RPMs sometimes run abnormally high

[Fords4Us]

Good afternoon all. I just finished up the duraspark conversion on my '83 Ranger with the 2.8L engine, and so far the road tests have gone pretty well. I have good power throughout the RPM band, nice, smooth, even throttle response, improved efficiency over my previous emissions-on-their-way-out TFI ignition system, and no performance hiccups under heavy load. I would have considered myself done and satisfied, except for an issue which came up during load testing.

My first real engine load test was going up a long moderately sloped hill, which my truck completed with ease. I got to the top and drove a few more miles, with occasional up- and down-shifts, to the next batch of hills. I was about to test on a steeper hill, the steepest in the neighborhood, when I noticed that once in awhile my engine RPMs would stay elevated even after I put in the clutch. This had not occurred prior to my hill test even though I did quite a bit of driving, with shifting all the way through the gears, prior to that particular test. The engine temp was rock solid at normal throughout this period of time, and the engine itself never gave any indications of trouble, other than the slight increase in RPMs. It wasn't racing, but it was definitely higher than idle. If I just coasted awhile, it would not come down on its own. If I goosed the gas just a little, it would come back down right away. Then a little while later, it would happen again.

I decided to skip any further testing for the afternoon and just head for home. The only thing I can compare this to, is the accelerated cold idle speed when first starting up and then goosing the engine to knock the carb off the cold idle setting. But this happened well after the engine was at normal operating temps. Does anyone have any suggestions for why this is occurring and how I can correct it? Thanks as always........

 

[Spott]

The first thought that comes to mind is that the throttle is sticking open, perhaps because of a damaged cable, or a bent linkage, or something interfering.

After eliminating the computer, the throttle is the only thing controlling the idle (unless there something I don't understand, which is possible).

 

[Fords4Us]

Thanks, Spott. I have been tweaking the dizzy timing and I switched the vacuum advance from ported to intake (thank you RonD for helping me understand how those two sources would affect advance), and that seemed to solve all my low-RPM power issues. Then this issue came up. I couldn't see how changing the vacuum advance source would affect the RPM but that was the only change I'd made just prior to this latest round of tests.

If anyone else has any ideas on what it could be other than sticky or kinked/snagged throttle cable, let me know. Until then, I'll work with the throttle cable as my prime suspect and see if I can find a hangup somewhere with that.

 

[Big Jim M]

Perhaps adding a spring will correct any small linkage problems. My telehandler was doing similar things. The damn linkage is way up under the dash and has three lengths to it! So I had to look for a way to get the linkage to return to idle. I had to add a screen door spring as there was nothing nearby on the engine to attach the spring to and I needed the length. Has worked for several years.
Big Jim

 

[enjr44]

Hate to ask a dumb question; but..... are you sure this isn't a new fail and has nothing to do with the conversion?

Is your choke working correctly? If the choke's linkage went bad, maybe it is resetting itself for some weird reason.