Jetting a Weber 38/38 for a 200

[kerb12]

Hey all, I have a freshly rebuilt '66 200 in my mustang with a mild Comp Cams camshaft, a duraspark 2 ignition, headers, and I'm trying to run a Weber 38/38 on an adapter (I know, I know, but don't want to machine the head yet for direct mount). Anyway, I'm having a really hard time getting the carb to idle correctly. I see there's some stickys on jetting the 32/36, but just curious if anyone is running the 38 and might have some advice.

It seems like no matter what I do, I can't get it to idle at a reasonable rpm unless I have the idle speed screw in like 3 turns, and the documentation says if it's more than 1/2 turn then it's running off the mains and there's a problem. The idle sizes from the factory were 45's, which is probably for a 4 cyl and way too small. I upped them to 60's but couldn't get it to idle unless the mixture screws were practically all the way in, so backed it down to 55's and now the mixture screws are about 1.5 turns out, which is what they recommend.... but my idle speed screw still has to be in 3 turns... so something is still way off.

Any ideas on what I'm doing wrong?? Is it just too much carb for the small log head?

-Nick

 

[xctasy]

See Falconfanatics 38 jetting


viewtopic.php?f=1&t=49901

 

[kerb12]

Oh thanks, didn't even notice there was a 38 in that chart.. Oddly enough it seems like he kept everything pretty close to stock. Idle and air correctors are the same, only bumped the mains up slightly.

Just an update, I read somewhere that having to have the idle speed screw in too far like I do is often the sign of a vacuum leak. Not sure how true that is, but next step is to start spraying down all my seals and see if I can find a leak anywhere.

 

[xctasy]

The Weber hates vac leaks...for some reason, a Holley 1946 1-bb will operate even with two loose carb manifold bolts and not show any ill effects.

As background on jetting, see

viewtopic.php?f=3&t=66647

Plenty of Aussie 2-bbl cross flow Falcons ran the intermediate carb, the 2 liter twin cam Fiat 125/132/131 based Weber ADM 34, a staged 32/36 type carb with 27 and 28 mm venturis, very much a 121 to 147 hp carb. It was used on the commerical utility pickups from 1992-1992 and domestic sedans and wagons from 1982 to 1987.

I think the stock 4.1 Oz figures for XE were

Primary Idle: 60
Secondary Idle: 70
Primary Fuel: 160----->180
Secondary Fuel: 170----->195
Primary Air: 160
Secondary Air: 180


So be confident that a synchronous 38 DG series carb will take any thing from 45 to 60 idle jet for sure.

 

[falcon fanatic]

Hi,
I can't remember what I'm running now, but I think the numbers on that chart are pre-rebuild/pre-cam. In the end I found that the only way to get a really good jetting was by using a wideband O2 sensor.

 

[falcon fanatic]

I will say this: you can go a bit lean on the primaries (until you get some hesitation on launch) and a bit rich on the secondaries for extra power. My mileage, by the way, has been awful with this carb. Around 13mpg, which really isn;t worth it for what probably amounts to a 145 hp motor.

 

[xctasy]

The 38 DGES is a synchronus carb, not staged, and no different to a Holley 2-bbl 350 cfm. Unlike the Holley, its finely tuned not so much by power valve as by the emulsion tubes, the idel air bushings, and then the fuel and air jets. The Holley works more by jet, power valve, accelerator pump and then power valve channel restrictions. You rarely have to get into idel air bleeds and emulsion tubes on them, but you do on Webers, or you get problems with the idel, progrssion and main circuits working properly to mask air fuel mixture holes.

The idle air bushes can be replaced with 145's, and the main jets can then come down to 145 or less to get a leaner mixture.

Wide bands sounds all very grand, but an O2 sensor can have you running after some illusionary numbers, not testing for feel and conditions. Its nice to have all the facts, but you only need to ensure your not runing too lean or too rich. If mpg is lousy, you'll be tuning for leanness all the time. The simple way to jet is the method taught by a Californian dude on the Jeep forum, and its basically how the Italians sucessfully tuned down draft two barrel Weber for Fiats. It was a method copied by Chrysler Australia for the triple carbed 265 D-series Hemi Chargers. Basically, you take a car, and drive it under various hill loads untill you eliminate drivabilty problems, then cross check for rich or lean conditions.

