1981 GS 750 cam chain broke in 1989

[GregT]

Back in the day,Paul, Suzuki NZ issued a service bulletin for these motors recommending shimming the oil pressure relief valve spring with two 6mm spring washers.
This involves removing the circlip which retains the spring and it's existing washer, placing the additional washers on top and reinserting the circlip.
Prudence would dictate cleaning and checking for free movement of the valve while you're in there...
I can't give you the specs for free length of the OE spring sorry - I've never seen a spec for it but someone here may have that info.

[laverda1200]

1981 GS 750 cam chain broke in 1989

Hi Greg

thanks for that.

I attach a close up of the oil pressure relief valve from the 750 engine I am working on.

It looks to me like a simple pressure relief blow off valve, the oil pressure coming in the end with the plunger inside lifts the plunger off its seat, allowing oil to flow out through the holes in the sides of the external casing, reducing the oil pressure flow from the oil pump.

so, adding a couple of washers on top of the spring, under the circlip, will require more pressure to lift the plunger, keeping the oil pressure higher.

the little spring is quite stiff already, on this 750 engine after 31,000 kms of useage, the spring free length is 27.8 mm, useless info without knowing what factory spec should be.

thanks for the help

Paul

[laverda1200]

a photo of the current state of the engine build, waiting for a new cam chaina nd a gasket set, both still on order....

Paul

[chirobo]

YOU ARE MY HERO!! (I want to be able to do that...in reverse, of course!!)

[laverda1200]

laughing out loud...... once you have done this a few times (OK, maybe many times) it gets easier. Just pick up your tools, if possible for your first time find an experienced friend who actually knows what he/she is doing, and go for it!

what you see on the table took less than 3 hours from starting to remove the engine from the motorcycle, to its current state, but I cheat a bit and use air tools as appropriate.

actually asembling the engine and timing the cams is straightfoward, the real trick is to meticuously clean all gasket surfaces, properly measure bearing clearances with PlastiGuage, meticuously fit the ppston rings to appropriate end gaps in the cylinder bores, and use a highly qualified machinsit to do cylinder boring and honing, clearancing pistons to bores, and replacing or relining valves guides/recut valve seats, and it is very possible for the home garage mechanic to do a truly high quality job and be confident in knowing absolutely everything is correct.

you actually don't want to copy what you see in the photo, less experienced home mechanics should carefully organize and label and shoot a ton of photos as they go, labelled ziploc freezer bags are a great idea, I have just done so many motorcyle engines now by age 57 I have gotten a bit lazy from knowing exactly how everything comes apart and goes together.

Paul

[hampshirehog]

The 750 you've got is a really good engine - it is only crap maintenance that brought them down (or overloading the top end). It is the smoothest running motor of the range by a mile - really silky when compared to the roller bearing motors.

[chirobo]

I always wondered where the coffee cup went in an engine....I know you'll show me later....

[dueller]

Subscribed!

So it's not possible that he WAS going 160 at 13Krpm's? Sounds feasible if the engine didn't detonate....oh wait it did! lol I mean top end on my 700EF is 140 at I would assume redline which is 10Krpms...just sayin...Might have hit it for a few seconds

[laverda1200]

So it's not possible that he WAS going 160 at 13Krpm's? Sounds feasible if the engine didn't detonate....oh wait it did! lol I mean top end on my 700EF is 140 at I would assume redline which is 10Krpms...just sayin...Might have hit it for a few seconds

laughing out loud again. The previous owner was very animated telling his story, lots of beer bottle wavin in the air, various engine noises being made, and quite insistent about the 160 mph and the 13,000 rpm, especially when I said "really, are you sure" and he says yepp'ers she was a wailin

I was just happy to trade a bottle of whiskey for the bike, I wasn't going to challenge a good story and **** him off......

Paul

[chirobo]

and the reason it blew was the low tension on that oil pressure relief spring!!! lol...I don't think I'll bother messin' with the washers in my 750t....I'll just keep it below 110mph!!

[dueller]

and the reason it blew was the low tension on that oil pressure relief spring!!! lol...I don't think I'll bother messin' with the washers in my 750t....I'll just keep it below 110mph!!

You understand why the mod right? Increasing the pressure on that spring makes the oil go thru the passages and not blow off (back to the sump I assume?) creating a lack of lubrication up the chain of oil passages. Not trying to be condescending.....just makin sure you understood the idea

[laverda1200]

the full engine gasket set and the new cam chain I ordered finally arrived today. The new cam chain is a Japanese manufactured 120 link endless "DID" Daido Bush Chain, (DID219FTS(S)120L) and the cam chain box has a large warning on it all in capitals as follows:

CAUTION: DO NOT INSTALL THESE ENGINE CAM CHAINS BY YOURSELF. BE SURE TO ASK YOUR LOCAL DEALERS.

then goes on to say that incorrect fitting is excluded from DID's warranty

I was laughing out loud....

Paul

[chirobo]

You understand why the mod right? Increasing the pressure on that spring makes the oil go thru the passages and not blow off (back to the sump I assume?) creating a lack of lubrication up the chain of oil passages. Not trying to be condescending.....just makin sure you understood the idea

Got it...but I thought that the low pressure was only a problem with un-maintained engines...extended oil change interval specifically...I live to change my oil out!! so r u going to get a new spring and shim that??

[laverda1200]

Hi all

new cam chain and full gasket set and full seal set all just arrived, so beginning reassembly of this engine which hasn't run since the cam chain snapped in 1989.

and yes, I do know the manual calls for assembling the transmission and crank into the upper engine case then installing the lower, I simply find it easier and neater to lower the very light weight upper engine case onto the lower case. The only "trick" is to install the cam chain guide and rubber cam chain guide isolator bushes a couple of hours earlier into the upper case, with a dab of silicone on each of the two rubber isolator blocks so nothing falls out when lowering the upper case....

Paul

[Nessism]

Did you remove the rods from the crank and look at the bearings? Highly recommended if you didn't. Also, I'd plastigauge the rods at least to make sure the bearing clearance is to spec.

Oh, and did you get an OE gasket kit?