DHLA Jet Gauges

[barker_001]

Does anybody have a set of Dellorto Jet gauges I could borrow? Quite happy to cover P&P, etc.

I've got a bit of confusion over idle jet size going on. Jets that say they are 55s but I suspect are larger. And we all know that size matters...

Thanks in advance!

[Esprit2]

It's not uncommon for tuners to drill jets oversize. It's a distant second best compared to using jets of the proper size, but it's not uncommon.

The Dellorto jet's number is it's bore diameter in millimeters. A 55 jet is 0.55mm inside diameter, and a 130 is 1.30mm.

If you have a set of small drill bits, then use the shanks like feeler gauges. Find the largest one that will slip-fit into your jets. You're looking for light drag without force. Find the right fit, then measure the drill shank with a micrometer... worst case, with a caliper.

If you work for a manufacturer with an in-house Quality Control department, or a Tooling Department, inquire if they have a set of gauge pins. They're a little old school, but still very useful. They won't let you 'borrow' them, but perhaps you can take your jets to work and measure them over your lunch break.

Regards,
Tim Engel

[barker_001]

Thanks for your comments Tim. Unfortunately (despite working in the aircraft industry) there's nothing like that in our tool stores... in fact I had to special order a very ordinary 9/16" drill bit the other day because we didn't have anything like that either... Looks like I'll probably have to buy some gauges to answer my question.

I have a strange situation. My car had the original 55 idle jets fitted, and for years has suffered from a hesitation around 2000 rpm (see my many, many posts about this subject). Early this year I took the car to a highly recommended rolling road (Vince, at Mosley's Garage in Llandeilo). He ran the car up through the rev range, and diagnosed that it was basically running out of fuel at 2000 rpm, and was going very lean, causing the hesitation. Initially, he tried some 60s, but when he fitted them it quickly became apparent that this was too rich. So then he suggested trying some 58s, and I handed him the packet of 58s that I had brought with me. In they went (so I thought), and the hesitation went away.

However, during the very hot weather recently, I had some problems with hot starting and an erratic idle. I suspected it was either running too rich, or something was wrong in one of the carbs. I took the top covers off the carbs and checked the float heights, needle valve operation, etc. All seemed OK. While I had them in pieces I gave the jets a blow-through with some carb cleaner. Lo and behold - they are 55s! And yet my hesitation has gone. On checking the little bag of jets I brought home from the rolling road - they are my 58s! All I can think of is that he mistakenly picked up my original 55s rather than the 58s, fitted them, and adjusted the mixture screws accordingly.

I am intrigued to find out whether the jets I have fitted are actually 55 size, or, as Tim surmised, may have been "oversized" at some point?

[Esprit2]

We never heard the end of this... what happened?

Regards,
Tim Engel

[DavidOliver]

I have just read the book How to build and Power Tune Weber and Dellorto by Des Hammil (due to a previous post from Tim Engel) and can recommend it.

Without having commented to Angus, perhaps the Forum could purchase and make available a set of jet gauges.
For members info they are sold at Eurocarb for 40 pounds ish.

Dave the cog.

[amarshall]

I have just read the book How to build and Power Tune Weber and Dellorto by Des Hammil (due to a previous post from Tim Engel) and can recommend it.

Without having commented to Angus, perhaps the Forum could purchase and make available a set of jet gauges.
For members info they are sold at Eurocarb for 40 pounds ish.

Dave the cog.

Sounds like a potentially useful thing for us to have available - and well within budget. Anyone else think it's a worthwhile purchase? (or have a reason not to do it?)

[Pete Boole]

Sounds good to me - I've got some carbs now!!

Pete

[Lotus-e-Clan]

Referring back to Brian's post on the idle jet swap ...although these (progression idle jets) will still be in play @ 2000 rpm the main circuit will have come in too ..so it is a complex region to trouble shoot. As well as the possibility of fuel starvation cause by jet calibration, you have to consider air flow through the venturi influencing 'pull-through' on the mains and float chamber level, and air leaks. And that's assuming the ignition is spot-on.

On the jet gauge purchase ... the jets are clearly marked so unless you do suspect someone has drilled them you probably wouldn't bother. Also you would want a set that is specifically for Dellorto jets which progress in steps of 1 ..whereas Weber jets progress in steps of 5. That would mean the Dellorto guage set would need to be pretty large and complete to be any use.

[DavidOliver]

Back to the drawing board.
I agree with Peter K.
The available gauges cover 0.45 to 1.5 and 1.5 to 3.00mm so the Forum would need to shed out 80 pounds and not 40 pounds
as I indicated.
These gauges come in 0.1mm stages and are actually 0.05mm undersized to be able to measure a jet without friction interference.
Very few of our jets will have been reamed out, and very few of us require 100% precision on carburation as we do not circuit race
our cars. The jets are visibly marked for size. Other variants like air temperature or where we have inlet air take off points will have
more effect on carburation.
The original post by Bryan Barker about idle jet sizing cannot be measured with jet gauges, and I consider my earlier post about
the Forum purchasing gauges is retracted.
Back to the drawing board!

Dave the cog

[barker_001]

We never heard the end of this... what happened?

Regards,
Tim Engel

Well Tim, nothing else happened! I never got around to buying any jet gauges, and as others have commented, they probably would have been of limited use. My car has continued to run very well, and the hesitation has not returned. The only pay-off has been that the emissions are a little high at circa 1000rpm, but fall into MOT spec if the revs are raised to 2000 rpm.

Interestingly, looking through the previous history of the car (prior to my purchase) I found a handwritten note on an old MOT invoice stating "idle emissions @ 2000rpm" or words to that effect. So it appears the fault was identified, and to some degree, solved, many years ago - but probably re-introduced when someone tried to tweak down the emissions at 1000 rpm for an MOT.

As part of the long saga of chasing down the hesitation I had, I'd replaced the original 55 jets with a new set of 55s, and these were the ones refitted by my rolling road tuner. So my jetting is completely stock again.