Help! Engine troubles

[Sloan66]

I would appreciate any advise.
I have a 307 2 bl Truck runs good at speed, but at slow speeds "bucks" under accelration. I have checked all fuel connections, changed filter, cleaned carb 3x, changed accelerator pump, verified timing, adjusted air/fuel mixture, pulled plugs. It seems to be a fuel issue as vehicle idles fine, sustains speed, the bucking only occurs under accelration , most noticable in 2nd and 3rd gear. Could it be bad fuel pump?

[East Tex]

Check your plug wires too. When they're breaking down, it usually shows up under a load, like acceleration.

[Lee H]

Please, don't take offence, maybe your trying to accelerate at to low an RPM in to high a gear? Points or HEI? How long since a good old fashioned tune-up? Before going to the carb again I would make sure your ignition system is operating properly. New points/condenser, check OHMs of each wire and replace any that are not within spec. Make sure number 5 & 7 wire are not touching each other, they are next to each other in the firing order and can cross fire causing missing under power. Worn out plugs/cracked porcelain? Plug gap? Most of the old timers are going to say 90% of perceived carb problems are ignition problems.

[Sloan66]

No offense taken, I have HEI, plug gap was good, I will have to check 5 & 7. It appears to be a load issue, from the light, first gear not surge, second I start to see the buck, third the same until speed met and sustained. Comes back on any type of grade I go up

[Hugh Mongus]

Are you sure you dont have any trash in your tank clogging up the lines? Are you getting a good solid stream of fuel at the fuel pump?

[19673ontree]

These trucks get water in the tank from sitting mine acts just like you desribe after sitting a while I think the tank sweats I put a bottle of fuel treatment in it and drive it 5 miles and the bucking stops till next time ,,,,,

[knomadd]

I had a different problem, but still similar on mine. Turned out I was too lean on my air/fuel mixture. When you checked your plugs, were they the proper tan color? If they are white, you're too lean. Black and it's too rich.

When you adjusted your mixture screws how did you do it? If you have a vacuum gauge, hook it up to manifold vacuum and adjust your idle mixture screws until your vacuum is at the highest level. Give the gauge a few seconds to read the adjustment. Once you're satisfied with the adjustment, drive it again and see if it fixes your issue.

[Hugh Mongus]

Air/fuel mixture screws have nothing to do with driving. They only affect the idle mixture.

[knomadd]

Originally Posted by Hugh Mongus

Air/fuel mixture screws have nothing to do with driving. They only affect the idle mixture.

So then explain to me how adjusting the idle mixture screws will make an engine run lean or rich, if they aren't adjusted properly? I had a lean condition and did nothing but tune it with the vacuum gauge and adjusting the idle mixture screws and the problem was solved. I'll agree that a no load situation won't feel different, but you can sure tell under load of your lean or fat on your mixture.

I apologize if I don't understand the problem. I was simply trying to give a suggestion based on personal experience.

[Old Truck Man]

Originally Posted by knomadd

So then explain to me how adjusting the idle mixture screws will make an engine run lean or rich, if they aren't adjusted properly? I had a lean condition and did nothing but tune it with the vacuum gauge and adjusting the idle mixture screws and the problem was solved. I'll agree that a no load situation won't feel different, but you can sure tell under load of your lean or fat on your mixture.

I apologize if I don't understand the problem. I was simply trying to give a suggestion based on personal experience.

Im here to tell you that you or anyone else never solved a lean or rich problem by adjusting those idle screws. The idle adjusting screws are just that. The only affect idle mixture. they have no bearing on fuel mixture except when in the idle circuit. That's why the end carbs on tri power engines don't have a idle circuit. The lean or rich fuel mixtures are depending on the carb controlled by the jets , metering rods, & power valve. Ive worked on and tuned carbs of every kind for more than 40 years. and lots of it was in high horse power drag race & stock cars. I remember back in the day when almost every car that was stick shift when they shifted during normal driving from first to second and second to third would blow a puff of black smoke out the tail pipe. Everone wanted to see chalked tail pipes. they really actuallythought chalked pipes where a indication of good tune & a sound engine. When in fact all the chalked tail pipes where was the coating of lead residue from the leaded gas!

[Sloan66]

Plugs were tan, used vacuum guage to adjust. I have been driving truck daily since November. Was running great, had an oil leak on back of manifold two weeks ago, pulled manifold and resealed, leak fixed then I developed the current issue. When I first got the truck running in Nov. it did the same thing, ended up being a loose connection at tank and it was sucking air. I checked that first. Tank was cleaned before I put it back on road. I run a filter at the tank and the filter in the carb, both changed last week when I changed oil.

[geezer#99]

What exactly is your timing set at?
What is your plug gap?

[Old Truck Man]

Running out of ignition acts just the same as fuel. You stated you have HEI ignition. Did you replace the ignition wire to the unit with a solid copper wire? the original lead to the coil on chev trucks is a resistor wire. I keep a known good two bbl carb handy. with a variety of adapters I can install it on almost any engine. If I suspect a bad carb I swap it on and test that way. Is your vacuum advance working. Is your centrifical advance working. Did you cause a intake manifold leak when you had it apart?

[Sloan66]

Timing is 8 degrees, plug gap is .045
Wire harness has been updated and resistor wire replaced.
Not sure if I have manifold leak will check, but truck idles too smooth for me to suspect a vacuum leak.

[Sloan66]

Sorry 8 degree BTDC

[Old Truck Man]

Originally Posted by Sloan66

Sorry 8 degree BTDC

Keep hammering away at it. there isn't a piece of cast iron in the world that can fool you unless you let it. Ok a good idle doesn't necessarily rule out a vacuum leak. Chev V8,s Before 1963 didn't have a PCV. A PCV is a vacuum leak and the engine still idles fine? Why? because the air idle circuit is adjusted to compensate. Prior to 1963 the two screws on the carb where screwed out about 2 and 1/2 turns. after the PCV it became about 1&1/4. you may have adjusted your carb to compensate.? My son has a 72 Pk. it has a 305 2 bbl carb engine from a 85 monti carlo. it acted much the same as yours. he changed carbs, changed ignition. It had a aluminum intake. I suggested he replace it with Known to be good an cast iron intake and 2 bbl carb from a 66 283. It fixed it. The intake manifold was bad had a hairline crack. Probably from someone lifting the engine from the carb base bolts.

[70SBC10BOB]

Do you have vacuum advance? If so, disconnect the line, plug it and go drive. The diaphragm may have a hole and makes the timing jump back and forth under accel.

[geezer#99]

Originally Posted by Sloan66

Sorry 8 degree BTDC

Bump your initial up to 14*.

[knomadd]

Ok then. No need for violence So maybe my issue wasn't what I thought it was. I do know that I haven't had the issue since adjusting those screws (it's an edelbrock so maybe it's different??).

[litew8]

edited: didn't see you fixed it. nevermind

[Old Truck Man]

Originally Posted by litew8

edited: didn't see you fixed it. nevermind

Sloan 66 is the original poster. As far as I know he hasn't solved his trouble. I dunno sometimes I can be a tad abrasive.

[litew8]

Thanks for the correction, I thought it was knomadd. Before I deleted above, I had suggested checking the carb gasket since it sounded like it was torn down and reassembled - then the trouble started. Sometimes, you can isolate the trouble by backtracking and checking things that were last messed with.

[71K20chevy]

I'm putting my money on leaking intake gasket.

[Sloan66]

Lots of good advice. Any sure test on checking for a bad intake gasket?

[71K20chevy]

Typically what you do for that is with the engine idling, spray around the intake gaskets with starting fluid, and if the idle picks up, you know you have a leak there.