Hi all,
This is a new to me 1977 "high altitude" model with original carb and intact emissions system that came to me from Colorado. The original 2F was pulled and rebuilt in 1996, 17K ago. Odo currently shows what I and the PO believe to be 132K. I haven't done a compression test yet, but the motor feels strong and nothing looks suspicious with the exhaust. I'm in the process of baselining, but the issue currently stumping me is the truck sounds like it has a vacuum leak but doesn't really behave like it. Once the motor is warm, I get an intermittent whistling hiss that isn't particularly noticeable in around town driving, but increases in volume and frequency at highway speeds. it can be modulated with the throttle (more throttle = less/no noise, no throttle = more noise/higher frequency) and is sometimes not present at all. The PO was aware of the sound, and of the opinion that it was a vacuum leak. Definitely eminates from the carb/manifold/brake booster area.
Some relative points/observations:
* No adjustments to the carburetor yet (not my comfort area..)
* I replaced the plugs/wires/filter/rotor/cap with OEM. plugs 3 and 4 were sootier than the rest. Checked the new ones a few days after swapping in, again 3 and 4 were beginning to darken. My expectation (I hope..) is this is valve seal-related.
*Replaced all the emissions vacuum tubing. No changes.
*Missing the gasket between the air cleaner and carburetor. OEM replacement on the way.
* Idles just fine once warm with no choke, although it feels a bit low sometimes. Ordered a tach but not yet installed, so exact RPM's are unknown for now.
* The motor has cut out a few times coming off an exit ramp but starts right back up again. This is the only time it does this.
* Fuel/exhaust smell can get pretty heavy in the cabin. I attribute at least some of this to driving a 43 year old carbureted tractor in stop and go city traffic.
* I ran a vacuum gauge into the cabin (T'd off the gas filter on the front of the intake manifold). Assuming the connection is tight, the readings have been low at idle:
Engine cold, choke out: 17-18mmHg steady
Engine warm, choke in 13-14mmHg steady, occasional fluctuation +/- 0.5-1mmHg
Engine warm, stop and go driving/coming off highway 10-12mmHg, occasional fluctuation +/-0.5-1mmHg
Worth mentioning that idle vacuum specs for the 77' high altitude model are listed as "greater than 12.6mmHg", although I'm assuming the lower spec is assumes a much higher altitude. NYC is definitely at sea level!
Not a whole lot of change in idle character at the different vacuum readings; even at 10mmHg the motor doesn't feel like its about to cut out. I also haven't noticed much of a correlation between vac readings and the whistling hiss intensity/frequency.
Searching the forum suggests a leak between the carburetor and manifold, but the relative solid idle and intermittent character of the noise gives me pause. Any experience/input would be greatly appreciated. Sorry for the ramble/rant!
Luke
Forgot to mention.. its Beige!
This is a new to me 1977 "high altitude" model with original carb and intact emissions system that came to me from Colorado. The original 2F was pulled and rebuilt in 1996, 17K ago. Odo currently shows what I and the PO believe to be 132K. I haven't done a compression test yet, but the motor feels strong and nothing looks suspicious with the exhaust. I'm in the process of baselining, but the issue currently stumping me is the truck sounds like it has a vacuum leak but doesn't really behave like it. Once the motor is warm, I get an intermittent whistling hiss that isn't particularly noticeable in around town driving, but increases in volume and frequency at highway speeds. it can be modulated with the throttle (more throttle = less/no noise, no throttle = more noise/higher frequency) and is sometimes not present at all. The PO was aware of the sound, and of the opinion that it was a vacuum leak. Definitely eminates from the carb/manifold/brake booster area.
Some relative points/observations:
* No adjustments to the carburetor yet (not my comfort area..)
* I replaced the plugs/wires/filter/rotor/cap with OEM. plugs 3 and 4 were sootier than the rest. Checked the new ones a few days after swapping in, again 3 and 4 were beginning to darken. My expectation (I hope..) is this is valve seal-related.
*Replaced all the emissions vacuum tubing. No changes.
*Missing the gasket between the air cleaner and carburetor. OEM replacement on the way.
* Idles just fine once warm with no choke, although it feels a bit low sometimes. Ordered a tach but not yet installed, so exact RPM's are unknown for now.
* The motor has cut out a few times coming off an exit ramp but starts right back up again. This is the only time it does this.
* Fuel/exhaust smell can get pretty heavy in the cabin. I attribute at least some of this to driving a 43 year old carbureted tractor in stop and go city traffic.
* I ran a vacuum gauge into the cabin (T'd off the gas filter on the front of the intake manifold). Assuming the connection is tight, the readings have been low at idle:
Engine cold, choke out: 17-18mmHg steady
Engine warm, choke in 13-14mmHg steady, occasional fluctuation +/- 0.5-1mmHg
Engine warm, stop and go driving/coming off highway 10-12mmHg, occasional fluctuation +/-0.5-1mmHg
Worth mentioning that idle vacuum specs for the 77' high altitude model are listed as "greater than 12.6mmHg", although I'm assuming the lower spec is assumes a much higher altitude. NYC is definitely at sea level!
Not a whole lot of change in idle character at the different vacuum readings; even at 10mmHg the motor doesn't feel like its about to cut out. I also haven't noticed much of a correlation between vac readings and the whistling hiss intensity/frequency.
Searching the forum suggests a leak between the carburetor and manifold, but the relative solid idle and intermittent character of the noise gives me pause. Any experience/input would be greatly appreciated. Sorry for the ramble/rant!
Luke
Forgot to mention.. its Beige!
