Carburation and fuel stuff (8 articles)

Overhead view - Dell Ortos Overhead view - Webbers


It starts fine, but when the revs get to between 2500 and 3,000 RPM, I notice a stuttering in the engine noise. If I rev through the RPM's very quickly, it is not noticeable, but when revved up gradually, the engine note hesitates and stutters a little bit. It has a pair of Dellortos in it...

Sounds like the idle jets are a little lean. The OEM jets are a tad on the lean side to begin with. Add a few years of varnish build up and they are probably too lean now.

3000 RPM is about where the carbs transition from the idle progression circuit to the main power circuit. If the idle jets are too lean, the engine will starve out right at the top of the idle circuits range just before the main circuit kicks in. The result is the Lotus two step.

At the very least, give the carbs a good cleaning. Better yet, install the next size larger idle jets. Sometimes (rarelly) it takes two sizes larger to eliminate the problem.

The fuel pressure to the carbs is very important. There is a pressure regulator mounted high and just behind the intake plenum chamber.

Richer idle jets should cure the hesitation at 3000 rpm. A good cleaning/rebuild wouldn't hurt either. The valve lash shouldn't have anything to do with the transient hesitation you described. The valve clearances tend to get tighter with wear (the valve hammers it's way into the valve seat). Don't let clearances get too tight or you risk burning valves. Check for an exhaust leak to cure the post-shutdown bang.

Fuel pressure

On fuel pressure for Dellortos, for DHLAs, Des Hammill's excellent sidedraft carb book (available at R&D for $16, a treasure trove of info recommended by Ken Ritchie, thanks Ken) says 2 1/2 to 3 1/2 psi, preferably the low end. So that suggests to me the Facet pump would be followed in line by a fuel pressure regulator to keep fuel pressure downstream at the furthermost carburetor during maximum demand (on the straight at your favorite track) at about 2 1/2 psi. The carbs don't need much pressure, they prefer lower pressures in fact compared to fuel injection.
Betcha Dave Bean's catalogs ($6 for the Elan/Plus2) might have a lot of the answers to any questions you'll have on fuel pumps/carbs.


Just one more thing, do you have to cope without leaded petrol out there - there are moves afoot over here to stop leaded petrol from the year 2000. This is NOT going to be any good for my engine ! (time to put an unleaded V8 in....?!)

Not to worry. Been running unleaded in it since before I owned it. It was a California car and has never seen one drop of leaded petrol. Strict emissions laws there. Most modern cars, 1970's and up, already have hardened valve seats. My Eclat, built in Oct '77 was catalyst equipped from Hethel. You won't have any problems. I'm surprised you're still using leaded fuel. When the axe falls, tell your friends not to mess with tetraethyl lead additives as one drop on the skin is fatal. The petrol companies have worked out any problems with unleaded fuels 20 years ago, so it's not like a new thing to them. In Germany, taxes on non-cat cars is outrageously expensive. Porsche put cats on their 911's and tweaked even more horsepower from them.

Emergency cut off

If you have no fuel flowing and filters are new: Jack the car up a bit so you can get your head under it. Under the right side of the car, along the outer edge, you will see the fuel line. Just before it enters the engine compartment you should see a disk-like object with a wire connected someplace. This is your crash or overturn emergency fuel cutoff switch. It shuts off the fuel pump flow if you crash the car. Sometimes they go bad.

Fuel flow problems

Make sure you've installed a new in-line fuel filter. I have one in the boot and another before the carbs. If you overhauled the pump, then you saw a pump filter in there. Is it clean? Any crud in the pump will make it not pump correctly.

If the fuel keeps flowing into the carbs, you may have a bit of grit in the fuel bowls or on the tiny check valves and it is not allowing the bowls to pressurize, or, one or more of the floats in the bowls are leaking or saturated. This problem will cause the engine to flood out, since the fuel just keeps running into the carb throats. Be gentle with this float apparatus as the measurements are critical. Don't let that stop you though.

Having said all that, now it's time for the pump test. Disconnect the supply line to the carbs. Put the end of the hose in a container and turn on the key to see if it's pumping out. You may need to make the fuel line longer. Pressure should be about 4 psi., not much more. The pump runs all the time. When it's pressurizing..... whirrrrrrrrrrr, and when it's under pressure.... tick....tick....tick....tick. Since you're working with raw fuel, take extra precautions. Big towel in a bucket of water at least.

