craig442

65 442 with stock freshly rebuilt 400 and stock 4GC carb, About 250 miles on engine. The only thing not stock is the MSD ready to run distributor. I am unable to eliminate an off idle hesitation. It is not bad, but annoying of course. Happens cold or hot. Other than this car runs really well. Starts on a dime hot or cold, no hesitation if car is moving. It idles best and with highest vacuum (19) with the idle mixture screws 4-1/2 -5 turns out, yet engine seem to be lean based on the hesitation issue and spark plug condition (tips are clean, dry and light brown). No change in mixture screws adjustments eliminates this hesitation (it might get worse or better, but not eliminated)

I've checked the following:

No apparent vacuum leaks (idles smooth and car stalls if I cover the top of carb),
Initial timing set to 8° (seems best here - going from 5 to 8 helped a bit, but over 8 doesn't help).( With Vac advance disconnected.)
Timing mark is verified at TDC on compression stroke
Slow idle set to 550 rpm (in DR) per spec)
Experimented with various weight combinations and stop bushings in the distributor. Looser springs helped a little, but too loose causes pinging.
I get two instant strong streams of gas when moving the throttle
Idle vent lash is to spec
Steady 19 vacuum reading (needle may bounce 1/2 randomly) , other vacuum tests seemed fine.
Vac advance holds vacuum, and is hooked up to manifold vacuum as originally done (seems to add about 20° to timing when hooked up). Car does run a lot better with this hooked up, more responsive)
Plugs gapped at 035 (005 more than spec due to MSD ignition)

I've read all the theories, so was looking for who had this issue, and what turned out to be the solution. Lots of forum posts describe this issue, but all to often the posts end before a solution is posted. The guy who rebuilt the carb (professional) is doing some research on this specific carb to see possible causes/solutions, but I wanted to see what other Olds people did as well. Carb guy believes it is related to today's fuel not having the same properties, thus a slight adjustment in the idle circuit may be needed.

Thanks!

oldcutlass

Which bushing and springs are you running in the distributor? The distributor comes with the blue (21*) bushing installed. So 8 + 21= 29* total advance. Not enough initial with that setup, you need the initial set to 15/16*. At the instance you first move the throttle, are you getting 2 solid sprays of fuel?

sysmg

Could the carb be leaking at the throttle shafts or at the base? If it's a relatively small leak it could impact the idle circuit, but would make little or no difference off idle. I don't have much experience with 4GCs, but 4.5-5 turns out seems like a lot, perhaps that is another sign of a vacuum leak.

I've never tried it, but someone suggested turning on a propane torch (without lighting it) near the outside base of the carb and near the throttle shafts. If the idle picks up, you have a vacuum leak.

craig442

Thanks for the replies. As noted there is no vacuum leak at present time. This engine does not like a lot of initial advance, it will ping like crazy, regardless of the weight springs installed. This seems to follow what I have learned, that high compression engines do not typically like a lot of initial advance (or total advance as well). Car runs strongest with one heavy silver spring, and one light silver spring, with initial around 7 degrees (w/o vacuum hooked up). Any more than this and it will start to ping. The only issue is the slight off idle hesitation.

dragline

Maybe try 1 or 2 heat range colder spark plug. If the underside of the carb does not look as pictured you may need a richer idle circuit. A quick test while idling is to pull off a manifold vacuum line, if the idle goes up you may be too rich, if the idle goes down you may be lean.

Koda

Are you running crap gas? The engine may want more timing, but can't handle it on 87 ethanol blend crap.

craig442

Thanks for the replies. I am using 93 octane gas. If I pull off the vacuum line (distributor advance) the idle goes down even when I plug the port. It is definitely lean I believe based on the mixture screws being 5 turns out. I'll try and check the photos above this weekend.

The MSD vacuum advance adds about 15-20 degrees of timing when connected (at idle).

craig442

Another issue I noticed today. I went to play with the timing and noticed when revving the engine to 2500-3000 rpm, the timing marks jump around, although they occasionally get steady. What would cause this? At lower rpms I can get steady readings. This is of course with the advance plugged. Could the timing gun be bad, or perhaps the weights not working properly. They seem to move freely by hand.

oldcutlass

Please answer post #2 (I saw what springs your using)

friesjh

Those older carburetors were not meant to run on today's gasoline. I had a similar issue with my 71. After having it tuned for the motor and today's gasoline it runs amazing, no hesitation at all. Cliff at Cliff's High Performance did mine. Rebuilt and tuned the carburetor.

Just my $0.02.

craig442

To answer post #2, I get two strong immediate stream of fuel when moving the throttle. The bushing right now is the 19° bushing. As indicated before, this engine doesn't seem to like a lot of advance as it starts pinging.

What specifically was done to your carburetor to help with todays fuel?

oldcutlass

Do not hook up your vacuum advance until you get your mechanical timing setup. It will not hurt anything. See if it still pings. On your carb, does the fuel spray when the throttle first starts to move?

craig442

If it ever stops raining I'll drive the car with mechanical advance only and see how it responds.

Tedd Thompson

On 4GC's I have had many problems with the new accelerator pump cups going bad, sometimes almost over night. may not be your problem but it drove me crazy for awhile till I found a NOS leather cup, no problems after that...... Tedd

friesjh

As Tedd said there are correct parts which handle today's fuel. Besides this some of the passages/fuel circuits are drilled out larger to allow the proper amount of today's fuel to flow where it needs to go. I had Cliff at https://cliffshighperformance.com/ do the work. I was having similar issues with my car before I had the carburetor tuned. Now it runs like a dream!

Del70

Have the same issue - If I try to set the idle mixture screws to factory 4 turns out set factory RPMs etc, and adjust the idle mixture screws to get to the curb idle RPM, it will hesitate off the line under light normal driving. Screws are at about 3.5 out, otherwise runs perfect. Start with 2.5 turns out. Set RPM to and then worked to get factory curb idle RPM, with some very light idle mixture adjustments (screws are at about 2.75 turns out), and the hesitation goes away. I suspect it is a little lean. For now, I am trying to see if this works. (yes no vac to advance when setting the carb,, float is set, new accel. cup, carb rebuilt)...)

friesjh

That is the problem, the old carburetors run too lean and the idle mixture screws won't adjust enough to accommodate the new gasoline.

sysmg

Doesn't sound like the right way to adjust the mixture screw. On a quadrajet, I usually start 1.5 turns out. I hook up a tach. Then at curb idle, I turn them out 90 degrees (1/4 turn) at a time (both sides evenly). You will notice the idle will increase each time, repeat, when the idle stops increasing, turn both in 1/4 of a turn. Then reset the curb idle via the throttle stop screw (on a cast tab near the primary butterfly shaft).