Explain the "Jim Rowe Power Valve" modification, please

Paul Wegweiser

Paul Wegweiser

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I've seen this mentioned, but no link or explanation for it anywhere, EVEN using the search function here.

Anyone care to elaborate on exactly what the "Jim Rowe Power Valve modification" IS????

I've finally hooked a wideband / AFR gauge into my 3.0 with 38/38 Webers and have decent fuel mixtures until going for 3/4 or greater throttle position....then things get WAY rich like I'm dumping raw fuel into the manifolds. Car runs reasonably well (even with those nasty rich mixtures), but I know I can get more top end if I can dial in the mixtures above 4000 rpm. My goal is 12.9 at WOT / high rpm.

I'm going to re-check float levels AGAIN... but curious about this modification - and if it has benefits for my situation.

Yes: Checked for fully opened choke plates - so all good there.

Currently going with this set-up (after hours upon hours of trial and error and endless test drives)

3.0 motor (stock)
135 mains
200 airs (was running 185s... before that, running 170s)
60 idles


Fuel mixtures expressed in AFR (93 Octane street fuel w some Ethanol):
idle 13.3-14.9 (I've found my 2002s and tiis like 12.9 exactly... no more... no less)
transition 11.8-13.0 (smooth, seamless...finally!)
cruise (interstate, light load, 70 mph) 13.0-15.8
3/4 or more throttle: 10.0 or less (!!!)

I'm thinking either unmetered fuel (power valve or float level) is dumping into the motor.

Carbs are brand new and genuine Weber... so I have little reason to suspect float or Power valve "malfunction"...though it's possible, of course.

I'm well versed in Weber things after 35+ years of tinkering.... but this one Mystifies me.

Thanks in advance!

Paul
 
I can't find the original article Paul but here is a link on the 2002faq that discusses this mod:
www.bmw2002faq.com

Whats this port on my new (to me) Weber 38?

Just comparing this new carb that I am gonna install tomorrow to my existing Weber 38 and I DO NOT have this port on it...what is it?? My old one looks similar but no port there. Should I plug this on the new one? Maybe for vacuum? I have a mechanical advance Tii Dizzy, so no vacuum and maybe tha...
www.bmw2002faq.com www.bmw2002faq.com
 
halboyles said:
I can't find the original article Paul but here is a link on the 2002faq that discusses this mod:
www.bmw2002faq.com

Whats this port on my new (to me) Weber 38?

Just comparing this new carb that I am gonna install tomorrow to my existing Weber 38 and I DO NOT have this port on it...what is it?? My old one looks similar but no port there. Should I plug this on the new one? Maybe for vacuum? I have a mechanical advance Tii Dizzy, so no vacuum and maybe tha...
www.bmw2002faq.com www.bmw2002faq.com
Click to expand...
Mine definitely don't have that vent tube. I was thinking the Rowe mod I've heard about was regarding a passage in the body of the carb (only visible under the float bowl lid when removed?) that gets plugged... but I'm hesitant to do any "permanently disfiguring" modification on these shiny (and expensive) brand new carbs.
 
Okay, here is an oldie but goldie by the inimitable c.d.iesel.
 

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Interestingly - the hole he mentions on the 32/36 (I've scoured the interwebs for all DG series mods and photos over the last 48 hours) is (I THINK) different from blocking off the vacuum passage from the power valve... which is the somewhat oval recess w hole, found in the carb. I'm going to try for a "temporary" block off and see what happens, before committing to a more permanent modification. I'll report back. I just need to cure these wildly rich mixtures at higher rpm / larger throttle openings before it kills my oil with fuel! :)
 
Paul,
I don’t think it’s float level, but that’s easy to tweak 1/8” and test (free).

How abruptly does your WOT AFR go from 13:1 to below 10:1 at 4000? Does the shift happen within a few hundred RPM of is it a gradual richening trend?

If it’s somewhat gradual I think you are on the right track going to a bigger air corrector. As you well know you might have to bump the main up one size if the mid throttle high rpm range gets too lean.

If the transition is abrupt … IDK

John
 
I have sidedraft webers, and admittedly have a lot to learn, but a couple of things come to mind.

1) Do your afrs hit 10 and then come up, or are they staying at 10? My afrs will drop to 11, then come up to 13.5 at WOT. The initial drop is the effect of the accelerator pump jets. On my car, if I were consistently seeing 10 at WOT, I would have to change the main jets to get a leaner afr.

