Logo
FULL SITE MENU

tunning a 4 barrel carter carburator

circle hook

circle hook

Regular Contributor
can I adjust my carter carburetor on 1972 LM318 like a car engine, buy using a vacuum gauge and turning each screw to obtain the highest reading

Thanks bob
 
As for adjusting the low-speed fuel air circuits (as you would a car engine), the answer is yes.

As Kim suggests, make sure that Base Advance is correct.
Also, you may need to fiddle with the idle stop adjustment (as to maintain the correct idle RPM).

You will be adjusting the low-speed circuits only.
Anything past or above low-speed fuel metering, will or may require getting into the carburetor.

While doing this, check to see that the accelerator pump linkage is causing the accelerator pump to deliver a stream of fuel immediately upon any throttle plate action.
A delay here may cause hesitation, or even a slight stall, when leaving the low-speed circuits while entering the high-speed circuits.


.
 
Last edited:
It's VERY hard to adjust the idle with a vacuum gage since the 'too lean' and 'too rich' readings are so darn close. Also, having solid mounts, a marine engine doesn't rock back and foorth when the adjustment is way off! So it's a real test of one's 'mechanic's ear' to tell the what's correct.

I've often found that my "best" adjustment is often off--usually a bit lean--and I've have had to add a 1/2 turn out or so to keep the motor from stalling when I goose the throttle. That too is a way to get it done properly (though a bit irritating).

Jeff

PS: You won't like hearing this, but a "rebuilt" Carter is still a lousy carb compared to anew Edelbrock 1409. Though I'm fairly competent with carb tuning (after 50 plus years) I could never totally eliminate the 'built in backfire' that is a trademark of Carter AFBs. The Edelbrock has no such problem and gave better gas mileage right off the bat.
 
Last edited:
I've never had a backfire issue with my Carter AFBs. Generally speaking, backfiring is out of the exhaust only. "Coughing" or "Spitting" is what's referred to as coming out of the carb. If your carb is coughing even after the engine is fully warmed, then your accelerator pump is likely not working properly or needs adjustment.

Thanks,
JJ
 
No I have no problems yet haven't installed the carter 4699S my question was that can I set it up like a car and use a vacuum gage and obtain th highest reading and of course set the correct RPM, and timming. Thanks bob
 
circle hook said:
No I have no problems yet haven't installed the carter 4699S my question was that can I set it up like a car and use a vacuum gage and obtain th highest reading and of course set the correct RPM, and timming. Thanks bob
Click to expand...

You need to set your base timing FIRST. I've never had any success using a vacuum gauge. I try to find the "smoothest" idle with the leanest mixture. Then I'll tweak the mixture if required to ensure smooth throttle response.

Thanks,
JJ
 
Circle Hook, the vacuum gauge idea does work, just as it does for an auto engine.

However, JJ makes a great point regarding favoring the rich side, rather than the lean side, and I agree with his comment re; initial throttle response.
Favoring the rich side may also help prevent "run-on" and/or "deiseling"...... if that was to become an issue!



.
 
Yea my engines run good but had 2 different carbs I got a match and had it rebuilt just waiting for the weather to change and toss her on yea rich is allways a little better to lean you can run hotter & burn things. Thanks bob
 
circle hook said:
........ yea rich is allways a little better to lean you can run hotter & burn things.
Click to expand...
Keep in mind that you'll have two entirely different fuel metering circuits.
Low speed.... and high speed!

Once out of the low speed circuit, the fuel metering becomes dependent on the high speed circuits.
 
But is the high speed circuits built in the metering rods & seats really I should only have to adjust the low speed mixture screws and set the idle & timming if I am correct. Thanks bob
 
..."Generally speaking, backfiring is out of the exhaust only. "Coughing" or "Spitting" is what's referred to as coming out of the carb."

Correct. Been using the wrong word for years!

Jeff
 
fastjeff said:
..."Generally speaking, backfiring is out of the exhaust only. "Coughing" or "Spitting" is what's referred to as coming out of the carb."

Correct. Been using the wrong word for years!

Jeff
Click to expand...

With that being said, I've never had a coughing issue with my AFB's either. I don't see any major design differences between the Edelbrocks and the Carters that would lend itself to reduced coughing.

I would have suspected an accelerator pump issue with your Carters.

Thanks,
JJ
 
Well my old carbs ran pretty smooth with out any problems the reason for replacing is the I want both carbs the same right now I have a 6130s and a 4699s. Now I will have (2) 4699s and I am going to tune them the same. Thanks bob
 
circle hook said:
Well my old carbs ran pretty smooth with out any problems the reason for replacing is the I want both carbs the same right now I have a 6130s and a 4699s. Now I will have (2) 4699s and I am going to tune them the same. Thanks bob
Click to expand...

I have some electric chokes that may fit those. PM me if interested.

Thanks,
JJ
 
Back
Top