I hope you can help me identify the source of the problem.
Yesterday, a friend and I did a service on my Lotus Elan S3 S/E (1967). We did the following:
-Engine oil change
-oil filter change
-Air filter change (the new filter, bought from Keveldon, was 2cms higher than the old one in place. We had to tighten the original airbox straw pretty hard to fit the new air filter.)
We wanted to check the synchronisation of the carburettors (Weber 40 DCOE 151 fitted new 6 months ago).
This synchro was done with a cold enough engine (1st mistake?), and by playing with the idle screw/synchro screw, with a small depressiometer.
When I got home (50kms), the idle was high (2000 rpm). Apart from that, I had no problems with the engine’s behaviour, no misfires, no gaps when accelerating, nothing drastically new as compared to before the service. I had to park 5 km from home, and it was very hard to restart the engine when it was warm. I managed after 6-7 attempts if not more.
In front of home, I lowered the idle speed via the idle screw on the carburettors to reach 900 rpm at 90 degrees engine temperature, did not touch any synchro screw at all. I tried to restart the engine to park the car: again, I had all the trouble in the world to restart it. I pushed it into park.
Do you know what could have caused this problem, which I didn’t have before?
A bad carb synchro? The air filter being slightly bigger than the old one? Anything else?
Hi Niklas,
I suggest you buy a “Carbtune” it uses 4 columns. It connects to each choke inlet and balanced in 10 miniutes.
The 151 carbs have the inlets ports very easy. You don’t need to remove trunking on Carbs. Did you check Timing 12° btdc on Tickover.
Alan
how was the engine hot starting prior to the resynch ? did you follow the same hot restarting procedure as before (sounds like you may not have been depressing the pedal - without pumping - when hot restarting, or am I second guessing too far)?
It could be a clever purchase, i think we’ll have to redo the carb sync anyhow.
If i am right :
-engine shall be hot (80 degrees or more) when tuning carbs
-we shall take the air filter out for this sync
sometimes, even before this synchronisation error, the engine could (very rarely) be complicated to restart when hot if I wasn’t careful enough about how I restarted the hot engine.
For example, if I pressed the key a little without insisting and the engine didn’t start, then it could be complicated to restart it. On the other hand, I almost never had the problem, to be honest.
PS: when I took the spark plugs apart to have a look, I saw that 2 out of 4 were soaked in oil (the NGK BP6ES plugs are 6 months old). I wiped them off and tried to start the car, and it started right up with no problems. I think that the oil on the spark plugs didn’t help the car restart when hot…
So I think that, apart from the synchronisation problem, I’m going to have to find out why this oil is reaching these 2 spark plugs. In doing some research, I saw some information about valve stem seals that could be worn… to be seen.
Next step: try to restart the engine when it is hot to see if the issue comes back.
Interesting point, so please let me know more about this hot restart procedure?
do you have to push the accelerator pedal all the way down when you do a warm start?
I’ve never done this before, but I haven’t really had regular problems with a warm restart before this episode.
From cold full choke 4/5 pumps of throttle don’t touch throttle again and turn over car fires straight away don’t do anything and let the car start to gain engine speed. I leave mine running for at least a minute put choke back in half way and off you go
From hot crank engine touch throttle always fires and away you go if no fireing full throttle it will fire
Make sure mixtures on all chokes are correct
Look at the plugs should be a sandy grey colour
There most likely should not be valve stem seals - but from the look of the spark plug it seems the plugs were somewhat fouled (combustion residue rather than plain oil, you may want to investigate that keeping in mind that the engine breather is usually sucked up mostly by cyl #4 via the air box tube at the rear of the head), which does not help to hot start (even if the ignition system is in perfect condition)… this could come from a bit too rich tuning, and the recent adjustment may have emphasized that (e.g. if too rich on the “idle circuit” then after leaving the car idling a bit long before shutting it down you end up with a car difficult to restart hot). If not done already you may want to check the tune of your carbs (jets type and dimension) to check if there is a lead to follow there, but rather than swapping the spark plugs to the next hotter version (5) I would first try to reduce the fouling tendency by altering slightly the use of the engine (esp. idling).
As per hot starting, I press the pedal all the way down by slowly (to avoid squirting all the content of the accelerator pump down the intake) and leave it there when cranking, then release the pedal as the engine starts. You can practice that a day when you have time, if necessary taking the spark plugs out for a good clean if your going nowhere - ps: the spark plugs can foul real quickly (e.g. in traffic jams).
