Lotus Elan

Jet sizes

PostPost by: TBG » Wed Jun 17, 2020 5:44 pm

Hullo everyone - I am looking for advice on Jet sizes. I have a 1700cc engine running 10:1 compression and QED 420 cams - big valve head - at present it is jetted as for a 1558.

34mm chokes - Main jet 140 - Air corrector 155 - Emulsion tube F16 - Idle jet F8 45

Do I need to change any of these to cope with the increased capacity?

Very grateful for any help out there!
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PostPost by: SENC » Wed Jun 17, 2020 10:31 pm

I can't answer your question directly as my only Weber experience is on my "new" 65 Seven (pre-crossflow), but I would suggest joining Keith Franck's group:
https://vintagetechnologygarage.groups. ... ?id=103878

Keith has been developing better emulsion tubes for the Weber for his Elan (and other cars) for years. Lots of good information in the threads, plus a pretty active group discussing experiences. Not a forum, but more of a listserve model.
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PostPost by: nmauduit » Wed Jun 17, 2020 10:43 pm

I won't elaborate on KF jets here, but I would not go that route just yet... (been there, I commented on it after a dyno session)

I'm no expert on anything, but have tried a few things with jets and found them to be counter intuitive... a wideband lambda sensor is certainly a great help for tuning.

then I would try a smaller main (say 135)...
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PostPost by: webbslinger » Thu Jun 18, 2020 1:53 am

I'm even less of an expert, but +1 to getting an afr meter. If your carbs are in good shape and mounted correctly they will respond to jetting pretty much by the books, at least down to a "pretty good" tune. The afr meter will be like an expert helping you who can tell you how they are responding to changes.
My engine has stock rods and crank so it's down about 85cc from yours but otherwise similar.
I was unable to get mine running the way I wanted it on 34mm chokes and have changed to 33s. Getting away on hills wasn't very good and getting going in general wasn't much better. As a tuner I'm the equivalent of a 200 hour pilot so I probably missed something and anyway your engine may have enough torque that 34s are fine.
I'm using F30 mains, 50F9 idles & F16 E tubes. Let us know what you wind up with.
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PostPost by: Panda » Thu Jun 18, 2020 7:50 am

I was running a similar 1700cc engine a few years ago with Q450 cams and 1.625 and 1.400 valves and used 38mm chokes 155 mains 180 air F16 emulsion and 50 F8 idle. Worked very well on the road and track.
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PostPost by: Esprit2 » Fri Jun 19, 2020 7:22 pm

What are your expectations for the engine? So far, you're asking us which woman you should marry, and we don't have a clue about your tastes. You've gone to the trouble of punching it out to 1700cc, and you've chosen a QED 420 cam. You must have something in mind.

There isn't one answer to your question. Carbs are like spider webs... touch them over here, and they tingle over there. Every thing you do to the engine can impact optimum jetting. Even the air filter used, and your altitude above sea level will affect carb jetting. Don't get your hopes up for a plug-n-play answer that just works.

There are standard 'book' formulas for 'starting point' jetting, but getting to a final optimal setting will require much tuning... trial & error jet changes. You would be wise to have the carbs set-up on a rolling road.

How deep are you willing to go into your carbs? Chokes are one of the main foundational features that affect everything else. Your 34mm chokes are sized for a 1558cc Twink, and will be a bit small for optimum performance in a 1700cc. They would be civilized on the street if you wanted to tune for flexibility, but not best for optimum performance. Changing chokes isn't a 'big' deal, but it does involve deeper surgery. Are you up for that, or where you just hoping to screw in some new jets?

Regards,
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PostPost by: TBG » Fri Jun 19, 2020 9:25 pm

Tim - thank your that!! I have been to a rolling road and been attended to by an "Expert" who totally cocked up the engine, My response to this chap is here.

"I thought that you would interested to know that after your attention to my Elan yesterday the drive home was a nightmare. Hugely reduced power as though someone had stuffed a rag in the intake, very rough running and snatching around 3000 rpm and lower down and popping and farting on a trailing throttle and elsewhere.

I have sent your power readout to various interested parties and they cannot imagine how you let the car out of the workshop with a power band like that!

To cut a sad story short, I have reinstated the proper jets as recommended by QED, re-balanced the carburettors as they were way out, sorted the mixtures and had the timing redone. Your invoice states that you left it at 12 degrees – it was in fact 8.

The car is now running again brilliantly and I hope that you learn from my expensive mistake."

i hope that someone out there has a simple solution to the jetting of a 1700cc engine with my specification. Some say I would be better off with 32mm chokes..........??
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PostPost by: Esprit2 » Fri Jun 19, 2020 10:53 pm

Sorry to hear about your bad experience with one shop. But don't use that as a reason to avoid all shops. They aren't all turkeys.

TBG wrote:i hope that someone out there has a simple solution to the jetting of a 1700cc engine with my specification. Some say I would be better off with 32mm chokes..........??

There is no universal carb set-up, so the first task is still up to you... decide what you want.

Carbs don't have sensors and feedback loops working through an ECU to provide the correct mixture under all conditions. Carbs are narrow minded, and not particularly flexible about accepting... or delivering against a wide range of expectations. You need to decide what 'you' want, and tune for that. There is no one universally correct setting for a 1700cc Twink.

Smaller, 32-33mm chokes may improve low-speed driveability and make the engine easier to drive in commuter traffic. But the choice of QED 420 cams indicates you might have loftier plans for which small chokes aren't the best choice. The cams & small chokes won't be pulling on the rope in the same direction.

Good luck,
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PostPost by: mbell » Sat Jun 20, 2020 12:13 am

Any recommendation from on here are going to be hit and miss due to all the variables. The settings provide with the cams are likely to be as closer than as anything suggested here.

Your basic choices are:
- live with the standard recommended jetting from qed
- take it somewhere else and have it turned
- learn about Webber carbs, buy a selection of jets and carb components to test and tune it yourself (probably worth buying a afr gauge if you go this route)
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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PostPost by: 1owner69Elan » Sat Jun 20, 2020 1:26 am

Esprit2 wrote:There is no one universally correct setting for a 1700cc Twink.

Smaller, 32-33mm chokes may improve low-speed driveability and make the engine easier to drive in commuter traffic. But the choice of QED 420 cams indicates you might have loftier plans for which small chokes aren't the best choice. The cams & small chokes won't be pulling on the rope in the same direction.

Tim Engel

mbell wrote:Any recommendation from on here are going to be hit and miss due to all the variables. The settings provide with the cams are likely to be as closer than as anything suggested here.


As noted above, there are a lot of variables, so anything posted here should be considered as perhaps a starting point. But, you already have a starting point from QED.

Hopefully, to not add to the confusion, but to provide another data point, here are the settings for my 1700 Twincam. What is notable, and contrary to the common wisdom, is that the high power/torque derived are with 40DCOE's and 32mm chokes. But, a lot of variables at play. (And no, again, this was not from a "juiced" dynamometer).

1700 cc. Long stroke, stock block
Builder: Dave Vegher
181 Bhp @ 6900
143 lb-ft @ 5100 (~flat from 4000-7000 - dyno range), car pulls strongly from lower rpms
7000 redline, street engine
Very tractable, no hesitations from idle to 7K
Idle, dead smooth; @ <1000

32 mm chokes, 40 DCOE
New SAS head
Valves: 1.625 / 1.375
0.44" lift Kent cam, short duration
Compression: 10.3
Pump fuel

F16, 40 pump, 140 main

Intially: air 165, idle 45F9

Later, changed air to 180 and idle to 50F6
'69 Elan S4 SE
Street 181 BHP
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