Twin carbs

[alfadave]

Anyone got any tips on setting up twin carbs from scratch?

I've got a pair of Dellorto DRLA 36's on a 95 bhp engine.

I've set the timing with a strobe.New plugs/leads/rotor/cap. Bosch elec ignition.
Idle jets, and acc pump jets all clear and working.

It fires up, but is very lumpy, and won't tick over easily.

[Veesix75]

Sorry to ask, have you cleaned the jets ?

The idle jets could have done crap in which makes it poorer on low revs, these are in the top of the carb, item 28 and 4 I think on the diagram.

Then the others are the ones on the side of the carb, but screw all way in and back off a turn at a time to get a steady tick over, item 54 below I think.

https://www.carbparts.eu/exploded_view/DELLORTO_36_DRLA

[alfadave]

Idle jets all clear....had that crop up many times before....even with 2 fuel filters in line, plus the 2 in the carbs
Idle mixture screws opened up 2 turns each
Pump jets all clear and working

The rolling road guy takes half a day to set twin carbs up.

Just want it to get to the MoT station......soonish!

[Alfastrut]

Had something similar. When abjusting the idle mixture on one cylinders I was getting no increase or decrease in revs. Found the air bypass was blocked.

[junior]

I found from motorcycle days to soak them in a degrease bath or similar and then blast out with as high pressure air line as you can can make a big difference

at the bottom end and tick over. Check when running for air leaks.

I surmise you have new fuel pipes as well ? Good luck.

[alfadave]

There are small holes directly next to the idle jet holders.
Which then turn 90 deg into the idle jet hole.These are all clear.
Had these blocked in the past with foam from Pipercross filters.....I only use K&N now!

The air by pass screws are covered with plugs on one carb, and open on the other with funny looking screw heads, that don't look like they will turn

[Alfastrut]

You are right they should be covered by tamper proof plugs. They do screw out. I used a colour tune to confirm which cylinder was not firing and a air flow meter to balance it to the other cylinders

[Alfastrut]

This is what I used to check air flow. There are other types available but this was quite cheap and easy to use

[Sprint88qv]

I’ve got a multi-step process which I always follow. I’ll PM it to you when back in work (Tuesday).
Worth checking the mechanical timing to make sure one of the cams isn’t one tooth out, otherwise you’re wasting your time trying to get the carbs right. A couple of points to note: 2 turns works for me on dellorto 40’s with 1.7 and I believe it’s pretty much the same for all the boxers. Are you sure it’s got the correct jets and hasn’t been changed at some point.
Make sure the linkage is set to open both sets of butterflies across both carbs at exactly the right time, otherwise you get this sort of stumble when you start to open the throttle.
I also use a colour tube and the air flow meter above - essential bits of kit. Lastly, i’d agree that lumpy idle with no throttle being applied usually means some crap in the system somewhere. And popping is normally ‘lean’, so maybe an air leak somewhere.

Sorry if I’m teaching you to suck eggs, but I wasn’t sure where you were starting from and these basics always work for me.

[Sprint88qv]

Also, on the air bypass screws. Sounds like you don’t have a matched pair. All alfa ones were set in the factory and then the tamper proof plug added (which I think was molten lead) to avoid unnecessary adjustment. I guess the dealers got fed up having to constantly adjust them and asked for them to be set ‘near enough’ from new.

Another reason to check all the jets are the same and correct to the 95bhp boxer.

If the one with the screw doesn’t turn, then it may have seized in a position that is providing an off balance to the other carb. Best way is to check the airflow as the other guys say and see what the difference is. Key point to remember is that the mixture has absolutely no impact on the airflow. So you need to adjust the air bypass screw or via the linkage to get the air flow the same across both carbs.

Hope this helps.

[alfadave]

Many thanks for that
Cambelts recently replaced an timing checked
Will check all the jet sizes

Just noticed.....one carb is DRLA 36D (RH)
The other is 36S (LH)

Is that just designation for left and right...or a mis match?

[alfadave]

To answer my own question.....
just googled Italian for left is Sinistra
right is Destra

The air by pass screws with the tamper proof caps removed on the right carb, have the screw heads butchered, and won't turn.......should I drill them out and replace them?

[Sprint88qv]

I personally wouldn’t touch them until you have checked the airflow across both carbs with the meter that Alfastrut suggested. I only say because 1. You may not need to touch them, the flow might be fine (both venturis in that carb will have been set very close to one another) and any difference with the other carb may be able to be ironed out via adjusting the stop screws, and 2) from bitter experience, it is very, very difficult to drill them out. These carbs are precision pieces of kit that rely on accurately calibrated amounts of air flow. It is not easy to get seized screws out without knackering threads or valve seats.....you need proper precision engineering kit to be sure. I tried and ended up scrapping a body.

