carburators

What is your fuel pressure set at? These are carbs do not run well with a fuel pressure above 2.5-3 psi. Your issue with a stumble after a high speed run could be related to that. How many turns out are your mixture screws set at?

also what brand electronic ignition are you running? 

^^^What Anthony said about fuel pressure^^^Also, backfiring means unburnt fuel is reaching the exhaust; the engine could be running rich (which is why Anthony asked how far out the mixture screws are) and need smaller idle fuel jets, which will improve mileage (itself a clue the engine may be a little rich).

When encountering a problem like this, start by making sure the timing and valves are right. So many people have chased their tails (myself included) thinking "I just did a tune-up a little while ago so that can't be it" when it turns out to be something basic. And the valves are set to .006" (six thousandths, not sixty thou), right?

Back to the jetting- Have you fine tuned the jetting, or have the carbs been installed and ran as received? When bought, even though the supplier may have jetted them for your combo, the carbs could still be running rich. The jetting Weber carb sets go out with (even when jetted for "your" engine) is usually rich; no supplier is going to take the chance of them running lean and destroying your engine, so usually some further tuning is required to get it spot on. It means buying a couple sets of smaller idle and main fuel jets and doing some experimenting. It's a bit of a trial and error process, going a step leaner, adjusting carbs and seeing how it runs, trying it again (and sometimes again) until you go too lean and have to go back a step.

ALB is right on track. Jetting is critical with the idle jets. That circuit operates from idle to mid range of the throttle. at cruising speed you should at the edge of the idle circuit. Therefore as you accelerate you transition into the main circuit. Every motor is different for jetting. The cam, heads, compression, exhaust all play into tuning the carbs.

email me and I can discuss it further.

I don't even need to reply to this. Thanks Al and Anthony. Only thing I have to add is timing at 28-30 degrees at 3000?

And do you have aluminum or steel pushrods? If aluminum, lash is fine. But if they're steel, you need to go to loose zero. 

Anthony,Alb,Danny,

I got some free time this afternoon so I took your collective advice and believe that I found the main problem. Even though I had adjusted the valves about 1000 miles ago I rechecked them and discovered that the nut had come off of the adjusting screw of the #2 exhaust valve and it was tight! I couldn't get  any feeler gauge in. I reset it  and used loctight. I thought that the screw would have backed out but it was tight. All the other valves checked out at .006-I made sure that every thing was tight. Danny-I have stock aluminum pushrods and the timing is set at 30 @3000rpm.

As soon as the engine warmed up I could tell that it is running better. Idle is the best ever and I can hear the "loping" of the mild 100 cam. I took the tub for a short drive and it ran smoothly up and down through the gears  with no surging ,hesitation, or roughness. It did backfire a little-I hope the exhaust valve didn't burn! I ran out of time but my next steps are to readjust the carbs and to check the stock fuel pump pressure.

Anthony-The electronic ignition was in the car when I bought it. When I went to the auto parts store to get a rotor and cap the very experienced counter man took one look and said this is GM and gave me a perfect match. Pertronix?

Thanks for your input and I'll let you know my future adventures and try not to bore you too much.

Joel

Glad to hear you found it, Joel! I'll say it again- when a car suddenly starts acting up and a quick check of what looks to be the likely suspects reveals diddley, start at the beginning, and this means making sure the basics haven't changed. Any one involved in this hobby (or any fool messing around with older cars) will confess to saying "well I just did a tune-up a short time ago so it can't be that" and it being "that" more than once or twice!

I wouldn't worry about that exhaust valve; usually it takes a while for it to burn, so unless the engine is showing some obvious sign, drive it. And we're all friends here, call me Al (come on, you know the tune!). As for not boring us; I have a sneaking suspicion Danny and Anthony are like me. We live for this stuff! Definitely let us know the results next time you do a little investigating....

Keep in mind that if this same valve / rocker combo does the same trick again, you might have a valve stem stretching or a valve seat pounding itself to death.....  Now you know why you check your valves EVERY oil change....

Great advice, Leon! 

Yup, good advice all. I told Joel he could do it! I guess that tight valve explains a few things.

My stock mechanical fuel pump is putting out 3.5 lbs pressure.Would decreasing this possibly  eliminate backfiring at highway speeds? All my idle screws are about one turn out.However turning #2 screw (not  #1 as I previously posted) changes nothing on the snail or the idle mixture. When I had the carbs apart the needle and seat appeared free and clean. CB Performance said they drilled the main jets 1.30 and the idle jets .55. My idles say .50 but they may have drilled them out?

The engine idles nicely {valves are noisy} has excellent pickup and cruises smoothly about  90 percent of the time. It doesn't smell rich-I just filled the tank so now I can check mpg.

Any  comments?

Thanks, Joel

The popping your describing, is it backfiring upon acceleration or when you let off the gas or is the a random pop here and there?

could be an air leak in the exhaust? Are you J tubes or heater boxes flanged? Or are you using the stock round donuts? 

Most of the time I fine a dirty idle circuit, remove the idle jets and use some carb or brake clean in the port then blow it out.

I do the basic checks, exhaust, idle jets/circuit and check your plugs.

i also had a car a while back that the miss when cruising was a bad connection on the wires....at 2400-3100 motor had enough vibration to cause amiss