Explain the "Jim Rowe Power Valve" modification, please

Stevehose

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Because it’s downstream of the carburetor or injectors/manifold, it’s already mixed by the time it’s ignited. Afr gauges read oxygen and convert to afr so they are not 100% accurate. Timing does not affect the mixture of fuel and air, that’s impossible. So tuning afr with timing changes will give a false impression of whats going on. Same with exhaust gas dilution at idle affecting the afr display. I use afr as one tool of tuning, along with old school techniques like plug color, sound, idle quality, and butt dyno. It took me a couple years to stop obsessing over afr readings.
 
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Stevehose

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Another thing that has helped me with off idle lean spot is the 123 distributor. For dcoe’s this allows you to set the throttle plates correctly over the first progression hole and then select the proper idle jets so you don’t have to throw gas at the problem. Then idle advance can be adjusted to set a suitable final idle rpm instead of the mixture or throttle screws
 
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Ahhh... interesting point!

Whenever I plug in an AFR, I remind myself that people tuned these cars magnificently for 6 decades without whiz-bang technology...but I have found them beneficial at verifying my "butt dyno" and gut instincts..... which are "generally" pretty accurate.

...and as for the 1-2-3 units: I have installed and programmed them on several customer cars and I adore the >concept< of them, but I'm not ready to drop $500+ on one myself until the hardware / design improves a bit. You REALLY need a dyno to get the most out of programming them - and I'm not made of dyno money. I've read about a fair number of various unit failures over the years, and spending an ADDITIONAL amount of money for the "improved oil control shaft" to correct an issue that shouldn't be present at ALL in their design, leaves me a bit cold.* I've had terrific results with having my analog units rebuilt by "Advanced Distributors" in MN, where I can specify the advance curves I prefer, based on engine and fuel delivery methods.

*If you've taken one of the older ones apart - there's simply an oil ring INSTEAD of a RADIAL oil SEAL in the 123 units... to seal a part that spins and spins and spins... I mean: what were they THINKING? Of COURSE it's going to allow oil seepage eventually! That should not be the buyer's fault due to "bad rings forcing oil into the unit".... because it never happens with the Bosch original ones, right? When they fix that... I'll probably sign on to their product and spend hours staring at my phone while I program the car. "-D
 

Stevehose

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Yep I've heard of the issues of the 123 and I recently bought the spiral shaft that I shouldn't have to buy (before he probably stops making them) so agree there. I've had the regular USB programmable one and now the bluetooth one and have had no oil or reliability issues (knock on wood) and they've been great for me for 20k miles. But if someone reading this has one I wanted it out there for the jetting possibilities, it's the missing link that solves the idle jet/lean spot/rich cruise conundrum that has plagued DCOE's forever. The advantage over the analog one is that you can tweak the static timing to nail the jetting without sending it out each time for recalibration or jetting to a fixed static timing. Or borrow one, get the data, then rebuild an analog one :cool: And adding cruise advance for a specific manifold pressure range is cool. Also agree on the dyno for maximizing it's potential, you could manually press + or - while on the dyno and watch the results then code in the curve. I'd love to do that someday when I launder enough $$$.
 

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I have a US spec 3.3L with dual weber 38s running Idles: 60, Mains: 135, Air correctors: 200, Emulsion tubes : F50. It wasn't until after the 123 dizzy was added that the last of the stuttering at tip-in went away. Runs great throughout the range though I've not sniffed the exhaust lately. If I had to guess, I'd say it might be slightly lean at high RPMs (+5K) but I don't spend a lot of time up there. Getting old(er)...
 
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I have a US spec 3.3L with dual weber 38s running Idles: 60, Mains: 135, Air correctors: 200, Emulsion tubes : F50. It wasn't until after the 123 dizzy was added that the last of the stuttering at tip-in went away. Runs great throughout the range though I've not sniffed the exhaust lately. If I had to guess, I'd say it might be slightly lean at high RPMs (+5K) but I don't spend a lot of time up there. Getting old(er)...
Any interest in sharing your 123 advance curve? I'm thinking heaps of initial advance (14-16?) and around 36 deg full advance is where I'll be aiming...but I'll probably have a Bosch unit re-curved vs going with the 123 stuff.

