Learn me O Carb Tuning Gods...

[Raze]

My Carb-fu is weak, I am a disciple of the church of FI with MegaSquirt being my savior. I find myself in a quandary. I have a 32/36 DFEV carb on a 1608 in my 72. I have good fuel, fires right up, I fixed the timing on the cams and the dizzy (it was set to fire on #1 instead of #4 as the book says, but it did run with good torque, strange). It now 'sounds' like a Fiat motor should. Issue is it sounds like a Chevy with a hot cam at 800-1000rpm, but as soon as I get it to 2000rpm it sounds like a Fiat should. I had to adjust the idle speed screw to get it from 2000rpm down to 850ish rpm. I also had to adjust the mixture screw to get it to hold idle at that speed and not keep accelerating up. Fast idle is at 2000rpm (is that right?).

When I touch the throttle it's almost as if it 'bogs' for a second, if I blip it and then hit it it'll spin up nice.

Also, when I had it in 1st going up my hill out of my neighborhood it doesn't pull smooth, it trailer hitches or bucks, what causes this in carbs? Lean? Rich? Timing? It's at low RPMS under load, as soon as I rev it past 2000rpms it loves it, maybe a different cam? I just read crappy motor mounts plus old school mechanical linkage (which mine has) can cause this lovely behavior, which it does, the motor will move 'a lot' if it's about to crap out or running crappy when tuning, like 2+" side to side. apparently when the motor moves as it revs it moves the linkage, could this be the problem? At higher revs I didn't have this problem, though I was over 2k rpm at slow idle before.

I have an O2 sensor bung in the exhaust but I've got to pull my wideband out of the race car to use it and I've been running the race car so I'd like to get it 'running' well enough for now.

Also, I drove the car around in the above configuration to get gas and just tool around my town and then I pulled plugs, #1 is light brown, #2 was light brown almost golden, #3 was golden, #4 was black. So that didn't tell me much except I'm not horribly lean or horribly rich.

Honestly I'm more concerned with the trailer hitching/bucking thing as it's never done that until I started fiddling with the carb...

Other thing of note, the gas line was on pretty loose to the carb and leaking, I'm going to test more tomorrow...

[thefiatek]

Actually sounds like a valve losing it's seating moreso than a carb issue. #4 being black is a good indication of that, not burning the fuel efficiently in that cylinder. I would do a compression check both stone cold (wet and dry) and at operating temperature. It could be as simple as a bad plug wire too. Does it backfire when it's cold?

[Raze]

Doesn't backfire or seem to miss, I'll double check the wires, I did shorten them, but they were all firing as of last week when I had the timing light on them and it doesn't sound markedly different. When I measured my valve clearances the only one out of spec was the Intake on #1 which was .013/.014, all other intakes were .017 and exhaust .019 spot on...

[majicwrench]

How does it idle??? The #1 issue I see with these carbs the low speed/idle jet plugs. Makes em really doggy to get moving, and really excaggerates that problem with the mechanical linkage (I have a 72 too).
How does it idle?? Do you have to use the choke even when warm??
Keith

[baltobernie]

If you've got good spark at the correct time on all 4 cylinders and no vacuum leaks, I'd suggest tuning the carb "by the book"
http://www.piercemanifolds.com/category_s/313.htm As you've mentioned, it will be difficult to achieve a smooth idle with the linkage setup you have. It's quite easy to change that to the newer bellcrank style.

[Raze]

The idle is stable, but 'lumpy' as in it sounds like a Ford 5.0 or SBC cam'd. The engine isn't rocking or misfiring, at anything over 1000rpm it sounds much more 'smooth' and consistent.

I should have noted that the car has a catalytic converter and one small silencer, that's it, so it's a pretty loud exhaust (came like that from the PO)...

[So Cal Mark]

linkage won't have an effect on idle quality, it's the lurching when the clutch is engaged that is the issue with the early linkage. 2000rpm idle is way too high, it should be in the 900-1000rpm range. I'd look at the basics first and make sure you don't have vacuum leak