My advice, leave it unless you have to do something with it, you may not have to. And if you do, soak it for days/weeks in releasing fluid.

[StescoG66]

To answer my own question.....
just googled Italian for left is Sinistra
right is Destra

The air by pass screws with the tamper proof caps removed on the right carb, have the screw heads butchered, and won't turn.......should I drill them out and replace them?

NO!!!! Soak them a few days in releasing agent (Innotec Deblock is great stuff) and heat gently they should release. If anything like mine were then its likely a build up of carbon coupled with steel bonding to aluminium. I am only reporting what the chap who serviced my Weber 40's told me. .............

[Sprint88qv]

Wise words there.

[alfadave]

Thanks for the advice.....I'll put the drill away!
And get some release agent.

Just thought last night.....I had a pair of Weber DCOE sidedraft carbs with a poor tickover......checked all the idle jets which were clear.
It turned out to be blocked progression holes.
Easy to get at and see on the DCOE with the carbs fitted.

Better with the carbs off on the DRLA.....will have a look.

[Sprint88qv]

Good plan. Let us know how you get on.

[alfadave]

Checked the progression holes. Not too difficult to check with the carb in place, using a torch and mirror.
They are much larger than I remember on The DCOE Weber, and all seem clear

Would take a lot to block them.

Carbs are tagged
Right :- R6202D 536974
Left :- R6201S 536975

so, seem to be a pair?

All main jets are 122
All idle jets are 52

Seems to tally with the spec in my Haynes manual

[junior]

To answer my own question.....
just googled Italian for left is Sinistra
right is Destra

The air by pass screws with the tamper proof caps removed on the right carb, have the screw heads butchered, and won't turn.......should I drill them out and replace them?

NO!!!! Soak them a few days in releasing agent (Innotec Deblock is great stuff) and heat gently they should release. If anything like mine were then its likely a build up of carbon coupled with steel bonding to aluminium. I am only reporting what the chap who serviced my Weber 40's told me. .............

I may be forgetting something, but I had to have one air by pass screw drilled out on one of my carbs. Then Dave at mech tune and a Weber chap told me I wasted my time

as they screwed them shut. Dave also then sealed them with a blob of silicon to stop air leaks.

Sorry if that just confuses matters.

[Sprint88qv]

Looks like they are a matched pair and the right ones too. Your jet sizes look good. Pump jet should be 33 and the emulsion tubes should be 9164.2 with a 180 main jet air corrector.

Sounds like someone has had a go at the tamperproof plugs on one carb, so, as long as they haven’t managed to change the setting via the butchered screws, they should be fine. Which means you will be able to balance them through the relay and throttle stop screws.

Uneven idling can be down to over-fuelling, so try and check the float settings. Also crap in the bottom of the float chamber can impact on the floats. So have a good look in there.

One way to check if it is the idle circuit is to increase the revs to bring in the main and acceleration circuits. If the rough running disappears, the the problem is in the idle circuit.

Lots to check I know but that’s always the way.

Hope this helps.

[alfadave]

Haynes give the float clearance as 5-6mm

But I remember on the DCOEs that people remove a main jet holder, and actually check the fuel level using the depth gauge on vernier calipers.

This gives a direct check on fuel level, which of course is altered by bending the float tabs.

Wondered if anyone had a figure for this measured from the top of the carb?

Also,I'll check the needle valves on the floats are clear.....but I think I did this some time back.

[StescoG66]

I will ask a wee question - and hopefully this will help others going forward. (Mods - a carburettor sticky thread might be a good idea as there are not many left out there with the skills to adjust these things I fear).

Going by all the guides (and from what I can see) Screw 1 should be the idle adjustment. When I picked the carbs up I was told not to touch that as it determines the closed position of the butterfly valve. I was told screw 2 is the idle adjustment - but the guides don't seem to refer to this one at all......... However I have followed the recommendations on screw 2 rrather than on screw 1. Therefore I have adjusted until it is JUST touching the lever - I had put a sheet of paper between lever and screw so that it was just held in place. I removed the sheet of paper, then tightened a quarter turn as the guides recommend. Have I done right?

[alfadave]

Looking at my red Haynes manual p159/160

Your screw no2 should be adjusted to 1mm (40 thou) clearance.

