troubleshooting help please?

[the_63]

Hey guys,

I went out for a ride last weekend, the bike was running fine, I merged onto the dual carriageway and kicked it up to 70...ish, maybe 80, and after about 2 1/2 miles the bike started to lose power, then find it again, and it just kept dying and lurching until I pulled off at the next exit. The throttle was held open in the same position throughout.

Bike is back to running great. I'm wondering if it's the .13mm exhaust valves causing an issue at high speeds.

I've read about carb backfire and wondered if it could be that, but I don't know what the looks like, at that kind of speed I can't hear much past wind noise, engine revving and my singing.

I just don't now where to begin in troubleshooting this

Chris

[Watcher]

Ensure the carb breather is routed out of the wind, would be my first step.

[ajensen]

Did you try putting the petcock on prime?

[1018cc]

....after about 2 1/2 miles the bike started to lose power, then find it again, and it just kept dying and lurching until I pulled off at the next exit...... I'm wondering if it's the .13mm exhaust valves causing an issue at high speeds.

So you're thinking that a clearance that you can essentially count on being a constant / a fixed value while the bike is running (at all RPM throughout the rev range) is causing an intermittent problem like surging?

Sounds to me like fuel supply at high RPM / load. I run 0.08-0.13mm exhaust valve clearances and have never found them to be as much of a problem at high speed as wind resistance vs the GS' low power. To me it sounds like your bike can just supply enough fuel to the engine up to a 70mph load but as soon as you creep higher the engine demands more fuel (to overcome the higher wind resistance) from the float bowls than what can flow into them. Normally you wouldn't notice but the sustained RPM / load is high enough for long enough that the float bowls run out of fuel causing your problem.

[sledge]

What you are describing are the symptoms of fuel starvation caused by a faulty fuel valve. So that is where you need to start

Its another one of those classic and well known GS5 common faults and much has been said on the subject in the past.

Under the conditions you described the valve is not being held fully open, due to either vacuum leaks or a faulty internal diaphragm, consequently the amount of fuel that passes is limited. The fuel level in the bowls drops and the engine starts to struggle. If the tank filter, the valve itself or other parts of the delivery system are not 100% the problem will be compounded because even less fuel can get through.

Switch the valve to prime next time it happens. This in effect by passes the vacuum controlled element of the valve. If you see an improvement the fault is without doubt within the valve or the vacuum line that leads to it.

[the_63]

Ok, thanks for the input guys!

Chris

[iamhiding]

yeah 63 sounds like ye' old fuel starvation pain in the arse but next time it does it put the bike in prime and see if symptoms stop. also worth making sure fuel and vacuum lines aren't kinked etc

couple of options if that is the problem:

rebuild your secondary petcock,

upgrade to the post 2001 petcock,

most bikes use the same bolt spacing on the primary petcock so you can mix and match a lot of parts to get a system be it vacuum or an inline tap setup that's going to work for yourself. vacuum petcocks are great things but they are prone to issues.

[the_63]

I took the bike out last weekend to test the prime switch suggestion, and the bike ran smooth as silk. Didn't get the chance to switch the feed.

Something is sucking power out of the battery though

Chris