38 weber tuning
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Thread: 38 weber tuning

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    1000 Post Club Dale .D's Avatar
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    38 weber tuning

    Eh Folks. Yes I've searched all these jets & more. Most detailed info has been on the 32/36 & not the 38 electric choke. I know it all depends on the motor but was hoping someone with something similar to mine has had success with with rejeting . My motor is a 1.9 that has been bored out to 2.0 in a gt. Combo cam from OGTS,Head work & Porting to Rally bobs specs with larger valves & hardened seats. Intake ported mildly To Rally Bobs expert advice as well.1 & 1/4 inch sport exhaust . Ignition is XR 3000 Crane & PS91 coil. Vacuum advance has been removed. Have not put stronger springs in ignition as yet & haven't played with the power valve yet either. Carb is all stock. Had a fire & it's just ready now to go into get re-inspected. Carb is adjusted now to roughly what weber has recommended. Still doesn't like to idle cold but fine when warm. Any & all advice appreciated. Thanks

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    Just Some Dude in Jersey The Scifi Guy's Avatar
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    Just like mine

    Your set up is almost mine exactly. My additions were the power valve spring cut back and my cam is a split profile style that has similar specs to a combo cam. My engine is a factory 2.0, sprint exhaust, stock exhaust pipe diameter, no resonator.

    I have these jets:
    170 airs
    50 idles
    140 mains
    F-66 e-tubes

    45 idles gave a bit more economy and 55's made it start more reliably when I experimented. 50's were the compromise.

    My car tears new butt holes in other people's cars, so I'm pretty happy.
    Last edited by The Scifi Guy; 01-30-2012 at 09:30 PM.
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    Quote Originally Posted by thescifiguy View Post
    My car tears new butt holes in other people's cars, so I'm pretty happy.
    ERRR you still have those twin 50 caliber's mounted on the GT?

    Si vis pacem, para bellum "If you want peace, prepare for war"

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    1000 Post Club Dale .D's Avatar
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    Ok so what your saying is change Idle from 45 to 50 or 55. change e tube from F50 to F66. Main jets from 145 to 140 & air connectors from 185 to170? . You were saying The idle jet started better at 55 but you went to 50 for better economy.Was the performance the same or does it only mean less gas use at idle. I have the sprint as well. Curious, how much did you cut the spring down on the power valve? What Ignition do you use. Sorry for so many Questions. Just want to learn what I can. Thanks

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    What's your idle vacuum?
    The ignition timing has a huge effect with vacuum.

    Get your inital close to about 10 BTDC then measure.

    Si vis pacem, para bellum "If you want peace, prepare for war"

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    Just Some Dude in Jersey The Scifi Guy's Avatar
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    To answer Dan's question, my idle vacuum is really low, like 5. That's mostly due to the split profile cam and the fact that I run automatics in my cars. When I'm in park or neutral, it's like 10-12 inches of vac, but when I'm in Drive and stopped, my vac drops to 5 or so. Kinda hard to tell exactly because the vacuum gauge in my dash bounces because of the lope due to the higher lift cam. With a stick you can have a less than perfect set up because there's no resistance when you're idling at a light, but with the engine fighting the torque converter when idling at a light you REALLY have to have things set up well or your car will stall when you touch the gas. That low vacuum I have probably has caused me to trim more of the spring back than most people.

    I'm curious about this part of spring cutting: Do you count that first wind of the spring, where it's flat and doesn't "rise" at all as a "full wind" of the spring? I don't think you should since if you cut 1/2 of the first flat wind or all of it the effect is the same since virtually none of the first wind actually "pushes" on the valve.

    If you count the first wind as a true full wind, then I've cut about 2 1/4 winds off....so far. If you only count it as a 1/2 or 1/4 of a wind, as I feel you should, then I've trimmed 1 3/4 winds of the spring off. I think I still need to chop another half wind off.


    My jet choices came from the responses I got from starting a thread on the subject just as you have just done. Check my profile, select see all started threads, and scroll through and you might find the thread.

    First off, you have to install F-66 emulsion tubes to set it up for an Opel. Period.

