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Hello,
I have a question that I hope someone on here can help with. I have a 1968 Oldsmobile 442 with the 400 motor, I added an Edelbrock performer manifold and a Edelbrock 650 CFM Thunder Series carb #1806. I have the carburetor hooked up to the ported vacuum, can is having problems running at idle when hot, I have checked for vacuum leaks and nothing was found. At idle ported side reads less the 5", should I switch the vacuum to the dist. to the manifold vacuum side? would this help with idle? Old Qjet I believe was hooked up to manifold vacuum and possibly both. Can anyone offer some assistance? Thanks for reading.
You've started 3 threads for this issue. Have you checked your fuel pressure and float levels? There should not be any vacuum on the ported side at idle. What RPM is the idle set to? What ignition are you running, stock points, electronic replacement, HEI? If you are still running points, what is your dwell set to? What is your timing set to? How much vacuum does the engine have hooked to manifold at idle? Is it steady? Have you adjusted your idle air/fuel mixture screws properly?
Your old Qjet quite possibly had the vacuum advance canister hooked to both switched through a temperature controlled ported vacuum switch that looked like this.
Hello,
I have a question that I hope someone on here can help with. I have a 1968 Oldsmobile 442 with the 400 motor, I added an Edelbrock performer manifold and a Edelbrock 650 CFM Thunder Series carb #1806. I have the carburetor hooked up to the ported vacuum, can is having problems running at idle when hot, I have checked for vacuum leaks and nothing was found. At idle ported side reads less the 5", should I switch the vacuum to the dist. to the manifold vacuum side? would this help with idle? Old Qjet I believe was hooked up to manifold vacuum and possibly both. Can anyone offer some assistance? Thanks for reading.
First, it isn't your CARB that is connected to manifold or ported vac, it is your DISTRIBUTOR.
Second, ported vacuum is taken from a port (get it?) just above the throttle plates. That means that there is little or no vacuum on that port at idle and small throttle openings. As soon as the throttle blades open enough to uncover the port, it becomes exactly the same as manifold vacuum.
Third, ported vacuum was used for the vacuum advance only as a way to lower NOx emissions at idle. In stock configuration, your distributor was always connected to ported vacuum.
Fourth, ported vacuum (or more correctly, the LACK of vacuum advance at idle) WILL cause overheating. Olds bandaided this by using a thermal vacuum switch as shown above. If the engine started to overheat, the TVS would switch the distributor from ported to straight manifold vacuum. This advanced spark at idle, lowering engine temps. Do NOT confuse this with Transmission Controlled Spark (TCS).
Finally, if straight manifold vacuum is the preferred configuration to prevent overheating, why not use this all the time?
Hello,
My apologies for not stating correctly, yes my distributor is connect to the ported side on my carburetor. I have not tried manifold vacuum since placing the new carburetor on. Car was in the shop and the shop who put the manifold on and carburetor on hooked the distributor to the ported side. Here are the problems that I am experience. When I first start up the car, it will run great, idle and drive great. However when the car gets warm the idle will get erratic and will almost want to shut down. When I start to drive the car again it will go back to driving good however there may be some hesitation when pressing the gas pedal to start driving again. I have adjusted the idle mixture screws, check floats and check needle and seat valves and all seemed to be okay. Idle is set at 700 due to AC compressor. Yes I believe the switch shown above was on my original manifold but was not added again to the new manifold.
I have the same systems you are describing. After a year of restoring my interior, I start the car and am having this issue, I am going to sort it out this weekend. In the mean time maybe someone could pinpoint the rough idle problem. I am going to start with vacuum reading at idle, hot and cold. I did install anew vacuum advance module on my HEI distributor, might be that. Time to start trouble shooting
I started over and rechecked everything. I installed a heat insulator #9266, rechecked carb and found one float would drop one inch and the other float was dropping 1 and 1/2 inches. I fixed the floats, reinstalled carb and now is running better and no more erratic idle when warm. Good luck, if you find another symptom and you fixed your current problem please share. Thanks
looks like I still have a miss. Went through the set up procedure as per CSM. The engine idle at about 600 RPM,nice and smooth. A gentle push of the accelerator at about 700 RPM causes a rough idle to about 950 RPM. Seems to run OK after that. Carb rebuilt 6 years ago, no run time. Every part is new except the distributor itself. No wear on the shaft so I re-used it. I am thinking carb accelerator pump problem? Any thoughts on possible problem?
Believe it or not, I read an article about this very subject today. I wasn't paying too much attention to which magazine it was, Hot Rod I think. I probably won't be able to explain it very well but here goes. Using the ported vacuum makes it very difficult to get the idle settings correct and you have a poor transition between idle and main jet circuits in the carb. Having manifold vacuum allows a lower throttle blade setting for idle so the idle mixture screws are easier to set. As the throttle is opened, the transition between idle and main jets is smoother because most of the time you are not flooring it. Normal driving is a lot less dramatic. The article was toward the rear and it was about a Chevelle with a 350 crate motor having drivability issues. Look for it to get a much better description if the issue and resolution.
On the 77 dist. is connected to manifold vacuum. A dist. thermal switch will reduce vacuum by 1.5"-3.0" when temp get over 220*F. I am having trouble from idle to main jet transition. Looks like the carb might need some cleaning after sitting for 2 years with no operation. Was rebuilt 500 miles ago. I was trying to "SEE" the idle fuel enter the carb. venture, difficult to see. I can see the fuel squirting on acceleration but not at idle. Maybe not looking in the right spot
July 6th, 2017, 11:12 AM
