testing for spark plug misfire

[William F.]

Going to set idle speed and idle mixture on my car's AFB to make sure not too rich. How many turns out from all the way in on the mixture screws would be a good starting point? Also, manual states 650 rpm as idle speed for my stock 340 hp 327. Think I can get it to idle reasonably well this low?
Thanks

[Timothy B.]

I would start with one turn out on the emulsion screws and after it warms up fine tune them keeping them both approx the same turns out.

If the engine is using ported vacuum advance you may get the idle down to 650-700 because the engine will be less efficient and require a slightly richer mixture. You just have to experiment with it and see where it runs at a stable idle speed. I assume your engine has a ported V/A.

[William F.]

Concerning setting idle speed and idle mixture in my '62. It's 340 hp and they are initial timing and centrifugal advance, no vacuum advance. Will it idle at 750 rpm as manual suggests?Thankd

[Frank C.]

All, quick question related to this thread. I am trying the Denso 14-U Spark Plugs for the first time. What gap are you using? Thank you Frank

[Leif A.]

All, quick question related to this thread. I am trying the Denso 14-U Spark Plugs for the first time. What gap are you using? Thank you Frank

.035 should work just fine...and, you'll like your plug choice, as well.

[Duke W.]

All, quick question related to this thread. I am trying the Denso 14-U Spark Plugs for the first time. What gap are you using? Thank you Frank

I hope you mean Denso W14-U. It's equivalent to the long out of production non-resistor AC 45, and it's my first choice for any road driven iron head small block that requires a gasket seat type spark plug. (My second choice is NGK B4, also equivalent to AC 45.)

W14-U includes Denso's patented "U-groove" ground electrode that offers more sharp edges that sparks prefer to propagate to compared to a plain ground electrode spark plug.

Any auto parts store that sells Denso plugs should be able to order the W14-U. The "stock number" is 6008, which is a box of ten. Depending on your source you may be able to buy just 8 or you might have to buy the entire box.

Duke

[Duke W.]

Going to set idle speed and idle mixture on my car's AFB to make sure not too rich. How many turns out from all the way in on the mixture screws would be a good starting point? Also, manual states 650 rpm as idle speed for my stock 340 hp 327. Think I can get it to idle reasonably well this low?
Thanks

My recommended idle speed is "as low as possible commensurate with acceptable idle quality", so it depends on the owner's preference. IMO the recommended idle speeds for mechanical lifter engines in the C1-C2 era is a joke. If you can get a supposed Duntov cam engine to idle anywhere near acceptably at 650, it probably doesn't have a real Duntov cam.

The '63 Corvette Shop manual calls for an initial mixture screw setting of 1.5 turns out from the seat for the 3461S AFB used on 300 HP MT and 340 HP engines. I don't know if ST-12 has an initial recommended setting for the '62 3269S used on the '62 300 HP MT and 340 HP engines, but if you can't find an OE recommendation start out at 1.5 turns out from the seat and go from there.

The OE '62 340 HP max centrifugal from the OE non-vacuum advance dual point distributor is 24 crank degrees at a lazy 4600, so you need 12-16 initial to achieve the optimal 36-40 range for maximum power. I recommend you install lighter springs to bring it in earlier, as low as 2500, then use a dial back timing light to set total advance, a few hundred revs above the point of max centrifugal, as high in the 36-40 range as the engine will tolerate without detonation because the timing tab is inaccurate. It's two to three degrees low above 10, and if the centrifugal starts below typical idle speed (which it does on some mechanical lifter engines) you can't get the engine to idle low enough, long enough to measure and adjust the initial timing before the engine stalls.

If detonation is an issue it will usually be at low to medium revs so install stiffer springs first before you reduce total advance below 36.

Once you have the spark advance map dialed in optimize the carb mixture at the lowest idle speed you find acceptable. I tend to like a higher idle speed, like 900 with a high overlap cam like the Duntov, because the higher the idle speed the smoother the idle and it's easier to get the car moving from a dead stop with a higher idle speed.

I can't give you or anyone else a perfect setup. There are too many variables like available octane, actual (real, measured) compression ratio, and ambient air density range. If your engine is "stock" or nearly so I can offer decent starting points, then it's up to the owner or mechanic to optimize through experimentation.

Duke

[William F.]

One more thing about oil capacities:
My 62 owner's manual does say 6 quarts with filter change. I don't see mention of oil capacities in the '53-'62 Corvette Service Guide. My '66 and '67 chassis service manuals say 5 qts with filter change for 327's. Was there a change in capacities from '62 to '66-'67 although both engines are 327's??