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Trouble starting - many issues!

Kempy487

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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!
 
Coils are usually replaced in sets so get that second one. Old ones are also famous for working fine when cold then failing when they get hot and they get pretty warm stuffed under the tank right over the engine. I would recommend new plug caps while your at it as they can go bad too.

From what you said it sounds like the fuel flow from the tank to the carbs might be restricted or plugged with junk so that the flow can't keep up and the bowls slowly get too low. The petcock has a fairly coarse screen it and that's the only stock 'fuel filter'. Most of us add inline fuel filters past that. It's a tight fit and you got to make sure not to kink the lines but these will fit.

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Thanks for the help Dave. I did replace the inline filter for a decent one as I had 2 snap on me. One of the first things I checked was the fuel in the bowls and was good, but I'll double check for kinked lines as I've had the tank on and off quite a few times now!

Hopefully getting a new coil tomorrow so I'll post how it goes!
 
So starts and runs, but won't rev very high, and dies under throttle. Hoping it just needs warming up and running in....
 
Sounds like some dirt made it through and into the main circuit. Probably have to drop and clean the carbs again :(
 
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