Help needed 383 with 750 vac backfiring!

[powermyster]

Hi,

The mechanical timing is as you state, the pump shirt looks good though if I'm honest I never really checked it from the passenger side as I'm usually in the drivers side to operate throttle. I'll check it later.

I'll try that with the secondaries, I adjusted then with the carb off but hadn't considered they may be opening slightly.

The plus leads are usually kept seperate, I just threw everything across in a haste yesterday wasn't pushed about being near. I'll be sure and keep them seperate when I put all back.

As for the driving, I haven't driven her properly for sometime. Last time a had her out and gunned it. I think it was like you said. She popped, stumbles and took off again. Sorry if I sound vague but it's several months since I had it out properly, been busy with college and didn't want to drive it until I have the problem resolved statically.

 

[Lazerwolf]

+1 to what Early B said, accelerator pump cam. Not the accelerator pump nozzle. I was having a similar issue with my 400 and same 3310 carb. Tried everything from #25 to #37 accelerator pump nozzle. Nothing working. Talked to Holley and they suggested changing from the White (41R218) cam to the Orange (41R466) cam. If that doesn't work, they suggest going to the Blue cam. I put the Orange in and what a world of different. No lean backfire (through carb). Not done fine tuning it yet, but I've took the #37 nozzle out and put the #35 nozzle it with no downside yet.

 

[powermyster]

Thanks laserworf, have tried them all, I had the blue in and went back to orange. Are you using hole 1 or 2. I have it in 1 currently, but I'm 99% sure I tried it in 2 before.

I was thinking of ordering the 37 or even upgrading to the 50cc pump. maybe i'll play with it a little more first

 

[Lazerwolf]

I have the orange in hole 1. Like I said, mine is not perfect by any means yet, but here is how it's set up. Stock front jets (72) and stock rear metering plate (134-21). Power valve is 5.0 since I have an automatic and only pulling about 10 vacuum at idle. Idle screws 1 1/2 turns out. Pump nozzle #35. Accelerator cam is Orange in hole 1. I have the idle speed set at about 900 rpm. Also, the vacuum secondary spring is the original silver one, which I haven't messed with yet. For what it's worth, I also put in the vent baffle (Holley 26-89). Talking with Holley, they did indicate to me that the backfire through the carb is a lean condition. They also said to keep the fuel pressure about 6.0-6.5 psi. Less than 6.0 psi could cause the same lean issue and more than 7.0 psi could cause rich (flooding) issues. I think the biggest difference is my timing is set on 12 initial but my car had a Davis United street/strip distributor and that was their suggestion. It does have vacuum advance and I have not tried bumping past 12 initial yet. Last thing Holley told me is that the stock accelerator pump diaphragm should be fine and would only need to go to the larger 50 cc if you put a nozzle in that is #40 or above. Along with that, if your using #40 and above nozzle, you would need the hollow nozzle screw, which I am not using. Hope this helps.

 

[powermyster]

Thanks for sharing laserwolf, some of our settings will be unique, but given you have the same carb, nozzles size and cam and m on a bigger engine, you would think my set up would suffice.

I seems to be pulling a lot of vacuum which made me consider the PV but Holley seem to say a 6.5 is beg enough. I'm pulling 21inches

Before anyone asks. I have no idea what cam in is the engine. It seems like a pretty stock set up

 

[Jcharger]

Reading through this thread I agree that it seems like it is related to the pump shot. I have some questions and suggestions.

Questions:

• What RPM are you starting at when you mash the throttle? And is it a manual or an automatic (apologies if I missed that).
• Does the pump shot start as soon as the throttle is touched? Another way to ask this question is: Is there any play between the pump lever and pump cam? If there is, you might not be getting the shot in quick enough. If you think you have to bend the lever arm (between the cam and the pump) (because the screw is backed out to its limit) to make it work, make sure that is what needs to be done and also consider buying a spare lever arm for reference and to set it back if you later find out that is not the issue.
• Is the adjuster that connects the pump lever opened up too far? If it is, you might be straining the pump and also might not be getting the full amount of the shot for the specific cam.
• Have you changed the springs in the vacuum secondary? This can make a difference if the secondaries open too quick for the pump shot.
• Does the A/F recover after the backfire/hesitation ends and the car settles in to a WOT acceleration run? Does the A/F recover to ~12-13? This is for later and I recognize that you have not driven the car lately, but it will tell you about your main jets.

