Weber chokes and jets

[chrishewett]

Since my engine rebuild ( big valve 1971 +2s130) I have felt that the car is not as fast as it was. At the same time as the rebuild I fitted a pair of 40 DCOE 31 carbs that I bought on ebay as they had been rebuilt and mine were well used. Yesterday I checked the size of the main jets ( something I should have done when I fitted the carbs) and discovered that they are 110. I assume that the chokes are also smaller 30mm chokes. My book tells me that the main jets should be 120 and the chokes 33mm.
How much difference would fitting the bigger jets and chokes make?
Should I also change the slow running jets to 50F8?
And finally where can I buy jets and chokes?
Your advice would be gratefully received.
Chris

[mikealdren]

Chris,
I would expect a significant difference, smaller chokes will reduce the airflow and the smaller jets will give less fuel (appropriate with smaller chokes).
Have you still got your original carbs? You could start by swapping over the components to see how much difference they make. Don't do it piecemeal, if you change the chokes and not the jets for instance, it will run weak and that may cause other problems.
If they are like my Dellorto's there are quite a few components that vary from one carb to another, check the whole spec.
As to new parts, there are lots of Webber service centres who would have the parts. Also tuners with rolling roads usually carry the key tuning components like jets and I would expect the prices are fairly standard, I wouldn't think they'll be discounted much but I may be wrong............
Good luck.
Mike

[chrishewett]

I have done a bit more research and I find that 33mm chokes are rare as they were a Lotus thing and not generally available. How I wish I had known about this before I put my old carbs on ebay! I could simply have swapped over the relevant parts first.
I can obtain the chokes but they are more expensive than chokes ending with an even number!
Thats life.
Chris

[Dave-M]

Chris, New mains are only a few quid for a set. Regarding the chokes, it is a perfectly simple operation to turn them out to 33mm and will only cost a few quid, if you dont know anyone with a lathe I may be able to help you.
Regarding the idle jets I would personally change away from the 50 f9 down to 45 f8 as this jet contols the off idle progression and is the most used part of the fuel curve in normal driving. I have settled on an even leaner setting and the car drives fine and does more than 5mpg extra.
I have a reasonable selection of idle jets if you want to experiment. I am only a short way up the road from you.
It may be worth checking you have the correct emulsion tubes also as this is quite important for correct running.
Regards
Dave

[berni29]

Hi There

I would not even think about getting rid of those 30mm chokes. They increase air speed at low revs and reduce the tendancy to flat spot. You will only notice any possible power loss at high revs, and even then I reckon that you will need the jets/air correction right & ignition/timing and everything else spot on to notice it.

I would make sure everything else is as spot on as possible.

Berni

[rgh0]

Chris

The engine can be made to run well on 40DCOE's for road use on any choke size from 30 to 34mm as long as you have the correct jetting setup I beleive. 35 and 36mm chokes can also be made to work but offer no real advantage for a relatively standard road engine and you should go the 45's with these choke sizes anyhow. The bigger 33 or 34 chokes will give you a little more top end with your sprint spec engine but not much. Much more important to get the mixture and ignition timing set properly for the chokes being used as a wrongly set up 33 or 34 mm choke will give less power than a properly setup 30mm choke

The factory setting for the 33mm chokes work well and are better than the factory settings for the 30mm chokes at least on Australian fuels. If worried about the cost of 33mm chokes I would go with 32mm chokes and the setting for them in the 118hp SE engines as a starting point.

However to get the best out of any carb setting these daysyou need to set it up for the fuels you are using on a dyno or DIY it with a wide band A/F ratio sensor as fuel has change substantially since the days when Lotus developed the original carb jetting specs. Again if cost is a challenge to doing this then find someone in your local Lotus club using the same fuels as you who is happy with their engine performance and has perhps set it up on a dyno and copy their setup.

cheeers
Rohan

[chrishewett]

Thanks for the replies.
After a lot of research today I have decided to go back to the setup that Chapman originally fitted. After all you have to have faith in the man!
I have ordered new chokes and jets from fastroadcars.co.uk in Gwent at a cost of ?75. From there I will set it up myself.
The setup I have at the moment, which was set up on a rolling road by someone who aparently knows webers, as standard elan, means that the revs do not pick up until about 2,800, it then has a bit of a surge but then dies and doesn't want to rev over 5,500 readily. I think this is because the slow running jets are too small and then it is not getting enough fuel and air because of the small mainjets and chokes. Whatever the reason it is not the car it was before I had the knackered engine rebuilt and that job was done properly.
I will fit the new parts at the weekend and let you know the outcome.
Thanks again,
Chris

[chrishewett]

I have just fitted the new chokes and jets and it started straight away and sounds better. I don't think I will take it out in 60mph winds with bits of trees flying about so I will post again when the weather has calmed down.
Chris

[chrishewett]

I have now driven the car with the new carb parts and it is totally different. Much quicker over the whole rev range.
Unfortunately now one of the universal joints in the driveshafts has gone so that is my weekend job!
Chris