backfiring through the carb

[berg bronco]

when I let the rpms get low (if I dont down shift) and step on the gas she backfires through the carb? im running a truck avenger, 302, with a mild cam and 351 heads. it does good in every other situation. could it be timing or air/fuel mixture? if I down shift and get the rpms up it does fine. thanks for the help.

 

[NYLES]

How much timing?

accelerator pump cam, sounds like its still getting fuel when you let off therefore the pop through the carb

 

[broncnaz]

My bet is on to much timing. the only real way for a back fire in the intake to happen is when the valves are still open when the plugs fire. Stepping on the gas without downshifting with a small engine and a heavy vehicle will result in a bog mainly due to not enough gas form the accelerator pump but the back fire is from timing.

 

[Bay0726]

when I let the rpms get low (if I dont down shift) and step on the gas she backfires through the carb? im running a truck avenger, 302, with a mild cam and 351 heads. it does good in every other situation. could it be timing or air/fuel mixture? if I down shift and get the rpms up it does fine. thanks for the help.

Hey berg, I'm having the same trouble with my Cougar, Street Avenger 675, I tried timing, and adjusting the idle and leaning it out, leaning it out helped the most. But I just bought a used Edlebrock and I am going to make the switch just as soon as I rebuild it... tomorrow! Hows it going, did you get rid of the 66 yet?

Bernie

 

[berg bronco]

How much timing?

accelerator pump cam, sounds like its still getting fuel when you let off therefore the pop through the carb

its at 10 degrees right know. im going to set it back just a little

 

[berg bronco]

My bet is on to much timing. the only real way for a back fire in the intake to happen is when the valves are still open when the plugs fire. Stepping on the gas without downshifting with a small engine and a heavy vehicle will result in a bog mainly due to not enough gas form the accelerator pump but the back fire is from timing.

thanks for the explanation

 

[NYLES]

10 shouldnt be an issue........

hmmm wheres my book?

From my Demon book....

Backfires or Pops through carburetor these the remedies
A) Open idle-mixture screws
B) Increase squirter size
C) Increase jet size
D) Raise float level

What size a TA?

 

[berg bronco]

i think its a 600?

 

[NYLES]

Im gonna guess D

 

[broncnaz]

If chaning the timing doesnt work. I'm going to say just down shift. A small engine turning low RPM's at low ground speed suddenly trying to acclerate a heavy vehicle in high gear is the problem you can only go so low before the engine either cant or wont pick back up. yes accelerator pump squirter size or cam change could make a differance but there will always be a point that a small engine just cant pick back up. Thats what transmissions are for.
What axle gearing and tire size are you running? to big of a mismatch theer could be part of the issue as well.

 

[73azbronco]

Points to wide?

 

[DirtDonk]

Verify that the accelerator pump is working too. An over-lean condition can cause this type of behavior in some engines.

Back off the timing anyway, even though as mentioned, that 10° should not be an issue at all, it can't hurt to experiment.

Check the quality of your spark. If something is wrong and your ignition is weak, combustion events can be off too.
Or, if you're getting some sparkage scatter or crossover from one old plug wire to another (like plugs 7 and 8 on the older engines) it might be giving you a late or early spark to one or more cylinders.
Weak ignitions are responsible for more "carb" issues than you can imagine!

Check PCV valve to make sure it's functioning properly. Again, the lean mixture thing.

Check everything for vacuum leaks. There's your over-lean condition again.

And just for giggles... this is not a recently rebuilt engine by any chance, is it? Reason I ask is that the first time I experienced a flat cam lobe on an engine was when the engine started popping through the carb under acceleration.
Not a happy scenario that, but I had to at least mention it.

Good luck.

Paul

 

[berg bronco]

If chaning the timing doesnt work. I'm going to say just down shift. A small engine turning low RPM's at low ground speed suddenly trying to acclerate a heavy vehicle in high gear is the problem you can only go so low before the engine either cant or wont pick back up. yes accelerator pump squirter size or cam change could make a differance but there will always be a point that a small engine just cant pick back up. Thats what transmissions are for.
What axle gearing and tire size are you running? to big of a mismatch theer could be part of the issue as well.

ya when i down shift earlier its fine. just thought id get some opinions;D
Im running 35 with 4.56 gears and a np435

 

[berg bronco]

Verify that the accelerator pump is working too. An over-lean condition can cause this type of behavior in some engines.

Back off the timing anyway, even though as mentioned, that 10° should not be an issue at all, it can't hurt to experiment.

Check the quality of your spark. If something is wrong and your ignition is weak, combustion events can be off too.
Or, if you're getting some sparkage scatter or crossover from one old plug wire to another (like plugs 7 and 8 on the older engines) it might be giving you a late or early spark to one or more cylinders.
Weak ignitions are responsible for more "carb" issues than you can imagine!


it is rebiult but it has 3000 miles on it now,ive checked all possible vacumm problems and everything is good. is possible thats its to big of a carb for the engine? oh ya I changed the timing to about 8 and it didnt seem to change anything
Check everything for vacuum leaks. There's your over-lean condition again.

