Miss - No Load Driving & Reverse

[LuvNLife]

OK; my money gun is almost out of bullets, so I need some help brainstorming a problem, rather than tossing random parts at it.

'85 C10, with a new Chevy 350/260 long block. At idle, it's smooth as can be. Drop it in gear, same. Accelerate lightly, and I begin to hear a random miss; don't really feel it bucking or anything, I just hear it. Driving steady state at like 25 mph, same. Pull out on a main road, throttle response is fantastic, half pedal and it pulls great, with no hint of a miss. At steady cruise, 55-60 mph, runs and sounds great. So long as there's a load, like a small hill, it sounds great. When I drop it in reverse to back up my 150' inclined driveway is the worst. Not only do I hear the miss, I feel it bucking. The truck also smells pretty rich at idle, but zero black smoke.

Here's the hardware I have: engine with less than 200 miles on it, Edelbrock Performer EPS intake, 1/2" wood carb spacer, Edelbrock AVS II 650 (no choke) carb, fuel pressure regulator set to 4 pounds, Patriot shorty headers, 10mm new plug wires, R45TS plugs, new (inexpensive) HEI connected (now) to ported vacuum, valves lashed to 1/4 turn past stopping of "clacking". The trans is a Turbo 350 with about an 1800 stall converter (was in the truck already).

Thoughts and recommendations would be appreciated.

[geezer#99]

Cracked flexplate!

[Ironangel]

Make sure your plug wires are good. I was having the same issues with my new 260 HP motor, it seemed to have an inadvertent miss at idle. I replaced the HEI cap and discovered two questionable wires, replaced them and wahlah, the miss was gone. Another thing I noticed about this motor is that it likes a lot of timing, like 15 degrees with the vacuum port plugged. Make sure you get the dizzy clamp tight because mine would rotate to retard (clockwise) if I just left it snugged...

[Dead Parrot]

Pull the plugs and check for one off color from the rest.
Park the truck in a dark place. Let your eyes adjust. Start and check for sparks or blue around the cap, wires and plugs.
The 350/260 stock likes lots of timing, both initial and at speed. Try switching to manifold port. I have mine at about 12 initial indicated but I never verified TDC, just went with the timing marks as is.
Rarely a bad idea to double check the flex plate and related bolts on a new install.
Adjust the idle mix for best RPM or vacuum then turn in 1/4 turn to lean the idle mix. Idle mix is just that, idle. Has little to do with down the road performance.
Might need to check the tuning chart for that carb and go a step or two leaner.
Plug gap at .045?

[AussieinNC]

Make sure the engine is not tilting over when in Reverse causing a plug wire to lay over onto an exhaust tube,,,,

Can you actually feel the miss or just hear it? Check the bolts are tight on the headers...

[cadillac_al]

I thought it could be a lean miss at off idle but if you smell gas it could be the opposite. You should try the vacuum advance on manifold vacuum to see if it works better.

[Wrenchbender Ret]

Try driving with the vacuum advance plugged off. Ive had these engines surg & run rough at lite throttle. If that helps you may have to get an adjustable vacuum chamber & limit the amount of vacuum advance.

George

[LuvNLife]

OK; good to be back. First, the dog got sick; she got better. Then, the wife got sick (not Covid); she got better. Then, I dropped a brake rotor on my middle finger; good enough to type again, but ugly to look at.

First thing I did was go out one night and fire up the engine with the lights off to look for any arcing; nothing at all. Physically checked the plug wires for cracks, breaks, etc., and nothing. Then, I disconnected and plugged the vacuum advance. Fired the truck up, took it for a drive, and it runs out great. No missing no-load or loaded going up 2-3% grades. Backing up my drive, no bucking, no missing whatever. The only difference I noticed was the exhaust was a bit louder, but that may be the mufflers breaking in (Smithy glasspacks).

So, taking a shot in the dark, I'm guessing something is extremely wrong with the vacuum advance in the distributor. I've used 4 of these inexpensive HEI's in other engines, and never had a minute's trouble, so I guess I was overdue. For now, I'm just going to leave it plugged off, since there's no other issues with running hot, etc.

Thanks for everyone's input.

[geezer#99]

Maybe you just need to limit the amount of vac advance.
I used to limit mine to 10 to 12 degrees.
There’s a section in here on limiters. They’re cheap and easy to install.

http://www.crankshaftcoalition.com/w...EI_distributor

[Bigdav160]

Exhaust leak?

[toolboxchev]

Check your HEI coil and Module also. Exhaust manifold gaskets are a thought here also.