Carburetor Idle Adjustment 101

[Shepard]

I've read a lot over the last few days about high idle - no idle control. I'd like to help to understand the function of the air/fuel adjuster.
#1 == proper air to fuel ratio is 14.7 parts air to 1 part fuel. Doesn't matter if it's idling or under power driving down the road.
At this point in time the EPA requires that the air/fuel mixture screw be sealed in some manner. This has lead to problems in the way we use these little engines. I should say we don't use these little engines as they were intended. The way a governor is set up on a mower or tractor -- it never idles - at least not like a car - starts - idles - revs up on demand. The idle circuit is very lean on a small engine because of this reason. We try to use them like a car and it starts and idles like poo We try to adjust them and sometimes it doesn't go right.
Here goes -- having installed a throttle return spring to the butterfly control on top of the carburetor - this guarantees that the carburetor is closed properly against the IDLE SPEED CONTROL SCREW. If it's not closed properly you can;t adjust the idle mixture. Here's the theory - you are adjusting the air/fuel ratio to a 14.7 to 1 mix for the idle circuit. So you set the idle speed screw to a high idle say 1200 rpm -- then turning the mixture screw slowly to the right till it stumbles and stalls. Back it out a little - enough to restart engine - then turn to the left till it stumbles and stalls. Turn it a little to the right - enough to restart the engine. Now you know the range of the mixture screw at 1200 rpm -- now adjust the mixture screw so the engine will run as fast as it can with the idle locked against the idle speed control screw.
Now the engine is most likely turning a fair rpm -- turn the idle speed control screw on the throttle to slow the engine down to more of an idle like 1000 rpm.
Now you need to re-adjust the mixture screw till it runs faster -- or it doesn't slow -- it's a dance between the idle screw and the idle mixture screw. You may need to do this in 2 or 3 steps to get the idle down under a 1000 rpm.
Your attempting to keep the air/fuel ratio at 14.7 to 1 "at the rpm your idling at".
If it pops out of the exhaust on deceleration open the mixture screw a little at a time till it stops.
The idle and transition circuit controls the engines fuel needs to about 1/4 throttle. After that it's starting to run off of the main jet. Now this is why we run an adjustable main jet kit. So we can tune that 14.7 to 1 air/fuel mixture above 1/4 throttle.

[MrNeon]

Nice write up! Understand completely what your saying. It is a dance though of working to get the right spot...I need newer carb spring as well to make this work right.

[Shepard]

One thing I need to add is to clean off the end of the head of the mixture screw and put a magic marker dot on the head of the screw at 12-O'clock position so you can track the turns of the adjustment as you go. Remember = small adjustments - 1/8 turn at a time because radical adjustment = radical changes in the way it will idle.

[panteramatt]

One question Shep. On a double barrel carb should I adjust one side at a time?

[MrNeon]

I did mark mine and yes, little at a time is all it takes...

[Shepard]

Originally Posted by panteramatt

One question Shep. On a double barrel carb should I adjust one side at a time?

Yes - from my experience working on cars with two barrel setups -- turn the mix screw in to the bottom carefully on one side. It should struggle to run -- turn it out till it clears up -- just that and no more -- do the same for the other side.
Turn the idle speed control screw to the left and bring the idle down to 950-1000 rpm. You my need to re-adjust the mix screws again. Remember - your not attempting to control rpm with the mix screw - your trying to achieve that 14.7 to 1 fuel ratio at an idle speed. Mark you needles screws for reference and to open them about the same amount of turns.

[Shepard]

As air races through the carburetor bore you can see that the vacuum is the highest at the smallest part of the bore or venturi. The air is traveling at about 200 mph to lift it up and atomize it into the air stream. That goes the same for the idle port and transfer port. This is why a good sturdy throttle return spring is needed. The throttle must be closed on the adjuster screw to properly adjust the idle.

[panteramatt]

thanx bub

[sho305]

My 420 will idle with the compression release going off. Yes it took a lot of adjusting and lowering the idle speed. It keeps changing as you lower it. And then you need to run it a little richer than what it seems to like, that gets you better summer starting with no choke and no backfires/popping at least for me. I did have to enlarge the idle jet a hair to get enough fuel out of it. That limits the max fuel the idle circuit can get in a stock clone carb. Now I run a scooter carb that works a lot better.

[Rooster59]

My 625 has so much power I barely use 1/4 throttle cruising at 3500 rpm. At about 3000 rpm when I press the throttle down a little more to climb a paved hill the engine stutters for a split second. Makes me think the adj main jet might be a little on the rich side when it makes the transition from the low and mid jets to main.