Vacuum Advance Question

[Fred Wiesinger]

Hello Everyone,
Hopefully someone out there can lend me a virtual hand with an annoying question / problem that I’m dealing with. I’ve just got my 66 Tiger finished (well, are they ever finished?), tagged, insured, and back on the road after a 20 year vacation. I’m running the original 260 with basically all the stock parts that came from the factory. I’ve rebuilt the carb and performed what would in the old days have been a normal tune up. Point dwell is spot on a 27 degrees and the timing with the vacuum line disconnected is right at 10 degrees BTDC. When the Tiger starts and idles, it runs great, hard in the pedal she responds truely awesome. Unfortunately in between there is a considerably rough band. For instance, if I’m driving in a 25 MPH area and holding speed say in 2nd gear, no uphill or down, it sounds like it’s running on maybe 6 cylinders. If I pedal it harder, it responds right away. In investigating the possibility of a vacuum advance issue, I find that I have great solid vacuum in the manifold when measured at the carb spacer. But, the vacuum port in the carb come in to the manifold just below the throttle plate. At this location, I measure no vacuum at idle. I can measure vacuum at this port but it certainly responds differently than the true manifold vacuum. I did remove the vacuum diaphragm from the dist and checked it when hooked to a vacuum line on one of my other vehicles. The movement seems just fine. Any thoughts would be greatly appreciated.
Thanks,
Fred Wiesinger
66 MK1A

[Chuck & Wanda King]

Fred,

This is just a thought, trying to always look for the simple solutions. You said the car sat for twenty years. I assume you’ve got a 2BBL Autolite carburetor. Although you rebuilt the carb, if you didn’t clean the gas tanks and replace the fuel lines, there probably was (and is) a lot of corruption coming from there to the carb. Even with a new fuel filter, some small particles can get through to the carb’s float bowl and partially block the main jets in the bottom of the bowl. This means the idle circuit can be open, therefore good idle. The accelerator pump and circuit can be open, so you have good acceleration when you punch it or jazz the throttle. When your just moving along at a constant mid RPM the engine’s being starved. The check for this is simple; leave the carb in place on the manifold, remove the upper cover from the carb, look down at the jets and see if you can see anything blocking the holes in them. Even if you can’t see anything, take a screwdriver and unscrew the jets, remove them and hold them up to the light to see if the hole’s blocked, If so, take a wire, clear the opening, check the bottom of the bowl for more “crap” and clean it. Reinstall the jets, close the carb up, fire it up and take it for a test run. It takes about as long to do this as it took me to type this. If it sounds like I may have experienced something like this, I did. If you try this, let me know how you make out. Keep in mind, if this is the problem, you’ll need to address the fuel tanks and lines (not as simple).

[dancin]

I experienced the same problem with my Shelby Mustang. Just as your car, it ran fine at idle and full throttle, but ran erratic at steady speeds as well as gradual acceleration. I too found dirt in the carb – once clean, car runs smooth at all RPM’s. No further problems – has been over a year since I found the problem. In my case, I can’t explain how the dirt entered the carb as I have a new gas tank and fuel lines – perhaps dirt was not completely removed at time carb was rebuilt.

Good luck,

Dan

[Fred Wiesinger]

Gentlemen,
Thanks for the heads up. I’ll dig into the carb this weekend. The tanks were cleaned and I’ve installed a new fuel pump and all new fuel lines. But, it’s certainly possible that while performing all this work that I dislodged some junk from somewhere. I’ll let everyone know how I make out.

Thanks again,
Fred Wiesinger

[Author]

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