While driving in town, stop n go. My engine seems to load up slightly. When accelerating from a stop the engine stumbles and misses slightly then clears up quickly as rpm's climb. The car is an automatic. If I manually shift to second and wind up the rpm's beyond the normal shift point the engine smooths out quicker. If on the highway for extended time and slow to stop or turn. The stumble returns the goes away with acceleration as before. If the car sits in the driveway running at idle I can smell what I think to be rich exhaust fumes.

This has been a long term problem.

Here's some engine specs:
383 Stroker w/ 30k miles on it.
9.8 Dynamic comp ratio. (10.6 static)
Always run Shell 93 octane no ethanol blend (according to shell pump).
63 cc aluminum heads, 2.02, 1.6 valves.
Comp cam .458 gross valve lift, flat tappet with roller rockers.
Performer Air Gap intake.
Recently rebuilt Quadrajet carb (but had this prob before).
Long tube headers.
True dual exhaust.
2200 Stall converter.
Machine shop that did all the balancing said HP should be around 420.
Engine runs very strong.

What I have done to chase problem:
Checked all plugs, #7 fouls about every 4K miles, oily, I believe that is a separate issue (valve stem seal) I have yet to address. Other plugs are rather dark in color. I'm using the NGK racing plugs recommended by the head manufacturer.
Timing has been verified correct. Dizzy is new. Have not been re-curved but probably should be for optimum.
I set the air fuel idle screws by ear for max idle rpm. I want to re-set using the manifold vacuum method ( please advise this method and which port to hook vac gauge to).

Interesting facts: After carb rebuild the mileage dropped about 3 mpg (hummm??). Throttle shaft bores were sloppy, bushed them back to proper fit. Used a NAPA kit for rebuild. Performed all 29 (I think) adjustments to carb. feel good about the re-build overall. Prior to the carb rebuild the port in the front of throttle plate fed from the pcv valve was 80% plugged with carbon build up from prior to engine rebuild. The idle adjustment screws were about 4-1/2 turns out (now 2 turns).

Sorry for the long post. There's a lot to consider in chasing this problem.

Thanks in advance for suggestions.

 

One more thing. When I rebuilt the engine I used a high flow alum water pump and 160 deg. high flow T stat. It's cold here in northern IL and engine barely gets to 160 deg. I just switched to a 180 deg Stat. and I think it has helped a little.

Thanks guys.

 

If you give me your carb number and jetting specs, I might be able to give you a clue. Are you running initial timing in the high 'teens with total timing around 34? Do you have a vacuum advance that's correctly matched to your engine vacuum so you are achieving actual timing at idle of around 30? If you did not re-jet that carb and richen up the idle/transition circuit, you will get a lean misfire miss at lower rpm with those engine mods you've done. This, combined with inadequate timing advance, will make it smell "rich" when, in fact, you're lean (I've never seen a Q-Jet that will idle correctly at 2 turns out on the mixture screws on an engine like yours - you're grossly lean, unless you've drilled out your IFRs and plugged your idle air bypass holes)

Lars

 

Lars,

Thanks for chiming in. I know you are the Q-Jet expert and value your expertise. I'll check the numbers, timing and get back to you. I'm very weak re: Q Jet's in comparison to yourself. No I did not re-jet the carb nor did I drill out the IFR's. No mods other than the re-build kit and bush the throttle shafts bores. Stay with me on this thread, I'll get back at ya in a day or so.

Thanks.

 

Lars,

My carb number is - 17056206. The date code is - 0206 ADB.
Timing at idle w/vac advance connected is 25deg.
Total timing at 3200 rpm is 41 deg.
Timing at idle w/vac advance blocked is 2 deg.
What is the best way to determine the jetting specs?

I thought the air fuel idle screws were 2 turns out. I rechecked them and they were 2-1/2 out. I backed them on out in half turn increments and rpm's continued to increase. They are now at four turns out. I hadn't checked them since the carb re-build and initial setting.

 

I suspect you are running retarded. 2 degrees initial is not enough......I also suspect your 41 degree number is with the vacuum advance hooked up. What is it unplugged? You will find it is in the 20's which is no where near enough.
You need 36 degrees total, mechanical, @ 3000 rpm. Never mind what the initial is after you set this....but record it for reference.
Do this and post your results.
Adjust the idle crews with a vacuum gauge for highest vacuum....then just a touch rich from that.

Jebby

 

Jebby speaks the truth. You are running way retarded on your timing. As Jebby states, I assume the "total" number you are posting is the total with vacuum hooked up. That is not a correct number - you need to set total with vacuum disconnected, and it needs to be about 34 with those aftermarket aluminum heads. Your initial timing is massively retarded, and needs to be around 18. Not 2. You also have a really badly-spec'ed vacuum advance unit if you have 2 degrees initial without, and 25 degrees with vacuum. That means your vacuum advance is pulling in 23 degrees of timing. It should be 10 to 12. Further, if your vacuum advance is pulling 23 degrees of timing and your total number of 41 is with vacuum hooked up, it means your "real" total timing is at 18 degrees, which means you are running 16 degrees retarded. That will make your car run like crap... A car with timing that far off with be very high on exhaust hydrocarbons, and it's hydrocarbons that you smell as the "rich smell" (carbon monoxide, which is caused by a rich condition, is actually odorless).

