wirenut

The problem engine is a SBC350. 2.02/1.60 heads, 206/206 425/425 cam, Holley 750 carb, MSD pro billet dist. w/6al and blaster 2 coil.
The engine idles fine. When accelerating it pulls hard and runs smooth up till about 3500rpm. At this point it begins to run rough, like its missing. Not real bad, it's not backfiring or making any loud noises. It just gets rough. It will continue to accelerate but with a definite loss of torque.
I'm at a loss. Any ideas?

ghostrider20

Oh' Yeah, bean here.

I had almost the exact same problem. My vette cut out arund 5500 RPM and acted as though a rev limiter kicked in. This happened at night on the way home. The next day I removed the fuel filter and took the car out for a drive. All was back to normal. So I ordered a fuel filter. Then about 5 days later at night again the problem was back. So it was not the fuel filter. I checked the coil, plugs, all good. Wires checked out good with the VOM etc. Then the next day I took the car out at dusk and made a run, all was good. Then back in the garage and packaged the shield back up and went out for another run, with the headlights on, problem back. I turned the electric fan and the heater fan on and the car would not pull more then 3500 RPM without cutting out. The ammeter was showing a decent charge at idle (which fooled me) and when really loaded up I noticed it falling off line around 2500 RPM and with all the electrical equipment on it was showing a 35 amper draw. Pulled the alternator off and had it re-built for $40.00. Problem gone.

This problem had bad fuel pump/filter written all over it. I removed the fuel filters and even entertained the idea of a cloged screan in the fuel tank. I un-hooked the rubber hose just aft the fuel tank and blew compressed air into the tank, as to back flush it. I then looked to see if setiment was floating in the tank. I also wanted to confirm the screen clog by temperarely blowing it open/clear to see if the car ran better. It did not and the tank looked clean and no crap floating.

Your problem may be:
-Fuel Filter (cheap item to eliminate).
-Fuel pump (should be around 6.5 - 7.0 psi).
-Clogged tank screen, (back flush with air and look for crap in tank).
-Coil, check Ohm between terminals and tower (coil specific).
-Check Dwel and points gap (Dwell 28-32*)
-Check Alternator :)

Mark

Lickitty

To do a quick check the wires , spritz the plug wires at night in the dark . If it looks like the 4th of July , Wires are Bad

GregP

Check the simple stuff first (filters, loose wires etc). If none of those, then pull the valve covers and look for a cam lobe going flat. I had one on a SBC and it will give those symptoms. With time the miss RPM keeps getting lower.

-Greg

kyosho

My car is doing the exact same thing. Pulls strong to around 3000-3500, then starts missing/sputtering/running rough.

I have a 327/350, stock except I put in a Pertronix unit. I am suspecting that might be the case.

I might go back to points. Anyone in the SE Michigan area willing to help out a Forum member in setting up his points correctly?

I have no way of knowing this is the problem though.

I will try to check the plug wires tonight see if that is the problem.

Vetterodder

How much total timing are you running?

wirenut

I'm running the dist. with 21 degrees of mechanical advance, and have the initial set at 12. However i've tried setting the initial everywhere from -4 to +12. Didn't help.

Hitch

This must be more common than it seems b/c in the last two weeks two of the four corvettes that I've looked at have had the same problem. One owner thought that it could be the plugs or wires. I couldn't tell if it could be the wires thru all the crud in the engine bay. The second claimed it never did that before... Needless to say I'm very curious to hear what you find out. Maybe the Duke knows???? :cheers: :confused:

SWCDuke

It's likely either an ignition or fuel system problem. Ain't I a genius! ;)

Truth of the matter, it could be just about anything. I'd start with a compression and/or leak down test. Then scope the ignition system and check the fuel pressure.

Duke

wirenut

I've checked the pick-up filter in the tank, looks clean. The in line filter is new. The charging system seems o.k. I've got 14 volts at the battery terminals at 1500rpm. I then started to play with the distributor. I pulled the cap and found the mag pick-up covered in rusty colored powder. So I replaced it. I've always suspected that I didnot have the timing set right. I went back and re-read SWCDUKES post about timing. (I got to admitt I might need a little remedial tutoring) I changed the bushing in the MSD dist. to give 25degrees of mechanical advance, (there is no vac can). I then set the initial timing to 10degrees btdc. Using a pair of dividers I measured out marks on the HB for 10,20,30,&40degrees. (I need to get up to the auto store and get a piece of tape) Running the rpm's up to the point where advance stops, looks like I got about 32degrees total. Took the wife out for a spin. It runs a lot better. Still a little rough above 4000, but improved from where it was. I'm still wondering about the timing. How much total advance should there be?

kyosho

Have you had any progress?

wirenut

I have also changed out the spark plug wires. (see my light show posting) That help some. But it still runs a little rough at higher rpm,s. Still trying to find out what total advance should be. Any Ideas?

Vetterodder

36º total is often recommended. 36º isn't necessarilly optimum for every engine but it's as good a place to start as any. Though you might be missing out on some power at 33º, I wouldn't think that it would be far enough off to be causing a miss. Marking your balancer for 36º is easy. Measure it's circumference, divide that figure by 10, and put a make that distance from the tdc mark.

wirenut

The saga continues, here's what has been found to date.

The engine had tapered seat plugs in it when I got it (champions). I crossed over to Autolites but stayed with tapered seats.
The heads require flat seat gasketed plugs.

The air cleaner on the car when I got it was cut up pretty bad. The bottom flange looked like it was hacked off with an axe. I thought it had been done to make it fit around the carb. Any way I replaced it.
I spent a lot of time trying to find an aircleaner to fit under the stock hood. With the highrise manifold I had 2 1/8" to play with. I thought I found a combo that worked. The base fit down around the carb, I used the shortest K&N element they have, and a slightly domed cover.
Well it turns out that when I put the cover on I was partly closing the choke plate. Then as the engine produced vacuum the cover would suck down closeing off the air supply further.

A local speed shop ran the distributor in a diagnostic machine that showed a smooth spark curve up to about 3500 rpm, then it starts to bounce around 10-15*. I contacted MSD who offered to rebuild the dist.

I ran the car with new plugs and no air cleaner and could not believe the difference. It's still a little rough, but its getting there. I'm hoping that a dist. rebuild will take care of it.

Now I also need to find an aircleaner that will fit under the hood. I really don't want to lose the highrise manifold, anyone have any ideas?

JohnZ

I'm confused - it takes flat-seat gasketed plugs and you have tapered-seat plugs in it?

Vetterodder

A worn timing chain will also cause ignition timing variations. If your intake manifold is too tall to allow proper clearance between the carb and the air cleaner lid, I don't think you have any choice but to replace it with a shorter one or get a taller hood. I really doubt if your combination would lose any power if you used a Performer or similar manifold. From what you'e described, it sounds the the high rise you are running is probably costing you serious power already. There are plenty of drop base cleaners that should work if combined with a lower intake and a shorter air filter like you already have. On my `65, a standard 3" element was too tall but, with a drop base cleaner, the 2 1/4" K&N didn't provide adequate clearance between the carb vents and the lid. My solution was to put a gasket made from 3/8" vacuum tubing under the filter. The end result was no clearance problems at the carb or at the hood.

wirenut

It now has flat seat gasket type plugs.