Linkert Carb, SU Eliminator carb, intake manifold

The Linkert M74B was in quite good condition. It had been kitted at some stage in the past. However the motor ran poorly so a detailed sort through of the carb was done. The bad running problem turned out to be cracked cylinder heads .

The float had swollen slightly making it a tight fit to get into the bowl. It's made from some kind of composite foam, very dense. I carefully eased the shape with sandpaper. In my country ethanol isn't  in gasoline/petrol so I decided to keep the float otherwise the float would have been replaced with a modern alcohol/additive resistant type. So far it's working but am keeping close watch.
Update: Float was renewed, see later post.

Jar of petrol with the float shows it floating well. The pieces looked generally in good condition.

The float lever had a crude machine screw. (later replaced)

The float needle valve had a viton tip. It was suffering with visible sign of decay perhaps from the days when the bike was still in the US. This was replaced with another viton tip item of the same type.

The parts otherwise needed cleaning. No damage.

This is the idle/slow running jet drilling. It also is used as choke enrichment. The carb body exhibits a fabulous patina of age.

The float lever pin requires a copper washer to correctly space it and prevent jamming.. Pictured upper left. (Re-made: see later post)

Carb air cleaner face has four screw holes. I use Loctite 243 blue to secure them.

It's important to ensure they don't come loose and get swallowed into the engine!!

I slotted the carb support bracket. The reason is to be able to get the bowl off without letting the carb flange bolts go. In addition, to get to the carb flange bolts, the horn needs unbolting. The thought on slotting the bracket means I can fix bowl issues more easily and at the roadside.

I connected the fuel line and supported the bowl level. The bowl filled inside 10 seconds.

The float level looked good as well.

I'd discovered my RH vented fuel cap was restricting flow of gasoline causing the bike to stop from fuel starvation. I drilled a 1.5mm (.060") hole in the underside (see pic). To my amazement it made no difference. Studying the cap I noticed the rubber gasket was preventing breathing so I ground a chamfer on the outer edge of the rubber seal using a linisher. Chamfer faces cap. Then I cut multiple notches. Update: I ended up reducing the outside diameter of the seal.

Testing flow rates from the petcock/fuel tap gave 700ml (23.7 US fl oz), tank was half full. This amount is comfortably in excess of the requirement for a Panhead however I should have tested actual fuel delivery rate to the bowl itself. (Maybe next time the bowl is off...)
For comparison flow without fuel cap on was 850ml (28.7 US fl oz.).

The exhaust is running a gray color with no visible smoke. The spark plugs are tan color. This seems to point to reasonable air/fuel mixture and oil control. I'm erring on the side of rich for idle mixture, in other words find the midpoint between rich and lean then go perhaps one click toward rich. I do this with hot engine, bike vertical. The HD shop manual also recommends setting the LS idle mixture slightly rich to help starting. Same advice for an idle speed on the higher side.

Today after a ride to a classic motorcycle racing event, and after another frustrating time starting the bike I decided to test the manifold for leaks with Cold Start ether spray-in-a-can. The manifold and carb flange gave no response to the ether. Then I directed the spray at the idle circuit of the Linkert carb. I got apparent 'feedback' on the idle circuit 'body plug' so I removed it and coated the threads with Loctite 567 Teflon (white) paste. I was careful not to over-do it as I didn't want any to enter the idle circuit.

8Feb2016 Nope, it's not any better for starting! I re-sprayed the area and there's some response but now I'm not so sure what to think. I'll dismantle the carb and pressure test the manifold and go through everything one more time. It's going very, very well on the road and idling, throttle response, power, etc is great. Engine will restart if kicked over with no throttle within 60 seconds of shutdown and resume idling. If let sit for a further 60 seconds it will restart and resume idling... I'm missing something...

Screw driver points to the concealed, taper threaded body plug. I was careful to put paste only on the thread and not allow any into the idle gallery.

10Feb16. I pulled the carb. The gaskets looked reasonable but I'll replace them. The two other plugs in the idle circuit were removed and sealed with Loctite 567 Teflon paste. Pressure tested the manifold, was fine.

Lower center is a flange type plug. Again I carefully sealed the threads . Note a small amount of white paste squeezing out around the screw head.

29Feb2016 I weighed my old Gary Bang float in at 6grams. This is twice the weight of an original. I bought a new Durable from Tom Cotten at Liberty. A very nice item, weight is 3 grams and 2mm thinner.

The float pivot had the hole drilled out by a previous owner. Am yet to repair it to suit the new float and retaining screw.

Old Gary Bang float:

4Mar2016. The float lever hole had been drilled oversize so I soldered it to up..

I filed away the excess solder and drilled the hole to the correct size..

I did another test of actual fuel level. It was 5/8". I noted the float wasn't quite at full height and I realized the needle valve can close "early" from its' own weight helped by the fact that there's .003" clearance at the fingers.

In this pic I leaned the bowl to the left as if the bike was on the sidestand. I noted the float came up to full height and was now applying light pressure to the needle valve.

Float lever pivot pin repair...

I lathed a bolt head to accept the pivot screw for drilling to size. I used Loctite 243 to secure it enough for the drilling and I put the tailstock center in place while the Loctite cured..

After drilling I cut the bolt head off..

Using a drift, I tapped the pivot screw out of the bolt head..

The two pieces separated...

I cut a piece off the shank of the drill I used for the job. This will be my float pivot. I think it was 2.5mm. Loctite blue was used to secure it.

Result is a restored pivot pin made to size..

