I've been dreaming of Carbs (1 Viewer)

Joined
Aug 10, 2018
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Bozeman, MT
Well. No real forward progress yet for me.

I attached a vacuum gauge on the manifold side of the carb. Vacuum bounces all around at idle (575 rpm). But smoothes out quickly when the rpm is increased. I'm hoping that the bouncing vacuum at idle is due to the Comp cam shaft I have. I'm gonna try to dig up the receipt to figure out what cam I put in there and research a bit. Hope it's not an indicator of valve problems.

Should I be reading baseline manifold vacuum at a higher rpm, say 800 or 900?

Later this week, I will be checking to see if the vacuum advance can is working (once I get another vacuum line adapter to hook up the gauge). And will pick up a compression tester to rule that out. O2 sensor should drop in this weekend.

If we do a club event at dyno night, or get our own dyno night, every one that participates should chip in an extra 5 or 10 bucks to go to the dude (dudette) with the lowest horsepower run. :cool:
It was my understanding that "idle" in the context of a vacuum test is 800-1200 rpm, but that's based on one repair manual for a different engine manufacturer and it was years ago, so I may be completely wrong.
 
Joined
Sep 11, 2017
Messages
305
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Bozeman, MT
So I finally found the receipt from my Comp Cam. Cam Lobe separation angle is 110, which explains the erratic vacuum at idle. As I understand, lobe angles of +/- 114 and higher will have a steady vacuum at low rpm. Lobe angles of +/-112 and lower will have poor vacuum as the intake/exhaust have a bit of overlap. This particular cam should give me a bit more torque in the mid rpm range, trading off some horsepower at high rpm. That's a good fit for me since I'm not driving in the high rpm range. And my setup doesn't have any vacuum needs for engine componets at idle. Maybe electronic ignition needs the vacuum at idle, idk, but mine is still mechanical points.

Installed (mostly) the O2 sensor and permanent vacuum gauges last night. Waiting for a chrome guage panel to show up from Napa this week to mount them into. Should finally get some data points this weekend and start adjusting stuff. It has been really hard not to fiddle with things before I can start it up and take some baseline numbers first.
 
Joined
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Messages
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Bozeman, MT
Is the plan to leave these diagnostic gauges as a permanent install? I could see that leading to lots of fiddling. You may inadvertently become a carburetor wizard.
 
Joined
Sep 11, 2017
Messages
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Bozeman, MT
Yeah, the gauges will be permanent. If I become a wizard after this, that's ok. But equal possibility of me going crazy if it doesn't work.
 
Joined
Aug 10, 2018
Messages
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Location
Bozeman, MT
The only thing that sets apart wizards from nutcases is that the wizard has proof of success. (Or at least they are enough of a salesperson that they can convince somebody else that they have been successful.)
 
Joined
Sep 11, 2017
Messages
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Bozeman, MT
Thought about doing a project update for a while. Well, I'm still not finished. But making progress.

I installed the o2 sensor, new tach, and new vacuum gauge. All turned out sweet. My idle was too rich. Also adjusted the cold idle speed down on the electric choke carb. Improvements.

While I was installing gauges, Mrs 90WT offered to recover and refoam my seat. So after some order and install time, seat is complete. Replaced some badly worn seatbelts. Then I find a leaky master brake cylinder. Brakes are super spongy. So pushing to do some brake work this weekend. Then, just maybe, I'll install the fuel pressure gauge and see if we've got a good steady pressure off the mechanical fuel pump. Still looking to play with the total timing and see what adjustments i can make before turning my attention to the secondary jets/needles. I can smell the burnouts already...
 

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