Dellorto advice

[frogeyesimon]

I'm new to my Plus 2 and also to Dellortos.

My car has been laid up for 7 years or so and has only recently been returned to the road. Having had a lot of bother with crud in the fuel line, I now find that I seem to be running on only 1 carb (i.e.2 cylinders) - not good.
I have DHLA 45s fitted, and whilst idling, my front carb (the apparent non-functioning one), dribbles a steady stream of fuel into the venturis (both barrels). A puddle forms in the choke and clearly the front 2 cylinders are running so rich that the plugs foul up and no combustion takes place.

Needle valves seem to have black rubber tips and small springs, and from what I can see seem to be functioning ok (though it's hard to tell)

I'm guessing that it would be wise anyway to rebuild both carbs with new seals, needle valves etc. I have read that this is sometimes symptomatic of corrosion in the bodies, but I can see no evidence to suggest that this is the case here.

Is there anything else that I should be checking?

Thanks in advance
Simon

[Esprit2]

Simon,

First, never look down the throats of a carb on a running engine !! Any spit-back or backfire is going to blow a flame into your eyes/ face. If you must look into the throats, wear eye protection, then position your head off to the side and use a mirror to look down the throats.

If the carbs have sat around without running for many years, then yes, it's probably a good idea to clean and rebuild them both. Seals and rubber bits perish over time, especially when they sit dry. Also, if they were full of fuel when they were set aside, then there are most likely some varnish deposits in internal orifices and passageways that need to be cleaned out.

*~*~*
The carbs shouldn't just pee a steady stream into the throats... as I interpret your description. That sounds like the float height isn't properly set, and/or the inlet valve isn't shutting off... then the fuel pump is flooding the float bowl. From which orifice is the fuel peeing?

1) If it's pee'ing out of the progression holes near the edge of the throttle butterfly, or out of the auxiliary venturi/ choke, then the float bowl may be flooding. How is the float set?

There isn't one fixed float height for Dellortos, although Lotus specifies only one for it's application. There are three float weights available, and each weight gets a different height... and that different height can get revised/ tuned for each application (Lotus was pretty liberal about "tuning" their own heights). So, is the correct float weight installed (it's molded into the black rubber float), and is the float height properly set for "that" weight?... ie, do the weight & height match Lotus' spec? If the float weight and height are correct, then the next suspect is the inlet valve.

*~*~*
The stock inlet needles were solid brass (OEM Dellorto). The aftermarket rubber tipped needles tend to take a compression set shortly after installation. If you remove the needle and inspect the rubber, you'll probably see a depressed impression of the seat in the rubber... a ring/ groove around the rubber tip.

If you install new rubber tipped needles and set the float height all in one brief session, then the rubber tip will take a compression set after a couple of days. When the fuel level holds the float up, and forces the needle into the seat, the rubber deforms under the pressure of being pressed into the seat, taking an initial compression set. Then the float will have to rise a bit further to press the valve further into the seat, the fuel level will rise with it, and the carb will run rich over-all. Worst case, it will flood.

When installing rubber tipped needles, give it a couple of days after the initial install to stabilize, then go back in and re-set the float height. The solid brass needles don't have this problem.

> and from what I can see
...and...
> seem to be functioning ok (though it's hard to tell)

They're working or they're not... confirm it. "Seem to be", and "hard to tell" don't cut it.

2) If fuel is pee'ing out of the accelerator pump jets, then strip down that circuit and make sure it's right. Are the check-valve balls and weights in place?

3) Are the chokes firmly seated in the throats, and their set screws tightened? Loose chokes/ loose auxiliary venturi are leak paths into the throats.

1-3 are a reaction to your comment that the carb seems to be pee'ing a steady stream into the throats. Switching focus to your comment that one carb seems to be running, and the other not... are the two carb's balanced? The idle speed screw is on the rear carb. Then the linkage coupler between the two carbs includes a screw for adjusting the front carb's throttle opening to match the rear carb's. Also, there are air bleed screws on each carb that are used to balance the two throats on a single carb (only one per carb should be cracked open at a time, the other is to be fully seated). If you connect a 4-tube manometer to the carbs, all 4 throats should show equal vacuum levels at a slow idle.

When only one carb is running, it's usually the rear one, since it's throttle opening is directly set by the idle speed screw. The non-running carb is usually the front carb, since it hasn't been properly balanced to the rear carb via the adjustable linkage coupler.

Whenever someone reports that one carb seems to be running normally, and the other is not (ie, the engine is running on two cylinders), I tend to jump to the conclusion that the carb's are not balanced.

Does any of that sound familiar?

Regards,
Tim Engel

[frogeyesimon]

Tim,

Many thanks for taking the time to construct such a comprehensive reply. As I said, I'm on a bit of a learning curve. There is much for me to digest here.

