Part throttle bog then backfire?

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magnumdust

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I put a new carb on my car because the edelbrock was giving me too many headaches. But now I've encountered a problem I can't figure out.

With the new carb the car runs 100x better, but occaisonally the engine will bog badly(only at speeds while getting into the gas). If I let out, the bog will go away. If I dont it backfires, the wideband shows the car go very lean and the rpm dips to zero for a split second(only happened twice, now I just back off before this can happen)

Timing: 14 initial, 32 total.
Ignition: oem w/ 18 degree limiter plate in dist. + lightest dist springs
carb: summit 750 vac, untuned, idles at 12:1 cruise: 13-14:1 wot: 14-15:1(I know it obviously needs. Tune.
 
You need adjust accelerator pump to add more gas when you first step on it. Are you running vacuum advance on your distributor and is it hooked to port vacuum?
 
How large is the engine? Could be you are running too light of a spring in your vacuum secondary. Read your plugs whitish color indicates a lean condition. That is most likely the cause of your part throttle bog.
 
I looked up a previous thread and saw your combo, then i looked up the specs on the new carb.......The jetting, power valve and pump squirter look pretty good to me. I agree that the accelerator pump adjustment could be some of the problem, but i'm wondering if it's not a float adjustment issue. Slow secondary opening, while you may need to play with that at some point, should not cause the issue that you describe. Make sure the pump arm is adjusted and then i'd check the float adj.
 
Vac is hooked up, I will double check that it's the right port. (pretty sure though).

Engine is a 360. I'll pull a plug on wed of it's not pouring rain.

I did adjust the accel pump per the Holley guidelines last night. Didn't work.

Will check floats on wed too. Luckily this carb has a float window so you can adjust it properly with no disassembly
 
magnumdust said:
I put a new carb on my car because the edelbrock was giving me too many headaches. But now I've encountered a problem I can't figure out.

With the new carb the car runs 100x better, but occaisonally the engine will bog badly(only at speeds while getting into the gas). If I let out, the bog will go away. If I dont it backfires, the wideband shows the car go very lean and the rpm dips to zero for a split second(only happened twice, now I just back off before this can happen)

Timing: 14 initial, 32 total.
Ignition: oem w/ 18 degree limiter plate in dist. + lightest dist springs
carb: summit 750 vac, untuned, idles at 12:1 cruise: 13-14:1 wot: 14-15:1(I know it obviously needs. Tune.
Click to expand...
18degrees dist = 36 crank degrees. You need to fix the timing first then fix the carb. Shorten the slots to get around 22 crank degrees and use the factory light spring with an aftermarket light spring on the other side to start. After you get that sorted out you can turn your attention to the carb.
 
Backfiring out the carb is too far retarded timing -- add some advance.

Backfiring out the exhaust is too much timing -- take some out.
 
I would also check the secondary opening. Maybe the spring is too soft. Also, what color pump cam does it have?
It is lean at wot too. Which would indicate lean jetting, or a malfunctioning power valve. Which I guess could cause the lean spot as well.
 
goldmember said:
18degrees dist = 36 crank degrees. You need to fix the timing first then fix the carb. Shorten the slots to get around 22 crank degrees and use the factory light spring with an aftermarket light spring on the other side to start. After you get that sorted out you can turn your attention to the carb.
Click to expand...

I think he's saying he has an advance limiting plate installed which limits mechanical advance to 18*. Like the one offered by FBO (J685 disc near bottom of this page) i have the same plate.

http://4secondsflat.com/Ignition.html
 
magnumdust said:
Ignition: oem w/ 18 degree limiter plate in dist. + lightest dist springs
carb: .
Click to expand...

Try replacing one of the distributor springs with the light stock one and one light one. You may be getting too much advance too fast with the dual light ones.


My 340 ran better with this combination. I used the "stock" elect dist light spring and one of the light "recurve" springs.
 
krazykuda said:
Try replacing one of the distributor springs with the light stock one and one light one. You may be getting too much advance too fast with the dual light ones.


My 340 ran better with this combination. I used the "stock" elect dist light spring and one of the light "recurve" springs.
Click to expand...

Krazy, I'm experiencing a slight part throttle bog as well, my distributor has an aggressive curve. Do you know which color springs you have set up?
 
