What can you make of this? (Engine bog/stumble)

[Lucien Harpress]

I've fiddled, I adjusted, I've read other topics here, and I'm still not 100% positive what's up with my engine. Here's the deal.

I'll start the motorcycle up, and (once it warms up) it runs fine. However, after about 10/15 minutes, it seems to get leaner the longer I ride. For example, when I decelerate the first few times it'll pop a bit, nothing big. As time goes on, however, it'll start to pop more, and louder, becoming full-on backfires after about 20 minutes.

What's really strange is that, eventually, (while at speed) it'll start to bog down- almost like I've dropped a cylinder. My first instinct would be to check the pickup coil wires (which I will, once the motor cools down), but I've been doing a LOT of work down there lately. I've pulled the cover 3 or 4 times now, and at this point nearly EVERYTHING is insulated, wires and internals both. I'm just wondering if it's fuel-related.

I've had inline fuel filters installed for a while now (two years, ever since I had MAJOR carb issues when I bought it). Would it be possible that these are getting gummed up- letting enough fuel through when the bike sits to give it a few minutes of good running, but not enough to keep it that way? There HAS been quite a bit of crap running through them, ESPECIALLY when I had my old (rusty) tank. How often should you clean out/replace the filters, anyway?

Well, sirs- whaddya think?

[crazy steve]

Checked your gas cap vent?....

[ManagerMike]

Does sound like fuel stavation. What type of in-line filters are you using? Apparently they are not the visually inspectable type. Are they in addition to, or replacing the petcock filters? Either or both may be restricing flow. There are also filter screens on the inlets of the float valves in the carbs that may be silted up.
Some investigation seems to be in the offing........wish I had a more direct answer, but I lack more direct information

[Lucien Harpress]

Thanks for the replies. My luck it's something stupid (but simple) like a kinked or badly routed fuel line. My fuel filters are clear plastic, but I haven't taken a good look at them in a while. I went through the gas cap a while ago, and cleaned it out best I could. Might be worth another look, as I seem to remember having this problem before (but THAT might've been bare pickup coil wires...)

[BA80]

Checked your gas cap vent?....

+1 on that and the kinked line. Easy way to check the cap is leave it loose and see if the problem goes away.

You probably would do good not to have a full tank when you do that though so you don't get gas in your crotch.

[Lucien Harpress]

Sorta puts "Great Balls of Fire" in a whole new light, don't it...

[BA80]

[Lucien Harpress]

I'm thinking (hoping) the gas cap vent was the culprit. I just took it out for a spin, and it started acting up again. Pulled off to the side, popped the cap open, and took off again, and it settled right back down. When I get a second here I'll pull the sucker apart and give it a good once-over.

[Eveready1100]

Hopefully, the gas cap is the culprit. Barring that, how long has it been since you checked your valve clearances? As everything gets warmed up in the motor, the clearances close up slightly as the valves expand from the heat. If they're too tight, it could start holding a valve slightly open and cause it to pop or backfire.
But hopefully, it's just the cap.

[yamahansolo]

I would change the filters I use one since I have the zrx carbs. About every 3 to 6 months they get dirty. You can buy the 2 to 3 dollar ones from Wally world. The little ones clogged up faster so I started getting bigger ones.