Spitting through carbs

[boxer246]

I'm having hell trying to get my '70 911T carbs tuned properly.
The basic problem is from 1,500 RPM to 3000 RPM it just spits and pops pretty badly back through the carbs. I used Wayne's tuning methods with a Uni-Syn to adjust carbs. The engine actually idles relatively well, with only the occasional "spit". Then, on acceleration from 1.5k-3k it just pops and spits, then pulls very well and cleanly from 3k on.

Here's the setup, for reference:
- Weber 40IDA's freshly fully rebuilt/new throttle shafts, butterflies, the whole deal, by Bieker up in Oregon
-stock jetting
-fuel pressure roughly 3psi
-fuel level set
-compression at 120 or above on all
-valves adjusted
-new rotor, cap, points, timing adjusted with light, advance seems to be working properly, new plugs
-fuel tank cleaned/restored/relined.
-plugs have a grayish brown color on the electrodes.

I have tuned the carbs multiple times, just like Wayne shows....adjust airflow with the Unisyn on all carbs, adjust idle mixture, turn in till rpms drop, then out about 1/4, recheck airflow, use air bypass adjusters to even airflow, recheck mixtures, set throttle linkage equal. The only thing I haven't done is do a accelerator pump test, because I dont have a little vial at the moment.

Could a failing coil show symptoms like this? I wouldn't think so, but I'm just trying to eliminate variables.

The idle jets are 55's. I live in Houston.....air dense, hot humid. Maybe go up on idle size?

Thoughts?

[gsjohnson]

Weber Carbs Technical

[sleepy911]

I know I'll get hell for this but I wouldn't trust bieker with my lawn mower carb. He rebuild some zeniths for my customer. First he lost the carbs then finally got them done after 8 months. I was excited until they bleed gas out of several spots. Upon further inspection the carbs weren't the original ones. Then after I took them apart I noticed the Venturi s were wrong along with all the jetting. I finally got them put back together correctly by robbing parts from an extra set.

[fred cook]

Generally speaking, an engine is running rich if it backfires or pops in the exhaust and lean if it pops and/or spits thru the carbs. At this point, I would verify the sizes of the main jets, air correction jets and the emulsion tubes they plug into. Sounds like something is not the correct size or may be damaged.

[JohnJL]

It sounds like your problem might be not enough fuel coming from the accelerator enrichment circuit. Before tearing everything apart, check that Weber Tech book and see if richening up your accelerator function doesn't help.

[gsjohnson]

You're describing a lean condition. Try opening up the mixture screws...Note a slight rise in the idle when performing...If that doesn't fix it, you may have an intake leak of some type...possibly thru the throttle shafts. Last resort is you will have to increase your idle jets...Hope this helps.

[boxer246]

Thanks guys I will check everything a little closer. I notice that Performance Oriented idle adjustment procedure is a little different than what I used, so I will try that. He also says that the initial idle air correction base setting is two turns out, I think mine were set significantly out more than that, so I probably should begin there.

[1QuickS]

Idle jet size at 55 is good.

You can check accel output squirt by making a vial from an insulin syringe (fuse the tip closed) and suspend beneath each tip with wire. Look for .7ml/nozzle.

Difficult to get all even UNLESS you perform a series of optimization processes of all components in the system. If you do not get squirt then primary culprit is the check valve in the float bowl well are not closing and allow fuel to not be squirted efficiently. We routinely service these check valves and replace the check balls to assure good performance.

[procan]

When my Zeniths were spitting, all I did was make sure the floats were set properly. I had to add a washer or two to one, and remove a washer or two from another. Carbs ran beautifully after that.

[JohnJL]

Results? giving the accelerator pump linkage a tiny adjustment takes 5 minutes and costs nothing, so a quick fix if you try.

[Trackrash]

Are these the original carbs? That you had rebuilt? Were they running OK before?
Or are you replacing Zeniths with new to you Webers?

Have you checked the float levels?
How far out are the idle mixture screws when you adjust them?
What air cleaners are you using?
Is the motor totally stock?
Stock muffler?
Do you have a fuel filter just before the carbs?
Have you confirmed 55 idle jets and that they are CLEAN?
Are your carbs 40 IDA or 40 IDPT?

Try the idle airs about 1 turn out and see if that helps. If not let us know. Lots of variables to consider.

[calimedeiros]

I had a 74 corvette and used to spit gas out of the carb I just had to adjust my timing
Not sure if this applies here but I would make sure your timing is correct

[G50911]

i had the same issue. i upped my idles to 65 an it's gone. runs sweet now.

[RyanH]

G50911. What car and carbs do you have? I have a 1970T with Webers, I have constant popping and spitting and considering changing my idles to a larger size. Does changing the size affect anything else? Thanks...

[Trackrash]

Ryan, without knowing some background on you car, see post 11, it is hard to know the answer to your question.

[G50911]

i have 40IDA 3C's. i'd try upping your idles 2 sizes first. if that doesn't fix it then it might be your accelerator pump. if that doesn't fix it, you might have a leak. give pierce manifolds a call. they told me to try this out (in this order from least expensive to more) and luckily the first one worked for me.