Your screw no1 is to adjust idle speed

[Sprint88qv]

Hi, Alfadave is quite right. Here is the 'full' procedure I go through. Obviosuly I don't always start with checking the mechanical timing everytime, but if I'm working on a car which I don't know, then I start from the very start. I find it can save a lot of wasted time overall. Also, worth noting, I check the compression to make sure the valves are sealing correctly i.e., not burnt out etc. As this greatly affects the running of the carbs.

Carb Balance procedure:
• Check mechanical timing – cam & TDC positioning.
• Check cylinder compressions (min. 149 psi & max variation of 15 psi cylinder to cylinder).
• Check inlet vacuum system & lubricate vacuum advance mechanism – ensure working ok.
• Check throttle & choke cables are not acting on the carbs and there is a small amount of slack.
• Theory: 3 screws to set up:
o Relay lever screws (x2) affect synchronisation of the carbs through the linkage.
o Mixture screws (x2) affect richness of fuel (start rich and weaken until crimson blue flame at idle with Colour Tune)
o Throttle stop screws (x2) affect RPM and therefore airflow (check with carb balancer/airflow meter).
• Check linkage adjustment (refer to Haynes manual for 'A' and 'B' settings below):
o Linkage cross-shafts should be 61.2 - 62mm between ball centres (“A”). Can’t adjust plastic ones.
o With throttle pedal released – check gap between relay screw & relay lever (‘B’) is 1 mm (each carb).
o Check both throttle/butteflies open at same time on operation of the throttle pedal. With the air filter off, you can hear the suck start on one before the other. Also by eye, watch the relay levers act on the throttle spindles as you gently push down on the nearest black plastic cross shaft. Adjust both relay screws until you get them perfectly synched.
o With throttle pedal flat down (from in car) – check each relay lever should be within 1mm of its stop.
• Check/adjust idle speed to 850-900 rpm. Adjust via throttle stop (horizontal) screws – one per carburettor.
• Check/adjust ignition timing (8’ BTDC @ 850-900 rpm) – first ‘I’ before ‘T’ (for 1.7 8v 305.50 engine).
• Check/adjust mixture settings of each choke with colour tune (bright blue flame) and/or gas analyser (within required range). Should be around 2½ turns. Start rich (3½ turns) and turn clockwise, until bright blue colour.
• Check/adjust airflow – adjust throttle stop (horizontal) screws to balance airflow to be identical across both carbs (can’t adjust the individual chokes on the EU Alfa spec Dellortos).
• Re-check/adjust mixture settings to ensure still within range.
• Re-check/adjust idle speed to 850-900 rpm
• Re-check/adjust airflow to ensure identical airflow across both carbs.
• Re-check/adjust throttle synchronisation across both carburettors. See linkage adjustment section above.
• It might take a couple of runs through these steps to get it all right.
• Ensure butterflies fully open with full pedal depression (1-2mm gap to relay lever stop).
• Check operation of vacuum advance unit on higher revs (operates between 5-10 mm-Hg & holds vacuum).
• Re-check ignition timing, disconnect vacuum advance for higher revs.

[Sprint88qv]

And your 1 = throttle stop screw and 2 = relay lever screw

[StescoG66]

Looking at my red Haynes manual p159/160

Your screw no2 should be adjusted to 1mm (40 thou) clearance.

Your screw no1 is to adjust idle speed

Thanks Dave - I have the Haynes manual but why can't I find any of this stuff.

[NEG]

2 ensures both carbs butterflies are opening at the same time, 1 is the idle speed adjustment set on both carbs. You need the balance flow meter to ensue both carbs are drawing the same flow when adjusting it. And the same flow meter to adjust the bypass screw so the individual chokes on each carb flow the same...

[sudboy1490]

I find 52 idle jets to be very small and with old carbs can lean out. If your trying to get it running with the air filter box off that may well add to the problem. Idle jets are very cheap on ebay, I'd try going up a couple of sizes. Maybe try 55 or 60 to see if it helps.

I had to do this on my 1.7 when I fitted ram pipes and fast flow filters.

My 1.6 modified engine is running 70 idle jets, anything less and it's missing it's nuts off.

[alfadave]

Thanks.....just ordered a set of 60's

Have been running it with no filters.....and have a pair of K&N's to fit eventually.

Currently got one head off due to low compression on no2 .....looks like an inlet valve needs lapping or possibly replacing
and the carbs are off with release oil soaking (for weeks?) in the by pass screws.....2 are butchered, one is solid.

Apart from that the threads are stripped on LH water drain from the block....good job I found it....intention is to drill and tap it from M10 to M12

And the bonnet came down with a crash.....rear view mirror fell off and broke, and the bonnet catch spring flew out!

Apart from that everything is fine!

As Alfastrut said.......the car is fighting back!

[Mufasa]

Brilliant thread guys