New Weber 40 IDFs: sputtering in Ethiopia

[rik-shaw]

Sorry guys, meant to say that the Uni-Syn almost kills the engine when using. I can check that it does the SAME to all 4 barrels, but pretty sure it does. At first (2 weeks ago now) I did have the brake booster unplugged and that did cause the Uni-Syn to have the engine race when on that cylinder. But all should be good now and indeed the Uni-Syn is nearly killing the engine when attempting a measurement.

Second, yes I checked the air bypass screws. Turned in all the way.

John wrote back saying that as long as there is no vacuum at idle the "1/2 turn in" on the throttle screw max limit is bunk (meaning that it can be more, as I have found I need). I didn't check if I have vacuum at idle or not. Should be able to check my hose going to the distributor from cyl 4 idle port and could unscrew cyl 1 idle port to check there.

So he is telling me again to to start at the beginning:

[57BLITZ]

[rik-shaw]

Just got home today from driving it back and forth to work (with my "temporary settings" of mixture screws backed out to 3-1/2 turns out).

First off, it runs GREAT at higher rpm (3000+?). Plenty of power.

Second, it runs decent at idle.

Third, it is a bit weak (but NO detonation) at 1500-2400.

Re-reading all the setup advice on these carbs, it SEEMS that people find it has too small of idle jets because when you try to find "max idle" from the mixture screw adjustments you are out more than 1-1/2 turns.

But for me I find max idle around 1-1/4, but it definitely seems LEAN from 1500-2400 (detonation in the carbs, sputtery, etc.). So is this why John keeps telling me I am "doing it wrong"? Hard to understand, but I am wondering about TIMING and VAC. ADVANCE on the dizzy.

Can someone confirm to me what sort of timing I should have? I have been going with about 28 BTDC w/ vac advance DISCONNECTED. What readings should I then get with the vac. advance re-connected? Is there a chart somewhere? I would love to check it at 1500 / 2000 to see if it is where it should be. Note I am at 7500 FT so keep that in mind and help me confirm what adjustment for altitude I should make (I had been under impression I add 1deg for each 1000 above 4000, meaning add 3 to the 25-28 recommendation I had before: meaning I could advance it a bit more but was uncertain if I really should do so: the same recommendation said if my idle timing was larger than 11 I should lower it back to the 25 range, thus 25 + 3 for altitude is back at 28 )

I did replace the vac. advance on the distributor last year.... it is definitely "functioning" but what settings should I be seeing?

Thanks for helping me sort this out....

Rik

[dukewagon]

it sounds like you are getting closer. i was going through the same thing. i had the cheap syn tool. bought a snail tool. ($35 from amazon) adjusted one of the carbs less that a half turn on the idle, and it made a ton of difference. just saying.

[modok]

To guess the max advance it will like need to have some idea of the compression ratio and quench clearance(or deck height as some call it)

[FreeBug]

What news?

[sled]

ok so I re-read this thread twice to try and get the best grip on your issue.

What john is saying by 'doing it wrong' is that it appears you do not understand that the idle mixture screw has almost NOTHING to do with anything above IDLE. meaning once you crack the throttle at all, it exposes the first progression port which activates the progression circuit. These two things are NOT the same and are not necessarily direct cause/effect. Stop being concerned with how it idles, and only focus on the progression circuit. as you are finding out, with any load (driving the bus) it sputters and bucks until the main circuit kicks in (approx 3000rpm in your case depending on LOAD). this shows you the engine is running lean on progression circuit. definitely too-small "idle jet" (I think they should actually be called progression jets). When you clutch it an rev the engine, you are covering up that lean hole with accelerator pump squirt. This works to get you around, but it can also result in a flooding situation, which sounds like might have happened on that round-a-bout when you had to pull off and the engine was slow to start again.

the idle mixture screws only affect idle. you could get it to idle well with a 40 idle jet as well as a 55 idle jet, but that does not mean both jets will perform well anywhere off idle. by cranking the idle mixture screws out as far as you have, it allows enough extra fuel that the lean progression circuit can sort of function.

not sure if any of that makes sense?

make the investment and purchase a set of 47.5 and 50 idle jets. as well as a 'snail' sync gauge. you may also want to invest in some main and air correction jets if you want to chase the best possible tune.

oh and mentioned previously, make SURE you have no intake leaks. this will definitely fuck with any attempt at tuning.
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drive your split.

[rik-shaw]

First, thanks for the reading (and re-reading) of my lamentable problems over here and giving great help.

Second, I am no longer detonating at the 1500 - 2500 RPM. I realized I had absent-mindedly messed up my geometry so that while the carbs were even when closed, they were opening up at different speeds (I had re-centered the crossbar pivots a bit and screwed things up). So I re-did the linkage lengths so they are both opening evenly all the way up to the throttle stops. Then I got out my 12mm meter of vacuum tubing to use as a stethescope again and re-balanced the carbs (since the uni-syn does NOTHING good for me). The result is that the detonation is totally gone on a warmed up engine all the way from idle up to 3000 RPM. So for those guys saying "are you sure the carbs are balanced at idle and at speed??" you win the $64 prize and you are correct I didn't have things set right.

