Carb problem...

[Skyswimmer]

Hi, can someone please help.....

I have a pair of dhla 45's on my S2, suddenly I noticed that fuel leaked out from the ventilationcap on top of the rear carb.
when applying choke during coldstart.The fuel "puffs"/"sprays" from the cap so to speak
and continued to do so even after the engine started but fortunately
stopped when turning the choke off. I then decided to send the carbs for cleaning/rebuild. I received the carbs last weekend and mounted
them back again. Unfortunately the problem persists..but now the leakage stops as soon as the engine starts. Has someone got any clue
of what the problem could be? everything has been cleaned, floating height adjusted, needles changed tc.

[lotusguy]

Hi,

The DHLA 45's are pretty easy carbs to diagnose, rebuild and tune. Your problem sounds as if the fuel pressure regulator isn't properly adjusted or has gone south.

The standard DHLA is not meant to contain much pressure as opposed to the DHLA 45(M) which is used on the carb'd turbos. The (M) uses butyl rubber gaskets as opposed to the paper ones on yours allowing the carbs to be pressurized to 12.0 PSI really incredibly high for a carb.

For an N/A car such as yours, you should hook up a fuel pressure gauge to the line and want to see no more than 3.5-4.0 PSI with 4.0 being on the extreme high side.

Since it is only the rear carb which is leaking, I suspect the pressure is only slightly high and the rear carb relieves the pressure spike before it can overpressurize the front one as well.

You do not want to drive or run the car until it's fixed. The fuel will leak all over the live starter electrics - the single greatest cause of engine fires on the early Esprits.
Happy Motoring! ...Jim'85TE

[Esprit2]

lotusguy said:

(Snip)... Your problem sounds as if the fuel pressure regulator isn't properly adjusted or has gone south. (Snip)... For an N/A car such as yours, you should hook up a fuel pressure gauge to the line and want to see no more than 3.5-4.0 PSI with 4.0 being on the extreme high side. (Snip)...

Stock S1/S2 fuel pump discharge pressure is 3.8 - 4.0 psi. Pressure at the carbs will be lower due to the elevation difference between the pump and carbs plus flow losses. However, Lotus did not include a fuel pressure regulator in the stock system, so don't knock yourself out looking for one.

The vertical head loss from pump to carbs is the better part of 1.0 psi. Losses through the tubing, Flowlock valve and filter will reduce the pressure a bit more. Expect to see about 2.5 – 3.0 psi at the carbs. That's high-normal for a Dellorto and "shouldn't" over-power an inlet valve that is functioning properly.

Dellortos don't require more than 2.5 psi, but they can tolerate 4.0 psi without a problem.

If the original fuel pump has been replaced by a non-stock unit with a higher discharge pressure, then a pressure regulator must be added to the system up near the carb inlets. Set it for 2.5 psi.

Having said all that, I don't believe the problem is due to excessive fuel pressure, high floats or a defective inlet valve.

The float chamber is vented to the atmosphere by a passage in the cover, so the fuel pump cannot "pressurize" the float chamber above atmospheric. If the inlet valve doesn't shut off or is overpowered by excessive fuel pressure, the fuel level will rise until it flows out the vent and into the airbox. But it will not pressurize to pump pressure.

The various fuel delivery circuits (idle, main, accelerator, cold start) all must lift the fuel to some height above the normal level in the float chamber before fuel will flow. If the fuel level gets high enough to exceed the lift-over height, then fuel will spill over and gravity-flow through the circuits into the carb throat and out the float chamber vent. The carb would run "very" rich… probably too rich for those cylinders to fire.

All circuits except the cold start enrichment circuit are contained in the body while the enrichment circuit is partially in the cover. In order for the fuel level to get so high that it could flow out the enrichment circuit, it would be flowing out the float chamber vent and every other metering circuit as well. That would be a severe problem and pretty obvious both in terms of fuel dripping out of the carb/airbox and in the engine's poor running.

You cross-posted your question on [e-mail] and I replied that I suspect the fuel enrichment device itself. I still think that's the case.

The enrichment device very rarely wears out or requires service, but when they do it's usually the washer that has worn out. In that case, it can allow excess flow in either direction. Too much fuel is dumped into the carb throat when the device is on, and/or a small pressure "puff" can back flow. Since the little inlet grill in the small plastic cover is the sole air inlet for the enrichment device, it's likely any such "puff" would show there.

The cam timing includes an overlap period. Both valves are open while the piston is rising and starting to fall. At slow speeds like cranking to start and slow idle, there can be a positive pressure pulse in the intake runner during overlap. If the enrichment device's washer is worn, that could result in a small puff of air/fuel emulsion being blown back.

The enrichment device seldom needs attention and it's not usually dismantled during a carb rebuild. I wouldn't be surprised if the recent rebuild didn't include it. Ask your re-builder.

In both cases (before and after rebuild) you mention the puffing stopped when the enrichment device was turned off. That's reasonable since the plunger seats and totally closes the passage when off. After the rebuild you note that the puffing stops after the engine starts. That's as expected since the overlap reverse flow would be more pronounced at slower speeds like cranking on the starter. As soon as the engine starts, normal idle speed should produce sufficiently high inlet flow velocity for momentum to keep flow going in one direction.

Normal Dellorto rebuild kits don't include bits for the enrichment devices. Talk to Matthew Cooper at Eurocarb, Ltd. Ask him about his opinion on your problem and see if he can provide a replacement washer. The typical exploded parts diagram should provide enough detail for you to be able to dismantle the enrichment device, but if you need more info, ask Matthew.

Eurocarb Ltd (attn: Matthew Cooper)
256 Kentwood Hill
Tilehurst
Reading
RG31 6DR
United Kingdom
[phone] Phone
[phone] FAX
<[e-mail]> e-mail
<www.racecar.co.uk/dellorto/> website

Best of luck,
Tim Engel
Lotus Owners Oftha North

[Skyswimmer]

Thanks for your answer Tim!

Anyway, I have checked the manuals and I can't find any thing called "washer" in the enrichment function.
What part do you refer to? has it something to do with the starter jet or emulsion tube?

ps.I must add that I'm Swedish so I have probs sometimes to understand certain names or expressions for these things ds.

BR

Anders

Esprit S2

Sweden