Help! Bike loses power while riding

[fyberoptic]

So I bought a 1996 GS500 about a week ago, and it's been running ok so far. But I took it for a ride today and after a few miles when the engine got hot it would loose power while riding, and stall when I pulled the clutch in. If I let the bike sit for a bit and cool off then restarted it, it would be ok for a few miles then it would start acting up again.

I noticed that it runs ok on idle and full throttle, but everywhere in between it has problems. When it started acting up I added choke and it didn't make any difference. I tried putting the petcock in every position possible, no difference.

I'm thinking it's running too rich since the choke didn't make any difference. Could be wrong size jet, or improperly adjusted needle, but I can't figure out why it's only doing it when the engine is hot.

Any and all advice is welcome. If there are other threads where people have had this same problem please point me to them. Thanks

[Dr.McNinja]

So I bought a 1996 GS500 about a week ago, and it's been running ok so far. But I took it for a ride today and after a few miles when the engine got hot it would loose power while riding, and stall when I pulled the clutch in. If I let the bike sit for a bit and cool off then restarted it, it would be ok for a few miles then it would start acting up again.

I noticed that it runs ok on idle and full throttle, but everywhere in between it has problems. When it started acting up I added choke and it didn't make any difference. I tried putting the petcock in every position possible, no difference.

I'm thinking it's running too rich since the choke didn't make any difference. Could be wrong size jet, or improperly adjusted needle, but I can't figure out why it's only doing it when the engine is hot.

Any and all advice is welcome. If there are other threads where people have had this same problem please point me to them. Thanks

Sounds like a fuel problem. Check your hoses first (always do the easy stuff before the complicated). Make sure there's no fuel starvation between the tank and the petcock and the petcock and the rest of your bike.

Once you confirmed that, you're left with the carbs. The other thing it could be is a bad/clogged jet. A cleaning will tell you for sure. Follow the guides here to the letter and remember to inventory all parts before and after. You'll thank me later.

[adidasguy]

Got gas? It can look like you have gas when there's very little left. Rule that out by filling up.

[fyberoptic]

I made sure there's gas. It appears that the carbs are getting fuel, and that the lines aren't bent or clogged or anything weird like that. I just got done cleaning the carbs and everything look ok on them.

I'm thinking about measuring the resistance of the primary and secondary ignition coils to see if they could be a problem. Maybe increased resistance once they get hot from being near the engine causing weird things to happen?

It's just really weird that the problem only starts when the engine gets hot and that full throttle and idle work ok but everywhere in between doesn't. I'd hate to start replacing things without knowing what the problem is. 

[Dr.McNinja]

I made sure there's gas. It appears that the carbs are getting fuel, and that the lines aren't bent or clogged or anything weird like that. I just got done cleaning the carbs and everything look ok on them.

I'm thinking about measuring the resistance of the primary and secondary ignition coils to see if they could be a problem. Maybe increased resistance once they get hot from being near the engine causing weird things to happen?

It's just really weird that the problem only starts when the engine gets hot and that full throttle and idle work ok but everywhere in between doesn't. I'd hate to start replacing things without knowing what the problem is. 

The throttle thing would tell me you have a bad jet somewhere. That's the whole reason I suggested cleaning the carbs.

[adidasguy]

You can insure the spark plug wires are screwed into the coils and the caps screwed on to the wires. Sometimes people remove them, trim off 1/4" then put back together. If the wires have been manhandled for many years, the ends can get a little loose. Same happens in cars.

[fyberoptic]

Well I think I figured out what the problem was with my bike. I took the carbs apart and cleaned them, then put them back on the bike and went for a ride. No difference, once the bike got hot it would start acting up and when I pulled in the clutch it stalled just like last time.

So I checked the resistances of both spark plug coils and the signal generator (pulse generator, or whichever you prefer to call it). Everything tested ok while cold I tested everything hot after a ride around town to warm up the bike and it was all within limits. All of the resistances were well within the ranges they should have been.

However, I pulled off the cover of the signal generator to take a peek at it, and while looking at it I noticed that the gap between the rotor and one of the pickups was a lot larger than the other. So I loosened the two screws that hold the lower pickup on the SG and moved it closer to the rotor. Put everything back together and went for a 25 mile ride the other day with absolutely no problems.

So what do I think was happening? I think that since the signal generator uses inductance to tell the spark plugs when to fire, it has a tight tolerance for how close the pickup can be to the rotor. I think that maybe since it's an older bike it just vibrated over the years and moved a little. and it was no longer telling one of the plugs to fire when it should have been. I think the heat from a hot engine exacerbated the issue by causing a small increase in the resistance, and maybe even a slight bit of expansion and movement away from the rotor causing it to be weaker.

Either way, now she's running like a top!