 

[kerb12]

What about tuning by vacuum? That's what I've been trying to do so far, at least to get the idle circuit working well. I was thinking once the mixture screws are set well there, then i'll replace the main jets by feel like you suggest by just driving and see what feels best.

 

[xctasy]

The best method is probably the Jeep forums swatson454 on speed talk.

See http://www.jeepforum.com/forum/f8/weber ... n-1051137/

Please note that this is name dropping or profiling people with simialar ideals to me, its just an avenue that needs to be persued. I spent 500 bucks on a 1981 Sun engine analyser, and have the capaility of teeing into my 81 Mustangs O2 sensor, but its diluted by EGR, the other emissions circuts, and its all a bit of a lotery looking only at air fuel ratios. You'll probay be ware I've spend plenty of weeks mapping my old 98 Explorers air fuel ratios, so I;m not a luddite really, although I do have some aversions to some electronics. I use an air fuel logger on my Campbell Scientific CR1000, a 16 bit computer or some smarts.

swatson454's process was to use basic science and then cross check it with real world driving. Mapping an EFI engine can be done on an lambda sensor, but that's because the Thick Film Ignition or EDIS variants are used, and you can do everything on the fly. Most times the air fuel and ignition are interconnected, so running a lambie sensor like wsa111 does can be a huge advantage. I'd say people like Bill are a lot smarter than they let on, and in wsa111's case, being a Lexus dealer has hined his skills to leading edge. My thinking is that its better to gain some basics then build on them so you don't end up using the lambie rope to hang yourself. And you can when you keep running leaner without base lineing your ignition first.

With carbs, the ignition is usually set, so running around with a lambda sensor reading to xx.x is a pretty danagerous thing to do, and it doesn't help with trimming the mixture since you focus on the numbers, and not the conditions that cause it. You have to Loius Barillle or Helen Kellar the engine, and keep away from just the Inovate readings, as they are subservient to ignition and dynamic loads.

 

[kerb12]

Wow, great thread, thanks for the link. I really like the idea of popping the carb off and looking from underneath to see exactly where the idle speed screw should be at max... Much better than just hoping you have it at the right place and not exposing the mains. Also interesting to see that they recommend NOT tuning for highest vacuum, that it actually causes stumbles, and that it needs to run a little lean.

 

[falcon fanatic]

The issue I'm having right now is that if I run what I consider to be the correct idle jets the engine stalls on left turns. I have to run one step (at least, possibly two) more rich than I need, making for a smelly situation and poor mileage. I had it nailed when I lived a sea level, but since I moved to the boulder area (5200 ft. elevation) I just can't get that perfect tune. I presume this is an issue that I can alleviate by adjusting the float level, which is presently set dead-on factory recommended, but am open to any and all suggestions. I know other people are also having left-turn death issues with their side mounted carbs, but like I said, I had been able to tune that out at sea level.

 

[kerb12]

Yeah I've heard a lot of people have that problem.. my car isn't at the point yet where I can keep taking it on drives to see if I'm going to run into that either, but I'm hoping it doesn't. Which way do you have the carb mounted? Is the fuel bowl to the driver side or passenger side? At least that way you can try to determine if you're starving the engine or flooding it... I've read people say that having a fuel pressure regulator helps to avoid the stalling by keeping the flow more consistent, but that it doesn't completely solve the problem. Probably something worth trying out though.

 

[yoozeq]

kerb12 , is your car runnig well on this Weber 38 / 38. I also have 200 c.i. '69 + headers + 260H Comp Cams camshaftt. I already ordered DUI and Weber 38 / 38 (cllassicinlines). I would also run this carburator on a adaptor. Can you give me some info what jest etc.

