• Hi Guest! Being a Paying Supporter is NOT a requirement to participate at BCF. However, subscribing will help ensure we can continue to serve the LBC community for years to come. If you enjoy BCF and find our forum a useful resource...Please consider supporting us by **Upgrading**
    (PS: Subscribers don't see this banner)
Tips
Tips

HD series Jet Centering - a Caution

steveg

Yoda
Gold
Country flag
Offline
Last year, I used the Moss jet centering tool to center my new needles and jets.
JetCenterToolMoss.JPG


These are my "new" needles; the jets are also worn:
JetsWorn.jpg

That's $120 + shipping wasted + cost of the tool.

Learned my lesson. Centered my new jets & needles by observation.

The jet bearing nut is 18mm - a socket works better than a combination wrench. It has to be first tightened by hand so as not to disturb the alignment. It's a trial and error process. When properly completed the piston will close with a clunk or clank.

With the carb on the bench, remove the large spring inside the dome and replace the dome; remove the damper and oil. This way you can push the piston/needle assembly down more smartly than the spring does it. Then with the bottom assembly off the carb you can slide the jet into the bearing. When the jet seats observe if the piston is disturbed in any way. If so, that means there's friction between the needle and jet. Loosen the jet bearing nut and jiggle the jet. Tighten the nut by hand and push the jet in until it seats. If the piston doesn't move in any way the jet centering is good. Carefully and delicately tighten the nut such that the position of the jet bearing is not altered. Check again for friction between the fully seated needle and jet.

When the jet bearing nut is tight, test by pushing the piston smartly through the dome with a phillips screwdriver - it should make a metallic click or clunk which indicates no friction on the needle.

Addendum: make sure the jet diaphragm is in the same position every time you check it and when you tighten it up.
 
Last edited:
OP
steveg

steveg

Yoda
Gold
Country flag
Offline
You take out the piston, reattach the dome and slide it down through the inner tube. The narrower diameter goes through the jet bearing, as does the large end of the little pin thingy in the tool kit.
 
Country flag
Offline
You take out the piston, reattach the dome and slide it down through the inner tube....

Maybe that's the problem: you've removed the variances introduced by the piston. The little pin thingy goes into the piston in place of a jet needle, and allows for any offcenteredness--is that a word?--of the piston and the needle shank. These little kits are worth having; it's one way to balance the carbs without having to remove the air cleaners (PITA) to use a synchronizer--or a hose if you're old school--and, by comparing all methods they work pretty good. The little mixture adjustment wrench works where you can't get a spanner (on non-HD carbs).
 
OP
steveg

steveg

Yoda
Gold
Country flag
Offline
I just tried the little pin thing in my spare HD8. The small end was too large to fit in the jet needle receptacle, and the large end is .002" narrower than the Joe Curto jet.

IMO the overall problem is with a centering tool one adds another layer of error.

If either tool in one's possession is good enough to center the jet without rubbing, bravo - neither of mine will do the job without further centering by observation.
 

ozhealey

Member
Country flag
Offline
Steve,
Sorry to see the tool did not work for you.

I have both the full sized tool and the small insert from the balancing SU tool kit. I have found over the years both work brilliantly and result in the definitive “clack” of a well centred jet/needle. I have never had the wear problem Steve unfortunately had in his experience.

The two tools have different applications depending on the state of your carbs. The long tool used by Steve requires the jet to be removed from the carb as the thin section goes through the full length on the jet sleeve. It does a great job of aligning the jet to needle when you are rebuilding the carbs. The smaller tool is great if your carbs are already built with the jet installed or perhaps you just replace the needle or for some reason want the “check” alignment. It is also good if you have never centred the jet and want to give it a go without removing the jet. You just remove the needle, install the tool and then crack the jet nut to allow alignment and then tighten followed by a check for the “clack”.

Just my experience other may find the tools different.

Cheers
Tony
 
OP
steveg

steveg

Yoda
Gold
Country flag
Offline
As usual, Tony, thank you for the clarification! Evidently my technique needs refinement.

