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Discussion Starter · #1 ·
Does anyone know what the thread pitches for the spark plug hole on a 98 Yamaha Royal Star tour Deluxe 1300 so I can do a compression test on it9
 

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Super Moderator "Loose Nut" - Houston, Texas
2001 Vstar 1100 Classic (sold), 2006 HD Electra Glide Ultra Classic
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My compression tester has 14 and 18 mm end. Never needed anything else.

107048
 

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Discussion Starter · #3 ·
My compression tester has 14 and 18 mm end. Never needed anything else.

View attachment 107048
Is the manual correct for my bike in stating to run the engine a few mins essentially running all the gas out of the carbs and then removing all spark plugs and test compression with wide open throttle?


The whole reason why I want to do a compression test is after setting my pilot mixture screws and sinking the carbs bike has ran great for a few days then I go to start my bike on lunch and it almost seems like it wanted to hydrolock when I hit the starter button it stopped abruptly I thought maybe I had left my Bluetooth speakers on to drain the battery so I hit the starter again and it fired up but ever since then it seems like it runs differently I also heard that the starter cannot cause hydrolock reason is I think I might have a faulty float needle valve not seating properly that allowed some fuel into the intake wallet was sitting because my dumb ass left my petcock on was still almost doesn't even make sense because it's fuel pump only pumps fuel when ignition on but I could be wrong my bike's still runs as is but I'm trying to do a compression test now to see if I did any damage and if I did it looks like I'm going to be looking for a new cylinder head with valves.
 

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Super Moderator "Loose Nut" - Houston, Texas
2001 Vstar 1100 Classic (sold), 2006 HD Electra Glide Ultra Classic
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I don't do anything before compression test on a bike. Pull plugs, insert tester, crank at full throttle a few times. Go to next cylinder. Never had an issue. A compression test reveals the condition of your engine's valves, its valve seats, and piston rings and whether these parts are wearing evenly. Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings.
@Jaguartech, your thoughts?
 

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Discussion Starter · #5 ·
I don't do anything before compression test on a bike. Pull plugs, insert tester, crank at full throttle a few times. Go to next cylinder. Never had an issue. A compression test reveals the condition of your engine's valves, its valve seats, and piston rings and whether these parts are wearing evenly. Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings.
@Jaguartech, your thoughts?
Manual says 216 psi to be ideal. 🤔

Im worried i possibly bent a valve
 

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Super Moderator "Loose Nut" - Houston, Texas
2001 Vstar 1100 Classic (sold), 2006 HD Electra Glide Ultra Classic
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The 100 psi is a minimum for an engine to run. You are correct that optimal compression would be much higher. But older engines with ring wear and slight valve seat wear will have a lower reading. If you have a bent valve, you will probably have zero compression.
 

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I don't do anything before compression test on a bike. Pull plugs, insert tester, crank at full throttle a few times. Go to next cylinder. Never had an issue. A compression test reveals the condition of your engine's valves, its valve seats, and piston rings and whether these parts are wearing evenly. Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings.
@Jaguartech, your thoughts?
I don't really use one all that much, l have a differential pressure tester (leakdown) that gets alot more use.
Over the years l have learned to spin an engine with the starter and listen for a flat cylinder, if it sounds funny then l break out the testers. Also learned to leak test a motor when it's at operating temp, when valve/seat problems will show better.
In my field l don't see that many ring wear issues, when l do it's usually ragged out oil control rings.
Most of the compression related problems that I see are valve related.
People like to neglect valve adjustment.
Most engines get damaged by lack of maintenance.
 

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Is the manual correct for my bike in stating to run the engine a few mins essentially running all the gas out of the carbs and then removing all spark plugs and test compression with wide open throttle?


The whole reason why I want to do a compression test is after setting my pilot mixture screws and sinking the carbs bike has ran great for a few days then I go to start my bike on lunch and it almost seems like it wanted to hydrolock when I hit the starter button it stopped abruptly I thought maybe I had left my Bluetooth speakers on to drain the battery so I hit the starter again and it fired up but ever since then it seems like it runs differently I also heard that the starter cannot cause hydrolock reason is I think I might have a faulty float needle valve not seating properly that allowed some fuel into the intake wallet was sitting because my dumb ass left my petcock on was still almost doesn't even make sense because it's fuel pump only pumps fuel when ignition on but I could be wrong my bike's still runs as is but I'm trying to do a compression test now to see if I did any damage and if I did it looks like I'm going to be looking for a new cylinder head with valves.
Bigger issue is if a carb is leaking fuel into a cylinder is fuel will get by the rings and comtaminate the oil.
That will damage a connecting rod big end bearing.
If your oil smells even slightly of fuel change it,
Oil is cheap, motors aren't.