Fuel seems to be dripping from what I believe is the auxiliary Venturi.
The car ran well for 30 miles or so.

Needle valve "seems" to seal when I blow into the fuel Union and lift the float, but I take your point...

From your comment regarding rubber or brass tipped needles, should I opt for the former or latter when replacing?

I will swap the carb tops over tomorrow. This presumably will tell me whether I have a needle valve / float problem.

Simon

[Esprit2]

Needle valve "seems" to seal when I blow into the fuel Union and lift the float, but I take your point...

Simon,
That's cheating. You can apply enough pressure with your finger to seat the needle against just about any pressure you're likely to apply by blowing. The float is:
1) limited to the force of it's bouyancy, and
2) needs to shut off at a specific height, while your finger pushes as far as it needs to in order to create a seal.

Run the fuel pump until you're certain the carbs are as full as they're going to get due to the float setting and needle valve. Then switch off, disconnect the fuel hose (it's pressurized, so wrap a rag around the end to contain any sprayed fuel while you remove it), and blow into it... does it still shut off cleanly?

From your comment regarding rubber or brass tipped needles, should I opt for the former or latter when replacing?

Each type of needle has it's fans. From an installation stand point, the solid brass ones don't need time to take a compression set. Put them in, and they're good to go. On the other hand, some folk insist the rubber tipped needles seal more reliably... once you finally get the float height set properly.

If you're uncertain about that whole rubber deformation business, then using the solid brass needles would simplify the installation and adjustment process for you. If you're a bit of a beginner at this, solid brass might be easier on the learning curve. It's not like one type works, and the other doesn't.

I will swap the carb tops over tomorrow. This presumably will tell me whether I have a needle valve / float problem.

Swapping tops over presumes that one is right and the other is wrong. Both could be wrong.

Instead, start from scratch. First, check the float weights (engraved on the top side of the black rubber part of the float). If they're what Lotus specifies, then set the float height/ float drop by the book. If different weight floats have been installed, then the best next step is to install a pair of correct floats. If, for some reason, that's not possible, then we'll have to ponder an appropriate height for the float you do have. To be continued.

Regards,
Tim Engel

[frogeyesimon]

Tim,

But presumably if I swap tops and the dribble moves from the front carb to the back then I have a fault with the top?

Simon

[oldchieft]

Will the Weber method of checking the fuel level by pulling the main jet out at tick over work on Dellortos?

Jon the Chief

[Esprit2]

But presumably if I swap tops and the dribble moves from the front carb to the back then I have a fault with the top?

Simon,
Possibly. But you're focusing on one favorite possibility, and ignoring the other things that could be wrong. Do the swap if it makes you sleep better tonight, but keep an open mind to the other possibilities. For instance, swapping covers ignores whether or not the chokes and auxiliary venturis are secure.

Check everything.

And if the cover swap produces a clear difference (as in, the problem follows the cover), then don't presume one cover is right. It's more likely that one is wrong, and the other is more wrong. When you're debugging problems, look for the worst case and fix everything. You seem to be looking for the best case, and the least work to resolve it. Neither life nor old carbs are usually that easy to figure out.

Regards,
Tim Engel

[Esprit2]

Will the Weber method of checking the fuel level by pulling the main jet out at tick over work on Dellortos?

Basically, yes. Personally, I think it's better (safer) to switch the engine off first... but yes. Remove the small cover from the carb's top, and remove one main jet assembly. Let the fuel pump run long enough to fill the float bowl (with a mechanical pump, that means run the engine). Switch off (me), and use the depth gauge feature of a vernier caliper (the rod that sticks out of the end when the jaws are open) to measure down from the top of the jet pedestal to the surface of the fuel in the jet bore.

That's great if you know what the standard fuel level is supposed to be. I've never seen the Dellorto spec published, but I have an unofficial, grassroots dimension IF you first remove the carb's large top cover, then measure down to the fuel level. Removing the cover withdraws the floats from the pool, and lowers the fuel level. So the cover-off dimension is lower than the cover-on dimension.

If correct, that would be Dellorto's default dimension. As I mentioned, Lotus was pretty liberal about using float height/ fuel level as a tuning option, and the Lotus fuel level does not always equal the Dellorto default fuel level. (see more below)

The 'unofficial' Dellorto fuel level by the 'cover-off' method = 27 mm from the top-center of the jet pedestal (in the V-notch between the Main Jet assemblies) to the fuel level. I'd presume (just a SWAG) the cover-on fuel level would be about 2.0 mm higher.

The Dellorto fuel level dimension is not the same as the Weber dimension. That's a designer's choice, and part of the reason why a Dellorto DHLA's Idle circuit feeds the engine up to about 3200 rpm, while a Weber DCOE feeds it up to about 4000 rpm.

