WOT too Lean

[fastd]

I haven't changed them back yet; thanks to Gaz64 i see that others with this carb have had lean issues also.

It's not the rpm band as much as it is wide open throttle; say the last last quarter of the of the throttle.

Actually, my 351w is all good; it's for another engine of mine; a 461 (um, pontiac; sorry); it's a hurricane/tomahawk single plane manifold

[G_Edmonds]

If you have 30cc accel. pumps you could be running out of pump shot. Moving up to 50 cc pumps might help.

[fastd]

G Edmonds,

I think you may be right but it stays lean for longer than the pump shot would last; also, it's not really hesitating, just lean. I am going to focus on the major leaness first before the finer tuning of accel. pumps. Thanks.

[fastd]

by the way, I just order the high speed air bleeds from WELCOME TO ALLSTATE CARBURETOR & FUEL INJECTION on Long Island.
They were only $3.99 a pair! I sent another carb up to them that I buggered up the fuel bowl threads on; they did a nice repair job at a fair price with a quick turnaround.

Recommended them.

[lippy]

Something else may be going on here. The changes you mentioned are radical. I change my jets in two-size increments at first and then one-size increments.

FYI, my carb is approximately an 850 and I have a 487 FE, and I run 74 jets primary and secondary, with 7.5 power valves and size 57 PVCRs.

Is your spike to 16 just a brief transition, or does it persist? Depending on the situation, I sometimes get a brief lean or rich condition, but it is always brief (a spike) and corrects itself quickly. That shouldn't be something to worry about too much.

Also keep in mind that your LSABs and HSABs have only a small effect on mixture (remember you are changing the air component and not the fuel component) but they do impact when the circuits are engaged. For example, smaller HSABs will make the cruise and WOT mixture slightly richer, but importantly they will engage the main circuits more quickly.

[fastd]

lippy,

my spike to 16 is more than just briefly. It kinda of stays there and comes down to high 14s low 15s.

Ordered 26, 28, 30 and 32 HSABs vs the current 34s...

[cycleguy55]

Originally Posted by blykins

7 psi is on the high side and I don't really see the pressure making the difference.

If a huge jet size bump didn't make a dent in it, I would look for a vacuum leak at rpm or a fuel flow restriction.

It would have to be a BIG vacuum leak (20% of total airflow?) to lean out the mixture that much at WOT on a 461 CID engine (Pontiac - cough, hack, gaaack).

I agree with looking at fuel flow restriction somewhere. He'll have to start at one end and go through everything (lines, filters, fittings, pump, etc.) thoroughly and systematically to find out where the restriction is.

[cycleguy55]

Originally Posted by Jac Mac

Have you tried a full power run without the hood, is the carb breathing engine bay heat as well as scoop air, or do you use an airbox for scoop air only?

I stand to be corrected, but my take is those approaches would be to fix an engine that's not getting enough air - therefore running rich, not lean.

[lippy]

What power valve restrictions are you using?

Also, did you measure vacuum at idle, cruise, and say, half throttle? That would be good to do.

[PeteF]

PCV valve not closing at WOT could make you go lean. (?)

[Gaz64]

This is starting to sound like a restricted needle and seat issue.

If the fuel pressure does not vary below say about 4psi at WOT, AND it occurs anywhere in the rpm band from 3/4 throttle up, then main jet changing that doesn't sound like they are making differences points to fuel supply trouble to the bowls.

Fueltank venting, all hoses and lines for kinks, pump capacity, fuel filter capacity, fuellog, regulator etc.

One of mates had trouble a day after he changed a fuel filter, I found fuel line rubber in a needle seat.

Gary

[DanEC]

Originally Posted by cycleguy55

I stand to be corrected, but my take is those approaches would be to fix an engine that's not getting enough air - therefore running rich, not lean.

Oh good, I'm not alone in my after lunch duldrums/confusion. Welcome to my world Jac Max.

[fastd]

I don't think its fuel restriction. I have a fuel pressure gauge and its steady all the time had it at 5.75 psi and moved it up to 7 psi last night and adjusted the floats slightly higher. Does not drop at wot. Helped marginally. Stay tuned until I get my air bleeds in the mail the end of the week.

[Jac Mac]

Originally Posted by DanEC

Oh good, I'm not alone in my after lunch duldrums/confusion. Welcome to my world Jac Max.

Ha Ha, too make it worse we/I/You now find we are trying to diagnose a problem in a 461ci Pontiac which probably does not live in anything like a Cobra engine bay!! What next, sometimes we have to be 90% mind reader around here... at least its not OUR problem.

[Gaz64]

Originally Posted by fastd

I don't think its fuel restriction. I have a fuel pressure gauge and its steady all the time had it at 5.75 psi and moved it up to 7 psi last night and adjusted the floats slightly higher. Does not drop at wot. Helped marginally. Stay tuned until I get my air bleeds in the mail the end of the week.

Yes, but it could still be debris in a needle and seat.

Fuel pressure could still be perfect, but a bowl could run dry.

Just something small to check.

[fastd]

update:
removed the 34 hsab and installed 26s...did help a lot with the WOT leaness; still spikes but comes down quickly to high 13s/low 14s.

the power valve channel restrictors are next up: currently have 70s in there but have 73s and 76s on order. Also, after I change the restrictors, next step would be add a "high flow" power valve.

The initial spike makes me think that I might need a 50 cc accelerator pump; going to make the other changes one by one before I do this step.

[lippy]

Like I said before, I think there is sone thing else going on here. You already have big PV restrictions and big jets and you are making radical changes that are only helping marginally.

If I were you I would pull a few plugs and cut them with a hacksaw so you can see the whole insulator. Inspect for color (mixture) and specs (detonation). Plugs don't lie, wideband meters sometimes do.

[jhv48]

If you have access to a different carb, try a swap and see what your readings are.

[fastd]

Lippy,

Thanks for your comments. I am gonna try to take this one step at a time:
1. increase restrictors to 73/76 (in addition to the 02 sensors, the car feels better and is not fighting me to get 4k-5k rpm as much. It kind of max's out at 5-5.5k rpm anyway)
2. I think that I will be able to move back down to 86-90 from 94 secondary jets when I get the WOT dialed in better. My understanding is that the best way to adjust WOT is hsab and pvcrs; the secondary jets aren't that impactful on the WOT so the fact that they haven't.
3. I think the right PV is 6.5 based on my vacuum gauge readings. Gonna try the high flow PV if there is no noticeable change after I change the pvcrs. Trying to figure out if I change to the high flow pv before I change the pvcr.

I haven't thought of hacksawing the plugs; that may be a bit above my pay-grade...not sure I would know what I would be reading. I have more confidence in my ability to read the o2 sensor than the plugs .

[lippy]

You can calculate theoretical AFR changes by calculating orifice sizes and approximate what impact a change will have. I did some quick math. If you have 74 jets primary and secondary, and you simply change your PV restrictions from 70 to 76, you get a total increase in orifice area of +7.7%. If you keep air the same and put in 7.7% more fuel, this would move you, for example, from a 13.0 AFR to 12.0. *Huge* change. I know things aren't this simple, but this should give you an idea. Similarly, going from an 86 to a 94 on your secondary is a *huge* change.

Agree with jhv48 you should swap carbs and see what's up.