dommie doesnt like starting after a long run

[mark porter]

hi all. a intermittent problem on my 2001 dommie. sometimes after a long run where it gets really hot, it will refuse to start. its almost as if there is no spark as it will just turn over without any hint of starting. it starts immediately by bump starting so i presume not an engine problem. i have tried shutting the fuel off when parking thinking the fuel was getting hot in the carbs and frothing. seems to work sometimes or is it just coincidence. cdi unit? anyone have this problem before? any help appreciated as i have nearly toppled over a couple of times trying to bump it. not clever when people are watching. thanks mark

[Rubberchicken]

Not starting as in not cranking over? If you can bumpstart it I'd be tempted to say it should start on the starter, unless that's the part that's broken.

[mark porter]

it spins on the starter perfectly. makes me think maybe the cdi unit not working when hot but i have no way of testing it. i have tried looking at the sidestand switch incase its stopping it from starting even on neutral but it bump starts straight away. i have tried taking the plug out when it refused to start one time but nearly burnt my fingers off and with the glaring sun i couldnt see much. not sure if there was a spark or not. i'm trying to work out if the automatic decompression system can make the valves stick open and prevent it from starting. it never does it when starting from cold, always starts first time in a couple of seconds of spinning. i didnt know if this was a trait of a single as some kick start singles were a pain to start when hot. thanks for the reply rubberchicken. mark

[Bounder]

Sounds a bit like how my CDI unit failed. Have you tried tapping the CDI with a screwdriver handle while trying to start it?

[Bobn]

From your description of "blazing sun"I assume you're not in the UK?
If it's hot where you are it may be that the fuel in the carb is evaporating when the bike is stopped with a hot engine, have you tried using the tiniest bit of choke (careful not to flood it)?
Otherwise try a new airfilter

[Blackboilersuit]

I would check your valve clearances. It's possible you have an exhaust valve clearance tightening up. When were they last checked? I've seen the same symptoms that you have on a bike before because the valve clearances were too tight.
cheers
Stewart

[Bobn]

---

I would check your valve clearances. It's possible you have an exhaust valve clearance tightening up. When were they last checked? I've seen the same symptoms that you have on a bike before because the valve clearances were too tight.
cheers
Stewart

Yes - spot on It's more usually a problem on bucket and shim heads (where the clearances close up) rather than tappet heads (where the clearances open out) but I too have seen this exact behaviour. One possibility may be mal-adjustment of the right hand exhaust valve, it's quite common because of the confusion caused by the decompressor on that valve.

[willywombat64]

the valve and fuel suggestions would surely be a problem on bump starting as well...... no?
I just wonder whether in some way the current draw of the starter motor is affecting the CDI's ability to produce the sparking voltage required . This is obviously not normally an issue but as we know.. the CDI's are quite commonly problematic due to internal breakdown and this can often only manifest itself at certain temperatures . Just an idea but I thought i'd mention it .

[Blackboilersuit]

Starting with a bump but not on the starter explains the valve, compression, fuel faults because when bump starting the motor is turning over much faster than it does on the starter motor. Turning over faster increases the intake air speed which will get the most out of low compression or low fuel level in the carb. The same effect can be seen on a kick/electric start bike. A very minor fault will prevent starting on the kick starter but it will still start perfectly on the electric start.

I would doubt that the CDI is being effected by heat as it is mounted up inside the fairing. The pickup coil in the engine could be breaking down when hot but I would expect this to show up when the bike was running as well as when it was being started. I would expect so see an intermittant hesitation at high engine speeds and an uneven tickover.

Starter pulling the battery voltage down? I've seen this on bikes before but only on a cold bike with a dying battery. I've had a bike turn over reasonably quickly but no spark. A dead cell in the battery was dropping the voltage way down when the starter was pressed. Although I'd only expect to see this on a cold bike it is possible that the fault is caused by the starter motor breaking down when hot, creating an abnormally high load which is pulling the battery voltage down.

I would still always start with the (cheap) and basic checks. Valve clearances, new spark plug, check/replace air filter, strip & clean carb. Then I'd do a compression test. If that was good I'd look at starter motor, ign pickup and CDI in that order. If it was bad I'd pull the top end of the motor apart to find the problem.

cheers

Stewart

[mark porter]

thank you all for the replies. all good points. i have checked the valve clearances because i thought maybe one was tight as well but all were spot on. the battery is new. it used to do it with the old battery as well. i havent tried tapping the cdi unit so i will remeber to try that next time. i have tried with choke on, throttle fully open, halfway open, twist a few times, etc but no difference. i really would like to see if it has a spark or not when it does it so it will give a more accurate diagnosis so i guess i will have to prepare myself with gloves and germolene! i have a nagging feeling its to do with fuel because 9 times out of 10, when i remember to shut the fuel valve off after stopping, it starts with no problem. ah yes, the air filter is maybe past its best as it can be dusty in the summer. (southern spain. high 20's low 30's the last couple of weeks). thank you all once again. i will order an air filter and bring some gloves and have the plug spanner ready for next time and will post how i got on. cheers all. mark