Help spark plug reading

[cabledog]

It would have to have a pretty big cam for 5" vacuum at idle - what cam is it? Besides timing I would be suspicious of a vacuum leak somewhere.

If the cam really is that big - then your rich situation at idle could be due to a stock Holley. Once your have made sure the engine is properly timed and the vacuum advance is working again - if it still is idling rich and especially if you don't get any response out of the idle mixture screws or they are backed way out I would pull the carb and look at it. Turn it over and with the throttle closed to curb idle position look at the exposed portion of the transfer slot in the primary throats. The blades should be closed so that the exposed section is roughly square or about .020" exposed. If it's opened too much your transition circuit can be dumping fuel in at idle and really mess up your A/F mixture. You can gain on it some by opening the rear secondary curb idle setting screw (also on underside with slot head down in a hole) an equal number of turns as it takes to close the primary down. But - you don't want to expose any of the transfer slot in the secondary throats. If it's really a big cam and the secondary plates can't be opened enough to compensate for closing the primaries down - you may have to drill small holes in the primary blades to allow more air and get the transfer slot exposure reduced enough to be able to adjust the idle mixture. But do that only as a last resort.

By the way - Holley usually jets a 750 vacuum secondary carb with 70 primary jets and 76 secondary.

Nice looking car by the way.

I looked at the primary blades and you are correct. They appear to be exposing way too much of the transition circuit. I watched some videos on that and could explain why it smells so rich on idle. I will investigate into that further and attempt to get it right. Thanks for that tip!

 

[AR67GTX]

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I looked at the primary blades and you are correct. They appear to be exposing way too much of the transition circuit. I watched some videos on that and could explain why it smells so rich on idle. I will investigate into that further and attempt to get it right. Thanks for that tip!

Yeah - that's not good and probably about what can be expected out of a big cam. You can try opening the secondaries up some as long as you don't begin to uncover the transfer slot in the bores - which would start more fuel flowing in them. They have a small non-adjustable idle circuit to flow a little fuel to keep fuel from sitting stagnet in the bowl. If the transfer slot is exposed below the secondary blades it will start flowing more fuel. It's kind of a trial and error effort - opening the secondary and closing the primary and hoping a good balance can be found. Ideally you want to end up with the ability to adjust the idle mixture screws about 3/4 to 1 turn out from closed for a max vacuum reading. If you can't get there and you want to keep the cam and carb, you may have to drill the primary throttle plates to pass a bit of air.

Even on newer Holleys I've found the secondary throttle plate curb idle screw to be difficult to move initially. I usually set it upside down and pour a mix of 50/50 acetone and transmission fluid on top of it and let it sit overnight. They have so far moved after that although still stiffly.