Technical Carburettor mixture adjustment

[AndrewRL]

OK. So I'm after a bit more education ahead of giving the carb a bit of a service. I've looked everywhere on the forum and in manuals but not found (yet) the answer.

I understand the idle speed adjuster and the instructions to adjust this down until the generator light is just on. But how do I know if the mixture screw is correctly adjusted? What is the proper way to set up the carburettor - which of two do you adjust first and what are you looking for when adjusting the mixture? The workshop manual just says "set to the most suitable ratio" Carburettor is the standard Weber 26 for 500cc (I think)

More questions to come when I start on this, I am sure

Thanks, as always, for the advice and education

BTW Part of the reason for servicing/cleaning the carb is to try and stop the car cutting out when coming to a stop. From other threads I understand that this is worth trying as well as checking the tappets so that will be next on the list!

 

[IanEmery]

OK. So I'm after a bit more education ahead of giving the carb a bit of a service.......

I understand the idle speed adjuster and the instructions to adjust this down until the generator light is just on. But how do I know if the mixture screw is correctly adjusted? What is the proper way to set up the carburettor - which of two do you adjust first and what are you looking for when adjusting the mixture? The workshop manual just says "set to the most suitable ratio" Carburettor is the standard Weber 26 for 500cc (I think)

More questions to come when I start on this, I am sure

Andrew,

Here is a copy of the page from the Autobooks manual covering carb tuning on the Weber 26.IMB. The fuel/air ratio is correct when the engine is running at it's smoothest.

I hope this helps you.

 

[dk500]

Don't know how to copy a link to a post, but search for " Weber 26 carb rebuild / tuning "
I found it very useful

 

[AndrewRL]

Great - thanks Ian

That provides a bit more information than the manuals I have.

So I assume it doesn't matter which you adjust first - one adjusts the speed, the other the "quality" of the running.

Both should only be adjusted when the engine is hot/at normal running temperature.

I've never adjusted the mixture...is it really that obvious when running at smoothest? It's not exactly a silky smooth engine anyway...or is that just mine ?

 

[IanEmery]

Andrew,

How I would do it, others might have other methods

Adjust the tick over screw to give a higher rev than normal, not too high just a fast tickover. Then slowly adjust the mixture screw until the engine is running at its smoothest (do this by small adjustments and never force the screw in too tight as this can damage both the screw and the carb body) then unscrew the tickover adjustment screw until the engine just starts to rock on its mountings. That's good enough for me

 

[Bambino]

Hi Andrew,

I wrote a pictorial thread about the IMBs a little while ago. Might be useful.

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

Chris

 

[AndrewRL]

Thanks Chris
I came across that via the search dk500 suggested. Very useful and far clearer than any of the Workshop manuals (a picture paints a thousand words) . Giving me confidence to give it a go at removing/cleaning/servicing the carb (I already have the maintenance kit). Sadly mine is not quite as clean as the one in your pics .
Cheers

 

[fiat500]

I think this page is brilliant. It seems to be talking about big Weber carbs, but if the design logic follows for our carbs, then it's giving some definite recommendations. These are that the mixture screw should be in the range of 1 to 2 turns out from fully screwed in and the idle stop screw should be about one full turn from when it just starts to touch the stop on the carb body.
It even tells you where to go with the jetting if you can't achieve satisfactory running within these ranges. And there is a table from which you can virtually eliminate the carb as a culprit for certain engine running problems.

http://www.carburetion.com/Weber/adjust.htm

 

[AndrewRL]

Thanks to the excellent thread (especially from Chris) all is going well with the removal, cleaning etc of my carburettor (now that is tempting fate!).

So, just planning ahead to next weekend when I will hopefully be ready to put it all back. The "instructions" in Chris's thread say:

"Disassemble, clean everything, replace worn parts, calibrate the float, reassemble and replace on manifold with new gaskets, reattach everything, start, adjust and go for a drive ".

Is the putting back together again that straightforward? Somewhere in the back of my mind is something to do with putting some fuel back in the float chamber - or do you just reconnect everything, give a few turns of the starter to get fuel flowing and (hopefully) fire up (and then adjust the mixture/idle when warmed up)?

Thanks

 

[dk500]

Hi
I didn't fill float chamber, just cranked and started ?
Darryl

 

[fiat500]

Same as Darryl. But if you want to be logical and eliminate potential faults as you go, crank the engine with the plugs out and the fuel pipe removed at the carb until fuel spurts, refit and crank a bit more and then fit plugs and start. Mixture screw 1.5 to 2 turns out, throttle screw 1 to 2 turns in after touching the stop. It worked for me.

 

[Bambino]

Is the putting back together again that straightforward? Somewhere in the back of my mind is something to do with putting some fuel back in the float chamber - or do you just reconnect everything, give a few turns of the starter to get fuel flowing and (hopefully) fire up (and then adjust the mixture/idle when warmed up)?

Thanks

Yes it is and you don't need to prime the carby, though doing so will make starting quicker.

This may be an old wives tale but I was told ages ago that the quicker you got the oil pressure up, the less the wear on the internal components of the engine. So the faster the engine catches, the less the wear. Cranking it over and over with low or no oil pressure may not be good for it, nor for that matter, the battery. Hence maybe the use of electric fuel pumps. Be interested in any comments.

Chris

 

[fiat500]

Yes it is and you don't need to prime the carby, though doing so will make starting quicker.

