Topic: Fuel valve problems - thanks for the help

[mycb750k6]

When I remove my tank from my 71 K1 and turn the fuel petcock on to down standard, fuel only comes out of one of the brass connectors. If fact it only comes out one when I switch it to reserve as well. Shouldn't it comes out of both? Seems like one line feeds the right two carbs and one the left two. So if it should come out of both lines, why isn't mine? Is it clogged somewhere and if so can you tell me where to look and how to clear the clog??


Thanks

[pjlogue]

Fuel should come out both output tubes regardless of source (normal or reserve).  Something may be plugging one of the outlets or someone in the past, closed off one of the outlet tubes for some reason.  I would pull the fuel valve and clean and inspect it.  Not a bad idea to replace the gaskets and the rubber seal that faces the handle.

-P.

[HondaMan]

When I remove my tank from my 71 K1 and turn the fuel petcock on to down standard, fuel only comes out of one of the brass connectors. If fact it only comes out one when I switch it to reserve as well. Shouldn't it comes out of both? Seems like one line feeds the right two carbs and one the left two. So if it should come out of both lines, why isn't mine? Is it clogged somewhere and if so can you tell me where to look and how to clear the clog??


Thanks

This is normal: the only way to get it to flow out of both at once is (sometimes) to open the gas cap on the tank. But, the system is designed to drain one side at a time, with the other side draining when the first side is filled, thus putting the pressure back against that side. All dual-port gravity feeds are this way. The slight differential suction of one side vs. the other is just enough to halt the flow on the opposite side until the suction of one side stops. As the bikes burn fuel at about 1/8 the flow rate of one hose (if wide open), this isn't trouble in practice.

[mycb750k6]

Thanks Mark. Long time eh?, but I'm still running my Hondaman K0 motor you built.


Anyway I read your Fuel petcock physics from 2010 so I tried squeezing the line that was running gas out  and sure enough fuel started coming out of the other line - just like your note said it would so that was a red herring on my part. 3-4 line goes to forward nozzle on the valve and the rear goes to 1-2 on the left side of the bike


So my behavior is that taking off from a stop it behaves like it's starving for fuel. A week ago none of this  was a problem.


Would you suggest someplace else to check.

[BenelliSEI]

Check your tank cap vent. I had one plugged up and it took me weeks to realize that’s all it was. Try running with the cap open. If your problem goes away.........

I’ll assume you checked the screen in the petcock bowl is clean?

[mycb750k6]

Yes I checked the bowl and surprisingly there was a bit of rust particles in the bottom of the bowl. I'll need to flush my tank although they should get picked up by the screen.
Current problem is when I accelerate from start and all bowls full of gas, it feels like it's starving.
I will look at the cap. I tried to get a wire in there earlier today but couldn't figure it out. How do clear a clog in the gas cap anyway.
And thanks for your help. Keep in mind it ran great last week. No problems at all.

[BenelliSEI]

There are a bunch of discs in the cap, I assume each one has a hole in them, offset from the other ones. As a temporary fix, I ran an 1/8” drill up through until it hit the underside of the chrome cap. Later I just replaced it.

Before that, try a few miles with it cracked open. That will tell you if there is a problem.

[mycb750k6]

yeah unfortunately that wasn't it. I ran with the cap open to no avail.
Chk'd valve tappets
Chk'd timing
Chk'd slow and fast jets
Chk'd flow through float valves
New plugs
Idles great immediately at 850-900 rpm from cold start
I'll tell you something weird.
When I hold my hand over the 300 exhausts, pipes 3&4 have a lot more back pressure than 1&2
Since this came out of the blue, I'm starting to worry about my motor.

[BenelliSEI]

I ‘d do a compression test next. not terribly useful data, but it will tell you if there is an issue in one or two cylinders.........

[mycb750k6]

compression test OK, carb syncing OK using carbtune, Still sounds like it's not getting the gas- why I can't even rev to 5K anymore. I think I'm done. I give up. I'll just dispose of it. Thanks for your help.

[BenelliSEI]

Before you giv up, have you checked float heights and the actual fuel level in each bowl. Low fuel levels can show the symptoms you are having.

[mycb750k6]

Yes Bene I have and they are all correct. No 2 seemed a bit low but I just checked it again and it is fine. I made float level gauges from eighth inch poly plastic.. I also have the bowl fuel height gizmo I made that screws into the drain hole in the bottom which shows fuel height in the bowl through a piece of clear tubing. I also checked fuel flow into the bowl through the float valve with the bowls off and the petcock turned on and the flow is enough to fill the bowl quickly. I appreciate the suggestion because I'm stumped and don't know what to do.
Nevertheless it still seems like a fuel delivery problem to me. (Oh and I put a few ozs of SeaFoam in the gas tank around this time but I doubt that's a factor)

[HondaMan]

This is starting to sound like a dying coil...can you swap in another 2-3 coil?

Years ago (2009) my own 750 started doing something very similar - in heavy rush-hour freeway traffic, no less. I had run to work that AM, all was fine: then about 10 minutes into the (90 minute) commute home in heavy summer traffic, I suddenly could not rev past 5500 RPM in any gear. I thought it was running out of gas, and the 2-3 mufflers were much cooler than the 1-4 pair when I stopped at a gas station (wondering if someone stole my gas?) but with a 2/3 full tank. I pulled out the #3 plug just to look, and other than being slightly wet, it wasn't fouled. I wondered if the plug cap was bad: after limping home, found it was fine and matched #2 within 50 ohms. Out of other ideas, I swapped in a (used) 2-3 coil from another set I had and it was instantly happy again. (I bought some new ones from Honda then...).

Often when the coils are dying, as their insulation material was past its expiration date about 15 yeas ago, they seem to just drop to a lower voltage and shorter spark duration. Sometimes they double-spark, which makes the bikes buck and sputter, usually right around 5000 RPM or so. Lately (last 4 years) I have been asked this same question an increasing number of times (by e-mails), and the easy majority of cures has been new coils, nothing else changed. I think they are just getting old (like me?).