Recalibrating Carb Jetting
 

I'm wondering typically how much of a jetting increase folks see for modern fuel and light modifications?

Our 2bbl Ventura was running ok with the stock 53 jets and a homebrew dual exhaust built off the stock y-pipe. We installed a bigger Pypes dual exhaust system and started experiencing stumbling and hesitation due to a lean condition. As of now, I'm up 6 jet sizes to a 59 which feels like a big jump, but it made a substantial improvement. It's acting like it could take even more fuel.

Looking at a chart for stock 2GC jet sizes, it seems like most 2bbl jet sizes for 350-ish engines came with mid-50 to low-60 size jets so maybe I'm in the neighborhood.

The long term the plan is to swap to a Q-Jet, but for now I want to get the 2bbl running right.

Jet up 3-5% for E10 fuel.

Thanks. That would be a couple of jet sizes. I suppose the exhaust can deleted EGR valve could make up for the rest.

I have found there is seldom if ever a need for larger main jets.

What is needed is larger idle/lowspeed jets, enlarged by 2-3 thousands of an inch and in some applications slightly larger downchannels and discharge holes at the idle mixture needle holes.
This provides a strong idle and reduces jerking when shifting into gear.

Remember you are running on the low speed circuit to about 35 mph.

FWIW

One quick way to check jetting is this.

Get the motor fully warmed up.

Disconnect the carbs accelerator pump.

With the motor out of gear and idling wing the carb wide open.
There should be no backfire which would indicate a lean condition.

If you hear no backfire which then you at least in the start of ball park in terms of wide open jetting needs.

No one in their right mind should use your method.
It says NOTHING about how accurate the calibration is.
If it would work without an accelerator pump why do ALL carburetors have one?

Instead use the "tip-in" test with the choke blade or the palm of your hand at about 2200 rpm.
A small increase in rpm of about 50 rpm indicates a proper mixture.
If the test run turns out to be satisfactory then all is well.

FWIW

Maybe you should get your ass out of chair in front of your lap top and try it before you criticize someone Kennth .

Once again note that I never said that it was as accurate as reading exh temperatures or air to fuel ratio’s, just a good ball park.

This procedure works.

Try it and then you can apologize to me big mouth!

Am I barking up the wrong tree with the jets? If so, why did increasing them show an improvement? We were getting hesitation whenever you hit the accelerator at any speed, not just off idle. Maybe I'll hook up my wide band and see what's happening.

Main jets are a good thing to adjust for changes in fuel BTU`s / quality... main jets are just not the only place there is a low speed restriction you can modify, Make sure you get fuel from the accelerator pump with any throttle movement..

I may have some 2gc jets from my Tripower days laying around

I'm guessing that richening up the main circuit helps cover something that may be lacking in the transition or accelerator pump circuits. Depends on the RPM where you're seeing the issue. Below 2300 I'd expect a combo of transition slot and accel pump, with accel pump predominating. Above 3000 I'd expect a combination of main circuit and accel pump, with main circuit predominating.

But yea, the WBO2 and vacuum gauge reveals all.

Bad advice is always bad and some are worse than others and cannot be left unchallenged, or at least given alternatives.

FWIW

Drove the car around a little bit today with the 02 sensor attached. I'm still seeing a short lean spike off idle to high 14s-low 16s before the car settles down to 12:5-13:1 when accelerating. The car isn't stumbling or backfiring under average driving, but I slowed to about 15mph and stomped on it and got a backfire through the carb. The engine was cold which didn't help, but it shoudn't do that regardless. Cruise is around 14-15:1 which I think is fine, and idle also a little over 14:1.

According to the gauge, it definitely seems like the off idle could use some more fuel. I am getting a squirt of fuel immediately on any movement of the throttle, so I think the accelerator pump is fine (just replaced it). This brings it back to the point above about modifying the low speed jets. I picked up a spare carb for parts so I could experiment. Sounds like I should consider enlarging the pump squirters on this piece? Those are the two holes in the middle of the piece on the top photo.

https://forums.maxperformanceinc.com...1&d=1748301728

https://forums.maxperformanceinc.com...1&d=1748301746

Don't go any larger on the jetting if at full throttle just yet if your AFR is now in the ball park.

I would check the the power piston is not hanging up and that the passage under that power valve is 100% clear from the crap that today's fuel turns to so fast.

Use some floral wire to run down in there and scrape around .

If you pull up crap then you got more flushing and cleaning to do .

I don't understand how attacking a fellow Pontiac guy is helping the hobby. I mean how is helpful saying "no one in their right mind" so I guess calling Steve crazy is good for the hobby.
I went back and read all your reponses for the last few months and I see a pattern.

I don't know Steve, but he sure offers a lot of help here to fellow Pontiac fans and I for one appreciate it. No place for bullies here.

FWIW

Do you have pin gauges and the tiny carbide drill bits? I'd pin gauge those accel discharge holes and go up 5-10% on the area of the holes. Depending on diameter, about .002"-.003" larger should be a good starting point.

If you can figure out where the IFRs are, you can go up a scootch on those too. If the transfer slot is lean, then richening the IFRs will add fuel.

Usually when you have a brief stumble or lean spot off idle before the main circuit takes over as you described, it either the pump circuit or the power valve circuit.

If you increased main jet and it helped it's likely covering up the issue you can accomplish the same with the idle circuit as well by richening that. Typically not the best approach but some engines also don't like lean mains or lean idle circuits either. It's a game of back and forth.

If youve messed with the pump circuit with no improvement then the other likely cause could just be too much of a delay in the power valve circuit. It has to work in conjunction with the pump shot. The pump shot alone won't always cover the lean spot when the throttle cracks open. That momentarily drop in vacuum should also trigger the power valve circuit if for only a second.
You may just need to change the power piston spring.

Isn't the low speed circuit metering at the bottom of the smaller tubes pressed into the venturi cluster?

Yes, that´s correct: