Background / Details:

• SBC
• edelbrock Performer intake
• Hedman headers
• Old style points distributor with coil (age unknown on both)
• Unknown parameters on plugs and wires
• 4 speed
• Engine seems a little tired but I've not run a compression test. It will come out in Spring for an overhaul.

Problem:
Car overall runs fine if I'm just driving around. Issue is when I really push it. About 3500 rpms, it starts to shudder. I've pushed through it to 4,000+ a few times but am afraid of breaking something. Engine and tranny both seems to vibrate a lot.

I don't think it is driveline related since it seems independent of speed and stops if I get off the throttle.

I've planned on buying a new HEI distributor for some time and finally butched up and bought a Performance Distributors DUI street/strip unit and a digital timing light.

Before installing the new dizzy, I wanted to see how the old one as set up. Looks like it had only 9 degrees of advance in it without vacuum line attached. I changed this to 12 (couldn't go more since the dizzy housing hit the coil bracket. Locked down the dizzy.

Confirmed the 12 degrees and then ran up the RPMs to where it quit advancing. 2500 or so. This showed 26 degrees all in.

At first I'm thinking... "That seems about right. 12+26=38 total advance. But then I read the instructions of the timing gun again. It said to subtract the initial timing from the all-in timing to find out your mechanical timing. So looks like I've got 14 degrees from mechanical for a total of 26. Everything I've read says I should be around 36 degrees.

Questions:

1. Could not having enough advance be causing the stumble / shudder at higher RPMs?

2. What is a good starting point for initial? 14-16?

3. I've not be able to put in the DUI unit yet. It is set up to add 22 degrees. I plan to start with 14 and see how it goes.

4. Any other thoughts on what could be causing this problem if the new ignition doesn't fix it?

****

Thanks everyone.

LarMan
Houston, TX

 

to start with since you have points, did you set the dwell to 30 degrees?
you have a digital timing light but is it a dial back timing light where you can set the total degrees you are trying to achieve?
you are aiming for a total mechanical advance of 36 degrees. To do that you need either a degreed balancer or the dial back light.

 

With any motor, it's test and tune. Try a few degrees advance and check for improvement. Usually timing set too far retarded will bog off the line or will be lazy. Your problem sounds more like a carb adjustment to the secondaries but try the timing test first.

 

I would start with a full tune-up before going much further. You stated that plugs and wires were unknown. I would freshen everything up, adjust appropriately, then start tweaking.

 

Thanks RedHook98.

Finally test drove the car tonight after putting in new plugs, new LiveWires and the DUI dizzy from Performance Distributors.

Warmed it up and then really wrung it out.

Night and day difference. I've got 12-13 degrees at idle and ~36 all in. Not sure what made the difference. Probably a combination of all 3 changes. The motor pulls HARD through 5,000 (and sounds really good doing it)! There is still a minor vibration but barely noticeable.

I also want to give major kudos to Performance Distributor. I bought a set of their under the header wires. All were fine but I could not route #5 the shorter way due to the wiper motor. Had to go left versus to right. As a result is the wire had to travel over the top of the cap since it was too short. It works fine but looks bad.

I emailed them, told them the problem and asked if I could buy one #5 wire but 6" longer. They replied they'd get one in the mail in few days. I offered again to buy it and they wouldn't take my money. New wire should arrive tomorrow. 2 thumbs up for old school customer service.