Compression Test

[Garrett]

I picked up a Bandit with a "seized motor"
But I can crank it over. I'd like to do a compression test but my repair manual says to do a compression test only when fully warmed up .
The carbs are old and dirty so I'd rather not go through all the effort of trying to get this motor running (if it will at all) before doing a compression test.

Is it safe to do a compression test with the motor cold? If so, what numbers should I be looking for? Repair manual says a warm motor should be over 114.

[golden chicken]

Its not so much a specific number as it is consistency. You want them all to be about the same.

[stoinkythepig]

You can do a compression test cold. If it passes, great, if it does not, there might not be anything seriously wrong with it.

[Garrett]

Hm. I think I'll just try to get it running.

[Kurlon]

I do my tests cold, hot tests can mask some issues. I've got a tester if you need one.

[Garrett]

Aye. Will it fit on the a bandit 400? My Snap-On doesn't.

[Kurlon]

Well, it fits on my FZR400 sooo... maybe? Guessing you need a 10mm adapter? Worst case, Autozone/etc should have a cheap tester on the rack that comes with a 10mm adapter if you're in a hurry.

[Garrett]

Yep it's a 10mm thread according to the interewebs

[Tunertype]

If you can crank it you don't have a seized motor. That's just what the seller told you right? If you can crank it the first thing I will try is a little starting fluid. If it starts with that everything is relatively good accept for the fueling and then it's usually just a cleaning that is needed. Spraying fluid in the intake is a lot easier than a compression test. The next test I will do is take out the plug and look for a spark. While you have the plug out and if its making a good spark then also check compression. If its low you can drop some oil in the cylinder and try again. If this improves the compression it's the piston wall or rings if not its in the valves/head. But again it usually starts right up on starting fluid and all they need is a carb clean.

[xsiliconkid]

Another good diagnostic tool is a leak down test..... this way you know where you are losing compression (rings/cylinder, exhaust valve, intake valve)
ps Don't do a leak down on a HOT motor....warm OK....

[timmyho414]

I do my tests cold, hot tests can mask some issues. I've got a tester if you need one.

just curious, what issues do you think doing it warm would mask?

[Kurlon]

Warm means you've got oil pressure built up, things have expanded, etc. An ok hot test does not guarantee that everything is in spec.

Plus, some of my motors could already bury a 300psi tester cold, warm just sped that process up.

[carsick]

some of my motors could already bury a 300psi tester cold, warm just sped that process up.

You racing Diesel now?

[timmyho414]

300 psi cranking pressure! jesus christ what is that on?

I'm use to seeing more like 120-180 on cars. My 749R should be about 200-215 I'm told and I thought that was crazy high.

I don't know that I buy not doing it warm, plus as soon as you shut it off the oil pressure will be bleed off before you can open the door.

[Kurlon]

High compression 72cc OHC single on a short duration factory cam. Tiny bore means very little pressure loss around the rings, combine that with a 14:1 static compression setup and almost no overlap on the cam and you'll get high test pressures. Still ran on high test pump gas too!

On the oil pressure, I'm more referring to the fact that you'll have oil on the cylinder walls, etc. I've had motors that require 15 minutes of kicking to get enough heat in them before they'll build pressure and start. Once eventually started they run and will pass a warm compression test. Get the motor good and HOT and it may blow higher than the OEM specs. Let them cool down and you'll be well below the 70psi min you need to make anything fun happen.

200 to 215psi isn't crazy high. YZF600R calls for 220psi for comparison. Looking at Yam's procedure, they only have you run the motor "for several minutes" so warm but not hammered.

OT - I've seen Legends cars checked scalding hot right off the track with no cooldown as a means of calling someone a cheater at a local track...

[timmyho414]

That I can believe.

200 to 215psi isn't crazy high. YZF600R calls for 220psi for comparison. Looking at Yam's procedure, they only have you run the motor "for several minutes" so warm but not hammered.

Like I said, I'm use to cars and their compression is normally 9-10, sometime 12 which lands their compression well south of 200 most of the time.

Sooooo.what did you have that little monster in?

[Kurlon]

It was in the 70 that Savas has now. My first year racing I ran in a class that required a stock head, stock displacement, stock cam... anything else was wide open.

[stoinkythepig]

High compression 72cc OHC single on a short duration factory cam. Tiny bore means very little pressure loss around the rings, combine that with a 14:1 static compression setup and almost no overlap on the cam and you'll get high test pressures. Still ran on high test pump gas too!

Sounds like a neat engine. The small bore means the flame front has a short distance to travel from the spark plug before it hits the cylinder walls and can start pushing the piston down. This means the spark advance can be very small so you can get away with much higher compression and still use pump gas.

[Kurlon]

Sounds like a neat engine. The small bore means the flame front has a short distance to travel from the spark plug before it hits the cylinder walls and can start pushing the piston down. This means the spark advance can be very small so you can get away with much higher compression and still use pump gas.

It was fun, best setup was that top end with my Ti weighted auto clutch. Switching to a transmission mounted manual with a close ratio 3 speed gearbox was quicker, but that auto just got off the line consistently quicker and I grinned a lot more riding it. Some day I'll re-create it, maybe.

[Garrett]

Got it cranking over with spark. Didn't try the starting fluid though.

[Garrett]

Booooo. All I get is a backfire out of the carbs

[stoinkythepig]

Booooo. All I get is a backfire out of the carbs

Did you get compression numbers? Did you check the valve clearance? The 400s have screw nd locknut valve adjusters so they are really easy. Backfiring might just mean the cam tensioner failed and the chain skipped a tooth or 10.

[Garrett]

Did you get compression numbers? Did you check the valve clearance? The 400s have screw nd locknut valve adjusters so they are really easy. Backfiring might just mean the cam tensioner failed and the chain skipped a tooth or 10.

At 5k miles? ehhh

[stoinkythepig]

At 5k miles? ehhh

Oh, it's only got 5000 miles. Then just ride it, there can't be anything wrong with it with so few miles.

[timmyho414]

compression test