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1977 Yamaha XS750
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Discussion Starter · #1 ·
What's up guys!? I'm new here. Hopefully I've got this in the right place. So I got thise bike off of a buddy of mine and knew it would need some work. I wanted something to wrench on...it's been too long! That being said my last bike was an '01 Ninja so once again I'm in a new world with this 77 cafe racer.
176468486_287534492964301_3742427799752832306_n.jpg


At first I couldn't get the bike to run without feathering the throttle so I took the carbs off and apart. The fuel inside wasn't too pretty.I cleaned them and all the parts well via chem dip. I found a bent air needle so I replaced that and a broken needle jet keeper in the middle carb and replaced that. I put it all back together according to specs in the manual.
176567045_132045658906248_6406955781767434599_n.jpg
176530144_138132274958784_2367009164777524494_n.jpg
178916601_918456578914671_5664840274599403391_n.jpg
I then was able to turn it over and adjusted the idle adjustment until it'd hold it's own with my throttling. Then I found one pipe was hot and the others cold. I replaced all three spark plugs on that note and officially had my first crank (that we can record)! lol Anyways, on that, it was running rough...somewhat of a popping kinda noise coming through the back end of the exauhst accompied with small puffs of white smoke. I then started to hear a faint knocking....more of a ticking...coming from the foward part of the engine and had some white smoke up in the area as well. After, I cut it off, I wasn't able to crank it anymore. I'm curious if I've simply knocked more crud loose and should reclean the carbs or if my problem is deeper in the engine. I've gone about as far as I know to go and definitley don't want to spend hours on pointless projects so I decided I'd come here and get some help from the community here to hopefully make my process more efficient. Any and all help would be appreciated. Ive attatched some pictures as well. If there's any further questions that need to be answered I should be around! I gotta say...she sounded strong when I got her running with the new spark plugs in there!

** Also, new petcocks. Airpods have been installed prior to having it running! It did sit for a while before me.

Anybody want to try pointing there finger at a couple things they'd check next?
 

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What's up guys!? I'm new here. Hopefully I've got this in the right place. So I got thise bike off of a buddy of mine and knew it would need some work. I wanted something to wrench on...it's been too long! That being said my last bike was an '01 Ninja so once again I'm in a new world with this 77 cafe racer.
176468486_287534492964301_3742427799752832306_n.jpg


At first I couldn't get the bike to run without feathering the throttle so I took the carbs off and apart. The fuel inside wasn't too pretty.I cleaned them and all the parts well via chem dip. I found a bent air needle so I replaced that and a broken needle jet keeper in the middle carb and replaced that. I put it all back together according to specs in the manual.
176567045_132045658906248_6406955781767434599_n.jpg
176530144_138132274958784_2367009164777524494_n.jpg
178916601_918456578914671_5664840274599403391_n.jpg
I then was able to turn it over and adjusted the idle adjustment until it'd hold it's own with my throttling. Then I found one pipe was hot and the others cold. I replaced all three spark plugs on that note and officially had my first crank (that we can record)! lol Anyways, on that, it was running rough...somewhat of a popping kinda noise coming through the back end of the exauhst accompied with small puffs of white smoke. I then started to hear a faint knocking....more of a ticking...coming from the foward part of the engine and had some white smoke up in the area as well. After, I cut it off, I wasn't able to crank it anymore. I'm curious if I've simply knocked more crud loose and should reclean the carbs or if my problem is deeper in the engine. I've gone about as far as I know to go and definitley don't want to spend hours on pointless projects so I decided I'd come here and get some help from the community here to hopefully make my process more efficient. Any and all help would be appreciated. Ive attatched some pictures as well. If there's any further questions that need to be answered I should be around! I gotta say...she sounded strong when I got her running with the new spark plugs in there!

** Also, new petcocks. Airpods have been installed prior to having it running! It did sit for a while before me.

Anybody want to try pointing there finger at a couple things they'd check next?
Welcome! Neat project. I'm assuming you put fresh gas in it as well? If you put a new petcock in you should have a fresh fuel filter there as well. A couple things come to mind...

What does the inside of the tank look like? If it's full of crud you could be just stuffing the fuel filter full of it and starving it for fuel.
Does it have an inline fuel filter? If not I would install one. They're cheap and an easy way to see if you have crud in your lines.
Lastly, I would pour half a can of Seafoam in your gas. That stuff takes a little time but it works wonders dissolving crud, paint chips, etc. If you did break loose more junk in your carb, it will eat it up and spit it out. I would put the Seafoam in and run the bike for a good ten minutes if you can. This will get it throughout your carb and float bowls.
 

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"started to hear a faint knocking....more of a ticking...coming from the foward part of the engine and had some white smoke up in the area as well. After, I cut it off, I wasn't able to crank it anymore."
So are you saying the engine seized?
Do a compression test if possible, that will let you know where it stands.
 

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1977 Yamaha XS750
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Discussion Starter · #4 ·
Hi @Stablefull ! I did get some seafoam in there so that any running I could get would get that seafoam in there. I haven't checked inside the tank though. I love the idea of on inline filter and man is that easy, between the petcock and carbs yeah? I definitley should look through the tank and give the petcok/filters another once over! Thank you!


@Trials I haven't run a compression test yet and honestly never have. So I'm gonna edumacate myself on that process now so that I can also check that off the list! Thank you!

