carburetor jets

[sg1747]

What is the recommended jet size on the primary and secondary for a 4053?

[firez]

I run 68s and 72s in my 68 Z28. I think a lot of guys step them up to 72s and 76s but I find them to rich.I find each car is a little different in what they like. http://www.yenko.net/ubbthreads/images/graemlins/3gears.gif

[70 Forest Green Zee]

I think Eric Jackson would be the guy to ask.

[Mr.Nickey Nova]

They came with 68's and 76's from the factory,but some change out the 68's for 72's which is what i did.

[L78steve]

The 68s I find to lean .The 70s will cover the stumble but the 72s work the best .

[sg1747]

then what do you use in the rear.

[VintageMusclecar]

72 primary 76 secondary should work just fine. If the engine is stock (or close to it), stay with the 6.5 PV in the front and an 8.5 in the back.

Eric

[Chevy454]

Funny you mention the pv, Eric...I was just fixing to ask if pv selection was more critical on a 302 than a comparable bbc?

[Keith Tedford]

Stock, other than headers, I always ran #73 primaries and #76 secondaries with the stock power valves in our 4346 carb on our L78 and L72 engines. Our 427 came from the factory with #68 primary jets and there was definite lean surge and 60 mph. They worked fine with this setup. If nothing else, it is a good starting point for fine tuning. The engines ran/run clean with long plug life. Sort of duplicating what Eric said, but the combination does work.

[VintageMusclecar]

Rob;

Good question. The OE PV specs on the 4053 (and 4346) carbs flies in the face of "conventional wisdom" that states you should run a PV that's rated at 1/2 of what engine idle vacuum is. Since most 302's idle around 9-11", that would call for a 4.5 to 5.5 PV. However...stick a 4.5 PPV in a 4053 on a basically stock 302 and watch a mysterious off-idle hesitation appear...been down that road previously with numerous customers who tried it themselves. http://www.yenko.net/ubbthreads/images/graemlins/wink.gif

For consideration: The only differences between a 4053 and a 4346 carb are the vented pri. bowl and an 8.5 PPV on the 4346 instead of the non-vented front bowl and 6.5 PPV used on the 4053.

I would be interested to know what your L72 idle vacuum is, and what PV you're running up front (blocked rear, correct?).

Keith;

I've found that on a stock 302 with manifolds, a 70 primary jet seems to be enough to eliminate the lean cruise issue, and 72's are well-documented for best performance. Any time I build a carb for a 302/L72/L78 with headers, it automatically gets 72's in the front.

[Mr.Nickey Nova]

Eric,
Was told by someone who has been into racing a long time to even out the power valves on the 302 to 65 front and 65 in the rear.I have had no problems at all with this setup,what is your opinion on this?? Mark

[VintageMusclecar]

Mark;

I've always stuck with 6.5/8.5 with complete success, so I couldn't really say.

Eric

[Keith Tedford]

Eric, I ran the stock factory spec. 4346 carb except for the jetting. If memory serves me correctly, the PVs were both 6.5 but don't quote me on that. I don't know what the vacuum idle was. I couldn't have asked for the carb to work any better. I have a 4346 done the same way on our L78 engine and it works the same way. Having carbs that were already built for the application probably helped a lot as far as tuning went. A friend of mine does some racing and he gets into modifying the restrictors in the body of the carb, stuff that I know nothing about. When he is done, it is surprising at how well a carb can run, and how far off most carbs are when he first starts on them.

[Keith Tedford]

I just checked my old GM parts books. For '69 they list the 3877167 power valve for the rear in all applications with various PVs for the front. For 1970 they list the same 3877167 PV front and rear for all applications. Not that I trust everything in the parts books, but that is what they show.

[SSRSBOB]

For ultimate performance stagger jetting is the way to go. Old racing trick for stock runner intakes.

Bob

[Chevy454]

[ QUOTE ]
Rob;

I would be interested to know what your L72 idle vacuum is, and what PV you're running up front (blocked rear, correct?).


[/ QUOTE ]
Running a 65 up front with a plug in the rear...my notebook says my usual setup for Stanton is:
<font class="small">Code:</font><hr /><pre> Front
/ \
|
|
71 | 70
-------
85 | 82
</pre><hr />
I bumped it up +2 for the thicker in Virginia last fall, but the crazy high humidity killed it...as for vacuum, for some reason I can't find where I'd written it down, but I'm 99% positive it was over 12 @ idle...the only reason I remember it is, it was nearly identical to Stefina's R-code 427 [both in engine specs and vacuum readings].

[VintageMusclecar]

Vac. sounds right considering your engine's stats.

I'm surprised it wants a 3 jet spread in the back--I'm assuming the 163 still has the plenum divider, correct?

Have you tried Maxjets?

Maxjets (http://www.jegs.com/p/Comp+Cams/743611/10002/-1)

I've yet to try these myself, but reliable resources say they're considerably tighter in flow tolerance than standard jets.

[Chevy454]

Haven't tried the new jets yet...I was always worried about having to start over on my settings, as the LM1 said I was dead nuts on last time I hooked it up...but, I *totally* understand the concept, and feel I've ran into that problem before.

[VintageMusclecar]

Is your current jetting based on dyno tuning or track results?

[Chevy454]

Track results, naturally! Other than the standard plug checking [and now LM-1 when I remember to take it!], I basically watch my MPH, and check the DA every once in a while and make a note...

[VintageMusclecar]

That's what I figured. Seldom does optimal dyno jetting = optimal track results http://www.yenko.net/ubbthreads/images/graemlins/wink.gif