Awhile ago I posted about my carb backfiring...well I think I have it narrowed down now, but of course I still need help. I got the timing all set and vacuum leaks checked, but the carb still backfires at certain times. It's fine when just driving around lightly. It usually does it when I mash down on the throttle. So I think it could either be a bad accelerator pump or out of whack metering jets...does this sound right? I'm just starting to get a hold of how carbs work, I'm 20 and I've had all EFI until now. It's an Edelbrock 600 or 650 carb I believe.
Thanks,
Sam

 

The engine is what is backfire-ing and not the carb. The engine can backfire thru the carb or thru the exhaust/muffler. I have found that most backfire is caused by an ignition problem. Check the fireing order to make sure a plug wire is not reversed. Make sure plug wires are good. Look inside the distributer and at cap and rotor. Check for cracks or carbon tracking. Also check to see that harmonic balancer has not slipped and that timing is corrrect. These are things to check if you have not done so already.

 

Great advice, I totally agree it is probably igniton parts/timing related. I would be courious to verify fuel flow. You could do that by removing the fuel line after the pump and verify its ability to fill a container in short time.

 

Yeah, I haven't changed the fuel filter and it looks pretty damn old. Maybe I should start there and work my way around the plugs and the entire ignition system.

 

Alright, changed the fuel filter...still doing it. Next nice day I'm going to pull the cap off and check it out. I really hope it's not a bad valve.

 

It is probbably just your timing like maniac said. Do a good tune-up.

 

Sounds like a bad ignition condenser to me. You can get either separate points/condenser for this engine, or what Delco used to call a UniSet which makes installation way easy. Blue Streak brand is probably your best bet these days, unless you can find some pre-90s Delco Remy old stock.

Or do you have HEI?

 

Yes, there is an HEI unit. And the timing is set as correctly as I could get it as the timing marks are no where to be seen so I used a vacuum gauge to set it.

 

Do you mean the timing mark on the balancer is gone or the balancer itself is gone? Or, do you mean the timing tab is missing? That would be an easy fix, just replace it. It the balancer is a the issue is a more serious problem though.

 

On the balancer. I think I've narrowed it down to the carb though. The other day on a straightaway I floored it and it backfired through the carb as usual, but this time I didn't back off the throttle like I usually do. After the backfire the secondaries worked like a charm and that son of a B took off. It's just a backfire right when the secondaries kick on. So I think it's probably the secondary metering rods.

Any thoughts???

 

Its worth a try, but take the top of the carb off and see for yourself how much dirt is sitting in there.. Edelbrocks are easy to take apart.. 10 or so screws and about 3 clips.

 

My thoughts are...

 

I will take apart the carb, maybe today or tomorrow. I still need to take the cap off too. So little time, so much work.

 

I still suspect a spark problem...since it has HEI are you sure the distributer has a full 12 volts to it? To convert a points dist to an HEI you cannot simply use the resistor wire from ignition switch to the coil and connect it to the HEI. You should also check the carbon button inside the distributer cap.

 

Is there a way to test if it's getting a full twelve volts? I just took the cap off and it looks brand new under there, same with the wires.

 

I'll need to bust out the camera tomorrow and start taking pictures, I'm not good at describing.

 

The secondarys....Thats a good guess, wish I had more experience with your carb but I don't. This sounds like a typical lean condition. When a quadrajet does this, the secondarys air valve is coming in too soon, or allowing too much air for a given amount of fuel, but that carb is NOT what were talking about here.

I wish I understood earlier that it only backfires when you mash the throttle. Then we could have gotten past the electrical non-issues more quickly. or maybe not......edit......

Ok so I'll back up a little.....like what was mentioned you need to make sure the HEI is fed by a full 12 volts. Thats the easy part.......you simply need a volt meter. Notice on the top of the cap where the wire connects...it says..or it should say..
BATT
GRND
TACH

At the batt terminal it should have 12 volts with the key in the on or run postion.

That could expain why the fuel is not being iginited properly.

 

Well, I don't have to work tomorrow, so you bet your *** that's what I'll be doing. Can't wait to see what comes up. Thank you.

 

Okay, I'm trying to figure out which wire to test. I think you're talking about the green ignition wire. Do I get to it by taking the very top of the HEI unit off?

 

I am not familiar with HEI but if it is labeled IGN then yes, it should be full battery voltage when the ignition key is on. But, what is that yellow wire for, and is it just hanging loose like that with a terminal connector waving in the wind?
By the way, I am adding this link, it is fairly accurate and was convenient for me to find. Notice the potential causes sections and especially the second item in that section. The third item in the section could be because your timing is off and therefore the car is running too hot, causing the problem. At any rate, I hope you can find the problem.

http://en.wikipedia.org/wiki/Dieseling

 

That yellow wire is for a tach, but I dont have one, so I just leave it alone to hang out in the wind.

 

Well, cover it up with electrician tape because if that does have ignition voltage on it and it touches ground, you may have a dead engine pretty quick, or worse, a fire.

 

Damn, I need full coverage insurance of this car ASAP. I will do that, thanks for the heads up. Well, I checked and the HEI is getting a full 12V. So, now I'm back to the carb again,

I'm going to check again (third time) for vacuum leaks. I just heard a new strategy though. Get a clean towel and wrap it around your hands tight. Then slowly cover the top of the carb. If the RPMs go higher or stay the same you have a leak (possibly an internal leak?). But if they go down like the engine is going to die then no leak.

Does this sound right?

 

The best way to check for a vacuum leak is to use a Vac-U-Tec machine. It is a protable machine that makes smoke with a little electric heater. You hook it up to a vacuum line and press a button, then it pumps smoke into the engine and it will come out of any leaks.

Of course, this isn't a machine everyone just has lying around. I saw friends at school blow cigarette smoke into the vacuum line but it never seemed to work too well... stupid Ford guys. You can also use starter fluid and spray it around the base of your intake manifold and such. IF your idle changes when you spray the starting fluid onto the engine then you know you have a vacuum leak.

I am not sure about the towel idea.

 

Went down to my buddy's garage, used the smoke machine (wow is that thing sweet) and found out...no leaks. So he got his dad to come out and he told us that we were both idiots. 5 minutes later after adjusting the carb....problem solved.

Wow, do I ever feel worthless.

 

Thats good news....dont feel worthless....it was a learning experience. Keep at it and 30 years from now you can come out and do the same for the next generation of grease monkeys.