Saetun said:
Krazy, I'm experiencing a slight part throttle bog as well, my distributor has an aggressive curve. Do you know which color springs you have set up?
Click to expand...



I used the stock electronic distributor "light" spring (the one that comes with the carb new in the box) and then one of the two light recurve springs you get in the Mopar Performance plastic baggy.

The two light springs from the mopar performance baggy were too light when I used them together and I was using up part of the advance at idle. Recurving is a "balancing act". You need a spring combination that does not allow any advance while your engine is at idle, but then advances just off idle. If you have a big cam and higher than stock idle, you need a little stiffer spring so you don't advance while idling.

The stock distributor "stiff" spring does not kick in until later, that's why it has a big loop on one end. Throw that sucker out. Use two light springs.

Some of the springs do not have any color paint on them.

If you have a pile of distributors around, take the light springs out and measure the spring wire daimeter with dial calipers or digital calipers and find the wire diameter and then count the number of turns in the coil. The smaller wire will be lighter. More coils will also be lighter than less coils.

After setting up the distributor, put it in the car and start it. Then get an advance timing light and tuning tach and map the curve for each combination from idle to full advance. Then drive each combination and see how it acts. Use the best running combo for your engine. It's trial and error... Hopefully not so much error....
 
I think you're on the wrong track, your comment about "RPM going to zero" is something THAT SHOULD NOT HAPPEN. I think you have an electrical problem that is dropping spark entirely, or losing supply voltage.
 
krazykuda said:
I used the stock electronic distributor "light" spring (the one that comes with the carb new in the box) and then one of the two light recurve springs you get in the Mopar Performance plastic baggy.

The two light springs from the mopar performance baggy were too light when I used them together and I was using up part of the advance at idle. Recurving is a "balancing act". You need a spring combination that does not allow any advance while your engine is at idle, but then advances just off idle. If you have a big cam and higher than stock idle, you need a little stiffer spring so you don't advance while idling.

The stock distributor "stiff" spring does not kick in until later, that's why it has a big loop on one end. Throw that sucker out. Use two light springs.

Some of the springs do not have any color paint on them.



If you have a pile of distributors around, take the light springs out and measure the spring wire daimeter with dial calipers or digital calipers and find the wire diameter and then count the number of turns in the coil. The smaller wire will be lighter. More coils will also be lighter than less coils.

After setting up the distributor, put it in the car and start it. Then get an advance timing light and tuning tach and map the curve for each combination from idle to full advance. Then drive each combination and see how it acts. Use the best running combo for your engine. It's trial and error... Hopefully not so much error....
Click to expand...

Damn, of course I don't have the stock springs laying around anymore!
 
Saetun said:
Damn, of course I don't have the stock springs laying around anymore!
Click to expand...

I saw an assortment of springs on a web site somewhere and can't remember which one. I want to say Year One or Mancini.
 
67Dart273 said:
I think you're on the wrong track, your comment about "RPM going to zero" is something THAT SHOULD NOT HAPPEN. I think you have an electrical problem that is dropping spark entirely, or losing supply voltage.
Click to expand...

hmm, but why would it happen at part throttle? Its never occured at WOT when i would expect there to be the most load on the system.

I'll take a look at the ignition wiring tomorrow and see if i can't find anything visibly wrong back there. I know that wiring looked a little frail and ify from when we put the magnum motor in it. so maybe time to redo it or step up to an MSD distributor.

The float adjustment turned into a cluster eff and the car wont start at the moment. Awaiting some needle/seat gaskets from summit for tomorrow. :mumum: Which regardless of the resolution to this issue has put efi at the top of my to-do list. Either carbs hate me or i have the magical ability to do very little to them and make them stop working. :banghead:
 
krazykuda said:
I saw an assortment of springs on a web site somewhere and can't remember which one. I want to say Year One or Mancini.
Click to expand...

That 4 seconds flat website sells them. When i bought the limiter plate, i also purchased their springs. I believe one of the pairs is a stock stiffness.
 
Took some pictures of the wiring. Starting with those rusty chrome ignition box and it looks near the coil is where you find this old sketchy looking plug that goes to the coil/balast resister.
 

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