Idle problem

[dlphil10]

Should I increase the DCRs to 0.055 as the next step? It definitely doesnt want more air. I loosened the tiny screw in the air horn over the air bleeds and it breaks up. If I unscrew it just a bit, I can get a little better response from the screws, but still not enough to get full rich indicated by a drop in rpm.

[Cliff Ruggles]

I don't quit follow.  If you open up the upper idle airbleed screw in the airhorn it will lean out the idle system.  If the engine responds well to this and you start to see some control with the mixture screws then it's already got plenty of if not too much idle fuel.

What do you have the timing set at?  Are you using vacuum advance?  Manifold or ported vacuum?....

[dlphil10]

Initial timing is set at 15deg, 13 vacuum advance(attached to manifold), 35 total (20deg mechanical). New DUI distributor with plugs gapped at 0.050". Idle vacuum is 18.5inHg in park at 900 rpm amd 15inHg at 700rpm in drive.

When I backed out the air bleed screw it did get worse, hut then backed out the mixture screw to compensate and the control with the screws seemed a little better. Unplugging a vacuum line makes it worse so it's not overly rich. Covering air horn chokes it.

[Cliff Ruggles]

Why so much timing at idle?  Few engines respond well to that much timing as it can cause an unstable idle and loss of control of idle speed and sensitivity with the mixture screws....

[dlphil10]

I had it running at 12deg initial and the hesitation was a lot worse and it would backfire through the carb. I bumped timing to 15deg and acceleration is smoother in general. WOT is nice, tuning the air door opening rate is still a process but almost there.

I can still significantly slow the engine by turning mixture screws in. Manifold vacuum rises after about 1.5-2 turns out, (1turn out from lean drop) but then doesnt do much if I continue to unscrew the mixture screws. Should I be looking for a drop in rpm associated with being overly rich?

Also should I bump up the size for the accelerator discharge? It's currently sitting at 0.028"

[dlphil10]

Ok so I got the car back out today. It was warmer than in the past. It got up to 67 today. I noticed my secondaries wouldnt open all the way so I adjusted the linkage to do so with them on the car.

Good news is after heat soak I got the idle at 625rpm and backed the mixture screws out to when now I can remove the trans vacuum line amd it picks up 50 or so rpm. They're out 4ish turns, but it seems smoother than before. The car is popping on part throttle now before the power piston opens, but I'm guessing I may have too much timing now that its richer, but I didnt have time to mess with it anymore.

Now that the secondaries throttle is adjusted, the linkage keeps popping off going down the road. Any tips on keeping it in? I dont have the correct 4 barrel throttle cable or linkage, but I cant be the only one that didn't replace the 2 barrel throttle cable.

[tayto]

when rebuilding the carb you set both primary and secondary plates to open 90*. they usually go over centre or not far enough. you can file the base for primary and bend the pins so they have a positive stop. not sure what you've done to the linkage but i've never had that issue. the only time i had an issue was with a marine carb, there was no way to set a positive stop so i just tweaked the linkage on the bench so it was just right. this is all outlined in cliff's book.

[tayto]

are you referring to the secondary air door on the top of the carb?

[dlphil10]

It's the throttle linkage for the actual secondary butterflies, not the air door. Air door opening has been adjusted via the spring and opening distance.

[dlphil10]

Attached is a picture of the linkage where itconnects to the  secondary shaft. The cam/lever on the primaries shaft has enough play in it that it allows the linkage to travel toward the primaries in the slot at the secondaries and slides out. I'm assuming this happens when the throttles whip shut. 

[tayto]

it's installed upside down. you might have to disassemble carb to fix it. i usually install that linkage as an assembly when i reinstall the throttle shafts.

[dlphil10]

Well that makes a lot more sense with the stop on the shaft... That's the way I got the carb.. Thanks for helping to clear that up. Now to figure out the off idle studdering.

[Cliff Ruggles]

15 degrees initial timing is probably OK, but not adding another 20 degrees, especially at idle. 

The only engines I see that like that much timing have too much cam and not enough compression in them.

35 total plus another 20 from the VA is also a LOT for one of these engines.  I usually shoot for 42 to about 48 degrees, initial plus mechanical plus VA, and for most builds I use ported to the advance, not manifold.........Cliff

[dlphil10]

I should be right on then as I've got 48 total at cruise. 15initial, 20 mechanical and 13 vacuum advance.

[dlphil10]

Is it generally acceptable to reuse the base plate to main casting gasket when I fix the linkage? Looks like I'll be raising the float back up to .25in while I'm in there as I lowered it a bit a while back.

I noticed the engine sags when vacuum hits 7inhg and recovers at 5inhg. 7inhg corresponds to the power piston opening. Is there a great way to measure the proper power piston hanger height?