69 C10 l6 lack of power with 4bbl, offy intake, exh manifolds

[Option13]

Changed the throttle return spring and that seem to help significantly. I believe the other one was too heavy and thats why when I mashed the gas, it still wasn't opening as much as it should. It also made the gas pedal significantly lighter (about the same as the old carb). With the new spring, I ran a new light blue line to the HEI. All in all its running substantially better. I think now that is left is the timing.

Thank you for all the help!

Originally Posted by '68OrangeSunshine

I recommend a 12 ga line. Some hotrodders say only a 10 ga wire will do, but I don't think it's necessary. Choose a color not used elsewhere, or one that will be prominent. I have Orange on one truck and Red on another. The feed end is plugged into any open spade on Ign Unfused on the fuse block. It goes out the firewall and can join the rest of the harness on the firewall going over the hump and clip into the metal tabs on the valve cover as it runs to the BATT+ tab on the HEI coil cap.
I have still preserved the original wiring [Orange/purple/white resistance line and the Yellow 20s that split off it] so I could go back to the points distributor if I wanted. I taped off and folded back the OEM points wires. When my Pertronix II burned out, I went back to points for a year, before I sourced a 292-specific HEI.
The Yellow 20 connection on the Starter is no longer necessary.
On my 292, I got a ''low boy'' cap for Model Year '75-'77 HEI w/ Remote Coil for Chevy C/10. I had to make a bracket to hang my coil since the OEM between-the-lifter-cover-bolts-type was not available. [Could not find my filed L6 HEI pix.]
I made sure I got a Vacuum can for that vehicle, which approximated my '68 C/10.

['68OrangeSunshine]

Originally Posted by Option13

Changed the throttle return spring and that seem to help significantly. I believe the other one was too heavy and thats why when I mashed the gas, it still wasn't opening as much as it should. It also made the gas pedal significantly lighter (about the same as the old carb). With the new spring, I ran a new light blue line to the HEI. All in all its running substantially better. I think now that is left is the timing.

Thank you for all the help!

Here's a shot on my fancy throttle return spring. I had to cut off the last inch or it would have continued thru my valve cover. Much better than the first solution, an old brake spring to the brake lines out of the master cylinder.

Good that you're up and running.

[Steeveedee]

Good to see that it's working better, now. I was mistaken about the carburetor- I thought that it was a lot bigger than it actually is. 390 CFM on a 292 should be a good match.

[Caddylackn]

Your HEI needs to be recurved due to the horrible centrifugal and vacuum advance set up it came from the factory for emissions. The vacuum advance gives way too much vacuum advance at part throttle and the mechanical advance comes in way too slow. I doubt you can get full advance from it before you hit the rev. limit. You would want full centrifugal advance in by 3,000 rpm max, but no centrifugal advance starting at all until a few hundred rpms over idle speed. You would want to limit the total vacuum advance or you will have pinging problems when the base timing is properly set.


Here is a proven method from an old repost of how to correctly set up an HEI distributor from Barry425 on the Hot Rodding Forum. This is for a V8 but the same applies to all GM carbed engines. My comments from my experience using this method are italicized. It helps if you have a digital timing light which shows advance in degrees or use a timing tape installed on the crank pulley that shows degrees up to 40 ATD or so to allow you to use a standard timing light.

Get an HEI recurve kit from CRANE and follow the instructions: (I have used the Jegs kits, and the advance weights and springs suck, use the Crane weights and springs)


1. Disconnect the vacuum advance from the distributor and plug the line.

2. Power time the engine at 3,000 rpm's. That means, attach your timing light with the engine off, then set the idle screw on the carb so the engine is running at 3,000 (a little higher is okay, but not lower).Adjust the distributor for the highest rpm You really need to make sure you have no vacuum leaks, or this will interfere with this step , then back it off by about 2-4 degrees (using the light). You can back the timing off at idle if it is easier, since turning the distributor 2 degrees at idle will have the same 2 degree effect at 3,000 rpm's. That way, you don't have to put your head into a screaming engine compartment to adjust the distributor, and the timing marks will probably be easier to read at idle.

