Dellorto DHLA40E Fuel Level

[William2]

Can anyone tell me what depth of fuel should be lying in the float chamber with the cover removed? I have adjusted the float height in accordance with their weight (10 grams).

[Esprit2]

With the cover off, 27mm. That's a generic dimension for Dellorto DHLAs, but Lotus was known to take liberties with float height when they tuned their engines. Raising the float/ fuel level (ie, smaller dimension) would produce an overall richer condition in the carb, and Lotus often used that as a tuning tool. I do not know if 27mm will give the same fuel level result as following the Lotus spec for float height.

How you remove the cover is important. If you simply remove the top cover, and if the fuel line is still pressurized while you do that, then as you lift the cover and the floats drop, the inlet valve will open and admit a little more fuel. The result will be a fuel level that's a little higher than normal running.

Instead, disconnect the fuel line before removing the top cover. That obviously depressurizes the system, and also minimizes the amount of fuel present that might leak into the float bowl as the cover is lifted.

*~*~*
The better technique would be to measure the fuel level with the top cover on. Unfortunately, I don't have the dimension for the cover-on fuel level. I suspect it would be ~2mm higher (smaller dimension, or 25mm), but I don't 'know' that.

Just for the sake of conversation, the technique for measuring with the cover on would be to remove the small plastic cover on top of the large top cover. Under it you'll find the idle and main jet assemblies. The larger diameter ones are the mains, remove them. The engine starts and idles on the Idle Circuit, so removing the main jet assemblies won't affect how the engine runs at idle. So start the engine, and let it run long enough to ensure the fuel level is at it's normal operating level, then switch off.

Since the engine/ carbs are installed at a slight angle, high at the front and low at the rear, the fuel level measurement will vary depending upon whether you take a measurement at the front, center or rear of the float bowl... or in the front or rear main jet well. Therefore, it's best to take two measurements, front and back, and average the two.

Use the depth gauge feature of a machinist's caliper, and measure from the top surface of the jet pedestal down into both main jet wells to the surface of the fuel. Measure both wells and average. Again, sorry, but I don't know the correct cover-on fuel level.

Since you're playing with yours, how about doing the following:
1) Double check, and accurately set the float height to Lotus' spec.
2) Measure the cover-ON fuel level that results.
3) Measure the cover-OFF fuel level that results.
4) Let us know what you find.

If you end up with a conflict between the generic cover-OFF fuel level, and the level the Lotus float height spec produces, then I'd suggest staying with the Lotus spec. But do let us know what you find.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[William2]

Hi Tim,
I followed your advice and removed the carb covers having disconnected the fuel line. I measured the depth with a vernier caliper and got a reading in each float bowl of about 27mm give or take a mm. My twin cam engine has just been fully rebuilt and has a standard bore size, C type single groove SE Elan spec camshafts, standard pistons and the head has been skimmed. The valves are all standard size. So overall the engine is pretty stock but with a slightly higher compression ratio due to head skimming. It has also had an unleaded valve seat conversion. Ignition timing set to 10 degrees BTDC as a starting point.

The carbs are currently fitted with 120 main jets, 180 air corrector jets, main jet emulsion tubes are 7772.5, starter jets are 70, starter emulsion tubes are 7482.1, chokes are 30mm, idle jet holders are 7850.4 and idle jets are 50. Float levels set to 15mm. They are set up as per Mr. Wilkins book but the air correctors are 180 instead of 160 or 200 (E type carbs) . I have only fitted 180s as that is all I had available.

I kept my fingers crossed and with full choke applied I turned the key! After a bit of cranking she fired into life and seemed to run quite evenly but I only ran it for a few seconds gradually pushing the choke knob in.
So initial results look promising but the carbs will need balancing, etc. I read somewhere that on a rebuilt engine it is best to run it at fast idle until it is fully warmed up as I guess I shouldn't rev it too much until it is fully run in, although I will need to apply some revs to check that the distributor is advancing correctly.
I am assuming that if the jetting is more or less as per the book then I shouldn't worry about causing any damage to the valves and pistons, etc.
Any advice on what I should do next re the carbs would be most helpful. Many thanks for your help, William.