    Now, as far as all the rest of the jets I chose: They're within the range that other people with similar set ups use. As RallyBob, Wrench, and other will surely tell you: Everyone's engine is different and you may have to slightly change your jets, larger or smaller, to achieve perfection. If I understand it correctly there is only ONE perfect air/fuel ratio target point. Regardless, of how cool you've ported and flowed your engine and whether your ignition shoots lightning bolts into your engine, there's still only one ratio that's optimal. All flowing and porting does is get that perfect mix in and out of your engine more efficiently and with more volume.

    Your idle jets are for idling. Too big and it's tough to get a good lean setting and you'll waste gas when you're up to speed. Too lean and there's starting problems and stuff.

    I suggest getting an assortment of jets within these ranges:
    130-145 mains
    160-180 airs
    45-55 idles

    Then start with my set up, which you now know is a proven, Scifi Guy Approved, set up and then you can experiment from there.

    And let me conclude that I'm no rocket scientist when it comes to this stuff, despite being "thescifiguy". I've only gotten a handle on this junk within the last 3 years pestering guys on the site. I just want my engine to go and not worry about whether I'm as perfect as can be. I'm just trying to help out the REAL rocket scientists on this site by taking some of the workload off their shoulders.
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    Thanks for some good discussion on 38 tuning.
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    Quote Originally Posted by thescifiguy View Post
    ... That low vacuum I have probably has caused me to trim more of the spring back than most people.

    I'm curious about this part of spring cutting: Do you count that first wind of the spring, where it's flat and doesn't "rise" at all as a "full wind" of the spring? I don't think you should since if you cut 1/2 of the first flat wind or all of it the effect is the same since virtually none of the first wind actually "pushes" on the valve.

    If you count the first wind as a true full wind, then I've cut about 2 1/4 winds off....so far. If you only count it as a 1/2 or 1/4 of a wind, as I feel you should, then I've trimmed 1 3/4 winds of the spring off. I think I still need to chop another half wind off.
    Very nice post there Gordon!
    Lets see if we can go deeper with the power valve.
    The 38 power valve will need to open at the proper time. IIRC it opens around 10 inches.
    Start off by just removing one full coil. Then remove 1/2 coil at a time to fine tune.
    Always use a new power valve diaphragm.

    First off I've never have ran a choke so keep that in the back of your mind.
    Last edited by wrench459; 01-31-2012 at 06:58 PM.

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    Great info. Just need a starting point to go from. Should the power valve spring adjustment be done after you have the set up the way you like it or does it matter?
    Quote Originally Posted by thescifiguy View Post
    To answer Dan's question, my idle vacuum is really low, like 5. That's mostly due to the split profile cam and the fact that I run automatics in my cars. When I'm in park or neutral, it's like 10-12 inches of vac, but when I'm in Drive and stopped, my vac drops to 5 or so. Kinda hard to tell exactly because the vacuum gauge in my dash bounces because of the lope due to the higher lift cam. With a stick you can have a less than perfect set up because there's no resistance when you're idling at a light, but with the engine fighting the torque converter when idling at a light you REALLY have to have things set up well or your car will stall when you touch the gas. That low vacuum I have probably has caused me to trim more of the spring back than most people.

    I'm curious about this part of spring cutting: Do you count that first wind of the spring, where it's flat and doesn't "rise" at all as a "full wind" of the spring? I don't think you should since if you cut 1/2 of the first flat wind or all of it the effect is the same since virtually none of the first wind actually "pushes" on the valve.

    If you count the first wind as a true full wind, then I've cut about 2 1/4 winds off....so far. If you only count it as a 1/2 or 1/4 of a wind, as I feel you should, then I've trimmed 1 3/4 winds of the spring off. I think I still need to chop another half wind off.


    My jet choices came from the responses I got from starting a thread on the subject just as you have just done. Check my profile, select see all started threads, and scroll through and you might find the thread.

    First off, you have to install F-66 emulsion tubes to set it up for an Opel. Period.

    Now, as far as all the rest of the jets I chose: They're within the range that other people with similar set ups use. As RallyBob, Wrench, and other will surely tell you: Everyone's engine is different and you may have to slightly change your jets, larger or smaller, to achieve perfection. If I understand it correctly there is only ONE perfect air/fuel ratio target point. Regardless, of how cool you've ported and flowed your engine and whether your ignition shoots lightning bolts into your engine, there's still only one ratio that's optimal. All flowing and porting does is get that perfect mix in and out of your engine more efficiently and with more volume.

