Odd low vaccum reading on Zenith

[blackbird225]

I have to tell you folks, I am ready to give up.

Any idea why my #5 ( right bank middle ) throat would have a significantly lower reading with my syncro? I did a compression check and all the cylinders want to suck the same amount through, give or take 1 psi. I am still trying to get to the bottom of my loud clatter, and I am suspecting its a fuel delivery problem. I Set my timing ( per Warren ) back to about 31 BTDC @6000 ad I could barely get it to idle. Well, I fixed that and got a much better power curve ( thanks Warren ). But still rough idle.
When i rebuilt the carbs, I took every precaution to get everything spotless and blew air through every orifice and venturi and passage that I could get to. THe carbs are rebushed and there is 0 play in the shafts. New plugs, wires, distro, cap rotor ..... When you step on it, you can defininatlely feel an imbalance back there. Like a fan with a missing blade. Its got to be one cylinder, or one side.

Wishing there was a magic step or 3 steps to perfect.

Sigh

[911ctS]

Have you tried adjusting the air bypass screws. They are what controls the "suction" through each barrel at idle. It may be that your #5 just needs a bit more.

[blackbird225]

I can indeed get the airbybass screw to open enough to get the #5 to equalize. But the screw is out so far that its loose in its threads.

Any idea what would cause a major fluctuation like that. I mean , all the others are 3.5-4 on screws all the way in and this one is on 2.
I think its time to take it in. I thought it would never come to this, but I am afraid that I am going to do major damage with that major ticking.

[project 911]

Blackbird-

Two things I just went through with mine. One throat always seemed to be a problem and I found the intake valve on that cylider was WAY too tight.
The other thing I found was the bottom flanges on the intake manifols were really warped. I leveled them out with a file and put new gaskets in.
I just went through adding a new fuel pump and regulator. It ended my constant tweaking of Zeniths. I had a gauge that showed proper pressure, but found that the volume was too low, starving the carbs.

I know how you feel- I've been screwing with this thing since last Fall, changing, adjusting, replacing everything and it seems to never be right. Keep at it. Mine ran the best it ever has today, and after everything I've done, it turned out to be something simple that I neglected to check (like fuel volume).

[A Quiet Boom]

ticking you say, have you check your valve lash? If you've got a loose valve (ticking) that could certainly make one cylinder weaker than the others. Also check your idle jets just in case one is blocked up, I rebuilt my Zeniths 2 years ago and had a jet start to plug up last fall, I just fixed it the other day.

[blackbird225]

Will try all of these. What psi is enough ? I have mine set at like 1 or 2 psi.
I could only imagine waht a mechenaic would have cost me to do the things I have tried...

Thanks again

[project 911]

Fuel pressure should be 3.5
So far I had a guage that was inacurate and a faulty regulator that fuel just dribbled out of. Check the flow.

[911ctS]

Steve brought up a good point about the bases of the manifolds. Have you done the starter fluid spray trick? If not, then let the engine idle while lightly spraying starter fluid at the bases of the manifolds and where ever else you might suspect a vacuum leak. Are you very sure there are no openings or un pluged inlets on the sides of the carbs. My Zeniths tend to loose a big flat head srew (plug) every now and then. No matter how tight i put it on, it still manages to come out and make a whissling sound. Dont give up, yet you'll find it.

[blackbird225]

I will do the test again. Before I bought rebushed Zenith's off of this list, you could spray crab cleaner right at the throttle shaft nut and the idle would pick up immediately... I tried that with the new carbs and all is good. I will do it all over again tonight. I need to have my new turbo valve covers planed slooth, so why not fo the manifold bottoms too?
Michael

[BK911]

Maybe #5 throttle plate is slightly bent, closing that cylinder off while keeping 4 and 6 open slightly. If/when you pull the carbs, shine a light and see if they are all closing. BTDT.

Good luck!
BK