82 chev 3/4 ton pickup low rpm power

[caccia]

I have a 1982 chevy ¾ ton pickup with 350 ci engine. Recently I installed a new distributor. It runs ok but seems to have less low rpm power than it had before. I’m pretty sure the problem is with the timing but I’m not sure if I should advance or retard it. Can anyone help me out?

[Jsdirt]

Give it a slight tweak clockwise and take it for a test drive. I will do this until I start to hear heavy spark knock, then dial it back to where you just get VERY light knock at full throttle - that setup usually makes the most power. YMMV.

 

Of all the vacuum advance distributors I've run, they never ran well with the vac advance hooked up to vacuum. Maybe it had too much weight or spring to begin with ... but every single one I've run I had to disconnect the vacuum to the advance mechanism.

 

If you really want to dial it in nice, do as Doug said - check how much advance your getting. If it's not enough, play with spring & weight combos - with some patience you can get them to perform just as good as these computer controlled wonders.

[Doug_Scott]

You may want to verify that the vacuum port you are using is the correct one. Some use simple manifold vacuum(has vacuum at idle) and some use venturi vacuum (no vacuum at idle, but has more vacuum with more throttle applied). Venturi vacuum is more common from my experience.

 

There is a whole science to distributors with vacuum and mechanical advance. Distributors that have vacuum advance connected to manifold vacuum will generally have lighter springs and heavier weights in the mechanical advance in order to get full mechanical advance early while under acceleration. Once engine is at speed cruising, the vacuum advance is used to increase economy. As soon as any load is applied, the vacuum advance is reduced by the drop in manifold vacuum.

 

Distributors that use venturi vacuum behave differently. They are tuned to use more vacuum advance than mechanical advance. On those distributors if you connect the wrong type of vacuum to them, they will not provide a proper timing advance curve and will be sluggish.

 

Why was the distributor replaced in the first place? Do you still have the original distributor? Some vacuum advance diaphragms have an allen headed screw inside the vacuum port that you can adjust total timing advance with. Would not suggest you play with that unless you have some knowledge of timing concepts in addition to proper timing testing equipment.

[caccia]

Thanks everyone, I started by tweaking clockwise and took it for a test drive. Much better. I had to replace the distributor because of bare wires showing.

[Doug_Scott]

Depends on the distributor you installed. Does it have the original mechanical advance weights and springs in it? How about the vacuum advance? Is it the same amount as the original?

Is the base timing set to the same value as defined by the sticker under the hood?

 

Generally retarded timing will reduce power. But, you should not just advance timing just because you can. Verify what you currently have, and verify the amount of mechanical advance you are getting, as well as the amount of vacuum advance you have.

[Doug_Scott]

Thanks everyone, I started by tweaking clockwise and took it for a test drive. Much better. I had to replace the distributor because of bare wires showing.

 

If the bare wires were inside the distributor is was likely just the pickup coil. Pretty common for the pair of wires connected to it to break due to the movement of the pickup coil when vacuum is applied and removed. Takes about 15 to 20 minutes longer to replace the pickup coil that it does to replace the entire distributer.