Carb tuning.......

[Monte72Carlo]

Alright, I need some help from you guys. First off engine/ drive train specs:

 

Engine:

355

Vortec heads/eddy rpm air gap intake. Heads decked intake decked to match.

Melling 0.501 intake 0.498 exhaust, I'm fairly sure. I could go check if needed.

block is decked (pistons 0.005 below)

flat top pistons

64cc combustion chamber (somewhere around 10-10.5:1 comp)

stock head gasket thickness Felpro (0.042?)

timing- 10-11 degrees initial, 38-40 degrees total all in by 2500 if I remember correct I don't have a timing light to go check at the moment.

MSD ignition box digital 6 plus (no start retard engaged)

MSD e-curve distributor

 

Carb: Pro form/ Holley 750 mechanical secondary. Power valves unsure, jets 70 primary, 76 secondary

 

Drive train:

 

stock converter, stock 10 bolt for now, th350

 

ISSUES:

 

1. Poor starting but I figure that's due to the mechanical no choke carb. Thoughts? It rolls over for a good 5 seconds before it catches if lucky, takes longer sometimes. Gets a little better when warm.

 

2. Running very rich at idle, not too sure at part/full throttle.

 

3. Hesitation off idle (fairly bad) Power valve giving too much fuel and killing the engine???

 

4. Performance loss?? feels like my old tired stock 350 had more "seat of the pants" feel to it.

 

5. Runs fairly hot 200 range but not really an issue. Stock rad/ higher performance engine what can you expect. I was seeing 220 in 100+ degree weather during the western meet.

 

6. Diesels on shut down, but not on 94 octane, not MSD box related but engine temp/octane related. Once carb is tuned should go away.

 

7. High idle (1200) and drops to (500-600) in drive and wants to die out. Power valve issue? (drowning the engine in fuel?)

 

Thanks for any help.

 

Thanks,

Garrett.

[Winston Wolf]

Here is what I would do.

 

With no choke, the car will start a bit harder. Give it a couple pumps of the pedal before you start it cold, and you may have to hold the gas a bit to keep it running until it comes up to temp.

 

To test the power valve, when the engine is running, turn the idle mixture screw in all the way. If the engine dies, the power valve isn't blown. If it stays running, that's one of your problems.

 

 

I would set the E curve so both switches are on 1. This will give you a K1 curve. 15 deg mechanical advance, all in by 2500 with a 10 degree vacuum advance that starts to come off at 10" and is all off at 4" of vac. This should be close enough to make the car run well. Set the total timing with the vacuum advance unhooked at 35 degrees. At 1000 rpm idle with the vacuum hooked up at those settings it should read about 30 btdc.

 

Set your idle mixture screws with a vacuum gauge tee'd into the vac advance line off the carburator (make sure its on manifold vac and not ported) while it's in gear.

 

A hesitation off idle can be a rich bog or a lean stumble/pop. Are you sure it's idling rich, and if so how can you tell? Just because it smells like gas, doesn't mean it's rich.

 

Good luck.

 

[Winston Wolf]

A couple other thoughts. Don't worry about the initial timing. Set it by total and it is what it is.

 

The advanced timing at idle will help it run cooler.

 

Usually you want the primary jets about 10 sizes smaller than the secondary jets. If I'm wrong on this, someone correct me.

 

Holley changed the carbs in the early 90's to integrate a check valve that prevents power valve blowouts due to an engine backfire. If your carb is older than that, you may want to install the kit.

 

When you get the idle circuit tuned correctly and have the timing set to optimal, it will not pull the engine down more than 2-300 RPM's. Since yours does, I can pretty much promise you it's not set right.

 

Also if your float level is too high you can get fuel that drips out the boosters, messing things up. May want to check that too right away.

[Monte72Carlo]

Thanks for the good information.

 

The float level is adjusted correctly as far as I know, it was set by my boss who is big into racing.

 

My carb is maybe 2 years old, not too sure of this but it is definatly not old enough to need the kit. BUT my carb during initil timing setup was definatly backfiring and throwing flames out, multple times. Could this have caused a faulty power valve? The valve is brand new as the carb came with out one and is new to this engine. I will try the power valve test though.

 

I have heard as well that the step up from primaries is 8-10 sizes I beleive you are correct on that one. Mine is a bit less as I had surging under light acceleration. I was recomended to start with 68 primaries and step up from there if I had surging under cruise with light throttle.

 

I will check manifold vaccum soon as I buy a gauge.

 

I have tried peddling the throttle during start up but have seemed to get the best results with out.

 

Thanks!

Garrett.

[monte70car]

I know the E-curve book says to leave the dials set at 0 and set base timing, once base timing is set then you pick a timing setting from on of the chats. I want to say the dials on my e-curve are dial one is set on 1 and dial two is set on 6. My used to came on when shutting it down found if you turn it off while in gear it wouldn't carry on, once we redid the timing has it was set to high we are able to put it in park and turn it off I only run 87 octane in my big block. Most of the the high temp is in the timing 36° all in by 3500rpm should be ideal.

One thing I did this year with my e-curve I open the gap on the plugs up to .045 and get a tan plug .035 was a little on the black side, so check you plugs on the color.

 

Running very rich at idle= reset the mixture screws about 1.5 turns out for a start. I can check a 750 vacuum secondary I have sitting on the work table and maybe it can give you a base.

 

Hesitation off idle= not sure which pump cam you have but the pink color one is what I have on all three of my 750's. Also check the clearance of the pump arm should be .010 of a feeler gauge. Could also go up on the squirt size must lucky it has a #35 squirt.

 

Performance loss= to much timing and fuel

 

High idle= try turning the idle screw out. My idles right around 1000 in park and 900 in od.

 

 

[wallaby]

Boy, I dunno....it sounds as if you have the symptoms of a vacuum leak.

[Monte72Carlo]

hmmm, I guess I have some work ahead of me. I will try some of these suggestions this weekend and see what happens.

 

Thanks,

Garrett.