gsxr 750 running dramas-HELP

[mick355]

First off awesome forum, just got myself a 750 gixer 89 model, runs nice for about the first five minutes of riding and then goes all wrong. Huge flat spot at about 5,000 rpm. I've checked the carbys out and syncroed them, new plugs etc. Just done a comp test tonight and the results were a bit ordinary- looks like rings. Down to about 90psi and then with a bit of oil back up to 150 psi. In your opinion guys do you think its a comp problem or should i be chasing something else like coils. Any help would be great as this if my first oil boiler.
Mick

[Gix11]

What mods has it had done? Pod filters, can, jet kit etc or is it stocko?

[Tone]

Hey Mick,looks like its 89 750 week on here at the moment.See other post http://www.streetfighters.com.au/forum/topic.asp?TOPIC_ID=4477
Good luck and welcome.

[mick355]

Thanks for replying guys, absolutley stock std, including the carby. The only thing you would consider none stock is the four into one system but that still uses the std can. Are they meant to get vso hot or am i paranoid?
Cheers Mick.

Buy some new needles;)[8D]

Runs fine for 5 minutes then gets a huge flat spot around 5G? A fucked CDI or coil will break down when hot,causing that exact shit.Compression deffinately needs attention,give it a head job while yiour there.Has this bike had a hard life.???.
cheers and welcome.

[shift1313]

when you say 5 mins, is that with the choke on? or just 5 mins or riding?

[alfiestorm]

Low compression , If you are going to take the head off and give it a going over as davo has suggested then have the barrels off measure everthing and over bore if needed or at the very least hone them and fit new rings. You know it makes sense

[mick355]

Thanks guys, pulled the motor out yesterday. Will get the head and barrels off tonight and measure everything up at work, and give it the birthday it deserves. Where is the best place to source parts for these things?
Shift1313- just five minutes of riding, don't really use the choke.

Davo- Don't know about the previois life of the bike, Who could i get to check the CDI and coils, i don't really want to go throwing cdi units at it if you know what i mean.

Mad Mundy- what make you suggest needles. I've tried a few clip positions and all it seems to do is alter where the flat spot is.

[TurboKat]

Sounds like a classic case of split/pierced diaphragm(s) to me.

[unsane]

Like mad mundy said, needles could be an issue. The needles and tubes can wear out pretty quick if the carbs are getting old (something to do with the plastic bit that holds the diaphram wearing out).

Sounds like my 88 750 before i ripped it apart, flats spot would change spots with needle clips and had bad comp. When i did pull it down the needles and tubes were fucked, but the low comp came from 4 cracked chambers (some missing huge chunks of metal between valve seats).

Ditto;)
quote:Originally posted by mad mundy

Buy some new needles;)[8D]

[mick355]

Diaphrams are sweet, there is a bit of play in the plastic slides but how do i know when the slides, needles and emulsion tubes are fucked? How much play should the slides have and what should i be looking for in the emulsion tubes/ needles. Just but a bore gauge down the barrels and they are well within spec if you trust the clymer manual 73.052 to 73-060 wear limit claimed by CLYMER manual 73.090 do these figures make sense?
Mick

figures make sense,90psi sounds suss. raised compresiion with oil down the bore can indicate oil rings or valve seats/burnt valves.How many K's?Tubes are common point of wear in CV's.Sounds like you got yaself a rebuilder bike mate...also not un-common.
But the 5 minute factor still points me towards CDI/coils. Only folk that can properly test this is your local dealer,or someone that once worked for Suzuki...???