 

[kerb12]

Well, it runs... but I wouldn't say well. It's still running very rich, but if I lean it out anymore it stalls. My idle speed is also still screwed in too far, which means I'm idling off my mains too, so I'm thinking that's why it's so rich. This is with 55 idle jets. I'm going to try to go back to 60's and see if I can get that working. I can definitely say the 55's are much better than the stock 45's, but I can tell it still just isn't running as well as it should.

Aside from that, I haven't been able to take it on the road yet to see how it is once I'm past idle since I've been working on the interior the past few months, but I believe I put 155 mains and 180 air correctors in as an initial setting.

I also just ordered new plugs and an Accel "duraspark racing" coil, so I'll see if that makes any difference in idle quality. Hoping to give that a shot this week.

 

[falcon fanatic]

Update: Found that recurving my distributor so that I can run 18* initial with 34* total gives me a great idle, good power through the powerband, and alleviates the left-turn stalling problem. Also, with that much advance the previously "too rich" primary jets are now just about right. Basically, unless you are pulling at least 8 to 10" of vacuum it will be hard to get that weber idling smoothly.

 

[yoozeq]

kerb12: Thx for your info. Let me know, when you go on the road

I have head from '70 (bigger valves etc.) and I thinking about bolt on job for this carb. But at the beginnig I will try your setup

falcon fanatic: Don't you think that 18* initial it's to mutch - is it safe? I have 12*.

 

[kerb12]

18* with a duraspark? Did you do that Mr gasket spring recurve?

 

[falcon fanatic]

It is a DUI, not a duraspark, but the same principles apply. I had a bunch of weights and springs of various brands and an adjustable timing light and went from there with trial and error. If I recall correctly, the duraspark has weights with the amount of advance per weight stamped on it. So, you can really plan how much advance you want.

Do you have an automatic transmission, or a manual. With the mild cam you have and a manual, I'd think 12* inital would be fine. I presume you've followed all the steps for "lean best idle setup" for you carburetor. If so and you still can't get it to idle right, attach a vacuum gauge and see what you are getting. The weber starts to work best above 9" or so, in my experience, and advancing your timing may be the only way to get there.

No, I don't think 18* is too much, although in looking at my notes it's actually 17*. I don't start getting decent idle vacuum until at least 15*. Having exhausted all options for setting up the carb at 12* advance and still getting very little vacuum (like 6" - I'm not the only one struggling with this cam, by the way), the next logical option was timing. I started dialing in timing until things started working right, and then worked on the advance curve to limit my max advance to something safe.

My present curve looks like this

idle 700rpm - 17* - 9" of vacuum
1000rpm - 22*
1500rpm - 24*
2000rpm - 27*
2500rpm - 30*
2700rpm - 32*
3000rpm - 32*

So, I could use 2* - 4* more maximum advance and I'll keep working on it, but with this basic setup my car idles great (with a c-4, stall converter), and I have much more power from idle through 3,000 rpm than I did before. The car pulls strong all the way to 5,500 and then the log head starts to gasp. I figure with a better head this is a good 6,000rpm setup.

 

[kerb12]

I have an automatic, and I'm currently set to 12 initial with stock duraspark curve. Vacuum is above 9, maybe 13-14, but my cam isn't too crazy, I would think it should be higher than that. Maybe I'll try giving it a little more initial advance and see if I can get the idle speed and mixture screws back to where they should be. I'm also running the PCV, which has a big effect on carb settings, and maybe that's part of my problems... I've read mixed things on it - some swear you should use it since it helps the engine clean itself and others say it acts as a vacuum leak and screws up the Webers.

 

[yoozeq]

I already removed PCV and connected hose from valve cover to oil catch can. Seems to work fine, vacuum is better and carb is cleaner. At the moment I have Holley 1940 bolt on, but when I set up initlial for 18* I also notice better vacuum. So the total timin shouldn't be more then 36* ?

Could you give me a link to this "lean best idle setup" ? I will check my way to carb setup.

Mine is 3 spd. manual.
On 12* and 700 rpm I have 0,45 bar

Parts on the way ( DUI and Weber 38/38), after three weeks should be on place