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Things that run off engine intake vacuum:
Carburetor, vacuum controlled fuel tap, vacuum controlled ignition advance, vacuum pulse powered fuel pumps and test equipment.

Things that need to be vented to atmospheric pressure in order to function:
Your fuel tank, carburetor float bowl, engine crankcase, hydraulic reservoirs, lead-acid battery, engine air intake and exhaust.

This applies to All 4-cycle piston reciprocating engine powered motorcycles and it makes zero difference who built the bike or when, it's physics.
 

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They are most likely tubes used for carb syncing
 
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Things that run off engine intake vacuum:
Carburetor, vacuum controlled fuel tap, vacuum controlled ignition advance, vacuum pulse powered fuel pumps and test equipment.

Things that need to be vented to atmospheric pressure in order to function:
Your fuel tank, carburetor float bowl, engine crankcase, hydraulic reservoirs, lead-acid battery, engine air intake and exhaust.

This applies to All 4-cycle piston reciprocating engine powered motorcycles and it makes zero difference who built the bike or when, it's physics.
My point was; I'm not familiar with that bike, I didn't bother expanding the pictures to figure out where the lines were coming from, and that there are vent lines on carbs too.
 

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1977, Yamaha, XS750
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Discussion Starter · #24 ·
Things that run off engine intake vacuum:
Carburetor, vacuum controlled fuel tap, vacuum controlled ignition advance, vacuum pulse powered fuel pumps and test equipment.

Things that need to be vented to atmospheric pressure in order to function:
Your fuel tank, carburetor float bowl, engine crankcase, hydraulic reservoirs, lead-acid battery, engine air intake and exhaust.

This applies to All 4-cycle piston reciprocating engine powered motorcycles and it makes zero difference who built the bike or when, it's physics.
I went out and spent around 400$ on new air filters, new spark plugs and oil and oil filter. I bought vacuum caps and plugged each of those ports and the service manual says that those are for carb synchronization? But each are now plugged and I also purchased new fork seals and went to Sierra Motorsports in Grass Valley CA and am going to pay them to fully rebuild my forks and I brought up the fork bushings and they said they will check and replace them also. I cannot for the life of me find fork dust seals? Yamaha discontinued them and I found some on eBay from All Balls Seals and idk if I should buy them or just order gaitors? The inner fork tube is 35 mm OD also. I also want to thank you for all the information you sent to me it was beyond helpful.
 

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most everything i ride leaks oil
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you can find gaiters at motocross shops or off road truck places, where they use them for shocks and steering dampers. you will need gaiters with two diameters- the top is small to match the fork tube, and the bottom is wide to match the slider.

ive seen those bikes fitted with sidecars. its hard to find s frame much newer than them that will take a chair
 

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Fork gaiters will certainly give the seals and stanchions a longer life, but looks wise they're a bit marmite, you either like them or loathe them.
I think that I still have some dust seals laying around somewhere after fitting gaiters to the XS in the pic a, few years ago.
Wheel Tire Fuel tank Vehicle Automotive lighting
 

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1977, Yamaha, XS750
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Discussion Starter · #27 ·
Fork gaiters will certainly give the seals and stanchions a longer life, but looks wise they're a bit marmite, you either like them or loathe them.
I think that I still have some dust seals laying around somewhere after fitting gaiters to the XS in the pic a, few years ago. View attachment 106804
If u have some let me kno and we can figure something out because mine are blown out. What size ID are the bottom of those gaitors in the pic?
 

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If u have some let me kno and we can figure something out because mine are blown out. What size ID are the bottom of those gaitors in the pic?
I've just found the dust covers (think they're the one's) the id's are 36mm & 50mm.
Postage from the UK might be an issue.
 

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1977, Yamaha, XS750
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Discussion Starter · #29 ·
Things that run off engine intake vacuum:
Carburetor, vacuum controlled fuel tap, vacuum controlled ignition advance, vacuum pulse powered fuel pumps and test equipment.

