Lotus Elan

weber carb question

PostPost by: seniorchristo » Mon May 11, 2020 12:10 am

I purchased some size 50 accelerator pump jets for my twin cam trying to improve upon a low end hesitation. Yet the existing size 40 jets appear bigger than the 50's. I thought the jet size was proportional to millimeters. Not having a wire gauge to measure size, I guess it is possible the 40's were drilled out. Is the jet size proportional to the opening?
thanks
Chris :)
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PostPost by: SENC » Mon May 11, 2020 2:27 am

I'm fairly new to Webers, but I believe you are correct. I believe 40=.4mm and 50=.5mm. Was your low end hesitation at WOT or partial throttle?
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PostPost by: Craven » Mon May 11, 2020 11:13 am

Yes proportional but it’s 40 = 0.40 mm ( ie 100 = 1mm )
So 50 is just 0.1 mm larger or 0.004 inch.
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PostPost by: seniorchristo » Mon May 11, 2020 1:34 pm

Ok thanks! The flat spot starts at 2000 rpm most noticeably when throttle is open quickly.
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PostPost by: Certified Lotus » Mon May 11, 2020 2:07 pm

Chris, you need one of these tools for jet size measurement (I forget where I bought it, but’s it’s made in Germany)
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PostPost by: seniorchristo » Mon May 11, 2020 4:06 pm

Thanks Glen. I'm going to order from Pierce Manifold.
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PostPost by: nigelrbfurness » Mon May 11, 2020 5:13 pm

Did you check your float level height during your investigation into your flat spot? More than once I've found incorrect float heights to the cause of the problem you describe.
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PostPost by: davidj » Mon May 11, 2020 6:31 pm

I too had hesitation at low speed and car would sometimes cut out when slowing down for junctions. I eventually traced the problem to the float height as well.
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PostPost by: mbell » Mon May 11, 2020 7:18 pm

Standard pump jets are 35, so going quite a bit bigger. Assume you only get when increasing throttle noticeably?
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PostPost by: alan.barker » Wed May 13, 2020 6:20 am

Imho your problem is not the pump jet but the Idle Jet to stop hesitation.
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PostPost by: seniorchristo » Wed May 13, 2020 3:20 pm

From what I've read I didn't really think it would be the pump jets but thought I would experiment. I've set float levels but may recheck using Keith Frank method. I have different idle jets but undecided whether to go richer or leaner.
Thanks,
Chris :)
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PostPost by: h20hamelan » Wed May 13, 2020 4:44 pm

Might be worth drilling a bung for o2
I leave the sensor/sender in place, but only have the readable instrument in place for checking.
Do you have access to ColourTune?
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PostPost by: alan.barker » Wed May 13, 2020 4:57 pm

What colour are the Spark plugs after a good run. You'll know straight away if it's weak or rich.
when you go up 5 on the size you can see the colour change on the Spark Plugs easy.
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PostPost by: seniorchristo » Wed May 13, 2020 6:02 pm

I've used colortune to set idle mixture but didn't think to use for the progressive circuit. I have an O2 bung to install but I'm waiting to rework my exhaust. Never a lack of things to do on an Elan. :)
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PostPost by: alan.barker » Thu May 14, 2020 5:50 am

Colourtune is only for slowrunning mix tick over.
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