VVC Refusing to Rev

[fast al gordo]

Hi Guys

I went to start the bike last night, just to check if it would, it's been sitting idle for about a fortnight over Xmas. It wouldn't start, checked it was getting fuel and it was. Battery was starting to die so I left it charge overnight.

Went out tonight and the bike starts up first push of the button and will with a little bit of choke. But now it won't rev, any application of throttle causes the engine to die. The bike was running fine before the holidays.

Any ideas?

Thanks

[Squishy]

If you are sure it has enough fuel (try pri mode on the petcock for a bit)...then I'd guess clogged jet.

[fast al gordo]

It is possible there aint enough fuel, how many kms would you expect to get from a full tank of gas?

[Squishy]

It is possible there aint enough fuel, how many kms would you expect to get from a full tank of gas?

300km

[greg737]

I went to start the bike last night, just to check if it would, it's been sitting idle for about a fortnight over Xmas. It wouldn't start, checked it was getting fuel and it was. Battery was starting to die so I left it charge overnight.

Went out tonight and the bike starts up first push of the button

My thought on this is you have/had two separate and unrelated problems.

Problem #1 was low battery voltage, Problem #2 is probably a partially clogged carb passageway in one or more of your 4 Mikunis.

I wanted to explain a little about Problem #1, the low battery voltage problem.  The B4 uses inductive coils (as do most other motorcycles and cars). 

Inductive coils take time to charge up before they can properly fire a spark plug.  This period of time is called "Dwell time" and for our B4 coils that required time is about 2.8 milliseconds if, and only if, the bike's electrical system is running at normal voltage (about 13.5 volts).

For our Bandit, 13.5 volts flowing through the coil primary winding for about 2.8 milliseconds equals enough energy for a good, strong spark plug discharge right at the instant that the CDI removes/interrupts the path-to-ground (causing the induced electrical field in the coil primary winding to collapse, inducing an exponentially greater field collapse in the coil secondary winding, which grounds out through the HT lead to the spark plug and then across the spark plug gap into the metal of the engine's cylinder head).

The problems start when the electrical system is running at less than normal voltage, say, cranking at 8 or 9 volts (I really don't know how low a battery will go during cranking).  Anything below the normal operating voltage will require more time to properly charge the coil.

Today's modern ECU controlled engines automatically correct the Dwell time for low battery voltage but the B4 isn't that advanced.  Instead, with its very basic (dumb) CDI the B4 continues to use the same Dwell time regardless of the battery voltage.  Basically, Suzuki is depending on you to keep a good battery in the bike.

... if you want a good strong spark.

[Squishy]

I went to start the bike last night, just to check if it would, it's been sitting idle for about a fortnight over Xmas. It wouldn't start, checked it was getting fuel and it was. Battery was starting to die so I left it charge overnight.

Went out tonight and the bike starts up first push of the button

snip

Hm can you elaborate on why it would be a problem?
I had a pretty much dead battery in my 400 when on a mototrip (10-11v rest, pretty much 5v when trying to start) but I could easily push-start it.
If your battery is way below 9v during start you would definite notice that when cranking.

[greg737]

Hm can you elaborate on why it would be a problem?

I thought I had elaborated, maybe even too much (don't want to bore people to death).

I had a pretty much dead battery in my 400 when on a mototrip (10-11v rest, pretty much 5v when trying to start) but I could easily push-start it.

You've sort of explained it yourself here... 

You had a near-dead battery that couldn't muster up the cranking amps necessary to carry out a normal start sequence on the bike so you eased the load by removing the starter motor's normal electrical draw (by push starting).  Relieved of the responsibility to turn the engine over with the starter motor the near-dead battery had the necessary voltage to power up the bike's CDI (and the CDI had enough power to charge and fire the coils).

And that scenario doesn't even take into account the fact that a push start also spins the bike's alternator.  Who knows what the RPM threshold is for the alternator to begin pumping power through the Rectifier/Regulator and into the bike's wiring harness?  I don't, but that could be part of the equation.

[Squishy]

Ok sure but my point is if the battery is so bad that it's not giving your a proper spark then it's already dead enough to notice while cranking.

[greg737]

I wasn't saying that reduced battery voltage will absolutely prevent an engine start. 

Truth: Anything less than a fully excited primary coil field will result in less spark than normal.

I was just pointing out that low battery voltage will to some degree reduce your chances of success (and that reduction is spread across a range from not that bad to substantially impaired). 

I was pointing out that in the case of a low battery the route to failure to properly start is not a simple, single vector issue: sluggish cranking speed , but is also a function of reduced/insufficient spark due to lack of complete coil charging.

And, as I was trying to point out in my last post, a battery that can't deliver a good spark while simultaneously turning the starter motor might be perfectly able to deliver a good spark when it doesn't have to turn the starter motor.

[greg737]

Ok sure but my point is if the battery is so bad that it's not giving your a proper spark then it's already dead enough to notice while cranking.

Yes, I wasn't trying to completely diagnose and solve the original poster's problem.  As I said in my first post I believe he has/had two separate problems: the low battery and a carb problem.

It could be that in this case, with some sort of carb problem causing improper delivery of fuel+air, the slightly reduced spark resulting from low battery voltage was enough of an additive factor to cause failure.

Then, after he charged the battery, the bike had the optimal engine cranking speed and coil performance necessary to overcome the fuel+air problem.

[fast al gordo]

Update:

Bike will still start on BOTH Prime and ON positions of the fuel tap.

I did notice that the fuel flows more freely when the tap is in the Prime position, fuel flow in the On position is no less than what I observed pre Xmas lay up.

I'm looking at a blockage in the carbs I can hear a fair amount of shit swhishing around in the tank and I don't have a fuel line filter.

Any recommendations as to what kind of fuel filter I can put on the bike?

Thanks

[Squishy]

Any cheap paper filter will do, but it needs to be vertically or fuel will not pass fast enough, especially on low tank.
From what I remember it's pretty hard to find a good spot.. maybe if you shorten the fuel hose and put it directly above the carb. Not sure.

Edit: the petcock has a small filter in it already.. that should be good enough unless your tank is seriously filthy.

[fast al gordo]

Edit: the petcock has a small filter in it already.. that should be good enough unless your tank is seriously filthy.

Petcock was replaced by the previous owner, unsure if current one has built in filter.

[ventYl]

I don't know where exactly do you live. but... Isn't there any chance that place where your bike sits got below freezing point and some leftover water in floaw bowls frozen? Here it was pretty cold during Xmas and end of year...

[fast al gordo]

I don't know where exactly do you live. but... Isn't there any chance that place where your bike sits got below freezing point and some leftover water in floaw bowls frozen? Here it was pretty cold during Xmas and end of year...

I'm in UK, the Midlands area. We've had a pretty mild winter this year tbf.

I'm fairly confident the issue lies with all the shit that's swishing around the tank.