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G'Day Guys, apologies in advance if there's a million answers for my question in the FAQ's, I did check, but couldn't find the answer.

First off, a little background. I bought my 1986 Interstate with a ruined engine (PO didn't change the oil in ten years, and maybe 50,000 miles) so I bought a good used engine with 27000 miles from a wrecker in Michigan, and installed it over the Christmas break, using the original engine's carbs and starter. In hindsight, I probably should have cleaned the carbs, which have done 97000 miles, but there you go, I'm not the brightest bulb in the beer fridge..........

Anyway, the engine's in, everything is hooked up, and it runs sweet. I was away from home for the last 2 weeks, so when I got home on Friday, I decided that a Saturday ride was on the cards. Saturday was a great day, the bike started on the third press of the button, and I did a 160 mile ride with several stops and starts (gas, food, beer....) without any issues whatsoever.

This morning, I went to start it to move it out of the garage, and it wouldn't start at all. The battery was fully charged, the engine cranked fine, my killswitch was in the "on" position, I could see that my fuel pump was pumping, (clear fuel lines and new clear filter) and I pulled a plug and earthed it on the rocker cover, and it had lots of spark.

Occasionally, it would repay my efforts with a massive, ear splitting backfire, which was really cool, but frightened my neighbours, who have recently emigrated from somewhere in the Middle East, and apparently don't like loud noises.......

I scratched my head, walked around it a few times, then removed #3 spark plug (RHS closest to brake pedal) attached the cap, hit the button and it produced a lovely fat spark, but coincidentally, the engine started, and happily ran on the remaining 3 cylinders? Weird! I reinstalled the plug, and the bike ran great again. I had other stuff to do, so I didn't go for a ride.

Now I'm wondering if it's maybe a bad earth? Keep in mind that I really have no clue, but I'm wondering if the reason why the engine has fired up when I've earthed a plug to the engine (this is the second time this has happened, BTW) is that there needs to be a better earth betwixt the battery negative terminal, and the engine, and the coils, and the engine? Or am I barking up a completely inappropriate tree?

Thanks in advance for any suggestions guys! Cheers, Terry. :?
 

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The Irish Crew
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The backfire might have been caused by flooding, seeing as you know you had spark.
 

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i agree. if the plugs are sparking then look at the carbs. most common issue: float valves not closing causing flooding/hydrolock.
 

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You may very well have an intermittent ground. You can attach another wire from the negative side of the battery to a bolt on the engine. You can't overground.
 

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It aint rocket science
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The backfire might have been caused by flooding, seeing as you know you had spark.
+1 Why do I got the sneaky suspicion the original spark plugs were used.:)

Check for fouling.
 

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If it turns over good the ground (earth) is OK, the coils are not grounded (earthed).
 

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If it turns over good the ground (earth) is OK, the coils are not grounded (earthed).
Well, I'm at a crossroad. I don't generally disagree with Dave.
But, the coils do not require an external ground. I have a test set of coils that I hang from the bars when diagnosing. They fire just fine.
But, it is still possible it's a coil ground problem, in that, the cdi determines WHEN the ground is applied, based on an input from the PG's.
I suspect faulty pg/cdi. From your symptoms, it reminds me of an ignition system firing out of time.
You have fuel, you have spark, you have compression, but do you have spark at the right time?
It's not uncommon for pg's to lose their reference.
 

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The carbs should be full, the pump shouldn't need to pump when you start the bike. That could indicate your fuel is going somewhere you don't want it to. Next time it won't start, pull all the plugs and see if any are excessively wet...stick a flashlight down the spark hole and see if any cylinder is particularily wet.

Replace the plugs...kinda sounds like 'verge of fouling'.

Another thought...if there's a poor connection in a spark wire or resistor cap...you could be disturbing it during your spark test and allowing the connection to be made again. Might want to look closer at those.
 

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Well, I'm at a crossroad. I don't generally disagree with Dave.
But, the coils do not require an external ground. I have a test set of coils that I hang from the bars when diagnosing. They fire just fine.
But, it is still possible it's a coil ground problem, in that, the cdi determines WHEN the ground is applied, based on an input from the PG's.
I think you need to read my post a little closer. It says "the coils are not grounded (earthed)".
 

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could be why engine went bad in first place had a costomer who had leaks from every seal in engine took oil dip stick out and oil and gas mix shot up about 3 inches drained engine and it was about 2 gallons
 
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