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Only 1 or 2 cylinders fire up

Stripey

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Hi guys,

Newby coming online 👋

I've looked for related threads but didn't find anything, so if one does exist, please just point me in the right direction to it 🙏

I've always had bikes (DT175, RD350, Ducati 750 Sport, CBR600) but they went staight to a workshop for servicing/repairs.

My new baby is a 1980 CB750K.
All went ok for a short while, but then it wouldn't start.

It went to a local workshop here in Spain, who check it out. They changed the 4 sparkplugs, cleaned out the 4 carburetors and handed it back working, adding I should close off the fuel tap every night! But it was soon on the blink, now firing up but with only 1 sometimes 2 cylinders. I suppose being so out of balance, there is a knocking sound. It starts up, but if I try to move it a little, it dies immediately.

Does anything stand out as a possible cause?

Appreciate any guidance 👍
 

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Partly depends on how good the shop is, but maybe ask them? If you're gonna dive in yourself, I suppose the first thing I'd do is check for adequate spark on each cylinder. The ignition is situated where cylinder 1 & 4 share components as do #2 & #3. So, two cylinders are affected if a component fails. By placing a spark plug in the plug cap, grounding to the head, spin the motor only briefly and see if you get spark on each cylinder.

If so and the carb cleaning was recent, I'd be taking back to them.
 
I would guess it got some trash back in the carbs. These are very hard to get clean and many times it takes couple times to get them clean. Also if there is some stuff in the tank its easy to get into the carbs. get some spray type carb cleaner and spray some in the carbs with it running to see if they start firing. If so then you know its probably still in the carbs
 
Junk from the fuel tank clogging up the carbs is a common problem with vintage bikes. You can fix it by cleaning out your fuel tank of all rust and debris and cleaning the petcock screen, or be lazy and buy a cheap inline fuel filter to go between the petcock and the carbs. I usually do the latter and continuously keep ethanol-free gasoline in the tank to stave off the rust.
 
Thanks so much (jpdevol, dirtdigger and SenseAmidMadness) for the advice, I'll follow it and report back.
 
Hi folks,

I thought I would give you a quick update.

I eventually took the bike to an official Honda shop and their workshop solved the problem quickly by simply cleaning the carburetors again.

They apparently had remnants of a cleaning product used by the previous workshop ... anyway, that's what they told me!

All working fine now.

Though I do have another problem now, but as it is regarding a broken speedo cable, I'll start a new thread.

Thanks again for your help 👏👏👏
 
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