Lotus Elan

Which to use...DCOE 18's or DCOE 151's ?

PostPost by: berni29 » Thu Jul 28, 2005 7:46 pm

Hi Guys

I have the choice of either. The 151' have an extra adjustment screw (with a white cap) on either side. What is that for? And is the jetting different for a big valve because I will have to mix and match jets if it is as at least one set came off of a big valve, and they are going on a standard engine.......

Thanks for any advice!

Berni
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PostPost by: bill308 » Thu Jul 28, 2005 10:33 pm

Berni,

The extra screws, under the caps on the 151's, are air bypass screws. This is a desireable feature on the newer Webers, not found on the 18's. This feature allows airflows to be balanced between throats of a carb without bending the throttle shaft or drilling holes in the throttle plates. In practice, start with both air bleed screws seated. Measure the airflows of both throats common to the carb. If one throat is flowing less than the other, open the bypass screw on the lower flowing throat until flows are equal, then lock the air bypass screw in place.

The standard twink is jetted differently than the SE or Sprint carbs. You will likely have to perform some jetting changes. Even the fuel is different today and this could also drive you to rejet.

Bill
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PostPost by: berni29 » Fri Jul 29, 2005 2:49 am

Many thanks for that Bill. You mentioned 131's and I was not sure if they were different. I will set them up as best I can, and if things are still not right I will be off to see the nice man at the rolling road!

All the best

Berni
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PostPost by: berni29 » Mon Aug 01, 2005 3:21 am

Hi Bill

I started to clean and rebuild the 151's and then decided to look at a pair of 18's that had come of my first plus 2. Now that car went like stink and drove really well, so I cleaned them up and put them on my car. Sure enough the car now goes really well with no major flat spots. The only thing is that they have 30mm chokes. Is it worth putting 32's in?

Also I am waiting for my syncrometer to arrive to balance them properly, and have guessed at the mixture (cannot find my colourtune) but I have one of those Gunson exhaust analysers. It was a cheap (?65) thing. It has a tube you push up the exhaust. Have you had any experience of using them with a twincam?

All the best

Berni
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PostPost by: bill308 » Tue Aug 02, 2005 12:36 am

Berni,

You stated in a previous post that you have a standard engine. If that's the case, I'd leave the 30 mm chokes in in the carbs. The 30mm chokes should work better over the useable rev range. I believe the SE and Sprints went to larger chokes, but they also had different ignition timing, cams, jets, and inlet stacks, to presumably take better advantage of the extra flow potential of the larger chokes.

Although I have a Gunson exhaust analyser, I've never used it. The problem is it will only read the average mixture results of all 4-cylinders so for setting idle mixtures this is not too useful. I have used the colortune and found it to be pretty useful for setting idle mixtures. I supose once the idle mixtures are set, the gas analyser would be useful in checking/dialing in the mains. Here you are making wholesale, discrete changes (i.e. changing all main jets or changing all air bleeds, or all etc.) so you really do want the results of the changes applied to all cylinders. As I recall, this meter is portable so it can be used on the road. Don't expect too much from it looking at transient conditions because I believe there will be a noticable delay in it's response. For steady state cruise or wide open throttle in high gear conditions, you may get some useful information.

Bill
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PostPost by: berni29 » Tue Aug 02, 2005 11:37 pm

Hi Bill

Many thanks for your replies. My synchrometer arrived today. What a useful device. Having said that I was not far out having already done it by ear and plug lead pulling. When I get bored one day I will give the gunson a go while driving at a steady speed. It will have to be taped to the boot I guess, with an observer in the passengers seat.

I am going to have to get another colourtune. I know that many consider them a toy, but they work for me.

All the best

Berni
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PostPost by: berni29 » Wed Aug 03, 2005 10:05 am

Hi Bill

Just by way of completeness, my engine is the original one for the car and according to the Miles Wilkins book (just got for my birthday) on the Lotus Twincam engine is a Super SE. Its engine number starts with the letter "F". It would have had 32mm chokes on the Webers as standard, but to cure flatspots Lotus suggested 30mm chokes and a 200 air corrector jet instead of a 150. Not sure which one mine has.

All the best

Berni
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