4150 goes lean in gear

[Cudatali]

Hello, I am working with a 4150 style Quick Fuel 680 on an engine with a healthy cam, which I have jetted and tuned. Primary and secondary throttle openings are at a minimum and idles at 900. I didn't need to drill out the butterflies. 4 corner idle screws are 1-1/16 out. All my other A/F ratios are on the money and the car is very responsive. The engine seems happy with a idle mix of 13.4-13.8 AFR in park (8 in. vacuum) but it goes to 15.0+AFR when in gear (5 in. vacuum). Obviously a large drop in RPM and the engine doesn't sound happy when it goes that lean. Initial timing is at 23. The car starts right up.

I'm thinking it's the low vacuum signal once in gear which starves the engine of fuel.

Any other ideas of what it could be? Or if I am on the right path, how do I overcome that?

Thanks!

 

[Canadian1968]

Just curious do you tight converter on the car ?

 

[Cudatali]

It's a 2500 stall converter, I guess it could be a little more lose. You thinking it's just too much of a load? maybe I should just bring the idle speed up to see what it does

 

[dvw]

It's tough when the converter is tight with a large cam. My 2 cents would be tune the idle circuit in gear and not worry so much about neutral. The simple choice would be higher idle speed. But that may get into a speed range you don't like. The lower vacuum in gear means less fuel flow. So if the idle mixture screws won't feed enough fuel it may need the idle fuel restriction opened up and/or smaller idle air bleeds.
Doug

 

[BSB67]

It's tough when the converter is tight with a large cam.

This. You simply might not be able to make it work with your converter.

Here are some suggestions:
1) Increase the idle rpm as much as reasonable.
2) Make sure that your mechanical advance is not moving your timing around in the 0 to 1200 rpm range.
3) Try more initial timing
4) Do not use the O2 sensor for tuning at idle. Maybe a vacuum gauge and simply make adjustments for best idle quality.
5) Confirm that there is no vacuum leak by placing you hand over the carb.

What is your cranking cylinder pressure?
Advancing the cam might help a little
A smaller cam may do everything better and bring you joy

 

[RemCharger]

Hello, I am working with a 4150 style Quick Fuel 680 on an engine with a healthy cam, which I have jetted and tuned. Primary and secondary throttle openings are at a minimum and idles at 900. I didn't need to drill out the butterflies. 4 corner idle screws are 1-1/16 out. All my other A/F ratios are on the money and the car is very responsive. The engine seems happy with a idle mix of 13.4-13.8 AFR in park (8 in. vacuum) but it goes to 15.0+AFR when in gear (5 in. vacuum). Obviously a large drop in RPM and the engine doesn't sound happy when it goes that lean. Initial timing is at 23. The car starts right up.

I'm thinking it's the low vacuum signal once in gear which starves the engine of fuel.

Any other ideas of what it could be? Or if I am on the right path, how do I overcome that?

Thanks!

Everything said is good advice.
I wonder if the cam has way more overlap than you're thinking, and just needs the distributor locked in.
Had an unknown " circle track 340" like that.
No way in hell it would idle well without being locked

 

[Geoff 2]

With only 5" of vacuum, it is going to need a LOT of initial timing to idle optimally. Maybe as much as 50*. The extra timing will do a number of things: increase idle rpm, increase vacuum, smooth the idle. The increase in idle vac will help pull more fuel from the idle cct.

 

[lewtot184]

if timing is the issue, and could be with low vacuum, i'd suggest locking the timing. the only sensible way to lock timing is with a start retard. if your using centrifugal advance only then the advance may be noticeably lower at 600rpm vs 900rpm. check the timing in gear to verify this. i really believe there may be other factors than timing or tight converter. one of my cars has a converter too tight for the cam and i use an old mopar vacuum advance distributor with no fuel issues.