Agreed testing reveals a bad timing curve.
You need a couple more testing answers as above, But you are getting closer!

• You will need to change the springs. 1700 is too low.
• You may need to lower the initial to 12*-14* and lengthen the mechanical advance from 16* to 24*, So you still get 36* total. That means a smaller limiter, or grinding the slots longer. You already found out it likes 36* up top. Now experiment until you know what initial and vac it likes at idle, then fix the mechanical in the distrib.
• But first I would test and straighten out the vac can. You very likely need both a 10* limiter and an adjustable vac can. But test it first. That can easily cause dieseling from over advance. Those cans are very seldom ideal. for these kinds of cams. And a 12" vac at idle will take very careful vac can tuning to get it to run well. With a 12" vac idle, your vac can must be all-in and steady at 9" vac. And still only give 10-12* advance.
• Then get back to the initial and the mechanical, if needed, after some test runs.

 

Leigh,

Thanks for all of the detail. I have a spring/weigh kit in route and will start adjusting. I will order an adjustable can today. Any thoughts or suggestions on what to buy, as I've never even looked into this. I'm sure I can figure it out, but thought I would ask.

I drove the car to work this morning and it is running much better. Very mild bog/stumble off idle. Almost unnoticeable. So I'm almost there on idle. It does have a stumble on WOT unless you ease into it. Not noticeable under normal driving conditions.

I'm just happy to start seeing some improvements and finally feel like I understand the problem and now just have to play with the timing/curve to get it dialed. I'll keep everyone posted as I progress. It may be a couple weeks before I can get back to tuning it.

Thanks again,
Keith

 

Great!
You'll get there.
We had a saying at the speed shop:

"Timing first, carb second."

Without the right timing, it can't use the fuel.

And I have a lot of cousins in Homewood.
Do a Drive by.

 

Homewood? Small world. I have one more kid heading to college in another year and then we may move to Homewood, as it is closer to downtown Birmingham, where we spend most of our time.

Keith

 

Oh yeah it is a very small world!
My grandfather, Ed Smith, was Police Chief there.

Back when Homewood was farms!

 

I have tweaked it a few different ways and still don't have it right. It's not terrible, but not right. I tried different springs and ended up back with the stock springs. I currrently have it running well if you roll into full throttle. But if you just drop the hammer, it will fall on its face when the secondaries open.

I'm going to keep playing with it, but it will be a week or so before I can get back behind it.

Thanks
Keith

 

Basically you are going to want to set the timing to the specs above. I have not seen where you touched / commented on the vac advance can yet, that is just as important.
The vac can tweaking may fix all or most of the bog. Once all that is done, it should run much better. No use touching the carb until all that is done.

Then, if you still have a bog, it could be due to the carb running a little lean, and you should contact Lars for suggestions, since he did your Q-Jet.
IMO your 12" vac at idle is a little low for a Q-Jet, and it may need some tweaking. You can not turn a screw and adjust a Q-Jet that way. If lean, it will need new rods & jets.
Lars would be your guy.
I know he tests runs them, but his test engine has a milder cam and more vacuum at idle, more like a stock one. And he tunes them for a stock engine, IIRC.
If I were to guess, your cam specs have changed since he did it.

Your cam in your 383 is nothing like a stock cam that came with any Q-Jet. Not with 12" vac at idle.
It is more like an LT-1, LS6, L71 cam. And those are solid lifter / Holley motors, all with 12" vac. With much lower idle vacuum. Kind of maximum borderline low vac for a street cam.
But I would bet, if you tell him your cam and idle vac specs, he can fix it.
Good luck.

 

Vacuum advance on the distributor should be plumbed directly to the base plate of the carb in order to respond quickly to throttle movement. Plumbing it to the vacuum can system will dampen the timing response to throttle opening because the can system maintains a vacuum supply during low vacuum conditions. I trust that you have also tested the distributor vacuum pot to respond to a handheld vacuum pump while watching the advance plate rotate back and forth. You can't trust that new parts are functioning correctly. Bogs, as mentioned above, are also due to a momentary lean condition when throttle is initially opened. If a malfunctioning EGR, for example, opens at throttle opening, a big gulp of air leans out the mixture. Timing, too much air, and not enough fuel all need to be considered.

 

Quick update:

When I took the car out for the last test drive, I pulled into a parking lot to make some timing adjustments and scraped the oil pan. Unfortunately, this scrape put a hole in my pan. So, I'm on hold to take care of this issue before getting back into the timing.

