I have a 1986 F-150 with the 300 I6. I swapped to a vacuum advance distributor some time ago. I also have an Offy manifold with an Edelbrock 500 CFM 4 barrel. I've also gone to an Accel coil with a GM HEI ignition. So we are not dealing with factory parts. When I get up to around 3500 RPM, the engine starts cutting out and loosing power. My first thought was that it was either running to rich or lean, but then I began to question the ignition and timing advance instead. I have the distributor set at 10 degrees BTDC. I'm wondering if maybe I need to set it at 0 degrees. I'm not sure what the difference is in the way the Duraspark and HEI ignitions operate. I can't remember having this problem with the Duraspark, but I swapped the carb at the same time as the ignition.

 

The "GM HEI ignition" caught my attention. Are you talking genuine GM (or equivalent) parts?

Or was this a GM style ignition, with parts made who knows where? In other words, a no-name eBay special made in China? Some of these bargain versions have very poor reliability, often defective right out of the unmarked white box.

 

Accel GM HEI ignition coil and unit.

 

Most engines like something more than 8 or 10 degrees BTDC. What you want to check is where the centrifugal tops out at, at high RPM. Disconnect and plug the vacuum advance and spool it up. Be mindful of the fan. Somewhere in the mid 30s is what you want to see.

 

make sure the exhaust/cat isn't clogged. That can manifest as lost power at higher rpms.

 

So you got a dist. that looks just like the GM HEI dist.?
Some use the HEI module from inside the dist. mounted to a heat sink (son has a Jeep done this way) and a GM early HEI 250 six coil, it is not under the cap but to the side of the block.
Dave ----

 

Same here. Vacuum advance distributor, then an Accel coil and the Accel unit for a GM. I currently have it mounted at the back of the block but I'm getting ready to move it to the fender well away from engine heat. I made my own aluminum heat sink, but I have one for a Ryzen processor that might work so I'm going to try using that instead if possible. I think once I'm done with my latest tear apart campaign, I will do as suggested, and check the timing with the engine revved. I don't think the HEI ignition actually advances the timing, so the settings with the distributor should be good, but I'm still fiddling with the carb too. Yesterday I slowly brought it up to RPM in 2nd gear and it went up past 3500 RPM without cutting out, so maybe I'm running it lean under throttle. I still need to do some more messing around. What I would really like to install is a fast reacting O2 sensor with a Fuel/Air ratio gauge, but the ones I find seem expensive. I would like to know what the carb is really doing.

 

Yes, I would experiment more. If you can gently bring it up past 3500, you might have a fuel delivery problem. Once you warm it up take it out on the road and quickly accelerate and see if it will go past 3500, but if you keep ******* it see if it starts to cut out. If it does then I think you have a fuel delivery problem. When you start out and are not using the engine much, the carb should be full of fuel. As you demand more from the engine, the fuel system should keep the carb bowl full, but if it can't keep up the carb will start running dry. If you get out of it and quit using a lot of fuel, the marginal fuel system can catch back up again.

Just a theory.

 

You've basically removed/bypassed any computer control of the engine, right? Gone back to an earlier system, if I understand correctly.

Is this HEI distributor designed for an older non-computerized engine? If so, I'd think it would have an internal centrifugal (RPM-based) advance. To check, remove the cap and twist the rotor. If the rotor turns about 20 degrees or so and then springs back when released, that's the centrifugal advance.

Does it have a vacuum actuator on the side? This is the little doohickey (sorry if I'm getting too technical) that looks like a flying saucer and has a connection for a vacuum line.

Unless your HEI is designed for a newer computer controlled engine, it should have both features. And if it doesn't, then it wouldn't be the correct distributor for your application (edit) unless it was properly interfaced with engine parameters such as RPM, throttle position, manifold vacuum, etc.

 

Ok so you wired the GM module in place of the DSII system and after market coil.
This is nice to know when trouble shooting.

Did you check the float level of that carb yet?
Try checking that first if you think its a fuel issue.
If it was a Holley it would be real easy to check.
Remove the screw on the side of the float bowl and use the screw & nut on the top to adjust till fuel just spills out when you shake the truck.
Dave ---

 

I have a vacuum advance distributor, and an Edelbrock 4 barrel. I have set the float level in them, I had to because there is an optional Offroad float needle set I wanted to install which uses spring loaded needles. That way the carb doesn't flood out when hitting bumps. I already had that problem. It's possible that set it too lean. I ordered a narrow Fuel/Air ratio gauge but it will be a few weeks before I get it. I'm currently on a tear down campaign right now so can't drive it for a little while.

So now I have a plan. Double check the float level, I'm going to go through the choke settings again to be sure, then I'm going to do as suggested and check the timing in high RPMS. I will also hav the air fuel ration gauge which will give me a better understanding of what fuel ratio the engine is getting Thanks guys.