[Esprit2]

William,

I measured the depth with a vernier caliper and got a reading in each float bowl of about 27mm give or take a mm.

Float height (fuel level) is a foundational setting for the rest of the jetting, and it should be set accurately. "Give or take a mm" is a pretty generous tolerance for the fuel level, and you should make an effort to zero in on 27mm fuel level, or a correct float level.

Which weight float is installed? It's engraved on the top side of the float... the one side you can't read very easily once the float is installed. There were three different weight floats available for the DHLAs, and each weight worked with it's own specific complimentary float height. Then each weight at it's height would produce the same 27mm 'cover-off' fuel level. In Miles Wilkin's book, he gives both 14.5-15.0mm and 16.5-17.0mm heights, but neglects to mention the appropriate float weights. They should be as follows:

14.5 - 15.0 mm is for DHLA40 carb with a 10 gram float
16.5 - 17.0 mm is for DHLA-40E carb with a 8.5 gram float.

Confirm the float weight so that there's a more solid starting point for talking jetting.

I am assuming that if the jetting is more or less as per the book then I shouldn't worry about causing any damage to the valves and pistons, etc.

Jetting is more precise than, "more or less as per the book", but going with what you've written so far, I don't see anything that threatens to melt a piston. If you're tuning for performance rather than emissions, then the 180 Main Air Corrector is probably too large. Larger is leaner, and 180 is probably one or two steps too lean. I'd put my money on 170 for now.
.................................... --------------- Small Valve --------------
........................................................ European
........................................................ ECE-15
.................................... Domestic ... Emission ... Yours
Carb ........................... DHLA ......... DHLA ........ DHLA
Throat ......................... 40 .............. 40E .......... 40E
Choke ......................... 30mm ........ 30mm ........ 30mm
Main Jet ...................... 115 ........... 120 ........... 120
Main Air Corrector ...... 160 ........... 200 ........... 180 ... (170 more correct ?)
Main Emulsion Tube ... 7772-l ....... 7772-5 ...... 7772-5
Idling Jet ...................... 50 ............. 45L ........... 50L
Idle Jet Holder ............. 7850-2 ...... 7850-4 ...... 7850-4
.............................. (2nd richest) ... (7th richest/ 3rd leanest)
Pump Jet ...................... 40 ............. 33 ............. ??
Starter Jet .................... 70 ............. 70 ............. 70
Starter Emulsion Tube.. 7482.1 ...... 7482.1 ...... 7482.1
Needle Valve ............... 150 ........... 150 ........... 150
Float Assembly (P/N) ... 7298-01 .... 7298-02 .... 7298-01 ??
.................... (grams) ... 10.0 .......... 8.5 ............ 10.0 ??
Float Height .... (mm) ... 14.5-15.0 .. 16.5-17.0 .. 15.0
........ (fuel level, mm) .......................................... 27mm fuel level +/- 1mm ??
Air Trumpet Length ...... 40mm ........ 40mm ........ 40mm
Colour Code ................ Green ........ Green ........ ??
Idle Speed (rpm) .......... 900-950 ..... 900-950 .... 900-950
Cam ID ......................... C-Cam ....... C-Cam ....... C-Cam

I kept my fingers crossed and with full choke applied I turned the key! After a bit of cranking she fired into life and seemed to run quite evenly but I only ran it for a few seconds gradually pushing the choke knob in.

So initial results look promising but the carbs will need balancing, etc. I read somewhere that on a rebuilt engine it is best to run it at fast idle until it is fully warmed up as I guess I shouldn't rev it too much until it is fully run in, although I will need to apply some revs to check that the distributor is advancing correctly.

It's best that you avoid any more short runs, and wait to start the engine until you can devote some time to it.

Use an oil with a high concentration of ZDDP anti-wear additive for the first start and first few hundred miles.

Start the engine, then watch for proper oil pressure to show on the gauge. If oil pressure fails to register in reasonable time (~30 seconds), then shut down and determine why.

If there are any bad mechanical sounds, switch off immediately and figure out what's going on.