7Mar2016 The new float, pivot repair and fitting a copper mesh air filter have made a noticeable difference to the starting so I think I've taken a good step. The last two outings starting was normally within two kicks, hot, cold or warm.

13Mar2016 I marked the LS needle position so it could be returned to its' setting at any time without having to re-tune. Handy if you want to try a click or 2, or if someone messes with your setting. I used white marker pen.

6Jan2018 The air cleaner perforated inner ring was missing because someone had replaced the filter with a foam element. I believe oiled stock wire mesh filter runs best on Pans. The ring was fabricated from perforated sheet. I riveted it together to form the ring using 2 solid rivets.
The bracket to hold the air cleaner cover was hand fabricated. The 4 bolts that secure the bracket and inner plate are Loctited using #243

1 Aug 2020

SU Eliminator carb.

SU behind stock Pan filter cover.

Fitting it up. Made the manifold, support bracket, cable bracket, throttle lever etc..

Spec:

Needle: BBT

Spring: 4.5oz

Main jet: 0.100"

Needle&seat: Viton

Dashpot: ball bearing

MPG/US gallon: 42 - 48.5

In this pic, note the main jet mixture screw is marked with red. Also the hole orientation is marked as well.. In this case it shows the screw to be approximately in the 2o'clock position. The depth the screw head is recessed is also accurately recorded.

The reason to do this is to have a baseline, point of reference. If I make an adjustment I can easily go back to baseline.

Various pics of Pan and spare SU.

Needle and seat. In this case it's a Viton tip needle

Datestamp 1982

Primer/tickler for cold starting.

Carb kit bought on local auction site. Upswept Shovel manifold. Minor problems were corrected.

Oiled copper mesh air filter. These are very high flowing filters which is important for correct carb function. I use Motul chain lube on the mesh. This goes on really thin straight out of the can and once the light component evaporates it is super sticky.

Cleaning the needle seat is easy with a cotton bud in a cordless drill and some polish. It shines up a treat.

 I carefully chamfered the seat from above to give better flow

I drilled the carb body so it was the same size as the hosetail inside diameter. About 1/4" /6.3mm?

These mods gave a 10% improvement in flow.

A new needle valve was made with a 8mm Viton tip needle from a HD CV carb and the brass outer was turned from 1/2" hexagonal stock. It has a bore of almost 3.5mm, bigger would also be possible.

Yet to test and record flow.

13Sept 2020 

Intake manifold

The intake manifold failed a bubble test at 10psi.

Cleaning the head intake nipples with brass wire brush.

SU adaptor to OEM Pan intake. It is a few degrees upswept so carb fits under 5gal tanks. Pic shows underside.

"Aircraft" style manifold clamps. The overlapping edges have been smoothed of the sharpness.

Viton O-rings. I had to tip manifold over to get clamps in place, then rotate the manifold into position and assemble the adaptor. I applied 10psi pressure then began tightening clamps. I stopped once the leaks ceased, so the o-rings don't get over-squashed and pushed through the gap. Was advised this trick by forum posts, seems to work.

Clamps, rotate them to suit ensuring the ends do not foul jug fins or head. Check all around especially rear jug clearance.

Clamps maximum 3/4" wide

SU pressure test device adaptor tool. Used with adjustable pressure regulator & gauge.

Bubble froth. I agitated a detergent/water mix, let it sit briefly then dabbed froth with a paintbrush. I find it's best to avoid picking up excess water with the brush but concentrate on froth only. The froth as pictured stays put for longer.

Tanks spacer washers are very important to avoid stress fractures.

Method: Fit tanks ensuring there's no cables being pinched anywhere. Leave the front 2 bolts completely loose. Tighten the rear bolts allowing the front to rest at whatever gap to the front bolt mounts it wants. Measure the 2 gaps accurately and exact fit washers as pictured... see the 2 washers between tank tab and frame.

Top engine mount, Thin washer under, heavy washer over. 

Add any needed spacers to the top frame mount bolt so there's no added stress.

Jetting, Spark plug signs

My thoughts on carb jetting. This pertains to gasoline/petrol as available in NZ. It has no ethanol content. A SU carb'd Panhead runs best without intake leaks. Air leakage interferes with the carb's ability to effectively lift the dashpot piston so the air speed across the jet bridge is compromised.

My plugs:

Ideal insulator is a shade of brown. 

The plug insulator must have no speckling which could indicate detonation. Check with magnifying glass. If it's tiny metallic splattering/globules it's aluminum pocked out of the piston.

Possible solution: retard timing. Lean mixture will also exasperate the issue.

Fuel ring should have a full turn. Less than indicates lean.

The first 2 ~3 thread should be discolored indicating ideal heat range. More points toward too hot.

Anti-seize on the thread will interfere with the ability to see this.

This is the theory. Mine appear to be OK. N12Y have extended nose so earth strap is longer. Interesting, studying....

 24March2021 I fitted an O2 sensor for testing.. Kit is by 14point7, imported from Nth America. Gauge used is by Innovate.

Early results and thoughts. Gauge hard to read in direct sunlight especially at speed. The digits change rapidly making it difficult to follow.

Overall readings were between 11.9 and 15.1 AFR.

Idling 12 ~ 13

Light throttle on road typical 13 ~ 14

Full throttle on road typical 13 ~ 14

Over-run, zero throttle: 20+

Testing to resume later this evening. Also with fit a sun visor to gauge.

17Oct2021 Repaired the throttle friction shoe. The housing thread had stripped. I repaired with a 5mm screw, turning the necessary groove and point in the lathe.  Screw is soldered to thimble which was made from a flat washer with notches filed into it.