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Hey guys,

I have been searching around the internet for a couple weeks now and I haven't found anything that relates to what I am seeing here.

This is my first bike and I bought it back in April. I took it to a mechanic to have it ran through to make sure it was safe to ride. The guy had my bike for a month, but never touched it due to backlog in his shop. So I went and picked it back up from him and decided that this would be my project. I may have bitten off more than I could chew.

First thing I wanted to do was get it running. So I bought a new battery, charged it, changed the oil, set the points, set the timing, checked the plugs, checked the compression, filled it up with gas and kicked it over. It fired right up and idled fine. I took it out for a ride and it died on me about 5 minutes from the house. I got it home checked everything again and went out again, same results. It would bog out around 4000-5000 rpms in 3rd or 4th gear. I would bring it to a stop give everything a visual, decide nothing looked out of the ordinary and fire it back up. Ride along for another 5-10 minutes, dies. I was able to limp home and parked it in the garage. I decided it was a carburetor issue and took it all apart. I went and disconnected everything. Took the carbs off, disassembled them, found the floats were damaged, bought a new carb kit and floats, and reassembled everything. Installed new intake boots while I had everything apart and installed clear fuel lines. I put everything back together and I noted the fuel lines were about half full. I didn't think anything of it and thought they would just fill up while I was out riding. Not the case. Still had the bog out issue after about 5-10 minutes of riding at about 4000-5000 rpms. Fuel lines were still about half full. Next, I decided to look at the petcock. Took that off and cleaned it out. There was a lot of grit in the petcock so I looked at the tank. The tank shows tiny traces of rust build up, but nothing too out of the ordinary I would think. Reassembled everything again, no change. Fuel runs free from the petcock but the lines are not filling with fuel. It does not make sense. So I turned the petcock to the stop position and left it overnight. I come home from work and the fuel lines are full. What gives? I take the bike out for another ride and it dies, lines are still half full, but I am able to limp it back home.

I am dumbfounded to this condition. It just does not make sense to me. Before I throw any more money at this, I want to get a good idea of where to start.

TLDNR: Air in fuel lines, disassembled carbs and petcock, installed new clear lines, no change.
 

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The lines are often half filled with air...The fuel draw from the lines is slow enough that enough fuel squeezes around the bubble and continues to feed the carb. If you turn off the petcock, run the bike until the carbs are nearly empty (engine starting to misfire) then open the petcock, there chould be a fairly strong flow of fuel. the air bubble will just flow along and purge in the carburetor bowl.

Overnightm the air slowly makes it way back up the line into the petcock....

This is normal....

Your problem sounds like a carburetor problem. I know you went through them, but if there is grit in the petcock, there is probably some grit in the carb again....You may have other problems too....The air bubbles in the fuel line are not it though..
 

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Hi
first off check the oil for fuel thinning its not the cause of your issues but leaving the petcock on overnight might cause bigger ones if the oils bad

did you check the float heights? Did you check the new float bowl gasket isn't hanging up the float, did you check the float valve tip isn't worn or damaged? Did you try taking off the fuel cap to check that it's not a tank vacuume issue?

hope that helps
 

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Discussion Starter #4
Thanks guys. I went through the carbs again. The insides looked pretty good so i just wiped them down and sprayed them with some carb cleaner just for good measure. Checked the floats again, they are set at 26mm. Hooked everything back up and the fuel seems to flow nicely. Not too sure what the difference is. I'm taking it for a ride tomorrow night and we'll see what the verdict is.
 

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I have had this so many times over the years and have never really put it to rest. Last time I changed petcock and removed fuel filters and it seemed to fix the problem ... for about 2 months. Now it is back. Mine does not bog at 4-5K in 3rd, it acts like it is running out of gas.

Like you, all the carb is to specs and when I open drop the float bowl it appears OK when cold but a little low when checked immediately after the problem. So I replaced the float pin. Same thing. Best thing that helps me is an almost full tank.

Wonder if it is some combination of ethanol and vapor lock? Maybe it is how I route the fuel lines? All I know is when I take those lines off, the gas pours very quickly from the petcock although only one petcock barb at a time.

What ever it is, it keeps coming back once I think I have it beat. At this point I just wish I could buy a set of carbs but since I have an SL350-k1 I can't find any new ones. Maybe the jets need to be changed for ethanol blends we have now?

But back to your problem. Try a full tank of gas. Maybe no fuel filters if you have them. Petcocks are so cheap that it was worth the $25 just to eliminate that as a potential problem.

EDIT: One other odd thing about mine. If I ride it to the store and it has the problem, the ride back is "usually" a little better. Then the next day the bike is better still. Letting it sit for two weeks will bring back the problem. Maybe it is just telling me to ride more!
 

