, that higher value valve will open and dump fuel into the intake until the vacuum comes back up above the rated value. IMO, a better way to fine tune it is with the accelerator pump cams and squirters, which will still add fuel but at a way lesser amount than the power valves. Might even try adding another degree or two of advance.

 

[Ron816]

Ready for this one I took it for a ride today everything seemed okay until it got warmed up then started backfiring like a mother and got progressively worse to the point I barely got it back to the garage felt like it wasn't running on all cylinders after further inspection I put a timing light on each wire and found only 3 cylinders were firing could this be the ignition module