Third, with more road tests I can still confirm that there is NO detonation happening. However, from RPMs approx. 1000 - 2200/2500 it still is weak. After those shifts it is still not strong until the RPM creeps up (slowly!) and then once it hits maybe 2200 RPM it will shoot off like a rocket. This is particularly noticeable on a long incline, as I need to shift (so do it "late" in order to keep up the RPM but after shift it is still low enough it will lug along for a bit until the RPMs creep enough to "take off" again). So does this again confirm that my idle jets should be increased in size? I think so. I mean it COULD be something with the vacuum advance, but remember this thing is really running great at 2200+ RPM so at least by that time I am getting proper advance so it seems. MAYBE my advance isn't following the appropriate curve or something? But this seems trickier to diagnose and other things seem to point to undersized idle jets, right? Maybe next year I get a mechanical advance distributor or something, but for now idle jets??

Fourth, this may mean nothing but when starting in the morning the carbs *will* detonate in the throats for a minute or so while I hold the gas pedal at around 1500 rpm to get them warmed up (again no chokes here). Is this further indication that the idle jets are too small or maybe this doesn't mean anything?

Fifth, YES I will be trying to get a snail sync meter over here with the idle jets I think I need to buy. As sled indicates maybe I should get 47.5 (what I believe I need) as well as 50. I am inclined to agree even though John says "no way I need 50s", because the pain involved here is in shipping anything over... the cost of an unneeded set of jets is minimal and is worth the insurance to me.

FINALLY, not sure if I should include a different set of main jets or air correction jets for additional insurance.

Thanks again from Ethiopia (now driving it daily but just need to make sure an not shift early on hills).

Rik

[sled]

Ok here's the deal. Warm the engine up to full operating temp (drive it around for a while and pull some hills) then remove the main jet stacks completely. Then go drive around town for quite a while to show you how the isolated progression circuit is working. Say for example you're cruising around at 35mph in 3rd gear on flat ground..this would be LIGHT load. If it is sputtering or bucking at ALL, your idles are too small. But this is not the only scenario to check. The point is to learn how the progression circuit (fed by idle jets) behaves on its own, without the main circuit possibly feeding it. Also, it is best although not crucial, to back out your accelerator pumps so they are also not feeding the progression circuit.

Now, you will find that it will fall flat on its face at some point, probably 2500ish rpm depending on LOAD. This is where the progression circuit can't keep up with the demand and the mains would need to kick in..this is FINE because you have the mains removed. But, if it is smooth as silk UP TO THAT POINT, then your idles may be ok.

My point is, then you can start focusing on the TRANSITION from progression to main circuit, which is what it kind of sounds like your problem is. Meaning there is not enough vacuum to pull the fuel from the main wells soon enough..resulting in a lean (weak as you call it) spot until the mains finally dump fuel and you're off like a rocket! This is a function of your main venturi sizing, your air jet sizing, but most importantly your emulsion tubes!!

Starting to get quite a bit more complicated here but sometimes that's what it takes to properly tune a set of twin-choke carbs (choke meaning venturi)


Also, it is completely normal for the engine to pop/sputter on warm up because the fuel is falling out of atomization resulting in a lean condition. It goes away when everything warms up and the fuel can stay combustible.

I know it's more expense, and I'm sure everything is a bit more expensive over there, but this is where a widband/o2 sensor would tell you exactly what you need to do without all the guesswork.

Yes, having extra jets is never a bad thing considering they're not very expensive
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drive your split.

[modok]

[sled]

[Clatter]

I could also see fuel quality issues in Ethiopia?

If the fuel has different properties than what we are used to in the states,
It could very likely change what is recommended/needed.

Say, for example, that the fuel is 'weaker' (a.k.a. 'watered down'),
then a correspondingly larger jet will be needed to get things right.

Be ordering a couple of sizes bigger jets for just this reason.

I could also see a different emulsion being your 'silver bullet'.

Sounds like you're on the right track.
Keep going!

WRT vacuum advance.. Full advance, all in.
When the hose is off, your timing should be around 28-29, and solid/steady.
When the hose is on, you will see a bigger fluctuation in the timing,
But you should also see a good 8-10 degrees additional advance - up towards or around 40 even sometimes..

If you are only seeing another few degrees advance from your can,
It might be stiff, or stuck, or wrong, or hanging up, or..?

On a type 4 with a cam in it, and in a bus.
Some like to tweak the stops on the distributor to get more idle/initial.
Making sure to keep full-in/hose off at 28.
I like to give it 10 or even 12 initial, to help make it come in earlier, and more tame to drive down low before the advance kicks in..

FWIW..
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