Loss of power and shaking

[13matsc]

The last couple of trips I’ve experienced some issues with my 85 C25 (350+400).

The truck has very limited power above 30mph and shakes more the the faster you go. Kind of lags, and does not have even power distribution. Runs rough and probably misfires. Never had any issues with this previously.
Truck has an Edelbrock carb and intake + aftermarket headers for the last four years.
Truck starts and shuts down normally, no issues there. Idle is fine, and when reving in park there is no issue.

Pulled the plugs (replaced last year) and they were fine.
Checked spark plug cables and and they all checked out good with the multimeter.
Pulled the carb and cleaned it. No difference.
Fuel delivery is good from both tanks.
Fuel filter was replaced this year and since its the plastic type I can verify that there is no clogging, debris etc.

Whats the next move?

 

[Snoots]

Check for vacuum leaks.

 

[fast 99]

Sounds like RPM related such as broken valve train part or ignition crossfire, bad coil, ect.

Drive with a gauge hooked up to manifold vacuum. Use a long hose under rear of partially shut hood. Clamp gauge under a wiper. Go around the block. If vacuum is steady that will eliminate many mechanical related drivability problems.

 

[13matsc]

Thanks. How does one properly connect a gauge to the intake manifold?

 

[fast 99]

Applications vairy there could be a port on the manifold itself or may have to find a source on the carb. Running at idle remove lines or vacuum plugs until vacuum is found. May need a T.

Depending on engine condition and altitude should be in the area of 18 inches at idle. That will be a good diagnostic port.

 

[ButchM]

Mats, (based on my past experiences) with normal start, idle and stop and revving in neutral, sounds like your basic mechanical such as timing chain slipping etc are ok. The idea of putting a vacuum gage on for a test drive is a good recommendation. You just need a non-ported (below the throttle blades) vac connection, almost any one will work for a quick drive. My thoughts; clogged fuel filter or fuel pump getting lazy or any other lack of sufficient fuel usually causes a drop in power not running rough, it kinda just lays down on you, not really runs rough, it almost feels like it going to die. I would suspect/chase a thing or two... when you get on the gas (open throttle) it puts more air/fuel in the cylinders which is more load on the distributor to fire the plugs, since you've eliminated the wires and plugs: 1. I would pull the distributor cap and check for carbon tracking in the cap. That will cause the symptoms you described. Some electrical contact cleaner and a little brush work should work to clean the inside of the cap. Check the rotor for same. 2. While the cap is off, pull the cover and check the coil. Most shop manuals will give you procedure to check with multimeter. Its not difficult. While the coil is out check the little button and spring under it. They used to sell aftermarket buttons that were basically zero resistance (factory had resistance similar to plug wires) that would increase the output from the coil to the plugs. It made a notable diff in MPG and performance. 3. Not sure about Edlebrock carb, but a malfunctioning secondary dash pot on a q-jet could cause the secondaries to "flop open" and cause symptoms similar to what you described. One tell there would be black smoke coming out of the exhaust pipe while the rough running is occurring. If you're getting black smoke while this is occurring it is confirmation that excess fuel is the cause. Been awhile so some of the memories are a lil vague but I believe a failing distributor module could maybe possibly be a cause.

 

[13matsc]

Mats, (based on my past experiences) with normal start, idle and stop and revving in neutral, sounds like your basic mechanical such as timing chain slipping etc are ok. The idea of putting a vacuum gage on for a test drive is a good recommendation. You just need a non-ported (below the throttle blades) vac connection, almost any one will work for a quick drive. My thoughts; clogged fuel filter or fuel pump getting lazy or any other lack of sufficient fuel usually causes a drop in power not running rough, it kinda just lays down on you, not really runs rough, it almost feels like it going to die. I would suspect/chase a thing or two... when you get on the gas (open throttle) it puts more air/fuel in the cylinders which is more load on the distributor to fire the plugs, since you've eliminated the wires and plugs: 1. I would pull the distributor cap and check for carbon tracking in the cap. That will cause the symptoms you described. Some electrical contact cleaner and a little brush work should work to clean the inside of the cap. Check the rotor for same. 2. While the cap is off, pull the cover and check the coil. Most shop manuals will give you procedure to check with multimeter. Its not difficult. While the coil is out check the little button and spring under it. They used to sell aftermarket buttons that were basically zero resistance (factory had resistance similar to plug wires) that would increase the output from the coil to the plugs. It made a notable diff in MPG and performance. 3. Not sure about Edlebrock carb, but a malfunctioning secondary dash pot on a q-jet could cause the secondaries to "flop open" and cause symptoms similar to what you described. One tell there would be black smoke coming out of the exhaust pipe while the rough running is occurring. If you're getting black smoke while this is occurring it is confirmation that excess fuel is the cause. Been awhile so some of the memories are a lil vague but I believe a failing distributor module could maybe possibly be a cause.

Thanks a bunch! I will be checking the cap and rotor today and see. I hope the specs are listed in the Haynes manual. I’ll probably replace them anyway as they are original to the car as far as I know. Next step will be the vacuum gauge and maybe a carb rebuild if it doesnt solve the problem.

 

[13matsc]

Pulled the distributor cap and pulled out the rotor also. Coil measured OK according to the Haynes manual.
Did not seem to bad in there.

 

[WFO]

The distributor cap and rotor have seen better days.

 

[squaredeal91]

Is vacuum advance pod still working?

 

[13matsc]

Is vacuum advance pod still working?

Not sure… How can I verify this?

 

[squaredeal91]

Not sure… How can I verify this?

Im not the best at explaining. Vacuum pod is on distributor. With cap loose and hose off of carb you can suck on the hose and pull Vacuum to see if pod pulls advance plate or not. If it moves and holds its good. If it tries to move then springs back it's leaking. Hope that makes sense

 

[13matsc]

Im not the best at explaining. Vacuum pod is on distributor. With cap loose and hose off of carb you can suck on the hose and pull Vacuum to see if pod pulls advance plate or not. If it moves and holds its good. If it tries to move then springs back it's leaking. Hope that makes sense

You mean the cap needs to be loose or off?

 

[squaredeal91]

You mean the cap needs to be loose or off?

Only so you can watch it move. I never need to watch to test a pod, I just suggested it because I don't know what you know. It's just easier with cap off so you can watch it. Also I've had a working pod but the weights were frozen and not advancing. That I would not have seen with cap on still.

 

[13matsc]

Only so you can watch it move. I never need to watch to test a pod, I just suggested it because I don't know what you know. It's just easier with cap off so you can watch it. Also I've had a working pod but the weights were frozen and not advancing. That I would not have seen with cap on still.

Tried sucking on the hose, but did not see any movement. Should it be enough just with normal human lung capacity? As the issue seem to get worse the hotter the car gets and with load on the engine this could make sense.

Edit: connected a pump to to vacuum advance, and its verified OK.

I’ve ordered new rotor and cap with coil as they was looking not to good anyways.