Topic: ‘71 CB500 Fuel Delivery Issues

[Mmyriel]

Hey, folks.

I picked up a “ran-when-we-parked-it” CB500 off of Craigslist in March, having never ridden a motorcycle before. After putting in a few months of work to get it running and learn to ride, I stinkin’ love the thing. It’s everything I like about driving and working on cars, only better. However, I keep chasing one problem.

Whenever I go for a ride longer than ~20min, my fuel lines start filling with air. If the tank is low enough (always still above a gallon left), I’ll start to sputter at stoplights. Things to note:

-I fill the fuel lines full before attaching them to the petcock barbs, so I’m starting free of air.

-I tried cleaning the petcock, although I didn’t see any problems.

-I tried cleaning the lid vent, but didn’t see any obstruction there either.

-I’m running 5.5 mm clear lines with steady drops all the way to the carbs, and no filters.

Any ideas what could be causing my problems? I think I’ve ruled out many of the usual suspects. The only odd thing I’ve noticed is that I seem to be unable to properly tighten the bowl at the bottom of the petcock—it seems as though the threads have stripped and it’ll pop loose again before I get very tight at all. Could that be the source of the air? I.e., I’m not able to tighten the bowl enough to actually seal against the o-ring.

Thanks in advance for any help—you’ve all already been a ton of help in getting the bike up and running.

[scottly]

If the tank is low enough (always still above a gallon left), I’ll start to sputter at stoplights.

If you switch to reserve at that point, does it pick up and run normally? If yes, it just means you have more than a gallon for reserve.

[Mmyriel]

Haha that’d be ideal, but no. As it is, I have to fill up before I get to use reserve. Popping my lid open doesn’t seem to change things, either.

[scottly]

Put a hose on the petcock and let the fuel flow into a suitable container. Does reserve, which draws from the very bottom of the tank work?

[Mmyriel]

Yup, it works. When I took the petcock apart, I checked that everything was open, and fuel flows through on reserve. If I pop the lines off, fuel flows just fine. It’s like the lines get air locked, but I don’t know why it keeps happening.

[scottly]

Try letting the petcock drain into a gas can for a longer time and see if it slows down; if it does, open the gas cap and see if the flow increases.
EDIT: Also, make sure the carb vent tubes are clear.

[Deltarider]

The CB500/550s are very sensitive to correctly dimensioned and routed fuellines. Read: http://forums.sohc4.net/index.php/topic,164089.msg2158019.html#msg2158019

[Scott S]

 Do you have the two small vent lines on the carbs attached? Are they routed properly?

[Deltarider]

Scott S made a good point. The vent of these carbs is to be checked from time to time, especially on the CB550K3/K4 models.

[Mmyriel]

Thanks for the advice, all. I didn’t observe a difference with the gas cap open vs. closed after letting gas run through for a while. I also saw your previous posts on fuel line length before cutting mine, so I should be good there—thanks, Deltarider.

Great questions on the vents. I hadn’t thought of those, and I’m afraid I probably didn’t pay them much attention when I cleaned my carbs. They also definitely don’t have vent lines on them. The previous owner removed pretty much all vent lines, and the only one I’ve gotten back on is the air box vent (since the bike had pods when I got it). When I get the chance over the next few days, I’ll pull the carbs, check that the vents are clear. and see if I can find some lines for it.

Is it unusual for the petcock bowl to seem stripped when it’s tightened, or are they just shallow threads?

[Stev-o]

Is it unusual for the petcock bowl to seem stripped when it’s tightened, or are they just shallow threads?

I wouldnt say it's unusual, the metal is soft aluminum so easy to strip with a heavy hand.  Does it leak?

[newday777]

Do you have added filters in the fuel lines?

[Stev-o]

Do you have added filters in the fuel lines?

His first post stated "no filters"

[Mmyriel]

For anyone still following along at home:

I took the carbs off and gave the vents a liberal spray-through with cleaner. It seemed to run freely into the vents and out the bowls. I measured my fuel lines out according to the link above. I put lines on the vents and routed them so they weren’t kinked. I went for a ~30 minute test ride, and… there was air in the lines when I got back. I didn’t do it on purpose, but I had even left the gas cap popped for the duration of the ride.

So, the search continues. On the bright side, the bike still rode great.

[scunny]

I would put Plasticine around that petcock see and see if that cures the air into lines.

[Robbo]

Replace the petcock.


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