Carb Lines

[Colt Winchester]

Hey Everyone! I am working on my '71 CB750 Four that I just bought and I have been trying to find information about these fuel lines. So far, that's proving to be very difficult.

The two delivery hoses from the Petcock to the middle of carbs 1-2 and 3-4 were both hooked up. Where I'm getting confused is the small lines on the bottom of each carb. On carbs 1 and 3, there is a line connected that runs to the back of the bike and leads to nothing. Just dumped out the back. On carb 2, there is a line that connects back up to a fitting above between carbs 2-3. Carb 4 is missing a line all together.

Attached are some pictures. The first is the fittings under the carbs with each line. The second is above the carbs showing where the two (disconnected from petcock) main fuel lines are. And the third is the space in the back next to bike lift where the lines just dump out.

If anyone can offer some insight on the how these should be hooked up and what their purpose is, I'd greatly appreciate it.

 

[Loose Chain]

Run all 4 out the back of the engine. They just hang there at the back of the engine. They are drain/overflow hoses. I noticed in one of your pictures that the frame of the bike has been cut. What is that all about?

 

[Colt Winchester]

Run all 4 out the back of the engine. They just hang there at the back of the engine. They are drain/overflow hoses. I noticed in one of your pictures that the frame of the bike has been cut. What is that all about?

Well, to be completely honest... I have no idea. I just picked up this bike for a grand yesterday. I just started to dig into it. Ive never worked or owned a bike. Only classic cars. It's been sitting in a garage for a decade and has been driven about 50 miles each year. The guy just didn't have time. If you can give me any idea of what you're referring to, I'd appreciate some insight from someone who knows more about it. What picture and where do you see the cut? I'll give some more pictures of it.

 

[Loose Chain]

Look on either side of the large main tube that runs up the middle. There are smaller tubes that have been cut off. You can't miss it. Seriously weakens the frame.

 

[Loose Chain]

Here is what it should look like.

 

[Colt Winchester]

Look on either side of the large main tube that runs up the middle. There are smaller tubes that have been cut off. You can't miss it. Seriously weakens the frame.
View attachment 18672

You're absolutely right. I talk to a buddy of mine who builds these bikes and he said that it's commonly done as a way to get to engine without pulling it. He sent me a link to replacement pipes that I can weld in. They have links that bolt end to end so you can remove them when needed. This will likely be the route we take.

As a general question about it, is it a serious concern to ride the bike with those missing? We don't plan on putting many miles on the bike before this is fixed, but we were planning to put some drive time on it to diagnose any potential issues.

 

[Loose Chain]

I would not recommend riding it but that is your call. I wouldn't say it is commonly done. It must have been owned by some hack at one time. Not a good thing to do.

 

[Nylundarn]

Just make sure that where you hang the hoses, if they overflow, they won't drip fuel on your exhaust.

 

[Colt Winchester]

Just make sure that where you hang the hoses, if they overflow, they won't drip fuel on your exhaust.

Okay. I was going to (temporarily) tie them to the frame on the opposite side of the exhaust to have them drain out under the bike.

I read somewhere that there should be two "vent" lines that also just end randomly. Is there anywhere specific I should place them?

 

[Colt Winchester]

Run all 4 out the back of the engine. They just hang there at the back of the engine. They are drain/overflow hoses. I noticed in one of your pictures that the frame of the bike has been cut. What is that all about?

Since they're just over flow lines, have you seen anyone who ever built something to clean up the look under there? I was thinking it might be possible to connect all four to the same line and have a single line flow out the back instead of having four randomly hanging out back there.

I'm wanting to clean up that look a bit.

 

[brettp]

To keep things simple, I suppose you could. Worse case scenario, you have multiple carbs overflowing simultaneously, you'll want a hose size to accommodate that or the fuel will end up running into the cylinders. Plus, with individual hoses you can more quickly identify which carb is overflowing.

 

[Colt Winchester]

To keep things simple, I suppose you could. Worse case scenario, you have multiple carbs overflowing simultaneously, you'll want a hose size to accommodate that or the fuel will end up running into the cylinders. Plus, with individual hoses you can more quickly identify which carb is overflowing.

That's a very good point. I plan on rebuilding the carbs fully so I hope that I have no overflow, but I figured in the event that it happens, I could let the bike cool down, then remove the hoses to diagnosis which carb is having the problems.

I know very little about these bikes at the moment, so do these lines commonly have fuel overflow or is that only indicative of a problem?

At the moment, I've got one that is leaking at the bowl, so I plan on pulling them off and I've got a rebuild kit that I'll use to swap out any old worn out hardware.

 

[brettp]

The overflow tube inside the bowl is where the fuel goes when it is too high in the bowl. It runs into that tube, then out the hose. If/when these carbs' float valves do not stop the fuel entering the bowl, it will exit the bowl via the tube, then the hose.
So, is it common? Yes, actually. If a carb overflows, it is always that float valve.

 

[Colt Winchester]

The overflow tube inside the bowl is where the fuel goes when it is too high in the bowl. It runs into that tube, then out the hose. If/when these carbs' float valves do not stop the fuel entering the bowl, it will exit the bowl via the tube, then the hose.
So, is it common? Yes, actually. If a carb overflows, it is always that float valve.

Gotcha. So even with a full rebuild, I can expect some sticking every now and then?

 

[brettp]

Depends. When bikes sit and fuel varnishes it can happen. Or if you're unlucky enough to get some crap from the tank into the bowl, or the valve rubber tip gets old. I wouldn't say it is common if the carbs get a good cleaning.

 

[Colt Winchester]

Depends. When bikes sit and fuel varnishes it can happen. Or if you're unlucky enough to get some crap from the tank into the bowl, or the valve rubber tip gets old. I wouldn't say it is common if the carbs get a good cleaning.

I've got a full rebuild kit with all new needles and o-rings and gaskets. I was going to ultra sonically clean each carb before rebuilding.

 

[brettp]

I wish I had the carb cleaning docs at the ready. I could have used them when I rebuilt mine early last year. Search this forum for that document, it is invaluable.

 

[Colt Winchester]

I wish I had the carb cleaning docs at the ready. I could have used them when I rebuilt mine early last year. Search this forum for that document, it is invaluable.

I took a look around the forum and only found one about ultrasonic cleaning. I'm most concerned with tuning them back after cleaning and getting them set properly.

 

[brettp]

Your top concern should be a thorough cleaning, tuning is made easier when the carb is ready for it. I'll see what I can find.

 

[Nylundarn]

Here is Macgregors excellent guide : https://boldorclub.nl/wordpress/wp-content/uploads/2015/01/honda_carb_manual_revf.pdf