What would cause this surging?

[magnumdust]

I was doing a little tweaking to my carb and this intermittent surge appeared. Basically i would crack the throttle(in park) and the engine would have an rpm surge that would go from say (1500-2k)-0-(1500-2k)-0 even after i had let off the throttle. Another crack and it'd go back to normal. The first time it did it, i shut the car off immediately and it puked fuel up out of the carb.

Adjustments: set the floats per the manual, adjusted my idle mixture to read in the 12-13:1 range.

 

[Abodybomber]

Does it act the same,driving in gear?

 

[OldmanRick]

If i'm understanding correctly, it sound like the primary needle/float could be hung open? Didn't you put a new Summit 600 on it?

 

[Abodybomber]

Good possibility on that one,if the newer Summit carb.Ran a double pumper 600 like that on a 5.0.Did exactly your previous statement,Rick.The carbs are sensitive to float adjustments.

 

[magnumdust]

Does it act the same,driving in gear?

I'm not sure, i didnt want to take it out and have it do that on me(and who knows maybe quit) and then be pushing it home.

OldmanRick

yeah, except its the 750 vac. Originally the car was doing a similar surge then backfire at highway speeds w/ mid throttle and ran fine otherwise. I asked around awhile back and under some advice i adjusted the floats(goofed up on it, and ruined those seals for the lock screw/nut, bought new ones) and then i had the car running, but today realized that the float level was too low, i set it per the instructions(which is fuel level in the middle of the window)

 

[Abodybomber]

Magnum,sounds like a classic case of too lean in the main metering ciurcut.Can't remember your cam specs.Have you adjusted the mixture screws,to highest manifold vacuum with a vacuum guage?Idle mixture adjustment,actually effects everything up to about 2500,3000 rpm.

 

[OldmanRick]

I agree "bomber" as far as the surge goes. Can't quite figure the puking fuel though. These carbs are copies of the 4010/4011 Holleys with the one piece top. I have heard that they are sensitive to float levels? Magnum, maybe try lowering them a little at a time and see if you can hit the sweet spot? Only other thing i can think is the pressure is too high, 7lbs absolute max, would probably prefer 5lbs, but i believe your running a regulation mechanical pump so that shouldn't be a issue. Just my current thinking, any other suggestions may help and teach me something too.

 

[magnumdust]

Magnum,sounds like a classic case of too lean in the main metering ciurcut.Can't remember your cam specs.Have you adjusted the mixture screws,to highest manifold vacuum with a vacuum guage?Idle mixture adjustment,actually effects everything up to about 2500,3000 rpm.

I have not, i was trying to tune solely by the wideband, but i guess i'll have to see if kragen rents them tomorrow or just buy one next week.

rick,

I'm running an electric pump @ 7 psi since magnums dont have the right part on the cam to use an mech pump, unless you get the hughes adapter. I can try to put on the cheap regulator tomorrow from when i had my edelbrock on it. and see how it does at 5-5.5psi. I had removed it because the instructions for the carb said they use about 7 psi. And it only puked fuel once during that surge. Otherwise it does not do it, but diesels occasionally.

 

[Abodybomber]

Reg it down,to 5.Post,if you need help on adjustments.Commend you,for sorting it out.Please be patient.

 

[OldmanRick]

You may be ok as far as pressure, although i think your close to the max that carb will tolerate, jmo. I reread and saw that the idle mixture was at 12/13.0. That's pretty rich for idle. I'm thinking more like the 14.5 range. I think with the 7lbs, i'd try and drop the floats a little and recheck those mixture numbers.......That might lean it up a bit. I'm not saying there can't be other issues, just trying to focus on one thing at time to see if a better result is possible?

P.S. Not trying to add to the confusion, but what is your idle speed? You mensioned dieseling. That's usually a result of a lot of initial timing and a higher idle speed.

 

[1wild&crazyguy]

5psi is plenty, after checking fuel bowl level and adjusting if need be...i would regulate the fuel psi to 5 lbs.
Also, what r the idle screws turned out to? and how initial timing?

Just to be sure, ur saying it will rev up and down 1500-2000 then almost die and then do it all over again?