And just for giggles... this is not a recently rebuilt engine by any chance, is it? Reason I ask is that the first time I experienced a flat cam lobe on an engine was when the engine started popping through the carb under acceleration.
Not a happy scenario that, but I had to at least mention it.

 

[berg bronco]

wow! messed that post up

 

[DirtDonk]

Hah! been there, done that too. You can go back and edit it if you want, just adding the proper quote marks after each of my questions, but being sure not to bracket any of your sentences. Or you could just change the color of the type in your answers.
If you even feel like it that is. Otherwise, I can see the differences.

Hmm, so no vacuum leaks, eh? Too bad almost. Would have been an obvious fix.
Carb too big? What size is it? Thought you said 600 cfm somewhere, but can't find it now.
A too-big carb can do funny things, but usually not that. And even then, if yours is 675 or smaller, I don't think it's so big to cause major issues. Smaller is often better, but bigger won't always cause spitting on it's own.
So what size/model is it?

A cam that's properly broken-in (if a flat-tappet type), but using oils that don't have enough of the right additives, can still fail. It just takes longer. And 3000 miles is not out of line from what I hear.

I would bump the timing up to 12 to 14 then, just to see if it responds better. Perhaps the combination of your cam and those heads and whatever intake/carb setup you're running likes more advance?

Paul

 

[berg bronco]

Hah! been there, done that too. You can go back and edit it if you want, just adding the proper quote marks after each of my questions, but being sure not to bracket any of your sentences. Or you could just change the color of the type in your answers.
If you even feel like it that is. Otherwise, I can see the differences.

Hmm, so no vacuum leaks, eh? Too bad almost. Would have been an obvious fix.
Carb too big? What size is it? Thought you said 600 cfm somewhere, but can't find it now.
A too-big carb can do funny things, but usually not that. And even then, if yours is 675 or smaller, I don't think it's so big to cause major issues. Smaller is often better, but bigger won't always cause spitting on it's own.
So what size/model is it?

A cam that's properly broken-in (if a flat-tappet type), but using oils that don't have enough of the right additives, can still fail. It just takes longer. And 3000 miles is not out of line from what I hear.

I would bump the timing up to 12 to 14 then, just to see if it responds better. Perhaps the combination of your cam and those heads and whatever intake/carb setup you're running likes more advance?

Paul

im running a 600 truck avenger holley with a elderbrock intake with 351 heads and headers ill adveance the timing a little more and see what happens
i used the correct oil when I broke in the cam but know i run 10w 30 castrol oil. thanks for the advice!

 

[DirtDonk]

Yeah, lots of cam problems since they removed most of the zinc(I think it was?) from most modern motor oils. Roller cams don't need it and it's a component of air pollution, so it's mostly bye-bye.
I would start adding a zinc component or use oil specifically stated to have it, or be compatible with "all cams" or something similar. Castrol's great stuff, but I'm sure some of their stuff is not going to be right for your app. At the very least, read the can.

If nothing you do or check works, take a valve cover off and measure some lobe lifts. Kind of a pain, for sure, but if nothing else pans out, it's something worth thinking about. You would think you'd notice a big drop in power, but if it's only one or two lobes, and the engine is pretty healthy, you might not notice right away.

Carb is fine. 600 is not too big even for a bone stock engine. It's borderline unnecessary at that point, but it's not so bad as to make it run poorly. You'd simply feel a slight improvement with a smaller carb.
With even a mildly better-than-stock engine however, that 600 should be perfect.

Not sure what diff the 351 heads make, good or bad, other than (possibly) having better breathing characteristics with slightly larger ports and/or a better combustion chamber shape. But that might all depend on what year and model 351 was the donor.

Good luck.

Paul

 

[broncnaz]

If you cam is going flat then you will get major ticking in the valve train. trust me you'll know its the cam going flat if thats the case.
The Holley TA only comes in either a 470, 670 or 770 cfm. the 670 should be fine on a 302 as was said more than it needs but still should be fine. once thing about bigger carbs is when you open the throttle especailly on a small engine in a heavy vehicle the AF ratio goes lean very quick the acclerator pump is supposed to cover that but If I remember correctly the TA has a #28 squirter I would step up to a #31 see if that helps I usually find that #31 squirters work best. Now some people do have good luck with changing pump cams but I've never had much luck with that myself. If it was only a slight issue then the pump cam may fix it.
351W heads are all about the same as 302 heads for most the only differance is head bolt size. some of the early ones(69-74??) had slightly larger valves and even then its only slightly larger not big enough to cause any issues. I run 2.02/1.60 valved heads on my 302 and dont have any issues.