Linkert dc question

[DonIII]

I have a 66 shovel with a linkert dc 7 carb. Good running carb for not having a squirter. My issue is this, upon start up it spits a lot thru the carb...that’s ok.....after warm up it eventually goes away...that’s good...then after some time riding, it occasionally spits. Is this normal for a dc? Reminds me of the bendix I had years ago that would spit at 2500 rpm consistently. Or am I asking to much from a 56 year old carburetor?

I have already replaced the intake o-rings, went to aircraft style clamps. I do have the required spacers between the intake and the carb. First time around I believe I had an intake air leak. I feel that issue is resolved. So now I’m trying to dial it in. She pulls good almost to good. Just picked this bike up last fall. Went thru a few things over the winter. I don’t know if it’s stock bore, or if there’s a hotter cam.(feels like it may be, been long time since my other shovel was stock).

I don’t know how many members are running dc linkerts. I’m sure I’ll figure it out eventually, but if someone has gone through this before please let me know. I wasted a lot of time trying to stop the bendix from spitting. I’m older now and don’t like to waste time. Thanks. Don

[T. Cotten]

DC's are awesome, Don!

The reason they have no 'squirter' is because the main nozzle discharges a metered gob o' gas upon acceleration. I call it the "accelerator sump".

I'm not sure what "aircraft" clamps are these days, but if they only tighten with a #10 screw, they should do.

New O-rings are "manditory", but true vitons will out-last common ones dramatically.

You can't wish away a leak; Did you bubble-test while tightening the clamps, so you didn't overtighten?
Http://virtualindian.org/11techleaktest.html

You might chase your tail otherwise, Good luck!

....Cotten

[chuckthebeatertruck]

I adore DCs and interact with the one on my 59 nearly daily. They are the least fussy factory carb of their era and remarkably tuneable to many motor sizes. Not really much to wear out and when you do, they can be rebuilt to go another 50 years.

Anyways, what you describe is not uncommon. Apart from intake leaks, the most common cause of the lean pops is a mix of overlapping things. In essence, the D.C. Is quite sensitive to float and fuel level. A bit too much or too little and it lets you know it.

What you are describing is likely a combination of three things

1) idle speed set too low. Make sure your tick over is above 900/1000.
2) float too low, raise it by 1mm/1/64 over spec
3) needle adjustment - specifically high speed needle.

It sounds EXACTLY like you are lean popping during the first part of the transition and it's oft cured with a 1/4 to 1/2 turn on the high speed.

Happy motoring.
To put this another way, when we were in the process of cutting a new throttle disc for my 78 inch sportster, I ran the bike on a dc- that's how flexible they truly are. I then bolted it right back onto a stock 55 inch bike and three kicks later was in the wind.

Third tip, make sure your throttle disc is oriented correctly. Many are assembeled upside down. Bike runs, but tends to tip lean when you crack it off idle.

[DonIII]

Cotten, thanks for the reply
The aircraft style clamps I’m referring to are the one piece straps with a pivoting threaded rod and a locking nut. Not exactly stock looking. I do have the vitron o-rings installed. I did do a bubble test originally over the winter while I was installing the carb.....but I think I had loosened the clamps to install the carb support bracket. I’ll check it again when I get a chance. I did use your “tutorial” about the bubble test when I was rebuilding my 57 seri-car...thank you for that,
thanks Don

[T. Cotten]

If the 'pivoting threaded rod', Don,..

Is a quarter-inch instead of #10, beware they were intended for the exhaust.

It doesn't take that much to squeeze an O-ring.

Beware most of all, that many of the commercial stainless clamps are metric width, and must be shaved down to 3/4" for later "Y" manifolds.

....Cotten

[DonIII]

Chuck, thanks for the reply,
I’ve been playing with the idle mixture screw, after well warned up, some in and out till the highest rpm.
Also been backing out the main adjustment quarter at a time and doesn’t seem to be doing much. I only have a few hundred miles since I installed the dc, and started from scratch a couple of times. Been riding side roads mostly...35 to 50 mph, some hills, “relatively” easy on the throttle. All in all carb is good. Just by coincidence turned the gas on the other day and started pissing out of the carb. Took it apart and cleaned up the float pivot rod assembly because it was a little stiff. When I replaced the float assembly I didn’t recheck the float setting. My bad, something to do next rainy day we have. I will double check the throttle plate position, but I was aware of that, and may turn the idle speed up a bit. Thanks for the input, Don

[DonIII]

Cotten, I believe these clamps came off an S&S setup. I will check the widths...but question...why would I not want to squeeze the o-rings some, as long as the intake port and the manifold lined up correctly

[T. Cotten]

Cotten, I believe these clamps came off an S&S setup. I will check the widths...but question...why would I not want to squeeze the o-rings some, as long as the intake port and the manifold lined up correctly

Naturally, Don,...

