rpms hang when off throttle...

tmbg · Post by **tmbg** » Wed Jun 14, 2006 8:19 pm

I've been screwing around with the timing a lot lately, trying to get it to run right...

Now, for whatever reason, when I take my foot off the gas, the engine tends to stay near the same RPM and drop very slowly, not quickly and smoothly like I'd expect. Is this timing related? It seems like it started doing that after I adjusted the trigger angle down 10 degrees to more closely match the actual timing.

Post by **grippo** » Thu Jun 15, 2006 7:24 am

If you adjust the trigger angle without physically moving the sensor then you have changed your timing by the same amount. That is why you should get the right trigger angle before tuning the spark tables. I would try changing the spark table and you should be able to get back to the same performance as before you changed the offset, only now you should be able to read the correct timing on your timing light.

tmbg · Post by **tmbg** » Thu Jun 15, 2006 8:12 am

I understand that moving the trigger angle moves the table, problem is that I don't really fully understand where my ignition SHOULD be.

Does the hanging rpm indicate too far advanced or too far retarded?

I've been battling this horrible problem where the car jerks and bucks between 3000rpm and 4000rpm, even though my AFR is under 13 the whole time. Changing timing in either direction doesn't seem to help, and I'm starting to wonder if it might be something stupid like a bad connection to the power transistor.

Post by **grippo** » Fri Jun 16, 2006 9:29 am

The rpm staying high sounds more like a stuck throttle. I only mentioned the timing because you said the rpm sticking started when you changed iginition trigger. Does it get better if you put the trigger back where it was ? Secondly, you need to do a short data log when this happens and see if the rpms and pulsewidths and timing look reasonable. IUf your timing is all over the place you may have a bad tach input where the noise comes in when you hit 3000 rpm. You will see this on the data log.

tmbg · Post by **tmbg** » Fri Jun 16, 2006 10:05 am

Actually, my datalogs show that the rpm stays pretty smooth, even where I'd expect it to be jumpy and weird. In that 3000-4000rpm range where it was bucking, the dash tach would bounce a good bit, but the datalogs don't show it at all. The only thing that's weird in my datalogs is my CLT has a lot of spikes. Sometimes the PW and AFR go up and down a couple points cyclically, that may be causing a problem, but I don't know if it's related to my tach signal. It's weird to me that the rpm datalog is so smooth and clean, when it feels like it shouldn't be.