1981 Honda CB650 only firing on cylinders 1 and 2

[Dom P]

Hello all, new to the forum and new to motorcycles. Just picked up this bike a short while ago that had sat for a while with the carbs off the bike. I reassembled it and the bike is only firing on cylinders 1 and 2. All the carburetor bowls are receiving gas and their seems to be solid spark and compression in all the cylinders. I took the bowl off carburetor 4 and cleaned it, the center jet (emulsion tube?) was clean and open and I shot gumout down the other two jets. It didn't seem to help much, could a eroded vacuum T between carbs 3 and 4 cause them to not be pulling fuel? Any feedback is appreciated thanks!

[MiGhost]

Just picked up this bike a short while ago that had sat for a while with the carbs off the bike. ...their seems to be solid spark and compression in all the cylinders. I took the bowl off carburetor 4 and cleaned it, the center jet (emulsion tube?) was clean and open and I shot gumout down the other two jets. ...could a eroded vacuum T between carbs 3 and 4 cause them to not be pulling fuel? Any feedback is appreciated thanks!

The T fitting between carbs 3&4 is not a vacuum source. It is there to provide atmospheric pressure to the float chamber to allow the vacuum from the engine to pull the fuel up through the jets into the airstream. If it is plugged to the point of not allowing air pressure to enter the chamber. It could keep the carbs from working correctly. The two T fittings between carbs 1&2, and 3&4 are normally connected together with another T fitting that routes a single hose back to the area above the air box. You can use two seperate hoses if necessary.

Another likely problem area is the slow/pilot jets. These are the pressed in jets that the service manual says are not servicable. They can be easily pulled out with pliers (Just twist, and pull at the same time), and cleaned. They are often plugged, and cause a lot of the issues with a poor running engine. Just spraying carb cleaner through them will not get them cleaned out properly. They need to be pulled out to open the tiny holes on the sides that allow the air to mix with the fuel as it is drawn through jet.

Pull, and clean those slow/pilot jets, and make sure the vent hoses are open. Then you can work on the leaky carbs.

[Dom P]

thanks for the insight, I actually have it running on all 4 now. Low end power is good but I couldnt get it to push past around 50 mph. Probably gonna need to go through the carbs again. Maybe they're out of sync or a high speed jet is clogged.

[MiGhost]

Low end power is good but I couldnt get it to push past around 50 mph. Probably gonna need to go through the carbs again. Maybe they're out of sync or a high speed jet is clogged.

You are barely getting into the midrange at 50. It sounds like there is still some cleaning to be done. A good bench sync will go a long way toward getting it running.