Since swapping the cam to a TA 284 on my 1971 350 it has been a real pain getting the car to idle properly under load. I know that I have lost vacuum because of the cam change but didn't realize the effect it would have on the car overall. I have gone through a couple of Qjets to date with not much luck addressing the idle issue which needs to be set to high (900-100) not to drop off in drive with your foot on the brake. The latest carb is a remanufactured early 70's Pontiac Qjet unit which has an electric choke set up. My old manual choke set up never worked right with the TA manifold. I just had the unit retested and calibrated by the builder after putting in a new 2004r trans but still have the idle issue. I was considering readjusting the carb as recommended on other post to see what happens as it is real rich at the moment. However, I don't think things will change much. As a side note if you cover up the carb You Will shut her down so I don't think there's a vacuum leak in the carb itself but at this point who knows. I've also checked the rest of my connections for a possible vacuum leak with none found. Any ideas or recommendations??
Have you tried setting the idle mixture screws with the car in gear and wheels blocked? This is how I do it and it works great. Be sure to block the wheels very well or better yet have someone hold the brakes and put it in gear while you tune the mixture screws. Use a vacuum gauge on any source that gives vacuum at idle. Before you even try the above, make sure your timing is set correctly or your idle will never be correct.
The only problem with the timing being set correct is it is no longer OEM with the cam. I have to check what the recommended setting would be vs. what I'm at. I know its a lot different now as a recall it no longer reads within the OEM plate by the flywheel and I purchased a digital timing light to measure it last time. Happy Thanksgiving
What is the initial timing set at? I bet that cam would like more initial and less mechanical than the dizzy has now.
There are a bunch of things that can cause your issue. Most aftermarket cams will really like more initial timing as Ceaser suggested. At least 12* initial. To do that requires less mechanical timing in most cases. Also, if the springs are too light, and you are into the mechanical advance at idle, when you put it in gear, you lose some advance. Ditto with the vacuum advance, if it is hooked to manifold vacuum, depending on how much vacuum you have, you can lose some or all of the advance from the canister when you put it in gear. That is what causes it to idle roughly, or stall. If everything is right, you shouldn't lose more than 150-200 RPM when you put it in gear. You might also want to make sure that the power piston is fully seated at an in gear idle. You can put a thin screwdriver down the vent, and angle the tip towards the front of the carburetor, and rest on top of the piston. With the engine off, you can push the piston down, and feel it spring back up. If you can do that with the engine running, the spring is too strong for the vacuum you are making.
Changing carburetors isn't the answer. That cam in a 350 engine requires idle system modicications, or there will not be enough fuel to the mixture screws at idle speed. Increasing timing helps, but it is not the cure all by itself. I would recommend going back to the factory carburetor, and setting it up exactly for what you are doing. Specific details are outlined in my book, we can help out with parts and tech support as well......Cliff
Cliff, I bought your book awhile back. I need to give it a good read as I didn't see any information related to my problem when I took a quick look. Steve
Your 284-88H cam will provide a reduced vacuum signal compared to a stock cam. Read Cliff's recommendations with regard to idle system modifications. Minimal enlargement of the idle tubes and idle down channel restrictions will make a big difference in the amount of fuel provided to the idle circuit, and will compensate for the reduced vacuum signal. If you find your idle mixture screws are having little or no effect on idle quality, these modifications should restore their function.
Idle mixture screw adjustment does affect idle quality. My main problem is that when in park the idle needs to be greater than 1K so when the car is put in gear and the idle drops down she doesn't stall. As it sits now I believe the cars initial idle is too high but if it wasnt the drive idle would drop 150-200 rpm's and the car would stall. I'm going to do a little reading of Cliff's book and try some of the suggestions made this weekend. I hopefully will get the carb right before I start playing with the shift adjustments on my new 2004R.
I ran the timing light with the engine running and the vacuum advance disconnected / plugged. The timing mark on the fly wheel was nowhere to be found on the measurement grid plate. I then made a mark where the vacuum advance was pointing on the fan shroud so I had a reference point. I loosened the distributor up and moved it slightly in both directions to see what effect there was at idle and the motor seemed to like where it was originally set. The digital timing light I have did not give me a timing degree so I don't know what I'm set at this moment!! I played around with the carb while the vacuum advance was first disconnected then connected going in on the idle adjustments till the engine ran rough then back out until the motor ran a bit smoother. Tomorrow is another day as nothing really got better but at least things didn't get worse. LOL
If you don't have a dial back light, then you make a 30* mark on your balancer. You either haven't read my power timing thread, or you don't understand it. Looking at the engine, from in front of the car, 1.76" clockwise around the balancer, from the stock mark =30* of crankshaft rotation, 2" = 34* Simply make a second mark, point the light at the running engine and see where your second mark lines up on the timing scale. I've repeated this many times, but set the total timing to 30-34*. Your initial timing will then end up where it must be for the particular distributor you are using, and the total advance you have set. Use the lightest set of springs you can find to set the total timing. Then substitute springs that allow the total in early, but not any advance at idle speeds.
yep first check the timing and make sure youre not in advance at idle rpm. Thats what I usually think of when I hear of the big rpm drop into D. as for setting the idle mix, I use a vacuum gage and tune for maximum vacuum reading at desired idle rpm. The sweet spot I look for is where If I go a half turn richer (ccw) there is no real effect, but if I go a half turn leaner (cw) the idle rpm drops. This is where I find max vacuum readings. IF the setting is too rich for your setup and fould a plug or whatever, you can lean it a half turn. You can tell where the engine likes it, just like the timing.
If the car needs to idle more in drive I would get a convertor with some stall in it. It needs to free wheel up to 1200-1500 in drive. I had a truck with a 700 trans and it was a dog untill I put like a 1800 stall in it. Idled great and got the rpms up when I took off normal from a stop.
That was my thought some time ago. However, my current set up with the 2004r has a 2400 stall converter. My old set up with my turbo 350 a 2600 stall converter. At this point I believe my carb is not set up to run with the low vacuum my 284 cam provides. Carb is going to get modified and we'll see what happens.
