Trouble re-starting when bike is hot

[Guest]

Hi all,

I've got a 1981 GS550L with a re-starting problem that I'm hoping someone might have some ideas about.

First, the basics: The bike starts up immediately (with choke on) if it's been sitting for a while (like overnight). I let it warm up with choke for a few minutes, and then can push the choke in and it will idle perfectly (~1100-1200rpm). No issues at all while I'm riding. Other maintenance items: brand new AGM battery this year, disassembled and cleaned the carbs last year, adjusted valve clearances last fall, new spark plugs and spark plug caps.

The problem: After I've been out for a ride (it can be as short as 15 minutes or as long as 1 hour), if I stop and turn the bike off (like to get gas), it won't start back up. The starter sounds like it's turning over well, but it just won't start. When this happens, I can push start it and it will catch and idle just fine. Alternatively, if I let it sit and cool down a little (usually >30 minutes), I can also then get it started again using the starter button.

My thought is that when the bike is hot, there's greater resistance in the electrical wires so that I'm getting some voltage drop before the coils that I don't get when the wires are cool. That might explain why it will push start (the starter isn't drawing current then), but I'm not entirely sure about this.

Any ideas?

[Buffalo Bill]

I hold the throttle more than half way open for hot condition restarts.

[fbody_mike]

I think your on to something with the coils acting different when hot and the starter motor is used. Carry a spare spark plug in your pocket, when hot and not starting with the starter, pull the #1 or #4 plug cap and stick your spare plug in it and rest the ground strap on the cooling fin. Hit the starter button and look for a spark, if it doesn't spark then you can confirm it is ignition related.

A couple thoughts, if it is ignition, then it is probably not as simple as a bad coil, because both would have to be bad, you can fire and idle these engines on 2 cylinders. My suspicion would be on the battery first, but it seems like you just replaced it, probably for this exact reason.

[mitosis]

I hold the throttle more than half way open for hot condition restarts.

Yeah I've seen people suggest that, but it still doesn't work for me.

...when hot and not starting with the starter, pull the #1 or #4 plug cap and stick your spare plug in it and rest the ground strap on the cooling fin. Hit the starter button and look for a spark, if it doesn't spark then you can confirm it is ignition related...

I'll give this a shot to see if there's spark. My thought was the fact that I can push start it suggests that there's spark, but maybe with the draw from the starter motor when I hit the starter button is making it too weak.

I should be able to check for voltage loss by measuring along different points in the ignition circuit right? Like from battery negative to the fusebox, ignition switch, and coils? I don't have the numbers right in front of me, but I quickly did that a couple weeks ago -- from what I remember, with the key off, there's really no voltage loss from battery to ignition switch (maybe .2V). With key on (headlight on), I was getting 1-1.5V loss at the ignition switch (this is with the bike cold). Cleaning up the fusebox connections (even though it looks good) has been on my list of things to do.

...My suspicion would be on the battery first, but it seems like you just replaced it, probably for this exact reason.

Exactly - the previous battery was 4-5 years old, so I got a new one just to take that part out of the equation. I've been meaning to do the quick start electrical tests as recommended by posplayr, but it's been raining like crazy for the past few weeks here.

[Uncamitzi]

When was the last time you checked your valves?

[Buffalo Bill]

Swap out the coils. I had a similar problem with my usually very dependable GS1100G but only lost 2 cylinders after riding a few minutes in summer weather. After replacing the stator and R/R, still had the issue. I had some GS coils with wires on hand so I tried that and it worked. Had another simple issue this summer, the stator connectors slipping out of the wire loom connectors, resulting in battery drain. Easy fix after I found it.
I had issues with my GS550T as well. Decided to never buy a GS550 again.
Track down all the coil connectors first of all, and all other connectors too.

[TxGSrider]

It sounds like it may be an electrical problem. When youre using the starter it drops the voltage and may be enough to keep it from sparking properly. I mean if you can push start it, that should indicate that it has enough voltage to trigger the coils when the starter isn't in use.
Maybe check our battery and all of your connections.
Question, have you tried using a jump box or anything like that when it wont start?

[Boriqua]

somewhere around here is a coil test. When I was first bringing mine to life I was having the same problem. I bought one of those light up inline spark testers for about 5 bucks from Harbor. Yup one coil kept dropping out when it got insane hot leaving me without 2 cylinders. Swapped my coils and all done. Like I said .. do a quick search as there are some tests you can do.

[mitosis]

Thanks guys for all the replies!

When was the last time you checked your valves?

Did this last spring -- had to swap out some shims and then everything was near the upper limit of the normal clearance ranges. Are you thinking that if they're a little tight then I'm losing compression?

It sounds like it may be an electrical problem. When youre using the starter it drops the voltage and may be enough to keep it from sparking properly. I mean if you can push start it, that should indicate that it has enough voltage to trigger the coils when the starter isn't in use.
Maybe check our battery and all of your connections.
Question, have you tried using a jump box or anything like that when it wont start?

I haven't tried to jump it using another battery but I should try that. I'm gonna check all the connections and clean up the fuse box this weekend. Also gonna maybe do the coil relay modification to see if I can bypass the voltage drop. I'm pretty convinced that my coils just aren't seeing enough voltage when everything is warm.

somewhere around here is a coil test. When I was first bringing mine to life I was having the same problem. I bought one of those light up inline spark testers for about 5 bucks from Harbor. Yup one coil kept dropping out when it got insane hot leaving me without 2 cylinders. Swapped my coils and all done. Like I said .. do a quick search as there are some tests you can do.

I'll look into this too. What's so weird is that the bike runs so well otherwise -- it's literally just the issue with restarting after I've been riding.


Thanks again everybody!

[Uncamitzi]

Someone once told me to make sure of two things when I started working on this bike... make sure you start with a charged battery and the valves have the right gap. Seems to have worked as a starting point for me.

[mitosis]

I checked the voltage to the coils this weekend, and with the bike cold (key on): battery -- 12.4V, at coils: 10.5V. So 2V of drop between the battery and the coils. I did the coil relay modification this weekend and the problem is now fixed! I'm going to clean the fuse box and connections at some point too.

I think what was probably happening is that when the bike was on while riding, the electrical system is heating up (combination of heat from the bike plus resistance through the wires) and then when I stopped and tried to restart, there was even less than 10.5V at the coils (especially with the draw from the starter).

I also did the battery quick test too:

Key off: 12.8V
Key on: 12.4V
Idle: 13.1
2500 rpm: 14.0
5000 rpm: 13.5
Key off: 12.9V

So looks like I have some voltage drop in the charging system too - hopefully cleaning the connections and fusebox will fix that.

Thanks again for all the responses!