Poor Throttle Response at Low RPM

[aj81spider]

I've got the car running pretty well, but have been having a problem for quite a while now that I can't run to ground.

Background: I changed the head and rebuilt the carburetor (DMSA) last year. Since then the car has been sluggish when cold. It really bogs down at low RPM and builds slowly. Once it gets above 3000 it is reasonably responsive. When it warms up it's better and drives pretty well.

Today I put the Computronix back into the car (it was out as a result of debugging a previous problem). The problem still exists. Basically at RPM under 3000 you can stomp on the accelerator and get very slow response. As the car speeds up it gets better. At 3000 RPM it's pretty responsive.

Last summer when debugging it I increased the size of the idle jets.

Any thoughts of areas I should look at?

Thanks for the help ahead of time.

[RRoller123]

Mine did this when I changed the head and added new cams. Exact same symptom initially. Degreeing in the cams with the engine in the car was a pita and pretty much impossible for me to get right, after 4 tries, but I was able to get it pretty close, so I did and then iteratively set it. The symptoms indicate the Exhaust cam might be too far retarded, the LSA too small. In my case, the exhaust cam needed to be advanced a few degrees and it snapped right back to attention.

Pete

[RRoller123]

https://www.youtube.com/watch?v=HrtQVhcTPFc

I have watched literally many dozens of these recently, and my head is spinning , but overall, this one is pretty good. At about 18:30 or so there is a good summary chart of effects of changing timing. I was surprised at how dramatic the effect on my engine was of even small changes!

This one is pretty good too, and shorter, more simplified:

https://www.youtube.com/watch?v=cAlw9RpTZ4I

Pete

[aj81spider]

Thanks Pete,

Got some research and viewing to do. I guess I'm going to need to put some adjustable cams on order too.

[RRoller123]

The thing that points me towards my suggestion is the mention that you changed the head. So the belt came off, obviously. If you don't yet have adjustable cam gears, check the alignment of the setup again. On my car, the proper alignment was always located in between 1 tooth, so it wouldn't align perfectly, and was a compromise. Had to be to one side or the other. You might try just loosening the belt and advancing (rotate CLOCKWISE) the exhaust cam 1 tooth, being careful to keep everything else exactly in place. Tape the belt to the other wheels or clamp or something, but keep it in relative position due to hunting the tooth problems. This is really easy, and only would take a few minutes, with almost no risk of damaging the engine in this small manner. If this make a positive change you are on the right track! If worse, go the other way. Probably will need a new belt afterwards, but that is small potatoes if it leads to a fix of the problem!

Viva Tutto at The Larz! After a 2 year absence, I am back on the road and will be there this year come hell or High Water! Also the Foxboro and Raynham cruises on Thursdays

Pete