Idle mixture on 36IDFs...Worn needle seats?

[wizrod]

Hi all,

I have a pair of Italian 36 IDFs on a type 1 motor. 2 of the idle mixture screws are not responding.

The motor is running on all 4 cylinders, so I'm guessing that the idle circuits are not blocked, rather that the seats that the idle mixture screws should close on are worn, and are letting fuel through even when the screws are wound all the way in.

Is there a fix for this or have the carbs had it?

I was wondering whether, if I were to machine up some slightly longer screws, they may account for the, I assume, now slightly deeper seats....?

Cheers

P.

[Slow 1200]

my first guess is that the throttle plates are open too far

[Dale M.]

Not sure how one wears out a needle seat when there are nothing touching them most of the time, doube seriously they are eroded by fuel flow....

I have to go among with previous poster and believe improper idle speed screw adjustment, and you are on the mains...

At what RPM are you idling engine at....

Dale
_________________
“Fear The Government That Wants To Take Your Guns" - Thomas Jefferson.

"Kellison Sand Piper Roadster" For Street & Show.
"Joe Pody Sandrover" Buggy with 2180 for Autocross (Sold)
============================================================
All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[wizrod]

Idle speed is set at about 900 rpm (45 Idle jets)

The thing is that one barrel on each side is fine...one is not.

I was just wondering either some previous owner may have been too heavy handed and tried to screwed them too far or maybe screwed the wrong screw in, resulting in some damage, which now means that they no longer stop the fuel flow when they are now fully screwed in.

I should say that I also tried swapping the screws from a working barrel to an unresponsive one, with no change, indicating the problem is not a worn screw, but the carb body itself.

TBH, I'm just exploring possibilities at this point, as I'm out and about enjoying the camper

P.

[wizrod]

So let me get this straight...I'm on a learning curve here...

I have a matched pair if 36IDFs fitted with a set of 4, 45 Idle jets
The motor idles at about 900rpm and both carbs are balanced.
The motor sounds as if it is running on all 4 cylinders, and all for exhaust headers are hot.
Cylinders 2 & 3 respond to adjustments to their corresponding idle mixture screws.
Cylinders 1 & 4 don't respond to adjustments to the idle mixture screws at all.
Swapping the idle mixture screws over from 1 to 2 and 3 to 4 does not have any effect.

My assumption, based on what I've been able to read, is that the Idle Mixture screws are effectively a valve. As you screw them in, they cut off the flow of the fuel, and as you screw them out, the flow increases...the pointy end on the screw giving a gradual increase in flow, as the screw is wound out.

The behaviour I observe...the cylinder running regardless of the position of the screw, suggests (to me) that there is fuel flowing past the screw, even when the screw is fully in....?

Now, as I understand the operation of carb, as the butterfly opens, the progression holes are exposed and when this happens the idle mixture screw is effectively bypassed. As such, it could be the case here that the throttle shafts on both carbs are bent, such that one barrel of each carb is exposing the progression holes early....? (I should say that I don't recall them being bent or the butterflies opening out of sync, but I concede the possibility.)

Could fuel flow out of the progression holes before the butterfly opens past them? If the idle circuit is blocked between the progression holes and the idle mixture screw, would the cylinder idle even if the progression holes are above the butterfly?

Are there any other possibilities...?

P.

[laker]

[wizrod]

Thanks Laker,

I won't get a chance to dig deeper than thinking about this until I get home at the weekend.

Will post on Sunday evening / Monday.

Cheers

P.

[Dale M.]

Spring bind keeping idle mixture screws for seating?

Dale
_________________
“Fear The Government That Wants To Take Your Guns" - Thomas Jefferson.

"Kellison Sand Piper Roadster" For Street & Show.
"Joe Pody Sandrover" Buggy with 2180 for Autocross (Sold)
============================================================
All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[wizrod]

[modok]

[FreeBug]

Do you have a snail? How does the airflow compare between all 4?

[hobbybob517]

[oprn]

Just wondering if by chance you are running a dual cannon exhaust? They can cause mixture imbalances at idle.

