as the title says, having a small problem with twin dcoe's on idle.
They work fine on level ground, however if the car is put on a -ve gradient (like 20% or so) or if while driving I shift into neutral and brake hard, the engine coughs and splutters, once back to level again it works normally.
I have checked the float travel, tried different supply pressures, changed needle valves and the same still happens. Any Ideas?

Setup is on a 1700 x-flow. Idle jets 50 f9, mains 145 airs 180 tube F16, currently running at 4psi.

All help appreciated, I cannot figure this one out.

I know you've fiddled with supply pressure, but isn't 4 psi a bit low? Have you checked the pump as well as the regulator and what state are the filters in?

Hi Phil,

Don't shift into neutral before hard braking!

On the other hand, one off-the-wall thought:

Hard braking or pointing downhill can cause oil starvation with a modified xflow sump.

If you have the problem only after the engine's been running for a while could it be related to overheating and oil starvation? Not a good scenario that one! I'd want to check oil pressure, whether there's enough oil in the sump and whether the oil pick up pipe is fully effective.
Fingers crossed that isn't the cause and the bearings are OK.


Otherwise:

Was it always like this? Or is it related to a change you made?

Could there be fuel sediment shifting somewhere?

best
Dave

It sounds more like progression from idle to mains, you're losing something somewhere in between. Does it only do it when you are pointing downhill or when you lift your foot off the accelerator quickly to apply the brake?

does this happen when engine is on a slope or after driving/stopping on a slope?

how are the carbs attached to the manifold?

Hi Guys, thanks for the usefull info.

This happens whenever the car is on a slope (downhill only). The carbs are attached with the standard xflow intake manifold and the standard rubber mounts and washers to go with it.

As for mods done to the car Dave, I have just changed the cam, using a 244 now. As for the sump I have a lotus orginal sump on the engine, oil pressure remains at a constant 60PSI even on the downhill slope so I doubt its the oil.

As you can see from the setup I have to use quite a large idle jet in order not to get any flat spots when driving normally.

Peteff, It only does it on downhill or if I brake hard while engine is in neutral (dont do that often).

If 4 PSI is low, then what should I use? I have a facet pump and a king fuel pressure regulator.

Thanks guys

Does it matter if the tank is full or not, could be fuel starvation if the pick up in the tank is near the rear. Have you got a fuel cut switch could it be faulty and switching off your pump.

Checked the tank, made no difference (full or nearly empty)

i know it sounds odd but have you checked the float heights if the level is very low it could be uncovering the base of the fuel pick up just a thought also if its under braking have you teide clamping the servo pipe off if you have one see if its vacuum related

Hey everyone, problem solved, I thought I'd post this in case anyone ran into a similar issue.

Basically it was a problem with the fuel level in the carb bowl. I had followed the webber DCOE manual regarding what height setting to keep the floats.

I started the engine, let it run for a few seconds then removed the top of the carbs.
Looking at the carb bowl from the front, the idle fuel pickup (which is about half way up the bowl) was just about covered by the fuel level.

I adjusted the float so that I get that level a couple of mm above the fuel pickup. Done that I had to retune the idle screws and problem solved

Basically the webber DCOE's need to be at the correct angle for the standard float settings to work. (an incline of between 5-10 deg) I had them at perfectly level, because I thought it would help while cornering. (and also due to bonnet space).