Lotus Elan

Weber Cold Start Unit

PostPost by: type26owner » Sun Aug 22, 2004 12:48 am

Now that my Weber 40 DCOE throttle spindles seals don't leak in a huge amount of air I'm finding that having the cold start choke actually working while the engine warms up would be a nice feature. Right now it wants to die all the time at an idle. Having to blip the throttle constantly or elevating the idle speed by depressing the pedal really sucks. This is because of a lack heat to offset the latent heat chilling of the fuel. It will not vaporize in the induction system when it's stone cold and the resulting mixture is extremely lean. Lean enough so the idle speed is reduced and the engine is prone to die. Weber did engineer a solution for it but it only actually works if it is tuned correctly. Did that today and it's easy to do.

The cold start units are their own complete carburetor within the carburetor. By that I mean they supply more fuel and air to embellish the mixture towards the fat side. The air portion can only correctly utilized if the throttle spindles seals are leak tight and the throttles remain closed though. Suspect this is the reason for the myth that the cold start units are useless on the Weber. Here to tell you if your Webers weren't grossly air leaking through the spindle seals then the cold start units work great and you do need them to make the cold driving a lot easier. Only problem is Lotus jetted the cold start unit wrong too! The orifice for the fuel is a 1mm in diameter. That will deliver twice the amount of fuel then even the typical sized idle jet required for the progressive holes circuit to operate over it's full rpm range. It's so overly fat that the before the openings to allow more air are exposed to adjust the mixture correctly the engine is flooding. To get the mixture leaned out enough so the engine will rev up via the choke manual cable I had to reduce the fuel orifice by about half. Did this by inserting two pieces of music wire through each jet's orifice. One at .018" and the other is a .020" diameter. Now when the choke knob is pulled out about 3/8" the engine idle speed increases to 2k rpms and if pulled out more the engine will stall from being too fat. Think this will be about correct for the lowest temperature during a California winter. Now have to wait for the really cold weather to test it.
-Keith
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PostPost by: gjz30075 » Sun Aug 22, 2004 12:40 pm

<!--QuoteBegin-type26owner+Aug 22 2004, 12:48 AM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (type26owner @ Aug 22 2004, 12:48 AM)</td></tr><tr><td id='QUOTE'><!--QuoteEBegin--> Only problem is Lotus jetted the cold start unit wrong too! [/quote]
Keith, I suspect the cold start jetting is generic to a particular model, ie, DCOE 18 for example, and Lotus didn't have anything to do with it. Who knows. Maybe jetting varies by model type and when you find the correct diameter wire to close down the orifice, a swap from a different model type will put you in the ballpark.

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PostPost by: type26owner » Sun Aug 22, 2004 1:25 pm

Hi Greg,
What, you think Lotus had no say as to what the jetting sizes were that Weber was to supply them with? I forgot my Weber Tuning Manual (which lists the size and dimensions of every jet made) at work but I'm fairly certain the cold start units have a broad selection of different jets available for them too. I find that assumption highly unlikely. Lotus just chose the size poorly again.

IIRC, Mike Pierce of Pierce Manifolds was telling me earlier this week that there are <span style='color:red'>13 billion</span> possible jetting combinations for the DCOEs. Only one of each type is correct though for each application. This tends to kinda complicate things if a person does not know what they are doing like recognizing two phase flow effects for instance.
-Keith
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PostPost by: type26owner » Mon Aug 23, 2004 5:45 pm

I actually should not claim the original starter jet 100FS are the wrong ones. They would probably be the right ones if I were to try and start a cold engine in mid-winter and I lived above the Arctic Circle. :rolleyes:

The Weber Manual surprisingly does not list the starter jet sizes and dimensions. It does mention on page 44 they come in a size range of 0.6 to 2.0mm for the fuel orifice. The 060FS ones (Weber P/N #75605.060) are available for those of us that live in warmer climates.

