| Carb settings apt adjust [message #297520] |
Thu, 17 March 2016 09:02  |
Tilerpep
Messages: 404
Registered: June 2013
Location: Raleigh, NC
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Does the picture on page 32 of the following q-jet write up match expert opinion on what should be happening with power valve height in primary venturis on an "RV cam" otherwise stock 455?
http://www.corvette-restoration.com/resources/technical_papers/Q-Jet_Carb_Tuning.pdf
My rods are no where near that deep. Movement marks from the jet on the rod are showing just barely gets near the taper fat part, much less sits within it.
Backstory if you want to keep reading:
Many attempts (2 years) at getting my gas mileage up from 6 and 7 mpg and occasional but persistent overheating. Those two symptoms, plus nozzle drip and just barely a hint of surge/unevenness at idle. I bought a second carb (correct number but center section warped) to compare and learn from, and did a bunch of reading here and general web.
Several of our experts over time have recommended that I check the power valve movement, and it always moved, so we moved on. Now, I am thinking it is not seating far enough down. Both carbs I have are the pre-1975 style that have the APT adjust screw press fit covered. I pulled one press fit cover off the extra carb and voila, another q-jet adjustment.
I read that you can't mix rods from 1973 carb and 1975 carbs, as they used different lengths. I read this on a non GMC site, and not sure if our carbs were consistent over their production run. This got me thinking about rod height and seating more than simply stuck or not stuck. Next thought was that even if the non-oem rebuild kit gasket I have been using on these multiple rebuilds and cleanings are even a bit too thin, that is the same as adjusting the APT up (because the stop is on the bottom plate and the power piston comes down from the middle section, gasket thickness and density would affect this), which causes rich at all but heavy loads (when power piston would rise fully as expected).
My inexact method of verifying has been to mount carb bottom plate to center section with gasket, then observe jets in position while manually pushing the power valve all the way down.
And the full confession paragraph is that I hypothesized this before I learned of the press fit covered adjustment at the front of the bottom plate (next to pcv vacuum port). Many web sites said "early carbs" had no adjustment and I thought that was me. Apparently some 1960s carbs had no adjustment, early seventies had this factory sealed adjustment, and post 1975 had one geared up to the top of the carb for access without disassembly. So, on my good carb I literal persuaded the apt adjuster down with some hammer and screwdriver intricacies. ahem. put it all back together and first time ever absolutely no nozzle drip on passenger side, idle mixture screws behaving as expected and can kill motor. So it seems I am on right track and need to adjust with proper APT turn screw. Thus, the question of where the rods should sit.
Will I need to re-seal that adjuster hole? It does not seem that much vacuum could get by the adjuster screw.
TIA
1975 Glenbrook, 1978 Royale rear bath
Raleigh, NC
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