Full throttle knock, jet help

[cj7]

Hi all, I had this in the wrong section and thought to move it here. I got a quadrajet on a stock 350 vortec in a old jeep with heavy tires, and some armor, so heavier than a stock jeep. its got a fairly low geared transmition with no over drive and 3.54 in the rear. duel plane intake. the problem is it has a knock at full throttle that i believe is form being too lean. I thought it was a timing problem at first, but after re curving and changing timing a million times, someone suggested it might be a carb problem. ive got a 17058213 carb, 36L primary metering rods, 71 jets, and DR secondary metering rods. other then the knock at full throttle, it runs pretty good. it does seem to have a light flat spot in the mid range, then picks up after 3000 rpm. And my APT is adjusted out a lot so it doesn't stumble off idle, like 6 to 9 turns almost maxed out which leads me to think I need different metering rods. the float is set to 15/32. I am completly new to the jetting area, so im not sure if i need to replace the jets or meteing rods, and if it needs primary or secondaries. as much as id love to buy an entire tuning kit with all the jets and metering rod set, i cant afford it on a college student budget. so if you guys could suggest a few sizes to try, or any other advice, that would be awesome. I found a set of 72 jets, but still think I need to hire

[cj7]

mainly im just looking for a good starting point for jets and metering rods for a vortec engine, and how i will know that i have the right jets installed

[cj7]

tried the 72 jets, knock is still there

[cj7]

also, my secondary hanger is an L if that helps

[Cliff Ruggles]

Stock jetting were much smaller, 63 or 64 in those and small metering rods as well.

71 jets would be HUGE and teamed up with 36B rods pretty "fat" at light part throttle and PLENTY of fuel coming down from the secondary side at full throttle.

15/32 is pretty low so reduces fuel volume in the bowl when effects metering some the other direction.

If you are using an over the counter kit it's also going to have a small N/S assembly which lowers the fuel even more at any given float setting, plus you can quickly suck the bowl low or empty on hard runs and lean it out (pinging).

DR metering rods are pretty fat so lean there as well.

Since you have working APT I'd call the shop when you get time and we'll set you up with a more desirable jet/rod combination.

I'd also like to know the static compression ratio, quench distance if you know it, and camshaft specs?

They made different "Vortec" engines over the years of production and there are several versions of heads dubbed "Vortec" as well.  The early versions were "swirl" intake ports and very efficient for low RPM scenarios, but didn't flow well at higher RPM's.

They settled on much better flowing versions in later years, higher compression and improve combustion chamber shape as well.......Cliff

[77cruiser]

How much total timing? Vortec heads like about 32, I think swirl port heads less maybe 28.

[cj7]

its 9.6:1 cam specs are Camshaft: Hydraulic roller Cam Duration @ .050 in.: Intake 191 / Exhaust 194 Cam Lift: Intake 0.414 in. / Exhaust 0.428 in. Lobe Separation Angle: 112 lsa. total timing is 29 degrees. but according to your book and the recipe for success, the jet should be 71-76, so 60s seem low

[Cliff Ruggles]

You are working with a single main airbleed carb, so the "recipe" for success woln't be the same as it would for an earlier Chevy model with two pairs of MAB's.

Those are the first SMAB units and only used in 1977 and 1978 in the divorced choke truck units......Cliff

[cj7]

in one of the notes, it states the later SMAB units bigger jets 74-76. if not, what do you recommend for jets, and primary and secondary metering rods? ive got a few laying around, the rest ill need to order from you

[Cliff Ruggles]

The model you are working on is referenced on page 98 stating that they used smaller jets/metering rods, still used the early casting design, etc.

I don't use or recommend putting used jets in any carburetor one is building or tuning.  They will seldom pass a pin gauge test (worn internally from the metering rods sliding up/down in them).

When you are ready for tuning parts, let us know, and using the APT system in the baseplate you can fine tune on the vehicle without having to take the carb back apart......Cliff