It was 1 and 3 on my engine so probably not.

[wizrod]

So...small update...
First off, the the exhaust is SS 4 to 1 header & single quietpack.

Ok. I'm back from my trip and have removed 1 of the carbs for inspection & cleaning. The 1st obvious issue is that the springs on the idle mixture screws...one has 5 coils, which I think is correct and the other (non responsive) screw's spring has 6 coils, so Dale may have called it correctly. I'll order up a new spring.

However, a slight fly in the ointment is that both idle mixture screw springs on the other side have the correct (?) 5 coils.

That all said...neither springs allow the screws to fully seat before they bind up.

Re: the other points, yes I have a snail meter and yes, both sides were flowing the same, at least as accurately as you can tell with a snail meter. I didn't check all 4...I will do when they go back on.

Its getting late now in the UK, so I'll postpone re-fitting the carb (And removing/cleaning the other) until tomorrow...

Cheers.

P.

[wizrod]

Been doing a little more research...

It seems that IDF idle mixture screw springs & idle speed screw springs are very similar in overall size, but the latter seems to be made of slightly thicker wire. As such, they would be slightly longer when fully compressed. I wonder if a PO has mistaken one for the other, resulting in the springs now binding up before the screws seat when screwed in...?

Also, I have also discovered that there are 2 types of IDF idle mixture screw - a long version designed to use a spring (plus an o-ring & washer) to stop them vibrating loose - and a short version that just uses an o-ring. Are these interchangeable?

Anyway, I got home yesterday and removed the RHS carb and gave it 30mins in a hot ultrasonic bath, followed by a good blow through with compressed air. I'll remount it this evening, get it re-balanced & see if I can get both the idle mixture screw to react (following the guidelines on the Redline Weber site)....

cheers

P.

[hobbybob517]

The short o ring style mixture screw is for the old italian webers
(Roller style accell pump)
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[wizrod]

[wizrod]

Ok...progress, of a sort.

The cleaned carb is back on so I thought I'd see if it had made a difference, before removing and cleaning the other.

Well...initially, no...none of the idle mixture screws did anything...screwed in or 4-5 turns out, no difference.

So given that I'd seen that the springs were binding, I removed all the springs, washers and o-ring and re-fitted the screws bare.

This time much better. 3 of the 4 screws react strongly. In each of the 3, the motor struggles noticeably w/o the the screw fully closed, and an each case the motor picks up when opened a 6th of a turn. 2 screws then continue to increase the revs when opened further, for maybe a full turn.

1 screw did nothing. The odd thing was this screw was working last week...and it is in the side that was cleaned. I also swapped the screws front to back, to eliminate the possibility that the problem was the screw itself...no effect.

I also checked the airflow on all 4 barrels. Both sides of the uncleaned carb read the same. The cleaned carb's barrels are out a small amount...If #1 reads 3, the #2 is a little over 2.

So....I think I'll both order a set of new springs for the idle mixture screws and grind a little off the existing screws, to see of I can get them to fully close without binding.

P.

[laker]

[wizrod]

Hi Laker,

Compression, ignition and timing are all fine on that pot (checked them all) but I completely forgot your hand-over-the-inlet test. Will try this evening.

Also, I noted that while I started with the idle speed screws set to 1/2 a turn on both sides, they needed a bit more...maybe a turn ... to idle at all, so will need more, to idle at the correct speed....so I guess the idle jets (45s) are too small. I have a set of 50s and 55s, so I'll see how they affect things this evening, too.

Would you anticipate that larger idle jets would make the idle mixture screws more responsive? My thinking is that currently to idle the butterfly is open enough to start to expose the lowest progression holes...reducing the effectiveness of the idle mixture screw (?) . Larger idle jets will result in idling with the butterflies more closed, so the idle mixture screws will be more 'in the game', so maybe more responsive. By that I mean that they would not be largely unresponsive beyond a 1/6th turn...?

This is of course speculation....I'll find out this evening

P.