Some electric fuel pumps will allow fuel to flow through with the power off, usually impulse type.
Not sure if yours is like that.

plug hole is 12×1.25 thread.
 

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Discussion Starter · #9 ·
The 100 psi is a minimum for an engine to run. You are correct that optimal compression would be much higher. But older engines with ring wear and slight valve seat wear will have a lower reading. If you have a bent valve, you will probably have zero compression.
So how do you keep fuel from going in the cylinders when you do a compression test and is it possible to still have compression even if a valve is a little Ben my bike only has 19000 miles on it not even do for a valve clearance check yet
 

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Super Moderator "Loose Nut" - Houston, Texas
2001 Vstar 1100 Classic (sold), 2006 HD Electra Glide Ultra Classic
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When doing a compression test you are only turning the engine over a few times. The small amount of unburned fuel will evaporate very quickly. If you were doing this on a V8 engine it might be a concern, but not on a 2 cylinder engine. A bent valve would not seal, at least the ones I've seen before didn't.
 

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A little fuel in a cylinder won't bother anything.
Seriously doubt you bent a valve, it's highly unlikely in a good working engine unless it jumps timing or a valve sticks in a guide.
Make sure to open the throttle during a comp test and spin the engine until the pressure stops increasing.
 

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Discussion Starter · #12 ·
Rstd is a 4 cylinder 1300cc 1998
A little fuel in a cylinder won't bother anything.
Seriously doubt you bent a valve, it's highly unlikely in a good working engine unless it jumps timing or a valve sticks in a guide.
Make sure to open the throttle during a comp test and spin the engine until the pressure stops increasing.

Here are my compression readings.
Manual says this is on the low end with one being slightly lower as you can see. Is this terrible and does it suggest i need to do a valve job asap?

20210605_144056.jpg
20210605_144153.jpg
20210605_144336.jpg
20210605_144459.jpg
 

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I'd say that's decent.
No two guages will read the same and the placement of the schrader valve in the tool will affect the actual number.
The fact that they are all nearly the same is good.

What is the bike actually doing?
 

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Discussion Starter · #14 ·
I'd say that's decent.
No two guages will read the same and the placement of the schrader valve in the tool will affect the actual number.
The fact that they are all nearly the same is good.

What is the bike actually doing?
Yesterday i was worried i hydrolocked the motor. I went to start it and it immediately stopped immediately after after pressing the start button although my first thought was battery was drained from my Bluetooth speakers. I pressed the button again and it started right up but is running a little different not as peppy as before. I now know that I had an issue from the left rear carburetor because I left the fuel petcock on and it had put fuel in the cylinder. Its a 1998 royal star tour deluxe 1300 with 19,xxx miles on it.

So my concern was doing the compression test was to see if I had bent a valve and my source of information from a manual I'm not sure is 100% correct but I think it probably has so that's why I hopped on here to ask some questions to know whether or not that had messed up my bike up.

I disassembled the carbs before I put it on the bike everything looks good and everything was clean but apparently I will be looking into a rebuild kit for the carburetors so I can solve this potential issue as to whether or not the the needle seat on that carburetor is bad or it really is because I just didn't turn the petcock off I noticed that on the cylinder that I had an issue with previously with fuel overflowing because the pilot jet was clogged and that cylinder spark plug was a little wet with gas even after not having run in approximately 15 hours. I appreciate you responding back quickly and your knowledge and opinions on these bikes and motors
 

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Make sure there isn't fuel in the oil.
If there was a cylinder that got filled with fuel l would change the oil regardless.
 

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Discussion Starter · #16 ·
Make sure there isn't fuel in the oil.
If there was a cylinder that got filled with fuel l would change the oil regardless.
Ive changed it three times now before i thought the problem was fixed and had fuel in it already.

Ive got two more questions about these 28mm carbs.
Is there a link on this forum where i can visually see how to properly set the floats on a bench? Essentially a dry float measurement off of the bike and is there a particular carb rebuild kit you would recommend that has new needles and seats?
 

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As far as float level adjustment and specs, I'm sure someone watching this thread can post a page from a manual or would know where to find a downloadable manual for a Royal Star.
@lesblank may know who to ask about that.
As far as carb kits go, K&L used to have decent aftermarket kits. Sudco has genuine Mikuni parts, would be worth calling them first.
The stuff on Ebay and Amazon is mostly Chinese and isn't good quality.
Aftermarket gaskets and o-rings are probably ok
but I recommend genuine Mikuni or OEM Yamaha float needles/seats.
 

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Discussion Starter · #19 ·
You wouldn't bend a valve if hydro-locked you would bend the connecting rod. For an engine to hydro lock the valves must be closed other wise you would just force the gas out one of the open valves.
I was able to do a compression test its good
 
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