Author Topic: Mikuni BST40 starting problems  (Read 309 times)

guest1667

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Mikuni BST40 starting problems
« on: July 18, 2015, 04:00:35 PM »
Unsure if an Aprilia Moto 6.5 qualifies as a thumper - but it is a 650 single!  I bought the bike (a 1997 model) a few months ago and as yet haven`t got it to start properly. Cold, and with the choke applied, it runs (ticks over) for a few seconds and then dies: it`ll do this forever without actually firing up properly. On removal of the spark plug after the attempts, the plug appears to be dry. It doesn`t sound or feel as though it`s getting enough fuel - it`s as though the bike is trying to be started  from cold without the choke. Fuel is getting to the float chamber. All the usual things have either been replaced (HV coil,spark plug HV lead and cap) or checked; wiring, compression, ECU unit etc. The original carburetor, a Mikuni BST40, has been thoroughly cleaned and overhauled with a new float assembly, rubber diaphragm,`O`rings etc.  Convinced it was the carb, I replaced it with a new one but the problem still remains. Can anyone help please?         

JOOLZ

  • Posts: 910
Re: Mikuni BST40 starting problems
« Reply #1 on: July 18, 2015, 05:04:33 PM »
Is the filter in the fuel tank partially blocked its incorporated into the fuel tap feed pipe, take the pipe off and see if petrol runs out, has it got a vacuum activated fuel tap if so check that that works, suck on it and see if the fuel tap opens. I cant see that it could be anything else, as you have said the carb has been cleaned and even replaced

SteveC#222

  • Posts: 1900
Re: Mikuni BST40 starting problems
« Reply #2 on: July 18, 2015, 06:43:45 PM »
What Joolz said really..


If the electrics are OK and you've cleaned/changed the carb it must be a fueling problem somewhere. The only ideas I can throw into the ring are
 1/ is the filler cap vent blocked ( might cause a vacuum lock).

 2/ Could you have water contamination in the tank? water will sink below petrol and the 6.5 tank shape looks like it would gather where the tap is.  Have you completely emptied and flushed the tank?

 3/  Have you thought about rigging up a small remote fuel supply going directly into the carb to try and rule out the tank/petrol tap?


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Propellor

  • Posts: 1187
Re: Mikuni BST40 starting problems
« Reply #3 on: July 19, 2015, 07:08:32 AM »
float heights ? Can you rig up a manometer up the side of the bowl to look at the fuel level?

Blocked breather?

If it's a fuel supply flow rate problem it would have to be pretty bad for it not to sustain tick over.

Does the bike run good once you're under way? Does it tick over reliably once up to temperature? These things would give clues.



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Propellor

  • Posts: 1187
Re: Mikuni BST40 starting problems
« Reply #4 on: July 19, 2015, 07:13:28 AM »
Not read this yet, but it may help?

http://advrider.com/index.php?threads/the-bst-40-bible.347184/

Quite a few big singles seem to use this carb so there's a few leads on Google.

You running standard exhaust and airbox?
« Last Edit: July 19, 2015, 07:32:27 AM by Propellor »
BEIGE is all the rage