Accelerator pump tuning

[Gunner]

I've got this Speed Demon 750 pretty well dialed in - I will be taking it for dyno tuning in the spring, probably, but I'd like to get it tuned as best I can until then.

Everything is good except that it still bogs a little off the line, a problem the previous owner reported with the Holley 4160 as well.

Given that it seems to be running very well once off idle - I can get solid acceleration at any speed, no detonation and the plugs are clean - I think it's time to tweak the accelerator pump. It has the standard 30cc pump, pink cam and .031 squirters. Where to start? Is this engine likely to need the bigger 50cc pump? (Seems unlikely given that it's relatively mild... but at 446ci it's at the upper end of cubes for this size carb.)

More aggressive cam profile? Bigger squirters? Both? Neither? What's worked for you?

[RICK LAKE]

Gunner 2 things How much does the timing change under WOT, Need a timing light to see if the timing retards too much. The other thing is to get a flatter cam for the accel pump. 30cc is fine you really don't need a 50cc unless you are running a 700" cam, 12.5 and up compression. 6,000 rpms hole shots.
You can also bend the lever or adjust the screw spring if it's like a holley to slow down the squirt on accell. An LM-1 meter in the tail pipe will really tell you which way to go with the setup. The car may run OK, but A/F ratios could be off by 1-2 points and still show no driviability problems. Go to the dyno before you change any more things would be the safe thing to do.

[Jerry Clayton]

The pink cams , 30cc pumps and 031 squirters work pretty good on most all demon setups---you might try leaning the primary jets 2 numbers, putting in a 4.5 power valve and setting the butterflies where you just barely have the transfer slots open and then adjust the idle eze screw for an 850 rpm idle with the idle mixture screws about 3/4 to 1 turn open but not past 1 1/4 turn

[*13*]

Have you checked to see if the parts are stock for the carb? That's where I'd start. Then you'll have an idea how it's been tuned & have an idea where to go from there

[Jerry Clayton]

the demon carbs all have pink cams and 031 nozzles---most holleys have 031 nozzles also but cams vary

most off idle bogs are ignition advance related or the carb has issues with the thrittle plates and the transfer circuits

[Bobcat]

Don`t know if the Demon has the same circuits as the Holley , but on the Holley , the float levels have an effect on when the off idle circuits come into play . Lower float levels delay actuation and higher float levels bring the circuits into play sooner .

Bob

[Gunner]

Thanks for all the suggestions, but I'm generally past all of these points and down to either accelerator pump tweaks or a more subtle overall setup problem. I have parts for the pump on the way; as they take about two minutes each to swap it's pretty simple to experiment with a larger squirter and then one cam profile and another. No fix, I go back to the .031s and the pink cam and look elsewhere.

For the record:

• Stock, out of the box carb.
• Primary jets dropped two sizes (76-74); will do more tweaking later if it seems necessary.
• Pink follower cam (stock), 0.031 squirters (stock)
• Full bench prep done including butterfly adjustments and accel pump follower adjustment.
• Full baseline idle mix adjustment made using vacuum gauge.
• Idle-Eze tweaked a little, to no effect; returned to base setting.
• Ignition timing checked; 15* initial gives 38* total at about 3000 (builder mark on damper for total advance used).

Oh, yes, I did tweak the float levels to a little on the high side as well.

The engine is running beautifully - perhaps a little off optimum, but not by shade-tree standards. But it maintains a little bog off idle and my current suspect is the squirt system. I'll know in a few days.

I don't want to waste time and money on a dyno tune until spring, for a number of reasons. I'll take my personal best with no obvious flat spots for now. Once I have this bog solved, I need to tweak the vacuum advance - the engine is lower vacuum than the stock spring assumes and I need to get into the secondaries before 4000.

No one quite addressed my original question (other than to confirm that a 50cc pump is probably unnecessary)... is it likely to be squirt size or timing, or both?

[*13*]

I think it's a tough question to answer. I'd try a size up on the nozzle & see what happens. If nothing try a size down. one way or another you should notice something. I had a 850 on a BB chev that bogged a little as you describe. The car had huge gearing so there was just no getting around it. I had the carb set up as good as it could get. I switched to a 650 carb & the car ran great. Your car isnt a 59 Cad with 2.92 rear gears though. My guess is that the squirter may remedy the problem.

[lovehamr]

Gunner, the first thing I'd have looked at would be the accel pump system, the squirter being high on the list. Something that hasn't been mentioned yet would be the pump arm spring. Sometimes going to a stiffer spring will bring the squirt in quicker than the stock spring. Let us know what you come up with, got my curiosity up.

Steve

[Gunner]

I can't see a smaller squirter doing much good (except that it would extend the acceleration squirt)... it really feels like not enough dino juice for that critical half-second. We'll see. One way or the other.

I'm a lot more excited to tweak the secondary vacuum setting. I don't even know where it comes in, except that the actuator *starts* to move at about 4000. I don't think I've ever had secondaries come in on this engine! But the stock spring is set to start at around 1900 on a slightly smaller engine (with normal idle vacuum in the 15-20 inch range... this one has about 11) and fully open at 6k, which is the rev limit. I'll back up one spring at a time until I get the secondaries started around 2200, that should be good for some fun on the bayou... as in blow bayou on the freeway.

