I have a 1979 Honda CB750F which had been sitting around for some time before I bought it and then sat around in my garage until I got my motorcycle endorsement and title. When I tried starting it recently, it had great difficulty starting. The engine would crank but not turn over. Eventually I did get it to turn over and run for about a minute, then it died again.
I gathered that the gasoline in the bike had gone bad, so I drained the tank and the carbs. I replaced it with new gasoline, and the engine cranked much faster but still had difficulty starting. However, I did get the engine to run four or five times with the choke all the way out and the throttle all the way open.
The problem is that it ONLY runs if the throttle is open all the way. I let off on the throttle, and the engine dies.
So, I assumed that the problem was that there was not enough air in the fuel mixture. I looked at the air cleaner filter and it was perfectly clean, so I did a little research and gathered that the carbs were probably in need of cleaning.
I pulled the carbs off the bike and opened the float chambers, but to my surprise and frustration, they were not dirty. I had seen pictures online of float bowls coated in “gum,” and thought that I would find the same, but they were pretty clean inside the carbs.
So here’s the question: is there something that still needs to be done with the carbs? Are the jets inside clogged? Is there some other part of the carbs besides the float chambers that I need to open up and clean out? I opened the cylindrical chambers on the opposite side of the carb from the floats (vacuum pumps?) and they were also quite clean. Is there dirt in the carbs that I cannot see with the eye that needs to be cleaned out?
Or is this problem caused by something other than the carbs?
I am basically new to motorcycles so try to explain in layman’s terms as much as you can. I appreciate any and all feedback and possible help. Thanks.
It’s probably easier to replace the carbs, but it may be that the fuel gummed up to the point of varnish - on the jet(s) or the idle circuit. Symptoms point to a problem in the fuel system.
I usually remove the jets and float valve and let them sit in gasoline or (better) carb cleaner. Then, if there’s still anything in there, blow through them and hopefully they’ll be ready to go.
I had a similar problem (bike sat for awhile then wouldn’t start). Gas sitting in the carb and going bad/gumming everything up. I had a shop fix it (beyond my mechanical expertise), so I’m not sure if the jets were clogged, or just the floats.
I once bought a 1979 Honda CX500 for a dollar. ‘Ran when parked’. I had the carburettors rebuilt and it ran as if charmed. I eventually lost first gear, but the engine wouldn’t die.
Should I attempt to rebuild the carbs or should I have them taken in?
The local Honda shop here doesn’t work on motorcycles older than 10 years. (?!?!)
There are some other small shops around, which I’ll try.
Since I have completely removed the carbs from the bike, I could bring them in and not have to worry about paying for the labor of taking them off the bike. So, with this said, how much should I expect to pay a shop to rebuild these carbs?
It does sound like a fueling problem, and a carb rebuild will probably take care of it. Another thing to check is the inside of the fuel tank itself. Is it clean, or is there any corrosion? How about the fuel filter? How do the spark plugs look? I’d probably also go ahead and change the fuel lines. Even if they’re not plugged, they’re probably dry-rotted and in need of replacement anyway.
How mechanically inclined are you? It’s not too hard to rebuild a carb, actually. If you know enough to pull the carbs, you probably know enough to give it a go.
Well, it’s not certain the carbs are the problem, just leaning that way. But if this is the problem - If you price what labor costs to have the carbs rebuilt, you might find it easier to just replace them?
So are the components in a carb rebuild kit like this one sufficient to completely overhaul one of the carbs, or are there additional components that I’d need to get? From the looks of it, it’d cost me a little over $100 to buy the carb kits to do it myself - is that more or less than the average mechanic would charge to do it?
“Rebuilding” the carbs is basically just taking them apart, cleaning all the gunk out (including all of the minute needle-thin passages) and optionally replacing the needle valve/seat, float, and gaskets, then reassembling. If you’ve had the carb apart, you’ve already done most of the work.
I also recommend that you go to a NAPA store or a welding supply house and get a set of acteylene torch cleaners. These are sets of tiny wire files that will clean out the tiny carburetor passages without damaging the metal (if you use them properly). I did this on the carbs on my outboard with huge success. It also pays to clean all of the carb parts with a solvent–I used a fuel additive.
All of the other suggestions are good as well–spark plugs and fuel line are cheap, and you can eliminate those as possible issues as well.
How many carbs does a CB750 have? Three? I’m just guessing, I’ve never owned one myself. From that page it does look like you’d need one kit per carb. But yeah, that’s pretty much what a carb rebuild kit includes and that’s all the parts you need, assuming there’s no unusual damage of course. Baron is right, rebuilding is basically just cleaning, you don’t need the rebuild kits if you clean everything meticulously, so long as the gaskets and O-rings are in good shape.
You’ll also need a can of carb cleaner. I also recommend finding a clean, well-lit area to work, like putting down newspaper on the kitchen table. You don’t really want to do this on a garage workbench because there’s lots of small bits that you don’t want accidentally getting mixed up with other, unrelated small bits. Springs have a way of shooting off into the unknown (ask me how I know).
When you’re all done, you will want to balance or synchronize the carbs together. That means making sure they pull the same amount of vacuum for a given throttle position. Being out of synch won’t stop the bike from running, but it will run smoother if they’re in synch. For that you’ll need a manometer. A professional mercury manometer will run about $100, or you can make one yourself for a couple of bucks with some clear tubing and two-stroke motor oil. I’ve done the latter and have used it with good results.
Not sure what a shop would charge to do the rebuild, but the bill would probably include the words “arm” and “leg”. It’s not hard to do, but it can be time-consuming, and it’s the labor costs that will kill you.
Replacing the carbs with new ones probably isn’t a viable option, considering the age of the bike. It is a very popular and long-running model though, so I wouldn’t be surprised if some aftermarket company made a set, or if carbs from another model year would fit. But that’s getting into model-specific information that I don’t know. Probably best to find a CB750 owners group or message board for that.