Warning! If there is a plastic fitting connecting the fuel line to the carbs, replace it now with a metal fitting. The plastic has been the number one cause of the "Flaming Lotus Syndrome". You'll notice the starter is just below the carbs. (BOOM!) Don't compound any damage. If you must use the starter, disconnect coil, remove plugs, crank a small bit at a time till very loose. When you do manage to start it, don't allow it to run for more than a few seconds at a time. I can only caution you that these are delicate, close tolerance engines and any miscalculation will be expensive at the least, totally disasterous at worst.


If it's too lean, the engine will miss, spit and backfire. The exhaust note will have a sharp stacatto note to it punctuated by backfires and misses. In that case, the mixture is too lean to ignite properly. That delays compustion, resulting in incomplete burning of the fuel. Un-burned fuel and soot is then evident on the plugs and at the end of the exhaust.

If it's too rich, the engine will also miss it shouldn't spit and backfire. The exhaust note will have a fat sloppy sound to it and the idle speed will hunt up and down a bit.

Plug color is a more effective guage of mixture at higher power levels and speeds. When checking plug color, don't check them after extended idling. They will probably be black. Run the car hard at highway speed for several minutes (5-10). Then pull over and shut off the engine with minimal low speed driving. Pull the plugs and check the color.

I don't recall the specific jet sizes in the Dellortos on my cars (Eclat and Esprit), but they match the manual's specs for the stock 2.2 912. They idle best with the mixture screws turned out (counter clockwise) 3 1/2 turns. 3 3/4 turns actually gives better idle emissions readings, but the engines seem to idle better at 3 1/2.

Unleaded ?

From Lotus Technical....
Thank you for your enquiry about fuel specification for the 1976 Eclat.

The Eclat 2 litre type 907 engine is fully compatible with unleaded fuel, but requires a minimum octane rating of 97 RON. You may therefore use without adjustment, either 4 star leaded (97 RON) or 'Super' unleaded (98 RON). Premium unleaded is only 95 RON, and should not by used whilst the other fuels are available.

The following extract from a soon to be released 'Technical Topics' internet page may help to answer your concern.

2. Fuel Requirement Some owners are concerned about the phaseout of leaded fuel and its ultimate banning in the U.K. in year 2000. All Lotus 900 series engines (i.e. all 4 cylinder 2 litre and 2.2 litre variants, and V8) from its introduction in 1974, are fully compatible with unleaded fuel. Lead was first introduced as a fuel additive in the 1920s as a cheap method of increasing its octane rating, for a greater resistance to detonation ('knocking' or 'pinking'). An additional benefit was also discovered to be a reduction in exhaust valve seat wear, referred to as Valve Seat Recession (VSR). Alternative methods of raising the octane rating were developed with the introduction of catalytic converters which are incompatible with leaded fuel. In order to combat VSR, cars designed to run on this unleaded fuel were fitted with hardened valve seats. All Lotus 900 series engines use hardened valve seats. The harmful effects of lead absorption into the human body, especially on children, has prompted the coming ban on the sale of leaded fuel.

Octane rating:
The octane rating of a fuel is a measure of its resistance to detonation. In the U.K., 4 star leaded fuel has an octane rating of 97 RON (Research Octane Number), Super Unleaded is 98 RON, and Premium Unleaded, 95 RON. Most early Lotus 900 series engines require a minimum octane rating of 97 RON, and so can use either leaded 4 star, or Super Unleaded. There is some speculation that Super Unleaded may become difficult to source due to the limited demand for this fuel in the future. If this were to occur, premium unleaded can be used in these early engines together with an aftermarket octane boosting additive. Alternatively, retarding the ignition by 3° should allow satisfactory operation on premium unleaded for all normal road driving outside of prolonged wide throttle operation.

A complete list of fuel requirements for Lotus models follows:

Lead Replacement Gasoline (LRG): This product is available in certain territories, and has been produced for use in cars with non-hardended valve seats which would otherwise suffer from VSR without the protection of lead in the fuel. LRG has an octane rating of 97 RON. The fuel may use one of several valve seat protection additives, each of which may produce some other characteristic. No engine durability work has been carried out by Lotus using this fuel, and so its use cannot be recommended at this time and is in any case not necessary - see above.

