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Trying to start this old 750...

  • Thread starter Thread starter janksolid
  • Start date Start date
J

janksolid

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I'm trying to start an old 750ES that has been standing 18 months. The bike was in daily use prior to that.

Carbs are clean - new plugs - new battery. Engine turns freely and compression seems OK.

At first I got nothing. Now I'm getting some pretty loud backfires with black smoke from the exhaust every now and then. And some loud pops out the backs of the carbs with grey smoke.

I'm guessing that oil has seeped into the combustion chambers hence the black smoke.

The bangs and pops are getting more frequent as I crank it (on to battery number 2 now) and sometimes I feel like the motor is 'trying' to start.

Should I just keep cranking away until it starts?

Any thoughts appreciated...
 
What does "clean carbs" mean, exactly?

Until you figure it out, stop grinding away at the starter. You can't possibly learn anything useful, and you could damage your starter.
 
Hey,

usually when an engine backfires and smoke and such, it has to do with the timing of the bike. So you might want to check the timing or retime the bike using factory standards. Just a possibility.

Good Luck
 
bwringer said:
What does "clean carbs" mean, exactly?

Until you figure it out, stop grinding away at the starter. You can't possibly learn anything useful, and you could damage your starter.
Click to expand...

Carbs stripped and dipped. New O-rings.

Last time I tried to start this bike about 4 months ago it eventually started after going through this same banging and popping routine. It ran so rough though which is what prompted a proper carb clean.

Not really expecting someone to wave a magic wand. Just like to knock ideas around... :cool:
 
What is your Gas tank like? Wondering about rust. Does it have a filter on it? Any idea if the valve clearances have been checked? Also what exactly in the carbs was stripped and dipped and what were the jets? Do you know if the spark at the plugs are good?

Thanks

Terry WBL, MN
79 GS750L
 
Tank has just been cleaned and sealed. I'll check the valves tomorrow. I last did them about 1500 miles ago. I'll take a proper look at that spark tomorrow too. It was sparking but so sunny here and no shade to move in to made it hard to see how bright and what colour.

If the spark is weak what are the possible causes other than ropey wiring? How can I test my HT leads? How can I test my coils?

Would oil leaking past the valve guides cause this hard starting? If so, what can I do?

This bike was my daily commuter until 18 months ago when I took it off the road to rebuild the forks. Now I need to sell it, hopefully as a runner....
 
Last edited:
popping and backfiring usually mean the 1-4 and 2-3 plug wires are mixed up/

put ether quick start to it see if it comes to life and then re-think the clean barbs statement.
 
When you get oil in the cylinders, it is usually blue smoke that comes out. Don't ask me how I know this. :D Try swapping wires like trippivot suggested.
 
Take your main fuel line off, put some rags below the line, now crank it, does fuel come out as the engine turns???? providing this is a vacuum feed petcock, fuel should come out as you turn it over, if it doesn't, try switching the vacuum line to another outlet on the carbs, I think from memory, it's between 3/4 carbs (to the right if your sat on bike)
If you have fuel coming out, put line back on and try some easy start on the intakes of the carbs, if it cranks right up, you have carb issues, if it doesn't start, you have electrical issues, ie weak spark, coil wires mixed up, timing out, CDI breaking down.......
 
No problem with fuel delivery. Float bowls have fuel.

I'm definitely leaning towards electrics.

It's 9am Sunday here and I'm ready to go. I'll report back later.
 
Swapped the HT leads as suggested. Exactly the same banging and popping.

Rather forlornly swapped the leads back again and it fired up first press of the starter!

How does that work???
 
I have previously had storage issues after 12mths in medium humidity areas.

The valves got rust over them and leaked so badly that they wouldnt seal at all, that was what caused my banging out the exhaust.

The only solution in my case was to remove the head and reseat all the valves.
 
janksolid said:
Swapped the HT leads as suggested. Exactly the same banging and popping.

Rather forlornly swapped the leads back again and it fired up first press of the starter!

How does that work???
Click to expand...

Got them in the correct positions the 3rd time!

It's the minor things that drive you mad.
 
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