How far down the rabbit hole will I go... `78 GS750

[Tim Tom]

You may want to check the diameter of those pistons and maybe the lift on the cams. Those look like Yoshimura pistons. Do they say POP Y on the side next to the wrist pin bore. The pistons look to have more dome than stock. If you have a stock head gasket it may not fit the bore. The cams look like they have some serious lift also. You just may have yourself a hotrod.

Pop Y stands for Pops Yoshimura. As TeamDar mentioned earlier in this very thread. It looks like the PO has indeed made it a bit of a hot rod.

These bikes are easy to work on, but you can't cut corners. That never ends well. Take the time to do the job right and you'll soon have a great motorcycle. Seriously listen to the advice here, guys like Nessism can probably rebuild a GS motor in their sleep.

 

[DannyMotor]

70mm? From a stock 749cc that'll make it damn close to a 770cc! Nice! Those pistons and rings look brand-new, and look to be Yoshi (sized up of course).

The other thing... was this bike running before you did the compression test?

It should've been at operating temperature (260-300 degrees) with the throttles held WIDE open, after a few seconds of cranking they should all have been between 110-140psi for a bike in decent running order.

On a cold motor, you can expect to see anywhere from 50-90psi at best.

How do the skirts of the pistons look? (The lower part under the rings.) If there is any scoring, or 'hot' marks, you'd have a compression leak... what about the inside of the jugs? It may not be the rings, but a hack-job bore with uneven surface area...

Take a metal ruler (or something VERY true, and very flat) and run it around the inside of the cylinders... the ruler should touch the cylinder wall from top to bottom with no gaps all the way around... if there's a gap there, your cylinders are toast.

 

[Guest]

70mm? From a stock 749cc that'll make it damn close to a 770cc! Nice!

What ?

56.4mm stroke
70mmm bore

=868.2cc

I thought the cylinder block could have been an 850 block but looking back a few posts, Those cylinder walls look really thin.

How thin are they ?

I believe stock cylinder walls are normally in the 4-4.5mm thick range. So if 5 mm were taken out of it, lets say 2.5mm of sidewall was taken out to make it easyer, than there should be is 2mm left.

The middle of the sleeve is fatter than the bottom of it. It tappers up just slightly after it goes into the block.

 

[bottobot]

What ?

56.4mm stroke
70mmm bore

=868.2cc

I thought the cylinder block could have been an 850 block but looking back a few posts, Those cylinder walls look really thin.

How thin are they ?

I believe stock cylinder walls are normally in the 4-4.5mm thick range So if 5 mm were taken out of, lets say 2.5mm of sidewall was taken out, than I'm there is 2mm left.

The cylinder walls certainly look like they have more than 2mm left on them. I'll measure it tomorrow when I get in the shop. Either way, holy ****. No kidding this is a hot rod in disguise. The disguise being rust of course. Now if I can just figure out how all this oil is getting in there.

 

[Guest]

The stock head gasket is 65mm.

You should have a 70mm head gasket for that . I don't like the idea of 2.5mm of thin gasket hanging around inside the cylinder.

Cometic can make one for you that matches the thickness that you have.
I wonder what the compression ratio is on this with those pistons and that new bore size.

Does the block say 750 or 850cc?

I wonder if its an 850 head?

I'm not an expert on those two engines I'm just trying to understand what someone would do in order to put those pistons in a 750 engine.
It would change the compression ratio way too much unless an 850 head was used.
(I'm assuming the combustion chamber is slightly larger on the 850 and a 750 would be too much compression.)

On your to-do list I would suggest:

New rings for the pop's
Quality valve seals
lap valves if needed
valve shim's adjust
hone cylinder slightly
New head gasket with 70mm bore.
New oil stud seals all around but you already have those to go in.
Check head for Trueness with a good metal straight edge
Check block for Trueness

When your done there is a specific way to wear the new rings in before you hammer on it.

There are gaskets out there that work ok but eventually fail. OEM seem to be the only frontrunner for quality,durability and no leaks.

