Hot engine misses on acceleration

[loueelotus]

Hi, Lotus Elan S2 weber carbs.. Maybe someone out can enlighten me on an issue I have. When driving in very hot weather only (Engine really hot) I notice that on a kick down (quick acceleration) on the highway the engine will miss a couple of times then drive great .. Do it again than it happens again.. Everything seems fine otherwise.. I checked plugs, timing seems fine too.. Cooler weather runs strong..

Thanks,
Lou

[h20hamelan]

Which o2 sensor do you have installed, the wideband would be the best choice.

[mbell]

What pump jets do you have fitted? Are they working?

Might be going too rich and need smaller jets, or not working and going lean but that probably wouldn't be limited to hot weather.

[benymazz]

I also had a similar problem on my Elan. Car ran like a scalded cat in 50F weather but above about 70F it had a definite "stutter" when accelerating, getting worse as it got hotter. My issue was worst if I had my foot completely off the throttle and then quickly tipped back in to a small throttle application (say, between 10 and 30 percent). You got a rather nasty jerk back and forth as the car would accelerate and the lash/slack in the drivetrain ran out, then the engine would miss for a split second, just long enough for all the lash to run back to the coast side, and the engine would come back and the lash went back to the accelerate side.

Anyways, I fixed my issue by switching to smaller accelerator pump jets. I believe standard jetting on an S2 is #40, I went to #35.

BTW, a lot of advice on the internet will steer you in the wrong direction on this. Some DCOEs especially later ones have issues with "transition lean spots" or "tip-in lean spots". This is most likely NOT that. As temperature increases the air the engine is drawing in becomes less dense and the fuel the carburetors are metering flows and atomizes better. So when it's hot, the engine is drawing in a smaller mass of air, but the accelerator jets are metering the same amount of fuel no matter the temperature, so you end up with a less than ideal air-fuel ratio.

-Ben

[alaric]

Hi. I have a similar problem with my car - as I start to accelerate from no acceleration there's a slight hesitation, and it only happens once the car is hot. I tried larger pump jets, but the problem was still there. When I took the carbs off I found an ignition lead barely clinging on to the distributor cap - I couldn't see that before removing the carbs. The lead and cap contacts were burnt. So, my issue may have been ignition, not the carbs. I haven't got the carbs back on yet to confirm, but I would check your distributor leads and cap - it may be a weak spark on one cylinder.

Regards.

Sean.

[webbslinger]

My engine wants to stall on take off after being driven hard (er), and after max breaking, like after stopping in a timer box. After a couple runs, the engine can quit with just the tiniest touch of the throttle when trying to gently increase revs while idiling.
A friend told me this can be max breaking forces causing the Webers to flood resulting in fouled plugs for a bit. He said this is pretty normal when hard stopping like that. Has anyone experienced this problem? I would like to believe it is possible. Perhaps something like that is what's going on.
Otherwise, this engine engine also likes #35 pump jets. I wouldn't have got there without an AF guage.

[loueelotus]

Thank you all for your information.. I will see what I can pick out and use to try to correct the issue. I wonder if vapor lock or over heated internals has anything to do with it as mentioned. I am also using NGK cold racing plugs. This issue seems to start when driving down a country road for a while then get on the highway and start to push at higher rev's..

Lou

[rgh0]

Thank you all for your information.. I will see what I can pick out and use to try to correct the issue. I wonder if vapor lock or over heated internals has anything to do with it as mentioned. I am also using NGK cold racing plugs. This issue seems to start when driving down a country road for a while then get on the highway and start to push at higher rev's..

Lou

What plugs are you using "cold racing plugs" on a road car seems like a potential problem

cheers
Rohan

[loueelotus]

Hi Rohan,
I am using NGK B8EV Gold Palladium V type.. They do not make them any more.. I checked the plugs a couple of times after hard driving and in great condition.. Recommended by Lotus Ltd USA years ago.. I took the Elan for a run with me wife and the temperature outside was about 25 degrees Celsius and ran beautifully, though the car never got super hot..

These plugs came in 6, 7 and 8EV

Thanks,
Lou

[loueelotus]

Well.. just tweaked the #3 carb a touch and all is well now.. Drove it hard today at 33 degrees Celsius (91 Fahrenheit) and purred along Beautifully.. Thanks for all your comments.

Thanks,
Lou

[mbell]

Great. Out of interest (and to help future readers who might have the same issue) what did you tweak on the carbs? Idle mixture? Richer/leaner?

[loueelotus]

Hi, I tweaked the carbs #3 Idle mixture up a tiny bit. and leaner.. On start up, starts great, but is a little rougher at idle for about a minute or two then starts to smooths out within another couple of minutes and idle is good.. It seems to be running leaner and plug check is good after City / Highway driving ..

Lou

[mbell]

Interesting. Thanks Lou.