Weber 40 idf - idle mix screws

MotoCarlo posted:

 

... I find backing it out 1.5 turns it runs ok but a little sputtering. If I back it out 2.5 it runs fine...

 

What happens if you close the screws almost all the way (tighter than 1.5 turns)?

More sputtering? Popping? No effect?

Another thought.

What happens if you pull the plug leads off, one at a time. Does each one make the revs drop, or does one have no effect?

Thanks Sacto. No difference if I send the idle mix in all the way. Have not pulled the plug wires, but not sure what you are thinking?

I did rebuild the engine about 1000 miles ago with new p&C set, rings etc... Compression perfect on all cylinders...

My thought is the float is off. Not sure what would have caused it to shift out of placement. Been perfect for almost 2 years....
And I drive it twice a week. Not much , but I do exercise the car. It does not just sit.

In a 'normal', healthy carb, closing the screws more than 'slight sputtering' should make more sputtering and they should start to pop. If that doesn't happen, something's definitely wrong.

Have you checked that the bypass screws are completely closed, as they should be when starting to diagnose problems like this.

If pulling a plug lead makes no change, a few things could be wrong, but it tells you on which cylinder to continue your sleuthing. It could be that plug isn't firing, a jet is plugged, or the mixture in that barrel is way off. Also, it could be time to start checking for leaking intake manifold gaskets.

I'd think if the floats are off, you'd see more problems at wider throttle openings than at idle.

Bypass screws are closed.
As I keep thinking about it : Anyone have any experience with the idf cold start? I know empi makes a block off plate. I have gone and forth with the block off  plates and the cold start unit on these carbs.
Wondering if that could be the problem with the idle mix screw.

Anyone remove those little springs in the cold start?
Any hands on experience with that?

Sounds like an intake manifold gasket leak to me. 

Have you checked to make sure you’re not missing a brass thingy?

Another thing I should mention is, it is running fine.
No missing, sputtering, no idle flux, etc. Seems a manifold leak would cause the idle to waver. But all is good.
The only thing that drew my attention to it in the first place is , when I was doing my seasonal tuneup - the idle mix screws did not do anything.

It's odd that both #3 and #4 carbs are doing the same thing. They both share the float and the cold start (and manifold).

I guess I could look for the manifold leak by spraying carb cleaner around it to see if it speeds up.  ?

Keep the ideas coming. I appreciate the help guys!

How high is your idle set? Are the idle stop screws turned in a lot? Is the one on the 3-4 carb turned in more than the other one?

I think the more the butterfly is held open (by the idle stop screw), the less effect the mixture screw will have.

That’s true, but it would have to be a long way off idle to get an effect as he is seeing.

And if it has a manifold gasket leak between the intake ports it will act like that and spraying carb cleaner around the base of the manifold won’t effect it.  The leak is between the head ports at the base of the manifold runner where it bolts to the head.

Gordon do you know a way to test it?

Not if it's between ports  -  The leak is between cylinders with no path to the outside world other than right down the throat of the manifold runners.

That's a reasonably common leak, though, especially if it's a bigger engine and/or has been heavily ported leaving only a sliver of metal between head ports.  There isn't much there to hold the gasket material in place, so the vacuum of the engine pulls it side-to-side, eventually weakening it and causing a hole between ports.

Popping in a new gasket usually cures it.

Gordon is probably correct here. If you put a snail in the throats, do they read the same for all 4? That would be where I would start.

Either there is an intake leak, or the throttle plate is closed on that one carb, or it is so far open that you're beyond the idle circuit's influence.

Or you could be missing the brass thingy, spring, and little press-fit split brass ring that holds it in place(top of carb body, under the top plate).......

When I first got my car and started tweaking my carbs, I tightened the idle screw down too much and the brass seat got stuck on the needle, dislodging it. This made the idle screws useless.

Fortunately, I was able to reseat them, polished the needle, and everything is good again.

Live and learn.

Thanks for the advice guys.

Nice to be able to talk and think about it before attacking the carbs. I think Lincoln said, "Sharpen the ax for 1 hour, chop wood for 10 minutes."  Something like that. I appreciate that advice more with every year added onto the old knees /back.

Gordon I'll pop in  a new gasket and see what happens. Great to know about the gasket being a potential problem. Well explained.

