Hi guys I'm new to this forum and I'm the proud (not so sure at the moment) owner of a '79 CB750K DOHC.
When I got the bike earlier this year it started on the button first time from cold (checked the engine temp as soon as I arrived, the seller had just got in and it was stone cold) and rode fine. Thought I was on to a winner.
I'm not the most experienced rider, so didn't want to ride until the better weather. I did a basic service of oil, spark plugs and filters. When it came to start and run it for the MOT, i couldn't get it past 20mph or 4000rpm in neutral. The seller had told me it needed the carbs re-jetting and gave me the parts to do it. I did this, being a mechanic by profession, with no problem. I balanced them, refitted them to the bike and she started and ran perfectly! I booked the MOT and rode down to the petrol station to fill up as I was on the reserve tank. It started to splutter as I went in to the forecourt and cut out. I figured I was just out of fuel as it was very low, but it wouldn't start again. After much swearing, a drained battery and a long push home I was left with a dead bike. I checked the plugs and they were black as hell. Cleaned everything up and still nothing.
Checked for a spark, got nothing from cylinder 4, but a spark on 3. Checked the feeds, had battery voltage at the blue wire and the yellow wire, and the black and white common (?), so I figured not a wiring issue. I bought and fitted a new coil, checked, had sparks from both (although stronger on the new one). I then had some near starts, horrendous backfires, then back to nothing.
Thinking of replacing the second coil.
Engine cranks at a good speed, compression seems good.
New to bike mechanics, any advice would be most welcome!
When I got the bike earlier this year it started on the button first time from cold (checked the engine temp as soon as I arrived, the seller had just got in and it was stone cold) and rode fine. Thought I was on to a winner.
I'm not the most experienced rider, so didn't want to ride until the better weather. I did a basic service of oil, spark plugs and filters. When it came to start and run it for the MOT, i couldn't get it past 20mph or 4000rpm in neutral. The seller had told me it needed the carbs re-jetting and gave me the parts to do it. I did this, being a mechanic by profession, with no problem. I balanced them, refitted them to the bike and she started and ran perfectly! I booked the MOT and rode down to the petrol station to fill up as I was on the reserve tank. It started to splutter as I went in to the forecourt and cut out. I figured I was just out of fuel as it was very low, but it wouldn't start again. After much swearing, a drained battery and a long push home I was left with a dead bike. I checked the plugs and they were black as hell. Cleaned everything up and still nothing.
Checked for a spark, got nothing from cylinder 4, but a spark on 3. Checked the feeds, had battery voltage at the blue wire and the yellow wire, and the black and white common (?), so I figured not a wiring issue. I bought and fitted a new coil, checked, had sparks from both (although stronger on the new one). I then had some near starts, horrendous backfires, then back to nothing.
Thinking of replacing the second coil.
Engine cranks at a good speed, compression seems good.
New to bike mechanics, any advice would be most welcome!