Rumpity Rump

If so, what was your car doing before...and did you go up or down on the Power Valve vaccuum rating number ?

Thanks, RR

467-Ratman

I had some stumble two weeks ago, after an unfortunate back fire! I replaced both power valves with Holley 8.5 size, seem to work well with my motor that has about 14 inches of vacuum. The previous ones were 10, and not made by Holley. How do you like your Speed demon 850 carb?

Paul 75 L82

Take your vac at idle divide in half and add 1" to that and thats the one to buy.

Rumpity Rump

Paul,
Holley website says to take reading and select one that is 2" lower. But, i might try your approach. Seems like i need fuel enrichment sooner than what im getting it now which accounts for the stumble in the 1500-2500 rpm range, when i depress the pedal suddenly. Hope that cures the problem.
Ive gone down on the jet size from 80/85 to 76/78 which gives me the right color on the spark plugs...and ive increased the squirter nozzle size from 42 to 47 on both primary and secondary. That helped the situation, but i still have a momentary stumble.

RR

Rumpity Rump

467,

i like my 850 Speed Demon, but wish i could eliminate this stumble.

I really like the idea of a chokeless carb -when i start it up cold...it idles well at 700 rpm. (I never liked it revving at 2000 rpms right away with the oil cold, using a choke) . After its warmed up, the idle comes up to about 950 rpms which i like too.

RR

gkull

I followed the above formula. I have 13-14 at idle so I called it 6.5 for roughly 1/2 and added one so I installed a 7.5 front and rear and the stumble from off idle to WOT went down. My Demons both came with 6.5's

Right now I have 76/81 jets in my 750 My 825 Race Demon liked 77/ 82

Fevre

I am pulling 16.5 on the ole vac gauge so I went to a 9.5 from the stock 6.5 on my front one and the stuble went away. Might have to adj your pump shot again too if you tried to use the acc pump to get rid of the stumble like I did. I was also able to re-jet down a size too. The carb is a system and changing one part affects the whole thing. Can't wait to install my headers and Edlebrock O2 sensor. :yesnod:

Rumpity Rump

If i understand this correctly then...the higher the Power Valve rating in inches of vaccuum, the SOONER the enrichment of fuel will take place (thereby eliminating the stumble) ???

RR

Langadorf

Stumble and bog show very similar symptoms. The wrong size too high or too low of a power valve can cause either. Generally, you want a power valve rated at half the vacuum you pull at idle. If you have that and are still getting bog/stumble, you need to play with your accelerator pump settings (cam, nozzle, etc.).

[EDIT] For the street you typically only want a power valve in the front and keep it blocked off in the back.


[Modified by Langadorf, 10:22 PM 8/14/2002]

gkull

the higher the Power Valve rating in inches of vaccuum, the SOONER the enrichment of fuel will take place. This is a true statement

As I understand it using the roughly 1/2 of your idle vacuum is to keep it from adding additional fuel durring small throttle blade changes. I drove around with my vacuum gauge on the window to actually see how vacuum is durring normal driving.

427V8

I think you are tuning the wrong circuit to remove stumble.
Stumble is caused by a lean condition.
So either your accel pump isn't giving enough gas off idle, or your secondaries are coming in too soon...

Rumpity Rump

427,

I dont have a problem off idle with stumble ; the stumble i have is between 1500 to approx. 2500 rpms while holding steady , then pushing suddenly on the accelerator pedal.

With the engine off, i slowly moved the accelerator linkage, and my secondaries are opening BEFORE my primaries are fully open ; it has always been this way since it was new and i assumed its the way Demon works it. (I have mechanical secondaries).

Another question : Will Holley Power Valves fit in the Speed Demon carb ??

Thanks, RR

Fevre

The power valve is like a long period helper, it opens when vac drops and stays open until vac is above the powervalve rating, while the acc pump is like a short period helper, it shoots a shot of fuel just long enough and rich enough to help the engine until the main's and powervalve's fuel flow can reach the engine. If your powervalve rating is too far off it will either run too rich at idle, meaning your powervalve is rated too high for your engine's vac level at idle or it will not open at all causing your car to stumble after the the acc pump shot it used up unless you have jetted it too rich in order to compensate for the lack of powervalve enrichening. I believe the order of tuning is: powervalve then mains then idle then acc pump then sec. This is straight from the Guru himself, Lars. I think many either do it in the wrong order or leave out the powervalve check all together. I followed Lar's tuning in the tech section and once I replaced the powervalve with a higher rating I was able to tune it too almost perfect, well at least much, much better. Thanks Lars! ;)

Rumpity Rump

Fevre,

SO then, if the engine is still stumbling after the pump shot is used up, then one needs a Power Valve with a HIGHER rating or LOWER rating , to help compensate ???

Does Lars talk about tuning a carb in the Tech section?

Thanks, RR

MotorHead

I have to agree but I may add that vacuum leaks are notorious for causing of off idle stumble, make sure you have no leaks anywhere before you start swapping out parts.

Like the quote above a leak ( sucking in air somewhere, coulld be anything from a worn carb base gasket to a hole in a hose ) will cause a lean condition





[Modified by MotorHead, 10:32 PM 8/14/2002]

Fevre

:yesnod: Look under the c-3 in the tech section. After changing my powervalve I was able to tune out my stumple.

Matt Gruber

i bought the TRICK KIT which has a bunch of shooters, and pump cams for tuning pump shot. worked good :yesnod:
i also staggered the sizes
'
note; primary opens ~1/2, then 2nd's start to open

Paul 75 L82

Many times a stumble of hesatation is caused by the throttle blades being set to far open in the idle condition. The little slot that you see inside the body just needs about a 1/16" or so showing when the plates are agaisnt the idle stop. Any more of the slot showing than that and you will have a stumble. I corrected this and it really helped off idle.

427V8

Rumpity
Mechanical secondaries are your 'problem'

With mechanicals you can't just mash it and go!
You have to know who much you can open it or you WILL get a lean condition as the secondaries open before the engine can use all that airflow.

My understanding has always been that mechanical secondaries are for drag racing where the engine is always screaming allong at WOT and the driver doesn't have to worry about part throttle driving...

Rumpity Rump

427

Ive always read that, you should go with mech. secs if you have a 4 speed, and vaccuum secs if you have an auto trans.

Im not sure what setting my Power Valves are in my 850 Speed Demon , but im going to increase them so they come into play sooner under heavy acceleration demand to see if that clears up the stumble within the 1500-2500 rpm range.

I had the engine rebuilt with a cam that has greater overlap, so i suspect i need a Power Valve change.

RR