Topic: CB750 is starving for fuel

[hiljak]

I have a 1971 CB750 that is presenting some fuel delivery challenges. It is a stock set up with 4 stock carbs with pods though, no airbox. It has duel lines from the petcock to the carb rail.

I have two inline filters that are clear plastic. The two filters seem absent of any fuel all the time. The bike is kickstart only. The bike wont start without removing the two lines from the petcock and turning the peck on and letting some fuel collect in the filters. Once the bike fires it seems to run well and is responsive to throttle twists. I can ride it at average pace for about a mile and a half and then it seems to run out of gas on me. I can repeat the fuel line/petcock process and it will fire up and go another couple miles.


I have plenty of fuel as I have topped off the tank.

When I remove the fuel lines I can see a steady stream of fuel when I turn the petcock position to on.

When the bike is running well I still cannot see fuel collecting in the clear plastic in line filters.

Is it possible that the fuel is running through the center of the paper filter and not sufficient flow to fill the smallish inline filters? If so you'd think that when I stopped the bike (i.e. limited to no throttle) that the filter should fill up.

Is vapor locking a possibility?


I'm confused as to why this is happening. Can anyone offer help.

Thanks in advance.

[mcpuffett]

Hi it could be the fuel filters that are giving you the problems ? I've heard this before , have you tried without the filters ?, cheers Mick

[david 750f]

Check that the gas cap vent is working. Loosen the cap and go for a ride, see if that helps.

[martin99]

Check that the gas cap vent is working. Loosen the cap and go for a ride, see if that helps.

+1

[Terry in Australia]

How do you "loosen" the fuel cap on a K1? It's either open or closed. I wouldn't ride it with the cap open, unless you want a crotch full of gas...........

[hiljak]

So it is most likely a vent issue? I suppose I can try to make sure the tank/cap is venting properly. I'll try that this weekend. Stay tuned. Thanks for the help.

[BobbyR]

It can also be fuel line routing related. Make sure everything is running downhill. No loops in the fuel line. It should be a smooth downhill run from the tank to the carbs. 

[goldarrow]

inline fuel filters are not needed on these bikes.  lose them and you're one step closer to resolve your issues

[hiljak]

It can also be fuel line routing related. Make sure everything is running downhill. No loops in the fuel line. It should be a smooth downhill run from the tank to the carbs.

So Bobby, how is one supposed to rout fuel lines to all be direct "downhill"? I don't see a way to do this. So yes, I currently have a small, neat loop which is where the inline filters are located.

Anyone have any pics of different routing? The petcock seems to lend itself to going straight down as there would need to be almost 90* angle coming off the petcock pipes.

[hiljak]

inline fuel filters are not needed on these bikes.  lose them and you're one step closer to resolve your issues

Ya know, I thought about that too but this tank is not in the best condition on the inside. I decided on inline filters to protect the carbs. I guess I can just clean out the petcock screen periodically.

[goldarrow]

inline fuel filters are not needed on these bikes.  lose them and you're one step closer to resolve your issues

Ya know, I thought about that too but this tank is not in the best condition on the inside. I decided on inline filters to protect the carbs. I guess I can just clean out the petcock screen periodically.

should start from cleaning the tank

[hiljak]

inline fuel filters are not needed on these bikes.  lose them and you're one step closer to resolve your issues

Ya know, I thought about that too but this tank is not in the best condition on the inside. I decided on inline filters to protect the carbs. I guess I can just clean out the petcock screen periodically.

should start from cleaning the tank


I have cleaned the tank. Flow from petcock seems more than adequate. Screen in petcock is unobstructed. The inline filters look brand new (of course I'm not certain they're working as designed because the filter isn't filling up with fuel.)

 

[Don R]

I've had new looking filters give me fits. I could blow air into the tank through the lines but fuel would not flow out. I replaced my in tank screen and removed the filters. That said, I've installed small filters on my other bikes. They can flow downhill if you try, and use small filters. My brother has always run his lines down and through the filters then back up. He never has a problem. Just lucky I guess.

[vames]

It can also be fuel line routing related. Make sure everything is running downhill. No loops in the fuel line. It should be a smooth downhill run from the tank to the carbs.

So Bobby, how is one supposed to rout fuel lines to all be direct "downhill"? I don't see a way to do this. So yes, I currently have a small, neat loop which is where the inline filters are located.

Anyone have any pics of different routing? The petcock seems to lend itself to going straight down as there would need to be almost 90* angle coming off the petcock pipes.