    Your idle jets are for idling. Too big and it's tough to get a good lean setting and you'll waste gas when you're up to speed. Too lean and there's starting problems and stuff.

    I suggest getting an assortment of jets within these ranges:
    130-145 mains
    160-180 airs
    45-55 idles

    Then start with my set up, which you now know is a proven, Scifi Guy Approved, set up and then you can experiment from there.

    And let me conclude that I'm no rocket scientist when it comes to this stuff, despite being "thescifiguy". I've only gotten a handle on this junk within the last 3 years pestering guys on the site. I just want my engine to go and not worry about whether I'm as perfect as can be. I'm just trying to help out the REAL rocket scientists on this site by taking some of the workload off their shoulders.

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    Quote Originally Posted by Dale .D View Post
    Great info. Just need a starting point to go from. Should the power valve spring adjustment be done after you have the set up the way you like it or does it matter?
    Yes it does matter. You'll need to set the sucker up fat on the bottom end to
    keep it from stumbling.
    I had to pour on the fuel at the lower rpms then lean the mixture out for power.
    fuel curve.jpg

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    Just Some Dude in Jersey The Scifi Guy's Avatar
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    It's been said to me that "every Opel engine with a Weber downdraft carb needs to have the power valve spring mod" because Opels have inherently weak idle vacuum. 32/36's and 38's need to do it to one extent or another. But, as many people, including myself know: Your Opel will still run without doing it....just not as well. That mod is mainly to get rid of the "stumble" when you first touch the gas after a stop. I would say do it last.

    As Wrench said, getting the timing right FIRST, before monkeying with the carb is essential. But sometimes a person's carb is so out of whack that you can't time it until you get some of the carb stuff right. Compression and vacuum leak issues also play a part.

    I'm not the guy to ask about timing. I've read all the threads, put marks on my pulley, used a timing light, and it's still a complete mystery to me as to what constitutes perfect timing and how to determine if it is. Once you start adding fancy cams and mess with the mechanical and vacuum advances the rules seems to fly out the window.

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    Quote Originally Posted by thescifiguy View Post
    It's been said to me that "every Opel engine with a Weber downdraft carb needs to have the power valve spring mod" because Opels have inherently weak idle vacuum..
    Not only with the downdrafts but also fuel injection!
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    Last edited by wrench459; 01-31-2012 at 09:32 PM.

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    Simple way to establish your power valve tuning. Get the engine running as well as possible, then measure the vacuum at idle. This means that your ignition timing and idle speed need to be correct for your specific combination.

    Now, the stock power valves on a 32/36DG-series or 38 DG-series carb opens at about 14" of vacuum. If your engine idles at 15"-16" of vacuum or higher...leave the power valve alone!

    If your engine idles at or below 14", you will need to modify the power valve. Cut ONLY enough coils so that the power valve opens 2" below your existing idle vacuum level. Example: Your big-cammed Opel idles at 9" of vacuum. Your power valve should open at 7" of vacuum. Using a vacuum gauge to test, cut the coils in small increments until that value is reached.

    That's it.

    This is the vacuum port to test the power valve 'cracking' point.
    Using a vacuum tester held tightly to this opening, pump the tester slowly until the power valve spring *just* moves. Note the vacuum level on the tester's gauge. That is the cracking point when the power valve begins to open.
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    Quote Originally Posted by RallyBob View Post
    Simple way to establish your power valve tuning. Get the engine running as well as possible, then measure the vacuum at idle. This means that your ignition timing and idle speed need to be correct for your specific combination.
    I think that Bob would agree that's more to this than meets the eye.
    Back when I was doing tuning I would never leave the ignition timing close.
    In fact almost always I kicked it back into a very safe range..then played with the fuel.
    Only when I was satisfied did the spark timing get advanced.

    Dang tuners...

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    I've been messing with my 38 for the last week or so, trying to fine tune it. I have Bob's 2.0 L recipe engine. I wasn't happy with the transition from idles to main and once I got that smoothed out I addressed the stumble on accel after decel, not from a stop. I lowered the accel pump jet to correct this. The following is what I ended up with:

    60 Idles
    150 Mains
    170 Air Correctors
    55 Accelerator Pump
    F66 Emulsion Tubes

    I just got back from a spirited Saturday morning drive and am now very pleased.
    N61WP and wrench459 like this.

