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

[bottobot]

Well I can get it started and running and it sounds good, but it will not idle for love nor money. I've tweaked the carbs a million ways to Sunday and there doesn't seem to be a combo that works. Right now the pilot air is at 2 full turns out and the fuel is at 1 1/4 out. That seems to work to get it started, but not idling. I'm getting pretty frustrated and defeated.

Put new grips on, fixed the turn signal and put new brake oil in the front today which is some positive movement forward at least.

[BigD_83]

I bet that once you get the valves sorted out it'll run well enough to move forward. Do you have feeler gauges and any spare shims?

[bottobot]

No spare shims but I do have feeler gauges. I could definitely ride it around today if I wanted to take it off lift. Tomorrow is valve clearance day!

[BigD_83]

No spare shims but I do have feeler gauges. I could definitely ride it around today if I wanted to take it off lift. Tomorrow is valve clearance day!

I can also bring some shims and a micrometer over for you so you don't have to leave the buckets empty when you do your inventory. I don't have any that are particularly thin, but even one spare is better than none.

[crapwacker]

I have to ask, did you replace the orings between the carb mounts and the head?

[bottobot]

I have to ask, did you replace the orings between the carb mounts and the head?

I did, the old intake boot o-rings were garbaged so I'm glad I took care of that.

[bottobot]

So BigD_83 came by this morning and we had a nice chat and he left me with some tools. I started with a compression test and Oh My Goodness am I glad I did.

Cylinder 1: 0 PSI, Cylinder 2: 150 PSI, Cylinder 3: 30 PSI, Cylinder 4: 60 PSI

So this may completely explain why it won't run at idle. Because it can't possibly with compression that low! Now its time to order the piston ring set and take the head off again. ****...

[Nessism]

Cylinder 1: 0 PSI, Cylinder 2: 150 PSI, Cylinder 3: 30 PSI, Cylinder 4: 60 PSI

So this may completely explain why it won't run at idle. Because it can't possibly with compression that low! Now its time to order the piston ring set and take the head off again. ****...

Sorry man but this thread reads like a train wreck...

You pulled the head but didn't bother to check the valves nor replace the valve stem seals.

You slam the engine back together with old rings and didn't even bother to check valve clearance.

Did you at least adjust the valves before performing the compression check?

BTW, if those are Yoshimura pistons they may take special rings. You first need to understand if the engine is over bored (measure the bore diameter), then go from there to see if you can get rings.

[bottobot]

Sorry man but this thread reads like a train wreck...

You pulled the head but didn't bother to check the valves nor replace the valve stem seals.

You slam the engine back together with old rings and didn't even bother to check valve clearance.

Did you at least adjust the valves before performing the compression check?

BTW, if those are Yoshimura pistons they may take special rings. You first need to understand if the engine is over bored (measure the bore diameter), then go from there to see if you can get rings.

It is a train wreck.... I tried to cut corners because of a lack of proper tools. It must be frustrating to read. I am just a noob trying to learn and each bike is a serious challenge for me but I learning and trying to move forward. I am going to start over and do all the things you have mentioned. I thought I was going to be able to just solve a couple of problems and do a major carb clean but it looks like I'll have to go further than I anticipated. I appreciate your input and advice.

[bottobot]

Measured the cylinders like Ed said I should. Get your 'I told you so's in dude I deserve em! :P They are 70mm so thats a full 5mm upgrade. Somebody definetly went through some effort to make this baby scream at some point in its life. Any ideas where I should order some rings from? Not even sure what to order to be honest.


Double checked the piston cuz I had the tool out might as well. Yup they are 70mm too.


Here is the piston. No idea if what POPY means, but I am guessing thats an OEM piston and not a yoshimura or a wiseco.


Top view of the pistons. They were all oily when I pulled the head off, so gaskets weren't the problem of plug fouling obviously if its still happening.

[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.

[Mekanix]

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.

[Mekanix]

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.