Backfire upon acceleration

[ryan053]

My charger backfires when i accelerate from idle. its a new engine. 383 stroked to 434. the carb is a 750 double pumper. It only happens when i try to accelerate hard from an idle. i have it idling at 700rpm right now. if i accelerate hard from above 2000 rpm it wont backfire. is the proper way to fix this to change the accelerator cam on the carb or what?

[bk72]

i am not sure why it does that but my roadrunner does it when you first start it but after a while it goes away but it is fun when i drive the car in north wilkes-barre and people think it is gun shots and the run to hide           

[firefighter3931]

You need to tune the accelerator pump circuit. The pump arm could be out of adjustment, the power valve improperly sized or the squirter needs upsizing. Lots of possibilities....but it's definately lean at the transition. 


Ron

[ryan053]

i checked the arm and it moves right when i press the throttle. the power valve is a 4.5. the engine is making 6" of vacuum but it isnt steady. its fluttering between 5 and 7. i pulled out the squirter and it was a 31. I checked to make sure the power valve isnt blown by turning in the idle mixture screws and the engine died. The next size squirter i see that holley has is a 35. should i get one?

[bk72]

i wish my roadrunner was not at my other garage. it it was here i could have looked at it

[1969chargerrtse]

Retarted timing will do that, can you tweek it a hair advanced?  I time by ear.    Old school stuff.

[ryan053]

I have changed the timing around and i dont think its related to that. I guess tomorrow i will start by trying one of the other positions on the accelerator cam and see if that helps.

[1969chargerrtse]

I have changed the timing around and i don't think its related to that. I guess tomorrow i will start by trying one of the other positions on the accelerator cam and see if that helps.

My buddy had that with his small block Chevelle and solved it, I can't remember what he did but I will ask him today.  Easy fix if it's the accelerator pump.

He said it back fired only when cold so he tweeked the choke and retared the timing a hair and the problem went away.

[ryan053]

So i tried the other positions on the accelerator cam and didnt notice any changes with any of them. it still backfired and stumbled in all the positions. Is it time to buy a larger squirter or are there other things i should try before i do that?

[firefighter3931]

Time for a larger squirter. 


Ron

[ryan053]

ok i will try to pick up a 35 this afternoon

[ryan053]

I went to the store and got a 35 and a 40. i installed both of them and it is still backfiring. it seems like it might be more responsive with the 40 in there but it is definitely backfiring still. any ideas?

[TexasStroker]

I've seen this come up and as it is off idle the usual thought goes to the carb.  I've also seen ple go thru so much heck trying to remedy the situation with the carb that they forget about other sources. 

I agree on double checking the timing etc.  If you do that and continue to tweak on the carb with no progress consider a new cap and maybe a new rotor.  What kind of distributor are you running?  If it is just an MP unit go and pick up a brass distributor cap and put it on..See what that does.  Even if it doesn't cure your situation it is good to have.

I'm sure you'll be very happy with the stroker's torque when you get it broke in and can experience the full force of the 434 

[ryan053]

I started from square one and redid the timing. its at 15 initial. i redid the idle mixture screws as well. the engine runs great other then the off idle backfire. it really happens only once in a great while when im driving the car. Generally im not flooring it from idle because im letting out the clutch and what not. It might only do it once every few months max while driving around. It really only happens when the car is in neutral and im flooring the pedal. i appreciate everyones help. Any other ideas.

[bk72]

you can do what i do: 1 go to a place where they do not know  your car, 2 drive really fast, 3 when it backfires (if it is like my roadrunner) watch people run away thinking it is gun shots     

[1969chargerrtse]

I started from square one and redid the timing. its at 15 initial. i redid the idle mixture screws as well. the engine runs great other then the off idle backfire. it really happens only once in a great while when im driving the car. Generally im not flooring it from idle because im letting out the clutch and what not. It might only do it once every few months max while driving around. It really only happens when the car is in neutral and im flooring the pedal. i appreciate everyones help. Any other ideas.

