Seeking Validation On Jetting Change

[brianstj]

Specs first -

FRESH motor (less than 500 miles)

E12 Head

9.5:1 Pistons

292 Cam

Step Header w/ 2" exhaust

Stock (vacuum advance) distributor with pertronix

I believe the timing and valve lash is set properly

I am running a 38/38 Weber DGES with the following jetting:

185 Air

142 Main

45 Idle

I've been having idle issues. I set the idle by the Redline instructions, ended up at about 1-3/4 turns on the mixture screws. I set the idle speed around 1000 RPM - ran fairly rough, which I attributed to the cam at the time. Otherwise the car ran very strong and smooth above 2500 RPM. Coming to a stop the idle would stick around 1500, sometimes eventually coming down to the 1000 rough idle. I am very sure the linkage was not sticking - I could duplicate this with my head under the hood, and verified the throttle was all the way against the stop. I also hunted around for vacuum leaks with a stethoscope, but I couldn't find any strange hissing anywhere. And actually pulling the vacuum hose off of the distributor did not seem to make a noticeable change at the high idle or the lower idle.

I went back to the beginning and readjusted the idle based on the Redline instructions. I found a much smoother idle at 2 to 2-1/2 turns on each idle mixture screw, and it took about 2 turns on the speed screw. The idle was MUCH smoother. The car felt much smoother below 2500 RPM - but, I was then starting to get a bit of a stumble at higher RPMs under a load.

According to the Redline instructions, with 2+ turn on the mixture screws and 2 turns on the speed screw I am running lean on the idle jet. They say anything above 1-1/2 on the mix and 3/4 on the speed indicates the jet is too small. I am thinking I should go to a 55 and redo the idle adjustments again. Anyone have any other advice or similar experiences? The main thing I am feeling weirded out about at this point is the higher RPM stumble I was getting once I richened up the idle. This definitely was not there with the leaner idle setting.

Any suggestions welcome!

[Guest Anonymous]

I think your instincts are correct. I run stock cam with 10:1 compression and a 38/38 and 60 idle jet. Idles fine at 600 rpm I recall running F11 emulsion tubes - which made a big difference with progression issues along with the larger idle jetting.

Another thought: You are obviously too lean at idle. You might want to try widening the gaps on your plugs to .040 or even bigger - on the off chance the larger gap will ignite the leaner mixture.

hth

hth.

[TobyB]

Yeah, up your idle!

Umm, I mean, bigger idles will probably help.

And the reason is, the idle mix is actually set by the screw,

but it draws through the transition jets (whuich we all call idles).

Also make sure your dizzy's really good-

hunting can be a sign of loose springs or worn pivots

in the mechanical advance.

t

[Chris_B]

What do you mean by "higher RPM"? The idle circuit has no effect on high RPM or WOT operations. Further, the idle mixture adjustment _only_ has an effect at idle speeds, not on any higher RPM operation. Changing the idle jets could cause a stumble in light throttle/light load conditions during the operation of the transition circuit, but you haven't changed the idle jets yet, right? If you have a new stumble at higher RPMs under a load, it has nothing to do with the idle mixture adjustment.

[c.d.iesel]

re check your valve clearance - too tight will effect idle

greatly

re check your ignition timing - with a light - REALLY

with your idle too high - it begins to feed the mechanical

advance to hang and creep up higher.......

So ignition timing set correctly - in your case "BALL " mark

in the inspection hole at 1500 rpm, idle speed at 800 ,

and THEN adjust the idle mixture screws - all the while

maintaing the 800 rpm idle speed.

THEN if your idle mixture screws are too far IN, or

too far OUT, begin to try different IDLE JETs

and as stated above - your 3/4 to FULL throttle

performance is not on the idle jets - but now with

the MAIN and AIR CORRECTION combination.

no timing light ? then settle with what you have

now because your just chasing your tail with guess

work and not realising any power gains with all

your Hi-PER motor parts/rebuild

[Guest Anonymous]

the idle mixture adjustment _only_ has an effect at idle speeds, not on any higher RPM operation.

Your comment is a bit of an overstatement. The thoughts of a man named Bernoulli has a lot to do with it.

It is true that the idle circuit has minimal effect at high RPM operation. As the name implies, the idle circuit works at idle. However, it ALSO complements the main circuit through the rev range where partial throttle is involved. Depending upon many things, including jetting for the idle and main circuits, progression-emulsion tube selection and float levels and ignition timing, it is quite easy to get an engine, like the poster's, to run at 2500 RPM on partial throttle.

There is much interdependence on various things to get an internal combustion engine to operate acceptably. One variable may mask another and in this case, richer idle jets may satisfy the poster or may mask(indicate) the need to modify something else such as the main circuit jetting or, as CD (Sorry He sold it!)'s ignition timing suggestion.

Who knows, the issue could be as seemingly unrelated as spark plugs of an improper heat range, a whimpy condenser, or plug gaps that are too small!

[brianstj]

To add some more info -

Plugs are NGK BP6ES, gapped around 0.032 if I recall correctly

Timing was first set statically to the first mark BTDC on the crank pully. After reaching operating temp. the timing was checked with a light (on the crank pulley), vacum advance disconnected and plugged, confirmed to be at 25 degrees BTDC @ 2700 RPM per the underhood sticker.

I think I am going to try the idle jets first and go from there, since the indications seem to be those are definitely too lean.

I'll let ya know how it goes.

