Help MC22 loud thunk when starting

Hi all,

I have a 1990 CBR250RR MC22 that's giving me a bit of grief when I try to start it. 90% of the time it's fine, but sometimes it will try to turn over and then there's a loud THUD, then nothing. If I leave it for a few seconds and try again it will sometimes start, but the battery seems pretty lacking at this point and it's struggling to do it.

I've ordered a new reg/rec and will test get the multimeter out tonight to see what the battery is like, but does anyone have any other ideas as to what it could be? My internet sleuthing is saying it could be the starter motor itself, but nothing I've found sounds like the exact same situation.

Cheers

 

Is it hydro locking ? Fuel
Could be filling the cylinder causing it to lock, maybe check your float valves for wear , or pull a plug and see if it is wet

 

@my67xr I'll give the rocking back and forth a go when it happens again, if it works I guess I need a new starter or get the current one reconditioned?

@kiffsta I hope it's not cuz I just paid out the ass to get carbs rebuilt (old ones were pissing fuel everywhere and had to get new ones in the end). If it's not the starter then I guess I'd have to look into that. Is there anything obvious that would happen while it's running if it's washing the cylinder walls down?

Cheers for the help guys, really appreciate it.

 

That was my first thought as well.

 

Didn't get time to measure the battery last night but tried to start it up this morning and it did the same thing, turns over a couple of times and then CLUNK! Shimmied it back and forth as much as I could in first and then tried again and it started but battery was pretty much dead at that point so I didn't take it out, will try again tomorrow.

Have a day off work on Friday so might see if I can pull the plugs to see if they're wet. Would I need to try turning it over and then have a look at the plugs, or will the fuel not evaporate in that situation?

 

It will but slowly. If the cylinders are full of fuel, you will need to dump the oil as it will be contaminated and thinned out by the fuel getting past the piston rings.

 

if the engine rotates a few times then it is not hydrolocking, usually it will do a half a stroke and lock up if the cylinder is full of fuel.

Is it your starter solenoid clicking is it due to low voltage in your battery ?

Can you post a video of the sound it makes.

 

Tried to get a video of it leaving work yesterday and it fired up fine and completely forgot to video this morning. Will try to get a video but I'm quietly hopeful that charging the battery works.

I'm also going to be blindly optimistic and take the last few messages as implying that it's a low chance of being a hydrolocking issue!

Really appreciate the help.

 

My first thought & brought back memories of mine doing this exact thing. Cylinder was full of fuel which in turn filled the ENTIRE engine with fuel, went to start then THUD.

It would actually kick over if you held the starter on long enough but this forced the fuel into the engine.

So they replaced the needles & seats ?

 

The second carb on the old set had a munted float seat and they're non-replaceable I think so they had to get a set of second hand carbs; replaced needles, jets, and seals.

I put it on charge for a little bit last night and fired it up this morning and it did it again, albeit a lot quieter. Managed to get it on video this time.

 

If I am not mistaken, that is your starter solenoid, that usually happens when there isn’t enough voltage in your battery or your starter solenoid is failing , the latter being more likely


Starter solenoids are pretty easy to get and change out , basically it sits inline between your starter motor and your battery

As linkin said, make sure your battery is fully charged then try again, you can easily test the the 3 phases on your stator with a alter and your voltage reg to make sure they are working as designed. Also stick a multi meter on your battery before you start the bike and get a battery at rest reading , then crank it over and watch the readings, that will give you a good idea if it is your battery or not. Once started also measure the battery voltage with the bike at 5000rpm

Kiffsta

 

Righto so I charged the battery overnight and had a voltmeter on the battery and it seemed fine, was alright when it was cranking and seemed like it was charging (~14v @5000rpm). Still acted up every other time I tried to start it but would start occasionally.

Have just bought a new battery (MotoBatt MBTX7U) and installed it, might be the placebo effect but feel like the lights on the dash are brighter.

Cheers for all the help everyone - hopefully this solves it and this thread can die peacefully. If not, I'll check out the stator (although it kind of scares me to be honest) and no doubt bother you guys again.

 

Stator testing is dead easy.

Disconnect reg/reg plug, start bike with fully charged batt, measure AC voltage across all 3 stator leads at RPM specified in manual. If output is uneven, stator is faulty. If output is low, might be faulty or loss of magnetism on the rotor/flywheel.

With the bike now off, measure resistance between each stator lead against spec in manual. If it reads higher or has no resistance across any winding, it is faulty.

Finally you measure resistance from each stator lead to earth (batt neg or earth point on motor/frame) - anything less than infinite resistance (no continuity) means the stator is shorted ie faulty.

 

That's what I get for reading/posting from my mobile