Weber 38 Idle Jet Tuning Question

afeustel

afeustel

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Was wondering whether it is adviseable to select different size Idle Jets for the Weber 38 carb since the mixture screws are set differently on either side of the carb. This effect would appear to be a function of the differences in manifold/airflow path length on the inner barrel (closer to engine) versus the outer barrel (closer to driver fender).

With my current setup, the inner barrel idle mixture screw is at 1.5 turns, while the outer mixture screw is at just over 1 turn. I think this implies that the inner barrel is running slightly leaner, hence the need for more turns out on the mixture screw. Am I making a correct assumption here?

By changing the size of the idle jets, can I get the mixture screws to be about the same, and is this even desireable?

Thanks
Drew
 
38's mixture screws will not likely be the same; within reason.

Keep the jets the same size. From what you describe, they're in the ballpark.

Is there something in particular you're trying to address?
 
I have 38's on mine with a 3.5L . the inner idle jet is a 55 and the outer is a 50. My adjustment is about the same as yours. Look in your tailpipe, if there's carbon you might go with a smaller idle jet
 
It is good to be open to re-jetting, as gas formulations are changing, and they certainly have changed since these cars were new. If you really want to get it right, Innovate technologies sells a very handy piece of kit that lets you record your mixture richness at idle and while driving.

Although most of my experience is with DCNFs, most of your normal driving and cruise is done on the Weber transition circuit, so the idle is important is what I was taught.

Webers are fun!
 
Thanks for the comments. Found this information on another site, its actually worth reading the whole section if you have an interest in Weber tuning, though it is fairly consistent with the other references out on the web:

"The 38/38 DGES carbs can be a little tricky because you are idling on both barrels at the same time. You have 2 mixture screws, which WILL NOT be set the same on most carbs. This is because the plenum type log manifold distributes fuel unevenly. By having 2 mixture screws you are delivering fuel from 2 places in the intake manifold. Do not stagger the idle jets (in other words, do not use 2 different size idle jets) even if you need to adjust the screws differently, as this can cause part throttle driveability issues."

Found the info at the following site:
http://www.classicinlines.com/WeberTune.asp

To answer Jerry's question: I believe I am chasing a lean condition overall, both in the idle circuit as well as the main jet for larger throttle opening conditions.

For the idle circuit, my clues come from reading the plugs, in that they appear a little too chalky/white for my tastes. This is not to say they are obviously way lean, its just that they seem to be more whitish than tan. Also, if you read the info on the link above, there is some reference that for 38s, the idle mixture screw may actually need to be set to about 1/2 the turns as is normally seen on other Weber carbs (and I have seen reference to this idea elsewhere). As such, I believe I may be a little lean on the idle circuit.

For the mains, my first clue is that both of my 38's are running 142 main jets. I find this interesting because based on the research I have done, the carbs come in the box with 145 jets. Furthermore, I cant even find 142 jets for sale for these carbs, so not quite sure what the deal is there.

The next indication regarding the mains is that when I swap the current NGK plugs (I think they are BP9ES but dont recall) for the Bosch Platinum +4 plugs, the car will not run well as I advance the throttle into the RPM range above about 3500. It basically starts to just fall flat on its face and hesitate and stumble...put the old plugs back in and runs just fine. So I think that the +4 plugs require a slightly richer mixture.

Received some alternate jets today so I will give it a try over the weekend and see how it all turns out.
 
Err..mmm... throw those platinums away. They don't work in a vintage motor.

Many have tried. My guess is that they really need a hotter burn as they just foul or never run right.

Because of ethanol and no lead, your plugs will be a whiter shade of pale.( Sorry couldn't help myself...old song :grin:)
 
Hmmm...wish I would have know that previously...the folks at Mesa spoke highly of the platinum plugs.

I may just do a little experimenting since I have some jetting options now.

Any thoughts about the current 142 jets for mains? The motor is an 1985 3.5L with some alternate cam that I know nothing about.

BTW, nice song reference...oldie but goodie.
 
Right - No Plat plugs.

I have used NGK BP5ES or BP6ES for upwards of 30 years in both my M10 and M30 engines - and worked very well.
 
I don't know why but platinums run crappy. Since I do a lot of stop and go driving I use nos german made Bosch W8DC so far with much satisfaction.

David said:
Interesting. What is this issue with platinum plugs, and what do others suggest ?
Click to expand...
 
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