weber gurus wanted - sputter under light load

[G50911]

hey pelicans. i've been trying to dial in my carbs and have it set to as best as i can at the moment. the pick up is pretty good (aside from low end flat spot).

my problem is at light load, i get this sputter or spitting up the velocity stacks. if i get on it, it smooths out and seems fine. i've searched both pelican and other pcar forums and i've read a few potential fixes. i'm hoping someone here, a weber guru can help me out. here's my current set up...

motor:
1976 2.7
MSD ignition
MSD coil
fuel pressure is 3.5

carbs:
40IDA 3C carbs
32mm chokes
180 Airs
F5 emulsion
140 mains
55 idles
PMO manifolds
opened to 36mm and blend

one post i read, the guy had the exact same symptoms and he dropped down to the next size idle jets and it fixed his problem.

here's a direct quote...

By reducing the idle jet from 65 to 60, you are reducting the amount of air entering the circuit and therefore richening the mixture coming out of the circuit. Therefore, the lean condition registering on the O2 sensor gets richened up and voila, runs great!

[ddinham]

G50,
Aren't those 34mm chokes?
What size were the intake ports on the heads?

[G50911]

i swapped out the 34mm chokes to 32's per a wrenches recommendation. not sure about the intake ports. how can i find out?

here are cam details:

Intake valve overlap 0.47
Intake opens: 6* ATDC
Intake closes: 50* ABDC
Exhaust opens: 24* BBDC
Exhaust closes: 2* BTDC

[lindy 911]

Fatten up the idle jet and see what you get. Sounds lean on the bottom to me.

[lindy 911]

The quote above about reducing the idle jet form 65 to 60 and reducing the amount of air is simply not true. The jets regulate the amount of fuel, not air. If you reduce the idle jet from 65 to 60, you are leaning the mixture. A sputter up through the carb is a classic sign of being lean.

[jpnovak]

Classic symptoms of a lean mixture.

What rpm? Just a certain rpm range? Is it about 2800 to 3500?

If it is just a certain rpm range then you might try opening up the idle mixture screws slightly so the transition from idle circuit to main jets does not run out of fuel.

Another option is to try the f4 emulsion tubes to drop the transition rpm so the mains take over slightly earlier.

If it happens at all low rpm low load sites then try to go up one size in idle jets.

[G50911]

its less of an rpm thing and more dependent on the amount of throttle. when i drive easy on the gas it tends to sputter and sputter. when i'm on the pedal it drives great.

[G50911]

just a quick heads up i spoke to mike pierce at pierce manifolds and i got some insight to check out.

apparently the throttle shafts can wear the sleeve they fit in causing an air leak. this leans out the mixture and is a very common problem for weber 3bbl carbs.

it's not a cheap fix so i'm going to start with getting the right set up for my motor. here's what i'm going to try:

F25 emulsions (from F5)
60 or 65 idle jets (from 55)

[racing97]

Usually if it backfires up the stack it is lean and out the tailpipe it is rich. Make sure that the linkage opens the right and left carb the same with a meter and someone holding the throttle open to 2,000 then turn each one of the air screws until you hear it come on sound, with certain regard for Idle.

regards

[Jcslocum]

Popping out the intake is a lean condition. If this at a constant opening then up the idle jets one size. Maybe turn the misture screws out a little bit and see if it helps.

If it's when pressing the gas pedal, maybe check the accel pump delivery. It might not be getting enough gas squirt.

Lastly, float levels, have you checked them??

[G50911]

yeah float levels are good. i haven't checked the accelerator pump. what can i do to confirm delivery is adequate or not?

[Jcslocum]

You can look for the first check and then measure with a small vial how much comes out per pump of the throttle.

Here is Wayne's tuning guide. Carb Tuning

Some more:

http://www.performanceoriented.com/technical.htm#ProgressionCircuit

[tom1394racing]

It sounds like is the idle circuit. Checked for blocked idle jets by removing them and blowing them out with compressed air. If that does not help, open up the idle mixture screws a turn or two and see if things improve.

[G50911]

the verdict is in... upped my idle jets from 55 to 60 and the sputter improved significantly. dropped in 65's and the sputter is GONE. i leaned out the airfuel screw 1/8 of a turn after and the throttle is as crispy and snappy as it was with the 55's. huge thanks to this forum for the feedback!

did i mention this thing rips?!?!

[lindy 911]

There we have it.

[G50911]

ok riddle me this...

looking at another case where someone has my exact set up, a reputable shop suggested a slightly different carb configuration. they suggest F25 emulsions, 135 mains and different plugs. according to that owner his car is now dialed in and is finally satisfied.

my question is, should i continue to tinker or leave it be? i have the tubes and new main jets already, just not in.

i understand climate and elevation can make a difference but are there many ways to skin a cat when it comes to carb tuning?

[lindy 911]

Emulsion tubes change the way idle jets transfer to mains so you'd have to play with it and see if there is any noticeable change. I went through the process and honestly couldn't tell the difference for the most part when changing ETs. If yours runs well now and your A / F ratios are decent, I'd drive it and have fun. If you were looking for that one tenth on the track, different story. Bottom line is if you like the way it drives, drive it. Repeating my earlier statement, error on the fat side as a cold front can lean it to perfection or detonation.

[G50911]

thanks lindy.

so here's some additional info:

1) on cold starts the car does pop a little at idle (a little rich). the transition to the mains seem ok so maybe i'll hold off on the ET's for now.

2) if i drop in the 135 mains should this help with the low end richness? or should i be working with the idle jets only?

also, i'm curious why at cold starts the idle seems a little fat?

[lindy 911]

If it's popping through the carbs it's still a little lean but if it only does it at cold start, I wouldn't worry about it too much. You do not want to correct idle problems with the mains. The mains should provide the correct AFR at wide open throttle. If it pops through the muffler it's a little fat but again, if it clears when warm I'd leave it alone.

I would balance the linkage from carb to carb, then do a really detailed balance of each throat with an air flow meter. A lot of the final details with Webers are in the screws.

[Ed Cavalier]

Make sure that all of your transition ports are free of blockage. I went through symptoms like yours and spent a lot of time and money trying to figure-out what was wrong. I finally took things apart far enough to look at the transition ports and found some of them blocked by gum, varnish, whatever. I cleaned them out with a carb jet reamer kit and all of the part-throttle popping, sputtering, and backfiring went away for good. Be careful: you don't want to change the size of the transition ports you just want to make sure that they are open.