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1957 Chevy Bel Air Convertible; ZZ4 w HEI ign; 700r4; 9" rear w/3:50 gears; pwr disc fr w drum rear
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Discussion Starter · #1 ·
I drove car extensively today after upgrading ignition system.....good performance except for throttle at low RPMs. There is no problem "off the line" it is a problem with moderate-heavy acceleration at lower RPMs...I feel a MISS. For example at 60 mph, (700r4 cruises hwy at low RPM) if I put my foot into it, and speed is too high for kick down into 3rd so that engine has a lower RPM load....I feel a miss....no pinging, but a jittery miss.

327/275hp, 700r4, MSD distributor w SS Blaster electronic coil, timed to 10* non-vac baseline + 25* total mechanical advance. (+ vacuum advance) with new MSD super conductor plug wires....carb is QJ with new good quality acc pump. Plugs are gapped to .035 and are fairly new.

performance from dead stop with WOT is awsome.....roars with no MISS...it's just in the low RPM load zone. I just suspect it's a timing issue....but not sure.

Any info would help.
Thanks,
Ace
 

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I think you're right on the timing, it probably needs more if it's not pinging. Lighter springs in the mechanical advance, maybe a different can for the vacuum. Crane sells an adjustable vacuum can if you can find one.

But the first thing to try is more initial timing because that's easy and it costs nothing for parts. The stock initial timing spec was based on typical gasolines of the 50s. Even today's 87 octane unleaded may let you run more timing and get the performance you need for this situation.
 

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1956 chevy 210 del rey sedan
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with the new ignition it may be a little lean under light throttle medium load . might consider a jet change . or possible a colder plug
 

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The plug heat range won't affect light load response, unless the plug is too cold and it's loading up. If anything you'd want hotter for a light load problem.

Whether it's a fueling deal or not and what the fix is would depend on whether it's a transition miss or a steady miss. Transition would be affected by the accelerator pump. Steady would be your main jets, or if low enough rpm, the idle transition circuit.

Just follow Rick_L's first law of carburetors: Look at the ignition first, either the general condition of the ignition system (think plug wires, rotor, and cap), or timing.
 

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Premium Member
1957 Chevy Bel Air Convertible; ZZ4 w HEI ign; 700r4; 9" rear w/3:50 gears; pwr disc fr w drum rear
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1,996 Posts
Discussion Starter · #5 ·
Well, I will the the cheapest method first.....I'll increase timing. (I'm having difficulty getting my idle low enough at 10* initial timing though.)
Plugs are AC Delco R45S. I checked plugs today for gap, and found that they were light brown for burn quality. BUT 10 + 25 = 35*.....how much higher can I go?

Hard to imagine its carburetion.....the car romps at WOT....performs pretty well all-around....and has new acc pump (quality unit with spring collar)

Prior to ignition install, car pinged, but didn't "miss"....
 

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Premium Member
1957 Chevy Bel Air Convertible; ZZ4 w HEI ign; 700r4; 9" rear w/3:50 gears; pwr disc fr w drum rear
Joined
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1,996 Posts
Discussion Starter · #6 ·
So I increased initial timing from 10* to 14* (vacuum hose off & plugged)
I have a 25* limit bushing in the dizzy.

The missing at low RPM load seems to be worse (sputtery & feels like 6 cylinders, but it doesn't ping...at least i can't hear it from inside the car)

I had already installed 2 medium weight springs for mech advance (MSD gives you 2 sets of light, medium, and heavy) which should achieve total advance at approx 3300 RPM.

Logic says I need to go back the other way and lower initial back to 8* or even 6* and see how that works. (The vac adv gives a big timing boost at idle....) with initial at 8 and mech limit of 25 that would be total 33.
 

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Premium Member
1957 Chevy Bel Air Convertible; ZZ4 w HEI ign; 700r4; 9" rear w/3:50 gears; pwr disc fr w drum rear
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1,996 Posts
Discussion Starter · #7 ·
Update.....I took my car here last Friday since the "missing" condition seemed to be getting worse, and I started to get worried it could be a fuel pump or something....http://www.superiorautomotive.com/index.html
(I saw their shop in another thread, and they were very close to my office.)

They put the car on the dyno and found out it was running incredibly rich through primary jet range.....When they opened the carb, it turns out one of the primary metering rods was bent at tip (which I'm sure I did when I replaced the accelerator pump a few weeks ago.) But they also rejetted primaries with leaner jets/rods.

Distributor set up was fine....springs and timing limit bushings were left as I had adjusted....but they did replace the #1 spark plug saying it was was fouled or maybe cracked.

Car seems to be running fine....($340 bucks later, but I think it's fair for the "go through" and rejet, etc...) hopefully, that's the end of that, and onto the next project (rain gear wipers...which will probably be the worst project of all since I have A/C in already)

Ace
 
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