Since my HD8 jets are centered and the carbs are mounted, will disassemble one of my old HD6s and revisit the tools. Will follow up later.
 

red57

Jedi Warrior
Country flag
Offline
When the jet bearing nut is tight, test by pushing the piston smartly through the dome with a phillips screwdriver - it should make a metallic click or clunk which indicates no friction on the needle.

Steve, IMO there is your problem - the "clack" should be heard as the piston 'free falls' by it's own gravity. You can push the piston all the way down and force a "clack" even if there is contact between the needle and jet but
when they are aligned properly there will be no place the needle will touch the jet so there will be no friction to resist the free falling "clack" of the piston. Key here is 'free fall'.

Dave


 
OP
steveg

steveg

Yoda
Gold
Country flag
Offline
Steve, IMO there is your problem - the "clack" should be heard as the piston 'free falls' by it's own gravity. You can push the piston all the way down and force a "clack" even if there is contact between the needle and jet but
when they are aligned properly there will be no place the needle will touch the jet so there will be no friction to resist the free falling "clack" of the piston. Key here is 'free fall'.

Dave


You're right - "smartly" is the wrong word. I wasn't forcing it. I was holding the jet fully into the jet bearing with my finger while pushing the piston down with the screwdriver - while observing by touch if there was any friction between the needle and jet. The other way I checked it was, with the piston/needle fully seated on the bridge, I pushed the jet lightly into the jet bearing. Was the needle disturbed?

It was more convenient to move the piston back and forth by hand rather than relying on the spring and gravity.


I used this as a way of ensuring there was no friction and got the "clunk".
 

red57

Jedi Warrior
Country flag
Offline
Steve, I figured you were careful and didn't mean to imply anything less but since it is so easy to affect the centering, I thought I'd mention it.

I once rebuilt a set of HD6s and was very careful to be sure the jets were centered well but when I put them on the car, one had a slight bind that I finally concluded was caused by the slope of the carb - I had set the jet bearings with the carb body horizontal in my vise. There are manufacturing tolerances in everything - the bearing ID in the dashpot, the shaft OD on the piston, the jet OD, the jet bearing ID, plus wear of all the above - and, if you get unlucky and just he right combination of upper & lower limits on the individual parts, it can add up to looseness or tightness. In this case it was towards the loose side and the angle of the carb allowed the piston to run slightly off center. Now, no mater how sure I am on the bench, I always check for the gravity 'clack' after installation on the car.

Dave
 
Country flag
Offline
This method hasn't failed me in forty-five (45) years. It's the pin (jet-centering device) that Bob alludes to in post #2

IMG_1753-me.jpg


IMG_1756-me.jpg


IMG_1757-me.jpg


IMG_1761-me.jpg


Snug the jet-holding nut and make sure the piston still falls with a clack, tighten fully and test again.

IMG_1760-me.jpg
 
Country flag
Offline
At least thirteen (13) of the first fourteen (14) pages of this album are devoted to the overhaul of a single set of HD8 carbs, if anyone is looking for some visual aids.

https://spcarsplus.com/piwigo/index.php?/category/375

If I had them to do over again, I think I'd see how well just putting new shafts work in the original bushes. While I was able to eventually get a good fit, they were more trouble than they should've been (even using the correct SU reamer).
 
OP
steveg

steveg

Yoda
Gold
Country flag
Offline
Last year, I followed the directions in SU CARBURETTERS TUNING TIPS & TECHNIQUES and rebushed my HD8s with new PTFE bushes - no ream. They were pretty OK, idle-wise.

Since I had my carbs apart for the above-mentioned needle/jet problems, sent my carb bodies off to Tom Bryant to install and line bore his Delrin bushings - that should hold the carbs while I'm on this earth.
 
Country flag
Offline
Randy,
Was this jet broken or did you cut it off? If the latter, why?

View attachment 56316
Linky no worky. Can you just give me the image number?

For example, the image number for this one is "image 1368" (sometimes shows as "IMG1368") https://spcarsplus.com/piwigo/picture.php?/63292/search/133

Or, just copy & paste the whole address from your browser.

I can say with certainty though, that I didn't cut a jet, maybe I was just showing a picture of how far the jet inserts into the piston (?) and the image got, well, cut off?
 