Vapor lock is something entirely different having to do with boiling gas. But it really seems like you have your fuel lines and filters routed such that there is a big bubble (ie air in your filters) that is blocking the fuel flow.

You need to find a more direct downhill route from the petcock to the carbs so that the bubble doesn't get trapped at the top of the loop. There are 90-degree filters that can help you do that.

I don't think the 71 had an in-tank filter over the petcock inputs like later bikes did, but it did have a screen in the petcock that you should clean. If that's working, it's going to catch any rust or debris from your tank and you're free to experiment on running a direct fuel line without the filters. That will give you an immediate indication if your routing or the filters themselves are causing your fuel delivery problem. Trouble shooting! If it still has the same issue and the gas is free flowing from your petcock, then you know the problem lies inside your carbs.

One additional note: Some guys on here absolutely hate inline filters and will say they can never work. They're wrong because some of us have been running them for years with no problem.

Here is the 90-degree filter I was talking about

[hiljak]

Thanks Vames. I'm going to try eliminating the filter first then I'll score some of those 90* inline filters.

[BobbyR]

It can also be fuel line routing related. Make sure everything is running downhill. No loops in the fuel line. It should be a smooth downhill run from the tank to the carbs.

I have a 78 so it is a bit different. I use Tygon fuel line which is much more flxible than the rubber and make tighter bends without kinking.

The fuel line comes out of the petcock at a steep angle into a filter with a 90 degree inlet. The fuel line drops almost straight into the carb inlet.

You can get Tygon from any small engine shop.

[hiljak]

It can also be fuel line routing related. Make sure everything is running downhill. No loops in the fuel line. It should be a smooth downhill run from the tank to the carbs.

I have a 78 so it is a bit different. I use Tygon fuel line which is much more flxible than the rubber and make tighter bends without kinking.

The fuel line comes out of the petcock at a steep angle into a filter with a 90 degree inlet. The fuel line drops almost straight into the carb inlet.

Ok Bobby. I think I get what your suggesting as a refined fuel line route. Thanks so much.

You can get Tygon from any small engine shop.

[Terry in Australia]

I use clear fuel lines, (and inline filters, because I hate cleaning carbs) and that way I can see if there's fuel being delivered to the carbs. Cheers, Terry.

[BobbyR]

I use clear fuel lines, (and inline filters, because I hate cleaning carbs) and that way I can see if there's fuel being delivered to the carbs. Cheers, Terry.

Exactly!!!!!!!!!!

[Terry in Australia]

I'm guessing that hijak's problem is his hose routing Bobby, with clear lines he'd know just by looking. Cheers, Terry.

[cmills83]

I know this thread is really old but still comes up #1 on google when I was searching cb750 fuel starvation.  I had a similar issue on a '74 cb750k, one thing to double check is the internal filters on the carbs are clear.  I was having fuel starvation issues after a few blocks of riding, the carbs weren't filling up fast enough.  Floats were ok and at the right setting, I had plenty of gas.  Gave the carbs a quick clean and no luck. 

Then I pulled the floats/float pins and removed the internal carb fuel filter (the m10 brass float pin housing that has a small filter on top) and cleaned that filter and passage/fuel t between the carbs.  Bikes runs great now even with a 3/16 90 degree inline fuel filter

So lesson learned never give carbs a quick clean even if you cleaned them a few hundred miles ago.  Pull them apart and clean everything, preferably throw them in an ultrasonic cleaner too.

[Terry in Australia]

Yep, but the OP had a problem with his K1, which didn't have internal filters. Good advice for folks with later model bikes though.

[ekpent]

I've seen those 4 little filters that attach to the brass float needle seat on the 1975-76 F bikes but not on any of my earlier K bikes. Not sure exactly what year or carb model they came into play.

[Terry in Australia]

I've seen those 4 little filters that attach to the brass float needle seat on the 1975-76 F bikes but not on any of my earlier K bikes. Not sure exactly what year or carb model they came into play.

Mike Beltrami sent me a set of K4 carbs Eric (and I managed to break one of the float towers, bugger it) and they had them, but that's the only set I've seen with them?

[Doobie]

I've seen those 4 little filters that attach to the brass float needle seat on the 1975-76 F bikes but not on any of my earlier K bikes. Not sure exactly what year or carb model they came into play.

I had them on my K6 with the 657B carbs but I don't know if they were original or done as a rebuild at some point.  I've seen plenty of other 657B carbs that didn't have them.