 

[bottobot]

Oh another head shaker you guys are gonna have is when I tell you that I painted my exhaust. Oh don't worry it desperately needed paint. It looked like real dogfood. Guess what I didn't do before I painted it? No, I cleaned it. No, I sanded the heck out of it. Yes, I didn't read the darn instructions. After I let it dry I thought I would sand it and then give it another coat for good measure. As I was sanding all the paint just came dusting off. Oh ****.. what did I do wrong? Well after reading the back of the can apparrently with paint containing a ceramic component you need to bake the **** out of it. Cool it off and repeat in order for it to "cure" properly.

Well thats another mistake I won't be making again. Been chalking up quite a few of those.

 

[Guest]

lol I'm sure you'll do a great job next time lesson learned

 

[bottobot]

The stock head gasket is 65mm.

You should have a 70mm head gasket for that . I don't like the idea of 2.5mm of thin gasket hanging around inside the cylinder.

Cometic can make one for you that matches the thickness that you have.
I wonder what the compression ration is on this with those pistons and that bore size.

On your to-do list I would suggest:

New rings for the pop's
Quality valve seals
lap valves if needed
valve shim's adjust
hone cylinder slightly
New head gasket with 70mm bore.
New oil stud seals all around but you already have those to go in.
Check head for Trueness with a good metal straight edge
Check block for Trueness

When your done there is a specific way to wear the new rings in before you hammer on it.

There are gaskets out there that work ok but eventually fail. OEM seem to be the only frontrunner for quality,durability and no leaks.

Ok thanks for the to-do list. Going to start knocking this out tomorrow.

Quality valve seals - I have some from a gasket set I ordered but I mangled the gaskets taking the head off so I need a new valve cover and head gasket anyways. Do you have a good source for a gasket set that includes the valve seals?
lap valves if needed- now that I have some time as I am waiting around for parts I am going to take my head into Alecs Automotive for a lap dance. I've had multiple recommendations that they are the best around for that kind of thing.
valve shim's adjust- Pretty sure I can do this myself
hone cylinder slightly - Don't have a honer but I can get use one at Motomethod
New head gasket with 70mm bore. Cometic can make me one? Does he have a site or should I contact him through this one?
New oil stud seals all around but you already have those to go in. - I think you mean those 2 triangle'ish o-rings that go around the base of the cylinder studs, if so then yes they've already been replaced with new.
Check the head, cylinders and base for trueness. - Can definitely do this myself.
Oh you made an edit... it does actually say 748cc on the head so its definitely not an 850 block. That was my first thought too, that maybe there had been an engine swap or something fishy. Are they even interchangeable? Kinda rad if they are.

 

[bottobot]

Pop Y stands for Pops Yoshimura. As TeamDar mentioned earlier in this very thread. It looks like the PO has indeed made it a bit of a hot rod.

These bikes are easy to work on, but you can't cut corners. That never ends well. Take the time to do the job right and you'll soon have a great motorcycle. Seriously listen to the advice here, guys like Nessism can probably rebuild a GS motor in their sleep.

Hey Tim Tom I give big respect to the experience and knowledge here. I only cut corners as I didn't think the bike merited so much work. Turns out I was wrong and so now I am going to do exactly what I'm told cuz I know you guys know your stuff. I very much apreciate people going out of there way to help people like me. In turn I go and help others. A few weeks ago there were some young dudes out in the alley behind me having major troubles with their old crappy bikes. I was able to help them out quite a bit over the last couple of weeks. Showing them how to service and repair their new toys. In turn they pass that knowledge on to their friends. They think I know everything :lol:
So please trust that your advice and information does not fall on deaf ears. That ultimately I am like you, I like helping others with their bikes. While I can't repay you guys here, I try to pass it on to others less knowledgeable than me. These guys I met really have no idea how much help I've received. They just shake their heads when I turn down their offers of payment. I tell em the best payment is seeing them super stoked when we go for rides and their bikes don't break down

 

[Guest]

Do you have a good source for a gasket set that includes the valve seals?


hone cylinder slightly - Don't have a honer but I can get use one at Motomethod


New head gasket with 70mm bore. Cometic can make me one? Does he have a site or should I contact him through this one?

