Ok, I just rebuilt a "CMJ" 350. That's a 1974 Caprice Police engine. It has stock 882 heads, stock bore, a Holley Street Dominator intake, Edelbrock #1406 carb (600 cfm), stock HEI dizzy with Blue Streak cap/rotor/coil, Long tube headers. Cam specs unknown, but 99.9% sure it's a stock grind. Also using #26 autolites for plugs and Accel 8.8 wires
I set the timing at 8* as per the service manual I have and let the motor come up to temp, then I went for a test drive. Off idle acceleration was incredible, ripped 'em loose just by stepping on the gas. Here's the issue though, at cruising speed the engine breaks up, and pops through the exhaust like crazy. Totally undrivable. I get it back to the garage, and turned the dizzy clockwise a little bit and took it out again. Long story short I ended up having to set the timing way past the mark at like 14* to eliminate the popping, but now there's absolutely no low end.
I've triple checked for true TDC which it is, tried various combos of moving the dizzy a tooth ahead/behind, ported/full vacuum advance with no improvement.
Are you doing this with the advance defeated and plugged? If not 8 degrees will be wayyy wrong.If that's what is happening, reset it for 8 with no advance.
Another thing to check is have the module tested, It may be breaking down under load..Have them heat it up good by testing it several times..and If you need a new one, have that tested BEFORE you leave the store. Do not forget the heat sink compound or you will be doing this again in short order.
Check the cap for Carbon trails from the carbon element to the towers...clean and dry any that you find. Check the element , be sure it isn't past runout and barely touching the Rotor pickup.
Check the rotor (NAPA just had a bad run of these) Use an ohm meter and put one probe on the rotor pick up tang, the other on the tower tang..move the rotor tang up and down and side to side , If the meter drops out at any point, it is bad in the center where they injection mold it..
Check the coil..it may be going south and acting up under load..put your ohm meter probes on the Batt and Tach terminals..it should read less than an ohm, but more than 000, If above or below that range toss it and get a new coil.
Next test the secondary, from the rotor pick up to the BATT terminal, It should read 6000 to 30,000 ohms..above or below that toss it..
This won't show internal arc~over from windings that have lost their insulating paint..you need an oscilloscope for that..So if you suspect it, replace it.
If all that's good, I'd start looking in the induction system or valve train.
Well, I replaced the coil, rotor, module today. I noticed excessive end play in the distributor shaft, so I threw another washer in it to tighten it up. Problem still persists.
I did notice that for it to even run, the rotor has to be way past the #1 tower on the cap, like a full tooth infact. I really hope the cam isn't off a tooth. If it was off by just one tooth, could it be that big a factor?
Well, I replaced the coil, rotor, module today. I noticed excessive end play in the distributor shaft, so I threw another washer in it to tighten it up. Problem still persists.
I did notice that for it to even run, the rotor has to be way past the #1 tower on the cap, like a full tooth infact. I really hope the cam isn't off a tooth. If it was off by just one tooth, could it be that big a factor?
What exactly do you mean by end play? Side to side or up and down?
Check your mechanical advance for stuck condition..
If it still persists do a static timing check..#1 TDC, Compression, BOTH valves Dead closed and timing marks lined up..If you can't coordinate these , you have mechanical problems (chain/ gears)
Next check the rotor For #1 Plug tower, if off, reset the dizzy and re-time..
If you have runout play on the dizzy up / down (not advance / retard) It is past serviceability..replace it .
Play in the distributor was up/down. I'll try replacing it with one my friend has laying around. Exactly how much up/down advance/retard play IS acceptable?
I'll also pull the valve cover and check to see if both valves are closed on TDC mark tomorrow.
Big thanks for all the help so far
You can buy a package of distributor shims to reduce the up-down movement. I use .015-.020 as a rule of thumb as acceptable. You should also consider if the block has been decked. It doesn't sound like that's your problem though.
Well I didn't get to replace the dizzy today, but I was messing around with it and inadvertantly left the vacuum advance unhooked and it ran better than ever. I could even set it at 8* with NO popping out the exhaust, but it still lacks power from a dead stop.
Also as far as I can tell both valves on #1 ARE closed at TDC with the exhaust opening about 1/4 crank turn.
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