It's my recently bought Longchamp, with original 351C and Holley Vacuum (looks like a 600cfm).
I got the car a week ago. It's been standing still for probably many years. I filled gas on both tanks and went for the first drive. I drove 10 miles and it worked fine, except it stumbled big time when pushing the accelerator more than ½ way down, so clearly the secondaries of the Holley needed repair. But idle and normal driving was fine.
Later that evening I started the car in my driveway, just to hear the engine. But it took a while to start and it missed on at least one cylinder right away. So I took out a few spark plugs, very black, and they were the wrong kind, too long threads and too cold.
Next day I took the Holley apart. Everything looked fine on the primaries. The secondaries had strange liquid gasket on the fuel bowl, and the float level turned down fully. And the vacuum diaphram was hardened from age (part 135-4). I corrected all this, ordered new diaphram, and new correct plugs.
I got the diaphram and plugs today and put it all in. Expectations were high. Unfortunately I got disappointed. The good news was that the secondary stumble was gone
. But the idle quickly deteriorated. I stopped and pulled the plugs again. Plug 2, 3, 5 and 8 were very black while the others were perfect, or maybe a little lean, not easy to see with new plugs. 2, 3, 5 and 8 are fed by the top layer on the dual plane manifold, and the left part of the Holley. Note that since the Holley doesn't fit the manifold, DeTomaso put a spacer in between which is open on the secondaries so the dual planes are not totally seperated.How can one side of the carb be so different? I've checked the obvious, the idle adjustment screws are both 1-1/4 turn out. The jets look fine.
But it get's worse!
As part of diagnostics I put a vacuum gauge to a manifold port, it was steady but only 10 InHg, the lowest I've ever seen. This low vacuum can of course add to my rich condition since the power valve will easily open, much more than if the engine at idle ran 20 InHg. But it still doesn't explain the diffence between the rich condition on plug 2, 3, 5 and 8 while the others look OK/lean.I ruled out ignition timing because I already set it at 16 degrees, and I've never experienced wrong ignition timing lowering the vacuum that much. I also considered valve timing or other problems, but since the needle is steady, not fluctuating, I don't think this is about 1 cylinder, it's common for all cylinders.
I then started looking for the vacuum leak. I took off the carb, taped the 4 holes in the manifold and put air pressure on a manifold port. The pressure made the tape bulge upwards and stayed that way for several seconds, to me that means no leakage beyond the normal. Then I installed the carb, and taped everything above the aircleaner gasket base. And put pressure on again. Here it was a different result. The pressure made the tape bulge, but as soon as I stopped the air the tape got flat, clearly a major (comparatively) leakage. I then used a soapy mixture to try to see where the air got out, it had to be around the carb, but I only found a leakage around the primary throttle axis, and to me it looked small and all carbs have some leakage there, right?
So can anybody figure this out?
1. Why do I get an over rich condition on one side of the carb/manifold?
2. What does the steady 10 InHg vacuum tell me?
3. If 2. is a vacuum leakage, I think I've diagnosed it to the carb, but can't find the leak?
4. Is 1. and 3. the same problem? Can my Holley have an invisible vacuum leakage on one side only? And would that produce such a huge difference in mixture?