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    Quote Originally Posted by jmbinjax View Post
    I've been messing with my 38 for the last week or so, trying to fine tune it. I have Bob's 2.0 L recipe engine. I wasn't happy with the transition from idles to main and once I got that smoothed out I addressed the stumble on accel after decel, not from a stop. I lowered the accel pump jet to correct this. The following is what I ended up with:

    60 Idles
    150 Mains
    170 Air Correctors
    55 Accelerator Pump
    F66 Emulsion Tubes

    I just got back from a spirited Saturday morning drive and am now very pleased.

    Jmbnjax,

    still workin out for you?

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    I've been doing the same thing over the last few days, and I've come to realize what a difference jets can make!

    I tried some of the jet sizes recommended by others here, but using the factory methodology told me that the idle jet was too lean, which is surprising since I'm at 3,400 feet elevation. Anyway, here's what I ended up with, and the car now purrs like a kitten again.

    Primary: Main 140, Air 170, Idle .65
    Secondary: Main 140, Air 160, Idle .55

    No spring or emulsion tube mods here, and I have no stumble on acceleration, even when the secondary opens up.

    I found this guide to be very useful:

    Weber Tuning Methodology
    Last edited by zippy7575; 09-12-2013 at 10:49 PM.
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    Quote Originally Posted by zippy7575 View Post
    I've been doing the same thing over the last few days, and I've come to realize what a difference jets can make!

    I tried some of the jet sizes recommended by others here, but using the factory methodology told me that the idle jet was too lean, which is surprising since I'm at 3,400 feet elevation. Anyway, here's what I ended up with, and the car now purrs like a kitten again.

    Primary: Main 140, Air 170, Idle .65
    Secondary: Main 140, Air 160, Idle .55

    No spring or emulsion tube mods here, and I have no stumble on acceleration, even when the secondary opens up.

    I found this guide to be very useful:

    Weber Tuning Methodology
    Awesome! you have the 32/36 though right? what was it jetted before?

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    Sorry... just realized the topic was the 38. Yes, mine is a 32/36. I started out with the factory jets, which I think is

    Primary Main 140, Air 170, Idle .60
    Secondary Main Jet: 140, Air 160, Idle .50

    Then tried this:

    Primary: Main 135, Air 170, Idle .50
    Secondary: Main 140, Air 160, Idle .55

    then this:

    Primary: Main 135, Air 165, Idle .50
    Secondary: Main 140, Air 160, Idle .55

    Then to what I have now. So basically, I ended up back at the factory-supplied jets except that I upped the primary and secondary idle jets each up one size.
    Last edited by zippy7575; 09-12-2013 at 11:35 PM.
    Sunburst yellow 1970 GT, 2.0L w/ header, Tremec T5, Weber 32/36, Pertronix, Sway Bars, BWA Sportstar wheels. Owned since 1973.

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    Mods to the full power valve spring tension

    Food for thought, I don’t have the 38 I’m running the 32/36 but the same power valve is in both carbs I believe. In trying to get rid of the off idle stumble the other day, I had an understanding that the manifold vacuum keeps the power valve off, I’ve never had an issue with it at idle 17-18” and wondered how in the world trimming springs would have any effect in helping. I had an extra cover and thought what the heck. It didn’t cure the stumble but what it did do was let the carburetor run a little longer and leaner on the highway before the power valve would kick in. Easy to see on my afr gauge. So I got to wondering after reading the post I didn’t read before having a spring trimming party, Bobs post #13 that gives an excellent explanation of what to look for when doing anything to this spring. So I went back the new cover again and the afr returned to what it was at highway speeds. So I got to wondering, if I can add a spacer or two to slightly increase the spring tension shouldn’t that cause the power valve to come in a bit earlier under these same conditions? If the factory power valve spring is set at 14” I’d have a little room there to bring in the full power valve a bit earlier if that theory holds water. I’d have to double check the vacuum reading when it opens first then see if there’s any room. Has anyone else tried this? I also now realize that the power valve had little split plastic washers (spacers) at the bottom and I probably didn’t even need to trim the 2-3 turns that I did. It doesn’t matter it’s a spare power valve anyway. I don’t think this will help me with the off idle stumble in fact if I keep it under 16” it shouldn’t be any different just possibly get the power valve to kick in a little sooner on the highway going up those hills where I live. I’m reasonably happy where it is but just curious it seems like a harmless experiment.

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