Lotus Elan and Lotus Elan Plus 2 Forums

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

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?

Yes proportional but it’s 40 = 0.40 mm ( ie 100 = 1mm )
So 50 is just 0.1 mm larger or 0.004 inch.

Ok thanks! The flat spot starts at 2000 rpm most noticeably when throttle is open quickly.

Chris, you need one of these tools for jet size measurement (I forget where I bought it, but’s it’s made in Germany)

Thanks Glen. I'm going to order from Pierce Manifold.

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.

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.

Standard pump jets are 35, so going quite a bit bigger. Assume you only get when increasing throttle noticeably?

Imho your problem is not the pump jet but the Idle Jet to stop hesitation.
Alan

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

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?

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.
Alan

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.

Colourtune is only for slowrunning mix tick over.
Alan