dibbons
Well-Known Member
After several initial break-in periods (about an hour run time altogether) on the rebuilt 340 I recently installed in our '72 Satellite Sebring Plus, I installed an air/fuel ratio sensor/gauge in the passenger side head pipe after having also rebuilt the Thermo-quad with less than a satisfactory outcome (photo #3).
Before the gauge installation:
a) I found a carb vacuum leak I believe has been remedied by replacing the '71 style carb base gaskets (aluminum plate sandwiched b/t two paper gaskets) by using the later thick one-piece insulator gasket. Not sure yet if I don't still have a smaller vacuum leak somewhere else.
b) During the previous mentioned static/parked break-in period, I used the '73 style divorced choke with electric wire disconnected. It took about 8 minutes at fast idle for the choke to completely open. The engine fired up almost instantly with the choke set. The first drive on the highway was a ten minute round trip to the nearest gas station. On the return home from the station, it seemed to be missing more so than before.
c) Curious about what was causing the miss, I pulled the newly installed plugs (Champion RN9YC) and found all of them dry but black (photo #1). I performed a compression test and found six cylinders 130 psi, one 150 psi, and one 105 psi. When i squirted motor oil in the weak cylinder (105 psi) and re-tested, it measured 130 psi. Being the fact seven cylinders ending up reading exactly the same, I began to doubt the accuracy of the compression gauge and ordered an OTC compression gauge on-line (has not arrived yet).
d) I installed a set of new spark plugs and disconnected the choke in order to not take the chance of fouling the spark plugs again. I decided to adjust the metering rod tower a little by running it down (counter-clockwise) until movement stopped, and then turning the tiny screw back clockwise one full turn.
e) Now I primed the carb with a little extra fuel and it started and died. It started and died another five to ten times while occasionally backfiring through the carburetor. I guess it really missed the choke being armed. Finally got it to run but almost died a few times before I could establish a faster idle with my foot on the accelerator. When the motor warmed up, it ran OK, but did not sound very strong or stable and it tended to die when placed into either reverse or drive, even when idling 800 - 1000 rpm.
f) I checked the timing which was now exactly TDC. I had previously set the idle timing at 10 BTDC but had not tightened the distributor hold down, so I assume it moved from the previous setting on its own. I reset the timing to 10 BTDC which definitely made it idle smoother (but the idle still wandered some). I tightened the dist. hold down.
g) I removed the vacuum advance hose from the front port of the Therm-quad and found the timing did not change from 10 BTDC. I thought the vacuum advance module was defective, but later I found here on FABO (thanks to a few members ) that the factory vacuum advance tube was ported which I did not take into consideration. Presently then, I still need to do some further testing to find if the vacuum advance will function using another vacuum source. Oh, the total timing appears to be about 35 BTDC.
g) With the new plugs, and the engine warm, I drove back to the nearest gas station and added another 5 gallons of gasoline. Then I drove about 10 miles into downtown (about a 20 minute drive in traffic) to a muffler shop to have them install the bung for the air/fuel ratio gauge. Motor did not stall on the way.
h) The bung was installed in the driver's side head pipe (about 18 inches downstream). However, some welding slag messed up the threads and the sensor was unable to be fit. A second bung was installed in the passenger side header pipe and this time the sensor fit. I plugged the sensor gauge positive wire into an unfused slot in the very bottom of the fuse box labeled "acc". I had already put a 10 amp glass fuse in-line of the positive wire per the recommendations of the gauge instructions.
After gauge installation:
a) On the way home from the muffler shop, I referred to the newly installed gauge frequently like a kid with a new toy. The reading at idle was in the 12's and at cruise was in the 15's. I kept coasting once in a while to bring the reading down from the 15's so I would not chance burning a hole in a piston or whatever. The gauge almost never hovered in the 13's which is where I wanted it ride. Before shutting down the motor at home, I screwed the idle adjustment screw clockwise to raise the idle readings from 12.5's to 14.0's.
b) The following morning I pulled one spark plug (number two cylinder) and found the porcelain to still be white and not all black like before (photo #2). This was only after about 20 miles of actual driving at various speeds, so I know it's difficult to determine anything so early by simply performing a spark plug inspection (maybe premature at this point).
My plan now is to re-connect the choke, warm up the motor on fast idle. And finally, if need be, to adjust the fact idle speed to the factory 1,900 rpm specification. Not sure where to set the curb idle. I would like it as low as possible without having a chance of stalling. Factory setting is around 750 rpm i believe. And I will probably connect the vacuum advance hose to the rear port on the Thermo-quad where the factory connection was to the air cleaner stove pipe so I will have non-ported vacuum advance in the distributor. Then the hard part, I need to find a way to enrich the mixture during cruise and acceleration. Funny, I can find no reference online (neither forum or video) where someone tunes a Thermo-quad that is running too lean and describes all the trials and tribulations of finally getting it right.