Country flag
Offline
Steve, looking back at the pictures preceding the the one you're asking about, it looks like the "jet tube" (movable tube connected to diaphragm) had seized inside the jet bearing. I don't recall how much force was required to remove the jet assembly, but clearly I separated the pieces__it was not cut, but pulled apart.

i.php


i.php
 
Similar threads
Thread starter Title Forum Replies Date
R For Sale Fidanza Aluminum Flywheel for 1275 A-Series Spridgets Classified 0
M Series 3 E-Type Bonnet Jaguar Classifieds 0
L BMC A-series: Oil in coolant British Motor Corp 6
6 SOLD!! For sale 1966 Series 1 FHC $85,000.00 Jaguar Classifieds 3
ROARRR Series 2 XJ6 with cert for chev eng for parts or refurb Jaguar Classifieds 0
N Wanted Spats - XK Series? Jaguar Classifieds 4
S Series 1/2 Wiper Motor Needed Rover / Land Rover 0
steveg HS vs HD series carbs Austin Healey 2
W XJ6 Series I bumper swap to Series II Jaguar 0
S For Sale Mg t-series crankshaft tb tc td tf 1250 1500 MG Classifieds 0
D Source for XJ6c Series ll Ignition key blanks & remote Jaguar 1
Lin Fairly new YouTube series on AH100 restoration Austin Healey 0
H Lotus Super 7 series 2 Race car for sale Lotus 0
T For Sale Gear reduction starter fits jaguar 3.8 340 mark series 3.8l 3.4l - $100 Jaguar Classifieds 0
M E-type series 1 plastic dash vents Jaguar 1
M XKE Coupe series 1 Tub versus series 1.5 Jaguar 1
O Wanted Vauxhall Victor F series parts Other British Classifieds 2
O TR6 TR series transmission compatibility Triumph 4
B For Sale 1969 Jaguar XKE 4.2L Series II Coupe - Indiana Jaguar Classifieds 0
Basil About to re-install my fuel tank Series I E-Type Jaguar 6
Healey Nut "Father Brown" British television series Austin Healey 9
Basil Series 1 EType Spotted 2
J Wanted tri-SU carbs, manifold and linkage for '69 series 2 4.2 Jaguar Classifieds 3
Nunyas Salvaged Lotus Evora rebuild video series Lotus 8
D Series II E-Type tire size Jaguar 9
M Photos Needed of Series III E-Type Engine Oil Filter Clearance at Exhaust Manifold Jaguar 1
S M Series Bonnet Striker Pin failures TVR 4
C Mounting ignition switch to XKE series 2 steering column? Jaguar 4
G original radio 1961 e series jaguar Jaguar 3
Y M Series indicator lens TVR 3
Q Series 1 E Type Clutch Jaguar 2
F 1965 Humber Super Snipe Series Va Other British Cars 5
T 1971 Series I XJ6 parked for 12 years - fuel pump problem Jaguar 4
G '70 Series II engine bay wiring harness -- Jaguar 7
G '70 Series II engine bay wiring harness -- Jaguar 3
David Townsend T-Series MG Midget TC Series Limited Edition Illustrations MG 1
J TR4/4A TR4 Series Vehicle numbers Triumph 1
N GT6 GT6 I II III Series rear vent window sizes Triumph 4
P XKE series 1 FHC windshield installation ?? Jaguar 4
E Series 1 jaguar boot latch problem Jaguar 2
N A series expert's needed?? Spridgets 2
D Series III E-Type Overider Rubber Installation Jaguar 2
3798j 1957 Series 1 [A Love Story] Rover / Land Rover 2
H Joan Baez flat floor series I e-type roadster: what did the authorities dump it for? Jaguar 0
Nunyas Patrolicious - 1973 Land Rover Series III Other Cars 1
Simmo Series 3 E Type Brake Bleeding Jaguar 2
matttnz TR6 CR series car wiring diagrams Triumph 4
J Question about Series III XJ6 Motor in E Type? Jaguar 2
BillD 79 Panther Lima Series 2 turbocharged Other British Cars 16
B Looking for some XKE series 2 parts Jaguar 0

Similar threads

Top