GPZ BIG BANG ENGINE

.....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???

Marty and Maximus have hit the nail on the head mate. First thing - being a speedway motor, you can bet your house it was built to run on methanol. That means really hi-comp (maybe up around 13-14:1). Someone who can build cams must have done up a set to suit the big-bang configuration - I would be checking with someone like Ivan Tighe in Qld about the cams - he has been doing cams for 40-odd years and may know something about the cams in this motor ??.....

big lumpy cams and hi-comp, the idle on this motor when it was in an outfit burning methanol, would be well up over 1500rpm and probably closer to 2000rpm - they just are not designed to idle down low cleanly at 1100 rpm like a stocker.

remember that even on a stock Z motor, 1&4 and 2&3 cylinders fire at the same time - but it is a 'wasted' spark arrangement where the spark fires on the exhaust stroke of the second cylinder in each pair (ie as #1 fires on the power stroke, #4 will also be firing, but on the exhaust stroke, where it makes no difference)......Has your old man split the cases to check out the crank mods himself ? Is he 110% sure it is a big-bang motor ?

Good luck with this one, it sounds like a really interesting piece of kit alright !!

Thanks for all the help guys.

My knowledge on these engines is very rusty.
spoke to the old man, 1st time for a while, and the news is not good, he's not well...and not going to get better..
So this has been put on hold, as with the rest i now beleave, but he wants to finish the XS650, but i think he has his hopes up to high...
1and 4 do arrive at tdc together,as do 2 and 3,
When he spoke to me a couple of years ago about cam timing, i can remember him saying that it had different marks on the cam gears, and the cut outs on the cam ends didnt line up with the head.
I asked if the cam cover had been ground out to take high lift cams, he cant remember taking any notice as he was unaware that was the case with high lift cams.
My drag bike back in the 80s, ran from memory, 780 tho lift cams, and 520 dec duration, and we had to grind the cam cover..
We beleive the engine was built by a guy in dalby, west of brissy,
As he has never had experience with these motor's. is into old english twins, hence the reason for the interest in the XS650, he prob doesnt understand how these high comp, big cam engines work.
But anyway, it runs real well when the revs are up, so i gather the motor's fine.
I told him to give it to me and i will turn it into a old school superbike..... dont think he approves of that
that may have to wait untill...................
Anyway i will see what happens... as whats wrong with dad will only get worse, not better, but he is having a hard time accepting it.... spoke to my sister at length about it...

Anyway thanks for ur help and idea's.... this bike will live again... will just be a little while

Hey mate no good to hear about ur dad. Make sure you spend as much time as possible with him.

.....1and 4 do arrive at tdc together,as do 2 and 3......

yep, thats normal. The cam marks will refer to modified cam timing needed for running methanol. If the inlet cam has been advanced a bit - then the cut-outs won't line up with the head like they do in OEM configuration.

Here is a good .gif image of a 4-cyl motor with 1-2-4-3 firing order.



Easy to see how 1&4, and 2&3 rise and fall 'together'- but the cam and ignition timing controls the actual firing order of 1-2-4-3. You can also easily see from this, that if the motor had been re-configured to a big-bang - then some serious cam mods will have had to be done. The crank balancing and other related issues are really quite complex - I would love to see a photo of the cams if they have been re-phased to 90deg or 270deg !!

All the best with your old man though, that is far more important than getting this old bike sorted.

Last edited by Gosling1 on Tue Jan 03, 2012 6:51 pm, edited 4 times in total.

best wishes to your dad brother very sad to hear.

Cheer's guys.
AS i live in melbourne and he's in outback QLD, i havent spent much time with him over the last 13yrs..
My sister who lives about 3 hours away is going to take care of him.... they have allways been close.

Thanks again...

Hey Gos is 1243 common? I thought most were 1342??????

.....Hey Gos is 1243 common? I thought most were 1342??????.....

Common as - it is the standard firing config for every 4-cyl jap bike I have ever worked on mate. Pretty sure it is standard on every 4-cyl Jap bike ever made from day dot to now?

1-3-4-2 is used in some old pommie 4-cyl car motors, but thats about it. I don't know why they would bother really, as this config introduces some weird harmonics/balance issues. Maybe not such a problem in a car motor I guess.....

It is 1-2-4-3 in across the frame 4-cyl motors because this config gives the best primary balance - basically you have a pair of 180-deg twins next to each other. Any of the secondary balance issues - what is known as a 'rocking couple' - are pretty much eliminated through the use of counterweights in the crank webs. Normal 180-deg 4-cyl cranks do not have a primary rocking couple - if the weights in the crank webs are correct, they don't need a balance shaft either. That is why you don't often see a balance shaft in a straight-4 motor, but will see them in other types of engine designs - mainly V twins or singles, or other 4-cyl configurations (some V-fours etc).

The best design crank is a 120deg triple

Same reason a V6 needs a good deal of balance shafts. A straight six is a much betterer design in my opinion. Good balance, smooth....silky smooth....ooooh....the slippery fast revving small capacity in line 6 mmmmmmmm.........M3............VLturbo..........Skyline.........

Same reason a V6 needs a good deal of balance shafts. A straight six is a much betterer design in my opinion. Good balance, smooth....silky smooth....ooooh....the slippery fast revving small capacity in line 6 mmmmmmmm.........M3............VLturbo..........Skyline.........

Tickford enhanced XR6 u forgot that one mate..