Before the gauge installation:
a) I found a carb vacuum leak I believe has been remedied by replacing the '71 style carb base gaskets (aluminum plate sandwiched b/t two paper gaskets) by using the later thick one-piece insulator gasket. Not sure yet if I don't still have a smaller vacuum leak somewhere else.
b) During the previous mentioned static/parked break-in period, I used the '73 style divorced choke with electric wire disconnected. It took about 8 minutes at fast idle for the choke to completely open. The engine fired up almost instantly with the choke set. The first drive on the highway was a ten minute round trip to the nearest gas station. On the return home from the station, it seemed to be missing more so than before.
c) Curious about what was causing the miss, I pulled the newly installed plugs (Champion RN9YC) and found all of them dry but black (photo #1). I performed a compression test and found six cylinders 130 psi, one 150 psi, and one 105 psi. When i squirted motor oil in the weak cylinder (105 psi) and re-tested, it measured 130 psi. Being the fact seven cylinders ending up reading exactly the same, I began to doubt the accuracy of the compression gauge and ordered an OTC compression gauge on-line (has not arrived yet).
d) I installed a set of new spark plugs and disconnected the choke in order to not take the chance of fouling the spark plugs again. I decided to adjust the metering rod tower a little by running it down (counter-clockwise) until movement stopped, and then turning the tiny screw back clockwise one full turn.
e) Now I primed the carb with a little extra fuel and it started and died. It started and died another five to ten times while occasionally backfiring through the carburetor. I guess it really missed the choke being armed. Finally got it to run but almost died a few times before I could establish a faster idle with my foot on the accelerator. When the motor warmed up, it ran OK, but did not sound very strong or stable and it tended to die when placed into either reverse or drive, even when idling 800 - 1000 rpm.
f) I checked the timing which was now exactly TDC. I had previously set the idle timing at 10 BTDC but had not tightened the distributor hold down, so I assume it moved from the previous setting on its own. I reset the timing to 10 BTDC which definitely made it idle smoother (but the idle still wandered some). I tightened the dist. hold down.
g) I removed the vacuum advance hose from the front port of the Therm-quad and found the timing did not change from 10 BTDC. I thought the vacuum advance module was defective, but later I found here on FABO (thanks to a few members ) that the factory vacuum advance tube was ported which I did not take into consideration. Presently then, I still need to do some further testing to find if the vacuum advance will function using another vacuum source. Oh, the total timing appears to be about 35 BTDC.
g) With the new plugs, and the engine warm, I drove back to the nearest gas station and added another 5 gallons of gasoline. Then I drove about 10 miles into downtown (about a 20 minute drive in traffic) to a muffler shop to have them install the bung for the air/fuel ratio gauge. Motor did not stall on the way.
h) The bung was installed in the driver's side head pipe (about 18 inches downstream). However, some welding slag messed up the threads and the sensor was unable to be fit. A second bung was installed in the passenger side header pipe and this time the sensor fit. I plugged the sensor gauge positive wire into an unfused slot in the very bottom of the fuse box labeled "acc". I had already put a 10 amp glass fuse in-line of the positive wire per the recommendations of the gauge instructions.
After gauge installation:
a) On the way home from the muffler shop, I referred to the newly installed gauge frequently like a kid with a new toy. The reading at idle was in the 12's and at cruise was in the 15's. I kept coasting once in a while to bring the reading down from the 15's so I would not chance burning a hole in a piston or whatever. The gauge almost never hovered in the 13's which is where I wanted it ride. Before shutting down the motor at home, I screwed the idle adjustment screw clockwise to raise the idle readings from 12.5's to 14.0's.
b) The following morning I pulled one spark plug (number two cylinder) and found the porcelain to still be white and not all black like before (photo #2). This was only after about 20 miles of actual driving at various speeds, so I know it's difficult to determine anything so early by simply performing a spark plug inspection (maybe premature at this point).
My plan now is to re-connect the choke, warm up the motor on fast idle. And finally, if need be, to adjust the fact idle speed to the factory 1,900 rpm specification. Not sure where to set the curb idle. I would like it as low as possible without having a chance of stalling. Factory setting is around 750 rpm i believe. And I will probably connect the vacuum advance hose to the rear port on the Thermo-quad where the factory connection was to the air cleaner stove pipe so I will have non-ported vacuum advance in the distributor. Then the hard part, I need to find a way to enrich the mixture during cruise and acceleration. Funny, I can find no reference online (neither forum or video) where someone tunes a Thermo-quad that is running too lean and describes all the trials and tribulations of finally getting it right.