3. Set the idle screw back to a normal idle if you haven't already in the previous step.

4. Make a full throttle run with a hot engine and listen for spark knock or "ping". If it pings, back it off another 2 degrees. Keep doing this until there is no pinging. This will be your maximum MECHANICAL advance. WRITE IT DOWN! (advance at 3,000 rpm's). Use the timing light to read the total advance

5. Now with the idle set normally, adjust the distributor for the highest rpm with no ping, especially when accelerating right off idle. Usually, advance for highest rpm and back off 2 degrees, just like before. This will be your INITIAL setting. Again, make sure there are no vacuum leaks on the intake/carb, as this will give you false base timing Make sure when you are doing this, that the timing marks are steady. If they are jumping around at idle, then you need stronger springs. Or your centrifugal advance weight holes are worn out-replace. Rule of thumb, choose springs to BEGIN your MECHANICAL curve approximately 500 rpm's above your idle speed. You might need two different color springs to achieve this This will give you a steady idle speed. Recheck this setting AFTER every time you select new weights.I found that it took one of the lightest springs and one of the second lightest springs to work. The two lightest springs starting advancing at idle speed, which interferes with setting base timing

6. With a kit, select a set of weights that will give you the total MECHANICAL advance that you found in step #4. In other words, let's say that you found your INITIAL advance to be 14 degrees, and your total MECHANICAL advance to be 34 degrees. That means that you need to put weights in the distributor that will allow 10 degrees distributor/20 degrees crankshaft advance (remember, you have been measuring CRANKSHAFT timing with your timing light, but the distributor turns only half as fast as the crankshaft). 34 degrees total advanced timing is standard for a SBC V8 with cast iron heads, an L6 may have different total timing needs

7. Make a full throttle pass to check your work so far. WRITE IT DOWN!

8. Reconnect the vacuum advance (it has been disconnected until now, remember?)

9. At part throttle CRUISE, listen for pinging. It won't be very loud, so you might have to find a place with a hedge or some other barrier on the left to reflect the sound back to you with your drivers side window open.

10. If it pings, the Crane kit comes with a vacuum limiter (a small flat serrated disk about the size of a nickle that mounts inside the distributor on the vacuum advance unit). Adjust it to reduce the total vacuum advance until it stops pinging. I used the second to last notch to restrict total vacuum advance to 10 degrees at the distributor (20 degrees at crank)Don't get this confused with the allen wrench adjustment inside the vacuum tube port. The allen wrench only adjusts the RATE of vacuum advance. In other words, how fast the vacuum advance mechanism will move when exposed tio vacuum. Let it happen fast (all the way Clock Wise if I remember correctly), unless it causes ping when ACCELERATING (different than part throttle CRUISE) at part throttle. In that case, back it off 2 turns at a time until there is no ping at part throttle acceleration.

11. The intial advance gives you low rpm torque and drive-ability, the total mechanical gives you maximum horsepower, and the vacuum gives you the best mileage possible from your particular engine combination. Write down what your combination is, compare it to measured gas mileage. You can make changes to base timing without messing up the advance curve. Try different base timings until you find the best mpg, best power, or the best towing set up without pinging.

12. If you have a Holley carb, make sure that you set the accelerator pump on the front float bowl so that it has no slop, but no preload either. Do this AFTER you get your idle screw set where you want it for the last time.

This seems like a lot of work, but the results get you better gas mileage, more power, and best off idle acceleration. It helps if you have your carb dialed in first so that there are no fuel starvation issues at WOT that may be interpreted as pinging. Make sure you have no vacuum leaks that would interfere with the vacuum advance and cause idle issues. Best of all setting your own timing curve gives you a working understanding of how ignition timing works and how to troubleshoot

['68OrangeSunshine]

Originally Posted by Steeveedee

Good to see that it's working better, now. I was mistaken about the carburetor- I thought that it was a lot bigger than it actually is. 390 CFM on a 292 should be a good match.

Easy to get confused looking at the Holley 4160 series. Huge range of CFMs. 390, 450, 600, 650. If you want more flow, you can go for the 4150s and Double Pumpers. I think I blew the 350 V8 engine [PO's crate] in my other rig, a '71 GMC Jimmy K/1500, by putting a 780 Holley double pumper on a cast iron intake and then experimenting with ''Aviation/Racing'' gas (100 Octane) on my local streets. I blew out valves on both sides. #3 AND #6 IIRC. They said ''it sucked the valves...'' The tips of the spark plugs in those cylinders were smashed. I just bought me a fresh Mr Goodwrench crate and rocked on. On pump gas.