The good news is I am in no rush, though having my bike running sooner sounds better. However, I can take my time and really take this bike apart and work on it. I'm defintley excited for more oppurtunities to learn with this bike as well!
 

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Discussion Starter · #5 ·
@Trials also sorry, didn't answer your question. I'm not sure I'd quite say seized. It was still trying to turn over at that point it just wouldn't turn over enough to run! Hopefully, that cleared that up. My b!
 

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Hi @Stablefull ! I did get some seafoam in there so that any running I could get would get that seafoam in there. I haven't checked inside the tank though. I love the idea of on inline filter and man is that easy, between the petcock and carbs yeah? I definitley should look through the tank and give the petcok/filters another once over! Thank you!


@Trials I haven't run a compression test yet and honestly never have. So I'm gonna edumacate myself on that process now so that I can also check that off the list! Thank you!

The good news is I am in no rush, though having my bike running sooner sounds better. However, I can take my time and really take this bike apart and work on it. I'm defintley excited for more oppurtunities to learn with this bike as well!
Yes, between the petcock and carb. It's a secondary filter that will catch whatever your petcock screen misses. $10 at any bike shop or auto parts store.

A really easy way to tell if the inside of your tank is full of gunk is to drain your tank and then pull the petcock. If paint has flaked off and is floating around you should find it in there. Also, look inside the gas tank filler cap, around the edges. Often times this paint flakes off from gas splashing on it every time you fill up. I discovered this on my wife's Triumph America. I opened the float bowl and there were tiny little bits of black paint in there. Cleaned it out and she ran 100 times better. This was causing lots of pooping and backfiring, which was accentuated by the aftermarket pipes. There is always some of this; with stock pipes you can't hear it nearly as much. But hers was so bad that is sounded like it was going to explode. After cleaning the float bowls it sounds and runs great. Er, did, until she wrecked it. That's another story ;)
 

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That was lookin' up until the end there!😂 Ok, that's an easy project to get through! I'll knock that out and keep this post updated!
Don't look at it as work, look at it as a learning opportunity ;) If you do it right, like with a wide funnel right under the petcock, you might be able to pull the petcock and then drain it. This will purge the tank of most of the gunk all at once if it exists. Just prepare well for it before you let it loose ;)
 

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The carburetors are powered by the engines intake vacuum pressure. Compression test is the easiest non-destructive test to asses the condition of each cylinder without dismantling the engine. The compression will be slightly low if the valves are out of adjustment, even lower on any cylinder that might have valve or piston or ring damage. A bank of carburetors on a multi-cylinder engine requires balancing, the process of making each of the cylinders work equal. It is impossible to balance the carburetors if the compression on the cylinder pressures are all over the place because engine compression is proportional to the intake vacuum pressures, so you are looking for nice high compression values as would be indicated in your service manual and hopefully all of the cylinders yield close to the same numbers.

The testers are not cheap but they usually pay off with the first use. If the numbers are all good it saved you looking into the engine deeper and if results are bad then you know exactly where the serious problems are. :geek:


By the way, I never soak carburetors I think that is a poor excuse for a cleaning effort, I clean carburetors just like I would clean a gun. I never soak a gun in solvent, I take it apart and clean it by hand, mostly using honing stone oil. There is nothing in a carburetor that can not be cleaned manually, otherwise they would have never been able to manufacture the carburetor in the first place ;)


This stuff is great, buy some and use it on everything. It's even food safe.
 

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Yes, that’s good stuff. (y)
 
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View attachment 67200

Yes, that’s good stuff. (y)
It's also thin enough to go through an insulin needle, so you can slide it past the oil seals on your wheel bearings without destroying them and pump the bearing full of lubricant again. Makes them roll easy and last way longer :geek: the key to keeping a sealed bearing good is to displace any water inside where there is suppose to be lubricant.
 

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1977 Yamaha XS750
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Discussion Starter · #12 ·
This is why I came to a forum! I appreciate y'all! This is great information getting funneled right to me! I've got the tester coming now and I'm looking forward to doing my first compression test. I love learning how to acomplish things like this on my vehicles. It gets me closer to being 100% self sufficient. I grew up with an 81 c10 and I loved wrenching on that thing. So it's nice to have something that needs work again. I'm going to check valve clearances as well at somepoint, probably 2nd, as I have what I need to take care of that.
 

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Discussion Starter · #13 ·
After putting y'alls advice to use I've founda few things. Compression was low in all 3 (70 psi, 75 psi, 110 psi). One( no fire, spark is good, fuel is good, air is good??) One too rich? One running decent! These are the new one's I'd put in last week. You can kind of see the same conditions as the old ones already! So, anyways, checked valve clearances as well and all of the intake shims were too thick leaving too little clearance, one being only .06 mm of clearance! So, I need to pull those shims and size em up on the chart to get the right ones back in place and then we'll go from there. Thoughts are the two valves are for sure getting left open with the shims...ones almost not too big which I assume is my 110 psi! Any thoughts? Again, simply waiting on valve adjustment tool to get to those shims before I can proceed! Just thought I'd keep the post updated!
67368
 

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Discussion Starter · #14 ·
Air is good is not a question. The fact that cylinders not firing with the right conditions is the question!
 
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