Update on my insanity and other carb related issues ;0)

[T250tut]

OK settle in grab a brew here we go
1977 650d
Test meter on battery was showing 14v at idle and 19v above 3000rpm - fitted new combined unit and getting 13.7v at all rpm's
so far so good

Carbs cleaned, carbs clean again, carbs really cleaned with guitar sting in every hole and carb cleaner and compressed air
New diaphragms -fitted by a very reputable carb specialist - finger over hole drop test satisfactory
new genuine Yamaha butterfly shaft seals
new floats
new pilot and main jets
floats set with Vernier gauge at 25mm and contents of each drained and measured to confirm both the same
Butterfly's bench set with feeler blades and confirmed with vac gauges at 3000rpm - see below
carbs fitted to new manifold rubbers and gaskets
new coils and condenser
Advance unit pulled lube and tested - looks OK to me
Timing spot on to the marks with a strobe and and advances from 2000rpm onwards
Standard airbox with new filters
fuel screws 1 1/2 turns out
So what's it doing?????????? apart from my head and my sanity :0)

Needs choke to start from cold - starts on the button (count to 3 or 4 as it cranks) and revs to 3000rpm
drop choke and it holds 3000rpm - add choke when its hot and it adds revs even higher briefly then cuts out
Once warm blip the throttle aggressively a couple of times and it drops to approx 1500rpm starts back firing and missing and after a minute or so cuts out
If you work the throttle it will keep going but you have to keep it above 2000rpm
Out on the road it holds 3000rpm during gear changes but above that responds well and daft as it sounds rolls along very pleasantly
Tick over screw backed out till there is a gap between the end of the screw and the cam it acts on - no difference
Sprayed ether easi start all round and no air leaks obvious - fresh E5 fuel
While it may be ticking over at 3000rpm if you try to pull away with out adding throttle it dies
Whilst riding it, it has below 4000rpm a very faint misfire above that sounds great pulls well and is a pleasure to bend swing on - go figure

Ideas or suggestion please

 

[5twins]

Did you center the butterfly plates when you installed them after the butterfly shaft seal replacement? When you install the return springs on the shafts, they pull the shaft towards them. If the butterfly plate was already tightened down, it can get pulled into the side of the bore and bind (hang up). To avoid this, you need to "center" the plates. With the mounting screws still loose (just snug at most) and the return springs and other bits installed, open the butterfly plates fully and let them snap shut. Do this several times then tighten the butterfly plate mounting screws.

 

[T250tut]

To be honest no but they do seem to open and close ok but it's worth checking cheers

 

[5twins]

They'll only bind when almost closed, and that could be the cause of your "hanging" engine speeds.

 

[gggGary]

Do a vacuum sync is my first call.

 

[gggGary]

Valves checked, set, cam chain set, compression is good both sides?
What exhaust? Genuine Mikuni jets or aftermarket?
Timing advances smoothly, also retards smoothly and consistently?
with idle screws backed off if the throttle snaps shut you're prolly good on the butterflies.
Did you open, clean, check, rubber seals on the choke plungers?
Choke bodies snug, good gaskets?
Never hurts to do a spray of carb cleaner around front of carbs while running, leaks, even in new assemblies, happen.

 

[Highpower]

I have noticed that if carbs are not synchronized correctly, the idle will be erratic. Either that, or you have an air leak.

 

[Its been a long time]

I don't have tons of experience with different XS650s but my 77 650D starts so quickly on the electric start, it's running before I can take my thumb off the button.

I gave up with points and BS38 carbs after nearly a year of trying. I now have Boyer Bransden ignition and Kawasaki ER5 carburettors.

 

[T250tut]

Update - despite its issues put about 20 miles on it yesterday. Pulled the plugs this morning and while obviously an imbalance i have set both carbs up as precisely matched as i can left plug is fouled n right plug looks OK would a fault in the choke system explain all these faults??

 

[Highpower]

I vote for air leak

 

[bazz]

Check for air leak where the inlet rubbers bolt to head
Spraying easy start won't show the problem
At least it doesn't on my bike

 

[AUTOMAN]

You must center and orient the throttle plates. Make sure they are tapered the correct way.
Carb sync is a must. Do it after a few blips on the throttle.

cliff

 

[Tiggertoo]

Do the diaphragm drop test again (choke OFF), if either choke is leaking then the slide will drop faster. With the choke ON both slides will drop fast. The drop should take around 8-10 seconds, if its faster then there is a leak. Balance the carbs using a manometer or good quality gauges. The cheap gauges IME are often next to useless on carbs as they bounce around too much.

 

[Bruno_c]

With a twin cylinders engine, it is always useful to know which cylinder is misfiring or both.

 

[T250tut]

OK so when checking the butterflies they are installed with the number on the engine side of the carb and below the pivot shaft. With screws loose and shaft worked a couple of times under its own spring pressure - they look as central as they can be i can still get just a .004 or .104mm feeler blade under the bottom??? Should they seal??

 

[Its been a long time]

OK so when checking the butterflies they are installed with the number on the engine side of the carb and below the pivot shaft. With screws loose and shaft worked a couple of times under its own spring pressure - they look as central as they can be i can still get just a .004 or .104mm feeler blade under the bottom??? Should they seal??

Back off the tickover adjustment screw completely, the plate should close fully. Then use a feeler gauge to bench balance the two plates so that they're the same gap. I find a 0.004" feeler gauge gives me typically a pretty good initial setting for the tickover screw.

 

[Its been a long time]

OK so when checking the butterflies they are installed with the number on the engine side of the carb and below the pivot shaft. With screws loose and shaft worked a couple of times under its own spring pressure - they look as central as they can be i can still get just a .004 or .104mm feeler blade under the bottom??? Should they seal??

I gradually tighten the plate screws a bit at a time, flicking the plates open and shut using the return spring. Then finally tighten them when I'm sure they are centralised.

Back off the tickover adjustment screw completely, the plate should close fully. Then use a feeler gauge to bench balance the two so that they're the same gap. I find a 0.004" feeler gauge gives me typically a pretty good initial setting for the tickover screw.

 

[gggGary]

I point the intake side at a light source and look through the bore, you shouldn't be able to see light around the butterfly. Adjust the sync screw so BOTH fully close then add in idle speed with the idle screw..

 

[T250tut]

Back off the tickover adjustment screw completely, the plate should close fully. Then use a feeler gauge to bench balance the two plates so that they're the same gap. I find a 0.004" feeler gauge gives me typically a pretty good initial setting for the tickover screw.

Tickover screw is fully backed off and right carb butterfly closes but left can fit a 004 feeleblade under it would the balance / synchronization screw cause this??

 

[Its been a long time]

Tickover screw is fully backed off and right carb butterfly closes but left can fit a 004 feeleblade under it would the balance / synchronization screw cause this??

Yes it could do, but without it in front of me, it's hard to say.