Lean out under load?

[oino]

Ok, after help from LKGTO, Got the car going. Idels fine, and drives fine. Untill I punch it, or try to rev it up in gear to about 3000rpm. Then it backfires out the carb.
I did a vacuum check and it was at 9. I read on Holley's site that with a auto trans, and 9" of vac. I should use a 4.5" power valve. I changed that.
Idel is really really good now. But still backfires out the carb under load.
Timing set at 10. also tryed 12.
I was reading that its going lean under load. Thats what causes the backfire. If thats true. Could the wrong size mechanical fuel pump cause it.
I'm using a fuel pump from a 5.4L.. 6.5psi.
The one for a 5.7L. 9psi.
Maybe under load its not getting enough fuel.

Other then that.
Anything else?


EDIT:
oh ya.
350. RV cam, HEI dis. Had rebuilt. all new insides, other the crank.

 

[PotvinGuy]

Do you mean 9" of vacuum at idle? If so, that is just wrong. You should see 18-20 inches. Maybe a vacuum leak?

Also timing...do you mean 10 degrees of initial timing? That's OK, but how much speed advance? Do you use vacuum advance, and is it connected to the manifold? Backfiring is often associated with timing issues.

Any other info you can add about the motor or it's behavior might help us diagnose. Can't imagine the fuel pump or carb is the problem.

 

[RPM]

Have you checked the total timing on that motor at 3000 rpms? Should be in the 40-50 degree range depending on how you have the advances working, if they are working. That could very easily cause a back fire. as to your fuel pressure, rig that gauge so you can read it as you drive the car, and see what it is doing when all the back firing starts.

 

[oino]

Do you mean 9" of vacuum at idle? If so, that is just wrong. You should see 18-20 inches. Maybe a vacuum leak?

Also timing...do you mean 10 degrees of initial timing? That's OK, but how much speed advance? Do you use vacuum advance, and is it connected to the manifold? Backfiring is often associated with timing issues.

Any other info you can add about the motor or it's behavior might help us diagnose. Can't imagine the fuel pump or carb is the problem.

I get 9" of vacuum in gear.
Yes 10 degrees of initial timing. I'm using the vacuum advance connected to the vacuum advance port on the carb.
and carb it a holley 600.

I have a video of it idling on my facebook page.

https://www.facebook.com/video.php?v=1692453344726

If I do have a vac. leak. I'm not sure where it would be coming from. Don't have alot on the engine. Unless it the intake its self. Was a used one I bought from a guy in cali. when I first built the car.

 

[oino]

Have you checked the total timing on that motor at 3000 rpms? Should be in the 40-50 degree range depending on how you have the advances working, if they are working. That could very easily cause a back fire. as to your fuel pressure, rig that gauge so you can read it as you drive the car, and see what it is doing when all the back firing starts.

I was thinking of doing that tomorrow.
Maybe its losing all it fuel pressure when I punch the gas.

Haven't checked the timing at 3000 rpms yet. I'll try that.

 

[one finger john]

I had a really elegant and well thought reply to your problem but my computer (Mac w/Safari) locked up and I couldn't send it. Here it is in a nut shell.

If it is an ignition problem, remember 12* initial, 36* total @ 3000 rpms (vacuum advance disconnected), 10* vacuum. Total advance with vacuum connected should be 46* If you don't have those numbers, take it to a shop that can adjust it properly.

9 in. @ idle is too low for the cam you have, possible vacuum leak, might be manifold, might be carb to manifold, might be carb. Check with starter fluid spray. Check spark plugs for any that are lean. Could indicate vacuum leak from lifter valley into intake ports.

If the carb has been modified in any way shape or form, PUT IT BACK TO STOCK (Ain't mad, just emphasizing). Holleys like 4.5 lbs. of pressure IIRC. With a lean acceleration back fire check fuel pump shot and pump cam.

Best of luck, John

 

[oino]

I had a really elegant and well thought reply to your problem but my computer (Mac w/Safari) locked up and I couldn't send it. Here it is in a nut shell.

If it is an ignition problem, remember 12* initial, 36* total @ 3000 rpms (vacuum advance disconnected), 10* vacuum. Total advance with vacuum connected should be 46* If you don't have those numbers, take it to a shop that can adjust it properly.

9 in. @ idle is too low for the cam you have, possible vacuum leak, might be manifold, might be carb to manifold, might be carb. Check with starter fluid spray. Check spark plugs for any that are lean. Could indicate vacuum leak from lifter valley into intake ports.

If the carb has been modified in any way shape or form, PUT IT BACK TO STOCK (Ain't mad, just emphasizing). Holleys like 4.5 lbs. of pressure IIRC. With a lean acceleration back fire check fuel pump shot and pump cam.

