Technical weber 26 carb rebuild / tuning

[daemonoid]

I've rebuilt the carb on my 500L, after only a minor problem with the throttle linkage (I put it on the wrong way round meaning it revved like mad) it all seems to be just about working.

My problem though is trying to set it perfectly - the Haynes manual suggests tightening the needle valve to produce the fastest, smoothest idle. Which I have done and then altered the throttle stop to slow it down a little. The problem is, that the needle valve is as tight as it will go which doesn't seem right to me. I've then played around with both the throttle and needle valve to get a smooth idle without it being so tight, but then I seem to lose power at full throttle (too little fuel I guess).

Anyone got any advice on how to get this tuned correctly? Any hints & tips would be appreciated...

 

[Bambino]

You've done the right thing.
Both the throttle screw and the mixture screw need to be adjusted to give the smoothest, slowest idle.
If you get the smoothest idle with the mixture screw all the way in that is a little odd, but not unusual. You may find that the butterfly is letting sufficient air past at idle to give a smooth idle with fuel entering through the small progression hole as well. The taper on the end of your idle screw may also be worn down so that fuel still passes even when it seems to be screwed down tight.
As to the loss of power at high engine revs, this is not due to the idle circuit but may have more to do with a faulty fuel pump or dirty main jet etc.
I've attached a diagram of the IMB carby that may help.
Chris

 

[JakeF]

Question about the Weber 26. What are the differences in models 6 and 10? The IMB 10 is in L but Haynes says "incorporates CO limiter for idling". What does it mean?

Second question. How does the choke for generally. I am asking this because when I coldstart the car(it starts well) with choke it idles maybe 30 s and then the rews start to get low. If I dont close the choke it will die. After closing the choke I have to keep my foot on the gas pedal for some time to keep the rews abit up. Normal? Or should the car idle with choke on longer time?

Thanks.
Btw my one is equipped with 26IMB 10.

 

[jjacob]

Jake,

The "10" has a different idle mixture screw. It has steps in it. The older "6" style has a more common angle end (like a <) on the mixture screw.

The IMB does not have a choke. It has a mixture enrichment valve. When opened you are opening a valve that adds more gas to the mixture. Try not closing it all the way. There will be a spot where it should run fairly good after starting. I usually have mine closed after a few minutes of running.
John

 

[jjacob]

My problem though is trying to set it perfectly - the Haynes manual suggests tightening the needle valve to produce the fastest, smoothest idle. Which I have done and then altered the throttle stop to slow it down a little. The problem is, that the needle valve is as tight as it will go which doesn't seem right to me. I've then played around with both the throttle and needle valve to get a smooth idle without it being so tight, but then I seem to lose power at full throttle (too little fuel I guess).

Anyone got any advice on how to get this tuned correctly? Any hints & tips would be appreciated...

Check to make sure the "choke" is fully off by manually pushing the choke arm fully closed after the engine has run for a couple of minutes. If you find the choke is not fully off, then you need to adjust the control wire so that it turns it off fully when the lever is in the down position.

When the choke is fully closed, turn the engine idle speed down until it is running smooth enough without wanting to die. Then try turning the fuel mixture screw in or out a quarter turn at a time. Do this when the engine is fully warmed up. Turn in until the engine stumbles, then turn out quarter turns until there is no improvement in engine running. Do not turn out too far or the engine will run rich.
I usually turn it in until the engine stumbles, and then turn it out 1/2 turn from that point and it is almost always perfect.
John

 

[JakeF]

Jake,

The "10" has a different idle mixture screw. It has steps in it. The older "6" style has a more common angle end (like a <) on the mixture screw.

The IMB does not have a choke. It has a mixture enrichment valve. When opened you are opening a valve that adds more gas to the mixture. Try not closing it all the way. There will be a spot where it should run fairly good after starting. I usually have mine closed after a few minutes of running.
John

John,

Thanks for this info, helps alot again!

 

[JakeF]

Hi,

Does anyone have a exploided view of the 26 imb? I am not sure where the other small aluminium seal goes? Under the needle valve?

 

[Bambino]

Jake,

I'm not sure whether this picture will help, but here goes.

Is this the alloy spacer you mean?

Chris

 

[JakeF]

Thanks chris! Thats it

 

[JakeF]

Any gasket or o-ring etc(red arrow)?

 

[Bambino]

Yes

Chris

 

[JakeF]

I cleaned my carb with an ultrasound cleaner which I bought for 32€. I also changed every gaskets etc. Wow, what a difference. The car pulls better at low rews. I think some jet etc has been dirty or something! Cool.

 

[cspecken]

Does anyone know how to remove the float pin on the weber carb? One tab appears to have a cut down the middle, I do not think it is crack because it is nice and straight. The carb is a Weber 26 M.

Thank you

 

[tjmra]

Split is normal. I believe it is so you can GENTLY put a thin blade in and take the pressure off that end of the pin to take the pin out and then to squeeze it together to put pressure back on the pin when you replace it. Go gently. I doubt the alloy posts will take a heavy hand when doing any of this.

One end of the brass pin is very slightly rounded. This is the end that ends up in the split post.

You can start to push the pin out from the split post end and then pull it out with your fingers. Gripping with pliers or similar has the potential to leave burrs on it.

My pin is not the same diameter along its length. It only fits back in one way. Rounded end through the solid post first. If I try and do it any other way and it gets caught up. Hope this makes sense.

Regards

Joe R

 

[thenetr]

I realize that this is an old thread, but can someone help me?

My mother-in-law has an old Fiat 500 ('65), where the carburetter died. Without looking at it I ordered a Weber 26 IMB 10. But at some point, there was an 26 IMB 6 (9A) in the car?

What should be done to fit the new IMB 10?

Thanks,

Anders

 

[Bambino]

Hi Anders,

The carbies are completely interchangeable.

Chris

 

[thenetr]

Hi Chris,

Thank you. We took the parts from the old one and put it in the new housing (since it was the housing that had split open). We couldnt get the new housing to fit unfortunately.

Now the cars is not running very well. it dies and will not rev when flooring the gas pedal. The mechanic (60+) does not know what to do at this point other than getting a new carburetor exactly the same as before...? Should that be nessesary?

BR
Anders

 

[Bambino]

Hi Anders,

It's always difficult diagnosing these problem half a world away so all I can really advise is to check everything - fuel, timing, distributor, tappets etc.

I wrote a thread on the IMB a while ago - here is the link

https://www.fiatforum.com/500-classic/355169-weber-26-28-imb.html

Chris

 

[dk500]

Just completed a partial rebuild on the carb. All the info here is great ! Discovered that the mechanic that replaced my last float just soldered the holes with petrol still in the float ! Seems to idle great and drive much better ! A little unsure if I have the idle/mixture set correctly but it seems ok ?

 

[Luxe]

Same here. Not sure if the idle mixture screw is set right. I get some lurching at high revs and can smell some gas after driving. She idles good though.