*~*~*
Okay, I just looked up Lotus' Dellorto / Twin Cam float height spec, and it's an example of why it's necessary to follow the Lotus Float Height spec when tuning, and not just use the generic number that comes with the Dellorto carb rebuild kit, or in Des Hammill's book:

DHLA
Float ...... Float
Weight ... Height
10 gr ...... 16.5-17.0 mm
8.5 gr ..... 15.5-16.0 mm (Lotus calls for 14.5-15.0 mm = 1.0 mm higher = richer)
7.0 gr ..... 14.5-15.0 mm

By the way, Des Hammill gives one float height for all float weights... 15.0 mm. I suspect you'd have to do a little additional tuning to make that work universally. I always use the engine manufacturer's spec, and drop back to the three settings given above if the mfgr's spec isn't available. With Lotus, I also presume their setting will be 1.0 mm higher than the Dellorto default (that presumption works on the 907 & 912 as well).

A heavier float naturally rides lower in the pool. So for a given fuel level, a heavier float will be lower = larger number. Conversely, a lighter float will be higher = smaller number. So, in the pursuit of one correct fuel height, all three of the above standard weight & height combinations should produce the same standard fuel level.

Note that for the 8.5 gram float used with the Twin Cam, Lotus specifies a 1 mm higher float height, which will give a 1 mm higher fuel level, and an overall richer condition in the carb. If you wish to tune your engine the way Lotus set it, then use Lotus' specs, and not generic Dellorto specs.

Regards,
Tim Engel

[frogeyesimon]

Well I swapped the float chamber lids over and the dribble followed the lid which I think is probably good.
I will source repair kits this week and rebuild next weekend. Then I assume the fun will begin as I attempt to learn how to set the things up

Simon

[Esprit2]

The solid brass inlet valve needles are OEM Dellorto, and stock DHLA's can't tolerate high fuel pressure at the inlet... basically due to that solid needle's inability to seal against higher pressure. Keep the fuel pressure at the carb inlet down to 1.5 - 2.5 psi. If you install a non-stock fuel pump, then check the pressure at the inlet. If necessary, also install an inline fuel pressure regulator as close to the inlet as possible.

The aftermarket rubber tipped needle can shut-off reliably at up to 5 psi inlet pressure, but only if it's properly set up... and that includes re-adjusting the float height after letting the needle sit under load for a few days.

The solid brass needle is easier to set-up correctly. The rubber tipped needle is more fussy and time consuming to set-up, but seals more reliably once it's properly done.

Even with the rubber tipped needle, keep the inlet pressure down to 1.5 - 2.5 psi. Consider the rubber tip as a better seal (insurance), but don't set up a higher pressure system that depends upon it to work correctly.

The rubber does have a service life limit, where as the all brass needle works pretty much indefinitely as long as the fuel pressure, the float itself, and the float height setting are all properly maintained.

Figure out where you want to be in all that, and order your parts accordingly.

Regards,
Tim Engel

[frogeyesimon]

Thanks very much Tim, all duly noted
Simon

[PeterK]

Simon
Don't know what tools you have, but if you need a 4 tube manometer to check vacuum balance after your rebuild, give me a shout
Peter

[frogeyesimon]

Thanks very much Peter.
I have a Carbalancer which has served me well when working with SUs.
If I struggle with this then I may well be in touch.

Simon

[JohnP]

Hi All, and happy new year.

Please pardon me for hijacking this wonderfully informative thread but perhaps somebody may have a bit of immediate advice.

Car has been running really nicely since I got her. Carbs professionally balanced recently.

After a short run a few days ago it was a little more difficult to start from fairly warm than normal, then seemed to only run on 3 cylinders, occasional possible burst onto 4 at speed.

Further investigation has shown that at idle and up to just about 4000 rpm unloaded number 4 cylinder is not firing. Possibly no 3 intermittently as well.

Changed plugs thinking I may have fouled one. No 4 is sparking.

No change to misfire.

At idle fuel is coming out of the approx. 10mm hole at 10 O'clock to the left hand choke of the rear carb (I don't know what it is called but I think it leads to the emulsion tube).

Can anyone pinpoint a single likely sudden point of failure that may have caused this?

Is it worth me gritting my teeth and taking it full speed down the A316 for an "Italian tune-up", or should I try something more methodical?

Finally perhaps I should chug along to somebody who knows what they are doing?

Any pointers/ advice very gratefully received.

John

[frogeyesimon]

John,
I recently had a similar issue that turned out to be crud in the fuel line between tank and pump. Because the fuel is fed to the front carburrettor and then onwards to the rear, the front float chamber was filling correctly but the rear was being starved. I then had very similar symptoms to those that you describe.
There's a simple test that you can do: Take the car out for a run and when you get the misfire, stop and whip the float chamber covers off and check the fuel levels - is the rear carb low?
Simon