GPZ BIG BANG ENGINE

[born green]

A qeestion for gos or anyone else that may know...
My old man, who's been around bikes all his life, and rebuilt and restored many bikes, he was/is a mechanic by trade, he's now in his 80s.
Some time ago he got hold of a GPZ 1100 B2, origin unknown, now this beast has a big bang engine in it, as in 1and 4 fire together as do 2 and 3, it has a reconfigured crankshaft, now he has rung me many times to ask me about it.
But as i havent had much to do with air cooled kawaka engine for over 20 years, im rusty on there problems.
It has a big kitted engine, dont know what brand, nor do i know what camshafts its running, but is running 34mm CV carbs, now the problem is it doesnt idle very well, off idle it revs sweet and hard, but when the throttle is closed it runs rough.
The old man has done everything, stripped and rebuilt the carbs many times, he has even tried another set off his other kawaka, replaced all the rubber manifolds, ect, checked and rechecked timing, everything....
All he know about this engine, is that it was built by a guy that built engines for speedway outfits, hence the big bang engine.

Would this rough idle be normal with this type of engine???
The old man is a anal about getting things running right, and its pissin him off... as he said to me, there all going to be yours soon so i want them right...

Nice collection to.
BSA gold flash, with dusting sidecar... XS 650 jamy... CB 400 four... two GT 750....GPZ1100 B1... and this GPZ1100 B2 with the big bang engine...

So any idea's???

[MAXUMIS]

if the motor + carbs has been set up for methonal there will be a prob, but sounds unsyncrinised/carbs or idle circuit there are so many things that could cause a problem like that

[born green]

if the motor + carbs has been set up for methonal there will be a prob, but sounds unsyncrinised/carbs or idle circuit there are so many things that could cause a problem like that

Its on petrol, that i do know, the carbs have been stripped, syncrinised, ect, no air leaks, at one stage he thort it may have a lecky problem, so that was all checked, i havent seen the bike as he's in outback QLD and i havent been there since he got this bike, but from what he's tried to tell me over the phone, it runs some sort of different cam setup?? thats from what i gather.... off idle, as in as soon as u start to roll the throttle on, it revs clean and hard.. shut the throttle and returns to idle no problem, then runs rough, doesnt stall, but hunts on the idle, he says..
He has been trying to find the guy that built the engine, but it was built quiet a few years ago...

[Strika]

Maxumis has thought of something which slipped my gaze. That motor would most likely be tuned to run on methanol as most of the speedway outifts and road outfits did at the time. Perhaps give it a dose of that and see if it runs better. Otherwise, I would suggest that a heavier flywheel might also assist in maintaining crank inertia when at idle. The flywheel would be probably lightened for the outift, add to that the big bang firing order and it might not be carrying over fast enough and almost stalling before the next two fire. Just a thought. will it hold a steady fast idle and it's only at it's "Normal" idle speed that it's erratic, or is it on any even throttle???

[born green]

Maxumis has thought of something which slipped my gaze. That motor would most likely be tuned to run on methanol as most of the speedway outifts and road outfits did at the time. Perhaps give it a dose of that and see if it runs better. Otherwise, I would suggest that a heavier flywheel might also assist in maintaining crank inertia when at idle. The flywheel would be probably lightened for the outift, add to that the big bang firing order and it might not be carrying over fast enough and almost stalling before the next two fire. Just a thought. will it hold a steady fast idle and it's only at it's "Normal" idle speed that it's erratic, or is it on any even throttle???

Hi marty,
We dont know if it ran on meth, but as a race engine we gather it did, but as the motor was freshend up before it was put back in a frame, not by dad, it was done before he got it, i gather it would have run different carbs in the outfit??? but as he has tried different sets of carbs, the problem is still the same...
And yep, it will hold revs well at a fast idle, just when returned to the stop.
I have said to him, just wind the idle up to around 1800 rpm but he's not happy with that answer!!!

[born green]

Maxumis has thought of something which slipped my gaze. That motor would most likely be tuned to run on methanol as most of the speedway outifts and road outfits did at the time. Perhaps give it a dose of that and see if it runs better. Otherwise, I would suggest that a heavier flywheel might also assist in maintaining crank inertia when at idle. The flywheel would be probably lightened for the outift, add to that the big bang firing order and it might not be carrying over fast enough and almost stalling before the next two fire. Just a thought. will it hold a steady fast idle and it's only at it's "Normal" idle speed that it's erratic, or is it on any even throttle???

Hi marty,
We dont know if it ran on meth, but as a race engine we gather it did, but as the motor was freshend up before it was put back in a frame, not by dad, it was done before he got it, i gather it would have run different carbs in the outfit??? but as he has tried different sets of carbs, the problem is still the same...
And yep, it will hold revs well at a fast idle, just when returned to the stop.
I have said to him, just wind the idle up to around 1800 rpm but he's not happy with that answer!!!

I think ur idea about the flywheel, and the big bang firing order is prob closest to the problem, orther than the idle, he says it runs and sounds great, picks up revs real quick and clean..

[Head_Mechanic]

This motor probably was never meant to idle . Being a two pot firring set up and running ? cams it would be like any hot motor only idles, at higher than normal rpm. If at all. If you have ever seen the old speedway slider bikes they don't idle just go like hell.

