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Another Monobloc Tuning Question, lean #773863
05/16/19 1:38 am
05/16/19 1:38 am
Joined: Dec 2007
Posts: 113
PA
Oil Changer Offline OP
BritBike Forum member
Oil Changer  Offline OP
BritBike Forum member

Joined: Dec 2007
Posts: 113
PA
After being distracted on another bike restoration, I am back to finally tuning this 55 6T to ride. When it came to me as a basket case it had a “pre Monobloc” 276. I decided to treat it to a 55 6T spec Monobloc from Burlen. A 376/42 is a 1 1/16, 270 main jet, 106 needle at position 4. It is my understanding that a Monobloc wants 2.5 turns out at the idle screw vs. 1.5 turns out of a Concentric. Correct me there, if incorrect. Ramp cam engine, valve adjustment at .010. Complete fuel system is new, though anything is suspect.
Here is the issue. Bike starts first kick no matter what with partial choke. Idles easily. Runs down the road apparently well. Once warmed up, bump the choke off at a stop, idles. Go to leave the stop and it will not take the throttle, flick the choke lever a small bit, bike picks up and runs through the gears and gets me home. Doesn’t seem to smoke, the new head pipes aren’t blue yet.
I started at 2.5 turns out on the idle mixture, along the way made several stops to turn it in by increments to 1 turn out, with no improvement.
Changed the main jet from 270 to 290. No improvement.
Raised the needle 1 notch. No improvement.
I have checked for vacuum leaks, can’t find any, and I don’t twist everything to 100 ft lbs.
The engine is not modified.
L82C plugs gapped at .025, sooty by the time I get home on choke.
Below is an paragraph from an August 55 TSB.
[Linked Image]
This is the time when I typically go wildly back and forth between ignition and fuel, make wild, multiple changes per tuning session. Before I do that, I am wondering if a more experienced, wiser person would have a suggestion or point me in a direction before I start changing slides for a different cut-a-way, going bigger on the main jet, or hiking the needle up higher.
[Linked Image]

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Re: Another Monobloc Tuning Question, lean [Re: Oil Changer] #773880
05/16/19 3:48 am
05/16/19 3:48 am
Joined: Mar 2012
Posts: 126
Ontario Canada
B
Bob Buchanan Offline
BritBike Forum member
Bob Buchanan  Offline
BritBike Forum member
B

Joined: Mar 2012
Posts: 126
Ontario Canada
Even on new Monoblocs, I would take idle jet out to make sure top hole is clear to see thru and always turned in tight. Start with needle 1 1/2 open and if doesn't make improvements when turning, its still plugged. Use .015 spark plug guage to check the TWO tiny jets are clear in bore, then use .025 to check further. Also you will find idle circuit, slide cutaway, needle and needle jet ,main jet all overlap .

Re: Another Monobloc Tuning Question, lean [Re: Oil Changer] #773885
05/16/19 5:08 am
05/16/19 5:08 am
Joined: Jun 2007
Posts: 6,396
Maui Hawaii
HawaiianTiger Offline
BritBike Forum member
HawaiianTiger  Offline
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Joined: Jun 2007
Posts: 6,396
Maui Hawaii
Nice bike! Mine is going to look very similar to yours in a month or two.
Anyway, I have a pretty good feel for Monoblocs having had them on dozens of bikes. My favorite bikes are pre units and there ain't a Concentric to be seen anywhere. BTW I actually prefer the 276 as it is a better quality instrument overall.
One thing that will bite you in the butt is the idle jet. If it isn't seated properly you will soon be bald.
But they are fragile and brittle brass and tightening them is a delicate operation without damaging them.
If in doubt, put some mechanic's blue on the end taper and see if it makes contact all the way around.
Second is air leaks. Usually on new carbs, it's not the worn out slide. Just kidding...but anyway, sealing the o ring is important. WD 40 comes in handy for testing for air leaks.
Have a peek at how far in the throttle stop screw is. It shouldn't be opening the slide very much. Just a little and the idle circuit will do the rest. If you have it open a lot, that points to problems in the idle circuit, so then you can concentrate on finding the culprit there.
Cheers,
Bill
PS The SU was spec'd for home market and is a very fine instrument. On a stock T-bird, I would be very tempted to use one of those.
B.
PSS Stalling from just off idle is a classic symptom of too retarded ignition. As is easy starting.
B.

Last edited by HawaiianTiger; 05/16/19 5:54 am.

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Re: Another Monobloc Tuning Question, lean [Re: Oil Changer] #773912
05/16/19 2:07 pm
05/16/19 2:07 pm
Joined: Dec 2007
Posts: 113
PA
Oil Changer Offline OP
BritBike Forum member
Oil Changer  Offline OP
BritBike Forum member

Joined: Dec 2007
Posts: 113
PA
My 1st pre unit with choke lever on the frame, I am a dumb ass riding around with the choke on, then trying to reinvent Monobloc theory. I will put the dunce cap back on.


Moderated by  John Healy 

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