Lotus Elan

Hesitant acceleration when hot

PostPost by: davie489 » Tue Aug 15, 2017 8:18 pm

hi there

Can someone shed any light on a hesitant pull away when engine is up to temperature? Is this common with the twin cam?
Have been doing a little reading and thinking maybe fuel evaporation in the carbs maybe the cause . Would a heat shield help? I could be totally wrong as not mechanically minded. The car has recently had the carbs balanced and tuned. All seems fine and car going fine apart from pulling away.

Thanks guys
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PostPost by: gus » Tue Aug 15, 2017 8:57 pm

no it is not common

which carbs?

Webers it is probably to just go through them and make sure the jets are stock and clear

Strombergs look at the thermal compensators
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PostPost by: davie489 » Tue Aug 15, 2017 9:14 pm

Thanks for the reply...

I have webers and everything is stock and have checked all jets , I believe everything is how it should be.
Its like the car is juddering when pulling away at times then sometimes bogs down a lot then next pull away would be fine, not very consistent
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PostPost by: friedy » Tue Aug 15, 2017 9:29 pm

I had similar problem turned out to be fuel starvation, partially blocked in line fuel filter.
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PostPost by: Grizzly » Tue Aug 15, 2017 11:08 pm

Sure it's not plugs? or a breaking down coil?

If my car sits for any length of time i get a white crystal deposit in the carbs (i'm told it's the detergent additive in the fuel), the knock on effect of this is i seem to be constantly cleaning the Pump jets....... The symptom of ths blockage is a splutter when accelerating (harder i push the throttle the more dramatic the miss) so thats my first port of call but if you are sure their all working my money would be on Ignition rather than fuel.
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PostPost by: prezoom » Wed Aug 16, 2017 3:34 am

Is your hesitation in the 2000 to 3000 rpm range, and after that it runs strong? A flat spot or gasp is not uncommon for a Weber equipped twin cam. Generally speaking, it is a lean spot in the transition from the idle jets to the main jets. A quick and dirty check is to try pulling the choke/enrichment out a little bit to see if that smooths things out. Another help is to make sure the float level is set to the new standard, not the old standard of measuring the distance between the floats and the top of the carburetor. This new method was suggested by Keith Francks in a white paper, which can be found on his site, sidedraftcentral on yahoo groups. He started on this site before moving the discussion to his own site.

Basically, it is measuring from the flat section where the emulsion tubes screw into the carburetor, down to the fuel level in the float bowl. You remove one of the emulsion tubes and measure down. This measurement is 25mm. Keith's white paper describes how to do this and what tools are needed, which can be made at home. Keith's site is quite extensive, as he has been on a quest to make Webers run like fuel injection for quite some time. He has made his own jetting, which he has assigned his own naming. It takes a bit to understand the relationship between his jetting and the Weber nomenclature. There is enough information there to occupy a lot of your time, if you choose to start at the beginning of his adventure up to the current date.
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PostPost by: ericbushby » Wed Aug 16, 2017 10:10 am

Rob is quite correct about checking the fuel height in Weber carburettors.
Keith insists that it is important for it to be 25mm below the top of the float chamber, plus or minus 1mm.
However I have not found it easy to measure it Keith`s way.
The top of the float chamber is 16mm below the rim of the jet cover, so the fuel is 41mm below the rim.
I measure this with a Vernier depth gauge because I have got one.
It works just as well using a cocktail stick as a dipstick and a small torch. As the stick touches the fuel the surface shivers. A mark on the stick at 41mm is all you need.
Do it soon after the engine has been switched off and fuel has not evaporated.
I apologise if I bang on about this too much, but I find it so quick and easy, no tools are required, and it made such a difference to the smooth running on my car.
Hope this helps
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PostPost by: Craven » Wed Aug 16, 2017 10:53 am

Keith Francks is credited with the method of fuel level setting which in fact goes back to the beginning of this type of carburettor.
Here is an illustration from a Weber Tuning Manual (hope it?s not copy right infringement) an explanation is also given about the why and wherefore of the recommended level.
p1030239.jpg and
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PostPost by: davie489 » Mon Aug 21, 2017 5:39 pm

Thanks for the useful replies guys....
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