Compression test.

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Donut 77
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Compression test.

Post by Donut 77 »

Hi, I'm a bit of a bike mechanics newbie, so please assume that I'm an idiot..

I'm trying to establish if the Firestorm I have been given is worth saving?

Thanks to advice from on here, managed to open up the alternator casing.

First question is: should there have been some oil in there? There was. Not much (maybe a couple of spoons full) but I'm hoping this is ok?

Secondly, whilst I wait to sort out starter motor issues, if I connect a compression tester to the plug(s) and turn the crank anti clockwise by hand, will that actually give any sort of accurate reading, or does it need to be cranked at speed using a starter and battery?

I could just try it, but didn't want to remove plugs now if this hand crank idea was really daft..

Finally, I will need a new gasket as half of it stuck on each side. Do folks normally use paste too, or just paste/silicone, or just gasket and oil? Not done this before.

Thanks.
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tastethefloor
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Re: Compression test.

Post by tastethefloor »

you cannot do a compressione test by hand..., does the bike start ? if no and the bike has been seated for long time i would suggest a carb cleaning before try everithing.
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fabiostar
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Re: Compression test.

Post by fabiostar »

if the bikes upright no oil should run out when you take the genny cover of as it will be in the sump which is quite deep on these...
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Donut 77
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Re: Compression test.

Post by Donut 77 »

tastethefloor wrote: Thu Mar 02, 2023 7:07 pm you cannot do a compressione test by hand..., does the bike start ? if no and the bike has been seated for long time i would suggest a carb cleaning before try everithing.
You are quite right, it was a failure trying it by hand. The bike doesn't start atm. I blew the staring solenoid yesterday on my attempt to crank it over, as the +ve terminal was rusted to ground. In the process of rectifying this so I can try again.

It has been sat 5 years. With very limited knowledge at present I am trying to see if there's good compression, which there seems to be. I have already taken the fuel tank off as it needs to be flushed.

How involved is a carb clean? I am a bit wary of messing up the carbs. Any advice welcome here.
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Donut 77
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Re: Compression test.

Post by Donut 77 »

The starter that I rebuilt worked perfectly. I’m very chuffed about this.

Compression test reads 125rear, 129front

I’m ashamed to admit that I got a bit excited when the starter worked and I could hear it squeezing and popping, and I went straight to the test, forgetting to put any oil down the plugs.

Didn’t hear any horrible screeches - I’m hoping I haven’t done any damage.

There is also coolant still in the system (just not in the expansion tank) there was pressure when I took the rad cap off and it looks to be a green fluid in there. Is this normal?

Now I know it isn’t seized, I think next step is to get it running before spending any money on lots of new shiny things.

What would you do next? I welcome any suggestions..
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8541Hawk
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Re: Compression test.

Post by 8541Hawk »

Did you block the slides open and also hold the throttle open when doing the test?
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Donut 77
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Re: Compression test.

Post by Donut 77 »

8541Hawk wrote: Fri Mar 03, 2023 5:46 pm Did you block the slides open and also hold the throttle open when doing the test?
Hi, No I didn’t. What are the slides?

I will repeat it tmro (with some oil this time too)
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tony.mon
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Re: Compression test.

Post by tony.mon »

Throttle slides.
Remove the tank and airbox lid, and you can see the carbs. Lift the slides with your fingers during the test
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Donut 77
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Re: Compression test.

Post by Donut 77 »

Manual CCT’s now installed.
Checked timing and tension I’m happy with (not too tight, not too loose, about 6-7mm play between sprockets)
So, time to re-visit the compression test. I don’t know why, but I’ve been fixated on getting to this point to check it’s worth continuing.
The bike is stripped, so there’s max air flow.. good battery and jump leads. 4L of fresh oil. That’s when I made the mistake of tipping too much oil down the plug holes.

On the rear, I’d say just a bit too much. On the front, due to the angle, it wouldn’t flow in so I kept trying and it started coming out of the exhaust port. I was worried about turning it over dry, you see. (How do you get oil in the top of the front cylinder anyway unless it’s being pumped around?!)

So, onto the test. Rear first - good reading of 157 or so. All sounded quite good turning over. When I hooked up the tester in the front, it struggled to turn with the plug hole now blocked. Basically stalled the starter and/or welded my jump leads. I assume because oil is now filling the available gap for air to compress..

Any thoughts very welcome. Is it possible that I have ballsed things by pouring half a tin of oil in the combustion chamber? Will it just drain away or should I ring the scrappie and go and buy a working bike???!
Last edited by Donut 77 on Tue Mar 21, 2023 1:00 am, edited 1 time in total.
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8541Hawk
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Re: Compression test.

Post by 8541Hawk »

leave the plug open and crank it until oil stops coming out
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Donut 77
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Re: Compression test.

Post by Donut 77 »

Correct you are. I did what you suggest. This morning, smoke pops out of the exhaust but it does turn over.

Rear 158
Front 163psi
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tony.mon
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Re: Compression test.

Post by tony.mon »

Donut 77 wrote: Tue Mar 21, 2023 9:54 am Correct you are. I did what you suggest. This morning, smoke pops out of the exhaust but it does turn over.

Rear 158
Front 163psi
A lot more smoke will come out when you get round to starting it up!
All that oil has to go somewhere.
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Aly
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Re: Compression test.

Post by Aly »

thats an interesting situation to be in. You needed to put some oil in it so not to crank it dry but that oil will mask bad rings. If its not worth rebuilding you dont want to screw around with carbs and whatever else is wrong it just to find out the rings are flogged. I think id be inclined to try and wash some of that oil out with petrol and redo the comp reading. If the compression starts to drop... Anyone know the right reading for these?
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Re: Compression test.

Post by tony.mon »

It's in the Haynes manual.
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MK_WF
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Re: Compression test.

Post by MK_WF »

If you've put in too much oil in the front cylinder and you haven't cleaned that out by letting the engine spin without a plug installed, then there's a good chance you damaged something.

Why? Imagine the cylinder volume has to be compressed to what's left above the piston in TDC (combustion chamber volume) This already takes a good effort.
Now if you have a lot of oil in that volume, it first just increases compression, i.e. it becomes harder to spin the engine.
At that point, where there is more oil than the volume above the piston at TDC, it's physically impossible to spin the engine as you can't compress liquids like air.

What happens now depends on the actual oil filling. If it's tad above the combustion chamber volume, then the crank can rotate to a point short before TDC. If the starter delivers enough torque, you've build a proper toggle lever press and the only degree of freedom is bending the connection rod.
This is called a hydro lock.
https://en.m.wikipedia.org/wiki/Hydrolock

If you've been very lucky, the oil excess was plenty enough that the stop position was way before TDC and in this configuration the starter didn't have enough power to bend the rod.
Press thumbs this option occurred.
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