Lotus Elan

Flat Spot On My +2

PostPost by: JACKJABBA » Sat Sep 20, 2003 12:17 pm

Hi there. I have a 69 +2, with twin webbers. I have balanced the carbs with a synchrometer and set up the mixtures with a colour tune. I have set the timing to about 12 deg btdc at 1000 rpm as it has had an unleaded conversion. There is large flat spot from about 2000 to 2800 rpm when in this condition. If I lean the mixture off the flat spot is not as large but the car starts to pop and bang on the over run, especialy going down hill. The idle jets are standard and the car goes well above 3000rpm. I have checked for intake leaks and can not find any, but the silencer has a slight blow.

Any ideas? Could the slight blow in the silencer cause this condition?

The advance for the ignition is working and it has electronic ignition and new plugs.
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PostPost by: types26/36 » Sat Sep 20, 2003 2:06 pm

I am also having problems with some webbers at the moment but as to your problems I would think the "poping" through the exhaust is due to the exhaust leak,
as for the flat spot have you checked the pump jets to see if they are spraying or the acelerator pumps are working? although if they are not working I would expect you to have a flat spot from idle....... just a thought.
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PostPost by: gjz30075 » Sat Sep 20, 2003 8:32 pm

Yes, I agree that the pump jets are the first to look at. If all is standard in the carbs, the pump jet should be a size 40. One, or some, could be blocked. You can use the color tune to determine if you're rich or lean at that 2000 - 2800 range.
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PostPost by: types26/36 » Sat Sep 20, 2003 10:13 pm

Jackjabba,
I've just been re-reading your post and you say its had a unleaded converson,
have you just refitted the head after the conversion? if so it may be a stupid question but have you got the cam timing right? its easy to get it out by one tooth as the marks on the sprockets do not aways line up too well.......its also possible to mix up the inlet/exh sprockets.
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PostPost by: JACKJABBA » Sun Sep 21, 2003 8:05 am

Thanks for the replies.

The head conversion was done by the previous owner. But I have changed the head gasket since buying the car. The timing marks line up and are flush with the surface of the rocker cover face and both were marked for inlet and exhaust. I must admit it took a lot of time and cursing to get the valve timing correct as it seemed to not want to line up.

I will check out the accelerator pump jets and will post my results.

Regards Jack
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PostPost by: JACKJABBA » Sun Sep 21, 2003 2:34 pm

I have found that one of the pump jets was blocked. But I am unable to test the car at present as the ignition has died when I moved the car to access the carbs.

I was adjusting the mixture using the idle adjusters.
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PostPost by: elans3 » Sun Sep 21, 2003 9:11 pm

Hello, just read of your problem. Wondered which engine is in your +2 ? If it is the 118bhp unit that is the same as my Baby Elan S3 S/E, there may be some similarities with my car. When I bought it, I found that it had huge power when you were at full throttle, (quicker than the Sprints that I had tried), but had a flat spot at about the same revs as yours when under more normal driving conditions. It wasn't very nice to drive slowly !.
I was looking through the original, big loose leaf Workshop Manual, and came across a whole section devoted to "Flat spots on S/E engined cars", page 54, Section L. The 118bhp S/E engine is fitted with 32mm chokes, and 150 air correctors, and the manual says that this does create a flat spot. They go on to say that the remedy is to fit 30mm chokes with 200 air correctors. I duly did this, and the flat spot disappeared completely. The top end power was slightly reduced, but the car is now much more driveable, and I've still got the old bits if I want to change back. The manual says that the power is reduced by 3 bhp only, and I would say that they are about right.
Wish you luck with it !
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PostPost by: JACKJABBA » Mon Sep 22, 2003 7:36 am

Thanks Steve, I will look into this, as I have the normal valve head.

Has anybody out there had to re-jet due to doing an unleaded conversion?

What static timing are people using with unleaded?

Regards Jack
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PostPost by: JACKJABBA » Mon Sep 22, 2003 8:36 pm

This is all starting to make me feel dizzy. :blink: :unsure:

The car was manufactured late December 1969, +2s, (checked the chassis number and log book)

The engine is an F prefix with red head. as you would expect.

The carbs though are jetted for the S/E model and not the S model. So may have
been changed sometime in its life, or were what was on the parts shelf that day.

So now I will try to get the 30mm choke's, 110 main jets and 155 air corrector jets. I think this may be about to hurt the wallet.

The Workshop Manual says that all the +2's should have size 35 pump jets, mine has 40's.

Please chime in on this, as any of your thoughts are appreciated. :D
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PostPost by: JACKJABBA » Mon Oct 06, 2003 3:15 pm

Well, I have now replaced the blowing silencer and replaced the choke's for 30mm items, 110 mains and 155 air corrector jets. Set the idle mixture with the colour tune and took it for a run. The car still had a flat spot but not as bad as before, the performance above 3000 rpm has dropped off a little.

Any idea's on the effect of changing the air correctors to 200 as Steve has mentioned.


Anyone in the North East who has a +2s that can compare my performance with theres?
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PostPost by: gjz30075 » Tue Oct 07, 2003 1:41 am

Sounds like you might be masking the real problem. Check your slow run jets, also called idle jets. They could be clogged. What size are they? 45F9? Could try a 50F9, as some manuals suggest.

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PostPost by: steveww » Tue Oct 07, 2003 8:48 am

It might be worth investing in a day at the dyno. There you can quickly try out lots of different settings and have access to more advanced diagnostic equipment. I always set up my baby Elan at the dyno as mine is not standard and the factory setting make no sense at all.

May be you just prefer to DIY ?
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PostPost by: JACKJABBA » Tue Oct 07, 2003 3:45 pm

I don't mind having a go myself, I don't like to pay for something I can do. But it looks like I may as well pay for the dyno time, as the jets will all add up to what I will end up paying at the dyno.

Just checked the idle adjusters, and they are all 3/4 of a turn out, to give a good colour on the colour tune. Run the engine up from idle slowly and the colour remains unchanged. Blip the throttle and it goes yellow. I would have thought that if the adjusters are at 3/4 out, that the idle jets are the correct size? They are 45F9.

What effect does the air corrector jet have and at what engine speed?
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PostPost by: types26/36 » Tue Oct 07, 2003 6:06 pm

JACKJABBA,
Have a look here, its got a bit about A/C jets.
<a href='http://www.theape.com/jets.html' target='_blank'>http://www.theape.com/jets.html</a>
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PostPost by: gjz30075 » Tue Oct 07, 2003 7:26 pm

Good website. 45F9 are about right. These idle jets also handle progression and progression varies if the Weber is a type 18 vs a type 31, both common to the Elan. Your model type is on the top cover. I like the dyno idea. You really don't know if you're going rich or lean when it stumbles and a good dyno with a fuel/air meter could tell you. If you're seeing a yellow flame at blip time through the colortune, you could be too rich. Time to move to a fuel/air meter.
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