Struggling With Dhla Emission Setup

[Gopherit]

I am struggling to setup a pair of Dellorto DHLA Emission carbs.

Engine is a Vegantune Twincam VTA, 1700cc 8-valve running “sporty†cams. At the moment it starts well (have disconnected the choke arrangement, it doesn't seem to help, just makes a loud whoosing noise when operated) and will idle at around 700rpm when it gets up to about 70/80 deg.

 

The engine will not pull or cruise below 2000rpm and it is running lean throughout the range, plugs are nearly white, very small amount of soot after idling. Vacuum advance is currently connected although have run without some of the time with no obvious difference in performance. It does seem to be relatively happy above 4000rpm, suggesting the progression range is the problem.

 

The consequence of running lean appears to be some overheating, gentle throttle and no static idling for any time is needed to keep temperatue below 100deg C. On acceleration the noise is strangely raucous, hard to describe and not, I think, attributable to the exhaust setup.

The carbs setup is:

 

0 DHLA “H†and “Nâ€, ie the emission carbs.

Idle – 62 with the emission holder

Main 145
Air Corrector 200
Emulsion 7772.11
Chokes 34

Auxillary Venturi 7848.1

Float level in carbs – as far as I can measure about 28mm from top of tower on engine side. Floats set at 16.5mm.

 

Ideas welcome!

[CHRIS211083]

Three letters  E_F_I

 

I know you like your carbs but finding some one about to fiddle them correctly is hard.

 

Just a thought.

 

Chris

[ChrisV]

From the knowledge i have acquired from my problems for the last 1.5 year...

 

I am talking about the idle circuit. The goal is to have the smallest idle jet with the best progression. The dellorto's mixture screws have to be at 3.5 to 5 turns from fully shut with the best idle possible. Do you have a good idle ? At what number of turns are the mixtures ? How is the progression ?

 

The emission carbs because of the idle holder are a bit trickier to tune. In small engines you can solve the problem with a bigger idle jet. And it settles from 5 to 6.5 turns from fully closed in this case. That's what the Des hammil book indicates and he is right. From the things you write i understand that the progression is not good. This on its own indicates that you need a bigger idle jet. Try a 65 for instance.

 

For all this to work you have to be sure that the carbs are not clogged, there are no vacuum leaks, no problems whatsoever. For instance my chokes were stuck open...This is your starting point.

 

Also i can not say for certain but maybe you need a 32 choke instead of a 34 for the 1700cc

[Gopherit]

Thanks Chris for the helpful reply. The progession phase is the problem. I can get a acceptable idle below 1000rpm and the engine will rev cleanly and freely without load.

 

I have tried a 65 idle jet without any significant improvement, along with a 155 main - my understanding, thanks to this forum is that the idle jet is fed directly from the main, but when the main starts working it stops the idle and in fact there is reverse flow through the idle jet. Hence thinking that if I went to a 65 it might need a bigger main. No combination of 62/65 idle, 145/155 main did anything different.

 

The engine runs lean in both phases and is hotter than it should be.

The Chokes are not small at 34mm, but I run 33s on my QED 420 spec 1558cc Twincam very successfully (using Webers). This engine (Vegantiune VTA) is 1700cc so I do not think 34s ore radical.

 

The idles mixture screws are around 4 turns out. Significantly, screwing in  1 & 2 does not stall the engine, 3 & 4 do stall the engine. Still working on this but there was a possible very slight air leakage through the mounting O-rings for 1 & 2. 

 

My latest area of attention is the float levels. Although I had gone through the procedure given in the Sticky in here, the fact that it is lean and the progession is bad leads me to look at this. So far I have been conservative in upping the fuel level (about 1.5mm) but there is a hint of improvement and I note this posting:

 

http://sideways-technologies.co.uk/forums/index.php/topic/5514-flames-at-200rpm/

 

...where he seems to have a similar problem and there is a suggestion of raising the float level. In that respect I am finding it difficult to judge when the float arm touches the needle head as you have the spring loaded needle and the moving holder to take into account. Most accurate method, but time consuming, seem to be to fill the fill and then remove a main jet and stick a cocktail stick down the orifice, mark the depth with a pencil and then measure. Doing this I get 27mm (coincidently) which seems low considering the floats are in the fuel. I am also going to measure fuel line pressure.

[Gopherit]

Think I may have found the problem. The fuel system is re-circulating, something I think the PO did - why I do not know!

 

Arranged like this the fuel pressure with the engine running is only 1psi at the carbs. My understanding is that the DHLAs need a minimum of about 2psi and preferably 3psi. If I cut off the return line to the tank I can get 3psi out of the quite small Pierburg pump. Doing this whilst idling the revs immediately go up about 2-300rpm.

 

Have not had time to set this up so I can run on the road and reset the idle etc but that is the next step.

[ChrisV]

Have you tried to go up to 6-7 turns ? My engine best idle was at 7,5 turns with minimum emissions. 

I am just asking because i thought that worked at 4-5 turns but couldn't understand why the idle was rough until i went that high and understood that the problem was elsewhere. 

