New carb driving me crazy

cfd2bc · 10-13-2009, 11:29 AM

I have read all of the threads on this and have tried different things. I have a 1406 that I changed to manual choke. I have adjusted the idle mix screws and put the accel pump in the highest hole (for the biggest squirt). It is still stumbling when I give it gas.

I can give it a little choke and it runs fantastic. So it appears that it is lean on the off idle to gas transition. Am I correct? If so, how do I correct this?

I have a 289 with performer intake, a small rv cam and a pertronix 3.

Thanks, Jeff

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kalli · 10-13-2009, 12:06 PM

first one with the pertronix 3 ? :-)

as to your question, you say it's better when choke (manual) is a bit on. The difference is not only that choke causes it to run richer, but also that idle is at higher rpms. that might be the difference (the throttleplate being opened wider)
unfortunately i don't know any other techniques on an edelbrocj to adjust the pump volume. is there anything on the linkage that you could modify so it would kick in sooner ?

THUMPIN455 · 10-13-2009, 02:06 PM

Try raising the float levels. While it isnt as easy as a Holley to adjust the floats, it can still be done. Other than that, cant tell you much without being there.

urban_cowboy · 10-13-2009, 02:55 PM

Assuming your idle AFR is perfect and your low speed AFR is perfect, you need to look at the transition circuit which is the accelerator pump. There is a hesitation in fuel flow and a drop in the atomization of fuel during vacuum loss that causes a lean condition when going from one circuit to another, especially when the throttle is moved quickly. All but the best tuned carbs will hesitate when you move the throttle very quickly. The accelerator nozzle helps with that. Now, if either of the above assumptions are wrong and the AFR of those two circuits is not correct, then this all goes out the window. By having the choke on, you raise the idle rpm which puts you more into the low speed circuit and less in the idle circuit. You are not transitioning from one circuit to the other because you are already in the second circuit. This on top of restricting some air flow is why you do not have an issue when you put a little choke on. This is a poor fix for the problem.

Have you checked your plugs or had someone monitor the AFR during those circuits with a wideband O2?

Things to look at:
Carb too big (poor atomization)
Idle AFR wrong
LS circuit AFR wrong
Transition circuit shot to little or curve too slow
Fuel bowl level (this affect emulsification)

Starfury · 10-13-2009, 05:08 PM

I'm going to back up a step and ask, is your timing set properly? Not only initial (@ idle), but the entire curve? Poor timing can easily cause hesitation. Until your timing's set properly, you'll be chasing your tail trying to get the carb tuned.

When exactly is it stumbling? Off-idle, or when going from cruise to power? The first thing I'd look at, assuming the timing is correct (which I kinda doubt, assuming you're running a stock distributor), is the primary jets. Too lean during cruise mode and you'll never be able to get enough fuel from the accelerator pump to iron out the miss/bog when transitioning to power mode. After that, I'd start looking at stepup springs. Too weak and you won't get enough fuel when transitioning to power mode, causing a misfire or bog. Too strong and you'll get too much fuel too soon. Not as big of an issue, but it can cause a dead spot, especially when going from cruise to power mode.

Edelbrocks are a bit finicky and much different than Holley designs, but not difficult to tune at all once you get used to them. Being able to do everything on the side of the road is a plus

cfd2bc · 10-13-2009, 05:48 PM

The timing is set at 14 d BTDC. Stumbling occurs when I say, pull away from a light. It takes off fine and shifts into second. If I go to give it a little more gas, it stumbles. If I go REALLY slow on accelleration in 2nd it feels like it wants to stumble. If I get on it hard it runs great. No stumble on kickdown up through 3rd to the floor. Off the line WOT all the way up is good too.

I will play with the timing to see if that helps.

I am going to adjust the floats as well.

Thanks for all of the help.

Starfury · 10-13-2009, 05:54 PM

Like I said, the entire advance curve needs to be checked, not just advance at idle. Speaking of which, 14* @ idle is a lot for an RV cam. I run that much with a Magnum 280H cam (lots of valve overlap) and aluminum heads. You shouldn't need that much.

That said, it sounds like your cruise mix is lean. Swap rods for a richer cruise mix and see what happens. What stepup springs are you running? If richening the cruise mix doesn't completely fix it, you may need to go to stronger springs.

AJ_68 · 10-13-2009, 06:03 PM

Out of curiosity, why did you convert it from electric to manual choke? I have a 1405 (manual choke) that I was planning to add an electric choke to.

Starfury · 10-13-2009, 06:04 PM

I have an electric choke on mine and I don't like it. I have to adjust it too often, and the erratic weather conditions on the coast make it a PITA. Either I adjust it to work in the cold and it doesn't choke in the middle of the day, or I adjust it for warm weather and it chokes too much and over-revs in the cold. At some point I'll probably convert to manual.

cfd2bc · 10-13-2009, 06:13 PM

I read threads on here about the elec vs man choke and decided to go that route for the same reasons as above.

I swapped rods earlier today to 1455's (7342) and with everything else the same it really ran bad. Stumbled worse. I am using the 4 in. springs. Should I try the rods I have in it with different springs?