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alanmacging

Joined:

May 12

Posts: 9

alanmacging says:

yzf 750 missfire

help!!!!!!!!!!!!

I have a 1995 yzf 750r which has a missfire when it warms up. I have changed the coils, plugs and caps,air filter,intake rubbers,vacume fuel tap. the carb have been stripped ultrasonicaly cleaned and balanced.I have stripped and checked the loom and cant find a short. The only part of the ignition system that hasnt been changed is the coil pick up and the ecu. I have also changed the the down pipes and exup valve, the battery and a chain and sprockets so im fed up throwing money at it.

Has anyone got any ideas???????????????

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  • Posted 3 years ago (10 May 2012 19:04)

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jaffa90

Joined:

Mar 09

Posts: 8820

jaffa90 says:

missfire

Welcome alan,is the missfire a definite pattern or is it intermitant?

Does it make any noises like spitting back,coughing,backfire or hesitation?

When did the valve clearances last get checked ?

Is it worse in colder weather?  

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smidget

Joined:

Nov 07

Posts: 2411

smidget says:

Hi

Have you changed the HT leads?

 

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alanmacging

Joined:

May 12

Posts: 9

alanmacging says:

m

The ht leads were changed with the coils, I dont know when the clearences were done but that wouldnt be an intermitant fault. Im leaning towards the ecu...more feckin money

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smidget

Joined:

Nov 07

Posts: 2411

smidget says:

Did

you fully refurbish the carbs or just rebuild with the old parts?

Ignition pick up has to be the cheaper of the two remaining parts, so I would go for that first.

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alanmacging

Joined:

May 12

Posts: 9

alanmacging says:

next step

carbs were built with existing parts but diaphrams seemed ok and the fault started quickly so i dont think its a wear issue. I really dont want to spend any more money on it. im going to get it warmed up then swap the diaphrams and slides to see if missfire changes cylinder before that i will swap ht leads between 1 and 4 im thinking the pick up sends a pulse to the ecu and if it send it to plugs so i think the pickup is ok. if swapping the parts between the cylinders doesnt move the missfire its got to be electrical???????

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James600zx

Joined:

Sep 07

Posts: 2759

James600zx says:

YZF 750.

"...misfire when it warms up."

Does that mean as soon as it comes off choke and onto idle, or when you've ridden it for a bit?

If it's the latter it sounds like a classic pick-up coil fail.

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James600zx

Joined:

Sep 07

Posts: 2759

James600zx says:

YZF 750.

Another possibility is that it's the charging system. I had a similar problem with my R1 which I eventually fixed by replacing the whole damned lot but I never really understood exactly what the fault was. Posts like yours prompt me to look into it again and I've just come across two discussions on a Yamaha XS650 forum which seem relevant (and also involve the pick-up coil).

http://www.xs650.com/forum/showthread.php?t=3487&highlight=sputters+backfires&page=2

http://www.xs650.com/forum/showthread.php?t=3707

Try a quick test by disconnecting the charging system so you're running on battery only (that's how I traced my R1's problem). If it smoothes out you're on the right track.

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alanmacging

Joined:

May 12

Posts: 9

alanmacging says:

found it

finaly got the bike running sweet. swapped the ht leads, still missfired on cylinder 1. swapped the slides and needle on 1 and 2 still missfired on 1. kept the carbs on the bike and took the float bowl off  carb 1 and started the bike there didnt seem to be a lot of petrol flowing, took the floats off and found a piece of debris in the float valve.Put it back together and ran sweet.I dont think this was the initial fault but what with changing all the parts must has sorted it along the way. cheers for your posts

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