Carburetor Adjustment Problems

Nauset13 · Apr 11, 2009

I have a brand new built 331 stoker motor with a Speed Demon 650 Carb.

I'm having some trouble getting the carb tuned properly.

The timing is currently set to 13 degrees and 32 degrees advanced. It idles fine when in neutral and in drive. It accelerates fine when I accelerate smoothly. When the RPMs are higher (around 2500 or more) I can acellerate by pushing my foot down rather quickly with more or less no problem...

My problem is when I want to get on it: the engine chokes out and it will backfire when i stomp on the pedal. This happens when I'm at a stop or a slow roll while in 1st gear. It will also happen when I'm cruising and the RPMs are at a lower speed and I press down on the pedal quickly. Instead of going fast, the power drops out for a second or two and the engine will backfire. I then have to let off the gas and then re-apply the gas in a more smooth and slow fashion.

If anyone could give me some tips on what I ought to do, I would greatly appreciate the input! Thanks.

jlangholzj · Apr 11, 2009

Nauset13 said:
I have a brand new built 331 stoker motor with a Speed Demon 650 Carb.

I'm having some trouble getting the carb tuned properly.

The timing is currently set to 13 degrees and 32 degrees advanced. It idles fine when in neutral and in drive. It accelerates fine when I accelerate smoothly. When the RPMs are higher (around 2500 or more) I can acellerate by pushing my foot down rather quickly with more or less no problem...

My problem is when I want to get on it: the engine chokes out and it will backfire when i stomp on the pedal. This happens when I'm at a stop or a slow roll while in 1st gear. It will also happen when I'm cruising and the RPMs are at a lower speed and I press down on the pedal quickly. Instead of going fast, the power drops out for a second or two and the engine will backfire. I then have to let off the gas and then re-apply the gas in a more smooth and slow fashion.

If anyone could give me some tips on what I ought to do, I would greatly appreciate the input! Thanks.

hmm....few things

if you can get on it at a higher RPM, then i think it may have something due to air velocity at lower RPMS....any more info on the engine build, ie cam specs, intake manifold, heads, etc. ???

however, since your backfiring, sounds like you might be a smidge lean. holley tells you to screw the mixture screws 1.5 turns out from bottom for a baseline. did that on the 350 we just built and it was a bit lean around idle. had about the same charecteristics.

best thing to do is spend $20 on a vacuum gauge and tune per vacuum pressure, It'll help immensely!

woodsnake · Apr 11, 2009

Plus one for the vacuum gauge. Old skool is the only skool.

Try and adjust your timing also with the vacuum gauge, it almost sounds like you are to far advanced...

jlangholzj · Apr 11, 2009

woodsnake said:
Plus one for the vacuum gauge. Old skool is the only skool.

Try and adjust your timing also with the vacuum gauge, it almost sounds like you are to far advanced...

never heard about the timing???

baisically the same principle? tune for optimum vacuum??

D.Hearne · Apr 12, 2009

The mixture screws have nothing to do with it. Your accellerator pump's not set right. There's an adjustment nut/bolt on the lever on the side of the fuel bowl, you need to adjust all the slack out of it, certainly no more than .010" gap between the bolt and lever. When looking down into the throttle bore, you should get fuel squirting immediately upon moving the throttle shaft lever.

jlangholzj · Apr 12, 2009

D.Hearne said:
The mixture screws have nothing to do with it. Your accellerator pump's not set right. There's an adjustment nut/bolt on the lever on the side of the fuel bowl, you need to adjust all the slack out of it, certainly no more than .010" gap between the bolt and lever. When looking down into the throttle bore, you should get fuel squirting immediately upon moving the throttle shaft lever.

+1

i think he just wanted some general tuning tech too..

makes sense that it would be the pump shot though.

woodsnake · Apr 12, 2009

What I was thinking is that you will need to readjust the timing after you get the best engine vacuum, rather than trying to set the timing with the vac gauge.

Nauset13 · Apr 12, 2009

OK - Thanks everyone for the quick responses.

Here are some more spec's on the engine:

- Aluminum Cylinder heads 209CC intake runner - 2.055 Intake - 1.60 Exhaust
- Speed Demon 650 carb - mechanical secondaries
- H-beam rods
- flat top forged pistons
- about 10:1 compression
- single plane 'crosswind' intake
- 1.6 full roller rockers
- crane fireball ignition
- billet late model distributor
- custom hydraulic roller cam
- 224 intake - .550 lift - 110 degrees
- 233 exhaust - .565 lift - 114 degrees

I hope this is helpful - I should supposedly be getting about 430HP w/ about 400TQ

Also, my accelerator pump should be spot-on - i spent some time making sure that the pump arm had no slop. I used a feeler gauge to make sure there was no gap - about .005 if any gap exists at all...

Now, I do have a vacuum gauge. Do i stick it in the port on the back of the carb that the tranny goes to? Is there a chronological process that i need to go through and repeat for optimum performance; and what is it? how do i know that i'm setting everything up properly when i do set it up?

jlangholzj - do you think that my accelerator pumps are shot? fuel dumping out too fast perhaps? that's what my friend thinks.