2) Another thought on very rich WOT afrs relates to accelerator pump jets. On the DCOMs and DCOEs, the air flow at very high rpms lifts the ball that limits the fuel pumped on each push of the accelerator throttle off its seat. When this happens, fuel is allowed through the accelerator pump jets and they act as an enrichment device. I have not really explored this with my carbs, so I don't know exactly how to test for either when it kicks in or how big the effect may be, but I'll toss this out there for you to consider. Obviously, if this is the cause of (or contributing to) your problem, the fix would be a smaller accelerator pump jet.
 
Here's the latest update.... this is TEDIOUS work, because each time I change jets or air correctors it involves:

removing air cleaner
removing adapter plates
disconnecting choke rods
unscrewing carb lid
etc etc...

I've done this 4 times today ALONE - and it's only 11:30 am!

The latest is as follows - and I, too thought maybe it's just the pump jets causing a momentary rich condition, but the change to slightly smaller mains seems to help.

Current set up with associated AFRs:

130 mains (but when checked w my jet gauge are a weeeee bit smaller than the "other" 130 mains I had in there** (I have hundreds of jets...don't ask!)
back to 185 air correctors. The previous 200s introduced a lean spot just AFTER tip-in.

Full throttle after 3000 rpm = 10.0 +/- but gradually comes back down to high 11s and even mid 12s...which I can live with at full throttle for sure!

Idle AFR: 13.0-14.6 <--- better than any other BMW I own
3500-4000 (4 speed trans, etc) cruise AFRs range from 12.9-16.2 or so...again: better than any other car I own.

I have blocked off the tiny brass inlet for the power valve, so it's no longer in the equation at all... may have reduced my overall high rpm / throttle AFRs by 1.0 +/-.

I may even step down to 125 mains and keep the current 185 airs.

**I've read that jet gauges are useless, since Weber "holes" are arbitrary and not always based precisely on jet number... it has more to do with flow characteristics than absolute bore diameter... this may or may not be true...but I use gauges to verify some wingnut hasn't drilled tham out larger than the stamped number on them! (which avoids huge headaches!)
 
Indeed, changes are tedious. And testing at high rpms is rather tough without a landing strip to use. I often run out of road.

Overall, that data looks pretty good. What do your afrs look like on gradual acceleration, say from 2500 to 4000 rpm? I would try 125s and test whether you can tweak the WOT 11s and mid 12s if you are not going too lean upon gradual acceleration from 2500 to 4000 rpm. I stopped decreasing my main jet sizing because of my afrs on gradual acceleration, which I think is a function of having lots of holes in the middle of my emulsion tubes.
 
Paul Wegweiser said:
removing air cleaner
removing adapter plates
disconnecting choke rods
Click to expand...
I left all this stuff of when I was tuning my 38/38’s, and assumed a small fudge factor for the restriction of the air cleaner assembly. I also left off the tiny c-clips for the choke rods for fear of dropping one down a carb throat (still off one year later).

Sounds like you are in good territory on your tune.

Interesting that the AC jet at 200 affected your tip in so much.

How do you know your jet gauge set is accurate? I got a set from Pierce Manifolds ($40), and per my digital mic they were WAY OFF the intended diameters (in some cases as much as 0.1 or more in error).

John
 
Paul Wegweiser said:
Full throttle after 3000 rpm = 10.0 +/- but gradually comes back down to high 11s and even mid 12s...which I can live with at full throttle for sure!
Click to expand...
As Steve says, that initial dip down to. 10:1 is the accel pump “squirt” you are seeing. Mid 12’s at steady-state-WOT is pretty darn good.

Now I’m inspired to got chase ADR number a bit more on my 38/38’s. Wanna sell some of your spare jets?

John
 
Tip-in still yields some pretty fat numbers (10-10.9) as does driving it slowly through town... 15-25 mph yields 10-11 AFR numbers. If I have to run that rich to attain smoothness, I'll get over it. I just don't want to dilute my fancy zinc infused motor oil.

My jet measuring gauges were bought from Redline Weber (presumably the same US importer that Pierce uses)...and they are WAY off compared to the nifty set of 700 jet drills I bought off some unknown source online. I have yet to put my very-near calipers on either the drill bits OR the gauge set...but that's probably a smart move! *Doh!*

My purpose for reinstalling air cleaner etc, is because that's how I'll be driving the car - and cool outside air is more better than hot engine bay air. I want any data I get to apply to the same conditions under which I'll be driving the car henceforth. :) I doubt the air filter is restrictive... but I'd rather take any doubt out of my readings, since I only install the AFR gauge set-up on THIS car for tuning... (I don't want any modern gauge-digital-gizmos in the car next spring.)
 