Fuel Requirements
1. Elite/Eclat S1 (non-USA);
Elite/Eclat S2; Excel (all versions);
Esprit S1/S2/S2.2/S3 (non-USA);
Esprit H.C. (naturally asp.);
Esprit Turbo (low comp. carburettor);

These non-catalyst models require fuel with a minimum octane rating of 97 RON, and can therefore use, without adjustment: either LEADED 4-star (97 RON) or SUPER UNLEADED (98 RON) Note that regular ('Premium') unleaded (95 RON) should not be used, but see 'Octane rating' paragraph above.

Hope this information helps.

Yours sincerely,
Dave Massey - Technical Service

Smelly boot (trunk)

I think they all had the vapour tanks. When the petrol is full up, it expands in the heat. Expanding liquid and gases needs a place to vent and dissapate without causing an explosive hazzard, or breathing the vapour, which causes cancer and liver damage. The vapour is drawn up through a hose (long gone)to a charcoal canister and the engine air box. What you should do now is replace the loose hose with a longer one, route it to the inside corner of the spare tire well. Drill a hole straight down. push a short piece of copper tube into the end of the hose, then put the tube into the hole you drilled. It should be a tight fit but a bit of adhesive should hold it. Why the corner of the spare tire well? Because the tube won't show from outside and it is away from the exhaust pipe. It isn't correct, but it will get the gas smell out of the car.

Carb jet tinkering

Here's "the rest of the story" [thanks to Paul Harvey] on my 907 carb setup to go with the plugs, and so on. My car is *a work in progress* especially with tuning. So, I'd like to learn from dialogue and experiments...

FYI -- my Dellortos were jetted in the old stock 1976 European Emissions, but with the 55L idle (long tail) uprated to a 60 -- apparently to cover what appeared to be a slight lean-surge in the cruising range, after I changed to 56L idles to cut down on the soot. :-o Idle holders were .1, mains 142, airs 110, emulsions .5, pumps only 45. All this with 38 chokes (what they used to fit) and 48mm long trumpets (good). Probably the richer mix was to feed the cat (missing???) and my car has no air injection pump or rails. [see Workshop Manual, page 11, as of 0976.]

Now 36's or 37's are the more modern wisdom. I've got Richie Longo's suggestions, and some notes from Ken at DBE, which correlate well.

The "revised engine specification for 1975" [page 18a as of 0976] went to 35 chokes, with mains down to 130, idles up to 56, and .6 idle holders (a/k/a idle correctors).

I'm still running original ("C" ?) cams, with no markings, but not sure if grind is stock. I've got a pair of 107's to put in someday.

FIRST CHANGE -- idles and accel pumps: 60 idle in a .9 holder, and 60 accel pump. Richie's notes suggest a 58 in a .9 holder. Remember, I'm running 38's on a 2.0 liter. So they won't develop as much velocity as a 2.2 and/or a 37 choke. *PLUS* I should tell you about my "trick" idle tuning first, so you know what I'm doing with the fuel.

The primary key to tuning the idle circuit is to get the progression mix right. Then adjust the idle screws for best idle. Now, if you have to crack the butterflies open to get the idle where you want it, you'll be giving up some progression and may find a flat spot or stumble opening up off idle. So, the jetting sets the progression mix.

The Dellortos have an extra screw on each idle circuit -- the "air bleed" -- usually used for fine-tuning the balance across one carb. I'm running these open about 1 turn. That leans out the idle mix a bit, so I've also got the idle screws open more -- to 3-1/4 turns.

With a "healthy" spark advance, that makes my specimen of a 907 idle around 1200-1400. Right where I like it to keep the alternator busy when I'm stopped or creeping in traffic. It drops a bit with the A/C (haven't put in a "throttle jack" solenoid yet to compensate).

The "trick" process is to close all air bleeds, fiddle with everything, then start opening up the air bleeds and adding more idle screw out. With the air bleeds closed, smoothest idle was about 2-1/2 turns of the idle mix, but the rpms weren't hi enough without cracking the plates.

MY NEXT STEP: Switch main jetting to 160 fuels, 230 airs, and .8 emulsions. If I can't get the 230's, I'll buy smaller and drill them out. I have 225 & 235 drills.

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