Best of luck, John

Maybe I'm not checking the vac. right. Other then the Vac. Adv.. Should I also be disconnecting the trans, power brake booster, and PCV?

 

[one finger john]

If you are checking vacuum, check manifold vacuum (vacuum from below the carb throttle plates). Is this carb a new carb with instructions? There should be two vacuum ports. One is ported vacuum that comes from above the throttle plates (used for certain smog set ups). The other is manifold vacuum that comes from below the throttle plates. That's the port you want to use. For the vacuum gauge use a take off from the manifold, T off the manifold vacuum port. The gauge just tells what the amount of vacuum there is. Leave everything else in place. When you set your timing (12* btc) make sure the vacuum line from the manifold vacuum port is disconnected and plugged so there is no manifold port vacuum leak. All timing adjustments MUST be made with the vacuum advance disconnected and the port in the carb plugged. When the adjustments are made and finalized, then reconnect the vacuum advance. If you check the timing after you have RECONNECTED the vacuum line you should see that the timning is another 10* advanced for a total of approx. 22* @ 850 rpms. If the rpms are higher, idle it down to 850 without changing the timing, i.e. use the idle adjustment screw. How is the adjustment for idle richness? I believe 1.5 turns out.

Also are all the unused ports plugged with those little black rubber plugs? If not, plug em.

 

[oino]

If you are checking vacuum, check manifold vacuum (vacuum from below the carb throttle plates). Is this carb a new carb with instructions? There should be two vacuum ports. One is ported vacuum that comes from above the throttle plates (used for certain smog set ups). The other is manifold vacuum that comes from below the throttle plates. That's the port you want to use. For the vacuum gauge use a take off from the manifold, T off the manifold vacuum port. The gauge just tells what the amount of vacuum there is. Leave everything else in place. When you set your timing (12* btc) make sure the vacuum line from the manifold vacuum port is disconnected and plugged so there is no manifold port vacuum leak. All timing adjustments MUST be made with the vacuum advance disconnected and the port in the carb plugged. When the adjustments are made and finalized, then reconnect the vacuum advance. If you check the timing after you have RECONNECTED the vacuum line you should see that the timning is another 10* advanced for a total of approx. 22* @ 850 rpms. If the rpms are higher, idle it down to 850 without changing the timing, i.e. use the idle adjustment screw. How is the adjustment for idle richness? I believe 1.5 turns out.

Also are all the unused ports plugged with those little black rubber plugs? If not, plug em.

New carb. and adjustment for idle richness are 1.5 turns out as of Thrusday.
Other then that. I've been doing just what you said. Its 12* btc right now.
Humm. Must be a leak. I'll try removing all the vac. lines and plug all vac. ports tomorrow. See if there is a change.
I'm leaning toward the intake, and maybe I should change to a stock fuel pump for a 5.7 engine.
But first the leak.

if it is the intake. Was thinking of getting this one.
http://www.oreillyauto.com/site/c/d...ake+manifold+-+performance&pt=2321P&ppt=C0258

 

[oino]

Oh and to check for a leak?
starter fluid spray. Can I also use carb cleaner or brake cleaner?

 

[Differen T]

Ok, after help from LKGTO, Got the car going. Idels fine, and drives fine. Untill I punch it, or try to rev it up in gear to about 3000rpm. Then it backfires out the carb.
I did a vacuum check and it was at 9. I read on Holley's site that with a auto trans, and 9" of vac. I should use a 4.5" power valve. I changed that.
Idel is really really good now. But still backfires out the carb under load.
Timing set at 10. also tryed 12.
I was reading that its going lean under load. Thats what causes the backfire. If thats true. Could the wrong size mechanical fuel pump cause it.
I'm using a fuel pump from a 5.4L.. 6.5psi.
The one for a 5.7L. 9psi.
Maybe under load its not getting enough fuel.

Other then that.
Anything else?



EDIT:
oh ya.
350. RV cam, HEI dis. Had rebuilt. all new insides, other the crank.

Have you checked your ignition componentssuch as cap, rotor and wires? If I understand you correctly you also have a backfire under a steady load at 3000 RPM as well as under acceleration from idle. Usually at a steady RPM that is less than full power (wide open throttle) a lean condition will result in a drop in RPM's. Plug wires are my first though, especially since you are running them through metal looms. Check them in the dark for fireworks under the conditions that are causing the backfire. HEI's can overpower the insulation on some types of ignition wires fire the wrong cylinder especially in metal looms. Carbon tracking inside the cap or on the rotor can also cause this type of problem.

