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.

[firefighter3931]

Try opening the secondary throttle blades slightly and see if that helps with the idle quality. Also have a look at the float level to make sure fuel isn't dripping out of the boosters. Does it smell rich at idle ? You should be able to get away with more base timing....bump it up to 18*.

Ron

[ryan053]

Ok i will try opening the secondary blades. should i close the primary blades a bit also so the car wont run on with the idle above 800rpm? i will check the float levels. It doesnt smell rich at idle. I will bump up the timing also tomorrow.

[oldcarnut]

One problem a couple friends had found was that the power valve would go bad with the first backfire and need replacing.  I put in Holly's anti backfire pressure kit in mine to keep from having to replace them when it occured for whatever reason.  If all else fails to correct it, it might be something to check out.

[Supercharged Riot]

My 383 backfires on idle too   

is it because of a fouled spark plug?

[ryan053]

ok so i checked the float levels and they were good. I looked at the distributor and it is the black bushing. Should i order one of the spring and bushing kits so i can put a different one in there? I opened the secondary throttle blades until there was a square sized opening in the transfer slot. I need to spend sometime readjusting the throttle blades though because the car idles to high for the primary idle screw to work. I bumped up the timing too

[ryan053]

I ended up leaving the secondary throttle blades so the transfer slots are just covered up. I have the car idling at 800rpm now. It doesnt seem to be backfiring anymore. there still is a slight hesitation though. The problem im having now though is the car will diesel after i shut off. I had the idle lower before so it didnt run on. Whats another way to get rid of that problem?

[maxwellwedge]

Better gas, shutting it off in gear if it is an automatic, give it a few healthy "throat clearing" revs before switching it off, rig in an idle solenoid like Chrysler did....these are a few tips that worked for me over the years.

Glad to hear it's better!

[firefighter3931]

Good news Ryan...run-on is usually caused by crappy fuel. You could have fuel dripping out of the boosters so check your float level. Blipping the throttle as Maxwell said helps to clear it out before shutdown.


Ron

[ryan053]

I checked the float levels and they were fine. I also hooked up a fuel pressure gauge and the gauge read 9 psi. Im going to go to the store and pick up a fuel pressure regulator. Maybe the high pressure was causing some problems. When i put gas in the car i only put chevron in and i use 91 octane. I guess i can try fuel from a different station around here

[ryan053]

Also would it be beneficial to order a bushing and spring kit from msd so i can have more total timing and change up those springs?

[ryan053]

so i got the spring and bushing kit and put in the blue bushing and changed one of the springs. The car hasnt backfired yet but it still has a stumble off idle. Is there anything else i should be trying?

[firefighter3931]

Try the #40 squirter and make sure you use the hollow screws that go with the larger squirter or it won't dump enough fuel. 

Is it stumbleing at full throttle or partial throttle ?


Ron

[ryan053]

I put the 40 squirters in last night with the hollow screw. The stumble was still there. Its just at partial throttle just when i try to floor it. Do you think i need to get the 50cc accelerator pump?

[maxwellwedge]

That and/or a different pump cam or repositioned pump cam should fix you up....wonder about vacuum advance? Do you have one?

[ryan053]

No the distributor doesnt have vacuum advance

[maxwellwedge]

What is your total timing and when is it all in by?

[firefighter3931]

You might be able to use a 50cc pump but try bumping up the timing to 18* at idle and see how it responds. Get a vacuum guage on it and advance the timing for maximum vacuum then back it off a hair. Let the engine tell you what it wants. 


Ron

[426HemiCharger]

Have you checked your timing? Or tune your carb, it may be too big for the engine to handle.

[ryan053]

I will redo the timing tomorrow. I have it at 18 right now. I took it up to 20 but the starter was kicking back so i bumped it back down to 18. I have that 21 degree bushing in there now so i am going to recheck the total timing to see if it is actually getting 21 degrees from that bushing.

[maxwellwedge]

Try setting the total to around 35 and let us know what you have at 800-900. Also let us know where it's all in by. Erratic (too loose) spring could be contributing as well.

[426HemiCharger]

16 degrees BTDC at 750 M/T or 800 A/T

[HeavyFuel]

I'm curious about timing recomendations.

The manual says 5-7 degrees BTDC, but many suggest more than double that.....say 16-18?


Why is that?   Does vacuum advance play into it when advancing that far, and must be disconnected?


I'm leary of pre-detonation.

[odcics2]

Used to have the backfire issue, back in the day.

Pulled off the single plane manifold and put on a dual plane high rise.

Problem solved...

[Back N Black]

I'm curious about timing recomendations.

