All Small Six Idle and tuning adjustments

[twodogs]

Straight to the point -- my idle adjustment screw doesn't let me dial my engine down to the spec of 750 RPMs.

I have a Carter YFA that I rebuilt last year. It's attached to a 200 cid L6 in my '74 Bronco. The thing runs great but it seems to idle high (850 is the lowest I can get it), and to the point that the idle adjustment screw isn't even touching the stop. Here's what I have in the rig.

Pertronix I ignition
Pertronix I coil
Plugs gapped to .039 (spec is .034)
I've got it timed to 8* BTD (spec is 6* BTD)
I ran the idle mixture in all the way and then backed out 1.5 turns
I ran the idle speed screw just to the stop and then an additional 1.5 turns in
I disconnected the throttle linkage just to make sure it wasn't hindering anything

I've been trying to set the rig to best lean idle by doing the following. I hooked up a tach to my coil, as well as a vac gauge to manifold vacuum. I hook up my timing light, unplug the hose from the vac canister and plug it. I verify the timing is at 8* BTD. I unplug the line and hook it back up to the vac canister. I then start backing out the mixture screw while watching my tach. The tach wavers all over the place initially but starts around 750. I back out the mixture screw 1/4 turns and watch to see what my highest reading is. It will climb and eventually stay at 900 rpm. I then turn the mixture screw back in about 1/2 turn to get a drop of 25-50 rpm. At that point, I was hoping to use the idle speed screw to drop it to 750 or so. But as I've said, that idle screw doesn't seem to affect the idle at all.

I've used an unlit propane canister to check everywhere for vacuum leaks but I can't find any. I also removed the air cleaner, covered the carb throat with a rag, and the engine dies immediately, without a change in speed which I used as my second test for a vacuum leak. I have no idea if this makes a difference but I'm at 4,500' elevation.

I can just keep driving the rig since it runs so nice, but I can't sleep knowing that idle speed screw gets a free pass and doesn’t have to do any work. Once more I'm coming to this well of knowledge. Thanks much!

 

[twodogs]

....maybe I'll put it to spec tomorrow at 6* TDC, since I'm guessing that timing affects idle speed. It should be obvious that I feel like I'm on a nearly vertical learning curve at times.

 

[bubba22349]

10 to 12 degrees base timing would be better at 4,500. What kind of choke do you have? What kind of throdle linkage do you have can you take some good pictures of the linkages. When it tuned right it should be able to idle at 500 or 550 for a manual trans. 600 to 650 for a auto trans in drive brakes set. I think you throdle linkage is maybe set a little to short if your crub idle setting isn't going to any lower then 850 RPM but I would need to see it. The choke linkage might also cause this but again I would need to see what it looks like. Good luck

 

[frozenrabbit]

did you have the butterfly plates out of the carb? Might not be aligned to the bore completely. Disconnect the throttle linkage and see if it will idle down.

 

[B RON CO]

Hi, as mentioned, these engines run better with more initial ignition timing, and make sure the high idle on the choke side is not holding the throttle open. Advance the timing first, because you know this will raise the idle. Then make sure the choke linkage is all off and not binding. When the engine is warmed up the choke should be fully open and held open by spring pressure. Give the linkage a shot of WD40. Try adjusting the mixture screw by turning it in until the engine starts to stumble, then back it out one full turn. Your engine should run better this way then going by the factory specs. The spark plug gap should be opened up, like you did, because you modified the ignition system. Good luck

 

[twodogs]

Thanks guys. I took some pics today of the linkages but I ran out of time to get them transferred to the computer this evening. Good to know about advancing the timing a bit more than I have it. I did not remove the butterfly plates from the carb when I rebuilt it. I disconnected the throttle cable completely yesterday, and that wasn't the culprit. There's plenty of push in that cable on the return.

I know when I was playing around with the settings yesterday that the fast idle cam was completely out of the way when warmed up. Today I took the carb off just to double-check all of the settings from when I rebuilt it. Float level was good, fast idle linkage was good, choke pulldown, etc, etc. When I got to the section about setting the metering rod height, it gave me pause. I remember bottoming out the metering rod plunger, and then turning the screw as directed. I don't think I finessed bottoming that plunger before setting the screw. I think I now know that the metering rod only needs to touch the bottom of the casting. That would have resulted in a pretty lean metering rod setting I believe. I was also wondering if I had the metering rod too lean because yesterday I had to turn the idle mixture screw out 3.5 full turns to get the highest idle. And perhaps those two things aren't related . I'll get after it tomorrow and will post pics of the linkages, unless I miraculously fix it.

 

[bubba22349]

Sounds like your linkages are probably right. If you happen to have changed the metering rod position very much from what it was then you right to look there for the cause of the high idle. Best of luck

 

[twodogs]

It was the metering rod. As I mentioned above, I adjusted the metering rod to where it just touched the bottom of the carb casting, and then screwed it down one full turn. Today I set the mixture screw two turns out, and the idle speed an additional 1.5 turns in from just touching. I fired it up, let it get good and warm, set the timing to 12* BTDC, backed the mixture screw out until I got my highest RPMs, turned the mixture screw back in until I saw a drop of 50 RPMs, and then adjusted the idle speed down to a very soothing sounding 750 RPMs. (I did have to readjust the metering rod an additional 1/2 turn to the lean side.)

I took it on my test course which has a long hill where I can test for pinging while climbing in 2nd gear (thanks for that tip, B RON). So I'm going to leave it at 12* BTDC for now (thanks for that target Bubba). Everything ran very nicely, without any stumbling, good throttle response, and it returned to a nice idle each time.

That metering rod is an exercise in frustration. A 1/2 turn on the metering rod screw can cancel out the influence of the mixture screw, such that you suddenly won't be able to drop the idle speed low enough. I'm just glad I stumbled across the issue while double-checking my previous carb rebuild settings.

One last question. Aside from reading the spark plugs, how do you know that you've hit the sweet spot for not too lean or too rich?

 

[bubba22349]

The old school way to read the plugs was look for a nice light tan plug color its harder now days to know with the fuels we have. An narrow or wide band AFR Gauge can be used its hooked up with a sensor to read the exhaust or you can put it on a Dyno to tune it.

 

[twodogs]

The old school way to read the plugs was look for a nice light tan plug color its harder now days to know with the fuels we have. An narrow or wide band AFR Gauge can be used its hooked up with a sensor to read the exhaust or you can put it on a Dyno to tune it.

Thanks for that info. I may look into installing an AFR gauge at some point. I wasn’t aware of the difficulty in reading plugs, as that was how I did it in the past. Good to know.