Things that need to be vented to atmospheric pressure in order to function:
Your fuel tank, carburetor float bowl, engine crankcase, hydraulic reservoirs, lead-acid battery, engine air intake and exhaust.

This applies to All 4-cycle piston reciprocating engine powered motorcycles and it makes zero difference who built the bike or when, it's physics.
Just did my oil and filter change and put 20w-50 in instead of 20w-40. I also went to replace the spark plugs but they were already new. While the plugs were out I compression tested the engine. Now I did not do the test at operating temp and I am at around 2500’ above sea level but I’m getting around 90psi in each cylinder. So that’s where my engines running condition is coming into play. I do understand the engine is damn near 50 years old and obviously looking at the bike no one has taken maintenance seriously before me. I also bought Seafoam fuel additive to hopefully help clean the engine and carbs. I found an engine with better compression for around 300$ nd was thinking of buying it and taking this one out and rebuilding it.
 

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3% per every 1000 feet above sea level, there's a formula to calculate it, and if you remove the carburetors to do the test or hold the CV carb slides wide open you might get a more accurate reading.
 

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Discussion Starter · #32 ·
3% per every 1000 feet above sea level, there's a formula to calculate it, and if you remove the carburetors to do the test or hold the CV carb slides wide open you might get a more accurate reading.
When I went to school to get my ASE when we did compression testing we left all the other cylinders spark plugs in and used those recorded numbers. My cylinders psi with the other two cylinders spark plugs removed were 1:90, 2:92, 3:89. With the spark plugs in I was at 1:120, 2:122, 3:120
 

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..............................90 psi is completely normal, when you don't have throttle open
You probably did the test wrong?
At least 2 of the vacuum lines are for fuel taps, one either side.
It's a 'shim over bucket' motor so the valves may need adjustment.
One thing people don't realise with shim OHC motors, the valve seats wear over time but the shim wear isn't measurable so, if valves get 'quiet' it means the clearance has closed up.
It's pretty much the opposite of a screw and locknut valve adjuster system where valves get louder as clearance increases.
A pretty surefire way to know you need a valve adjustment is when bike is incredibly difficult to start but runs well when it's hot and also is much easier to 'hot start' (the alloy head expands way more than steel valve stem so gives clearance when hot)
 

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Discussion Starter · #34 ·
..............................90 psi is completely normal, when you don't have throttle open
You probably did the test wrong?
At least 2 of the vacuum lines are for fuel taps, one either side.
It's a 'shim over bucket' motor so the valves may need adjustment.
One thing people don't realise with shim OHC motors, the valve seats wear over time but the shim wear isn't measurable so, if valves get 'quiet' it means the clearance has closed up.
It's pretty much the opposite of a screw and locknut valve adjuster system where valves get louder as clearance increases.
A pretty surefire way to know you need a valve adjustment is when bike is incredibly difficult to start but runs well when it's hot and also is much easier to 'hot start' (the alloy head expands way more than steel valve stem so gives clearance when hot)
I had the throttle pulled wide open doing each cylinder. And I believe u are tight because cold starts are a pain on this bike but hot starts and idling it runs like a champ besides it being rich
 

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I had the throttle pulled wide open doing each cylinder. ...
It is a CV carb and has 2 throttle valves, you only have control over the butterfly valve but the engine vacuum opens the round slide valve. If you hold the both valves open or remove the carbs completely, you get the highest possible compression for the engine.

In my experience valve clearances tighten with age wear on everything 4-stroke.
 

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Just did my oil and filter change and put 20w-50 in instead of 20w-40. I also went to replace the spark plugs but they were already new. While the plugs were out I compression tested the engine. Now I did not do the test at operating temp and I am at around 2500’ above sea level but I’m getting around 90psi in each cylinder. So that’s where my engines running condition is coming into play. I do understand the engine is damn near 50 years old and obviously looking at the bike no one has taken maintenance seriously before me. I also bought Seafoam fuel additive to hopefully help clean the engine and carbs. I found an engine with better compression for around 300$ nd was thinking of buying it and taking this one out and rebuilding it.
You should test with the engine warmed up and the throttle open for an accurate reading
 
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