Weber Cold Start Elimination

[simonknee]

Just pushed the button on these:

http://www.webcon.co.uk/shopdisplayprod ... rchfields=

Nearly ?90 the pair delivered And most of the parts are not needed! However it's the only way to get the bits that actually blank off the cold start jets. I am not keen on the fuel that makes it way out of the back plate and this should do the job.

I'll let you know how I get on...

(Should this have been in mods, dear admin? Not sure people look in there as much as the other forums)

[elanner]

I'll let you know how I get on...

Thanks, please do. I'm definitely interested to know.

Nick

[oldchieft]

Just a thought.

Why not maintain the cold start device in good working order?

Then you can run your idle and progression at closer to correct mixture?

Also you won't be stamping on the throttle control to get the acceleration stage operating to prevent cold stumbles?

And you won't be jetting neat fuel past an open inlet valve to wash the oil off the bores on start up?

It may even be the case that the guys at Weber knew a thing or two about carburettors, could of known what they were doing when they made them?

Jon the Chief

[simonknee]

Good valid points Jon and that list is indeed things you would want to avoid doing.

So I am very glad non of it applies to my state of tune or my start up routine - even though I don't even have a choke cable attached

Now come to think of it I used to have to pump away on the accelerator and if that didn't work I might have even stamped! (now I just mildly tickle) That was before I got rid of the inaccurate stock Weber jetting and stopped following Weber instructions on setting float level. It's almost as though the guys at Weber don't know what they are doing! (ha ha sorry I could not resist )

Anyway that cold start circuit is getting eliminated come what may and the inability for the fuel to leak from here ever again is only a good thing for me. Of course Weber didn't mind relieving me of a lot of cash for what is a lot of redundant parts to achieve the job - I asked Webcon and they say Weber only supply the bits needed as part of the kit to which they add a bunch a gaskets and panels and screws that you likely already have.

Simon

[billwill]

I seem to recall that on this forum we discussed a fix for that leak, that didn't involve removing the coldstart stuff.

I think it involved drilling a hole in a cover plate,

Ah, found the topic, have a read & see if it is relevant:

lotus-carbs-f40/weber-hole-return-spring-plate-t24632.html#p158479


HMMM, that's not the topic I was thinking of. I think the hole described is the fix, but this topic has all the wrong reasons for it.

[AdrianSi]

On the topic of maintaining the cold start system, how do you do this?

I have the cold start system still installed on my Webers and ideally want to keep them. When I bought the car nearly a year ago the 'choke' worked perfectly, until one day I accidentally left it warming up on choke. It went 'off song' a little at this time.
Then a few days later, the cable screw let go on the front carb so the choke only worked on the back one. Some how in the tiffling to get the choke cable back assembled I managed to unbalance the two carbs.
So I've got them back balanced now and idle mixture adjusted to give a good even idle. However the chokes are a different matter. If you pull the choke lever out even just a little amount the rpm drops rather tha rises as if the engine Is flooding. Pulling tje choke out any further causes a stall situation.

I've had the mechanisms off and all is well with the timing and meshing of the ears. All the plungers are free and return to the same positions.

So what is the issue do we think? Butterflies closed too far preventing a good air mixture when the cold start is operated? Insufficient air bleed by on the idle mixture?

Any help or past experience welcomed.

Thanks in advance.

[simonknee]

Hi Adrian,

My guess (and it is purely a guess based on what you describe) is that you still need to ignore the cold start circuit in your attempts to diagnose this and address the normal running of the carburetors. There is no reason that running for a while on cold start would then take you "off song" nor should reattaching a cold start cable have interfered with the balance. And then putting the balance back should not have led you to adjust the idle. Sounds like everything you "put back" led you to have to adjust something else. Which makes me think you have another fault that started this merry-go-round.

And as it's traditional that 95% of carb faults are electrical I have to ask - have you checked the ignition?

Simon

[AdrianSi]

Hi Simon,

I have a the carbs running sweet now. I think the main issue is that I must have tweaked the balance screw instead of the base idle screw sending them off well off song.

The ignition is all new and advance set.

As I say, it all runs sweet now, just the cold start seems to flood the engine and cause it to stall. My only conclusion is that there is insufficient air in the mixture?

[oldchieft]

I seem to recall there is a gauze air filter at the bottom of the device, is it clean?

Jon the Chief

[simonknee]

Here's the diagram of the cold start operation

weber-cold-start.gif (31.11 KiB) Viewed 3177 times

Good call on the guaze Jon, it's just where the air goes in between where the numbers 33 and 34 are printed on the diagram.

You can also see that it is a bit crude compared to the other metering in the carb. I'd be worried about exactly how rich I was going if I was using this. The twink is quite capable of running at 8:1 AFR (8 parts air to 1 part fuel) but that is not very healthy for it. The optimum ratio for power is 12.5:1 (and for efficiency is 14.7:1).