Motor oil, fancy or not, needs to be considered even more expendable with Webers.

At low speeds around town you are probably relying a great deal on your idle circuit. Might it be possible to use a smaller idle jet, use the idle adjustment to get back to a similar idle afr, and lean out both progression and WOT?
 
IIRC my idles are 45’s. 60 seems kind of big to me.

I assume you already tried turning in the mixture screws to lean out that 15-25mph range, and found the transition/progression unacceptable?
 
"I assume you already tried turning in the mixture screws to lean out that 15-25mph range, and found the transition/progression unacceptable?" - I definitely don't want the idle any leaner than it currently is - especially with colder temperatures here in Pennsylvania (30-51 deg F)...but may revisit this in the spring.

Smaller idle jets yielded weak / lean transition issues...but again, I may revisit this when warmer temps arrive next spring.
 
Paul,

Back to your original question on Jim Rowe’s Power Valve modification, here’s a summary of my notes from a discussion I had with him back in 2009 when I was trying to tune my 38’s on a 3.5 liter:

”The PV doesn’t work right as it comes from the factory in 38’s. It works off the absence of vacuum, and the spring is too weak causing too much fuel to flow through. To correct this, insert a 7-8mm piece of nylon tubing over the shaft to compress the spring. Spread the tubing over a nail so it will slide over the shaft, then once installed it should compress back into shape. Once the PV is fixed, you should be able to drop the mains by 15.”

I tried this on mine, and no noticeable difference, but my problem at the time with the flat spots at mid throttle was the fuel pressure not the carbs themselves.

A couple of notes on my installation:

1. I thought the tubing was way too loose on the shaft, with the potential to come off. So I tightened it down with a tiny tie-wrap.

2. When I checked it 500 miles later the gas had deteriorated the tubing, and I removed it as again, it did not seem to make a difference. In hindsight I should have use a different material.

Let us know how you make out, and if you do this mod what material you used.

Good luck!

Jeff
 
Paul Wegweiser said:
I definitely don't want the idle any leaner than it currently is
Click to expand...

Right,
Interesting that I don’t have a flat spot with 45 idle jets, and you can’t go below a 60 without getting a flat spot. Do you have headers or a high performance intake/exhaust system?

At-Idle mixture is mostly controlled by the screw and much less so by the jet size. So, like you, I pick the idle jet to enable good transition/progression behavior. For me, a 60 made it too rich for Highway cruising (progression). The 45 jet size was rich enough for smooth transition/progression, and also raised my Highway cruising AFR to a more acceptable range.
 
I did not modify the power valve at all.. I simply took it out of the equation entirely. ;-) Good to finally know what the Rowe modification truly IS though! THANK YOU!

I have extensive experience with DCOE carbs, so I may be mistakenly using my experiences with THOSE to dial in the downdrafts... and I'm used to installing BIG idle jets to overcome a lean spot "just off idle" whereas these 38/38 carbs exhibit a lean spot slightly farther up the progression throttle position / rpm range.

Adding frustration to this, is the fact that the emulsion tubes don't want to come out of the carbs...and I have not yet made a tool to extract them for inspection or changing them! That's where my solution may lie!

I will revisit the idle jets and if richening up the idle mixture to mid 12s at idle eliminates the lean spot during tip-in highway cruise... I'll consider it...as well as leaner idle jets with the adjustment screws backed out a bit more (I started with exactly that,) but since I have swapped main jets and air correctors since, I should go back and play with idles again. I may also tinker a bit more with ignition timing, as I've seen it affect fuel mixtures in other cars with positive effects. I'm currently at about 14 deg at idle (850-890 rpm) and full advance is about 36-37 deg (admittedly a bit aggressive, though this is a low compression USA motor as far as I know) Perhaps backing it off to 32-34 will yield fatter transition mixtures. The distributor SEEMS to be advancing and retarding predictably, though I have not graphed it throughout the rpm range, as I hav with other cars.

Again - it's 34-48 degrees here in Pennsylvania right now...and soon the car will go into storage to avoid road salt for a few months.

Thanks all for the tips... I will continue to grind away at it!

Paul
 
Timing doesn’t affect mixture, mixture does. Timing may affect your afr gauge display slightly, but I doubt it.
 
Oh it DEFINITELY affects the AFR display - by as much as a full point or two... (I've worked extensively with timing on my tii to affect mixture "readings")... so if it's data reflected in the AFR display, how is that not affecting combustion mixture?
 
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