Lotus Elan

Jetting Webers for 285 cams

PostPost by: richardl46 » Tue May 29, 2018 9:19 am

My rebuilt engine is settling down nicely after 1000 careful miles so I'm getting more fussy about the way it behaves.

Should I expect some hesitation 1500 to 2000 rpm, especially at town traffic speeds. The head has bigger inlet valves, is gas-flowed, 10.5:1 compression and Piper magnum 285 cams? The engine is easy starting, carbs fully balanced with a vacuum meter and accelerates very cleanly after the initial hesitation.

The carbs are 40DCOE 18's with exactly the same spec as the big-valve set up on page 186 of Miles Wilkins book; ie 33 mm chokes etc. On that same page the author refers to a known flat-spot condition and using a larger accelerator pump jet on the SE. He actually implies that a 35 jet is larger than the 40?

Should I try some size 40 pump jets if these are the next size up from my 35's. I know there are many other variables to consider but this seems like a good starting point.

Thanks, Richard
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PostPost by: nigelrbfurness » Wed May 30, 2018 8:08 am

Check the float levels and maybe play around with them a little before you start changing jets.
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PostPost by: richardl46 » Wed May 30, 2018 8:21 am

I haven't checked the floats since rebuilding the carbs two years ago, so will give it a try first. Many thanks, Richard
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PostPost by: nmauduit » Wed May 30, 2018 8:48 am

pump jets would be the last thing I would try to tune, it's only for the short period (less than 1 sec) when pressing the pedal (the amount of fuel stored for enrichment is squirted slower or faster dependig on the jet), this is rather fine tuning and difficult to feel unless very wrong. If you think your engine may be weak in the high revs you may want to look into richness of the main circuit, if you think the transition region has too bad of a flat spot you may want to look into the idle circuit (which will probably require retunin the idle). For tweaking carbs all other parameters should be set well (esp. ignition timing). A lambda sensor may be of help on this journey...
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PostPost by: HJF » Wed May 30, 2018 8:54 am

I read that too and believe it is an error, I understand there are one or two in the book. My understanding is that Weber jet numbers are the hole size in hundredths of a millimetre.
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PostPost by: CBUEB1771 » Wed May 30, 2018 1:03 pm

Hesitation during the transition from the idle circuit to the main circuit is very common. I recommend that you follow Keith Franck's jetting developments at his "sidedraft-central" discussion group on Yahoo Groups. He sells his jets and set-up tools on line:
https://www.webstore.com/user,pgr,82598 ... ther_items
You should check manifold depression at idle. With your engine modifications it may be in the region of 17 inches of water which will exacerbate a lean condition as you move off of the idle circuit. I have no financial interest in Keith's products, I just appreciate the depth of the analysis and engineering he has put into improving the performance of the DCOE carburetors. I am a year or so from having my +2 back on the road and I am really looking forward to testing his set up procedures and jets.
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PostPost by: prezoom » Wed May 30, 2018 3:52 pm

One thing of interest from a recent posting of Keiths, was his thoughts on having more progression holes in engines with lower vacuum at idle. I used DCOE 45 152G carbs, 36mm chokes, that have 5 progression holes on my Zetec conversion. It has absolutely no hesitation off idle, and will pull strong from just over 1000rpm. I had to be careful with throttle and clutch operation when starting from a stop, or I would get wheel spin. However, that was changed with the installation of the close ratio trans and the 3.54 differential Previously, Keith had stated that carbs with 5 progression holes would not work on the twin cam, but everything I read on the Zetec, said they were absolutely necessary. So I took a chance with the new carbs. I did set the float level at the recommended 25mm. Have not checked the vacuum at idle yet, as the Dunnell Engineering intake manifold has no port for testing, another reason for the MX5/Miata headlight conversion, which is excellent.
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PostPost by: pauljones » Wed May 30, 2018 4:44 pm

http://www.s262612653.websitehome.co.uk ... webers.htm

Ive found this a good source of info
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PostPost by: HJF » Wed May 30, 2018 7:32 pm

Richard,
I think I recognise what you are finding from my experiences having rebuilt my son?s Crossflow powered Caterham with twin DCOEs and 244 cam. The engine was rebuilt and run on a dyno? for initial setup. When I installed and ran it in the car it was initially a bit rough and had a noticeable flat spot around 2000 rpm. I began to tune it myself ... balanced the carbs again which improved it. Carefully set the float levels which improved it. Readjusted the dizzy timing which improved it. Adjusted the idle mixture and reset idle speed and balance again which improved it. So, incrementally it got much better until I had reached about the same point as I think you may be at, i.e. really starting and running well with slowly and steadily pressing the throttle accelerating the car smoothly but a more rapid attempt at accelerating still resulting in a now slight but still annoying hesition. So, next step was to tune the car under realistic road conditions on a rolling road. Best place for me was Aldon in the Midlands as they know carbs?. You can make an initial setup with the engine on a dyno? but need to then move to a rolling road to really see what is happening. Roger at Aldon did the work. He is the chap you see setting up the yellow Elan on the Car SOS TV programme. He wired the car into the diagnostics and off we went and indeed he could spot the hesition I had been on about on his first assessment run through the range. First issue was a mechanical problem with the dizzy advance curve. In short we ended up swapping it out for a new replacement Aldon dizzy which then took us forward to the next stage. He then began revising the main and idle jetting and idle mixture etc until the car was running significantly better. We got to a point where the hesitation was significantly reduced ...he did say that you usually cannot 100% eliminate it as it is a feature of the progression phase as the Weber?s transition through it ... I think Dellortos maybe slightly better (more holes?). We had now been at it most of the day with a break for lunch and he declared it to now be running very well as confirmed by the power printout curves. Now, pump jets: the ones it already had fitted were 45?s which I already suspected to be too big as the Caterham wisdom for that engine tune is for 35?s. Roger felt that when simply blipping the throttle in neutral and at idle it was not quite ?snappy? enough and decided to try 35 pumps. Unfortunately, when he went to the stores they had no 35?s left! So we left it that I would get some and play around with this myself when I had the time. When we got the car home (we trailered to and from Aldon) it was going really well, much better with a hardly noticeable hesitation now on rapid acceleration. It is so good that a year on I have not yet got around to playing with the pump jets. I do suspect that the current 45 is too large and the very slight hesitation that remains is a mild wet stumble so will be buying some 35?s over the Summer and seeing what happens.
Hope that helps.
Harry
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PostPost by: richardl46 » Thu May 31, 2018 6:48 pm

As usual a lot of good advice for me to follow up.

If the hesitation is typical of Webers I need to decide what is an acceptable stage to reach without looking for perfection. I'm not inclined to throw money at rolling road tuning as my "Boy's Toys" allowance" will not run to that.

I'll start with the float levels, move on to idle jets and see where that gets me. From the various publications on basic set-up, it's clear there are many permutations possible and some get well away from the original Lotus settings.

Again, many thanks to all who responded.

Richard
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