Dwell stuck below 30 deg.

[Clone TIE Pilot]

Dwell is stuck below 30 deg in my car, it varies between 5 to 20 deg which is a rich command. I have tried adjusting the IAB valve but it doesn't change the MCS dwell. The dwell goes a little richer with the air cleaner lid on, a little leaner with it off. Carb is rebuilt, the O2 and temp sensors are good, and no trouble codes. The MCS clicks in KOEO like it should. Guess I am looking at a vacum leak somewhere?

 

[DoubleV]

Are the rich and lean stops set to proper specs? How does the car actually run and drive?

 

[rustyroger]

I don't understand :? , how does the dwell vary with carb adjustment?.

I understood dwell angle is set by points adjustment or with electronic ignition a fixed function of the primary circuit.

I'm always pleased to learn more, can anyone explain how carb adjustments will affect dwell please?.

Roger.

 

[AlanB]

Rusty,

TIE Pilot is referring to mixture control (M/C) dwell, which measures the amount of time the MC solenoid is energized. 30 is about right.

TIE Pilot,

A few thoughts. As Double V mentioned, it's pretty important that the rich lean stops, idle air bleed, and idle mixture screws are properly adjusted. The procedures vary depending on the type of carb. There are 2, 3, and 4 point adjustment carbs and the procedures are all a bit different. Your sig says you have a 305. Is it a 305 (Chevy) or a 307 (Olds)?

Has the O2 sensor been recently replaced? Older sensors can go out of range by reading leaner than they should, which causes low mc dwell. This basically make the car run rich even though it appears to be a lean condition according to mc dwell. One way to test the sensor is to do an enrichment test with propane or maybe carb cleaner and pin the O2 rich. It should be at least 900 mv at full rich.

 

[Clone TIE Pilot]

It is a L69 HO 305 and the O2 sensor is new, it is a Denso thin type O2 sensor that is supposed to be better than the beer can style O2 sensors. As far as I know the the rich lean stops should be good. I had the carb rebuilt by Sean Murphy Induction out in CA, supposed to be one of the best Qjet rebuilders and equal to Cliffs. However I will check the M/C solenoid travel. I have one of those old adjusting kits for CCC Qjets that comes with the special gauges, such as the J hook for setting the IAB valve, the rich/lean stop, and M/C travel gauges.

 

[AlanB]

TIE Pilot,

Sounds like you've got a pretty good handle on this already. If I were you I would not touch the rich/lean stops just yet. Since you used a reputable rebuilder they likely set it right.

I believe you have a four point adjustment carb. If so, then there is a procedure to make the final settings while running at 3000 RPM. I don't have the procedure handy, but the Doug Roe book or a factory manual will provide the details. If you have no access to either I will see what I can come up with.

I would make this final adjustment before messing with the stops.

 

[Pbngem]

I had a similar problem once like you describe & after checking EVERYTHING at least 10 times, I found that the dwell meter I was using was not set at the correct scale. It was set for 6 cyl...... After setting it to the correct scale,the dwell readings were in range

 

[Clone TIE Pilot]

I had a similar problem once like you describe & after checking EVERYTHING at least 10 times, I found that the dwell meter I was using was not set at the correct scale. It was set for 6 cyl...... After setting it to the correct scale,the dwell readings were in range

Mine is one of those Actron dwell meters which has both 6 and 8 cylinders scales on the gauge face. They do not have a switch to change cylinder numbers like the older meters.

Also I think I hear a slight miss in the exhaust sound, maybe lean misfire?

 

[Clone TIE Pilot]

Well I found one leak, the vacuum motor for the damper in the Thermac air cleaner. I tested it with a vacuum pump and it leaks vacuum and slowly reopens. I compared the current V8's air cleaner to the V6 air cleaner from my Regal's old V6. It's vacuum motor holds vacuum and does not reopen by itself, so I am just going to swap vacuum motors and see how she runs.