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Troubleshooting cool cylinder

hbrown7332

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On my 1972 CB750K1, cylinder #4 is running cool enough that I can wrap my hand around the exhaust pipe. All other cylinders and systems are running ok. I swapped out the spark plug and it is not the problem. Visually the spark looks normal. No carbon deposits are building up on the plug -- it looks new out of the box (but it isn't).
I'm pretty sure the problem is carburetor but haven't figured out how to tell for sure. And if it is carburetor, I'm not eager to pull the carbs out and do a rebuild.
So my question is, does this sound like a carburetor problem, and if so, is there any reason NOT to dump a half can of Gumout carburetor cleaner into the gas tank and see if that makes any difference?
Thanks...
 
Check to see if you have spark first. Pull plug hook up to plug wire ground plug on engine case turn engine over see if you have sprark. If you dont check wire then coil if you have spark then start on fuel system. First make sure that carb has fuel in the bowl , just loosen the bowl drain see if fuel flows out. If you have fuel there then you need to look into the carb. That should get you started on trouble shooting. Let us know what you find. Good luck
 
I would guess it is carb. I run into the problem all the time with the fuel now days. If I dont start it up every couple of weeks I end up having to pull the float bowls off and cleaning the jets. Most of the time you can get by with just cleaning the jets and not having to pull the carbs. you can pull the jets out with a short straight blade screwdriver. The idle pilot jet is usually the one that plugs because it is the smallest. While the bowl is off I pull the main jet and the emulsion tube(part that the main jet screws into) because it has many small holes in it. It is an 8mm wrench or socket to get it out. When cleaning you jets out I dont recommend using a drill bit to clean them because you run the risk of changing the hole size of the jet, even a little is to much. I use carb clean and compressed air, you may have to soak the jets for a while in the carb clean but you will get them unplugged.
 
thanks for the tips on getting the jets cleaned out. Does this mean the jets are exposed if I just remove the float bowl (fortunately, it's #4)?
 
Yes. Pull the bowl off and you'll see the main and pilot jets. There is a lot more to the pilot circuit than just the pilot jet, but to start I guess you can pull it out and spray carb cleaner in there. It might work. If not, pull the carbs and clean them right.

If you hold the revs at like 2,500 rpm for a bit, does that head pipe get hot?

For any air cooled bike, it's a good idea to put a fan in front of the bike (blowing at the engine) if you run the bike in your garage longer than it takes to get things up to operating temperature. Air cooled bikes need air flow to stay cool.
 
If the cleaning doesnt help i'd look verifying they rack is sync'd as well.. Has a similar issue and found my needles sitting at different heights.
 
not sure about holding the revs at 2500, but noticed after a good long ride that head pipe #4 is hot -- not as hot as the other 3, but too hot to grab on to and hold easily. So something is happening in that cylinder -- ?
 
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