engine

andrewb · Nov 27, 2011

I have a mk 3 fitted with a Mikuni.
Starts easy (one kick)..runs for 3/4 mins then cuts out.
..will start againwith no issues then die
Checked fuel and carby.. all OK.
New spark plugs
Could some one list the poss cause for me to investigate..

Andrew

Torontonian · Nov 27, 2011

Andrew it's probably something as stupid as your fuse and fuseholder. Or like your old battery. Or the wires at the ignition pickup vibrated to bits. Or the crummy old harness with original crap connectors.Did you put gas in the tank and did you turn on the tap .Think stoopid/ obvious then get into finer tracing.

andrewb · Nov 27, 2011

thought that too..
Tried the fuse holder and fuse.

have sufficent and (fresh fuel)... tap on

Unclviny · Nov 27, 2011

The test is to see if the tap allows enough flow, there could be an obstruction in the tap.

Close tap.
Remove fuel line.
Hold a catch bottle under the tap and open it.

If it is not flowing enough fuel it will starve the engine and then when the demand goes away the carbs will refill.

Vince

Matt Spencer · Nov 27, 2011

Next time it does it , see if the coils to hot to leave finger on .Boiled in oil & so on .Ancient coils & 6 V tend to be kaput .

Wots the ignition system ?

pete.v · Nov 27, 2011

It's either lack of fuel or lack of spark. Duh!

My guess would be weak battery/poor charging. The battery recoops enough energy to fire up, then poops out. It may show 12v or more but will not hold under load. Load test it.

Pull the plug off the bottom of the carb, put a measurable vessal under it and time the flow with one petcock open, then the other, then both. I doubt venting would be the issue with such a short time to stall out.

Other issues would be coils heating up or the black box.

A plug reading will tell you things. Light and dry = lack of fuel, wet and dark = lack of spark

andrewb · Nov 27, 2011

Thanks for that... appreciate this.
Forgot to mention ,I have a boyer

rvich · Nov 27, 2011

Make sure the vent is not blocked on the fuel cap.

Russ

splatt · Nov 27, 2011

See if it still starts with the lights on

andrewb · Nov 27, 2011

splatt said:
See if it still starts with the lights on

It will start with the lights on... if it didn't what does that say?

splatt · Nov 28, 2011

dying battery,changes the voltage for your ignition, does the engine note change if you switch the lights on and off

Fullauto · Nov 28, 2011

rvich said:
Make sure the vent is not blocked on the fuel cap.

Russ

That's my first choice too.

grandpaul · Nov 28, 2011

Here's a thread that could use a quick edit to it's title.

johnm · Nov 28, 2011

Forgot to mention ,I have a boyer

Thats the most important piece of information.

You very probably have a broken wire at the pick up coils in the trigger assemble. When the motor cools the wires will join and you can start. As sson as it warms up they part and it stops. Firmly pull the wires to see if they are broken inside the insulation. It is easy to miss that they are broken if you do not check very carefully.

If it is fuel starvation then the revs will often increase just before the motor cuts out as it leans off.

cash · Nov 28, 2011

Does it cut out when you knock the choke off, and will putting the choke on keep it running ?

Cash

Torontonian · Nov 28, 2011

Since we're all guessing with minimal info feedback I side with Johnm's theory/prognosis.Use the FORCE young Luke.Battery terminals could be loose too.Just leave the gascap slightly unlatched to run and eliminate that theory. Then there's the Brontosaurus Theory by John Cleese,very very small on one end,much much bigger in the middle then smaller again on the other end, so waddaya wan't for free advice ?

andrewb · Nov 28, 2011

cash said:
Does it cut out when you knock the choke off, and will putting the choke on keep it running ?

Cash

starts without choke,usually one kick at most two.... runs for about one minute then dies.

andrewb · Nov 28, 2011

Torontonian said:
Since we're all guessing with minimal info feedback I side with Johnm's theory/prognosis.Use the FORCE young Luke.Battery terminals could be loose too.Just leave the gascap slightly unlatched to run and eliminate that theory. Then there's the Brontosaurus Theory by John Cleese,very very small on one end,much much bigger in the middle then smaller again on the other end, so waddaya wan't for free advice ?

ruled out blocked gas cap vent... unlatched the cap while running... same.
checked terminals... all tight,no loose wiring

Torontonian · Nov 28, 2011

So did you pull off gas line to observe if gas pours into a cup when you turn on the tap ? Put battery on charge overnght,do not try startup with charger attached.Did you remove points cover to wiggle- inspect for breaking up wires to Boyer pickup plate ?How many miles on bike and more details. Do you have a voltmeter? How ancient is battery ? This could take a while ,got any beers ?

1up3down · Nov 28, 2011

I had the SAME problem a couple years ago.

FINALLY< i replaced ALL the boyer wiring and presto problem solved.

Upon inspection, the boyer wires going through the timing cover and in to the points cover had fractured from vibration, and you could not "see" the fracture because it was inside the outer cover of the wire.

If you are stumped, just rewire, completely, the boyer set up.

Does not take long at all, and just maybe that will be the solution.

90% of what are thought to be carburation problems are solved with electrical solutions.

engine

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