Oh you made an edit... it does actually say 748cc on the head so its definitely not an 850 block. That was my first thought too, that maybe there had been an engine swap or something fishy. Are they even interchangeable? Kinda rad if they are.

I know some people have swapped the 850 top end onto a 750 base

There is a little bit of the case that has to be cleaned up so the sleeves drop in.

Cometic is the name of a company.
http://www.cometic.com/ They probably have your gasket template on file and make one from scratch for you. You can get it made out of the regular fiber gasket or out of coper. there is a $20 difference between the two but the coper is reusable.

I'd stick with OEM for just about any gasket and seal.

Why not just bring the block with you to Alecs while you have the valves done. I'm assuming since they do engine work that honing would be simple for them and its up to you weather or not you want to spend the money getting and owning the tool or having them do it.

 

[DannyMotor]

What ?

56.4mm stroke
70mmm bore

=868.2cc

I thought the cylinder block could have been an 850 block but looking back a few posts, Those cylinder walls look really thin.

How thin are they ?

I believe stock cylinder walls are normally in the 4-4.5mm thick range. So if 5 mm were taken out of it, lets say 2.5mm of sidewall was taken out to make it easyer, than there should be is 2mm left.

The middle of the sleeve is fatter than the bottom of it. It tappers up just slightly after it goes into the block.

Ok damn. I thought it was a 49-51mm stroke. Why not find an 850 cylinder and re hone it to match the pistons?

 

[TeamDar]

People put 70mm pistons in the stock GS750 sleeves. It made it 870cc. R.C. Engineering and MTC Engineering both made kits that bored into the stock cylinders. I was aware of Yoshimura making a 69mm kit for 850cc. I'm not saying that they didn't make a 70mm, just that I have never seen one. Please measure your pistons carefully at the bottom of the skirts to see what you have.

 

[Guest]

http://www.suzukicycles.org/GS-series/GS750.shtml
[FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3]
GS750 specifications
Overall length: 2,225 mm (87.6 in.)
Overall width: 870 mm (34.3 in.)
Overall height: 1,170 mm (46.1in.)
Wheelbase: 1,490 mm. (58.7 in.)
Ground clearance: 150 mm (5.9 in.)
Dry weight: 223 kg (492 lbs.)
Fuel tank capacity: 18 l (4.8 gal.)
Reserve: 2 lt. (0.5 gal.)
Engine type: Air cooled inline-4, DOHC, 2 valves per cylinder
Bore x stroke 65,0 x 56,4 mm
Piston displacement: 748 cc
Compression ratio/test range: 8.7 :1 / 128?171 psi (15 psi var.)
Maximum opwer output: 72 hp/ 8,500 rpm
Maximum torque: 60 Nm/ 7,000 rpm
Carburetors: Mikuni VM26SS x 4
Carburetor settings: 1000 rpm. pilot air 1 1/4 T.
Carburetor float height: 0.98?1.1 in.
Main/pilot/needle/jet: 105 /22.5 /P-1 /5F21-3
Starter system: Electric and kick start
Lubrication type: wet sump, 3,400 cc. 10W-40 (20W-50) preferred
Oil pressure: 1.42 psi @ 3,000rpm.
Clutch type: Wet multi-plate
Transmission: 5-speed
Primary reduction: 2.152 (99/46)
Final reduction: 2.733 (41/15)
Gear ratios:
Low 2.571 (36/14)
2nd 1.777 (32/18)
3rd 1.380 (29/21)
4th 1.125 (27/24)
5th 0.961 (25/26)
Top speed: 125 mph
Drive chain: 630SO 96 link, O-ring .8 - 1.2 in. free-play
Steering angle: 40 degrees
Caster: 63 degrees
Trail: 107 mm (4.21 in.)
Turning radius: 2.6 m (8.5 ft.)
Front brake: Disc
Rear brake: Disc
Front tyre: 3.25H X 19 4pr.
Rear tyre: 4.00H X 18 4pr. 120/18
Ignition type: Battery/coil
Ignition timing: 17? BTDC at 1500 rpm
Points settings: 0,35 mm / .014" point gap
Spark plug type: NGK B8ES
Spark plug gap: .024?.028"
Battery: 12V 14AH/10 hr.
Generator: 3 ph. AC


Yeah I just threw the numbers in the engine calculator and 862 came out.



