Thought I had my recently rebuilt 427/390 running good and then it started stumbling on hard acceleration a few weeks ago...not a miss or backfire in the exhaust, but more like it is just cutting out at WOT and higher rpm...then comes back smooth when I back off. I would think if it is erratic firing, caused by intermittent spark, I would have unburnt fuel in the exhaust when it fires and know it.

Everything in the ignition system has been rebuilt or is almost new, so I hate to just start replacing parts. Timing is at 15 initial, 34 total. New B26 vacuum can has a brass bushing and seems to be free and working, running on a ported vacuum source, nothing at idle and comes in as the throttle is opened. I have the original springs back in the centrifugal advance, nothing at idle and all in by 3000 rpm.

To me it is acting like fuel starvation, but I've never had a problem there. It has a new fuel pump, front hoses, filter and carb. The accelerator pump squirts fine and the car starts right up after sitting for weeks. The car still has the original tank, pickup sock, rear hoses and metal fuel lines.

Should I continue looking at ignition (cap, rotor, points, condenser, wires, plugs)...or start checking fuel flow, float level, then pressure and suction?

 

Try this - from a stop accelerate to red line in 1st then 2nd 3rd etc and note what the car does, mine was fine up to redline in 1st and 2nd and then started hesitating/holding back in 3rd thru 5th, there was silicon stuck in the carb, it had enough fuel for the first 2 gears then couldn't keep up.
See if your car does this which could indicate a fuel issue.

Good luck.

 

Not so long ago my 454 was doing the exact same thing. I removed the filter from my carb (Q-jet) and found it to be almost completely plugged. I installed an in-line metal filter in the flexible line between the tank and the frame rail and the problem was solved. Atleast it's solved until the new filter plugs up.

 

With it running, spray the plug wires with a water mist. My BB gets a little stoopid with a bad plug wire. And #1 and #3 wires ( the only ones that ever have any problem) are really close to the headers. Your mileage may vary.

 

If the sock is original, I would look at that, you can see it with a mirror into the tank, original is black

 

Mine does it before 4000 in 1st gear. Forget 2nd. Where did you pick up silicone?

 

I don't have a filter in the Q-Jet inlet. I have a new one of those GF-432 expensive metal filters between the pump and the carb. Could be plugged already, but I've never seen any dirt in the tank. May disconnect the fuel line to the carb, attach a hose and pump some fuel into a can. Otherwise I've got a combo vacuum/pressure gauge, so I might as well check pressure there.

 

Stock manifolds and fairly new repro shielded wires. I'll try your suggestion in the morning. Car idles great and comes on strong, at least at lower rpm.

 

The sock is black and I'm sure it is original. Tank is 3/4 full right now and I'm not crazy about changing the sock. Was hoping to get some more driving done before pulling the tank and finishing my underside restoration from the trailing arms back.

 

I had something similar earlier this year. Ran fine at low speed and when I got up to speed it acted like it was starved for fuel --- It was the float.

 

With the carb having been recently rebuilt, you might want to take a look at the accelerator pump. Lars recently did some investigating into a problem with brand new accelerator pumps failing.

Here's a link - http://forums.corvetteforum.com/c3-t...t-quality.html

 

The carb is actually a new GM service replacement that I bought in 2005. It has a 1982 date code and was in the original box and plastic tray with yellowed shrink wrap intact. It ran fine for 4 years and then I had the engine rebuilt. Took the opportunity to have the local vette shop put a new accelerator pump in it. I'm going to check with him today to see where he got the pump. Sounds like I am going to order a super pump from Cliff R and put it in. Good time to check my float level. Thanks for the link and Lars is the man.

 

Try it without the gas cap - cap not venting acts like plugged filter.

 

Mine did the same thing. Had the points taken out and a set of pointronix(spelling?)
installed, never had any trouble since.

 

Took the ignition shield box off and checked out the distributor inside. Everything looked fine. Went for a test drive on the expressway before heading to the shop and guess what? Car ran strong at WOT all the way to redline in 1st thru 3rd and continued pulling in 4th without a hiccup. Until I figure out why that stupid shield box is evidently screwing up the spark, it is going to stay on the shelf. Thanks for all your suggestions.

 

That's an easy one- the plug wires are so close to the shield they're arcing to it. The water bottle will prove it. Could also be the cap, but my bet is on the wires. The coil wire usually rubs on the bottom of the shield too. It's taller than the rest of the wires.

And electricity is generally lazy- it'll take the easy way to ground.

 

The previous owner must have had a field day with the stuff, found it clogged up in the carb filters, regulator and fuel pump, all good now

 

Just wanted to give you an update on my problem. Rather than going thru the fuel system, I decided to go back to ignition. The car continues to cut out at higher rpms in 1st and 2nd, somewhat in 3rd but not in 4th, even up to 110, but the car just didn't feel that it was pulling as hard as before. Measured 11.33 volts at coil positive at idle. Decided to try a jumper wire from the 14.66 volts at the horn relay and put full system voltage on the coil. Went out for a spin and lit 'em up thru all 4 gears without missing a beat.

A little history. After rebuild last year, decided to replace an incorrect coil that was too large for the bracket and a little tall for the ignition shielding. Went with a new correct GM Restoration Parts coil #1115287, undoubtedly made in China. Next step is to try replacing this coil with a known good and see if I get similar performance with normal coil positive voltage thru the resistor wire. My feeling is there is something going on between the primary and secondary windings in this repop coil. I just hate it when new parts go slowly or intermittently bad...really screws up the troubleshooting and causes a lot of aggravation.

 

Think I may have found the problem. The coil primary wires go thru a grommet in the side of the ignition shielding. The grommet was old and hardened...but found it had a split in the groove, allowing the wires to come in contact with the shield. I had noticed the cloth covering over the resistor wire was missing in that area but just assumed it was due to excessive handling. My messing around and running full voltage to the coil evidently burned the Blue Streak points and probably the condensor. Burned another set of NAPA/Echlin points messing around. With a new set of Standard points, a new NAPA/Echlin condensor and a new grommet, the car seems fine with the repro coil and all shielding in place. Static voltage readings (just learned from 69427 that it is the only way to measure without an oscilloscope) at coil positive seem normal with points both open and closed, so the resistor wire was evidently not damaged. Time will tell. I just thought I would post this little tidbit of information, embarrasing as it may be, for any future ignition vs fuel troubleshooters.

 

Glad to hear that you've gotten to the bottom of the problem.