lean and variable A/F ratio

[Shark Racer]

On that note - how much "wiggle" is normal on the APT screw in an M4M carb?

[blarson]

Hooray!  I finally solved the problem with being ultra lean all over except idle.  I can't find reference to this part at all but a brass sleeve that was screwed into the airhorn was holding the power piston down all the way against the APT screw.  I didn't notice before but the tab on the power piston actually left a little indent on the brass sleeve where they were touching.  It was hollow and I was able to fit a screwdriver through it but the APT wouldn't turn because of the pressure against it.  I omitted the part and surprise surprise I get 13s WOT.  I easily turned the APT to 5 1/2 without the brass sleeve impairing it and now part throttle hovers around the 14s.  Still no idea how high I can go with that thing but it looks like I need to turn it back down just a touch anyway.  AFR does still fluctuate quite a bit with std dev of around .7 at all throttle opening.  It was much worse before the throttle bushings but it's still happening.  I hit the intake with carb cleaner and it doesn't appear to be leaking.  I'm going to fix an exhaust leak and maybe block off the brake booster and see if it stabilizes.  Should I be looking for something else as well?  Now we are making some good progress!

As a side note so that the knowledge is out there on the internets: Doug Roe's book references 4psi as the minimum fuel pressure off idle.  My fuel pump was indeed previously hosed.

[TommyK]

On my carb the APT is about ready to fall out at about 7 turns out. This may vary from carb to carb. IMO if you have to turn it out more than 5 or so turns you probably have the wrong jet/rod combo.

[blarson]

I discovered yesterday after driving it around now that I get a good mix off idle that sometimes if I come off full throttle the thing richens all the way out to 10:1 and start running rough when I come back to idle.  The problem appears intermittent and appears to have no relation to how the idle screws are set.  Turning the idle screws does nothing when it has gone full rich BUT does affect AFR during the times when it hasn't.  If I give it some throttle it leans back out to where it should be off idle.  If I let it idle or drive it around for a few minutes idle appears to lean back out to normal again.  I never seem to have the problem with idle mix if I don't use WOT.

My first thought was that the power piston is hanging up but that didn't make sense because it shouldn't be pulling fuel off the booster at idle.  Also, the mix is good everywhere off idle even if not completely stable.  Choke is confirmed all the way open.  It has a new float needle, seat, etc per Cliff.  Perhaps that is getting stuck?  Secondaries appear to be closing all the way and manually pulling on them to be sure they are closed doesn't appear to change anything.  Any ideas?

[TommyK]

Try slightly lowering float height?

Is the clip on the needle properly attached to the float arm? (on the back not through one of the holes)

Is there anyway you can monitor your fuel pressure while this is happening?

Richness immediately after overrun is not unusual but for it to last several minutes after returning to idle is.

[blarson]

Ahh, it sounds like I didn't attach the clip correctly.  I unknowingly ran it though one of the holes instead of over the back of the float.  Full throttle is in the range of 13.5:1 so perhaps the clip position is making the float a little artificially high.  Crazy thing is that I drove the truck about 150 miles today and the amount of time spent super rich seems to be getting shorter and shorter.  I really can't explain that.  The carb hadn't been run for years and was pretty corroded.  I did soak it in carb cleaner and blow it out with compressed air.  My original hypothesis was some corrosion that I didn't succeed in getting out but now that you point out the correct clip position.......  Time to pull the airhorn again.

I'm not sure if this is related but I am also getting some backfire through the exhaust that is progressively worse the faster I lift the throttle.  I perceive that to be related to a leak between the manifold and exhaust pipe due to a stud failure that I'm taking care of on Monday.  When that happens AFR goes lean while it pops and then rich once the engine slows to the point where backfire stops.  We'll see what happens when the stud is fixed.  The O2 sensor is on the other side and there isn't a crossover in the system so it's a little perplexing.  I was assuming that exhaust was contaminating the intake.  We'll see.

As a side note I've been turning in the APT a little at a time to set cruise now that I've had more time to drive it.  I'm back down to 4 turns now with freeway cruise fluctuating between 14-15:1

[blarson]

I lowered the float level to 3/8 and the extreme rich idle condition following full throttle decel appears to be gone now.  My AFR fluctuates quite a bit at constant throttle settings but watching a bunch of youtube videos and seeing essentially the same thing leads me to believe that to be normal.  Normal like a worn out ignition system I cannot say.  I'm going to leave well enough alone with that particular issue until I can get it on a dyno.

My last two problems are this exhaust backfire on full throttle lift decel and some choke issues.  I'm going to fix the exhaust leak before blaming anything on the carb.  I think all I need to do with the choke is replace the metal coil.  It doesn't seem to want to close all the way anymore but also doesn't want to open fast enough to avoid a very rich condition about a minute after start either.

Thanks again for the help everybody!

[429bbf]

couple of things the round brass thing that you backed out with a screwdriver in one of your earlier posts is the top limit adjusment for the power valve.when mash it to the floor and loose vacumn your spring maybe pushing the needles up to high out of the jets.i had to play with my carb a little bit to get it set right. one last thing is your vacumn pulloff working correctly on the choke?or when you step on it is the choke flopping in the breeze causing crazy readings.imo

[blarson]

Thanks!  I did screw that limit screw back in a few turns it leaned the truck out a little at WOT.  I kept it around 14.1 but it sounds like a little richer than that is actually a touch safer and won't materially impact my fuel economy or emissions.  I resolved the exhaust leak today and that actually smoothed things out considerably to the point where I'm not bouncing around a full point on the AFR anymore.  I essentially disabled the choke until I had funds to buy a new one by turning the cap all the way open 100% of the time.  I'll be sure to double check the pulloff at the same time as soon as I replace it just to confirm that last little bit of bounce isn't related.  Thanks again everybody!