Please only leave the cold start in for the absolute minimum you need to get the engine going.

[oldchieft]

My Weber tuning manual graph has about 12.8 for Max power and about 16 for best fuel economy.

Also says engines can run OK from 6 to 18 AFR.

Jon the Chief

[simonknee]

Thought I'd add the notes for that diagram for others that come looking:

The fuel flowing from the bowl (4) arrives to the starting device through the ducts (32) and the starting jets (30). Emulsified with the air coming from the hole (29) it reaches the valves opening (35) through the ducts (31) and definitely emulsified by the air entering from orifices (34) is then carried by means of the ducts (33) to the carburetor throats below the throttles.

I actually didn't realise that there were starting jets to emulsify to fuel within the carbs in addition to the air intake with the gauze. It's definitely not neat fuel like the accelerator jets - you do have a point with these.

Jon you are correct that 16:1 will be highest economy not that you should be tuning an twincam for highest economy, buy a Prius! 14.7 is when you get full combustion (with normal fuel) and that is the baseline I head for when tunig. My AFR meter (did I mention I have a wideband O2 meter in the exhaust!) confirms that the engine will run in the range the weber manual states (well actually 6:1 would be a ridiculous sooty mess) but I would be alarmed if it ever went richer than 9.5:1. I am tuning for 12.5 under power and just below 15 cruising. It will wander up to 18:1 on lift off. However we are heading off-topic here - this is something I am going to revisit this year when I move up from 32mm chokes to get a leaner WOT.

[simonknee]

That was quick...

So from top left we have:

a pair of two-peice blanking jets - i.e. a solid rod - to go in location 30 on the diagram above.
a pair of blanking screws to fill up what I _think_ is an airway but I'm not sure how it relates to the diagram
as an alternative you can bang in the pair of ball bearings. Why? The blanking screws require tapping an M5 thread so is really a carb off job to remove all swarf whereas the ball bearing is doable in situ but is kind of permanent I reckon.
a pair of replacement "pistons" with springs and o-ring to replace item 15
a blanking plate for the "cog" assembly (I already have one of these)
a blanking plate for the throttle spring cavity that does not feature the hole (suggesting that the hole is important for the cold start billwill - maybe I'll be able to see why)
A selection of gaskets depending on whether it is a 40 or 45 series body.

Oh, I missed something in the picture. There is also a replacement accelerator pump actuator arm. Why? I don't know but maybe it'll become obvious as to what part the accelerator arm plays in the choke circuit. I am guessing that the original has some mechanism that prevents the pump operating whilst you have the cold circuit running. Just a guess.

[oldchieft]

Jon you are correct that 16:1 will be highest economy not that you should be tuning an twincam for highest economy, buy a Prius!

I do own and drive a Prius!

Jon you are correct that 16:1 will be highest economy not that you should be tuning an twincam for highest economy, buy a Prius! 14.7 is when you get full combustion (with normal fuel) and that is the baseline I head for when tunig. My AFR meter (did I mention I have a wideband O2 meter in the <a class="vglnk" target="_blank" href="http://www.eurocarparts.com/car-body-parts-and-car-exhaust" rel="nofollow"><span>exhaust</span></a>!) confirms that the engine will run in the range the weber manual states (well actually 6:1 would be a ridiculous sooty mess) but I would be alarmed if it ever went richer than 9.5:1. I am tuning for 12.5 under power and just below 15 cruising. It will wander up to 18:1 on lift off. However we are heading off-topic here - this is something I am going to revisit this year when I move up from 32mm chokes to get a leaner WOT.

14.7:1 is actually stoichiometric, that is when you count the atoms of carbon and the atoms of hydrogen and then work out how much air you need to get enough oxygen for a full chemical reaction.

This will never be achieved in a engine during the 4?5 milliseconds of combustion. you will always have unburned fuel, (carbon monoxide and free hydrocarbons) and un-converted oxygen, (seen as free oxygen an nitrous oxide)

Also the dreaded E word comes in, ethanol has a stoichiometric of around 9:1.

so the more the E number the leaner you will be running, one reason why modern cars need a closed loop control.

I have no idea how we will address re-jetting Weber carbs for what will be coming out of the pump when we come to fill up in the near future.

Jon the Chief

[simonknee]

They'll provide us with a special "real fuel" pump at the station Jon, surely!

As I understand it they already have no choice but to tune for the E content in the US. At a basic level it means that you tune for an idle mixture of 13.5 rather than 14.7. (it seems we both have a good understanding of what different AFRs mean)

Check out all the posts by type26owner back in the day (start with his tuning white paper). He is still active but over on his own group at Yahoo as this site proved a bit too confrontational for him and that wasn't his style. Check out my thread about Hypojets to see how I tune my Webers.