WOT too Lean

[fastd]

My WOT A/F spikes to like 16; everywhere else it's fine.

I increased 2ndary jets to 94 from 86, the PV from 4.5 to 7.5 and the 2ndary squirter from 30 to 42 but still no benefit.

I am thinking about changing high speed air bleeds and the PVCR; anyone had any experience with tuning these two?

Any other ideas?

Thanks,

Phil

[DanEC]

Could it be it's leaning out at high rpm due to a restrictive air filter/cleaner? I would have thought a WOT lean condition would otherwise be fairly simple to address - just go to larger jets like you did.

[blykins]

A restrictive air cleaner will actually richen it up....

Sounds like you have something else going on. Checked for vacuum leaks?

[fastd]

i think the vacuum is OK; running 5.75 psi fuel pressure; gonna try 7.0 psi to make sure i am not running the fuel bowls dry.

If that doesn't work, gonna mess with the high speed air bleeds; i haven't checked yet, but I hear that they are easy to replace on the XP and they aren't very expensive.

[blykins]

I didn't think about the fuel pump……

I wouldn't raise the fuel pressure but I would check to make sure you have enough flow.

[fastd]

7 psi too much for holley? I thought that it the upper end of "OK"

[Jac Mac]

I would be checking the fuel line from tank pickup to carb, could be a restrictive line or fitting. The carb doesn't still have those OE compressed bronze filters in each end does it? those will block fuel flow despite high pressure indication. Also check tank filler etc for venting, saw a slant 6 cyl Chrysler collapse the steel factory fuel tank once due to blocked breather vent.

[fastd]

Jac Mac,

I feel pretty good about the fuel delivery; high capacity in tank electric pump to fuel regulator with a return line; no OE compressed bronze filters. Tank is vented pretty well also.

Thanks.

[DanEC]

Originally Posted by blykins

A restrictive air cleaner will actually richen it up....

Oh yeah - well, there's that after lunch duldrums/dyslexia slipping in again.

[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.

[Gaz64]

Originally Posted by fastd

i think the vacuum is OK; running 5.75 psi fuel pressure; gonna try 7.0 psi to make sure i am not running the fuel bowls dry.

If that doesn't work, gonna mess with the high speed air bleeds; i haven't checked yet, but I hear that they are easy to replace on the XP and they aren't very expensive.

Is the fuel pressure at idle from a gauge under the bonnet, or dash mounted at WOT?

Sounds like something else going on here, the engine would have melted itself by now at 16:1 at WOT.

What AFR gauge are you using, and where is the sensor mounted?

What carburettor have you got on your engine?

Gary

[fastd]

Carb is a Holley 850 Xp
Fuel pressure under the bonnet but on a 4 ft extension so not too hot.
Afr gauge is afx with ngk O2 sensor.

Increase in psi to 7 and increase to float helped a little but not enough.

High speed air bleeds are 34s...gonna try 31s or 28s?

[Gaz64]

Is the carb at factory emulsion bleeds and high speed bleeds?

Does it feel "lean", or does it go as good as you would think?

Where in the exhaust system is the O2 sensor mounted?

You could try some 28s first, and sneek back up to 31 if necessary with a number drill.

Gary

[fastd]

Carb is at factory for emulsion and air bleeds.

It definitely feels lean in wot.

The O2 is bunged about 18" downstream from the end of the exhaust manifold.

Gonna try both 28 and 31s. Anybody have any experience with changing the air bleeds? I assume going from 34 to 28 should be impactful. But not sure whether to start with the pvcr...?

[blykins]

Bleeds are usually changed in .004" increments.

[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?

[Gaz64]

If you are happy with the A/F ratio at low engine speed under load 2000-3000rpm, I would leave the PVCRs alone.

For some reason, there is a few complaints of lean "out of the box" Ultra XP 850s.

Here is one thread on the subject:

new holley 850 ultra hp WAY out of wack - Yellow Bullet Forums

So I'd start experimenting with the high speed bleeds.

Gary

[fastd]

Jac Mac,
Have not tried without hood and no air box.

Gaz64,

Thanks for the link. Makes me feel a little better about my moves so far.

[vector1]

You made all these changes, did you change them back? A good rule is to make one change at a time, if it doesn't have the desired effect, put it back.

What part of the rpm band is the lean out? Is any portion fine with respect to a/f ratio?

What type intake and size engine? 351W is in your sig.

[G_Edmonds]

I'm by no means a carb expert but 94 jets in a 850 is a lot unless you plugged the power valve. That's up in dominator territory. You can help a lot of things on a carb. by blowing carb cleaner thru all the passages to make sure they're actually open.