Stumble off idle -
 

I have a stroked 428 (447) with a mild cam. It has a holley vac secondary. It runs good, side pipes are tan inside and plugs indicate fuel mixture is about right.

The accelerator pump gives a good shot and both sides are equal. I am pretty certain it is a lean stumble.

The carb had a .031 squirter that I drilled in .002 increments. I am now at .037. The stumble is better but still there.

My gut tells me that I have gone far enough and may be chasing the wrong part.

What squirter size are you running in your setup that is similar? Should I go a few more drill sizes up or change accelerator pump cams?

John

What color cam do you have and are you in hole #1 or #2?

Go to the big acc. pump (50mm), took care of my stumble.

Grubby, watch the instructional video first: Holley TV : Mr. CarbTune's Garage - Accelerator Pump Tuning Make sure everything is adjusted properly first, then follow the advice.

Orange, screw in the top hole.

I suspect your set up should be exactly what I need.

John

Grubby .... rather than going bigger on the squirters and changing cams , look at the location of the primary butterfies on the transition slot . You should have .020" or less showing . If you are too far into the transition slot , you can get an off idle stumble . Another real simple "cure " to try ... richen up the mixture adjust screws about 1/4 to 1/2 turn ( out ) from where you get best idle .
Once you are sure you aren`t too far into the transition slots and going richer on the mixture screws doesn`t help , then look at changing cams/squirters . I`m at 31 on my squirters and no stumble at all on a big block 482 . Transition slot can barely be seen . Believe it or not , sometimes it is the simple things that work .

Bob

Make sure there are no vacuum leaks. Check your initial timing. Once that is done set your idle and adjust the mixture screws. The mixture screws should have an effect on the quality of the idle. If turned all the way in the car should stall. If not you'll need to pull the carb off and look at the throttle plates. The transition slots should be seen under the plates. about .020 is good but there is leeway. I try to get them balanced. Sometimes I'll set the primary at .020 and the secondary at .020 and use the secondary for minor adjustments on the idle. The primary plate may be open too far and out of the transition slot range causing the car to stumble due to ---well---no transition from idle to main circuit. To correct this you will have to open the secondaries up a little and you will be able to close the primaries--hence the balance I stated earlier.
Good Luck
Lou

Listen to Bobcat on this. I'm at 31, orange cam with the screw in hole #1, with no stumble. I would concentrate on making sure your carb is set up as per his post, of course double check your float levels, check your pump arm clearance as per the vid, and clean everything (yes, air bleeds, down the vents, etc.), richen the mixture a bit and see what happens. Honestly, if it was in front of me and you told me that you had been drilling the nozzle yourself, I might just buy a new one for $20 just to make sure there wasn't a problem in there somehow.

Make sure your timing is spot on also.

BTW, I think 99% of all carb problems are dirt related, or rather "goo related." You really can't keep them too clean. Today's gas turns in to a goo that is like maple syrup. Here's a shot of my recent leak and that brown stuff is dried gas that is sticky and gooey, just like syrup. And I use the Lucas Ethanol Gas Treatment, that has seemed to help over the last couple of years....

http://www.clubcobra.com/photopost/data/500/goo001.jpg

Get ahold of the edelbrock carburetor tuning manual. It's a great primer for all things carburetor. It came with their carbs at one point.

Immediately off of idle?

Where do you have your transfer slots set up on the primaries and secondaries?

I started under the assumption all was well and the carb was adjusted mostly correct. Should have known better. I will start from square one and go through the initial install instructions and check all of the typical stuff. Should get it done this week.

My timing is spot on. I checked that against the builders suggestion and it is right.

I will go through the check list this week and let you guys know what I find.

John

I just spent two hours of quality time with my Cobra.

Removed carb.
1. No indication of vaccum leak
2. Transfer slots were at about .028. So I backed off the idle screw to get them to .015.
3. The clearance under the accelerator pump bolt is .015
4. Confirmed float levels. Fuel just sloshes out when the car is shaking at idle.
5. Set the idle screws at 1.5 turns out.

Then I fired it up and warmed the engine.

Adjusted idle mixture screws with my vac gauge. Best vac was about 7 at 550 rpm idle. Screws are pretty close to 1 turn out. My vac gauge is old and beat up - it may not be accurate.

I do like the idle somewhat faster. Another 150 rpm would be nice, but that would open the transfer slots further.

The stumble was immediately off idle. That has moved. Off idle is now crisp.

Now if cruising at 1500 - 3000rpm and you get into the throttle pretty hard it stumbles. Moderate or less throttle and it pulls just fine. The stumble is a single, quick loss of power then it goes. The stumble decreases as you approach 3000 rpm. It is most noticeable at 1500.

I could not see any sign of black smoke under hard throttle. Primary jetting seems to be dead on. The side pipes and spark plugs are both a deep tan.

What are your suggestions? This does not seem to be the idle circuit.

John

Try putting a stock, unmolested, 31 nozzle back in before you do anything else. BTW, I idle at 750 with just under 13" of vacuum. Before you get side tracked with stuff like when your power valve opens, etc. I would first go back to the original nozzle.

Stumbles are lean conditions. I would add a bigger pump shot and also think about sucking the secondaries open slower.

If you want a higher idle, you can do it a couple of different ways. You can add a little more initial timing, which raises the idle speed. You can also richen the idle mixture a tad and jump it up.

Brent,

Would you add a bigger pump shot with a cam change? What color would you choose?

How much is a tad timing?

Thanks for the help.

John

Brent ... at 1500 rpm , isn`t he still in the idle circuit , and doesn`t it still have some effect up to about 2500 rpm ? If so , maybe richening up the mixture screws about 1/2 turn as you suggested will help a bunch .
On my 427 Galaxie with vacuum secondaries , if they dumped in too fast , the engine stumbled badly and then came on strong . Changed springs to delay opening and solved that issue . I also run 22 degrees initial timing and 35 max ( modified the advance curve ) .

John,

In your point 3 above, " The clearance under the accelerator pump bolt is .015", please say you understand the .015 is NOT under the bolt as some think.
The .015 is ADDITIONAL travel of the pump lever measured at WOT. It ensures the pump diaphragm won't bottom out at WOT.

If you have ANY slack/clearance in the pump mechanism, you'll have a flatspot off idle.

A tad is vague, I know. What are you at now? If you are at say 14 or so, I'd bump up to 16 and see what that does for you.

But...

I'd leave that part of the tuning towards the last and as an option.

The goal now is to get rid of the stumble and you really need to change one variable at a time. Do you have a bunch of parts at your disposal?

Bobcat, up towards 2000, I would think he's in the transition circuit and coming out of it, especially when he's nailing the gas.

Gaz also has a good point....