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Weber 38 Jets, Timing And Wideband O2!
https://slantsix.org/forum/viewtopic.php?t=53928		 Page 2 of 2
Author:  /'d Box [ Wed Nov 27, 2013 4:03 pm ]
Post subject: 
For /'d box, I'd get the springs changed first so you have a known curve that is better suited to your vehicle, then change the carb settings (as the timing curve change may cause your lean areas to richen up by lighting the fire a little later than the current curve and you may not need to jet up. - D.I.

I'll be changing the curve before messing with mixture for sure. But, the pinging is showing up during sustained 2300-3000 RPM with half to full throttle going up hill (low or no vac signal to dist.) I don't hear anything during throttle tip-in or flat highway cruising. Plugs don't show any signs of overheating. These symtoms/conditions seem more related to mixture than the early timing curve.
It will be interesting, 63v, to read about your results with the 180 & 190 mains. Good luck.

BTW, I've worked on factory distributors before with no problem but I can't figure out how the MP comes apart. Any hints?
Author:  DusterIdiot [ Wed Nov 27, 2013 8:32 pm ]
Post subject:  Easy...
Quote:
I've worked on factory distributors before with no problem but I can't figure out how the MP comes apart. Any hints?
Two flat screwdrivers to pry the reluctor off the governor. Two phillips screws to remove the vacc. pod pull out of body until it stops then gently use large flat screwdrive to unhook the round vacc. can arm "peg" out of the advance plate...remove the two other phillips screws to remove the pickup/advance plate from the body...use needle nose pliers to remove spring clip from inside the nose of the governor...pull governor off and do the spring change (if you are meat fisted you will need to drive the roll pin out of the gear and remove the main shaft to get better leverage on the springs and weights). Replace everything in reverse and make sure that you clock the rotor slot toward the light advance spring for correct phasing.

Quote:
But, the pinging is showing up during sustained 2300-3000 RPM with half to full throttle going up hill (low or no vac signal to dist.)
That would be where the heavy looped stock spring would keep the curve "conservative" (not all out) by about 4 degrees. I would garner the guess that a little less timing there and maybe a little more main jet would do the trick. Be careful reading plugs with the ethanol gas I've had plug have a nice green/brown glaze on them and be lean (thankfully I was in the ballpark I'd hate to see my pistons if they were clean or had ash on the electrodes...)

:wink:
Author:  mpgFanatic [ Fri Nov 29, 2013 12:59 pm ]
Post subject:  Re: Power Valve/Enrichment?
Quote:
Problem 1 - I can't set mine to 10 BTDC. I get SCREAMING detonation.

No one has mentioned that your timing tab might be off? I assumed you checked that...
Quote:
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Should the Carb CONTINUE TO GET LEANER as the RPM goes higher/engine works harder??
When the load goes up and the engine works harder the vacuum reading should be dropping (and even worse if leaned out)...at a certain point the power valve or circuit should open up and help put the carb back on track by dumping a large shot of extra gas into the air stream to enrich the lagging/sagging condition...
Exactly! Note the phrase "back on track". After the power circuit kicks in, you should see the result at (or near)Wide Open Throttle. However, before the power circuit kicks in, YES YOU ARE CORRECT that the carb continues to get leaner as engine works harder. The textbooks say that a venturi effect is how the fuel is delivered, such that in theory more fuel is drawn into the airstream as the air velocity increases. But my experiments show otherwise, at least when looking at a very small portion of the demand curve, say for example from 15" to 10" manifold vacuum. There is a very distinct slope which correlates strongly to manifold vacuum... the only thing I can surmise is that the two vacuum levels (venturi + manifold) interact with each other.

Imagine if the power circuit were disabled, imagine the simplest configuration just for demonstration: if you had only one fuel circuit, it provides fuel depending on the vacuum across a discharge nozzle. You won't draw as much fuel out of the bowl at 10" vacuum as you would at 15". The "curve" at that moment (between 10" and 15") is actually backwards: more demand, less supply. This is why carbs require stages, so the next one kicks in as the previous exceeds capacity, yet within each stage it must taper from "too rich" to "just right" as you apply more demand. Numerous attempts to get around this have resulted in ever more complicated beasts (for example, Holley's two-stage or even Gradient power valve, or Carter's long slow taper on metering rods) that become more and more finicky to tune, yet it's really just a series of compromises. It was this realization that finally convinced me to embrace EFI. :)
Quote:
Part Throttle under load -> ULTRA LEAN (15.8-16.5)
This says the power circuit hasn't opened quite soon enough. It needs to kick in at a higher manifold vacuum level.
Quote:
Full Throttle -> ULTRA LEAN, albeit a bit less (15.7-16.0)
This says that even after the power circuit has opened, it's not big enough. My familiarity is not with Webers, but this is just physics. In a Carter or Holley, my experiments showed that the power circuit feeds the overwhelming majority of the fuel demand, at WOT... any changes to idle and low speed circuits contribute diddly-squat. They are too small a percentage of the total.
Quote:
Uh, he also starts to get into drilling emulsion tubes, which is, uh, waaayyy beyond me. I'd prefer not to go there. These things are supposed to be insanely customizable without drilling!
I can think of no faster way to ruin a carb than to grab a bunch of microscopic drill bits from a hobby shop and enlarge the orifice in an emulsion tube to see what happens, then epoxy it shut and try another smaller size, hoping you can regain whatever functionality used to be there.... don't ask. ;)
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Any thoughts? Anyone? Bueller?
:D

- Erik
Author:  DusterIdiot [ Fri Nov 29, 2013 2:33 pm ]
Post subject:  Yep...
Quote:
any changes to idle and low speed circuits contribute diddly-squat. They are too small a percentage of the total.
That's carb 101.

