Crackling sound?!?!?!?!

[boze]

hey ppl, i got the old SR500 on the road and everything, ive just got back in from a run and ive noticed something wierd, if i accelerate hard and get to a certain speed i hear a kind of crackling coming from the front of the bike, its very disconcerting and i was wondering if anyone could possibly shed some light on the matter, anyone else had a problem like this? It doesnt appear to affect its performance or anything its just a little worrying.

regards

damo

[guest18]

pinking/detonation?? Give me a shout if you're through East and I'll have a listen!!
Meantime do a plug chop and see how your mixture is..

[boze]

pinking/detonation??

i dont know what pinking is...  i do think she might be running a bit rich though, she starts from cold no problem at all but if ive been a run then switch her off for a moment she doesnt really want to start till ive left her for a bit to cool down....does that make any sense? how do i adjust the mixture? any help is greatly appreciated

damo

[themoudie]

Aye boze,

Pinking is the noise you get from the top end of the motor when you put it under load (accelerate) and the air/petrol mixture explodes or detonates before the piston reaches Top Dead Centre, or before the spark plug sparks and ignites the mixture causing it to burn. This can blow a hole in your piston, burn out the valves so that they no longer seal, break the big end bearing!!!!!!!!!!!!!!!!!!!!!!!!!!

Take your plug out and have a look at the porcelain insulator around the central electrode. Is the spark plug the correct one for the SR500? Does the insulator look black, sooty, wet (Rich)? Light brown/yellow, slight deposit, dry (OK)? White, melted, crisp with bits flaking off (Weak)?

Was the plug loose when you removed it, can you hear air hissing on the inlet side of the carb or cylinder head area (weak mixture)? Exhaust gases leaking from exhaust side of the head (weak mixture)?

Is the original air filter and exhaust still fitted, have either been modified or removed and replaced with alternatives?

Buy a manual (Clymer, Haynes or Yamaha) from Ebay and read it from cover to cover at least three times before laying a spanner on the bike!

Have a browse through this website: http://www.thumperpage.com/articles/sr500faq.html

Have a browse through this website and enjoy the pictures as well: http://www.sr500club.org

Have a browse on this website and go and have a blether with them: http://www.ayrclassicmc.com

Ayr club meet in the Glenburn Miners Club, Glenburn Road, PRESTWICK, every second Wednesday in the month and have a very diverse range of interest and knowledge. The craic is nae bad either!

I hope this helps and that you get sorted. I'm not trying to be a BOF!

Regards, Bill

[guest18]

Beat me to it
Cheers Bill

[Steve H]

To me pinking always sounds like someone smashing glass on the engine.

[boze]

thats almost definately what it is, ill sort the air/fuel mixture today, ive got a haynes manual so ill be ok

thanks for all the help people!! ill get some pictures of the bike sorted as soon as possible

regards

Damo

[kenny]

Have you done anything that may have altered the timing or the compression?

Ken

[boze]

Have you done anything that may have altered the timing or the compression?

Ken

not that im aware of, i did strip and rebuild the carb a few months ago as the fuel pump diaphragm was shot to pieces but i did it in accordance with the manual. everything is completely factory standard, no air filter mods/engine mods/exhaust mods.

Steve H: it does sound like glass smashing on the engine its almost definately pinking.

Bill: Ill check out those links, cheers!

damo

[Steve H]

Fuel pump diaphragm ?. Do you mean the accelerator pump ?, this is a small cylinder/pump which squirts fuel into the intake when the throttle is opened quickly. This could be be the source of your problems. The idea is that when you open the throttle quickly the fuel does not respond as quickly to the new throttle postion as the air does, and it weakens the mixture off. The accelerator pump is supposed to compensate for this sudden mixture weakness with a squirt of fuel. If it isnt working properly this could explain the pinking when you accelerate.
As and aside I first found about accelerator pumps when I looked down the barrel of an XL500RC carb and opened the throttle to check the slide movement, an eye full of petrol is not to be recommended !

[boze]

no, its a small flat rubber coated piece of fabric with a flat circular metal piece in the centre, it secures a spring on the other side of it, i am led to believe it creates the vacuum needed to start, or something like that.....but anyway, it was all crumbling away and i managed to order another online to replace it with.

damo

[guest18]

Dunno if the SR has an accelerator pump but what you're describing certainly could be one... either way I'm sure we'll get to the bottom of it

[guest27]

Accelerator pump - or a vacuum tap diaphragm maybe? - Either way sounds like pinking to me - says he being really useful and adding to the conversation.

R

[Steve H]

Look at this and tell us which bit you changed http://www.siue.edu/~rsutton/SR/mikuni.htm

[boze]

Look at this and tell us which bit you changed http://www.siue.edu/~rsutton/SR/mikuni.htm

um you cant see it on that pic, its part number 7 on both pictures on this page http://slash5.net/sr500/b0000dcc.gif

and part number 18 on this diagram (both the same part btw) http://slash5.net/sr500/b0000dcb.gif

damo