After I got the Jetfire done this past winter/early spring, I could not get it tuned just right. I suspected the dwell was not correct and did not trust my meter. I got a new dwell meter about a month ago for another project I was working on so tonight I hooked it to the Jetfire. Turned out it was way off. I adjusted it correct but it is bouncing around about 5 degrees. I have not had this in the past but thought it could be the bushings allowing the shaft to move around but it feels tight. Also, if I increase the RPMs the dwell drops another 5 to 6 degrees. I have never increased RPM when checking Dwell but did not think that should happen. This should be a simple issue but I am having fits with this one. Any thoughts?

 

Dwell is supposed to be set at a certain RPM if I remember correctly. I think if you play with the throttle the needle will move slightly but get the correct RPM then as Billy Mays would say "set it and forget it"
I would recommend looking at your timing afterwards. Boz

 

Edit - You may have a weak/cheap set of points. If not, you probably have worn bushings in the distributor shaft. You can just set it in between the plus and minus 5 degrees, erring on the higher side, and it will probably run fine. Or rebuild your distributor.

 

Eric did you replace the points?

 

^^^This. Dwell should definitely not change with RPM. If the spring in the points is weak, they will bounce. Also, how is the condenser? Finally, while the shaft bushings may be tight, what about the bushings between the main shaft and the part the rotor bolts to? I've seen slop there.

 

I did not change the points over the winter but they only have about 2000 miles on them.... I think.... I had a Pertronix in it at one time and though I had troubles with it so I pulled it back out and put the points in (I had wrong voltage to it and didn't know it). I am almost sure I put new points in it at that time. I would bet they are cheap China junk. I have an old NOS set I will put in it and go from there.

Good point on possibly having slop between the shaft and advance mech.. I will check that also.

Thanks guys

 

Also check the top distributor shaft bushing for wiggle. Watch the breaker plate while its idling to see if its moving around. You do have the point adjusting door in the cap?

edit didnt see Joes post. I had a 71, 8.2 liter Eldorado that did this. Couldnt believe what I was seeing when I lifted the door. This one was so bad it actually seized and took the breaker plate for a brief twist till it pulled the wires off. One loud back fire and that was it. Pulled and repaired it on the side of the road and managed to Macgyver it to get us through the weekend vaca. Later installed a 76 HEI.

 

I found that the points were bad. I replaced them and the meter is now consistent but... I am still having the problem of when I rev it up the dwell angle moves up as the RPMs go up. Not sure what is causing this. I did not have a new condenser when I changed the points so that will be my next change.

 

Physics, to some extent.

The arm on the points has inertia. The faster you turn the point cam in the distributor, the faster you accelerate the point arm away from the cam. As the rubbing block on the points crosses the peak of the cam, inertia prevents it from following the cam exactly. At low speeds this is a minor issue, and it gets worse as RPMs increase. This is one place where an electronic distributor does have an advantage.

 

See if you can find Vette Blue Streaks by Standard Ignition. Stiffer spec spring. Echlin has the better points as well.

 

When I bought out the barn full of cars and parts there were several sets of NOS points. The guy told me they were part numbered for the Corvette and had stiffer springs in them. I have no idea if this is true or not. I still have them somewhere but can not locate them at this time.

 

I'd use those in a heart beat.