Hi Niklas,
To expand a little on nmaudit`s explanation above this is what I believe happens.
The Weber carburettors have a large hole above the level of the fuel to the airbox. When a hot engine is stopped then petrol evaporates from the warm carburettors and fills the airbox, duct and filter with petrol vapour. The mixture is now too rich to burn.
There is not enough air in the mixture and the vapour will not ignite in the engine cylinder. Cranking the engine with the throttle fully open will pump this rich mixture through the engine. After a few seconds when air has been pulled through and the mixture is correct the engine will start.
This is also what causes the smell of petrol in the garage after a run and why on a cold start we have to crank the engine for a while to fill the float chambers ( unless you fit a priming pump).
As above press the accelerator pedal slowly to the floor so that the accelerator pumps are not activated.
This seems to work for me,
Cheers
Eric in Burnley
1967 S3SE DHC
Thank you so much for the explanation. FYI the 2 spark plugs that are “fouled” are next to the windscreen. So that those are cylinders 3 and 4 if i am not mistaken.
I think i will do the following:
-check the carb sync when hot, and maybe even try a couple of jettings adjustments (ex: Idle jets)
-Buy 4 other NGK BP6ES in case they are needed
-Try to restart the engine when hot with the procedure you guys have shared
PS: Please find the jettings i have used for this Lotus Elan S3/SE (1967):
-chokes 30mmx4
-Main jets 115x4
-Air corrector jets: 200 x 4
-Emulsion tubes: F11
-Idle jets: 55 F8
-Accelerator pump jets: 40x4
-Needle valve: 175x2
-Back bleed jet x 2 around 50
-Fuel level : 25mm below the top of the float chamber
I found great difficulty with the fuel level at 25mm, it was always too rich and fouling plugs.
I was following Keith Francks advice and set 25mm. but could not cure the problem. With advice from Alvadave on here, I lowered the level, first to 27mm and then to 29mm. I am sure that 25mm fuel level would work correctly with Keiths own jets.
I am also using 30 mm chokes.
I settled on 45F8 idle jets and BP5ES spark plugs. The plugs are now a good colour and lasting a year or two.
There used to be a problem on 151 carbs with a hesitation when moving off from idling., called an off idle stumble.
One way to fix this was by running too rich. Again following Keith`s advice I drilled a fourth progression hole 1mm downstream of the first one to fill the gap between idling and progression modes.
My carburettors are about 14 years old and this may have been fixed on later ones.
best of luck
Eric in Burnley
Hi Richard,
yes, I know, that is the question. I will say it took a long time.
First though, Keith Franck sells a jig for drilling the extra hole. It is $150 with half back if you return it.
He also states the hole has to be 1mm diameter and 1mm downstream, which was all I needed to know. I also found someone on the internet advising the same modification.
I spent about two days on and off wondering if I could do it myself, measuring the jet hole where they are situated,
making drawings and plucking up courage.
I found that the existing holes are not so accurately positioned anyway and thought that I could do it as good as that.
Careful marking out, scribing and finally a centre punch mark.
A Dremell turning a pin vice with a 0.8mm twist drill made the hole in about 10 seconds each.
I am a scaredycat so I went under size at first but I have never needed to enlarge it.
Oil on the drill collects some swarf and any I missed just gets blown through the engine. There is very little.
The stumble was immediately gone.
I don`t know of anyone else who has done it so if you do please let us all know.
Good luck
Eric in Burnley
It would be interesting if one could duplicate the progression hole pattern of the 152G carbs. When I installed the Zetec in the Plus2, these carbs were recommended by the vendor, and included the the correct jetting. Sceptical, but upon inspection I found them to have 5 progression holes, and now I understand. Having 5 holes, makes a huge difference during the transition phase. The first time I started the engine, it ran incredibly clean, and when I drove it away from the garage, the engine ran as smooth as silk. No stumble or any flat spot. Can pull away in a higher gear when driving through town without having to change gears to avoid the hesitation. This is with a 3.5 diff and a close ratio gearbox. For the life of me, I cannot understand why all the 150 style carbs do not have the 5 progression holes. If they did, I would chuck all my 18 and 31’s for this design.