If it were me:

• I would work with the cam first (this will determine the amount of gas that the pump delivers). You can switch to a 50 cc pump, but I think that is probably overkill unless you are really having the secondaries open quickly.
• I would try a test with the secondaries wired shut to see if it backfires. You may get a bog (too much gas) rather than backfire and hesitation which can be too little gas in this configuration, but it will give you some information.
• Once it is known that the primaries without secondaries are OK, then I would switch to the secondaries, and probably start with a heavy spring (slow opening) of the secondaries. I would then incrementally work my way up in pump shot and springs to the point where it hesitates and probably back off the spring one step or up the pump shot a little.

I would hold off on adjusting the size of the nozzle or pump size for now. I found the size of the nozzle makes a difference in the duration of the pump shot. I did some tests with different squirters and filmed with a camera that I then timed using the videoplayer on the computer and examining frame by frame. For my set up, the total shot could vary on the order of tenths of a second to maybe a second going from 28 to 46. The total time for a full throttle shot was on the order of a second. I could see more with the lean spikes in A/F traces than with the timing on the camera for squirter changes.

If you are way out from the normal settings for the carb, I would think carefully about whether it could be something else that needs adjusting. There seem to be many ways to get to a similar point, but the initial settings are there for a reason. The first time I did this, I covered up a problem with main jets and the transition using the pump shot. The car ran, but it ran much better when I figured out the real issue.

 

[powermyster]

Jcharger thanks for the reply, to answer you questions;

Questions:

What RPM are you starting at when you mash the throttle? And is it a manual or an automatic (apologies if I missed that).

The can is a manual transmisison - A833 4 speed. The can seems to like to idle at about 850-900, this would be the RPM I'm kicking off from.

Does the pump shot start as soon as the throttle is touched? Another way to ask this question is: Is there any play between the pump lever and pump cam? If there is, you might not be getting the shot in quick enough. If you think you have to bend the lever arm (between the cam and the pump) (because the screw is backed out to its limit) to make it work, make sure that is what needs to be done and also consider buying a spare lever arm for reference and to set it back if you later find out that is not the issue.

The pump shot kicks off as soon as the throttle is open, I was using a feeler gauge for this, but have done it by eye the last few times as I was constantly adjusting between cams. I can double check this later and make sure it not coming in too early.

Is the adjuster that connects the pump lever opened up too far? If it is, you might be straining the pump and also might not be getting the full amount of the shot for the specific cam.

As above, Ill double check this.

Have you changed the springs in the vacuum secondary? This can make a difference if the secondaries open too quick for the pump shot.

I haven't really looked at the secondaries, whatever spring was in it when I got it is what I have now. I'll look at wiring when shut to confirm the primary circuit is ok as you suggested.​

Does the A/F recover after the backfire/hesitation ends and the car settles in to a WOT acceleration run? Does the A/F recover to ~12-13? This is for later and I recognize that you have not driven the car lately, but it will tell you about your main jets.

Yes the AF recovers back after the backfire in neutral in the garage anyways.


I hope to get a little bit done tonight and report back. I'll start with swapping the original cap back across and see how it performs with the vacuum secondary back on. If she doesn't pop but still leans out I will check the pump arm and then wire the secondaries shut and see how that goes.

 

[powermyster]

AARRGHHHH!!! I give up (for tonight anyway).

I started the car reversed it out and checked secondaries and pump shot, pump shot was strong. snapped her open a few times and there was one small pop. held the secondaried closed and snapped her open a few times all seemed good.

Swapped the good cap and rotor across and she popping out the exhaust a bit, checked the timing and the light is showing misfires and timing jumpin a bit. Checked all the pug wires onto the cap. Still the same.......

Bearly drove back in, it was popping back and front. Just had to walk away from it, she wasn't missing before i put the old cap back on to check the phasing.

There is a little play in the rotor, is this normal??



