Lotus Elan

Flooding Carbs

PostPost by: carrierdave » Mon May 25, 2009 3:43 pm

Good afternoon all,
I was wondering if anyone could help me with a problem I am having with my Carbs?

I installed a new set of 40DCOE151?s about a month ago and have just started to set them up along with the engine.

The problem I am experiencing is petrol dripping back out of the chokes. I have taken the trumpets off and with each one you can see where there is petrol between the chokes and the body of the carb ? It looks as though they are flooding.

I have checked the floats and adjusted to the book and I still have an issue ? Any thoughts would be welcomed.

Thanks
Dave
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PostPost by: Carlos A » Mon May 25, 2009 4:23 pm

My advise would be to check the pressure of your pump with at least two different gauges.

My carburetors were overloading for no apparent reason. It took me a month to realize that the mechanical fuel pump was putting about 5 psi (go figure why). I got a replacement?. Guess what? The new pump was delivering gas at 4.5 psi plus. Way, way more than the 1.5-2.5 you need. I learn that virtually all aftermarket mechanical fuel pumps (some made in India some in China) are not properly calibrated to deliver the pressure they are supposed to deliver.

I removed the damn thing, and decided to install an electrical pump rated at 2.5-3.5 psi. Problem solved. This electrical pump is delivering about 3 psi.

I have Stromberg carburetors. But I understand that Webbers are even more sensible to high pressure.

Best Carlos
Last edited by Carlos A on Tue May 26, 2009 6:20 pm, edited 1 time in total.
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PostPost by: bill308 » Mon May 25, 2009 10:07 pm

Dave,

I've never seen float heights for the 151's published. IIRC, the floats are a different shape and material from the early Webers, which have a pair of brass, round floats. I suspect the float heights may be different than the Lotus wsm values.

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PostPost by: types26/36 » Mon May 25, 2009 10:14 pm

bill308 wrote:Dave,I've never seen float heights for the 151's published. Bill


The float level for the 151/152 is quoted here
http://www.fastroadcars.co.uk/shop/inde ... oductId=30
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PostPost by: europatek » Mon May 25, 2009 11:53 pm

Have you taken out the needle valves to check that there is nothing stuck under them?
I've had this once or twice before.
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PostPost by: gjz30075 » Tue May 26, 2009 1:24 am

Make sure your auxilary venturi is in tight. It's held in place with a stubby screw and locknut, around the bottomside of the barrel.

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PostPost by: carrierdave » Tue May 26, 2009 2:44 pm

Thanks for all of the suggestions guys ? When something is under your nose it is often difficult to see the wood for the trees.

Carlos - I have thought about the pump issue and having fitted a new aftermarket pump your comments now raise my suspicions further ? so something I will definitely look at tonight.

Thanks for the information on the float heights ? I had tried to set them up as per the manual but its impossible with the new shape of the float ? I assume this would also be the same for the replacement floats in old carbs?

Anyway thanks guys, I will keep you posted

Dave
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PostPost by: prezoom » Tue May 26, 2009 3:05 pm

As for setting float height, Keith Franck, type26owner, has it nailed. The use of a sight gauge is the easiest tool to use and gives an accurate measurement. Takes all the guess work out of the equation. Do a search, it will take time, and find his white paper on the subject. Well worth the time.

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PostPost by: mastijo » Tue May 26, 2009 9:16 pm

If you find the white paper by Keith Franck please help me find it. I just looked at the archives and had no luck.

My problems is hard starting after I have driven for a while . I need to start it right back or wait until it cools. This is especially true on hot days over 90 F
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PostPost by: gjz30075 » Tue May 26, 2009 10:01 pm

It's over at Sidedraft Central on Yahoo groups. Try this:
http://f1.grp.yahoofs.com/v1/UFgcSpafSe ... _Paper.pdf
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PostPost by: bill308 » Tue May 26, 2009 11:55 pm

Brian,

Thanks for the link/info. Very helpfull.

So, float lavels are:

12 mm for the 151/152's
5-8.5 mm for the traditional round/brass floats

I assume these are measured in the same way, lid held verticle and the float tang in light contact with the ball end on the needle valve. The measurement is made between the lid + gasket and the surface of the float.

The Elan WSM states 8.5 mm from the gasket to the float.

I would guess the 12 mm measurement with the new Webers are roughly equivalent to the 8.5 mm level of carbs using the brass floats. This is a pretty big difference. A higher measurement means the floats will be lower in the resevoir, assuming the same boyant force on the needle valve.

Dave,

This seems consistant with the problem you are having. Your resevoir is too high. If the new floats are set at say 8.5 mm, they allow a relatively higher level of fuel in the resevoir, resulting in flooding and hard warm/hot starts, and under worst conditions, liquid fuel leaking into the inlet tracts.

I'd try the 12 mm setting on your floats and note the engone response.

It may be that your 13 mm floats are not quite equivalent to the 8.5 setting on the early carbs, but it should be close. I wouldnt be too reluctant to experiement +/- 1 mm on float leve, just make sure both floats are set the same way.

Bill
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PostPost by: Carlos A » Thu May 28, 2009 4:19 pm

Any news? I am curious. Carlos
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PostPost by: bill308 » Thu May 28, 2009 11:27 pm

Carlos,

I'm curious too.

One more thing on float heights. They are sensitive to fuel pressure. Webers like fuel pressure in the 1.5-3.5 psi range. Dellorto's?

A relatively higher fuel supply pressure will result in a correspondingly higher resevoir level and a relatively richer mixture at all rpm and load ranges.

One result I've not seen discussed is the effect of fuel starvation, or over supply, resulting from float height variations. One would think that high g-turns might lead to a leaner mixture in one direction and maybe a richer mixture in another. Maybe richer under braking or acceleration, maybe richer in right hand turns or left hand turns?

My DCOE's always seemed pretty good under all conditions. But then I never looked at the A/F ratio under these conditions. I think I need an LM-1,2.

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PostPost by: carrierdave » Fri May 29, 2009 10:29 pm

Good evening all,
Not had an opportunity to look at the float heights but I probably have them set too high.
I had a discussion yesterday with someone who has build a lot of engines and he commented that the mechanical pump was no good for Webbers due to the pulsation on the fuel pressure; this would cause the floats to bounce and petrol to flood back!

His recommendation was to fit a facet and filter king to even out and regulate the fuel pressure.
Any thoughts
Dave
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PostPost by: CBUEB1771 » Sun May 31, 2009 4:31 am

bill308 wrote:One result I've not seen discussed is the effect of fuel starvation, or over supply, resulting from float height variations. One would think that high g-turns might lead to a leaner mixture in one direction and maybe a richer mixture in another. Maybe richer under braking or acceleration, maybe richer in right hand turns or left hand turns?


It is not a "one would think" matter, it is a certainty. Turning to the left moves fuel to the outboard side of the DCOE float chamber and increases leverage of the fuel on the float. Turning to the right has the opposite effect. This is probably not a huge issue for road use but perhaps so for competition. I suspect it is one reason that mechanical fuel injection turned up in racing automobiles long before anyone was thinking about digital engine management for improving on emissions and brake specific fuel consumption.
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