So my engine has been running very well and my existing Qjet runs well too. I did experiment and put on a 72 Qjet from a junk yard that I disassembled, cleaned and installed new internals and gaskets. The engine Runs rough under 1000 RPMs. When I put the original carb back on, it runs great again, so I can say that the problem lies with the 72 Qjet I tinkered with. Thoughts? Sent from my iPhone using Tapatalk
what are the numbers on the carbs? each one is a little different in calibration in stock form. a 72 BBB qjet is very different from a 68 400 carb, for example. also, there a a bunch of common problems with old qjet cores that need to be countered. For example, throttle shaft bushings to eliminate primary shaft vacuum leakage. clogged idle tubes that restrict idle circuit fuel.
I turned the idle mixture screws liberally in both directions. Initially started 1.75 turns out. Still ran rough. In my haste, I didn't check vacuum yet but it's on my list of things to do. Sent from my iPhone using Tapatalk
Whats the intake carb mounting pad look like? heat channels at all? 1968 carbs had a wider, different baseplate than the 70-74 version. Its possible there may be a mismatch there causing a vacuum leak. which basegasket did you use?
I forgot to mention that the good carb is a 1971. I'm using the base gaskets below Sent from my iPhone using Tapatalk
Manifold vacuum with the good carb is the following: 650 rpm = 16 inches 800 rpm = 17 inches 1000 rpm = 19 inches Sent from my iPhone using Tapatalk
Operator error on idle mixture screws. Too far in. Now Manifold vacuum with the "bad" carb: 800 rpm = 16.5 inches 1000 rpm = 18.5 inches
You've figured out your issue. The carb seems to have an internal vacuum leak. Ken mentioned the throttle shafts. how tight/ loose are they?
I incorrectly stated the problem and just changed what I wrote above. Car is in garage idling fine now. I thought the idle mixture screws were close when I started but they were way too lean. I'm going to take the carb off again later this week and see how things look. Sent from my iPhone using Tapatalk
Another quarter turn out and I'm at about 17 inches at 800 rpm and 18.5 at 1000 rpm. Sent from my iPhone using Tapatalk
My issue now is the driver side idle mixture screw is stuck and won't turn out any more. I knew this when I put it back together but wanted to try it anyways. Planning on fixing this later and checking the throttle shafts. Sent from my iPhone using Tapatalk
that's not bad. Out of curiosity, what is your timing like at 700 rpm? as you dial the mix screws, you should be able to get a "feel" for what the engine wants and likes, and what is too much or too little. generally though, I do go for max vacuum at idle rpm, and timing plays a big part too.
you could try blocking off the idle air bypass holes in the baseplate, or reducing their size. That would make you open the throttle angle more at idle, exposing more of the transfer slot and adding more fuel. There would be less idle air bypass for your idle screws to overcome.
I messed with timing for a year and now I can't remember what initial is set at. I believe 12. I wanted it set as low as possible but still reach the 30-34 total recommendation. Sent from my iPhone using Tapatalk
Took it on the road at lunch today. It ran rough for an few minutes in the garage then it started to run smooth at idle. Had to push the accelerator quite a few times so it wouldn't stall. Once on the road, no problems with the primaries. It severely bogs when attempting wide open throttle. Secondary Air flaps might be opening too fast? New primary choke pull off on the front. The rear choke pull off is the original and doesn't respond to vacuum. Going to put the good carb back on soon and have it ready for racing on Sunday. It's really nice to have a good running backup carb. Makes the experimenting fun with no pressure of having to get it running soon. Sent from my iPhone using Tapatalk
