Need help identifying Dellorto origin

[Gary73Super]

Hello,

If anyone has a reference that will help in identifying where a pair of Dellorto DHLA 40 F series carbs I am running (as installed when I got the car last fall), I would greatly appreciate it.

This is a 2.0 liter in my '73 Super and the carbs are just not tuning right. The car refuses to hold an idle--either over-revving or dying are the two choices. Ignition system is brand new electronic and is timed correctly and engine is freshly rebuilt. Other tests such as for vacuum leaks have been done so it's been pinpointed to the carbs (which run fine off-idle other than a mildly annoying flat spot). Carbs were also swapped front to back and the one barrel that refuses to adjust more than the others also moved from cylinder #4 to #2.

Here are the numbers on the identifying tags attached to the cover plate screws:

5208 P
105300401101

5209 A
105300401001

This seems to match FUSI for a GT Junior, meaning a 1.3 liter. But, my engine is from an Alfetta so I was thinking the carbs might be post-FUSI as well. They seem to be jetted for a 2.0 liter with 32mm chokes, 148 main jet, and idle air corrector 210. Unfortunately, no history is known on the carbs or the engine rebuild, other than it being done in Europe a few years ago.

I'd also appreciate any help in identifying the engine. It's stamped with

AR01655134050

behind the exhaust manifold indicating an Alfetta original application.

Thanks,
Gary

[seani]

According to my reference chart the carbs were originally fitted to a 1300 gt junior and you are correct that they have been rejetted for the 2000. Your idle problem may be down to the throttle plate angle being incorrect for a 2000. If you remove a progression port plug you will be able to see the edge of the throttle plate, if this has uncovered the first progression hole at the idle setting the idle speed will be erratic as the progression circuit will feed blobs of fuel and upset the mixture.
A way to fix this if your carbs have idle air bypass screws is to set the throttle plates with the idle screw untill the edge of the plate is directly under the first progression hole and then open the air bypass screws slowly to flow enough air to achieve a resonable idle speed. This will take a lot of trial and error!
If your carbs do not have air bypass screws you can drill a small hole in each plate to flow more air but this is a last resort!
The 01655 engine was indeed fitted to late model alfettas, I had a GTV with this motor and DHLA 40 H carbs which ran well on the following jetting

main 150, emulsion 7772-11, air 210, idle 58

If, as I suspect, your carbs have the earlier progression drilling of 4 small holes instead of 5 large holes then then try this jetting

main 135, emulsion 7772-5 or 7772-7, air 200, idle 50, idle jet holder 7850.1

Fiddling with carbs can be hours of fun but sometimes you just have to find a good rolling road with a carb guru

[Gary73Super]

Thanks for the advice Sean. Mine have the 5 progression holes so series F must be close to series H. Also, it looks like mine are jetted very close to what you had setup in your GTV so maybe no changes are needed there to at least get it to hold an idle, if not ideal. Thankfully, I do think I have idle air bypass screws since the last thing I want is to start drilling holes in the throttle plates. I never thought to touch any of the adjustments beyond the mixture screws, balance screw, and idle rpm screw since none of the write-ups for tuning dual carbs I've seen mention them. I'll apply your advice and start making small changes to the air bypass screw settings and see where that leads.

Perhaps the issue with one barrel/cylinder being very hard to adjust via the mixture screw is because that one has a really bad setting for the idle air bypass and the others are at least in the ballpark? Just a wild guess.

So, it appears that 1300 carbs can be made to work acceptably on the 2.0, but never as well as carbs originally made for the 1750 or 2000? Or, is it more just fighting the progression hole emissions compromises of these later carbs that makes them a challenge when adapting between engine sizes?

Gary

[seani]

Ok, looks like we are in the ball park here as the F and H bodies seem to be the same. As you say these carbs were jetted on the lean side for emissions, when I fitted 11mm cams I went up to 60 on the idle jets which eliminated the mid range flat spot, might be worth a try?

[101 Alfa Mike]

Quote:

So, it appears that 1300 carbs can be made to work acceptably on the 2.0, but never as well as carbs originally made for the 1750 or 2000? Or, is it more just fighting the progression hole emissions compromises of these later carbs that makes them a challenge when adapting between engine sizes? Gary

[Gary73Super]

Next to the progression hole cover there are two screws. One is a brass plated screw like the progression hole cover screw, except much smaller. These are screwed in all the way for all cylinders and I assume they are just inspection covers and not for adjusting as there is nothing to hold their setting. The other is a regular metal screw with a screwdriver slot at top instead of a head and then it has a locknut holding the current setting. I've been playing with these opening up all 4 of them several turns. But, I'm not getting any obvious changes in how the car is running. It's just continues to hold a very high idle when I push the throttle down and only after 30 seconds or so will the idle speed slowly come back down from 2500-3000 to around 1200. The engine must be getting a good volume of air when it shouldn't be to hold such a high idle.

My assumption was that I wanted to adjust these air bypass screws out rather than in. In fact, the lock nut was nearly all the way to the top of the threads, but I suppose I could get the bypass screw in a bit more if that's the direction I should really be going?

I also tried turning out the mixture screws from their current 5 turns to a more typical initial setting of 2-1/2 turns and cutting back the idle rpm screw to see if an over-rich condition was masking changes to the air bypass screw. No change--can't get it to come down in rpm after blipping the throttle.

If you are interested in more information on what the symptoms are and what's been tried, this thread:

Difficult Dellorto's?

is for the same car.

Thanks,
Gary

[Gary73Super]

For an update on this issue, please see the following thread: Dellorto idle question

Gary