I usually run a little 750 QJ on my 462 for the street. It runs great and I get about 10 inches of vacuum in gear. 290-08h cam. So I think that is pretty good. This carb idles in park at around 1000 at 15 inches and in gear at 850-900 at a steady 9-10 inches. Smooth transitions and no stumbles. Any way, I also have a John Osborne carb that he built for me 4-5 years ago for my current setup. For whatever reason, it has just been sitting around. Anyway, I decide to put it on after putting on a new fuel pump. The car runs great with the carb but I can't figure out my idle situation. I can get it set up for about 16 inches in park at about 1000-1100. In drive it sets in about 800-850 with about 10 inches. All tuning done completely warmed up. I go out driving even after it is completely warmed up I get these symptoms. At one light I will suddenly idle up to about 1000 and have erratic vacuum down around 5-8 and can jump as high as 10 at times all within seconds. Drive to the next light after giving it some decent throttle and I am back to 850 or so and 10 inches steady the entire stop. Sometimes it will start to idle down to 750 range and 5 inches and seems like it is going to puke, then it recovers back up to 800-900 but still has poor vacuum. Go again and it gets erratic the next time, next time smooth and steady. I am not sure what it going on. I have really never used this carb and now I remember why. LOL I think I had similar problems with it in the past. I don't know enough to know what could be causing the problem. My guess is too much fuel but why up and down and light to light?? I can't see how it can be vacuum since it will set in right at times? It never seems like it is going to stall or anything but it can even cause my oil pressure to bounce which I don't like as it seems like it is going to stall, just never does. Off idle and the transitions are very smooth and it runs great on the move with WAY more power than the 750. I would just like to figure it out to get my monies worth out of this thing! Otherwise back to sucking air through a straw with the 750. I don't remember what size carb John used for my car. He built it based off all my engine specs I gave him. Thanks for any ideas. Could be simple, could be hard, I just don't know enough.
OK, so I went out and started over. When I come off a good clean out and come to a stop I will run about 11 inches. The longer I sit there the more the vacuum degrades down to about 8, but sometimes it will continue to hold 10ish. I guess with my cam this may be as good as I am going to get? I still have no explanation about the idle. It will run between 850 and 1000 depending on stops. Just wanted to provide some more info in case it helps. I have about 15 inches in park at about 1100-1200 now.
My first guess is that the power piston spring tension may be on the ragged edge of being excessive. This means that the primary metering rods may be rising slightly at idle, richening the idle mixture and further causing manifold vacuum to drop. The easiest way to check this is to have someone hold the car in gear at idle attempting to replicate the idle problem. When it occurs, while standing BESIDE the car, with a small screwdriver attempt to push the power piston down fully via the vent on the air horn. If you feel it move and the idle suddenly cleans up, you may have found your problem. If that is the problem, the solution is to use a lighter spring under the power piston or clip a coil off the one you have. Devon
99.9% of engine problems in this era of points ignition cars are caused by faulty ignition. YMMV. Do you have electronic ignition? You have changed two things. Which one may have caused the problem?
MSD ignition for several years. Only thing changed was the pump and the carb. 750 carb has no issues. This JO carb does. Still think it is rich at idle as the exhaust seems to irritate my eyes even though it is set at highest vacuum. I may try to lean it out some and see what happens. I could bolt the 750 back on and be good to go. The car runs fine, just don't understand the erratic nature I am having? I had my temperature gauge take a puke in the heat yesterday so had to order a new one now also. I don't like driving without the temperature gauge working when it is 90+ outside.
Next time it's fully warmed up, idle it down to 750-800rpm's, fine tune the mixture screws, and take a gander into the primary side of the carb and verify you don't have any fuel coming from the boosters (nozzle drip)......Cliff
I think Cliff is on the right track, if the speed is a bit high it could result in this. Also try adjusting the idle mixture screws when the car is in gear, works for me.