 

[magnumdust]

You may be ok as far as pressure, although i think your close to the max that carb will tolerate, jmo. I reread and saw that the idle mixture was at 12/13.0. That's pretty rich for idle. I'm thinking more like the 14.5 range. I think with the 7lbs, i'd try and drop the floats a little and recheck those mixture numbers.......That might lean it up a bit. I'm not saying there can't be other issues, just trying to focus on one thing at time to see if a better result is possible?

P.S. Not trying to add to the confusion, but what is your idle speed? You mensioned dieseling. That's usually a result of a lot of initial timing and a higher idle speed.

ok so here's the update, picked up a vac gauge and gave it a try. I adjusted the idle screws for the max vacuum which was about 17 give/take(needle shook, but a site i found suggested that this minor shaking is probably because of my cam:
http://www.secondchancegarage.com/public/186.cfm
(scenario 3), i have video too, but i'll have to chase down the usb cable for my phone.

After that adjustment it showed a steady 13:1 range at idle, came inside to post that and it occured to me to try lowering my floats. I did so and the idle hoves in the 14.2:1-15:1 range at idle.

Hopefully i can go for a little drive on tuesday and see how the car feels.

 

[OldmanRick]

Those new numbers sound much better to me. Hopefully you'll get good results on your test drive.

I will just mention something i've found on Holley's that i'll pass along, although some may not agree with me.......On occasion, setting the A/F to max vacuum will put it right at the "lean" edge. If there seemed to be any softness in light acceleration coming off idle, i like to back off (richen) the fuel mixture screws 1/16 to 1/8th of a turn to keep just this side of being too lean.

Good Luck.....you've had enough carb issues, now it's time for some fun .

 

[magnumdust]

Those new numbers sound much better to me. Hopefully you'll get good results on your test drive.

I will just mention something i've found on Holley's that i'll pass along, although some may not agree with me.......On occasion, setting the A/F to max vacuum will put it right at the "lean" edge. If there seemed to be any softness in light acceleration coming off idle, i like to back off (richen) the fuel mixture screws 1/16 to 1/8th of a turn to keep just this side of being too lean.

Good Luck.....you've had enough carb issues, now it's time for some fun .

I'm really hoping so, It gets frustrating when you have a car that looks great, but is awful to drive, but you can't sell it because the potential is there.

At this point efi is not just a possibility, its a definite upgrade in the near future. However, i'll be thrilled if this little bit of work can help me avoid the efi for a few months while i re-do the ignition and trans.

 

[magnumdust]

Okay test drive update. Good news: I think i can stand to put off efi for a month and get my msd distributor. Bad news: still issues

First one is a minor nuisance. My elect. choke is not adjusted right so it idles too high and for too long.(i know i just need to look up the inst. and adjust, no biggey)

Now onto driving, after finally getting my car to warm up, i got to cruise and at low speed ran fairly good but a little rich at 13.1-13.5, and at WOT the a/f ratio was staying around 15:1. And it would have an off idle sputter/backfire that seemed to occur without a pattern. Sometimes it did it, sometimes it ran fine from a light.

 

[OldmanRick]

Okay test drive update. Good news: I think i can stand to put off efi for a month and get my msd distributor. Bad news: still issues

First one is a minor nuisance. My elect. choke is not adjusted right so it idles too high and for too long.(i know i just need to look up the inst. and adjust, no biggey)

Now onto driving, after finally getting my car to warm up, i got to cruise and at low speed ran fairly good but a little rich at 13.1-13.5, and at WOT the a/f ratio was staying around 15:1. And it would have an off idle sputter/backfire that seemed to occur without a pattern. Sometimes it did it, sometimes it ran fine from a light.

Hmmm. As far as the off idle, first place to check always has to be the accelerator pump adjustment, especially if you've changed the idle speed screw location at all. If that checks ok, i would try opening the mixture screws a little as i talked about above. For some reason i found it to help with some transitional issues. Another shot in the dark since you mentioned the dizzy, are you using ported vac advance? And could it be causing the stutter as it kicks in with the throttle opening.

Still thinking about the reason for the lean WOT. Just so others know that read this, as delivered this carb should have.....

77 prim. jet
79 secd. jet
#35 pump nozzle
9.5 Power valve

Plus the fact that you have ample fuel pressure it's kind of hard for me to find a reason for it to go lean.