[MAXUMIS]

Maxumis has thought of something which slipped my gaze. That motor would most likely be tuned to run on methanol as most of the speedway outifts and road outfits did at the time. Perhaps give it a dose of that and see if it runs better. Otherwise, I would suggest that a heavier flywheel might also assist in maintaining crank inertia when at idle. The flywheel would be probably lightened for the outift, add to that the big bang firing order and it might not be carrying over fast enough and almost stalling before the next two fire. Just a thought. will it hold a steady fast idle and it's only at it's "Normal" idle speed that it's erratic, or is it on any even throttle???

Hi marty,
We dont know if it ran on meth, but as a race engine we gather it did, but as the motor was freshend up before it was put back in a frame, not by dad, it was done before he got it, i gather it would have run different carbs in the outfit??? but as he has tried different sets of carbs, the problem is still the same...
And yep, it will hold revs well at a fast idle, just when returned to the stop.
I have said to him, just wind the idle up to around 1800 rpm but he's not happy with that answer!!!

Mate if the motor has not been tuned for street fuel it will not run good at all.Stouch for normal fuel is 14.64 to one and power is 12.9 depending meth is around 7 to one stouch and around 5/6 to one on power it will run like shit if not tuned for the write fuel

[born green]

This motor probably was never meant to idle . Being a two pot firring set up and running ? cams it would be like any hot motor only idles, at higher than normal rpm. If at all. If you have ever seen the old speedway slider bikes they don't idle just go like hell.

I think thats about it to, but the old man is anal about things been right...
I dont know what sort of comp its running, but i bet its high...
Its just that someone put it back in a B2 frame, sometime ago, as a road bike, and thats how the old man got it..
He's never riddin it, as he's not young, and their heavy... its just something he's restoring...

Remember the old jawa 500s, rode one for a couple of years back in 87/88!!!!! scary fucker's

[born green]

Maxumis has thought of something which slipped my gaze. That motor would most likely be tuned to run on methanol as most of the speedway outifts and road outfits did at the time. Perhaps give it a dose of that and see if it runs better. Otherwise, I would suggest that a heavier flywheel might also assist in maintaining crank inertia when at idle. The flywheel would be probably lightened for the outift, add to that the big bang firing order and it might not be carrying over fast enough and almost stalling before the next two fire. Just a thought. will it hold a steady fast idle and it's only at it's "Normal" idle speed that it's erratic, or is it on any even throttle???

Hi marty,
We dont know if it ran on meth, but as a race engine we gather it did, but as the motor was freshend up before it was put back in a frame, not by dad, it was done before he got it, i gather it would have run different carbs in the outfit??? but as he has tried different sets of carbs, the problem is still the same...
And yep, it will hold revs well at a fast idle, just when returned to the stop.
I have said to him, just wind the idle up to around 1800 rpm but he's not happy with that answer!!!

Mate if the motor has not been tuned for street fuel it will not run good at all.Stouch for normal fuel is 14.64 to one and power is 12.9 depending meth is around 7 to one stouch and around 5/6 to one on power it will run like shit if not tuned for the write fuel

It has been rejetted, even when he took the carbys off the other bike, to just see if that solved the problem, it didnt, but yer the carbys that are on now are jetted right i beleive, the old mans very clued up on most things with engines, he's been playing around with this bike for about 5 years now, so a lot of things have been tried.He should just give it to me now... so i can turn it into a hot old track bike,,, idle wont be a problem then

[MAXUMIS]

if a motor is set for meth it WILL not be the only prob(jetting it) comp/timing is way differant as well.Mate your best bet is to find out what has been modded to the motor on it's own.

[Strika]

Mate, if it is a full speedway motor, it will have cams that are way wild. Wild cams don't make for a good idle, which is most likely why it will idle ok at 1800 and not lower. It's as Maxumis says, it's not meant to idle. Even my race bike won't idle. It will fuel up and stall. But that's jetting more than cam grind, as it still has stock, all be it slotted, cams.

But, if he changes cams to get it idling properly, he may have to redo the ignitor as I imagine that has been altered according to the cam grind. But, if he is running little carbs again, the thing is going to run out of puff, just when it should be climbing onto it's fat part of the rev range. A real waste. Perhaps go to some big bore flat slides and a decent pipe and see what it's like?

[MAXUMIS]

Strika is 100% write!

[Six Addict]

Just to out the big bang thing into perspective...

Kawasaki released patents for their own big bang 1000cc engine... It had an electric motor to keep the crank turning over at idle speeds and I dare say that engine wouldn't have had any crazy cams in it... Along with everything Marty and maximise have said (which is mostly over my head)... I can easily why the engine wouldn't idle smoothly

[Strika]

BG, just a thought. Gos is God with this stuff. Not only in terms of his knowledge of the internals, but also his vast knowledge of who built what using what. Why don't you PM him, or put him in touch with yr old man. I reckon your old man would enjoy it as would Gos knowing what yr old man is into. Plus, he may know something about the actual motor. He's a bit freaky with this sorta stuff. Bit like Phil with who won the 1998 Italian SS championship and on what bike with how many wins. Fuggin freaks I tell ya!