I dunno if this was in there before i put the FBO plate in, IIRC i had to open the weights a little to slip in on.

 

[powermyster]

 

[Jcharger]

I do not have a solution to the rough running and hope someone else provides a solution.


Regarding the play in the rotor. I just checked the one in my car which is a relatively new MSD unit and it also has play – which I think is related to the mechanical advance (at least I see the advance weights swing out when I twist the rotor back and forth). The rotor snaps back to the initial position because the springs on the advance weights pull it back. I assume the springs pull your rotor back when you let go.


Returning to the testing discussion, I also have a manual and had a backfire through carb that was related to a combination of things. I had an ignition problem with dwell that you don’t have, but I also had a problem that the jets were too lean and the pump shot was a little small.

My suspicion (like what others have suggested) is that your situation is related to the pump shot and possibly jetting, but it sounds like you have the latter under control.

It is not clear to me how you are using the feeler gauge to set the accelerator pump. My understanding of using the feeler gauge with the pump was to make sure the pump did not bottom out and potentially damage the pump diaphragm. I believe the feeler gauge is used to set the screw when the throttle blades are all the way open (full throttle) to give a little play at the end of the travel and stop the pump from bottoming out. I do not know if I misunderstood your approach. The way I read it, you are using the feeler gauge to set it when the throttle blades are closed. This could produce a situation where the gas will not squirt right when the blades open. I found my car to be very sensitive to this and if there was any play , it would have a lean spike on tip in.


I don’t know how the pump shot would come in too early. If it is running out and needs to be extended further into the throttle range, I would put in a bigger cam for a longer shot. The orange one one has a nice long shot.


Also, I do not know if I was clear about the WOT acceleration runs. I was suggesting that this would be while driving, not in neutral. I was interested in knowing whether the A/F when you get to the end of the runs (I assume around 5000 RPM) is reasonable. I would think the A/F should be somewhere 12’s or 13’s if your jets are right.


Good luck with the car. I hope you find a solution to the miss and rough running.

 

[powermyster]

Thanks again for the responses. That play is in the dissy, before the weighs engage.

It was running ok before I swapped out the caps the other day. I haven't replaced the coil either yet. I might get anew could and plugs, the other ones were pretty fouled up from having the idle set overly rich.

I'll go back at it with a fresh head later, just got pissed off yesterday when the timing seemed to jump about and she was popping.

Another thing I noticed was the dissy was bringing in vacuum advance real easy. I could suck on the vacuum line to the dissy would bring in a fair bit of timing.

 

[powermyster]

Ok I got a ready to run dissy and new coil. Sorted the recent issues. Tried a street demon 750 and still backfiring.

Pulled valve covers and nothing obvious wrong. I'm gonna go at the inlet today and start splaying vacuum lines see if I can't hunt this down. It must a small vacuum leak somewhere

Is there anywhere common and what's the best method for checking. Smoke?? Carb cleaner?? I noticed the connection on brake servo has a little play where it mounts to the servo

 

[Jcharger]

I don't know the best strategy for vacuum leaks. I would probably start by connecting a vacuum gauge, then seal up all vacuum ports to see if there was a change, and then to start checking mating surfaces, gaskets with something like WD-40 or carburetor cleaner. I have read about propane but never used it.

Another test might be to wire the secondaries shut and see if you have the same backfire.

 

[powermyster]

I got out to the garage this evening had to shave the head a little to get the new Pertronix the dissy to fit properly, didn't realise it was touching slightly when clamped. I removed the manifold checked gaskets, refit manifold and rocker covers. put new plugs in while I was at it (weird the rear plugs were fouled more than the front, front looked lean enough)

the only thing I didn't reconnect was the PCV, the hose was braided and on close examination the end nearest the PCV didn't look great. Set the timing by vacuum this time and snapped that throttle open a few times all seems good.

Cant take her out to test her because my throttle cable doesn't fit the new Street Demon properly, nor does my air filter. I need to fettle a little and tidy the wiring, only connected dissy and electronic choke loosely while testing. I'll get a filter spacer ordered and crack on hard wiring everything, take her out and set the timing curve.

Fingers crossed that's me sorted.