I need the jet/rod specs and the float level that you set up your '76 Q-Jet to - you need to determine that by opening the carb up and looking at the stampings and measuring the float level. If the carb is original and unaltered, you are running extremely lean in addition to have some really bad timing specs. These 2 factors combined will cause all the problems you are experiencing and more...

Lars

 

Just did this today (sort of). I turned the idle screws in until RPM dropped off then backed them off until RPM picked back up, this also was highest vacuum (15mmHg). What do you consider 'a touch rich from that'?

 

The technique you're using and describing is the GM-recommended "lean drop" method for idle mixture adjustment. Obtain best quality idle (which you can do "by ear" or using a vacuum gauge) and then lean it out until a 50 rpm drop is achieved. This is the GM-recommended setting. For best idle and throttle response, back the mixture screws out (richer) 1/4 to 1/2 turn from that lean drop setting.

Lars

 

Lars, I have to give you credit for that, I read about it in one of your previous posts. Thanks for the prompt response.

 

Thanks Jebby & Lars.
Tonight I'll re-set the timing with vac advance disconnected and open up the carb for jetting info & measure float height (stay tuned). Where's the proper location to hook up a vac gauge for idle screw setting? Also, I have heard of the availability of adjustable vacuum advance modules. Should I consider picking one up and if so after installation how would I go about setting it?

Thanks again, love this forum.

 

 

Please allow me to save you a lot of trouble guessing what air fuel ratio your particular carb is running. I put this on my C3 Corvette four years ago and I instantly know, what A/F it's running at any throttle setting. No need to pull the plugs, read them, etc. Totally easy to hook up, with two wires: One to the 02 sensor you'll install (included) and the other one to the aux power on your fuse box. AEM even supplies the bung that accepts the Bosch 02 sensor ($25 to put on my exhaust at the muffler shop).

 

Bosch 02 Sensor. Included, with bung.

 

Here's an update. From a previous post Lars felt the vac can that came with dizzy was giving me way to much advance 23 deg's. I ordered an Accel adjustable vac advance module from O'reilly ($27). It has an adjustment range from 0 to 20 degrees. Requested Lars timing and vac advance papers and boned up on those (big thanks to Lars).

Installed the new advance module into the dizzy. Not difficult with distributor in engine, removed plug wires, cap, rotor and two mounting screws.

With vacuum adv. disconnected and engine warmed up I set mechanical timing to 35 deg. @ 3000 rpm. This gave me 17 deg. at idle. As stated in the papers the timing at idle is less important than max. Then I screwed the vac adjustment screw all the way out (ccw) and connected manifold vac source. Restarted engine and checked timing again 17 and 35 deg. as before. Now I screwed the adjustment screw in (cw) 3 turns, just a starting point to see what I get, reconnected vac source and rechecked timing. I picked up an additional 8 deg. advance from the module. That added to the initial mechanical setting put total at 25 to 42 deg. The vac papers indicate 10 to 12 vac advance degrees is a target to achieve so I continued to go back and forth adjusting and checking until I had 11 deg. of vac advance netting a total of 28 at idle and 46 at 3k rpm.

I then reset the idle adj screws using the lean drop method stated above by Lars. Put me at 3-1/2 turns out.

Reset idle rpm to 700 using the throttle linkage idle screw.

Time for a test drive. First priority was to check for pinging, all good. The stumble and misfires off idle I had experienced before were gone. Throttle response was as good as ever.

I've never gone this in depth regarding engine timing. I feel it was time well spent and I'm gonna call success!!

Lars, you had asked what distributor I had. Here it is: https://www.ebay.com/itm/Distributor...item51e054c943
I thought I'd try it for the price, 65k volt and well constructed (seemingly). I read one of your distributor test reports where any distributor ( pricey or not ) could net the same dyno performance results if they were all curved identically so why drop $300 or more.

Do you feel I've done all this correctly??

Big thanks.

 

Outstanding job - good work! You may still be running a little lean, but you've nailed your timing, and the car should be running significantly better. Did the timing tuning resolve all your issues? Often, fixing the timing will take care of it.

Lars

 

Yes Lars I believe the timing corrections have made a huge difference in the behavior of the engine and driving performance. Curious why you think I may still be a little lean. And I have yet to open the carb and identify the jetting . Lastly, now that ignition is pretty close what can I do to tweek for perfection and finish the job?? I have designed products and have a saying, ''God is in the details and the last 5% of the design process is where most drop the ball'' LOL.

Thank you for your support and advise.

 

If you are running headers, a cam, and good-flowing heads with a stock-jetted carb that's set up to stock specs with stock float level, I can assure you that you're running lean. The 1976 model year cars were set up lean on the stock engines from the factory to pass emissions, and the carb was set up and jetted to run on 100% gasoline - not 10% ethanol. If the carb was lean on the stock engine, you are grossly lean on your modified engine running modern pump gas. Just my suspicion and observation based on 40 years of seeing, racing, and tuning this stuff... But that's why I asked for your actual jetting and setup specs - if you have already compensated for those issues, you could be OK. Impossible to know without knowing the specs you are running. If you'd like me to make comment based on my experience, I need to know your primary jet size, primary rod size, float level, APT setup height, secondary rod code, and idle mixture screw setting. I also need to know if you have altered or drilled your IFR tubes, and if so, the diameter you drilled them to.

As a point of interest, and perhaps you already know this, but your carb is one of the rare high performance 800 cfm carbs. Properly set up, you can't run a better carb for that engine.

Lars