I will say that the best I have had this run was with the vacuum advance disconnected. If I set the initial timing with the advance disconnected, I set it at 15 and the total timing is 32. Connecting the advance makes it hard to crank when hot and causes it to fall on its face at WOT unless I ease into it. Also, I tried changing springs to impact when the full advance comes in and couldn't see any real change. Each option came in under 2K, which makes me believe I did something wrong. I also added a stop on the vacuum to reduce the amount of advance down to around 10-12. I need to get under the hood and document some of this, but need to get the oil pan corrected first.

To me, the advance has issues. To get to 36 all in, I have to set the timing to 20 initial. At 20, it is hard to start when hot. Next, reaching 36 too early in the rpms. Last, not positive about the total amount of vacuum advance yet, but dropping it down to around 10-12 didn't seem to correct it.

Move to come when I can get the oil pan corrected and can document some things.

Keith

 

Did you contact Lars to get his distributor set up paper?

 

Keith -
That carb was built and set up for you back in 2019, so it's not exactly a recent rebuild... If it's been sitting on a shelf for 6 years and you're just now putting it into service, the carb needs to be torn down and cleaned up.

Here is what I would do if I were you:

First, fix your timing. E-mail me for my timing papers and follow them - you have my e-mail address from when I built the carb. Your total timing needs to be 36, and it needs to come full-in at 2800 - 3000 rpm. Not 1700. That's way too quick. Vacuum advance has no effect on the engine being hard to crank or start - there is no engine vacuum at cranking rpm. But you need to limit your vacuum advance to no more than 12 degrees. I'll bet your vacuum advance unit is allowing 16-18 degrees of additional timing, which is too much. Fix your advance curve so you can run 12-16 degrees of initial timing while achieving total of 36 at 2800-3000. Run your vacuum advance with 12 degrees off manifold vacuum to produce actual timing at idle of about 24-28 degrees.

If it still stumbles, we should probably replace your old accelerator pump and make sure the discharge holes are not obstructed in any way. If it falls on its face going into WOT, check your choke pulloff diaphragm: Put a long vacuum hose on it and suck on it until it retracts. Verify that it is functional, and then check and see how it responds when you release the vacuum: Does it extend almost instantly, or does it take about 1.5 - 2 seconds to fully extend? If it extends instantly, or if it does not hold vacuum, it is defective and must be replaced with a unit with a correctly calibrated bleed orifice. You can also check your secondary airvalve spring windup and make sure it has not loosened up. The spring should be wound up 3/4 turn.

If you suspect a carb condition, which could be the case after 6 years in use and/or storage, you might consider sending it back for a "refresh" and test.


Lars

 

Lars,

Thank you very much for the detail. The carb has been on the car and in use since the rebuild. I have had a few other issues that I have worked through on the car since then that was causing issues with it not running as expected. Once I got it to run much better than it had previously, I have just settled for the performance I had, including the bog.

I truly expect this to be a timing issue. After some adjustments, I was able to get the bog to go away, but it didn't solve all issues. So I ordered a spring kit and the vacuum advance stop plate. I was in the process of making my adjustments and test driving when I put a hole in my oil pan. I should have that repaired in a few weeks and can get back on the ignition.

One question. I seem to get about 17 degrees of mechanical advance currently. To get 36 degrees of total timing, I'm going to have to set my initial around 19-20 degrees. At that setting, it is hard to start when hot. How many degrees of mechanical should I get? How do I adjust the advance curve to get it where I need it? I have a cheap HEI that I think is part of my issue, but that is a guess. I've considered a DUI distributor to attempt to repair, but don't want to just throw money at it if it's not necessary.

Thanks again. Still learning as I go.

 

Yes, your cheap Chinese HEI is a problem. You're better off getting a good, used GM HEI and setting it up right. The length of the centrifugal curve is determined by the shape of the weights and the "flat spot" that determines the peg-out point - the standard HEI does not have an actual hard-stop for the curve, which can cause problems at elevated rpm. By installing a positive stop pin in the advance slot, or by welding and grinding the slot, you can set the curve up the way you want. This requires a bit of work, or you can have DUI set up a distributor with the hard-stop in it to your specs and requirements. Cliff Ruggles is also very good with HEI setup and curving, but he's not going to touch that Chinese paper weight...

 

Thanks again. Sounds like my suspicions might not be too far off with this not being a great HEI. Instead of fighting to get the performance out of it that I need, I'll either find a good used GM HEI or go with DUI.

Thanks,
Keith

 

Wise choice, Grasshopper....