My '73 Blazer (which I use as a plow truck and have owned since new) has developed a problem I have not been able to track down -- the only one in forty years.

It started out like a low power problem, but developed into a situation where if you accelerate slowly from idle, you can accelerate to any RPM, if you accelerate either rapidly or under under any load, the engine bogs, backfires (both intake and exhaust) and will stall if you don't come off the gas. Basically, the truck runs and sounds great in neutral, but it can barely move itself up a slight grade. So I really can't plow right now. Which sucks.

The truck was starting to run bad last season, with insufficient power, so I did a full tune-up (plugs, rotor, cap, condenser, wires, timing and dwell). The timing was really off, advanced 20 degrees or so. To confirm the plate/marker were right, I pulled the plugs and confirmed #1 at TDC on the comp stroke had the timing mark right on 0 on the plate and the rotor aligned generally toward #1. The engine was rebuilt at 230k, with a double roller timing chain so it is not prone to jumping as the old nylon-tipped gears were. I thought maybe the timing wasn't advancing, but I've got good vacuum, the inertial advance functions (that is, its springs resist torque and it snaps back). Also triple-checked I had the firing order right on the wires.

Next I thought maybe I lost a cam lobe (wearing one lobe round was why the engine was rebuilt the first time), so I pulled the covers and rotated the engine to confirm all the rockers appear to be working and to the same extent.

At this point I found out that what I though was just a low-power problem had something to do with how fast you accelerate, so I wondered if the accelerator pump had failed, so I looked down the bores while rapidly accelerating at the carb (engine off) and I didn't see a good fuel stream, just a puff, so (since I needed to get the truck running ASAP) got a rebuilt carb and installed it. Didn't fix the problem. Somewhere in all of this the fuel pump died, so I replaced that also.

I'm going nuts with this and have been ducking the snow for a month. Won't be able to do so this weekend. Anyone got any ideas? Only thought I have left is bad distributor or worn distributor gear.

Thanks,
73

 

i would GUESS a clogged catalytic converter from what you are describing but that truck being that old i would hope you dont have one on it.

you can check the distributor, take the cap off and make sure the little springs on the wings under the rotor didnt break or that they are seized in place. same thing for the vacuum advance, make sure its getting vacuum on acceleration and apply manual vacuum with the cop off to make sure everything moves like it should.

ya know one thing you can try that i dont think you mentioned is try replacing your coil. i had a old suburban that had a coil go out on it that kinda did what you were describing. it would backfire and run all weird when driving it. i kept it in my glove box as a spare and had to use it to get home one day when another truck i had later on had a coil go out on it

it would be nice if you had a entire distributor HEI unit to just throw it in there to test out.

 

This truck will not have a catalitic converter being a 73 lucky for him. i would do a compression test on each cylinder first. If you still have a mechanical distributor I would check to see if your tach and dwell readings are where they should be. If they arent then the distributor gear may be worn out. You could easily swap the mech dist. for a newer style HEI distributor from a 75 and later truck. Very easy swap and much better performance.