aaronz28

OK gang - got a quick question on some carb jetting...

i've been tuning the 4053 Holley on my 421 stoker - -

i ran it with an Innovate AF meter and was running a tad rich in part throttle - but lean WOT - so i reduce primary jetting by 3 sized, and increased secondary jetting by about 6 - -

NOW, the AF reads closer - BUT i have this part throttle stumble thing happening.... and NO amount of squirter (i'm running a 37 now - and i've tried 4 different pump cams) none of which seem to solve the issue...

that being the case - I'm finding that its responsive immediately off idle - but the hesitation occurs around 1800-2200 rpm.....

SO - is this a pumpcam/squirter issue? Or jetting/power valve issue?
I didn't change the primary power valve - although (now that i think about it) i did put a secondary power valve block off and increased the secondary jetting - still wouldn't think that would cause the issue -

car idles without issue - and other than that hesitation - which does not seem to be affected by timing either -

SO - is it just too small of primary jets?

Pop Chevy

You might try a bigger air bleed in the front ?

aaronz28

they are non adjustable - and i really don't want to drill them out and try experimenting....

larrywalk

Power will drop off quickly when it goes lean. IMO, I would have left the primary jets alone and richened the secondaries by 2 jet sizes.

tbarb

Go back to the factory Holley jets and start over.

If you remove the P/V in the secondary make sure the area of the sec. jet and PVCR is taken into account when selecting a sec. jet at least to start.

If you have a small drill bit set check the IFR/IAB and main jets/MAB so you have a baseline and know whats there now.

It seems to me you have leaned the main circuit to much, does the motor have a surge at 3000rpm cruise? Make sure the primary throttle blades/transfer slot relationship is set right. You want the slot at the bottom of the blade so the curb idle emulsion is set by the side screws, how far out are they now at ? idle RPM.

It's all baseline stuff but if not correct you can chase problems using one circuit to cover another problem or adjustment in another circuit like the accelerator pump you mentioned.

Is the 4053 a double pump or vacuum sec. Holley?

DansYellow66

I have no idea if this is relevant to your issue for sure but you might pick up a copy of the Dec 2015 Hemmings Muscle Machines magazine and look at the article by Jeff Smith on the Holley idle circuit. I did not realize the extent the Holley idle transition circuit operates. It discusses that at a steady 10 to 15% throttle setting which could generally hold a 3000 rpm cruise speed, the carb is typically running only on the idle transition circuit. It discusses that often this circuit is a little rich on an out-of-the-box Holley because of the universal state of tuning necessary for it to work on everything from a warmed over 307 up to a 454 hi-po engine.

So, in theory the rich condition at part throttle may have nothing to do with the primary jets and may all be due to the transition circuit. And the idle mixture screws have no influence over this - but the idle feed restrictors in the metering plate do.

Reducing the size of the primary jets may have just reduced the fuel charge during rapid throttle opening enough to induce the stumble.

Again - I know just enough about carb mechanics to be dangerous but it's an interesting how-to article that might have some relevance to your issue.

tbarb

It's amazing to understand how a carburetor works and this tuning is like a disease because the leaner you can get a engine to run the better it responds except power enrichment of course. Spark plugs stay clean etc. the limit will be a misfire caused by poor fuel distribution.

All circuits overlap each other that's why you need to start with a baseline. For example if the primary blades are open to far to idle the engine you loose the adjustability of the curb idle emulsion screws plus you use the fuel available through the transfer slot up too soon before the main circuit starts feeding and you can get a lean stumble.

Vizard's book describes tuning a Holley and it's a good read, I usually need to read things a few times to learn them and I recommend it if you want to learn more.

65tripleblack

Can you give us the cam specs and static compression ratio and timing specs. Can you post the Innovate output graph here, as well as tell us exactly what part throttle condition you are talking about. As a rule of thumb, your cruise a/f ratio should be about the same as your idle a/f ratio. By cruise, I mean running @ around 2000-3000, depending on diff ratio, steady state on the highway.

Here is what you should be looking at. This is a sample from my Zeitronix ZT2:

jim lockwood

That's new news to me. And it certainly does NOT apply to Rochester FI engines. Carbureted engines? Well, I'm not an expert on these, but I remain skeptical.

On an engine with Rochester FI, engine will idle the happiest with an AFR between 13.5:1 and 14:1, waaaaay too rich for a cruise mixture. At cruise, you want to see something in the 15.0:1 to 15.5:1 range, with 15.5:1 being perfectly acceptable.

Jim

PAmotorman

doing a carb engine is hard to do unless you can do it on a dyno where you can keep a constant load on the engine because the carb responds slowly not like electronic FI which responds in milliseconds. WOT on a carb is easier on a long up hill or a drag strip.

65tripleblack

Trust me, Jimbo.

It has more to do with the color of the spark plugs than quantitative values given for lambda. As well as the way the engine idles, makes power, and responds to throttle input. My engine cruises at between 13.7 and 14.5 and idles @ around 14.5. I purposely have my cruise ratio set a bit fat to preclude any throttle response issues. The spark plugs like the ratio with no fouling and a tan insulator.

My engine has a "bigger" cam than the vintage 346, 097, and 178 solid lifter cams. It's better to have a slightly richer fuel mixture than slightly too lean. It sounds like your a/f ratio is either inaccurately measured or your engine is in danger of burning a piston.

L78racer

4053 OEM jetting IIRC is 68/76. been through a few sessions with them on big and small blocks. it is really hard with them to get the part throttle where you want it without getting into the stumble you are experiencing. i would go back to 68's up front and go with 73 or 74 in the rear. should get you pretty close. i have adjusted the idle air bleeds on a few and got the part throttle a little leaner without the stumble. the bleed screws are directly in front of the air horn with the sealer in them. you will have to adjust you mixture screws after adjusting the bleeds.

solidaxel

I'm sure you have addressed your distributor vacuum and advance weights at these stumble RPM's

jim lockwood

15.5:1 was the design spec AFR for Rochester FI at cruise. That's the AFR for optimum fuel economy and there is no danger of burning a piston.

DansYellow66

It sure sounds like a classic case of the issue addressed in the magazine article on idle fuel restrictors I mentioned above. It does state in it that some marginal improvement might be made with opening up the idle air bleeds a little, but too much and it will shut off fuel flow through the transition slot earlier - and create or worsen a transition stumble.

65tripleblack

Was that with pure gasoline or E10?

jim lockwood

Not sure E10 was readily available back with Rochester FI was developed.

To avoid confusion, I'll state it this way: Lambda of 1.05 was the design spec and is an acceptable upper limit for the cruise mixture for Rochester FI.

PAmotorman

we burned a piston in out 57 FI chevy 150 at the February 1957 NASCAR Daytona beach race because the early FI had a fuel shut off at coasting. chevy gave us aniline to put in the gasoline but it still burned a piston. towed it over to smokey's put in a new piston and drove it back to Pa.

jim lockwood

That Coasting Shutoff Valve was notorious for causing burned pistons and was deleted early on.

PAmotorman

smokey's car was leading when it also burned a piston.