GPz running problems

[Danny]

I've kind of got my GPz on the road. It failed an MOT due to a few minor things (zaust gaskets leaving badly).
But the bike seems to have serious problems above 60mph in high gears.
It stutters/jerks when I hold any speed above 55mph and gets worse the faster I go. But if I accelerate the problem almost goes away.
I've been told it might be carb icing or dirty carbs or both.
The bike runs fine in low gears, albeit VERY LOUDLY! When my neighbour heard me start her up she thought it was a low flying aircraft.

Oh and I stuck some redex in the tank, it seems to have helped a bit, but not that much.

Any ideas?

[Kickstart]

Hi

Could be carb icing, but not that likely I suspect. Tends to be worst in cold damp weather, but generally air cooled bikes are not too bad for it (although Kawasakis tend to be worst for it). Not sure what the weather was like when you took it for a run.

My old FZ would be fine at low speeds, and fine at high speeds for a couple of minutes, then start to suffer. It would be fine at higher revs except for drinking fuel. Just that when you slowed down it did not want to know below 4000rpm.

Have the carbs been balanced? This would mainly affect idle, but would not make it particularly great at small throttle openings (ie, trying for a constant throttle). Easy to fix but probably not that likely.

Also check the rubber diaphragms in the carbs. These had a reputation for failing in 1980's Kwaks. With these holed the mixture will vary fairly randomly.

Have the carbs been cleaned out? If not first try dumping a can of carb cleaner into the tank.

Also check the rubber carb mounts. If these crank the mixture will go all over the place.

Valve clearances tend to firstly give problems with starting. Would also probably be noticeable from a compression check (I assume compression is not too bad).

I would hope it isn't electrics, but ignition problems normally seem to mean the bike just doesn't run (not always, but most of the time).

All the best

Keith
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[Danny]

[Kickstart]

Hi

Probably not carb icing. 55mpg is too good. The FZ would drop from 45mpg to 25mpg, and you could watch the fuel gauge go down.

Carb balancing takes a few minutes with the right gauges.

Redex is not a carb cleaner. It cleans out the combustion chamber (unless Redex have brought out a new product).

For the carb rubbers check for cracks. You can bodge this up with silicone sealant.

Most likely it is mixture somewhere. Compression problems would affect starting.

Carb rubber and the diaphragms would be my main areas, but AFTER cleaning the carbs out. Old petrol is horrible stuff.

All the best

Keith
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[Danny]

Sorry I meant to write 52mpg.

Looks I'm going to have me a good old carb cleaning session (unless the carb cleaner works).

Thanks for all your help. https://www.asdz68.dsl.pipex.com/smile/thumbs.gif https://www.asdz68.dsl.pipex.com/smile/xmas.gif

[Danny]

The rubber seal around the carb doesn't seem very good, I think this may be most of the problem. I found out that it runs worst in wet weather, I had to abort a ride to Kev's because it was running so badly.

[Kickstart]

Hi

Water will mainly affect the electrics, unless it is getting water into the petrol tank. Thats a point. Charlottes GPZ500 is a bugger for getting water into the tank which doesn't cause a problem until you switch onto reserve at which point you either need to drain the carbs or push.

Get a spray can of something and spray it onto the carb rubbers with the engine running. If the idle speed varies then you know there is an air leek (just be careful with spraying flammable liquids on a running engine).

All the best

Keith
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[Danny]

How did the water get into Charlotte's tank.

Hmm flammable spray near a running air cooled engine..... could be interesting. I'll try this tomorrow and see what happens.

[Kickstart]

Hi

Not sure. I can only think it accumulates around the fuel cap and then drops in when you open the tank. There is a drain hole around the rim of the fuel filler which was blocked. Haven't had the problem since, but then I avoid running it onto reserve.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Danny]

Right, had a little look around the carbs, the rubber seal looks very bad. I tried the spray thing, no effect. I pulled the rubber of one carb a bit, no effect. Might take the carbs off and try and get the rubber on properly or get some sealant stuff.

[Kickstart]

Hi

The important bit is the part between the carbs and the head. 90% of the time these are made of rubber and crack up with age.

All the best

Keith
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[Danny]

I've been told that boiling the old rubber should make it easier to fit, I might try that.

[Kickstart]

Hi

Yes hot water does make them easier to fit. Extends the time from them starting to be a pig to you having to replace them. The FZ750 needs hot water to refit the airbox if it has been off for a few days.

All the best

Keith
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[Milo]

those things were a right cunt to get back on after removing the restrictors. Very, very fiddly!
they look pretty much as they did when i had it, and it was running fine for about 10 months like that until it was garaged.
Damn! I forgot to take my old horns off the thing
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Shaft drives rule.

[Danny]

Well i've got the ones off the other gpz you can have if you want. Have you found the old restrictors?

Hmm if it was were running fine with the carb rubbers in that state then it probably just means that the carbs need a good clean. Do you know if there was a fuel filter?

[Milo]

I haven't looked for the old restrictors. but I know they're sat in my tool box.
I did get some better horns for that bike but don't worry about it - i don't think i could be arsed to fit them!
No idea about the fuel filter.
the carbs were fully drained before garaging to avoid them gunking up so i thought they'd be ok.
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Shaft drives rule.

[Dusty]

(Danny)

We've got the carbs off, they seem to be clean (generally in good nick), but we have cleaned them anyway. There was engine oil in the airbox, which was coming from the crank case breather.
We are boiling the carb rubbers now.
So we're a bit confused as to what the problem could be.
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[Kickstart@Work]

Hi

Oil in the airbox suggests worn rings or the engine too full of oil. Neither will do much for performance, althought the engine oil level is only going to have real nasty effects when it gets way too high.

What are the diaphragms in the carbs like?

All the best

Keith
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[EuropeanNC30R...]

https://www.grandillusions.com/media/penetropenny.jpg

https://news.bbc.co.uk/media/images/38413000/jpg/_38413457_petrol_can150.jpg

Go on, you know it makes sense https://www.asdz68.dsl.pipex.com/smile/nod.gif

[Milo]

Oi, that was a nice bike!
Definitly not a case off too much oil in there - not when i had it anyway!
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[Dusty]

[EuropeanNC30R...]

[Milo]

I can't wait to get mine on the strip.
All that torque and the weight up front I reakon i could do pretty well.
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Shaft drives rule.

[Danny]

Update:

Got the front wheel bearing and zaust fixed with help from G and Ste, thanks guys.

Still got the problem with the carbs (?) it has got a fair bit better now, but is still there.

I'll try to MOT it again and see what happens.

[Milo]

I know you've done the whole redex thing, but i'd take it out for a good hot run and put the engine under strain and rev high. Cos of the MOT legal thing i dunno if you've done this yet but it could be an idea.
Also check the vacuum hose that goes to the top of the carbs (right at the top in the middle) as if that's coming adrift it could affect the fuel flow. It's a short pipe and fiddly so could easily happen.
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Shaft drives rule.