This is a hard one.  How about this.  Is there any difference from it being cold or hot?  This would eliminate fuel issues.  If it never did it cold and the choke was on partially, verses being warmend up and the coke wide open?  Bigger carb needed, vacuum leak?  If there's no difference from cold to hot meaning it does it either way I would forget carb issues and think electrical.  make sure no spark plug wires are touching each other, what kind of plugs are you using?  In that area next.  Maybe you need to stop stomping on it so fast.

[ryan053]

Thanks for your guys help in figuring this out. There is no difference if its hot or cold. There is no choke on the carb though. I will do a thorough check for vacuum leaks tomorrow. Some of the spark plugs wire do touch each other. could that be causing the backfire? I am using ngk's. I was using champions but changed them to see if the ngk's are any better. It backfired with the champions though also.

[firefighter3931]

Ryan, what powervalve are you running in this carb ? How much vacuum is the engine making at idle ? How do the plugs look ; white/light grey ?

You might need to jet up on the secondaries....


Ron

[ryan053]

the power valve is a 4.5. the engine is making 6" of vacuum but it isnt steady. the needle fluctuates between 5 and 7. The champions i took out after about one thousand miles were black. the ngk's in there only have a few hundred miles one them now. i could pull them out and look at them though.

[firefighter3931]

the power valve is a 4.5. the engine is making 6" of vacuum but it isnt steady. the needle fluctuates between 5 and 7. The champions i took out after about one thousand miles were black. the ngk's in there only have a few hundred miles one them now. i could pull them out and look at them though.

The PV seems in line for the amount of vaccum the engine is making. 

The 6in vacuum reading seems quite low....have you checked for vacuum leaks ? What cam is in this engine ? What's the whole engine combination ?


Ron

edit ; I just found your engine build in the proven combo section...looks nice ! 

Based on that engine combination you have either a vacuum leak or the distributor needs some tweaking. The 275hl cam in that engine should pull 12-14in of vacuum, easily. Where is your initial timing set at ? Total timing and where does it stop advancing ?

[ryan053]

ok so i went over the intake and carb gaskets and the front and rear of the valley pan to check for vacuum leaks and didnt get any rpm change or change in amount of vacuum at idle. i checked by connecting a rubber hose to my propane torch and running the hose along the gaskets. Im not to keen on spraying carb cleaner or starting fluid all over the engine while its running.

For the timing, the initial is set at 15.
at 2000 rpm its 7 advanced
2500 9 advanced
3000 11 advanced
3500 15 advanced
4000 15 advanced

I pulled off the cap and rotor and they both look fine. The springs under the rotor are silver. I havent changed them and im pretty sure no one at the machine shop changed them either. I cant seem to find the bag of extra springs that came with the distributor so i will need to order some if they need to be changed. The distributor is the msd pro billet one with no vacuum advance.

On a side note i noticed when the car was idling at 850 the vacuum was at 10" and was pretty steady. When i leave the idle set that high the engine always runs on when i shut it off so thats why i have the idle set at 650-700 right now. Should I pull off the carb and adjust how much of the transfer slot is showing so i can run the idle a little higher?

[maxwellwedge]

Good idea - if you have the plates open too far for that low idle you may need to drill a couple of small holes in the primary throttle plates to get the blade/slot position closer to where it should be. I agree the vacuum sounds way too low.

[ryan053]

What about opening the secondary throttle blades a bit and closing the primary blades? Also i forgot to mention that my rockers are 1.6. Im not sure if that is an important detail or not.

[firefighter3931]

Ryan, that distributor appears to have only 15* of mechanical advance. Are you using the black bushing ? The springs could be lightened up a tad to have the total come in 500rpm sooner. 

That carb should have a 4 corner idle adjustmen so you need to tune both the primary and secondary side on the idle circuit. It's not getting enough air at idle so that's why the plugs are dark. I like to idle these engines at 800+ rpm in gear to help with cam lubrication.

Use a vacuum guage and adjust all 4 corners....that should be your next step. 


Ron

[ryan053]

I adjusted the 4 idle air mixture screws yesterday. I have them a turn plus a 1/4 turn out right now. Thats where i had the most vacuum with the engine idling at 700. Im at work now so tomorrow i will try to pull the carb and look at the transfer slot to see if i can adjust the blades that so the engine wont run on when i have the idle at 850. I will also check which bushing is in there. I didnt think to look at that today.