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wow, did an edit and got a whole new post. Sorry for whatever my fumble fingers just did.
 

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My guess for both of you is the fuel tank vent. It sounds like there is a vacuum forming in the tank preventing fuel to run through the lines to the carbs.
Try running with a low level of fuel and open filler cap. Or, when it stalls/stops, crack the filler cap and listen for air rushing in.
You may just need to clear the vent hole in the filler cap.
 
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Not my problem. Already tried the open cap test. But I don't want to thread hijack, let's hear from the OP.
 

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These are all great tips. My 350 stalls just as I think its warm. Carbs are clean and good, freshly lined tank + new petcock. There are bubbles in the filters and wondered about that.
Will try:
...to turn off the petcock, run the bike until the carbs are nearly empty (engine starting to misfire) then open the petcock, there chould be a fairly strong flow of fuel. the air bubble will just flow along and purge in the carburetor bowl.

Thanks everyone.

Overnightm the air slowly makes it way back up the line into the petcock....


 

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These are all great tips. My 350 stalls just as I think its warm. Carbs are clean and good, freshly lined tank + new petcock. There are bubbles in the filters and wondered about that.
Will try:
...to turn off the petcock, run the bike until the carbs are nearly empty (engine starting to misfire) then open the petcock, there chould be a fairly strong flow of fuel. the air bubble will just flow along and purge in the carburetor bowl.

Thanks everyone.

Overnightm the air slowly makes it way back up the line into the petcock....


Try eliminating the fuel filters. There should be a screen in the tank on the petcock to filter the fuel. Some add on fuel filters restrict 'gravity flow' carbs. On cars they work fine, but cars are fuel pump fed.
 

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I'm dealing with a similar issue. I can have the tiniest of air bubbles in the left side carb line and cruise along indefinitely at low rpm's. Everytime I open it up I'll look down and watch the fuel line get sucked dry going into the left carb. No issues with the right side, but the right side has a very short line and is a straight shot. I'm currently using the visu 1/4 filter on both lines. I just don't see any way to route the left side line without it having a downward, and upward arch that rise below and above the carb.
 

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Try it without the in line filters. There should already be a filter in the tank unless somebody has removed it.
 
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If the line is going dry, it means it is not being fed...Could be a blockage in the port in the petcock, or even vapor lock. Heat will boil gas so that the vapor keeps expanding, blocking the line...This is not the same as air in the line.
 

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Thanks. Today I tried without the filter on the left carb and no difference. Works great cruising, but sucks dry when I get the rpm's high. I also pulled off both lines and opened the petcock to observe that fuel is streaming equally out of both ports. Lastly, I compressed my fuel tank cap to its maximum to allow the most air getting into the tank.

My current thought is to get a 90 degree Visu filter and try to minimize some of the hose length and curvature. My experience with gravity siphoning fluids in general is that the longer the hose the harder it is to draw vacuum and initiate flow.
 

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Gravity, NOT vacuum feeds fuel to the carbs.... Vacuum IN THE TANK can overcome gravity and stop the flow, but otherwise is irrelevant to getting fuel into the bowl....


Topic moved to correct section for 350's........ (Thanks Simon....I didn't notice until you mentioned it).....:wink::D
 

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Sorry for the lack of clarity. Instead of using the word vacuum I meant to say something along the lines of draw, or pull. You know like when you siphon liquid (using gravity) you have to initiate the flow when there is an upward bend in the line? By initiate the flow my experience has been a mouth full of gasoline trying to get it started. Once it's flowing it continues to flow, but if the flow is stopped gravity will not start it on its own. It must be drawn again, but how can it be drawn if the line is empty? With the CB450 eventually the tank's gravity pressure overcomes the situation and slowly fills the line again. That is my troubleshooting logic for my situation and it may be ridiculous. You tell me. Anyways, I get the 3/16 line in the mail in a few days and I'll test to see if that works.

Also, I have a CB450, but tagged on to this thread as to avoid starting another similar topic. Thanks guys.
 

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Fuel flow is stopped and restarted semi-continuously by the shut-off needle and seat....
AS long as the highest point of the "upward bend" in the fuel line is below the fuel level in the tank, (and the tank vent is functional), gravity will automatically re-start the flow when the shut-off needle opens....
 

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Alright I switch the left side carb to 3/16 line and no lack of flow issues. I stumbled upon another thread in which fellow owners preferred using 3/16 over 1/4. Now instead of a line filled with air it's almost completely full with full, and only tiny bubbles at the upward curve. Carb doesn't go dry at the higher rpm. Worth a try for anyone else with excessive air in their lines.
 
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