Author Topic: Delco Carburetor Adjustment Procedures  (Read 115 times)

Offline silverminer

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Delco Carburetor Adjustment Procedures
« on: February 01, 2020, 03:16:58 PM »
How critical are the official "Delco Carburetor Adjustment Procedures"? I'm talking about the pictographic steps included in the M4M series instructions that detail proper choke angles, fast idle, and many other measurements. Since the mid 80's when I first attempted Qjet rebuilds I always payed little attention to these steps because I lacked the unique GM tools specified in the instructions. I was rebuilding carbs original to the vehicle and simply hoped everything assembled back within spec. Now I'm messing with carbs of questionable history acquired from either junkyards, craigslist or random piles at swap meets.

I've recently acquired a mint J9789-C box of the dealership Qjet specific tools in fantastic shape, and now with the benefit of Cliff's knowledge and kits I'm not just a NAPA kit level clean and reassemble carb rebuilder anymore. It seems clear to me that the adjustments are every bit as important to the overall recipe as the items detailed in Cliff's book, or they wouldn't be so different depending upon CARB ID. There isn't much discussion of the adjustment steps either in Cliff's book or here on the forum, unless I'm not using the correct search terms.

Anybody got useful tips on how to best use my new tools and how to source the correct adjustment info? Specs for carbs from the 1708XXXX series on seem hard to source.

Offline Kenth

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Re: Delco Carburetor Adjustment Procedures
« Reply #1 on: February 02, 2020, 03:24:19 AM »
On close to factory original rebuilds i relay on Delco Parts & Service Manual 9.
They show up on ebay occasionally.

And, most carbkits includes instruction for many years of the carb style.

Offline Cliff Ruggles

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Re: Delco Carburetor Adjustment Procedures
« Reply #2 on: February 02, 2020, 06:32:56 AM »
All adjustment, choke pull-off unload angle, fast idle speed, idle speed, idle mixture, etc, etc, require custom settings beyond what information will be supplied in any written instructions.  The factory settings were there to get you close right off the bench, but further fine tuning will be required if you want perfect setting and flawless operation in all areas.

This fact is further compounded by many things, including removing factory emission control parts, engine modifications, differences in fuel blends, etc.  It's also pretty rare these days to see anything that is completely "stock".  Many of these vehicles are a half century old or close to it, and many changes will have been made to the engine and associated parts over the years.

When adjusting the choke and related parts for instant cold starts w/o stalling out, it may take several attempts from dead cold to warmed up to find the perfect settings.  One has to work pretty fast as the engine and intake heat up pretty quickly unless someone has blocked off the heat crossovers (bad idea for most set-ups).

Also most older carbs require bending of linkages in lieu of just turning the choke dial or adjusting the angle of the choke flap with the screw on the pull-off.

Also interesting that as "dated" as divorced choke set-ups are that were used on early models they work flawlessly once one finds the most ideal settings.

For the most part I use and prefer later carburetors converted to electric choke on my own engines, but have owned a number of earlier vehicles with electric choke set-ups and they never gave any issues and worked fine once I found the most ideal settings for them......Cliff

Offline silverminer

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Re: Delco Carburetor Adjustment Procedures
« Reply #3 on: February 03, 2020, 07:34:38 AM »
On close to factory original rebuilds i relay on Delco Parts & Service Manual 9.
They show up on ebay occasionally.

And, most carbkits includes instruction for many years of the carb style.

Kenth - thanks for the heads up. I went back to the local gentleman who kindly sold me the J9789-C tool box and darned if he didn't also have a giant thick 3-ring dealer binder of that exact service manual. Now I'm armed with so much info I'm vapor locked. I was initially intimidated imagining over 5" of binder on just Qjets, but it's so much more. Data for Varajets, Corvette TBI, the whole Delco fuel mixing shebang.

Another benefit I'm finding from using 1708XXXX series carbs is the simplicity of the adjustments required versus the multiple linkage bending on the older models. Making adjustments on the engine via screwdriver and allen wrench beats cranking around with pliers. Now we'll see if I can experiment my way to knowing which direction to twiddle to achieve perfection.