 

I have a stock 1800cc with one cam. The best idle with minimum emissions (3.46 co and 314 hc) was achieved with a 58 idle at 7,5 turns. The progression was terrible. The other jetting was 145 main, 7772.8 emulsion and 210 air corrector. i tried after this a 60 idle jet without changing the setting and the gap in the progression was filled but you could still see that the idle jet was small.

 

Now i am trying a 135 main, 7772.10 emulsion and 230 air with a 60 idle jet but i cannot get to the above excellent idle.

This is a leaner setup overall so maybe that is the reason why... I am saying this just for reference.

[Gopherit]

Chris -

I have now modified the fuel line and pump arrangement. I have put a convenient Dellorto idle jet - drilled out one from my 1st investigations into these carbs and probably about 68 at a guess - into my return fuel line. Total guess, but it has raised the line pressure to arround 2.5psi, just what I wanted.

 

This has given a definite improvement in both the progression phase and the main jet phase, tying in with what I thought was the problem. I now have to go back and set correct float levels and try the various jet combinations I have been using.

 

I can set the idle at about 700-800rpm although it is slightly lumpy - but I am still trying idle screw adjustment. As before I have this curious feature of the engine stalling if 3 or 4 is screwed in completely, but not 1 or 2. Seems to suggest that fuel is reaching 1 & 2 from somewhere else, but cannot think where. I have re-checked the air flows and they are all the same.

 

I have tried my 62 Idle/155 main combination and that is ok but still not picking up from low (1500rpm) revs. Above 2000rpm goes like a train. 65/155 is similar.

Latest thought is to slightly advance the static ignition as my previous changes in this have been totally masked by the lack of fuel.

 

Eurocarbs tech man thought 145 was on the small size for my engine, hence trying 155. I would think 135 definitely too small for your 1800cc and personally I would have thought the 230 air was too big - my old Weber rule of thumb was Main+50 for Air corrector - as a starting point. Also, I thought Des Hamill said that if you had to go further out than about 5 1/2 turns on the Idle screw you needed a larger Idle jet?

 

IF anyone else is reading this it would be REALLY HELPFUL if there was a guide to the fuel level in the DHLA float bowl when measured through the Main Jet orifice - ie without taking the top of the carb off - should be higher than the 27mm specified in the excellent guide at the front of this section - but by how much?

[ChrisV]

The information i am sharing is for reference because there might be something wrong that someone else might have encountered. 

 

Yes you are correct. If you go high on the turns you need a bigger idle jet. My problem was that i wasn't going (in my early explorations) that high ! Thus i didn't see any change on the engine and thought that my problem might be elsewhere. I sold A LOT of problems to be frank haha...

 

The floats is quite a big story... my manifold for example gives a big angle to the carbs. So all the fuel was at the back on the main jets... In order for my setup to work i had to lower the level of the gasoline. My initial setting was 17mm as the guide suggested and settled to 20mm (float setting). I set the droop at 26mm. Have in mind that there should be no leaking from the aux venturis. So check the height, see how far you are, test it 1mm at a time.

 

If the carburettors are flat it is a lot easier to set the floats.

[NakaGTV]

Chris -

I have now modified the fuel line and pump arrangement. I have put a convenient Dellorto idle jet - drilled out one from my 1st investigations into these carbs and probably about 68 at a guess - into my return fuel line. Total guess, but it has raised the line pressure to arround 2.5psi, just what I wanted.

 

This has given a definite improvement in both the progression phase and the main jet phase, tying in with what I thought was the problem. I now have to go back and set correct float levels and try the various jet combinations I have been using.

 

I can set the idle at about 700-800rpm although it is slightly lumpy - but I am still trying idle screw adjustment. As before I have this curious feature of the engine stalling if 3 or 4 is screwed in completely, but not 1 or 2. Seems to suggest that fuel is reaching 1 & 2 from somewhere else, but cannot think where. I have re-checked the air flows and they are all the same.

 

I have tried my 62 Idle/155 main combination and that is ok but still not picking up from low (1500rpm) revs. Above 2000rpm goes like a train. 65/155 is similar.

Latest thought is to slightly advance the static ignition as my previous changes in this have been totally masked by the lack of fuel.

 

Eurocarbs tech man thought 145 was on the small size for my engine, hence trying 155. I would think 135 definitely too small for your 1800cc and personally I would have thought the 230 air was too big - my old Weber rule of thumb was Main+50 for Air corrector - as a starting point. Also, I thought Des Hamill said that if you had to go further out than about 5 1/2 turns on the Idle screw you needed a larger Idle jet?

 

IF anyone else is reading this it would be REALLY HELPFUL if there was a guide to the fuel level in the DHLA float bowl when measured through the Main Jet orifice - ie without taking the top of the carb off - should be higher than the 27mm specified in the excellent guide at the front of this section - but by how much?

Brooklands manual for Alfa Romeo says that fuel height measured through main jet orifice should be: 1300 and 1600 engines 15-15mm; 1750-2000 engines 15.5-16.5 mm. It also states not to bend floats but to change the thickness of the washer under the needle valve body.