[*13*]

You are down two sizes on your main jets "may" mean that you are dumping too much, too fast from the nozzles into the carb causing a slight bog. I agree that you may need more fuel but like I said, it's tough to say without knowing everything. Try bigger first. If that does nothing positive, go smaller.
I didn't notice that you have a vacuum secondary carb. That also may be a cause. If you can't get the secondary to open correctly with the proper spring change try a double pumper :0)

[Gunner]

Nah, I very deliberately passed on a mechanical secondary, dual pumps or not. Other than instant WOT response, vacuums are generally easier to tune and live with. I'd have fixed that problem days ago but my local speed shop (the only one in quite a radius) was out of the tuning spring kit. So I have a whole care package coming from JEGS.

[eschaider]

Gunner,

My experience with the stumble you are describing has always been caused by a lack of dino juice. I would consider a step up in nozzle diameter and then a step up in pump volume if the diameter does not fix the problem.

When you hit the throttle not all the fuel goes into the cylinder. A small amount of it attaches itself to the manifold and port walls and evaporates from both because of airflow passing over it and engine heat. Your job as the tuner is to reach a happy balance where the liquid on the manifold and port walls continues to evaporate in sufficient quantity (without drying up) to fulfill the engine's acceleration enrichment requirements.

When you literally inject from the accelerator pump an insufficient quantity of fuel to properly wet the manifold and intake port walls for your engine displacement you will experience stumble. When you overdo it you will see a black smoke 'cloud' briefly out the exhaust and loud but lazy throttle response. When you are just about right you will see a quick puff of black smoke and the engine rpm will jump in a fashion that sounds like it is instantaneous. Your engine will sound crisp and responsive.

By the way the Detroit guys call that thin film of fuel on the manifold and intake port walls Tau(t). The greater the volume of air passing through the intake the faster this film will evaporate. As the tuner you need to maintain this film for proper throttle response. More air volume (larger displacement) needs more fuel (larger nozzle diameter and larger accelerator pump shot) to maintain the fuel film during the engine acceleration event.

Ed

[*13*]

Cool, Carbs are fun to fiddle with, especially when you get them dialed in correctly!

Originally Posted by Gunner

Nah, I very deliberately passed on a mechanical secondary, dual pumps or not. Other than instant WOT response, vacuums are generally easier to tune and live with. I'd have fixed that problem days ago but my local speed shop (the only one in quite a radius) was out of the tuning spring kit. So I have a whole care package coming from JEGS.

[Jerry Clayton]

I also did not notice that you had a vacume seconary carb

However--your ignition curve isn't fast enough to give you the advance you need off idle--try this to prove/disprove it---if you have a MSD dist, take one spring off and using a piece of wire tie the advance mech in full closed position, start the car and check timing, then advance dist to around 40 degrees, and see if the bog is gone--(you probably will need to reset the timing to shut off the engine without dieseling or kill it in gear)

work with this until you find the timing needed at idle to eliminate the bog and then set your curve with that as initial and whatever springs/weights you need to get all your advance in by 2000 rpm--or you will never have full advance at cruising rpm---

Also you didn't say that you tried a lower PV and generally I find the 750 nvac carbs work better around # 70 primary jets.

[elmariachi]

I am running a stock Mighty Demon 750 with annular boosters and mechanical secondaries and I had the same bog. After jacking around with the carb and getting nowhere I decided to take Don Gould's (4secondsflat.com) recommendation and make some drastic timing changes. I was at 16*+20 advance with medium/heavy springs but I am now testing things at 20* initial +14 advance, two blue springs, MSD mechanical. The bog then went away leaving me only with a slight hesitation off idle and a sooty base ring on the plugs. So we ran the Idle-Eze out to the 1.5-2 turns out zone, dropped to a 3.5 power valve and fattened up the mixture to 1 3/4 turns out. I know everyone will say that's a bit too rich but the plugs now look spectacular. And we reconfirmed the transition slots were correct/square after all this.

Car now idles at 850 rpms with an AWESOME pattern (uh baadda bump, uh baadda bump) and she'll jump out of her skin when I hammer it. This is with a MR side oiler, Sidewinder intake, stock heads and Comp 294S.

[Gunner]

Elmariachi, I expect to tweak many things on the dyno but for now I'm staying out of the distributor curve. I will likely run out of time, weather and temporary tags before I can get to distributor tuning anyway. (As it is, I won't get the carb junk from JEGS until Thursday, meaning lost days of weather and tags...)

If I have time, though, and if this final carb tweaking doesn't get me a sharper off the line response, I'll start with the distributor.

[elmariachi]

I was in the same boat you are in, I wanted to resolve as many of these issues ahead of time to get the car driveable and to minimize chassis dyno time/expense. But in my opinion, you cannot solve these subtle carb issues until the timing is right, including and especially the curve.

Not trying to be argumentative, but if it were me, I'd first lock the timing out of the distributor at idle. Then I'd baseline that carb to 1.75 turns out on all 4 corners, confirm the transition slots are correct and use the Idle-Eze to get a level idle. Then I'd unlock the advance and start trying to experiment with initial and advance settings. Once you get that done, then you can finish with the carb tuning. But I think Jerry Clayton is right, you are likely seeing issues caused by timing not matching the carburetion, not the other way around.

Good luck.

[Zoom This]

I tore my hair out trying to eliminate an off idle bog on my 418W when I had a Holley 850 on it. I did everything and more to work around it and gave up, sold the carb. I bought a 650 Mighty Demon with mechanical secondaries, modified the timing as elmariachi explained. Instant off idle response, no bog. I gave up about 12 HP at the top end for 15 lbs. increase in torque thru the midrange using the 650 MD. Not a bad trade-off. I tweaked the jets a little on the chassis dyno and have used this combo for 4 years now.

[jhv48]

Before you get all involved in changing everything else, try opening up the idle air screws about 1/8 to 1/4 turn and see if that helps. It worked on my Speed Demon.