I did try to set the air fuel mixture screws, but i made the problem worse. now it bogs down with light acceleration from a stop. could that be too little fuel?

what am i doing wrong? thanks everyone!

- K

jlangholzj · Apr 12, 2009

couple things,

pull your plugs, they should be slightly tan, if they're black, your rich, of they're closer to white, your lean.

the more you screw the mixture screws out, the more rich it becomes.

although I think i figured your problem out.

mechanical secondaries.

remember when i said that its sounded like an air velocity problem??? when you open the secondaries at too low of an RPM range, it acts ALOT like how you describe. When i force mine open with my hand, you can tell. it boggs pretty bad, b/c air velocity (baisically vacuum) is killed. motors operate off of vacuum.

not saying that anything else is bad, this is just what I'm leaning towards, like mentioned above, it could also be the accel shot.

so:

how long since last carb rebuild? (or since you bought it)

and for the vacuum, it should plug into a "full manifold pressure" . go to the spot where your carb lines into the intake manifold. put a t-fitting or someting on there so you can attach the vacuum line with the carb hooked up, and then cap it when its not in use.

these holley's are a blessing an a curse. If you know how to set them up (you will after a while ) there's not a whole lot better for adjustable. but if your having issues...be prepared...lol

1) timing
2) mixutre
3) check timing
4) idle screw
5) check mixture
6) check timing

after that, the pump shot and secondaries are adjustable on their own. with these carbs you just have to set, check, reset, and recheck. (in my experience anyway)

Nauset13 · Apr 12, 2009

Wow! thanks jlangholzj. Sounds like I have some fun ahead... I pulled a few plugs and they look like too much fuel is getting in the cylinders (plugs are more black than anything - i checked 4 and they were all black except one which was black and the side electrode was whitish). As for the vacuum lines; all I have is a line going from the back of teh carb to the transmission. nothing is going from the manifold to the carburetor.

I've had the carb for about 3-5 months... also everything else in the engine compartment is the same age.

So when i'm checking all these:
1) timing
2) mixutre
3) check timing
4) idle screw
5) check mixture
6) check timing

I should be using a vacuum gauge? Like, i should use a timing light AND a vacuum guage? and when i'm doing the mixture i should be using a vacuum gauge?

10secgoal · Apr 12, 2009

D.Hearne is right. It's your pump shot, no doubt. It's probably already extremely lean knowing Demons. You flick the throttle and it's going to go to 17-1 and backfire.

mikethebike · Apr 12, 2009

Ditto the accel-pump adjustment....but don't forget the power valve...rule of thumb is take vacum reading at idle...subtract 5...go to closest .5 valve size.

e
Example:

vacum at idle=...12
subtract 5.....=...7
select vavle...=..7.5 or 6.5

jlangholzj · Apr 12, 2009

Nauset13 said:
Wow! thanks jlangholzj. Sounds like I have some fun ahead... I pulled a few plugs and they look like too much fuel is getting in the cylinders (plugs are more black than anything - i checked 4 and they were all black except one which was black and the side electrode was whitish). As for the vacuum lines; all I have is a line going from the back of teh carb to the transmission. nothing is going from the manifold to the carburetor.

I've had the carb for about 3-5 months... also everything else in the engine compartment is the same age.

So when i'm checking all these:
1) timing
2) mixutre
3) check timing
4) idle screw
5) check mixture
6) check timing

I should be using a vacuum gauge? Like, i should use a timing light AND a vacuum guage? and when i'm doing the mixture i should be using a vacuum gauge?

well the vacuum gauge is what you use to CHECK the mixture screws. baisically when tuning the mixture, you tune for optimal vacuum.

but that line going to the tranny is what you want to hook into. (complete DUH that you don't have one to the carb....mech. secondaries )

the reason why i said what i did was that timing is first and foremost, usually, after you change the mixture, it *might change a bit* so then you reset the timing (if needed) then get your idle where you want it, and chances are you'll be too rich or too lean (usually rich) so then you set your mixture AGAIN>....then just to be sure, check your timing again. it should be 8-12 is and your full advance should be 32 ish. **this MAY be different for everyone, you won't know your timing untill you run the car, and if it pings or not.

after you get the mixture set, I'd check out that pump shot. then I'd adjust when the secondaries kick in (if it doesn't go away). guess i missed in your OP that it WAS backfiring. so take appart the accel pump and check it out, if its shot...replace...and you should be good, if not, its more than likely in the secondaries.

i'm just kinda a small fish in this pond...although...Sr. should step in and bash me if i tell you anything wrong. good luck !

D.Hearne · Apr 12, 2009

Check the accellerator pump linkage clearance first. If it's a lean condition like he describes, that's the most likely cause. If the pump diaphram was bad, he'd have a fuel leak too. The black plugs are most likely will be fixed with a jet and/or powervalve change.

blown65 · Apr 12, 2009

Listen to what D.Hearne is telling ya. You might need a bigger pump/nozzles/pump cams also if the linkage ends up right. Get your jetting right, and powervalve. Set your idle fuel, then get the accel circuit right. I always went by vacuum divided by 2 while in gear.

Nauset13 · Apr 21, 2009

OK - Thanks everyone! I will check on all these things when I have time; hopefully this weekend...

I'll let you all know what happens.

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