We want to squeeze the O-rings "some". But the clamps don't hold the carb on, the bracket does.

Over-squeezing is a common issue with non-OEM design clamps, but bubbles steer you clear. Heavy duty exhaust clamps often pucker out of round.

The width issue is typical aftermarket evil, but can easily be remedied.

....Cotten

[chuckthebeatertruck]

To be clear, fiddling with the idle mixture screw is not what I'm suggesting. Instead, you may need to adjust the high speed and test it at speed. If there is zero response to a high speed needle change it indicates leaks (throttle body or manifold) or the float level remains off/poor fuel flow making it run lean. It's become pretty common to find D.C. with sticky float pivots from ethanol swell. You may need to correct it several times before it always pivots correctly and keeps your fuel head at the right level.

D.C. carbs tell you what they need. Pay lots of attention to the response with each change and it will point you home in no time flat - then stay there for years on end.

[T. Cotten]

...Third tip, make sure your throttle disc is oriented correctly. Many are assembeled upside down. Bike runs, but tends to tip lean when you crack it off idle...

I would "adore" them too, Chuck!

But they never really needed me.

About the throttlediscs, can you tell me the differences between a 9/A and a common 9/X disc?

They are too subtle for me to guess, thanks!

...Cotten
PS: Other than the stamp is on the opposite side... Why?

[T. Cotten]

Many are assembeled upside down. Bike runs, but tends to tip lean when you crack it off idle.

Sorry for a second question just like the first, Chuck!

But since DCs have greatly eliminated the variable of a wear groove at the idle bleeds, your observation is a priceless clue about the disc bevel; Can you describe "tip lean"?

Thanks!

...Cotten

[DonIII]

Chuck, I carefully adjusted the float yesterday to 1 1/64”. Readjusted the mixture screws to starting point. Upon start very little carb spit while warming up. Still have the occasional spit at speed. Not under hard throttle at all. Still trying to pin point a consistent spot or action. So back to basics ,valve lash, timing, and check for intake leaks, and go from there . Thanks to both you and Cotten for your input, Don

[chuckthebeatertruck]

Cotten:

I haven't seen all three discs together -- ever. Generally only one at a time -- and generally with throttle body wear that has been "solved" with some poor hatchet work. For some reason -- no one seems to understand the throttle disc position is what governs the low speed -- not excess air from the shaft.

Anyways -- to answer your question - when we were trying to figure out exactly how S&S developed the early GB/GA carbs -- the first two things you notice is that there are fewer air bleed holes AND the smaller bore works really, really well on motors from 55 to 75 cubic inches without any modification -- but it fuels better on acceleration.

To get an idea of how much vacuum signal was being seen -- I tried the disc in four orientations -- notch up (facing motor or facing air cleaner) and notch down (facing motor or facing air clearner). To keep things "consistent" nothing was reset between tests other than the position of the disc. We even kept the idle speed screw at the same setting. Here's what we learned:

1) With the notch facing down the idle was more stable and the highest speed of all the positions. It was the same regardless of which way the notch faced. BUT the motor did not respond to retarding or advancing the timing as readily. The lean "tip in" happened as the throttle was cracked off idle -- if the air cleaner was off you could see the fuel stand off and it huff lean for just a quick moment. It still huffed inside on that initial tip in but was "cured" by using a metal mesh or a K&N instead of a paper element. Then, it barely became noticeable. This position also had the lowest response to the idle mixture screw. However, this position was also the easiest to prime the carb. Generally two kicks at 1/8 to 1/4 throttle and full choke saw it almost too rich.

2) With the notch up and facing the motor the lean tip in problem was gone -- idle speed dropped and the motor was more responsive to timing adjustments. BUT, it was "slow" until midthrottle.