Once you have oil pressure, check all around the engine bay and under the car for any signs of leaks. Don't worry about a few drops of coolant, but stop for any coolant gushers, or significant oil leaks. It's nice to have a helper who can be looking for leaks while you're watching for oil pressure. Once you're sure the engine is running normally, and there are no major gusher leaks, then increase the idle rpm to 2200-2300 rpm. Let it run that way for about 30 minutes before adjusting the idle back down to normal and switching off again. Constantly monitor the engine, for anything out of the ordinary. Run with the pressure cap off the cooling system, adding coolant as required, and constantly monitor the temperature.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[William2]

Measured the fuel level several times adjusting the 10gram floats to get the required 27mm depth of fuel. Phoned Eurocarb to order some 170 air corrector jets but the chap I spoke to said that as the main jets were slightly larger (120 as opposed to 115) this would offset the slightly weaker 180 air correctors.
I ran the engine today for about 15 minutes and checked oil pressure and coolant temperatures all the time. All looking good. Also checked the timing with the engine running at about 1200 rpm and it is reading 10/12 degrees BTDC.
The tick over is rather lumpy though and the carbs are popping a bit. If I puIl the choke out a small amount the popping ceases. I assume this suggests that the idle mixture is too lean. I haven't as yet tried unscrewing the idle mixture screws further than 3 1/2 turns. Also I am running the engine with the air trumpets fitted but not the air box/filter system. The Eurocarb chap said that I should connect all the air filter system up as this would also have some affect. Regards, William.

[Esprit2]

Measured the fuel level several times adjusting the 10gram floats to get the required 27mm depth of fuel.

William,
And when you finally achieved the 27mm fuel level, what was the corresponding float height?

Phoned Eurocarb to order some 170 air corrector jets but the chap I spoke to said that as the main jets were slightly larger (120 as opposed to 115) this would offset the slightly weaker 180 air correctors.

Matt is correct that the richer Main Jets will offset the slightly leaner 180 Main Air Correctors. The same can be said for the 170 Air Correctors. The difference is a matter of degree. The 170 Air Correctors are more correct for the 120 main jets, and 180 Air Correctors will result in a slightly lean mixture. You're not going to burn valves or pistons, but you're also not going to get the power you could have.

I ran the engine today for about 15 minutes and checked oil pressure and coolant temperatures all the time. All looking good. Also checked the timing with the engine running at about 1200 rpm and it is reading 10/12 degrees BTDC.

You seem reluctant to do the proper run-in at 2200 rpm for half an hour.

Set the static ignition advance at slow idle, 900 rpm or less (1200 rpm is too fast). Slow enough to ensure that no mechanical advance has occured yet.

The tick over is rather lumpy though and the carbs are popping a bit. If I puIl the choke out a small amount the popping ceases. I assume this suggests that the idle mixture is too lean. I haven't as yet tried unscrewing the idle mixture screws further than 3 1/2 turns. Also I am running the engine with the air trumpets fitted but not the air box/filter system. The Eurocarb chap said that I should connect all the air filter system up as this would also have some affect. Regards, William.

Have you balanced the carbs yet? The carbs must be balanced before attempting to do any other tuning. Otherwise you're just wasting effort.

Next, adjust the Idle Mixture Screws for peak manifold vacuum (if using a manometer), or for peak airflow (if using an airflow meter, like a Synchrometer or Uni-Syn).

The 40E carb's Idle Mixture Screw's needle has a long, gradual taper, so the screw ends out more turns than with the 40 carbs more blunt needle. But by 4 turns out, the needle is pretty much withdrawn from the orifice, and further turning of the screw accomplishes nothing significant. If peak vacuum cannot be accomplished and surpassed (vacuum starts dropping again) prior to 4 turns out, then the idle circuit is too lean. Adjust it richer, then repeat the Idle Mixture Screw adjustment.

