Re: 1973 Chevelle SS Station Wagon

by bucket Sun Aug 06, 2023 4:15 pm

After a bit of a vacation off the road again due to us moving, I'm happy to report the wagon is back on the road again. I recently removed the old electronic Accel distributor and installed a nice HEI distributor and a set of wires. A few of the old wires had tape on them, that I had assumed was the previous owners way of marking them... but it turned out the wires had been bit in half by a rodent and the tape held them together, lol.

1973 Chevelle SS Station Wagon - Page 3 20230711

1973 Chevelle SS Station Wagon - Page 3 20230710

1973 Chevelle SS Station Wagon - Page 3 20230810

I put about 500 miles on it in the last week or so. It runs and drives fantastic with the exception of the nasty tip-in stumble that it still has. I guess I'll try replacing the accelerator pump again. I have to keep talking myself out of a 4 bbl. The engine makes enough power to have a little fun, I really don't need to go down the slippery slope of engine modifications.

by 76Chevelle2Tone Sat Aug 12, 2023 1:40 pm

With respect to the engine stumble, I would suggest replacing the various vacuum valves if old. My 76 of course has more than your 73 with the EGR but the vacuum valves get dirty and weaken. Also what is the status of the timing chain and balancer?

by zucchi Mon Aug 14, 2023 8:32 am

bucket wrote:It runs and drives fantastic with the exception of the nasty tip-in stumble that it still has. I guess I'll try replacing the accelerator pump again. I have to keep talking myself out of a 4 bbl.

From looking at the one photo above of the engine, it appears yours is box-stock with a 2-bbl carb. If that's the case, I agree with 76Chevelle2Tone but go a bit further. The cars of this era are very sensitive to engine vacuum status. Unless everything is tight, snug, and buttoned up, engine operation will be less then optimal and ideal. Understand that every vacuum connection outside of the transmission modulator and distributor advance is compensated by adjusting the carb for "controlled" vacuum leaks. With that said, you may need to run the stock air cleaner with the vacuum lines that go the heat riser thingy (the one photo doesn't show what kind of air cleaner you're running). This whole vacuum thing is a marvelous balancing act that's as robust as it's weakest link. All it takes is for one failure anywhere in the system and…

If you really think the problem lies with your accelerator pump then swap the entire carb with a known, good 2-bbl. Otherwise, you'll need to go down the slippery slope of replacing every vacuum line, modulator, check valve, servo, filter (i.e. the bottom of the carbon canister), etc.

by bucket Mon Aug 14, 2023 9:02 pm

So, the hunt for the stumble continues.

As a refresher to what happened in the past, I rebuilt the carb and bypassed what little vacuum lines there were. I still aim to run the stock air cleaner, but it's badly rusted and needs a restoration. So I'm running a little Eddy air cleaner that I ran on my Impala's 2bbl 327 back in the 90's. Anyway, when I first got the car running it ran great with no stumbles at all. Then I went out of town for a couple months and then when I drove the car, it had a mild stumble. With more driving, the stumble became worse.

Yesterday I pulled the carb apart again, looking specifically at the accelerator pump circuit and also the transition slots in the throttle bores. I found absolutely nothing wrong. Float level was also fine. Accelerator pump shot was also good. So I put it back together and of course, it ran the same. However, I later realized that I neglected to check out the off-idle discharge holes, so my next step is to make sure those are clear.

by Iggy Tue Aug 15, 2023 9:23 am

If I suspect a vacuum leak, I start by replacing ALL of the vacuum lines and even replace the intake manifold gasket. Intake gaskets leak vacuum far more often than most think that is the issue. Once all of the lines are fresh and you know you have eliminated all possible external leaks, then you can properly deal with the carburetor if that is the issue. If there are any external leaks, you will fiddle with the carburetor forever and drive yourself crazy!

by 73ss Tue Aug 15, 2023 7:05 pm

Was the stumble there before the HEI install? Don't forget the HEI needs full voltage. The coil lead for the points system was a resistor wire.

by 76Chevelle2Tone Tue Aug 15, 2023 7:14 pm

73ss wrote:Was the stumble there before the HEI install? Don't forget the HEI needs full voltage. The coil lead for the points system was a resistor wire.

Yes that's what he said up above but the info was spread over a couple of posts. the stumble was there before HEI but has gotten worse with more driving.

by bucket Thu Aug 17, 2023 2:56 pm

Correct, the stumble was with both distributors and I have tried different timing settings. I have a 12v ignition source powering the HEI. I'll check for vacuum leaks again, but last I checked, there weren't any I could find. The only vacuum sources being used is vacuum advance, th350 modulator, brake booster and PCV. I suppose the intake gaskets could be leaking from the underside, potentially.

I haven't had time to mess with it anymore. Hopefully this evening

by zucchi Thu Aug 17, 2023 4:17 pm

bucket wrote:I later realized that I neglected to check out the off-idle discharge holes, so my next step is to make sure those are clear.

Did you go back and check those?

bucket wrote:I'll check for vacuum leaks again, but last I checked, there weren't any I could find. The only vacuum sources being used is vacuum advance, th350 modulator, brake booster and PCV. I suppose the intake gaskets could be leaking from the underside, potentially.

I suppose you know that you'll need to use a hand-held vacuum pump with attached gauge to check the dist. advance, trans modulator, and brake booster? Conned the pump/gauge to the hose that leads to the diaphragm assembly, not the assembly itself. A leaky intake gasket would manifest itself as a misfire or sputter at idle. Nevertheless, you can use a vacuum gauge to check for a leaky intake manifold by connecting it to the PCV opening on one valve cover and blocking off the opening of the other.

by bucket Thu Aug 17, 2023 6:23 pm

No, I haven't had a chance to check the discharge holes yet either. That's what I still plan to check first, then go ahead with checking for vacuum leaks, just to be sure.

I honestly don't suspect a vacuum leak as I don't have any other symptoms of a vacuum leak, just the stumble. Advance, modulator and booster all hold vacuum. Throttle blade position is normal, idle mixture screws responsive, etc.

by 76Chevelle2Tone Thu Aug 17, 2023 11:24 pm

bucket wrote:No, I haven't had a chance to check the discharge holes yet either. That's what I still plan to check first, then go ahead with checking for vacuum leaks, just to be sure.

I honestly don't suspect a vacuum leak as I don't have any other symptoms of a vacuum leak, just the stumble. Advance, modulator and booster all hold vacuum. Throttle blade position is normal, idle mixture screws responsive, etc.

On my 76 car all the various vacuum devices held vacuum. Nevertheless when I replaced them all with new components the difference in engine behavior was instantly noticeable. Things can be functional but not performing to operational specs in real time situations. The timing chain and balancer can also contribute. Have they been recently replaced or are they old?

by Sponsored content