Are you using the (any) vacuum feature for advance or retard?

Thanks!
 
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Well.... after hours of chasing ghosts... I have finally settled on a set-up that is working nicely for our 40 degree ambient November temperatures in Pennsylvania. This may change next summer... but for others that are chasing 38/38 smooth driveability issues, the following may help:

I had started out with 135 mains, 170 airs, 45 idles, F50 em tubes - as the new carbs came from Redline Weber. These were "generic", out of the box genuine Weber 38s... ie: not ones specifically jetted for a 3.0 or 3.3 M30 engine.

Quickly found out that the top end / main circuit would just flutter and fart out at 3500 rpms or higher...so I went to work sleuthing a solution. I have loads of DCOE experience, and this both HELPED and HINDERED my approach to the 38s. It felt way too fat (you can feel the difference between rich and lean crappiness, after decades of carb tinkering) and that raw fuel was indeed dumping into the cylinders.

I set to work blocking off the tiny ports that feed the power valve its vacuum signal....and that eliminated the tablespoons of raw gasoline dumping into the motor... I used very tight fitting drill bits in the holes, with "just enough" hanging out, that if I ever wanted to remove them w pliers, I could. As my friend said: "Eliminate the power valve function and let the main jets do their job".... and he was absolutely right!

putting in smaller mains (130s) and 185 airs got the top end and throttle crispness back where I wanted it. After some more fiddling, I increased the idles from 45 to 55...and then even 60. This was in an effort to chase a lean spot during the tail end of the progression circuit. I got it to the point where you could squeeze the throttle in any gear from 1200 rpm up to redline without a stutter or flat spot... but every once in a while the car would get ornery and cough at one fairly specific throttle position. Sometimes it was incredibly subtle at highway speed... but I could feel it and it drove me nuts. My DCOE experience has taught me that fatter idles will often clear up this lean spot... but after dozens of jet changes, the lean spot still persisted. Often as a faint flat spot at cruise position throttle at highway speeds. Thinking this might just be a main jet issue after all - I installed some 145 mains and 185 airs... but the car fluttered and immediately fouled it's plugs. "Way WAY too fat" I thought.

What I missed in the above scenario - is that the plugs had fouled (somehow - and only ONCE) JUST BEFORE the 145/185 swap...and I had mistakenly thought the fatter mains had CAUSED the fouling... so I tossed in the smaller mains and another week went by until I said - "dammit... I'll try fatter mains again, because just MAYBE the tail end of the progression circuit is ACTUALLY the MAIN circuit!"

(again... I really WANTED to believe it was lean idle jets at fault - as is often the case with DCOEs)

Yesterday - one last time, before I put the car in winter storage, I tried the 145 mains again...and it was like friggin MAGIC! I ran with 170 air correctors at first... then switched to 185s....and the car runs GREAT. Next spring I'll see what the AFR gauge tells me... but I refuse to be a slave to it. As long as I'm nice and fat at full-throttle 5000 rpm and not fouling plugs at traffic lights... I'm using the AFR numbers only as a rough guide. Modern fuels yadda yadda...

Throughout all of this, I've also been toying with ignition timing a bit (while I await a custom curved Bosch distributor to come back in the mail) and with ported idle vacuum at around 7-8 degrees BTDC and full advance at about 38-39 degrees... the car seems happy as a clam in the current weather conditions. The re-curved distributor will have more radical timing at idle and should make throttle response even better...

And yes... I must have checked float levels 3 or 4 times during the last 2 weeks.

Will be curious to see what it likes best, once the temperatures come back up into the 70s/80s around here.

Hopefully this helps others! I still love Webers!

Happy Holidays!

Paul Wegweiser
 

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