The 50 Idle Jets should be about right, but your The 7850.4 Idle Air Correctors (aka, Idle Jet Holders) stand out as being quite lean. They were standard to the small valve carbs for the Euro ECE-15 Emissions standard, as such they were quite lean, and the engine did have poor running characteristics as a result. The 7850.2 that was original to a UK domestic carb with 30mm chokes would be about right... "IF" you're seeking performance rich instead of emissions lean.

Follow this procedure to adjust the Idle Jet and it's Holder:

1) Carbs balanced before you start,
2) Idle Mixture Screws adjusted the best you can,

3) With the engine fully warmed up, set the idle speed to the slowest possible consistent with smooth running.

4) Open the throttle slowly enough that the accelerator pump is not much of a factor.

If the engine hesitates just off idle, then the Idle Air Corrector is too lean. Go a step or two larger and try again. Repeat until the hesitation just goes away. Given that the UK Dommestic carbs with 30 chokes used 7850.2 Idle Air Correctors, you could just jump to that and see how it works.

If the engine does not hesitate, then the Idle Air Corrector 'might' be too rich. Go a step or two leaner until the engine just hesitates off idle, then go back richer until it just goes away.

In theory, the point is to go just rich enough that the hesitation just goes away, no richer. In reality, what works for just idling in the driveway may prove to be too lean in real world driving. It may be necessary to go one step richer than having the hesitation 'just go away'.

Idle Air Corrector Jet Sizes:
7850.5 .. Leaner, going down the list gets richer.
7850.10
7850.9
7850.4 .. Yours... pretty lean.
7850.1 .. Normal
7850.3 .. Normal
7850.6 .. .6 & .7 are very similar, almost interchangeable
7850.7
7850.2 .. UK Domestic Twin Cam with 30 mm chokes.
7850.8 .. Richest

5) Take the car for a drive on a country road. Make a full throttle acceleration run, short shifting into 2nd (dropping the rpm down lower than usual) and let the engine pull long and hard up through the rev range. Third gear may work better, if the road and local speed limits will allow.

If the engine stumbles at around 3200 rpm, then the Idle Jet is too small. Go a step or two richer until the stumble just disappears, but no richer.

If the engine does not stumble, then the Idle Jet may be too large. Go leaner until a stumble just occurs, then go back richer one step until it just disappears.

As with the Air Correctors, the point is to go only as rich as necessary to eliminate the stumble, and no richer.

The Idle Jet and Idle Air Corrector (Idle Jet Holder) affect one another. If you make a change to one, then it will most likely be necessary to make a change to the other. So, repeat the above processes until the last round results in no further change.

With the idle circuit adjusted, go back and re-adjust the Idle Mixture Screws for peak manifold vacuum (peak airflow).

Regards,
Tim Engel

[William2]

Tm, I have bought the correct size 7850.2 idle jet holders and will fit those today. Currently I have set the timing at 10 degrees at 900 rpm. I will follow your guidance and balance the carbs first with the engine fully warmed up.
Eurocarb said that I should make sure that all the air filter system is in place before attempting to get the idling correct as this will affect it.
Initially I just need to get it running well enough to get it through am MOT and drive it 1 mile to the garage!
Thanks for all your on going help.
Regards, William

[Esprit2]

(Snip)...
Eurocarb said that I should make sure that all the air filter system is in place before attempting to get the idling correct as this will affect it.

Initially I just need to get it running well enough to get it through am MOT and drive it 1 mile to the garage!

William,
Eurocarb is correct about the air filter affecting the carb's final settings. It's also important to install the filter you plan to use long term. The pressure drop across a paper filter is greater than across an oiled linen filter, like a K&N. If you simply install a K&N on carbs that were tuned with a paper filter, there will then be less overall vacuum in the throats to pull fuel in from the float bowl, and the mixture will go lean.

Does a vintage car have to pass an emissions check in order to pass an MOT? I don't know what's involved in a MOT, but the stories told on the internet make it sound like a very thorough test. If that's the case, then I don't think "I just need to get it running well enough to get it through an MOT" gives enough respect to the importance of having the carbs well tuned. The carbs are critical to good running, and If they aren't right, then the engine may not pass.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[dougal9887]

For an historic registered car, the MOT emission test is a visual one only.
Dougal.