Hi all, I have a 1969 Buick Skylark Custom equipped with a 350 4 barrel and a TH350. At some point before I acquired the car the factory Quadrajet was replaced with a Holley 4175, which I'm sure most of you are aware is the Quadrajet replacement that's offered by Holley. For the most part the car runs and drives very nicely, but there's a few small problems I've encountered with getting the car tuned up that I could use some help with. First, I'm having trouble getting a stable vacuum signal, the vacuum signal I have currently at idle and in Drive is 20-21, but the needle bounces around. Would this be an indication of a vacuum leak or perhaps a cylinder isn't up to spec? (While the needle does bounce its within 1 or so of the reading I mentioned before) Second, the throttle response isn't as crisp as I think it should be. If I manually move the throttle linkage the accelerator pump discharge seems excessive, after the pump discharges the engine stumbles slightly and takes a few seconds to recover. Multiple "blips" of the throttle is quick succession will cause the engine to stall or nearly stall. I did do a compression test when I was inspecting the car, but it might be possible that a cylinder has lost compression and is causing some of my issues. Thoughts?
vacuum at 20" is good, so no vacuum leak. If the needle slowly moves around, your idle mixture may need adjusting. The accelerator pump may need an adjustment if it is excessive, it may be temporarily drowning the engine and it stalls from too rich a mixture. This instruction sheet has a lot of good information. Scroll down to accelerator pump adjustment. Have a look at the float levels too. https://documents.holley.com/199r10012-1rev1.pdf You can download and or print the above document (pdf.)
Thanks Larry, With adjusting the idle mixture should the adjustment be made in Park or in Drive? I've been doing the adjustment in Drive and adjusting the screws until I reach the highest vacuum reading and then I readjust the idle speed and adjust the screws again. Do I have the right idea or should I change my method? Thanks
You can do it the way you are doing it. Try leaning the adjustment in a bit from highest vacuum. If the vacuum needle is flicking down quickly at regular intervals, that points to a sticking or leaky valve. If it just wanders around, that's usually idle mixture.
The vacuum needle seems to be flicking like you say instead of wandering around. How can I diagnose and fix the issue? The car currently has around 66000 miles, and when the car reaches 70,000 miles I was considering pulling the engine and transmission to service both. What do you think?
You should be able to pinpoint the cylinder with a compression check. You can also try some Marvel Mystery oil. It might un-stick a sticky valve. https://www.marvelmysteryoil.com/ Maybe just pull the heads and do a valve job on them if all else seems good.
I'll give the Marvel oil a shot, when I get some wrenching time I'll do a compression check. Thanks very much for your help, I'll report back when time permits
I wanted to report back with my results! I gave the MMO a shot and it seemed to help with a clicky lifter I had but other than that I didn't see any major differences. What did make a difference was I changed the ignition coil, which woke the car up significantly. I also tuned up the mechanical advance with new springs and weights and it's running a lot nicer than it has since I got the car. Thanks again for the tips and advice!
I'll give a piece of advice, stop winging the throttle multiple times in a row in park. That proves nothing about nothing. If you can floor it from a dead stop with no hesitaion/hic-cup then you are good. Unless you floor it from a dead stop and black smoke comes out the tailpipe then your accel. pump is not too much. The accelerator pump does not refill itself quickly. What you're doing is like flooring it off the line, leaving off, then flooring it again and doing the same again and then wondering why the car finally stumbles. The accel. pump's initial shot should be its biggest and after that will get smaller but it is not expected to be floored multiple times in quick succession. Also, did I mention throttle response in park means almost nothing. Not saying you can't discern some problem in park with judicious use of the throttle to figure out a miss of something like that. Minor movement of the vacuum gauge means just about nothing. You've got pretty much nothing going on there. Good idea on the, I'm assuming, performance lighter springs and weights. Look into Larry's setting up your distributer guide and set it up with the correct advance total. Setting up your distributer will add nicer response. If I had a 60,000 mile engine it would be just ready for a cam/valve springs/headers/dual exhaust and small converter upgrade and it would be flying. Possibly an electronic distributer upgrade. If compression checks reasonable then no rebuild necessary. Add a good shift kit to the trans. For now the Holley is perfectly fine. Get a secondary spring kit after the other mods are done.
X2 . Possibly overthinking on flogging the gas pedal. Get a distributor recurve kit and add some timing. A few more hundred rpm on converter. Remember to get converter rated for small block, If vacuum is not dropping really low when it fluctuates, not much concern. A vacuum leak is usually constantly low . 15 or lower I would worry.