[Guest Anonymous]

Good move. Per your timing settings, if you are setting things at 2700, your timing may actually be a tad retarded. I have seen 2500/2400 used as a baseline setting for Tiis. But there is a fair amount of latitude especially given current fuel availability.

Good luck and post an update of your success.

[Chris_B]

You took my comment out of context. What I said was "The idle circuit has no effect on high RPM or WOT operations. Further, the idle mixture adjustment _only_ has an effect at idle speeds, not on any higher RPM operation."

I repeat that the "idle mixture adjustment" (i.e., the mixture adjustment screws) _only_ has an effect on operation at idle. A soon as you open the throttle plates only slightly (a few degrees) the transition circuit comes into play. Changes in idle jets have an effect on mixture in the transition stage (and to a very small [perhaps immeasurable] extent at WOT/high load); changes in the idle mixture adjustment do not.

[Guest Anonymous]

You took my comment out of context. What I said was "The idle circuit has no effect on high RPM or WOT operations. Further, the idle mixture adjustment _only_ has an effect at idle speeds, not on any higher RPM operation."

I repeat that the "idle mixture adjustment" (i.e., the mixture adjustment screws) _only_ has an effect on operation at idle. A soon as you open the throttle plates only slightly (a few degrees) the transition circuit comes into play. Changes in idle jets have an effect on mixture in the transition stage (and to a very small [perhaps immeasurable] extent at WOT/high load); changes in the idle mixture adjustment do not.

Weber takes a slightly different position from yours regarding the idle circuit.

"The secret to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other carburetors is the Idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation." http://www.redlineweber.com/html/Tech/carburetor_set_up_and_lean_best_.htm If you do a search you will find that other Weber franchisees use the same language. http://www.piercemanifolds.com/Carburetor_Set_Up_and_Lean_Best_Idle_Adjustment.htm

Notice the choice of words "low speed" circuit and 80%.

My point, as noted before, is that the idle circuit also known as the low speed circuit complements the main circuit and works in tandem with the progression ports at part throttle. You seem very dismissive of the significance of idle jetting to the extent it only applies to idle. I do not know how Weber arrived at the 80 percent figure, but the import of the statement is clear, the circuit is responsible for a large majority of driving operation, Even if the "low speed" circuit only affected 20% of the driving operation, that still suggests its importance.

[c.d.iesel]

Brian - please set the timing at 1,500 rpm,

and use the mark on the flywheel - at the rear.

BEFORE any other adjustments.

[TobyB]

WHat I found was that, for 60- 70 mph cruising in 5th gear with a 3.91 diff

and a moderate cam on the 38, I was running about half on the idle and half on the main jet. That's at about 3k, for those at home without a tach.

"What???"

Ok, so if I increased the idle (remember, it's really a 'transition' jet) jet a step, the mixture would fatten a set amount.

If I increased the main jet a step, the mixture would ALSO fatten about the same amount!

So the car was running on both jets about equally, at the amount of

throttle it took to cruise in the 60's.

Now, if I wanted to accelerate and opened the throttle further, then

the main jet had a much bigger effect, and the transition jet didn't make much difference.

The problem I had with the 38 was that, for my car, there was always a lean

gap between the jets unless I fattened the main so far as to be running

rich all the time. I seem to remember 145ish, but it's been a few years.

A really fat idle would help, but never fix, the problem.

And with the 145+ main, the car ran like a refinery, getting unpleasantly low

mileage and annoyingly little power until the air correction jets started to have real effect.

So I eventually gave up on the 38 for a street car, with sadness (for it makes nice power)

and am running a 32 now. Which is less fun, but a lot more economical

and makes more sense.

I want to make a 'twin single' manifold to see if the 38 will work better

that way- I'll be welding aluminum for other projects this winter,

so, as they say, 'watch this space'.

t

[Guest Anonymous]

Good move. Per your timing settings, if you are setting things at 2700, your timing may actually be a tad retarded. I have seen 2500/2400 used as a baseline setting for Tiis.

Oops, since we were discussing your carburettor jetting, I do not know why I assumed you were tuning up an ii. Your timing is actually retarded if you are using this setting! CD (Sorry he sold it!) has the right baseline answer.

Sorry.

[Guest Anonymous]

WHat I found was that, for 60- 70 mph cruising in 5th gear with a 3.91 diff and a moderate cam on the 38, I was running about half on the idle and half on the main jet. That's at about 3k, for those at home without a tach.

"What???"

Ok, so if I increased the idle (remember, it's really a 'transition' jet) jet a step, the mixture would fatten a set amount. If I increased the main jet a step, the mixture would ALSO fatten about the same amount!

So the car was running on both jets about equally, at the amount of

throttle it took to cruise in the 60's. . . . ."t

Now that The Toby has rechristened the idle jet nee low speed jet as a Transition Jet, what more is left to be said!

Well done Toby.

Chicken, chicken, chicken. lol

[Chris_B]

Either you are not understanding what I said, I am not expressing myself clearly, or some combination of the two.

I never said "idle jetting...only applies to idle." To the contrary, idle jetting has a profound effect on the transition circuit, which I would say is responsible for more like 90% of the "driving operation" of a street car.

The original poster said "I ...readjusted the idle based on the Redline instructions...and...the car felt much smoother below 2500 RPM - but, I was then starting to get a bit of a stumble at higher RPMs under a load." I concluded from what he said that he was only adjusting the idle mixture and speed. These adjustments alone, absent a change in idle jets, simply can't cause a stumble above 2500RPM under a load where one didn't exist before.