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Backfire upon acceleration

Started by ryan053, April 15, 2009, 02:54:19 PM

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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
68 Charger R/T "Black Pig" Street/Strip bruiser, 70 Charger R/T 440-6bbl Cruiser. Firecore ignition  authorized dealer ; contact me with your needs

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   :icon_smile_big:

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
68 Charger R/T "Black Pig" Street/Strip bruiser, 70 Charger R/T 440-6bbl Cruiser. Firecore ignition  authorized dealer ; contact me with your needs

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
68 Charger R/T "Black Pig" Street/Strip bruiser, 70 Charger R/T 440-6bbl Cruiser. Firecore ignition  authorized dealer ; contact me with your needs

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
68 Charger R/T "Black Pig" Street/Strip bruiser, 70 Charger R/T 440-6bbl Cruiser. Firecore ignition  authorized dealer ; contact me with your needs

426HemiCharger

Have you checked your timing? Or tune your carb, it may be too big for the engine to handle.
------------------------Cars I have now----------------------------
1969 Charger R/T
1998 Ford Econoline 150
2002 Hyundai Elantra GLS
-----------------------Cars I wish I had----------------------------
1969 Charger R/T 4-Speed or Hemi Clone
1970 Charger R/T Hemi Clone
1970  Dodge Polara IL State Police Car
---------------------------Future Posibilities------------------------
2010 or later Ford Fusion Hybrid (Replaces 2002 Hyundai)

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
------------------------Cars I have now----------------------------
1969 Charger R/T
1998 Ford Econoline 150
2002 Hyundai Elantra GLS
-----------------------Cars I wish I had----------------------------
1969 Charger R/T 4-Speed or Hemi Clone
1970 Charger R/T Hemi Clone
1970  Dodge Polara IL State Police Car
---------------------------Future Posibilities------------------------
2010 or later Ford Fusion Hybrid (Replaces 2002 Hyundai)

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...
I've never owned anything but a MoPar. Can you say that?

Back N Black

Quote from: HeavyFuel on September 21, 2013, 03:29:26 PM
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

Quote from: Back N Black on September 27, 2013, 07:04:38 AM
Quote from: HeavyFuel on September 21, 2013, 03:29:26 PM
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*?   :scratchchin:

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?   :shruggy:


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


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.