how to adjust carb when runs better with choke a little out

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

slickrock

slickrock

10 Year 15 Year 20 Year
Joined
Aug 15, 2005
Threads
776
Messages
5,125
Location
Lemoore, CA (south of Fresno) / Cortes Island, B.C
My timing is on the spot and running pretty well, but I notice it gets a bit more power when my choke is pulled out just a little bit and my idle picks up about 200 RPMS.(1973 FJ40 stock carb ported base to run a 60 series dizzy)

I take it my carb is running too lean and runs a bit richer when the choke is out a bit. On my 73 carb, what do I need to adjust to make it run a bit richer?

Thanks.
 
Do you have any vac leaks?
 
Timing is "on the spot"? You mean the BB? Maybe try advancing it a little?
 
I don't have any vac leaks. Tested all lines and did not increase idle. Timing was set following FSM and turned dizzy until idle sat between 700-500 RPMS.

I guess I will first try setting timing a little bit more and increase the idle a bit, maybe closer to 800. I like it a bit higher anyways so I don't need to fool with it from going 300 feet above sea level to 7,000 feet when wheeling. If that doesn't work, I'll try the larger jet. (best source for this, maybe Mark's off-road?)

I could have sworn there was an adjustable location on the carb that would increase or decrease richness. . . Thanks for the responses.
 
slickrock said:
I could have sworn there was an adjustable location on the carb that would increase or decrease richness. . . .
Click to expand...


The only readily accessible "adjustments" are the idle mixture (facing the valve cover) and idle speed (in the carb base facing the firewall). The idle mixture has very little effect on the mixture at higher RPMs when the thottle is open.

The possibility of a stock carb leaving the factory with main jets that are too small is very slim. Choking it just isn't going to increase power at high RPM because it also obstructs air flow (unless it is so lean that it won't run w/o the choke at all). If there is an actual improvement, it is going to be at low speeds. If you are sure that you don't have a vacuum leak, then maybe you have an idle fuel flow problem (bad solenoid or obstructed passages).
 
Or maybe a PO rejetted it for higher altitude at some point in its 30+ years......... Pull the jets and compare them to someone who has a similar setup. What altitude are you at?
 
Pin_Head said:
The only readily accessible "adjustments" are the idle mixture (facing the valve cover) and idle speed (in the carb base facing the firewall). The idle mixture has very little effect on the mixture at higher RPMs when the thottle is open.

The possibility of a stock carb leaving the factory with main jets that are too small is very slim. Choking it just isn't going to increase power at high RPM because it also obstructs air flow (unless it is so lean that it won't run w/o the choke at all). If there is an actual improvement, it is going to be at low speeds. If you are sure that you don't have a vacuum leak, then maybe you have an idle fuel flow problem (bad solenoid or obstructed passages).
Click to expand...

It idles right t 700 RPMS without the choke and nice and smooth. IF I am speeding up there is a slight hesitation unless I "just" open the choke a bit. It is enough to increase the RPMS to 1000 at idle.

At highway speeds, I get a tad bit more power and a bit more smoother running if the choke is opened enough to get the rpms at 1000 at idle.

I looked at my fuel filter (clear one) and it looks good, but I have a spare I could swap it with to see if it is restricting flow. I sprayed an either based can all around the carb and any vac lines/ports. No increase in idle (unless sprayed directly into the carb).

I will check that again tonight. Then I suppose the solenoid replaced.

Is there anything you could pinpoint from my description?
 
One thing Pighead has learned...
Usually, if just a little choke makes it run better, it's a vacuum leak. If it's the fuel cut-off solenoid, you need all the choke to keep it running...
So I'm thinking it's not the solenoid.
Have you put a vacuum gauge on it?
 
Pighead said:
One thing Pighead has learned...
Usually, if just a little choke makes it run better, it's a vacuum leak. If it's the fuel cut-off solenoid, you need all the choke to keep it running...
So I'm thinking it's not the solenoid.
Have you put a vacuum gauge on it?
Click to expand...

I will have to get one. the closest thing I have to a vac tester is a brake bleeder and that won't work. I will focus on this area first. Thanks!
 
Just a thought but could it be the accelerator pump not fueling properly on acceleration and the choke being out is limiting enough air to keep the mixture correct? That might explain the smoothness and feeling of more power.
 
No, the AP is only a factor during throttle transition. If pulling the choke improved performance while changing throttle, then the AP would be suspect.

If it runs better at steady throttle, w/ the choke pulled, then there is a steady state mixture problem.
 
FJ40Jim said:
No, the AP is only a factor during throttle transition. If pulling the choke improved performance while changing throttle, then the AP would be suspect.

If it runs better at steady throttle, w/ the choke pulled, then there is a steady state mixture problem.
Click to expand...

Your point is well taken. I was assuming that the "more power" the Original Poster referred to meant that when he added throttle and accellerated the truck had more power with the choke out. Obviously (for me now) that is not the only instance where we measure "power" and a constant throttle up a hill would be a fine measure and nullify the the AP theory.
 
Let me clean this up a little: When going at 2800 RPMS on a flat road, if I pull my choke a little bit, my speed would increase a bit (without chage to the throttle) and the engine would seem to smooth out a bit.
 
year JEt
9/72-12/74 FJ40,FJ55 1.20
From SOR. What would you suggest I change my 8/73 carb jet to?

I have the option of a 1.44, 1.36, 1.47
 
Oh recap, FJ60 Dizzy with a ported vac base by Mark from Mark's offroad so the dizzy and carb work together.
 
slickrock said:
year JEt
9/72-12/74 FJ40,FJ55 1.20
From SOR. What would you suggest I change my 8/73 carb jet to?

I have the option of a 1.44, 1.36, 1.47
Click to expand...
Find out what is in there, and try 5-10% bigger.
 
I would pick up a vacuum gauge from the suto parts store. I run about 19-20" of mercury at 650rpm with the idle screw adjusted for optimal vac. I set my timing at idle speed (I run HEI/DUI) check vac, then adjust idle screw for max vac (19-20"). I check and recheck the timing during this operation adjusting the idle mix as it will change rpm often. I use the 0 degree timing mark and an adjustable timing light at 12 deg. You will be adjusting timing to the BB mark as this is the proper setting (7 degrees maybe??).

I check for vac leaks at the carb base, intake manifold to head areas hoses etc. I use small amounts of starting fluid. If it speeds up rpm then a leak is likely.

If you set your timing at an rpm other than oem recommended your advance will be off. I have adjusted timing and idle mix with a max vac method. adjusting both disty and idle mix via max vac. I've had success with this method on many other vehicles. Check all this stuff before you get into jetting as it will only help.
 
You must log in or register to reply here.

Share this page

Back
Top Bottom