Just my thoughts,
Al

 

[Mike]

Oino, I prefer to use carb cleaner to check for vacuum leaks. Ether tends to float in the air and it can easily get drawn into the carb, which will increase idle and make you think you've found a non-existent vacuum leak. If you're going to use ether, try to devise a means to pull air into the carb from another area, which will require a stock type air cleaner, with a good air cleaner gasket and an air filter that will fit the housing properly. Then run defroster hose from the air cleaner snout out the door or a window.

That power valve is all wrong. Wrong, wrong, wrong.

Put a long piece of hose on your vacuum gauge, so you can get it into the car with you. Now go find a long, level stretch of highway and get the car up to cruising RPM (65-70 MPH). See what that vacuum level is. It should be fairly close to that idle vacuum level (which I agree is far too low).

Take the lowest vacuum reading of the two and get a power valve that is 1.5" - 2" lower than that reading. If 9" is the lowest reading, then you want a 7.5" power valve. The 4.5 valve is opening far too late and causing the lean condition.

If you get a power valve that is 1.5" below your lowest vacuum reading, it will not be trying to sneak open on you at idle or at cruise RPM. But it will be sitting right there, ready to tip in when you give it a bit of throttle.

The power valve circuit on a Holley is really great, because it is so easily tuned. Unfortunately, most people don't have a clue how the circuit works and never get it dialed in properly. They end up with blown power valves and a rough-idling motor, so they say Holley carbs are junk. Or they think they know more than the Holley engineers and remove the power valve entirely and end up with a car that is jetted too high and still stumbles under load. And once again, Holley carbs are junk. When it simply isn't true. I've said it before and I will keep right on saying it - if you're going to do carburetor work, you have to be smarter than the carb you're working on. That's really all it takes. And I'm yet to see a carb that had a brain, so you've already got a leg up on it.

6.5 lbs. of fuel pressure is plenty adequate. I don't trust needle and seat assemblies above 7 lbs., so you're where you want to be.

You need to be methodical about your testing procedures. Buy a new power valve, so you know you have one that is not already ruptured from backfire. Buy another 4.5, so you know it will be closed at idle. Now, check for vacuum leaks first. When you get that problem resolved, then check your idle vacuum level and install the proper power valve for that. Now, go do your road test and check your steady, cruise RPM vacuum level. If it is lower than the idle level, then tune the power valve accordingly.

 

[putz]

Another way to check for a vacuum leak is hold your hand over the carb intake and see if the idle goes up doesn't tell where it is just lets you know there is one. Use carb cleaner.

 

[2old2fast]

If you don't have a blowout preventer[check ball in the base plate] then ANY time the carb coughs "could" be the end of a power valve. Just as an aside, had a similar situation on a friend's car, chased the 'pop' for quite some time, only to find a couple bent[slightly] exhaust valves, point is ,don't overlook anything!


dave

 

[oino]

I did the hand thing over the carb and made no difference.

I'm giving up.

 

[oino]

I was just thinking. I wonder if any of this has to do with my oil level never reading right.
Remember when I asked about that.

I just don't know. Shoudn't be this damn complicated.
I've lived with firend years ago who had rebuilt engines. He did way less to them then what I had done and they alway got it running without problem like this crap.

 

[oino]

Just in case it was an exhaust clog.
I removed the ends with the muffler in them.
Didn't change anything.

Oh Where do I spray the carb cleaner to test for leaks.
I sprayed it around the intake gasket and got bubbles. I'm guessing thats just boiling carb cleaner.

 

[oino]

Chevy 350 crate engine 1980s
New Holley carb 600cfm
Edelbrock intake 2101 used.
stock HEI Dis.
Had engine rebuilt.
Block completely cleaned in and out.
New pistons, rings, bearings, freeze plugs, RV cam,
timeing chain, push rods, and hydraulic lifters.
Heads redone.

I think thats the just of it.
Just shy of buying a new engine.
I think when I get paid I'll replace the intake, and put a new carb. gasket.
I'll also remove the oil pan and see if I can figure out the oil level problem.

 

[putz]

Just in case it was an exhaust clog.
I removed the ends with the muffler in them.
Didn't change anything.

Oh Where do I spray the carb cleaner to test for leaks.
I sprayed it around the intake gasket and got bubbles. I'm guessing thats just boiling carb cleaner.

You can spray around the carb base and shafts in short bursts so carb doesn't suck it in the top

 

[Screaming Metal]

-------Any luck yet?------You've been given the best advice. Troubleshooting problems takes time. Thats just the way it is.
Your mechanic friend had some experience behind him. NOT everyone can build a motor....and have it run like it should.