Strife with a gpz250

**paul_skidmore5** · 14th June 2006, 22:09

I have a 1984 gpz250/305 twin that is giving me greif. for the last couple of weeks when riding it, it starts losing then gaining power, backfires hard out and than loses power completely and stalls.
I fire the bike up again, starts up no worries, you can sit there and rev it o.k but when you start to go again it does the same thing. Does anyony have any ideas what is wrong? I have replaced the coils to no avail, adjested the floats, cleaned out the carbs, tested the stator...What next?

**SwanTiger** · 14th June 2006, 22:12

That sounds like a Fuel Supply problem to me.

Have you checked the Fuel Tap Filter, the Fuel Hose and Gas Tank itself?

**paul_skidmore5** · 14th June 2006, 22:14

O2 getting into the tank o.k, tried gas on reserve etc

**SwanTiger** · 14th June 2006, 22:16

Well if the Coils are sweet and I presume you have replaced the Spark Plugs, then you are either loosing compression (strange) or have a problem with the carbuerator(s).

**paul_skidmore5** · 14th June 2006, 22:19

When i stripped the carbs they seem to be in great nick, what do i look for to see if the carbs are stuffed?

**SwanTiger** · 14th June 2006, 22:22

I'm not sure what would define a 'stuffed' Carb, but with old bikes its easy to get shit clogged in the jets and so forth. Was there much crap in the bottom of the float bowl when you cleaned it out?

Also did you use an Air Gun / Compressor to clean the jets?

I'm not experienced with carb's, hopefully someone else who is can provide you with some suggestions / solutions.

**paul_skidmore5** · 14th June 2006, 22:26

i would say, i just used carb cleaner. I think i will have to find someone with a compresser now

**Ixion** · 14th June 2006, 22:26

Does it ahve a vacuum controlled tap, and if so have you checked the vac hose to the tap - for both blockages and leaks?

It is unlikely to be carbs if it was previously running OK.

I assume they are twin carb. Imporbable that both would crap out at the sam time , and if only one was blocked etc it would still run Ok on one cylinder. Look for things that are comon to both cylinders.

**jazbug5** · 14th June 2006, 22:34

So it idles ok at standstill, but not when riding?

It does sound like fuel supply, but then my bike was doing something very similar and it turned out to have leaky spark plug caps plus the rain wasn't draining from the plugs due to some sort of blackage. Also, the reserve fuel line wasn't working properly (vac problem), so when fuel was low it crapped out... gah. The joy of old bikes.

I'm no expert, but thought I'd chuck that in in case it gave you any ideas?

**paul_skidmore5** · 14th June 2006, 22:42

you can give it as much gas as you want and no problems, then all of a sudden it starts crapping out. Could it be the cdi?

**grantman** · 17th June 2006, 11:45

Hi Paul
I have a GPZ400 (1984) and a spares bike
I know the motors are different but there maybe some parts you might want to try swapping to test your one.
I live in west Auckland so not to far

sounds like a CDI unit but could be anything
I had heaps of problems with mine stuttering and cleaned carbs changed coils and ended up being the airfilter was to restrictive!

let me know
Cheers
Grant

**T.W.R** · 17th June 2006, 13:24

Just read through all the symptoms your describing, it sound more like the spark is breaking down underload.
you've done the plugs, coils etc but have you changed the actual plug leads ?
If their stiff & hard ditch them for new leads

with the age of the bike & the amount of current that flows through them after a period of time the core of the plug lead becomes brittle & starts breaking down (this causes the symptoms your describing).

Also a lot of bikes around that era had ceramic isolators in the plug caps these too breakdown after a period of time causing the same symptoms.