Dellorto Idle Air Bypass Screw

[NYK]

I believe I’m correct in saying that some DHLA’s do not have Idle Air Bypass Screws, well mine don’t anyway! Why is is, surely it means that you can’t balance the two barrels of the carb?

[types26/36]

It’s only the Dellorto DHLA 40E that has the by-pass screws, if there is no letter after the 40 they are just standard carbs.
Some people bend the throttle shaft to equalise the air flow but I have never had any success there. You can always try to re-centre the throttle plate but again it never worked for me.
Have a read if this thread as it will give you a good idea what you are up against.
viewtopic.php?f=40&t=51577

[NYK]

Thanks Brian, I’m sure bending the throttle shaft is too crude an adjustment and very hit and miss to balance out any slight difference, I think I will give that a miss. Following a complete car (and engine) rebuild I’m just trying to get a good set up on the carbs. They were also stripped and rebuilt I’m pretty sure I managed to centre the throttle plates. It’s a shame that every dhla does not come with the air bypass screws, can’t cost much to include them and a no brainier really to ensure good balance.

[vxah]

When I rebuilt my Dellortos I found centring the throttle plates did not always give the same small flow through each barrel so, I made up a vacuum rig using my “Henry” workshop vacuum cleaner.
I made up a fitting to go on the hose and seal around the carb flange, I could then use a carb sync gauge to measure the flow at closed and low throttle openings on each barrel, re-setting the throttle plate changed the flow significantly so I knew all barrels flowed the same before they were fitted!

[NYK]

I’m going to do exactly that today although my workshop vac is not called Henry but I’m sure he (the vac) will be a decent substitute. Looking at the flows on my 4 column carbtune, it was clear that there was a large imbalance between the barrels on the front carb.

[NYK]

Took the offending carb off that appeared to have a significant difference between the two barrels. Made up my ‘Henry’ vacuum cleaner adapter and compared the flow between the two barrels.

The result was ………. both barrels had exactly the same flow!

Put everything back together thinking horrible thoughts about my recently rebuilt engine that might explain why No. 2 cylinder might be down on vacuum then had a rather obvious thought, one I should have had before I messed about pulling the carb off. In case you’ve not guessed, the problem was that the wretched manometer had one tube that was measuring low. Sure enough, when I swapped the manometer tube that I had used on number 2 cylinder to number 1, the low reading went with in. Whilst it was a waste of half a day, I at least had an answer to my problem, which was that I did not have a problem.

Funny thing was, I was only thinking yesterday how the old fashioned manometers were probably a whole lot more reliable than the solid state ones being sold now, less to go wrong. It may well be that the new electronic ones also suffer variation but I’m certainly far from impressed with the thing I bought which I have only used twice.
By the way, if anyone ever needs to borrow my Henry adapter, just drop me a note.

[vxah]

Hence when working on electric vehicles, test your absence of voltage meter, test the vehicle, test your absence of voltage meter!
Failure may be more severe than annoyance!

[mbell]

In case you’ve not guessed, the problem was that the wretched manometer had one tube that was measuring low.

The one i have, has you calibrate it by connect all four tubes to the same inlet and adjusting screws, before using it to balance the carbs.

Does yours not require something similar?

[NYK]

In case you’ve not guessed, the problem was that the wretched manometer had one tube that was measuring low.

The one i have, has you calibrate it by connect all four tubes to the same inlet and adjusting screws, before using it to balance the carbs.

Does yours not require something similar?

I’ve just rechecked the instructions, they say
“Gauges do not need to be zeroed or calibrated.All parts have been matched at assembly.”
There is no provision to make any adjustments. Might drop the supplier an email, but not holding my breath.

[mbell]

Sounds like they did match them correctly at assembly.

TBH I gone back to just using one of the flow meters as it was too much hassle to setup the manometers up.

[NYK]

Sounds like they did match them correctly at assembly.

TBH I gone back to just using one of the flow meters as it was too much hassle to setup the manometers up.

I suppose using a single flow meter eliminates one variable, that of differences between flow meters but the need to move it from one barrel to another introduces a different variable, that of time during which engine revs may have changed.

[mbell]

You are correct but flow meter is damped so it easier to read and few rpm variation doesn't effect the air volume that much.

With my manometer I found the readings are consistently moving a bit due to individual cylinder firing so its hard to get consistent readings for all cylinder. Might just be my manometer thou.

[alaric]

Hi all.

The referenced thread was re my Dellortos one of which has an imbalance between barrels, which gives the engine a lumpy tickover.

My plan with my old carbs is to replace the lead plugs at the ends of the mixture galleries, and see if that fixes the barrel imbalance. The current plugs no longer seal with a smooth or flush surface with the mating face of the carbs, and appear brittle. Apart from that nothing that I have tried has made any difference. I believe it's an air leak, and the only remaining candidate is the lead plugs. If I open the cold start circuits slightly the balance between all barrels is a lot closer, which is why I think it may be the plug on the idle mixture gallery.

But, I must admit that I've asked that nice bloke Rob at Eurocarb to supply a new pair of carbs, and specced the DHLA40E variant, which has the air bypass valves.

Regards.

Sean.