3) With the notch up and facing the air clearner -- the lean tip in problem was gone, idle speed was "normal" and the motor responded as normal to timing adjustments. However, this position required the most prime kicks and the most throttle during priming. About 1/2 throttle was the only way to consistently pull fuel through the circuit.


Simple carbs my (fill in the blank). Just minor changes in that edge position in relation to the bleed holes -- plus the shape/position of that notch really do make a big difference. Much, much more so than worn out shafts (unless of course the shaft is changing the throttle disc position that much!)

All of this lead us to understand the L series carbs eliminates one of the transition holes in order to fatten the mid-range immediately. This makes a lot of sense for the people who were buying those carbs -- they wanted to GO -- and Go NOW. That carb takes what the DC does well and multiplies it through size. We could only summize as the carb got bigger the need for as many bleed holes disappeared.

Coincidentally, the big L series carbs respond very similar to the notch up/facing air cleaner -- need about 1/2 throttle and full choke to pull the fuel. Not a coincindence at all they share this characteristic when you think it all through -- and why a mandrel had to be made to cut down GAL (1-7./8) discs to fit now obsolete GBLs (1-3/4 throat).

One day - I fear no one will be curious enough to try and understand how/why these carbs work and why they are so flexible.

[T. Cotten]

I compared all three 9° discs, Chuck,...

And could discern no differences.
They all have the chamfers, or facets (notch?) on both sides at the bleeds, opposite the stamp, as shown in my previous attachment.

There's only one way a disc can go in with the facets at the bleeds and the bevel parallel to the bore.
It could be installed 'upside down', which means the bevel no longer closes, but the facets remain symmetrical.
Installing it 'backwards' puts the sans-facet bevel at the bleeds, either parallel to the bore (as all earlier models were), or where it really wouldn't close.

I have never encountered any significant borewear in a cast-iron bore: One of the things that make DCs awesome!
S&S models are not familiar to me at all.

So basically, you really have me confused. Apparently installing it correctly works best; The mystery is why the facets are suddenly critical, when they weren't produced until somewhere around 1950.

....Cotten
PS: Once upon a time,
I received an M-5 for service, which not only had the disc 'upside down/backwards', but soldered into a shaved shaft.
The owner had been running it on a Chief, and said it always ran fine. S&Ss don't sound as forgiving.
PPS: "Notch" reminded me of the 'wildcat' in the attachment. Please note that the carving is entirely upon the inlet side, funnelling air at the bleeds. Perhaps ahead of its time!

[chuckthebeatertruck]

Cotten:

I haven't seen an obvious notch on both sides that was as apparent as your photos. This might be (probably is) me.

The few discs I have handled only had an obvious notch on one side -- and all above the stamp (9A). To make certain we're talking the same language, I'm talking about part # 27280-57 for sportsters. I've only handled one DC for the FL version and only one DC2 with the #12 for servicars. I have seen a 9X -- but don't recall two bevels/notches. So, my sample pool is quite small and not fully representative. I'm also only passingly familiar with M carbs -- enough to be dangerous but not enough to draw any conclusions about the bevel/notch and why/how it affects the DC.

The DC that is on the bike I did tests on was a 9A disc that came to me -- not sure if it was repo or oem. It only had a bevel on one side and allowed for all four positions. Even though as you point out two of the positions are the same -- it was one of those where I was personally curious if the notch being on the bottom made any difference. As we both have likely discovered -- sometimes logic doesn't play out in the real world.

That's part of why these carbs keep fascinating me. They shouldn't work as well as they do over such a wide range of motor sizes, compression ratios, etc.

I could be remembering incorrectly and you've certainly seen more of these than I have. It's also possible I was working with incorrect or later replacement/service parts. I have not taken apart a carb off a totally original bike and can't guarantee I've ever seen one :-)

So, many apologies if I've caused more confusion. My sample pool is small.

PS - The early S&S are for all intents and purpose nothing more than scaled up DC's and as the carbs got bigger -- three series were released about two years apart that progressively got less and less fussy. They are dead simple carbs that combine the best elements of the DC with even better flexibility. Unfortunatly, most people ignored S&S's own instructions and stuck the biggest carb they could on a motor . . .and in reality there are two series of carbs for a reason.