In Holley (and similarly in other carbs) terms the best analogy is:

The idle and transition circuits are like a squirt gun (that's why
tuning these any place other than in the driveway doesn't
make a difference).
The mains are like a gardern hose.
The power valve/power circuit is a fire hose.
The accelerator pump shot is like a bucket of water
to cover the lack of flow going from one circuit to the
other on throttle opening and stops when you stop
pushing on the pedal.

-D.Idiot
Author:  DCFAB.US [ Sat Nov 30, 2013 6:42 pm ]
Post subject: 
63... I have an 82 truck with a very similar set up with 38DGES and I also have "SCREAMING" detonation. I cannot for the life of me figure it out. I drilled two 145 mains to 192 and it did nothing. I have 9r govenor two medium springs (heaviest I have that arent long looped which dont even tension with the short 9r slots) and at 10-12 deg + the 18 deg from dizzy advance should put me around 28-30 total. I have verified damper mark at TDC, NEW timing set/chain etc. Please ask if you are thinking of something to try that I might be able to share my findings. I know this isnt "help" but more of a shared hinderance that hopefully we can find a solution.
Author:  DCFAB.US [ Sun Dec 01, 2013 6:41 pm ]
Post subject: 
Quote:
63... I have an 82 truck with a very similar set up with 38DGES and I also have "SCREAMING" detonation. I cannot for the life of me figure it out. I drilled two 145 mains to 192 and it did nothing. I have 9r govenor two medium springs (heaviest I have that arent long looped which dont even tension with the short 9r slots) and at 10-12 deg + the 18 deg from dizzy advance should put me around 28-30 total. I have verified damper mark at TDC, NEW timing set/chain etc. Please ask if you are thinking of something to try that I might be able to share my findings. I know this isnt "help" but more of a shared hinderance that hopefully we can find a solution.
Update... AGAIN NOT A THREAD JACK, JUST SHARED EXPERIENCE...
Locked pertronix so it could not advance with can... no fix
Tried two very stiff springs on 9r gov... no fix
Drilled jets to 230 mains... no fix and ran $#!+ rich
So im curious if ur 180-190's produce same results when installed.
Backed timing to "0" and "10-12"... nothing changed

Im wondering if the HYD cams in these things need degreeing or installed a tooth retarded on timing chain?
Author:  63valconvert [ Wed Dec 04, 2013 5:13 pm ]
Post subject:  The BIG UPDATE!
Alright, so I've made some changes.

Namely, I installed the 180 mains and now I'm getting a big flash (less than a half second) between 15.5 and 16.0, which quickly (and I do mean quickly) settles to 13.8 on heavy wide open throttle.

As a result of this, I've been able to move my timing to 10 BTDC with no problem. Not a ping to be found.

I also upped the idle jets to 150 from 145s. Smoother idle, and a reasonable (if sometimes rich) low speed cruise. We're talking 11.6-13.3 area.

High speed cruise is about 13.3-13.7. Not sure what I can do about that. I'd like a 15, as was recommended by Sam Powell, but I don't think that adjustment can be made. If it can, I'd love to do it.

I'm also running 170 air correctors, which I've read some posts saying "these are ONLY in use at WOT". They also say I should change them if I feel a "flatness". I don't really do, so, I'll leave them be.

I think just for giggles I'll try some 175 mains, but I think I may have this as set as it's gonna be.

Feels good, though. If only it wasn't erring to the rich side.

Still, at least it's no longer dangerous! :)
Author:  /'d Box [ Thu Dec 05, 2013 2:38 pm ]
Post subject: 
Good news, 63. After I fine-tune my timing curve I might try the 180 jets if I still have ping. Where did you get you jets from?
Author:  63valconvert [ Thu Dec 19, 2013 12:15 pm ]
Post subject: 
I always get them from Carbs.net. They deliver pretty fast, I find.
Author:  63valconvert [ Fri Jan 03, 2014 11:06 pm ]
Post subject:  Weber 38 Jet Update
Just as another quick update:

I leaned up on the Air Corrector jets to 180. I may go back to 175 when I feel like taking the thing off. But, it is good.

I also leaned up the idle jets to 140s.

Funny thing, I could only run 145s for a while. The 140s would stall the car when I slowed down for a stop.

Then, I found out I had a cracked distributor cap, and one of my Idle mix screws was broken off before the needle!!

Needless to say, it's running great now.

The 140s are perfectly lean at the right times (I found the 145s really heavy - 10.5-12 during slowdown). Just make sure your ignition is good.
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