Power loss and vibration at low rpm, take off

[13matsc]

A couple off weeks ago the truck suddenly started to feel slow and sluggish under take off, and first acceleration. Getting some vibration under certain rpm’s (don’t have a tach so not sure exactly where).
Seems to be around 30 mph when driving on flat and straight.
Acceleration is fairly good when rpm and speed is high.

Backed the timing off two degrees, from 16 to 14 btdc, did nothing
Checked for vacuum leaks around the carb, found nothing
Pulled the carb off and cleaned it, especially the accelerator pump. Tried different holes on the lever. Did nothing
Checked all the sparkplugs and gap, all were OK. Also just replaced them a couple of months ago.
Inspected and ohmed all the spark plug wires, all were OK
Removed the cap and inspected and cleaned the inside, also measured the coil. They were OK, and also new one year ago. Rotor looks good, also replaced one year ago.
Inspected the harmonic balancer, fan and clutch. Cannot see anything out of the ordinary.
Checked engine vacuum, and still stable at 21 inHg at idle

Vibration is only under engine load, and it seems like the problem gets worse when engine is properly warmed up, after maybe 25 minutes of driving or so. Seems to slowly become more «sluggish» at low speed and low rpm’s.

1985 GMC C25
350/400
Edelbrock 1406 og 2101 performer intake + headers.
Normally running 16* btdc and manifold vacuum.
Gives me about 35* @ 3000 rpm and 26* in idle.

HEI module dying? Failure of other parts in the distributor?

 

[Old Guy Bill]

The spark advance springs/mechanism was good?

 

[fast 99]

Checking the vacuum eliminated a lot of mechanical items. I would suspect an ignition issue, weak coil, wire breaking down, crossfire inside cap, that sort of problem. Unfortunately unless a scope is available would be hard to pin that down. If you have a source try a different cap/coil assy. Also look carefully at pickup coil wires inside distributor where they attach. Would be glad to scope it for you but Norway is a bit far away. Another possibility is a weak EGR return spring, try unhooking it temporarily.

 

[Chevy 88]

Check compression / try brand new plugs.

 

[Ricko1966]

My experience with GM ignition modules,is they just die,no warning. Check for lightning under the hood. Wait until dark,mist the distributor cap and plug wires all the way to the plugs themselves with salt water out of a spray bottle,look for lightning and listen for snapping noises. It helps if you have a buddy you trust to mildly power brake it while doing this. It's a cheap easy check and almost free but I've found lots of misfires this way.

 

[13matsc]

Checking the vacuum eliminated a lot of mechanical items. I would suspect an ignition issue, weak coil, wire breaking down, crossfire inside cap, that sort of problem. Unfortunately unless a scope is available would be hard to pin that down. If you have a source try a different cap/coil assy. Also look carefully at pickup coil wires inside distributor where they attach. Would be glad to scope it for you but Norway is a bit far away. Another possibility is a weak EGR return spring, try unhooking it temporarily.

Might purchase a new coil and cap just to try, good to have on the shelf anyways. The rest of the distributor is stock, I think. Maybe best to replace the whole thing? Forgot to mention that emissions has been deleted. One thing I did notice was some scraping marks around the carbon «ball» inside the cap. Did not take a picture, but looked like the plastic around had been riding on some part of the rotor. I can pull the cap again if a picture is interesting.

Check compression / try brand new plugs.

Checked compression earlier this summer. Six was 180 psi and two were 185 psi. Would not think much changed over a couple of months, but you never know. Might check again this weekend.

My experience with GM ignition modules,is they just die,no warning. Check for lightning under the hood. Wait until dark,mist the distributor cap and plug wires all the way to the plugs themselves with salt water out of a spray bottle,look for lightning and listen for snapping noises. It helps if you have a buddy you trust to mildly power brake it while doing this. It's a cheap easy check and almost free but I've found lots of misfires this way.

Thanks, might try this on the weekend. Also believed the modules shortet out completely, but also heard of them being unstable when hot.

 

[fast 99]

Only mentioned the cap swap because it is quick and easy. Be sure to look carefully at those pickup wires. If the vehicle has the original distributor best to retain it if possible. Aftermarket or rebuilt distributors may or may not have an advance curve that works for that vehicle.

 

[13matsc]

Only mentioned the cap swap because it is quick and easy. Be sure to look carefully at those pickup wires. If the vehicle has the original distributor best to retain it if possible. Aftermarket or rebuilt distributors may or may not have an advance curve that works for that vehicle.

I’ll pull it again and check better inside the distributor and under the rotor.

I’m looking for loose connections, bad connections or what?

 

[fast 99]

Where the 2 wires attach to the pickup coil they can break right at pickup coil. Cause is from the advance plate moving. I sometimes give them a gentle tug with a needle nose pliers. if about to break it is usually visible.

 

[13matsc]

Pulled the cap again. Attached some pictures. Not sure if anything obvious is visible? Wires seem good.

 

[Ricko1966]

Your cap is wasted

 

[13matsc]

Your cap is wasted

My thoughts when I saw it. I first thought it was bad molding from the factory. But maybe not so?

This is my old AC delco cap which went bad about a year ago. It does not have the same grooves around the carbon piece.