 

[magnumdust]

Hmmm. As far as the off idle, first place to check always has to be the accelerator pump adjustment, especially if you've changed the idle speed screw location at all. If that checks ok, i would try opening the mixture screws a little as i talked about above. For some reason i found it to help with some transitional issues. Another shot in the dark since you mentioned the dizzy, are you using ported vac advance? And could it be causing the stutter as it kicks in with the throttle opening.

Still thinking about the reason for the lean WOT. Just so others know that read this, as delivered this carb should have.....

77 prim. jet
79 secd. jet
#35 pump nozzle
9.5 Power valve

Plus the fact that you have ample fuel pressure it's kind of hard for me to find a reason for it to go lean.

yeah i do have ported vac advance.

 

[OldmanRick]

Bump for some more opinions..........

I have to admit that while i know A/F ratio's, i haven't used the newer electronics that are available now. I'm from the butt drivability, plug reading, track mph age. Hope someone will have some thought's here.

I am curious that the secondaries may not be opening at the needed rpm, but the WOT numbers shouldn't be consistently lean. Maybe leaner with more rpm, but not across the board.

 

[magnumdust]

Bump for some more opinions..........

I have to admit that while i know A/F ratio's, i haven't used the newer electronics that are available now. I'm from the butt drivability, plug reading, track mph age. Hope someone will have some thought's here.

I am curious that the secondaries may not be opening at the needed rpm, but the WOT numbers shouldn't be consistently lean. Maybe leaner with more rpm, but not across the board.

I can charge up my gopro tonight and get some video tomorrow. I'll point it at the A/f gauge and go for a drive.

 

[OldmanRick]

Where is everybody.....

Just so you know what i think, and using round numbers.........Idle in the 14s. Cruise in the 13s, WOT in the 12's. Obviously fine tuning would narrow those down. Is your A/F monitor a gauge only or can you download from it to map out the numbers. If it's the gauge only and you want to video, please be careful trying to run around and video at the same time. We don't need any issues trying to fix a stupid carb...:smile:.

 

[1wild&crazyguy]

Okay test drive update. Good news: I think i can stand to put off efi for a month and get my msd distributor. Bad news: still issues

First one is a minor nuisance. My elect. choke is not adjusted right so it idles too high and for too long.(i know i just need to look up the inst. and adjust, no biggey)

Now onto driving, after finally getting my car to warm up, i got to cruise and at low speed ran fairly good but a little rich at 13.1-13.5, and at WOT the a/f ratio was staying around 15:1. And it would have an off idle sputter/backfire that seemed to occur without a pattern. Sometimes it did it, sometimes it ran fine from a light.

What power valve and which spring in the secondary?

wait ..lol wtf kinda carb is this??

With 17 inches of idle in gear vacuum, you should run the largest power valve, which i think is a 10.5. The pump cam you should try is a orange one, thats to say this thing uses holley replacement parts...idk sheet about these slummit carbs.fyi

 

[OldmanRick]

Just so others know that read this, as delivered this carb should have.....

77 prim. jet
79 secd. jet
#35 pump nozzle
9.5 Power valve

^^^^^^^^^^^^^

 

[1wild&crazyguy]

what size carb is it though, see a holley 600 comes with 67's or something in the primary, this thing is reminding me of a demon with the big *** jets.

im assuming that with jetting so close, that is has 2 power valves?

if its lean at wot, id spread the jets 8-10 sizes=85 or 87 secondary

 

[magnumdust]

what size carb is it though, see a holley 600 comes with 67's or something in the primary, this thing is reminding me of a demon with the big *** jets.

im assuming that with jetting so close, that is has 2 power valves?

if its lean at wot, id spread the jets 8-10 sizes=85 or 87 secondary

This is a 750 vac.

The carb is basically an updated version of this holley/autolite carb:

And before you ask the obvious question here is the answer: Read several forum reviews, it had good reviews from the people who bought it and tried it on their car.

 

[OldmanRick]

Wild, i double checked Summit and they list a 6.5 for the secondary.

Nothing bad to say in this shootout....http://www.carcraft.com/techarticles/ccrp_1210_seven_budget_carburetors_tested/viewall.html