Can anyone help me with a idle problem I have on my 4.1? I have gotten 3 carb assemblies from tompco and they all still run very rough at idle. Also I got one to run half way decent but the fuel mileage was only 12 mpg. Any help would be fine.
Make sure the vacuum connections you've hooked up to are the right ones - ported vacuum for the vacuum advance - not manifold vacuum. Also make sure that there are no vacuum leaks at the intake manifold to head gasket.
I agree with Quenton. Check out the manifold to head seal. This is an area prone to leak on the OHC 6. Spray some ether or carb cleaner around that area and see if the idle changes.(surge and/or slows)Check all vacuum lines to verify that there is no cracks or leaks also. Good luck!
Rough idle is always the result of unequal combustion pressures between firing cylinders. A vacuum leak is certainly a candidate (because a vacuum leak will cause some cylinders to produce varying difference in combustion pressures but it is only one of many items. I would recommend the following steps:
1.) Perform a static compression check on all cylinders with the throttle valve held open at least 25%. More than 15 - 20% difference in compression will begin to noticeably effect the idle smoothness.
2.) There is no direct measure for dynamically measuring combustion pressures (which is very unfortunate). However, to determine if the problem is isolated to one cylinder, you can ground each cylinder's high tension wire one at a time, to see if one cylinder is causing you trouble. (Do ground each wire, an open circuit will cause High voltage feedback through your ignition system, this is phenonemon is visible on a scope). If it is one cylinder, and the compression is good, then it almost certainly points toward an ignition problem (bad high tension wire, etc.) all of which are easy to check.
3.) If 1 & 2 check good, then the problem must be the improper air fuel mixture to most cylinders. Each barrel of your carburator has an idle air bleed. This can be easially located by inserting one finger into the throat of one carburator barrels, when your finger begins to seal this air bleed, the idle should change very noticably, (work your finger over the air bleed slowly, this is a very sensitive operation). If partially restricting the idle air bleed causes the idle to improve, then the air / fuel ratio is too lean, which could be a vacuum leak or the idle portion of the carburator is simply not supplying enough fuel. It is not likely that the carbs you are buying are defective, but I have seen where rebuild houses consistently made a mistake, which causes all carbs to produce a problem. If everything points to a carb problem, it would be good to borrow a known good one and try it, if possible.
Also, vacuum leaks large enough to effect the mixture can almost always be heard and located by an observant person with very good hearing with the idles speed backed off to a low RPM. Spraying a small amount of oil with a directional nozzle can help (as mentioned before in another response). I have an ultrasonic listening device that I built many years ago which really can pinpoint and hear a vacuum leak with amazing accuracy and success.
