Lotus Elan

+2 dellorto carbs & spark plugs

PostPost by: kstrutt11 » Fri May 07, 2010 10:16 pm

After 14 years of never touching the carbs (it's always run very well and gives around 35mpg so I have left alone) Mine seems to have developed a similar fault after being stored over the winter, After going up hill cyl 4 starts misfiring at part throttle, reving it and holding a balled rag over the trumpet clears it, I suspect it's got some muck in the float chamber which is blocking the idle/slow running circuit, it looks pretty simple to remove the jets and float chamber cover to clean it out, is there anything I should look for?

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PostPost by: billwill » Sat May 08, 2010 12:32 am

Dellortos have a sort of double bottom to the fuel chamber and you cant really clean it out properly on the car.

Webers I seem to remember are such that you can do it from the top,

Use compressed air to blow through jets etc, do not stick metal needles in the holes that tends to enlarge them.
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PostPost by: Chris_3003 » Wed May 12, 2010 10:46 am

Hi again, after cleaning the carbs up and using a compressor to blow through everything it seems to be all clean and as it should be. So on went the carbs and now I am having some problems with smoke (petrol).

After looking at all of the parts carefully it appears I may have the wrong floats, they are 7298.1 gr10, and from looking into it a little i think that they should be either 7 or 8.5 grams?

Obviously if the float is heavey it doesn't matter what I set the level to. I am also considering getting some new main jets (there is little or no smoke on idle), reducing the size from 120 to around 110. Would these things help to solve the problem?
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PostPost by: rgh0 » Wed May 12, 2010 10:56 am

The standard Dellorto jetting is normally "OK" in a standard engine and should not need changing to get acceptable performance. If you have non standard floats then you should replace them. However setting the fuel level precisely regardless of the floats used is critical and it needs to be directly measured. Search the Yahoo group Sidedraftcentral for more information than you ever wanted to know about setting up your carbs.

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PostPost by: Chris_3003 » Thu May 13, 2010 11:16 am

I am at the stage where the old floats are going tobe changed, as I think they are wrong. what is the recomended replacement (original) float spec?

I know that they are 7298.1s but what wieght are they, 7 or 8.5 gram and is it likely to make a difference?
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PostPost by: Daholmes19 » Thu May 13, 2010 11:00 pm

Hi Chris

I doubt its the jetting, as Rohan confirms, and as I found out when I rebuilt my carbs the standard jetting is fine. Any jetting changes I did make had little if any beneficial effect. If there are any changes to be made they would be very minor anyway, and not a dramatic as 120 to 110. My engine was rebuilt over the winter to QED 420 spec and when we ran it up on the dyno we saw an indicated 145bhp. All we changed in the Dellorto's away from std was the chokes (33mm to 34mm) mains (120mm to 125mm) and the float needle valves (150 to 175). Its pulls cleanly and strongly all the way from idle to max rpm. In fact the idle at 800 rpm was the smoothest he had ever seen. So we only changed the main jet by '5' for 20 more bhp.

I say that just to confirm its not the jetting, unless the jets you have have been tampered with at some time by drilling out or improper cleaning.

There are only 3 DHLA floats with 7298.1 (8.5 grams), the ones you should have. The other two are 7298.2 (also 8.5 grams) and 7298.3 (7.0 grams). I am not sure where 10 grams comes from, from 45 or 48's? So by all means buy some new ones, for a supplier I recommend Eurocarb or a guy on E Bay in Italy called Alfa1750. Used them both with no issues.

While your at it you should also replace the float valves, the vitron tipped ones are best. That way you know the fuel is being fully shut off when the float chamber is full. When you set the float height (15mm) make sure the gap is measured when the float first touches the needle, when held vertically the spring should "just" hold the float out at the right distance.

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PostPost by: Esprit2 » Fri May 14, 2010 6:12 pm

There are three Dellorto floats available. They all look similar, but they have different weights. For a given fuel level, each weight float must be set to it?s own corresponding height.

A heavier float will naturally ride lower in the fluid, and a lighter float will ride higher. So, in order to produce one standard fluid level (27mm / 1.063? down from the carb body / float bowl's rim), a heavier float must be set to a lower dry-height, and a lighter float must be set to a higher dry-height. If the proper heights are set, then all three float weights will result in the same standard fuel level in the float bowl. IMHO, the heavier 10 gram float provides a more constant fuel level and running condition. However, float weight is a very real factor in the carb's overall jetting set-up, so don't go changing it unless you know how to jet a carb from scratch.

The weight & part-number are engraved on each float. The basic part number for all the floats is 7298-_ _, with two final digits denoting each weight.

7298-01 = 10.0 gr, set to 16.5 - 17.0 mm (bigger number = lower).
7298-02 = 8.5 gr, set to 15.5 - 16.0 mm.
7298-03 = 7.0 gr, set to 14.5 - 15.0 mm (smaller number = higher)
Float Drop = 25.0 mm

If the incorrect float weight is fitted, then the stock spec float height is no longer valid either. Change the float height along with the float weight... it's a package deal. However, Lotus wasn't shy about playing with the float height (deviating from standard) as a means of tuning the overall mixture. If in doubt about any float height differences (?) between Lotus model-specific and Dellorto generic specs, follow Lotus' recommendation for your engine. Stay with original spec unless you really know what you're doing.

I agree that the Viton inlet valves are better, but they're different and require a different set-up procedure. The rubber tips take a compression-set shortly after installation, and cause the eventual float height setting to end up higher / richer.

When using the standard, solid metal tipped needle valves, set the float heights and you're done. However, when using Viton needle valves, install them, set the float heights and finish the installation complete with filling the carbs with fuel. Give the needle valves a couple of days under load to take a set, and then re-adjust the float heights. You can cheat by initially setting the float heights about 0.5mm lower (larger number... ie, 17.5mm becomes 18.0mm) to compensate for the small compression set the soft tip takes, but then you're guessing, and it may not come out spot on.

This applies to new Viton valves. Used valves have already taken a compression-set and should be okay after the initial setting.

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