The manual says 5-7 degrees BTDC, but many suggest more than double that.....say 16-18?


Why is that?   Does vacuum advance play into it when advancing that far, and must be disconnected?


I'm leary of pre-detonation.

Timing depends on the cam, with a stock cam 5-7 degrees is recommended. With a bigger lumpy cam you need more timing, the initial poster in this thread has a 383 stroker, requiring more initial timing. I'm running 18 degrees with no vacuum advance, no issues.

[HeavyFuel]

I'm curious about timing recomendations.

The manual says 5-7 degrees BTDC, but many suggest more than double that.....say 16-18?


Why is that?   Does vacuum advance play into it when advancing that far, and must be disconnected?


I'm leary of pre-detonation.

Timing depends on the cam, with a stock cam 5-7 degrees is recommended. With a bigger lumpy cam you need more timing, the initial poster in this thread has a 383 stroker, requiring more initial timing. I'm running 18 degrees with no vacuum advance, no issues.

OK...I'm learning about how initial, mechanical and vacuum advance work together.

Back n Black, if you run 18* with no vacuum, do you have your total advance set at the 34-38* range?   If so, do you use a modern distributor that allows the mechanical advance to be adjusted down to like 20*?  

I have an older MP electronic conversion, and the mechanical advance is not adjustable, and is stamped 13 on the bottom, which means 26* total.  For me to keep from getting detonation at WOT, I have to back off my initial to about 8*.

But setting 8* initial doesn't give me enough advance when mildly acceling from a stop, so I increase my adjustable pot on the dizzy to add in another 10-12* to put me at around 20* at light throttle.

That seems to work under very light throttle, but anything heavier than that and I get a severe stumble/backfire.

I've tried different metering rods and springs to check for a potential lean condition.  That didn't really help alot.

I'm wondering it my timing has an overlap or lag condition.  My stumble is the worst when the engine gets under load, just after it have shifted into 2nd and 3rd gear.  I'm thinking that this is a time when the engine is not creating vacuum, my vac advance is virtually nil, and my mechanical has not kicked in yet....so it's at about 10* advance, and that's what's bogging the engine.  (my advance is all in at 34* @ 3000 rpm)

I've noticed that the car shifts to 2nd and 3rd really quick.  Maybe that's normal under light throttle.  Under very mild throttle, I'm in 3rd by 20 mph.  Could this be part of the problem?  


The engine seems to run great once I get up to speed, say 40 mph or so, and the rpms have climbed a little.    


Possible solutions to try:
A) Fix my dizzy so that I can adjust the mechanical advance down, which will allow me to have a higher initial advance, and in turn a lower vac advance setting.  This will hopefull fix the drop in advance during low vacuum periods.

B)  Adjust the springs in my dizzy to speed up the curve...be all in by 2000-2500 rpm.  Maybe that will help reduce the advance drop due to lack of vacuum under load.

C) Maybe the problem is still fuel related....still in a lean condition.  Changing out the meeting rods did have a small effect....it basically moved my bog from right when stepping on the gas, to when the tranny shifts to 2nd or 3rd.

D)  Adjust shift points on tranny?.....not sure how this is done.  

Sorry for the long post...sometimes it just helps to get it all down in black and white.  

[HeavyFuel]

Made some progress.

Turns out that some one in the previous life of my carb put in .089 primary jets, replacing the .101 that the 4429s is supposed to have.  It runs alot better now.  Not perfect, but moving in the right direction.

Gonna tweek the timing next by bringing the mechanical advance down to about 20*, then upping my initial to around 14*, and adjusting the vaccuum down to like 10*.

[HeavyFuel]

Last weekend I modified the distributor plates to give about 20* mech advance, and recurved the springs.   I set my initial at 16* and it timed all in around 36* at around 2600 rpm.  (which now that I think about it, I should have done it the other way around, set my total at 36* and let the initial fall wherever...)

I also took the carb apart (again) and checked the operation and flow of the accel pump.   The check valve between the accel pump well and the bowl might have been a little gunked up...seemed like there was some back flow into the bowl when testing the check valve.   It's cleaned up now, and I'm getting good squirt.   The discharge check looked pretty good, but I cleaned it up a little to make sure air wasn't drawing back into the circuit.

Took the car for a drive last night....45* out.

Feels like the car is running better than it ever has.   The stumble backfire is pretty much gone......it still needs some tuning in the carb.

The engine SOUNDS different with the advanced timing....seems smoother when driving, less lumpy on idle.

The choke it still staying on a little bit in this cooler weather because of the blocked crossover.   I'll have to work on that. 

Getting there.