It's nice to have a manual cold start choke which does not kill the engine when the knob is pulled on.
-Keith
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PostPost by: type26owner » Thu Aug 26, 2004 3:37 pm

Now that the cold start unit is jetted somewhat better for the mean temperature I've learned how to use it. First rule don't touch the throttle pedal. Secondly, pull the choke knob out by about 1/2". Third, start the engine. Fires up almost instantaneously everytime now and idles about 1500 rpms. Another problem has developed though. If I try to gently drive away immediately though it will stumble and try to die while releasing the clutch in first gear and only opening the throttle plates in order to maintain the 1500 rpms. This affect is annoying and perhaps I don't have it tuned to perfection quite yet. Don't have my AFM installed at this time so I can't see what the mixture strength is doing at that exact moment. Could be this behavoir is normal. Could be I simply should let the engine warm up first before driving off. Anyway a stealthy immediate departure from my neighborhood in the morning is tricky.
-Keith
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PostPost by: steveww » Thu Aug 26, 2004 8:16 pm

I find that giving it a minute or two to warm up a bit before driving off really helps, even with the Stromberg set up on my S4 :D
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PostPost by: type26owner » Fri Aug 27, 2004 1:55 am

Steve,
Suspect I don't have it dialled-in to perfection quite yet. It should allow me to motor away gently without balking and with no warmup required. That's my best I could hope for expectation but I usually have to settle for less. Fun stuff when it works. BTW, I've invested just 30 minutes time tuning this stuff so far.
-Keith
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PostPost by: type26owner » Fri Aug 27, 2004 2:44 pm

Now I think about it maybe the problem is just me. It very well could be a matter of practice makes perfect. The choke knob is extremely sensitive as to the position and how it affects the engine. Going for only my fifth cold start this morning. I'm going to try pulling it out a tiny bit more this morning.

It makes a fair amount sucking noise when the cold start plunger valves are raised and expose the outside air inlets. Enough so it drones out the sound of the running engine.
-Keith
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PostPost by: type26owner » Fri Aug 27, 2004 4:03 pm

That tiny bit more of motion of the knob turned out to be fully pulled out. Worked to perfection this time! As a baseline reference for about the right size(?) of starter jet when the engine bay is fully heat soaked and the choke is pulled out fully the idle speed will be right around 2800 rpms. I can't tease it up there in rpms with just the knob though. It gets stuck at about 1500 rpms for some unknown reason. AFM time again.
-Keith
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PostPost by: steveww » Wed Sep 01, 2004 10:44 am

Keith,

As you like to play with carbs so much I thought you might find this interesting <a href='http://www.triumphspitfire.com/carbchoices.html' target='_blank'>http://www.triumphspitfire.com/carbchoices.html</a> :D
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PostPost by: mac5777 » Wed Sep 01, 2004 2:07 pm

Hi Steve, I know the company you referred to above, PRI, they are in Oregon. I had a spitfire and almost had the carbs changed but purchased the Elan instead. Has anyone installed the Keihin carbs? And are they as good on a lotus as they are on the triumph spitfire. If anyone has some good results, I may attempt the change.
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PostPost by: type26owner » Thu Sep 02, 2004 1:19 am

Hi Steve,
Yep, I've seen that link before. Think I might have read ALL the ones which included 'carburetor'.

Whadya mean by 'play'? :D Unfortunately if you have to repair and tune one or a thousand Webers you still have to know the same amount of stuff. To bad all of the tuning books don't actually explain how to do the tuning part of it.

Best Regards,
-Keith
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PostPost by: type26owner » Sun Sep 05, 2004 7:06 pm

For the warmer climates areas out there like California a 0.6mm starter jet makes the cold start units function properly. Weber p/n 75605.060. This jet style is the early single piece type. Pierce Manifolds $7 USD each. 4 are required, 1 for each throat.

Look at the jet with the partially shown flat screwdriver just above it on the rightside of page 16. That's the starter jet.
<a href='http://www.lotuselan.net/uploads/weber.pdf' target='_blank'>http://www.lotuselan.net/uploads/weber.pdf</a>
-Keith
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PostPost by: type26owner » Sat May 14, 2005 1:14 am

The cold start chokes have stopped working for the last two tankfuls of fuel. Been scratching my head wondering what broke. Ah, turns out nothing at all I suspect. Bet Shell started distributing their summer gasoline stock a couple of weeks ago. I'm just jetted too lean now for that stuff. Really don't need the cold start chokes when the ambient temperature is above 10C in the mornings. :D
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