1968 Chevy C10--new carb problems

[1968]

So I swapped out engines in my 68 because of a main cap being busted and I'm having problems with the new engine. I bought a 350 engine off a buddy (numbers say it came from '77) and replaced the stock intake with a edelbrock intake and brand new edelbrock 1406 carb. It starts and idles fine but its backfiring out of the carb when goosed and seems like it lacks power. I put a new timing chain on and lined everything up correctly inside and got the dizzy hitting number one when i put it in. Problem is I dont have a timing tab on it(chrome cover didn't come with a tab), but I checked the tdc with the piston and marked where it was on the chrome cover. I set it at zero btdc with the vacuum advance unplugged. Is that right? Could being off a few degrees cause it to backfire? Should I be using full advance or timed advace? what else could be causing it? any help would be greatly appereciated. thanks

[68Gold/white]

Sounds like a hopeful minor problem....
Make sure all the plug wires are where they need to be, on the cap, and engine...

As a rule of thumb, an engine that accelerates sluggish, needs more advance. Get it running, try moving the dist around, see what happens. Be very careful, since this thing's in the backfiring mood...

Look at your old cover, compare it to the new chrome cover. try to make a mark somewhere w/ a sharpie, to give you at least a close estimate on timing.

Yes, check timing w/ vac advance disconnected. I'm guessing your combo is close to stock. The timing should be around 8-12 degrees BTDC idling.

Take the carb off, make sure the carb base gasket you are using, isn't leaking vacuum.
There should be an imprint on the top and bottom of the gasket, where it's making contact, it will tell the story, good, or bad.

Plug off the PCV valve, see if that helps, I used a cheapo valve here recently, it caused problems..wanna guess where it was made??? (not USA)

The backfire could be from a minor vac leak, check the carb, as stated earlier. New intake gaskets seldom leak. make sure the bolts are tight. 30 ft/lb is all they need. I never use a torque wrench, make sure they are tight within reason.

When you get all this under control, check the intake bolts for tightness, when the engine is completely cooled down (as in after it sets over night). The intake bolts need to be rechecked for tighteness. 2-3 times, until you recheck them ,and they don't loosen up any more (all this has to do w/ aluminum expanding when hot, more than iron)

[1968]

Originally Posted by 68Gold/white

Sounds like a hopeful minor problem....
Make sure all the plug wires are where they need to be, on the cap, and engine...

As a rule of thumb, an engine that accelerates sluggish, needs more advance. Get it running, try moving the dist around, see what happens. Be very careful, since this thing's in the backfiring mood...

Look at your old cover, compare it to the new chrome cover. try to make a mark somewhere w/ a sharpie, to give you at least a close estimate on timing.

Yes, check timing w/ vac advance disconnected. I'm guessing your combo is close to stock. The timing should be around 8-12 degrees BTDC idling.

Take the carb off, make sure the carb base gasket you are using, isn't leaking vacuum.
There should be an imprint on the top and bottom of the gasket, where it's making contact, it will tell the story, good, or bad.

Plug off the PCV valve, see if that helps, I used a cheapo valve here recently, it caused problems..wanna guess where it was made??? (not USA)

The backfire could be from a minor vac leak, check the carb, as stated earlier. New intake gaskets seldom leak. make sure the bolts are tight. 30 ft/lb is all they need. I never use a torque wrench, make sure they are tight within reason.

When you get all this under control, check the intake bolts for tightness, when the engine is completely cooled down (as in after it sets over night). The intake bolts need to be rechecked for tighteness. 2-3 times, until you recheck them ,and they don't loosen up any more (all this has to do w/ aluminum expanding when hot, more than iron)

Im thinking timing may be my problem, I set it at 0 degrees btdc instead of 8-12s degrees btdc. Would that cause backfiring under load? It idles fine and revs fine also unless you repeatedly rev it, then it backfires. Also, there are two ports for vacuum advance, one is for timed and the other is full. I am using the full for my dist. vacuum, and I have my transmission vacuum line hooked up to the timed. Is that acceptable or should I route the trans vacuum back into the manifold where it was stock and plug the timed vacuum? The carb and gasket are both new so I doubt they are leaking, I will check them though cause you never know. I'll also try the pvc valve just in case but it was my old one so i doubt it.

[oldblue1968chevy]

Ive always heard/read vad adv to full vacumn (driver side port)

[Wrenchbender Ret]

The timing is most defenitly the problem. Like the guys say you need to start out with about 8-12 degrees. Usually the advance works best on full manifold. Sometimes especialy with an auto. trans. the ported side might work better. It takes a little experamenting. The trans must be hooked to manifold vacuum. The port in the manifold behind the carb is the best place.

[1968]

Thanks for the replies guys, Im going to try and set the timing to 10 degrees btdc to see if that stops the backfiring and i'll reroute the trans vac into the manifold. I'll let you know what I come up with...hopefully it will fix the problem lol

[68Gold/white]

Originally Posted by 1968

Thanks for the replies guys, Im going to try and set the timing to 10 degrees btdc to see if that stops the backfiring and i'll reroute the trans vac into the manifold. I'll let you know what I come up with...hopefully it will fix the problem lol

Generally the vac advance needs to be hooked to a ported source. It takes all that extra advance out at idle, it's not needed there, not trying to start an arguement, just my experience.

Check all the little details, we've all been bit at least once...

I've seen carb base gaskets that just barely made a seal, when installed, and a little heat and time made them leak...When you start using aftermarket parts, there is no written book to guide you, just your skills and emails...