This may be an old wives tale but I was told ages ago that the quicker you got the oil pressure up, the less the wear on the internal components of the engine. So the faster the engine catches, the less the wear. Cranking it over and over with low or no oil pressure may not be good for it, nor for that matter, the battery. Hence maybe the use of electric fuel pumps. Be interested in any comments.

Chris

That's got to be a fact Chris. Having said that, I cranked my 500 ad infinitum when rebuilt it; I had no idea what was wrong and I can't remember what I did differently that made it start. But since then there's been no sign of impending failure although I guess you need to be right up in the 60's or 70's of thousand miles before you decide you ought to gave been more careful on the early days....a bit like my own human body TBH.

 

[Bambino]

I guess you need to be right up in the 60's or 70's of thousand miles before you decide you ought to gave been more careful on the early days....a bit like my own human body TBH.

Roger Waters wrote that 'the memories of a man in his old age are the deeds of a man in his prime', so in my book a few aches and pains are signs of a life well lived. Not sure it applies to engines though

Chris

 

[AndrewRL]

So thanks to all the help here I removed and stripped down and cleaned the carby. Sunday it will hopefully go back on and the car will start

As a total novice at this I thought I would share my experiences so far (mainly because I am bored at work!) but am fully aware that I am also showing my ignorance . I did end up buying a secondhand carby from ebay which helped with a few pieces in addition to the maintenance kit.

Removing carby - managed OK but clearly easier if you remove the rocker cover first (which I will have to do to get it back on); not important I guess but it does mean that if this hasn't been done for a while it makes sense to put on a new gasket (so should have ordered one at the same time as the carby maintenance kit).

Main jet - there was no way the jet was coming off its holder (even after lengthy soaking in penetration/releasing oil) and a few attempts just ruined the jet! The ebay carby helped here and provided a jet holder/mount for the new jet

Emulsion well - gave up trying to get it out before I damaged it!

Butterfly valve - geez those screws are stiff! Ruined my originals trying to remove them so left the valve and spindle on my original and ended up using the main carby body from the ebay spare - even those were tough to remove. I guess it makes sense as you wouldn't want them working loose and falling into the engine (note to self - make sure they are tight!)

Valve spindle - took more than a few attempts to put it back together again especially getting the spring to look halfway correct; the new spindle provided in the kit was not going to go through the holes so I reused the old one

Float adjustment - not exactly easy to measure accurately when the new gasket doesn't lie flat

Mixture screw - my original and the ebay spare both had a totally different screw to the one in Chris's pictures and different to anything I could see on a spares website; fortunately both look in good nick so fitted the new O ring and refitted

So I have one carby to go back and one for spares. All looking good so far. Not that I am tempting fate

 

[Labman]

Make sure the flange at the base of the carb is flat. Many are warped (as mine was). This will cause an air leak and it will affect the idle. I filed it as flat as I could and doubled up on the gasket and all is perfect now.

 

[Paolo66]

Make sure the flange at the base of the carb is flat. Many are warped (as mine was). This will cause an air leak and it will affect the idle. I filed it as flat as I could and doubled up on the gasket and all is perfect now.

Good advice. Always good practice to have a spare carb gasket to hand. You should replace it everytime you remove the carb and you always end up removing it more than you would want. The rocker cover gasket should be okay to reuse as long as it is not damaged. Always handy to have a spare carb float. These get small cracks or holes in them and fill with fuel thus flooding the carb. Always give it a shake to listen for fuel if you remove the carb. Those little brass screws on the butterfly are a bugger. Try and use a flat blade the same width as the screw head. I would put a dab of Loctite on the threads when you refit them. Try and tighten the two nuts holding the carb on evenly. You dont want to cock it over or warp the base. Just my two penny worth
Damian

 

[fiat500]

I know that wobbly feeling when so much has been disturbed that you lose a bit of confidence that it will work....but it will.
I've never removed the throttle butterfly...could never see a reason to; but if I did I think I would use threadlock on those little screws. Are they peened over at the back originally?
To set the float levels I came up with the idea of using plastic packing shims which are used by kitchen fitters and the like; they come in multiple sizes of 1mm up to 6mm and are easier than trying to use a steel rule.

 

[Vitesse]

I know that wobbly feeling when so much has been disturbed that you lose a bit of confidence that it will work....but it will.
I've never removed the throttle butterfly...could never see a reason to; but if I did I think I would use threadlock on those little screws. Are they peened over at the back originally?
To set the float levels I came up with the idea of using plastic packing shims which are used by kitchen fitters and the like; they come in multiple sizes of 1mm up to 6mm and are easier than trying to use a steel rule.

As you probably remember Peter I attempted to change the butterfly on an IMB 28 carb and both screws sheared off and it turned out the spindle on the shaft was an oversize one which I couldn't get a replacement for. New carb was the only solution.

Lesson learned stay well away if possible!!!!!! I only attempted it because there was one in the refurb kit, no other reason. Being brass they are just so soft it doesn't take a lot to snap them, they don't appear to be peened.

 

[AndrewRL]

No warped bottom ! All went easily back together and after only a couple of pulls on the starter Luigi fired up! I had set the carby as suggested (number of turns in/out etc) and he purred like a kitten (sort of). Went on a short drive and even the stalling when suddenly slowing/stopping seems to have disappeared (so much less stressful!).

And yes, regarding the butterfly valve, I only tried it because there was a new valve and spindle in the kit. I won't bother in the future (if I get round to this again).

Thanks all again .

Until the next challenge......