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Dellorto mounts vs heat transfer

PostPosted: Mon Nov 04, 2019 3:42 am
by SimonP
Apologies if this has been answered before, but I searched and did not fond the question.

The Dellortos on my '73 Plus 2 are mounted on aluminium spacers with rubber O rings. When the engine is cold - for the first fifteen minutes or so - they behave beautifully.

Gradually as things warm up, the engine fluffs in traffic after idling. Seeming flooded. Once cleared they pull like a train up to max revs. This is really only a running in traffic issue.

Some Alfa experts suggested the carbs are getting too much heat transfer from the inlet manifold. They say they have had similar problems and cured it with a fully rubber spacer instead of the aluminium. I'm in California with pretty poor gas/petrol quality and 91 octane.

Has anyone else experienced this cold vs hot running issue? And cured it?

Thanks for any advice.

Simon P

Re: Dellorto mounts vs heat transfer

PostPosted: Mon Nov 04, 2019 6:51 am
by alan.barker
The carb mounts are correct.
They should be set as workshop manuel and be a little flexible.
Fit new "O" rings and thackery washers.
Fit new "viton"" float needles and set fuel level .
Good luck

Re: Dellorto mounts vs heat transfer

PostPosted: Mon Nov 04, 2019 8:39 am
by rgh0
The issues are more likely to be due to plugs fouling at idle due to being to cold a plug or to rich an idle mixture. Too rich at idle may be due to idle jets and tuning or due to the fuel level going to high due to leaking needle valves or to high a fuel pressure.

The inlet runners on a twink which are much longer than on an Alfa engine run cool as they are effectively cooled from the inside by the vaporising fuel so heat transfer from the head up to the carbs is limited. The two rubber O-rings also form an effective heat transfer barrier also. So conduction to the carbs is not an issue.


Re: Dellorto mounts vs heat transfer

PostPosted: Mon Nov 04, 2019 6:43 pm
by SimonP
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Thank you both for the replies. This forum delivers such a depth of expertise.

In my case, I think the carb float levels / needles are the next suspect. It's good to be able to rule out heat transfer as the probable cause - even though the symptom only occurs when the engine warms up.

Thanks all
Here's a bonus pic of the Plus 2 playing with big boys at Willow Springs...

Re: Dellorto mounts vs heat transfer

PostPosted: Mon Nov 04, 2019 7:04 pm
by Esprit2
The Lotus soft mount with it's metal spacer and two O-rings is excellent, and very unlikely to be the source of the problem you're experiencing.

How are your Dellortos jetted? Are they original and stock, or has someone played with the idle jets and air correctors?

How are the floats set? If the floats are old and starting to absorb fuel/ get heavy, the fuel level will be high. That will cause an overall rich condition that could cause the engine to bog down off-idle and at low rpm.

Original Dellorto floats were available in three different weights, and each weight required it's own, different float height. Some of the floats are no longer available, or there are reproduction floats available that fit, but are different weights. If lack of availability forces you to install a float of a non-original weight, then it's important to set the float height correctly for the float you're using, not to the original Lotus spec height.

Is the fuel inlet valve's needle solid brass, or does it have a black, Viton rubber tip? The solid brass needle doesn't change, just set the float height when the new valve/ needle is installed, and forget it. The rubber tipped needle compresses under load, and takes a compression set. In the plastics world, that's called 'creep'... the material moves to relieve an applied load. Inspect an old rubber tipped needle, and you'll see a depressed groove around the rubber tip where the orifice pressed against it. It's a small dimensional change, but with the float's lever ratio, it produces a too high fuel level/ rich mixture over time.

With Viton tipped needles, install them as usual, setting the float height as you do. Then after they've been in service for a few weeks to a month, go back in and check/ re-set the float height. Failure to do that will result in a too high fuel level and overall rich condition.

Combined, the float & needle control the fuel level in the float bowl. Remove the carb's top cover and remove some of the fuel to lower the level. Replace the cover, start the engine, and let it idle briefly so the pump can refill the float bowl to 'full'. If you have an electric fuel pump, forget about starting the engine, and just run the electric pump for a bit.

Disconnect the fuel line in order to de-pressurize the fuel inlet, then remove the top cover again. Use a depth gauge, or the depth feature of a caliper to measure from the top face of the jet pedestal down to the fuel level. The standard Dellorto fuel level is 27mm / 1.063? down. Lotus was not adverse to playing with fuel a non-standard level as a tuning tool... lower is leaner and higher is richer.

Tim Engel