You might notice that when you measure your pistons that the skirt is wider than the top of the piston.

The place the measure a piston is .5" from the bottom of the skirt.

SO it might be 70.5mm there and measure 69 on top. (depends on the type of alloy the piston is made of and if its forged or cast) Its for thermal expansion.

My JE's are almost a full mm smaller at the top because of the low silicon content. They expand allot. But others just expand a bit and the piston to cylinder tolerances are closer.

While your at the shop maybe bring a piston and get them to accurately measure the piston to wall clearance, Just so you know what it is.




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[bottobot]

Regarding the pistons, I measure the top of the piston as you can see in the pics, however I also measure the inside of the cylinder wall. Both came to 70mm. If the piston is smaller at the top and bigger at the bottom, what significance does that have as far as maintaining the bike? Or is it more for curiosities sake. I'll measure it all tomorrow and let you know what I find.

 

[Guest]

Depending one the pistons, you need a certain amount of piston to cylinder wall clearance. Just so you don't hone it out too big and go past that clearance range.

 

[DannyMotor]

Did you check your sleeves for any wave in the hone yet?

 

[bottobot]

Haven't yet though I've confirmed the pistons are 70mm and top and bottom measurements.

Other than that I've been too busy destroying my cylinders to do much else. Follow this link to see my embarrassment.

 

[bottobot]

So I've been spending some serious time with my good freinds the cylinder heads. We splashed around in the sink for a few hours yesterday and another couple of hours today. I went to the store and picked up a variety of solvents. Which you can see in the pic here.



This is one of many stages of cleaning her out.



Now the valves are drying out while I do the other half here.


and another pic for good measure.


Girlfriend started to get noisy about how much it stank in the house today. Empty all six cans of that stuff spraying all the passages inside and out. The end result is very positive. Thanks for giving me the direction and confidence to make a good start you guys.

After I was out of my own solvents, I took it down to the local shop and used their solvent tank to do another hour of cleaning. I'm down to the last little stubborn bit where there are some tiny granules still hanging around.

No worries as tomorrow I plan on pressure washing and then dish-washing them.

I also checked the freeplay on the valve guides. Quite wobbly which I had confirmed by the veteran mechanic. So off to the shop to order some Suzuki valve guides and Suzuki brand guide seals as per a previous recommendation.

What's funny is that the title of this post is now more apt than I ever thought it would be. I was debating with myself how much service, in terms of cosmetics I was going to do, and now I find myself so much deeper into this engine than I ever imagined. It's been quite a learning experience so far.

One last thing to mention, not only does this bike have pods, 4-1 aftermarket exhause, jetted carbs and an insanely bored out 864cc engine. It also has a dyna RPM limiter and Dyna S ignition system.

Someone once upon a time made a serious effort with this bike so I am really looking forward to seeing how she performs when she's running right again.

Todo list:

• fork seals
• brake pads
• fork oil
• cylinder overhaul - valve guides, planing and trueing, valve seals, and valve adjustment
• turn signal switch and front brake light switch replacement
• soda blast cylinders
• paint cyilinders

 

[Guest]

Girlfriend started to get noisy about how much it stank in the house today.

I also checked the freeplay on the valve guides. Quite wobbly which I had confirmed by the veteran mechanic. So off to the shop to order some Suzuki valve guides and Suzuki brand guide seals as per a previous recommendation.

LoL its one of those things you do when she's not home and clean up the evidence before she gets back


There are better guides than stock

http://valvetrain.kpmivalvetrain.com/category/e-valvetrain-components-suzuki-gs%E2%84%A2-750-br-pre-1980


I believe they have a dealer out west near you.

 

[bottobot]

Well she was fine with the first day of it but the second day was a bit much :lol:

I wish I'd known about that site first. I've already paid for the seals and guides. Something I still need to replace is a cup/washer that sits at the bottom of the valve spring. Any suggestions on where I would find one of those? It was missing from the exhaust side of cylinder one. I guess the PO at some point felt he could do without. No amount of shimming could offset that kind of problem.