knocking sound GPZ750

[leep]

G Day
my 84 gpz750 has an issue when running into a head wind that I have noticed, when doing approx 100km/hr, or trying to accelerate hard into a head wind at slightly lower speeds the engine develops a knocking sound, this bike only has 25K on the clock, and it is genuine as it previously belonged to my uncle who garaged it more then he rode it since purchasing it new. once I back off the throttle it goes away as expected, and normally only will occur when trying to give it a fist full. when the knocking occurs it doesn't want to accelerate much either

I'm not sure if it could be a cam chain tensioner issue????? the best I can relate it to is when a car engine "pings" if the timing is set too advanced

has anyone had a similar issue with their gpz/gpx????

[Rossi]

What fuel you using ?

[leep]

standard unleaded as it says to

[Rossi]

switch to premium matey, high compression engines always run better with 98RON or higher

[leep]

cheers for your advice Rossi, i will try it out and see what happens.

has anyone else experienced such an issue on these old girls when running normal unleaded?????????

[phear8me]

nah sorry i dont run anything less then 98

but i dare say get a tank full of 98 or higher and see if it changes

[Kawawog]

Sounds like detonation to me,which is more likely to happen on hotter days with average fuel.. Another contributing factor could also be that your in too high a gear for the speed your doing and labouring the engine..
As suggested above give it a belly full of priemium and see how it goes

[dave#3]

I'm with Wog - what sort of revs are you pulling from?

[Gosling1]

....switch to premium matey, high compression engines always run better with 98RON or higher .....

they do, but this is not a high-comp motor. The standard c/r on an 84 Geeper 7fiddy is 9.5:1 (that is the monoshock model A2) - is this the one you have ??

The knocking sound you have described, sounds like detonation to me - needs to be fixed before you do any significant damage. These models did not have the old-school style advance unit on the end of the crank with weights etc - they rely on a specific circuit within the ignition unit - the IC Ignitor box.

You can check the operation of the electronic advance unit easy enough with a strobe light (attached to #1 or #4 plug leads) - the timing is checked behind the engine cover on the r/hand side of the motor. Take the cover off, shine the strobe light at the timing marks. At low speed (idle), the "F" mark on the rotor should align with the timing mark cast into the motor. At full advance revs (3800-3900rpm) - the advance mark (C) on the rotor, should align with the timing mark. If these check out OK, then the ignition timing is fine.

Its a really easy job to check the timing. BUT - If the IC Ignitor box is from an early model 750, it will still work, but these early boxes do not have the advance built into them. These boxes are easily damaged by reversing polarity on the battery, or disconnecting the battery with the ignition switch on 'ON'. If this unit has ever been replaced, this may be the source of the problems. Normally, these old girls rev like the shitter, it should easily pull 11 grand on a good day !!

If the ignition timing checks out OK, it may be worth throwing a tank of 98RON into it, just to see if it makes any difference. But under normal circumstances (with stock compression) - normal 91 or 95 unleaded should be fine. If the motor is badly carboned-up (ie carbon deposits on the piston crown) - this can raise compression and can be a source of engine knock when using low RON fuels.

Spark plugs also need to be checked - these old girls like B9 plugs (colder) not the standard B8's or B7's. If it is fitted with 8's or 7's, ditch these immediately and throw some B9's into it. This could help a lot.

See how these adjustments go first - gearing may be another potential issue, as could cam timing. But hopefully it will be something simple.

[leep]

cheers everyone for all your help
yeah gosling its the UK monoshock A2, well Ive finally been able to get back near the big girl and found this...........

timing is advancing sweet, at timing mark "F" for idle and just above and advances to the line just right of the 1 4 mark at approx 3800rpm+, didn't notice the "C" mark on my model

checked the plugs, it has NGK b8ES, I will grab some BR9ES plugs tomorrow and see the difference, I managed to locate a manual which references the BR9's for normal riding and the 8s for the U.S model and low speed riding.
If the above fails then 98 fuel will be the next step and check it out.

normally the knocking occurs around the 3500rpm approx mark, especially heading into a head wind, more so actually when heading into a head wind.....its almost like something is in front of you holding the bike back..

lets see what results the CORRECT!!! plugs will now produce

THANKS AGAIN EVERYONE

[leep]

Hi all

well I am pleased to report that the problem is pretty much sorted now. I installed the BR9ES plugs and no more surging occurred. I can run at 100km/hr and then power on and the "holding back" feeling isnt evident anymore, I have since run a tank of 98 octane and the old girl seems to like that a little better. if the revs are down near 3500rpm in top cog and I give it a fist full of revs it will still rattle slightly but nothing like before....not even close. a 170km ride today through various rev ranges held for periods of time has it feeling like a different bike.

problem is now that with my efforts to try and kill the old girl during testing after the plugs were replaced the tappets seemed to have woken up a little and are a bit tappy, so that's the next project...but other than that all is on the up and up so far.

thanks everyone who has offered there advice on this issue, I really appreciate it

cheers

[Kawawog]

Good to hear its sorted

[Gosling1]

..... the tappets seemed to have woken up a little and are a bit tappy, so that's the next project...but other than that all is on the up and up so far.....

Good to hear that the old girl is running better now.....

The shims on this model are under-bucket shims, so they are a bit of a pain in the arse to do.....

If you do this job yourself, take a lot of care when removing and replacing the bolts that hold down the 4 cam-caps. These only run a 6mm thread, and it is *real* easy to strip if you don't undo / do up the bolts in a cross-hatch pattern - this must be done *every* time, and the bolts need to be tightened up in sequence, a bit at a time. You really don't want to strip a thread out of these bastards, and it is really easy to do this.......because the cams have to come out every time you need to change a shim.......it pays off to measure and then *re-measure* all the clearances ONCE, and try and limit the cam in/out stuff to only needing to be done once......sometimes, but not always possible.

This is a good opportunity to replace the cam chain as well - this makes a big difference to how *crisp* the engine will feel - with cam clearances all done properly, and a new cam-chain - well it will be running about as good as you can get.....

Got any pics ??

[leep]

Cheers once again for your help Gosling

I will get some pics online asap