Backfire through carb among other issues
 

Good evening everyone looking for some advice,

Going through the process on getting my truck driveable while starting on my first engine build and running into some problems.

First the details of the truck its a 1968 long bed, 2wd, 3 speed w a 250 L6 and a rochester monojet carb/manual choke.

I have rebuilt the carb and set the float to what the manual calls for and set the idle mixture screw to 2 turns out from fully seated. I replaced the distributor cap, rotor button, plugs, plug wires, fuel pump, and all new intake/exhaust manifold gaskets. After resealing everything I have sprayed all around with brake clean and don't think I have anymore major vacuum leaks.

I set the timing with the vacuum advance plugged off to 4 degrees BTDC

I attempted to set the idle mixture screw with a vacuum gauge but when I turn it either direction it doesn't seem to change the vacuum a measurable amount so I've left it at 2 turns out from seated.

I am chasing several problems but one big one is when I start the truck and rev it up in neutral It backfires badly through the carb almost every time I rev it. This does not seem to happen with the choke pulled out about 50% or more.

My other problems include dropping way down below the normal idle speed after being revved and while driving it bogs way down and sometimes dies while I am braking (no vacuum on the brakes). Any advice is appreciated.

Re: Backfire through carb among other issues
 

Sounds like something isn't right with the carb. Maybe the mixture screw seats are damaged. How do the needles themselves look?

Re: Backfire through carb among other issues
 

Needle looks ok to me but outside of major damage I'm not sure I could tell.

Re: Backfire through carb among other issues
 

Check to see if the accelerator pump is spraying ..But it sounds like it is just running too lean

Re: Backfire through carb among other issues
 

It is spraying. I am confused about the backfire because I saw that = lean but the bogging after revving/braking points to flooding so it is confusing me.

Re: Backfire through carb among other issues
 

Timing will cause backfire, Did you remove & put the Dist in wrong ?, Plug wires not in the right order, Vac advance not working, Accelerator pump not pumping which would cause stumbling & backfire. Idle mixture adjustment not working means Vacuum leaks. spraying WD40 around carb base & any vacuum ports & Hoses, if RPM changes would be a problem. Hope this helps

Re: Backfire through carb among other issues
 

I didn’t remove the distributor and it wasn’t doing this before all the “maintenance” I’ll triple check the firing order. Sprayed carb clean all around and can’t find any vacuum leaks. Not sure how to check the vac advance. Accelerator pump seems to be pumping fine.

Re: Backfire through carb among other issues
 

Checking the Vac Advance, Pull the hose from carb, Should be connected to dist, Such the hose & see if the advance is working. The arm on the advance should move. Check hose for cracks. If you don't get any resistance when sucking bad diaphragm.

Re: Backfire through carb among other issues
 

I would double check the firing order. It's easy to get the wires crossed and will pop back through the carburetor like you described if they are crossed.

Re: Backfire through carb among other issues
 

1st question. How many NEW threads are you planning on starting for what may simply be a tune up?

Did you check the float? Stick it in a container of gasoline and see if it floats.
Don't over tighten the manifold bolts. If you have a small leak, you can usually fix by loosening any nuts/bolts that have been over tightened. Doesn't sound right, but try it.

#1 - Dwell. Point contacts must be clean and smooth or the feeler gauge will give you really bad dwell readings. Use small metal file or just replace the points.

#2 Timing, but with idle rpm @600.

#3 Mixture screw adjustment, but with idle rpm @600.

Any adjustment you do that increases idle above 600, reset idle to 600 and redo that adjustment.

Point adjustment with feeler gauge works just fine, you just have to be very careful till you get the knack.

Re: Backfire through carb among other issues
 

I am planning to do this this weekend. I have a dwell meter/tach in the mail on the way.

Re: Backfire through carb among other issues
 

Just checked, vac advance is bad there is no resistance. Got one ordered.

Re: Backfire through carb among other issues
 

This is usually caused by the mechanical advance weights not returning completely every time. To verify this install your timing light and while watching the movement of the timing mark Rev your engine. Vary how quickly you Rev the engine and down. The timing should always return to the same number at idle every time. If not the changes in timing at idle are causing the idle speed to vary.
This is usually caused by weak or broken springs, and/or rusty or worn weights that won't travel freely.

Good luck with your truck it sounds like you have several issues to locate and correct. Please keep us posted on your findings.

Re: Backfire through carb among other issues
 

Was chasing the EXACT same problem as you. Would backfire bad through carb and bogged real bad on acceleration. Turned out to be a bad coil. Good luck

Re: Backfire through carb among other issues
 

You can have Lean and Rich conditions at the same time...

Yes, backfire through the carb can be from a lean condition - especially since pulling the choke helps. Bogging down could be rich condition with throttle is closed...

Carbs have several circuits (or paths that the fuels travels within the carb) and each can give a lean or rich condition independently. Each circuit operates for specific conditions and can overlap. So, if you have a float set wrong causing a rich condition when throttle is closed and an accelerator pump not set correctly it could cause a lean condition when mashing down on the gas pedal.

Double check firing order and make sure distributor timing is correct to where the rotor is actually at the right spot inside the cap, pointing to the correct terminal at the correct time of spark.

You can set the engine by hand to 4-8 degrees BTDC on cyl 1, loosen the distributor, take the cap off, and see that the points are at the opening position (which causes the spark at the moment points open) and physically see the position of everything is in order. This is like setting the correct timing on a new engine before starting on new or rebuilt engine. Youtube vids may help? Point is, correct physical timing.

You may just be off a little bit somewhere with dwell, timing etc...

Once all that is fine, move on to carb and fine tuning last.

Bob

Re: Backfire through carb among other issues
 

I'll check that out thanks a ton.

Re: Backfire through carb among other issues
 

Makes sense. Plan to tackle it friday I appreciate all the info.