My mind was on float / cold start device, so this thread has sent me in another direction entirely. 

I'll fill you all in on  how it goes.

Have a good weekend.

Well... a long hot day in the garage and sweated through 2 t-shirts. Summer in Georgia.

So here is my story - 

Throttle plates - all open and close and working perfectly. (closed during adjustments) Took carb for 3 and 4 off , replaced manifold gasket and top plate gasket, cleaned all jets. Cleaned throttle body, etc, etc.. Put is all back together and decided to go over #1 and 2 at the same time. So did the gaskets etc on 1 and 2 too.
I rebuilt these Webers only 2 years ago, but ... why not.

So everything came off, cleaned, and with new gaskets. Cranks right up. But, still no adjustment with idle mix screws on the 3 and 4 carb.

At this point I simply started trying stuff.
I took the idle jet out and replaced the 5.0 with a 4.5 - no difference, then took an old 5.0 and filed it slightly larger - probably to 5.5 or 5.75. No difference. Went back to the original new 5.0 jet. 

So now, I throw a syncro on it. 1 and 2 read a healthy 5.5 .
3 and 4 read a anemic 2.5.

So now I figure I might as well sync the carbs.

Now as a side note - I have always done the idle mix without the idle adjustment screw making contact, so as not to move the throttle plate. Am I wrong?
I only had to put 1/8 of a turn on 3 and 4 idle speed screw to get the vacuum balanced.  Now everybody at 5.5 on the syncro. (Idle screw on 1 and 2, barely touching plate.)

So what the hell, for the 20th time, let's see what the idle mix screw does now on 3 and 4.   It works.  But not as dramatically as 1 and 2.   Even so, it does drop as it should when I screw it in , and it is optimum performance is in the proper 1.5 to 2 turned out range.

So f-it. 
It cranks and runs perfectly, not spitting and sputtering...
Not sure what is going on here. Every time I dive into the Webers (been doing this for more decades than I should admit) I learn a little bit more. (ex- I had no idea about the manifold gasket causing a problem, etc...)

But in this case - any ideas what is going on? 
My guess is , I am simply did some bone-headed thing.  Just be nice to know what it was / is.

This is probably a stupid question, but when you synced the carbs you had the linkage between the carbs disconnected, right?

Yep. I did Lane.

MotoCarlo posted:

 

 

Now as a side note - I have always done the idle mix without the idle adjustment screw making contact, so as not to move the throttle plate. Am I wrong?

Yes. Disconnect linkage from one carb. Both throttle stops should be touching the screws, and throttle cable should have a bit of slack. Then adjust air to be all the same. Then mixture. As idle speed changes, you may/will have to go back to the stop screws to even the air and get the idle where you want it. I like 900 or so.

Then after all that is done, reconnect linkage and hold the car between 1500 and 2500 and make sure you have exactly the same readings on your snail left carb to right carb. This part is usually a PIA especially if you have sloppy linkage. Plus the adjust changes when you snug the locknuts on the downlinks. But once you get it right, the car will drive so smooth you'll not believe it.

Also, have someone give it full throtttle to make sure you have enough cable travel, but not so much that you may twist the throttle shafts!

How certain are you that the idle air bleed ports are not blocked or partially blocked by the carb top gasket? This will cause your symptoms.

The gaskets sold these days often require some fine cutting around the port openings to ensure they are not blocking anything.

I appreciate the thought, but I did check the gasket. In my case it turns out the velocity stack was too tall for the carb. I went form the new 3" back to the 2.25" and all problems went magically away.  :-)

Hi MotoCarlo 

Can i ask what you did exactly to rectify your problem?

I have exactly the same issue on cylinders 3&4 with twin 40idfs and the mixture screw not responding. 

Thanks 

I completely disassembled the carbs and cleaned them with berryman cleaner . I found a couple of passageways clogged with 'varnish'. Then replaced all the jets. They needed all that, but the real issue was the secondary venturi were in backwards (google it) . Not sure when this happened. Easy error to make. Could have been wrong when I got the car. Also I could have done it at some point along the way. I don't know. But I flipped them around and now it is responding well.

Thanks really appreciate the response. I've stripped and airlined all the passage ways.

I've tried googling the second venturi, would you mind showing a picture?

Try "auxiliary venturi"

Cheers guys. I'll have a look , much appreciated