Good news - bad news

The good news for me is that the SO LS650 that I rebuilt (as a semi-rat
runabout) from a non-running parts bike has just passed its MOT. The bad
news is that I can't get the bastard running properly. It starts OK, ticks
over nicely and pulls well - up to about half throttle. Any more throttle
and it just hiccups then dies - until the throttle is reduced, when it picks
up and runs OK again. It has a new coil, HT lead and new plug so it is
unlikely to be an HT electrical problem.

Right - so it looks like a carb or fuel supply problem. The carb is a mikuni
BS40 CV type. So I swapped the carb for a spare one from the parts box -
didn't seem to make much/any diference. I have tried swapping out the main
jet for a larger one - didn't seem to make much/any diference. I have tried
it with the normal size jet but with air filter removed - didn't seem to
make much/any diference. So now I am struggling a bit. I have checked the
fuel flow - the tap has a "prime" position that bypasses the normal
vacuum-operated route and turning to this position didn't make any
difference (but fuel flow is fine in this position). Basically I have
swapped carbs, tried making it richer, tried making it leaner and the
problem seems to persist. Of course sods law allows for the possibility
that both carbs have the same problem but I have checked for blocked jets
etc and can't see an obvious culprit.

The only other thing that I can think of is that the CDI/igniter unit is
flaky but that doesn't seem too likely. Anyone got any helpful suggestions?
It is getting personal now - I know its a heap of crap bit I WILL make the
fucker run.

 

Have you checked for air getting in behind the carb somewhere?
When you checked the fuel flow, is this sufficient for a full open throttle?
I had a Trophy 1200 that really didn't want to play on anything above half
throttle. Fuel flow _seemed_ ok through the tap, but I put a new one in
(over 50 bloody quid!!) and that sorted it.

--
Greybeard

FLHR -03 UK (95 cu-in Stg 2. Big Boy 2!)

Garmin Zumo 550, To get me home!

ukrm@foxtails[dot]co[dot]uk

 

Check carefully for an air leak at the inlet; the BS40 is set really lean so
an air leak won't help at all.

 

Diaphragms are pefect

I don't think that badly adjusted tappets would give this kind of problem

Yebbut different size main jets don't seem to make a difference, nor does
removing air filer altogether.

I have checked and they are clear. Anyway, if the pilot jets were at fault
I would expect difficult starting and poor idling and slow running - none of
these is a problem.

 

Hmm...the fuel tap might be flaky. Worth a try - ebay here I come!

 

BTDT on the 400/4.
Crap behind the floatvalve seats restricting fuel flow into the float bowls.
Remove and blow through with compressed air.

 

Carefully clean all low tension connectors in the ignition circuitry,
including the pickups. Even if it's not that, you really need to be
utterly sure the LT side is properly sorted.
I've had dirty LT give me exactly that symptom.

 

Thanks for this suggestion - I will check out the wiring and connections.

 

Right, I have,
Checked the LT side of the igniton - seems OK (voltages and resistances)
Big fat spark at plug
Dismantled and checked the fuel tap - clean as a whistle and flow is very
good
Swapped back to first carb after dismantling, thoroughly inspecting and
cleaning it - all airways, jets and needle valve are clean and clear
Checked for leaks around carb - all seems good

SO WHY DOES THE FUCKING THING DIE AT ANYTHING OVER HALF THROTTLE?????

It seem to be lean rather than rich at around haf throttle - opening the
choke slightly improves running a bit but full choke kills it.

I'm baffled and bewildered.