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

  • Thread starter Thread starter bottobot
  • Start date Start date
Not gasket thickness, but bore diameter. If you put a stock GS750 head gasket with a 65mm bore on those 69mm cylinder you will have problems.
 
IF you need to get any machining done, I can recommend the guys at Alec's Automotive. Good shop, decent rates.
 
TeamDar said:
Not gasket thickness, but bore diameter. If you put a stock GS750 head gasket with a 65mm bore on those 69mm cylinder you will have problems.
Click to expand...

Ah ok.. I realize what you mean now. I just checked the old gasket and its a 65mm so I think I'm safe. Thanks for the good tip though. Always better to be safe than sorry.
 
BigD_83 said:
IF you need to get any machining done, I can recommend the guys at Alec's Automotive. Good shop, decent rates.
Click to expand...

I've heard good things about those guys. Paul at Motomethod recommended them to me. When I do my next teardown and resto/mod of this bike I am going to get the to do a few things for me.
 
Put it all back together today and it won't start. ****....

I'll pull the carbs off tomorrow and double check all the settings.
 
So I've checked a bunch of different settings and tried a variety of configurations. No luck. The bike turns over but doesn't fire except when it makes a hella loud backfire. Had to check my ears once to see if they were bleeding. A backfire in an enclosed space is no party.

I'm pretty stuck here. I've check and re-checked the tutorials and pretty much ruled out everything I can. There is spark, I am pretty darn certain there is compression but I will double check it later, and there is definitely fuel and air but I have no idea if I have the right settings.

If anyone wants to chime in their 2 cents I am happy to hear it. Its a 4-1 exhaust with #127 main and 17 pilot with pods.

Before you give me grief about the set up please be kind, the PO did this and if I could put it back to stock I would in a heartbeat.
 
bottobot said:
So I've checked a bunch of different settings and tried a variety of configurations. No luck. The bike turns over but doesn't fire except when it makes a hella loud backfire. Had to check my ears once to see if they were bleeding. A backfire in an enclosed space is no party.

I'm pretty stuck here. I've check and re-checked the tutorials and pretty much ruled out everything I can. There is spark, I am pretty darn certain there is compression but I will double check it later, and there is definitely fuel and air but I have no idea if I have the right settings.

If anyone wants to chime in their 2 cents I am happy to hear it. Its a 4-1 exhaust with #127 main and 17 pilot with pods.

Before you give me grief about the set up please be kind, the PO did this and if I could put it back to stock I would in a heartbeat.
Click to expand...
Those numbers for your main and pilot seem about right based on what I've gleaned from other people's experiences. Maybe MisterCinders would be able to provide more insight. He's done extensive work on his '78 750 with 4 into 1 and pods.

What are the valve clearances at the moment?
 
That's next up. After dialing the pilot air and fuel screws way out I'm getting really close. It actually fired up for a brief second. I am off to Lordco to get a battery as the battery I inherited is pretty weak it barely starts the bike even while connected to a charger
 
bottobot said:
That's next up. After dialing the pilot air and fuel screws way out I'm getting really close. It actually fired up for a brief second. I am off to Lordco to get a battery as the battery I inherited is pretty weak it barely starts the bike even while connected to a charger
Click to expand...
Before you hit Lordco, call Modern. Murray has/had motorcycle batteries at a pretty decent price.
 
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.
 
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?
 
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!
 
bottobot said:
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!
Click to expand...
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.
 
I have to ask, did you replace the orings between the carb mounts and the head?
 
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. ****...
 
bottobot said:
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. ****...
Click to expand...


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.
 
Nessism said:
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.
Click to expand...

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.
 
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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.
 
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