Stumped - Only wants to start when plug is removed

[Guest]

This is on a 1977 gs750 stock (New to me, unknown history)

This is how it wants to start.

On the first press of the button it acts like it wants to start and then just spins, and doesn't want to start.

Here's the kicker. If I pull a plug (any one doesn't matter) It wants to start right away and will run. Put the plug back in and try to start again, all it does is spin, doesn't want to start. (remove plug again and it wants to start, you get the idea)

Now, If I using starting fluid to get it running, once it starts I can turn it off and restart anytime, until it sits overnight, then the same issue happens. Only wants to start if I pull a plug or use starter fluid.

Anyone have a similar experience like this and a fix. I would appreciate any input, Some other people and I are dumbfounded by it only wanting to start with a plug removed.

Thanks

[barnbiketom]

battery voltage to the coils is low.

when you unplug one wire, it actually increases the coil output voltage since there is a huge gap to jump, so the coil voltage will increase respectively in order to jump it.

Check the Positive + side of the coils to make sure it is within 1/2 volt or so of battery voltage....

incidentally, back in the days of 2 stroke dirtbiking, if a plug was fouled, we would take off the plug wire ever so slightly and the increased double gap would sometimes make the things start!!

[Larry D]

Also...adjust the valves.

[halz426]

I am getting full battery power to the coils, I just performed the coil relay mod, I checked the voltage and I was happy with the result. (originally it was 1.3 volts low)

Its when I pull a plug out of the cylinder, (not disconnected from coil/spark plug) is when it wants to start. I discovered this when checking for spark.

battery voltage to the coils is low.

when you unplug one wire, it actually increases the coil output voltage since there is a huge gap to jump, so the coil voltage will increase respectively in order to jump it.

Check the Positive + side of the coils to make sure it is within 1/2 volt or so of battery voltage....

incidentally, back in the days of 2 stroke dirtbiking, if a plug was fouled, we would take off the plug wire ever so slightly and the increased double gap would sometimes make the things start!!

That is not a bad idea, Ill have to look into that. I have never done it on a bike before

Also...adjust the valves.

[BigD_83]

When you say it will spin, do you mean the starter spins, or the motor continuously turns over when you depress the start button?

[halz426]

When I hit the start button for the first time it sounds like it wants to start, then the motor just turns over.

The starter button operates fine.

[AJ]

When I hit the start button the motor just turns over.

The starter button operates fine.

I think BigD was asking if the engine is actually turning over, or is the starter motor whirring around but not actually turning the crank? Either or both of these conditions make sense with your "works when the spark plug is pulled" mystery, but the repair will be different.

1) If the battery is low, removing one plug will decrease the torque needed to spin the engine because you're removing compression from one cylinder (any cylinder), letting the engine get up enough spin speed to start. DC motor torque is directly related to electrical current through the motor, therefore, low battery will produce less torque. Fix for this is a new battery.

2) The same applies if the starter clutch is slipping or the rotor is working loose from the crankshaft. Removing a spark plug means less compression to overcome, so the starter can engage without slipping. Fix for this is to remove the rotor, clean up the crankshaft end, apply loctite and bolt back together. Or remove the rotor and clean up the bearing/pawls that mechanically engage the starter.

Good luck!

[BassCliff]

Hi,

If you don't know the history of the bike then I think it's best if you get caught up on ALL of the maintenance. You'll find the maintenance lists in your "mega-welcome". Or just visit my little website and click on the READ ME page.


Thank you for your indulgence,

BassCliff

[BigD_83]

I think BigD was asking if the engine is actually turning over, or is the starter motor whirring around but not actually turning the crank? Either or both of these conditions make sense with your "works when the spark plug is pulled" mystery, but the repair will be different.

1) If the battery is low, removing one plug will decrease the torque needed to spin the engine because you're removing compression from one cylinder (any cylinder), letting the engine get up enough spin speed to start. DC motor torque is directly related to electrical current through the motor, therefore, low battery will produce less torque. Fix for this is a new battery.

2) The same applies if the starter clutch is slipping or the rotor is working loose from the crankshaft. Removing a spark plug means less compression to overcome, so the starter can engage without slipping. Fix for this is to remove the rotor, clean up the crankshaft end, apply loctite and bolt back together. Or remove the rotor and clean up the bearing/pawls that mechanically engage the starter.

Good luck!

Thank you, AJ. That's where I was going with that question.

[halz426]

Thanks for the reply's, ( I will look into the starter clutch) It has a new battery and good voltage, starter engages and spins strongly.

I can hear the compression through the exhaust. ( Sounds good)

I can start the bike with starter fluid every time (1st try) / or pull a plug. lol

I am making my way through "the maintenance list" about to check the valves and rebuild carburetor.

I had a few respectable people not understand why it starts and behaves the way it does. And also a few jokes, lol

[hjfisk]

Can you start the bike with the kick starter?

[halz426]

I can start the bike with the kick-starter once I get it fired up with ether.

I will try to kick start it cold and with no ether.