Low compression

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Zx4 h1
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Joined: Wed Sep 02, 2020 12:14 pm
My Bike: Zx4 h1

Low compression

Post by Zx4 h1 »

My bike was running fine until lockdown and now won’t start so i changed the oil and cleaned the carbs and still wouldn’t and got annoyed and sprayed easystart into the air filter and it fired straight away so i took it out to blow the cobwebs off and the week after it wouldnt start again and i heard a slight mechanical sound so sent it to the garage and they’ve checked everything and said the compression test was low on all 4 and its either take it back or take the engine apart which could be upto 1000
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banner001
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My Bike: ZXR400 L4
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Re: Low compression

Post by banner001 »

Hi there, if the engine genuinely has low compression then there is very little you can do about it unless you want to spend either your time or your money to investigate the cause.

It could be timing chain, valves, valve seats, rings, bores, head gasket etc...

If you can get the bike to me in the midlands we can go through it together and I will be a lot more reasonable than a proper garage.

Your other option is grab another engine and swap it in. Zxr400 engines will fit, but need some electrical fixes, so it's still not a cheap thing.
UK ZXR400 L3 (1993) - Fully restored and on the roads, my green beast!
JPN ZXR250 A2 (1990) - Revs to 19,200rpm... 'nuff said :smt003
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banner001
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Re: Low compression

Post by banner001 »

Might also just be carbs as well.

Honestly, most "bike mechanics" dont care about older carbed bikes, and they can't diagnose issues unless they can plug in a computer and be told what's wrong...

If you know how to change your spark plugs you can also check the compression, a compression tester for bikes costs about £20, as I would be wanting a second opinion as it seems odd that you would lose compression like that.

Did they tel you what cylinders were low, or the psi readings per cylinder?
UK ZXR400 L3 (1993) - Fully restored and on the roads, my green beast!
JPN ZXR250 A2 (1990) - Revs to 19,200rpm... 'nuff said :smt003
Zx4 h1
Posts: 13
Joined: Wed Sep 02, 2020 12:14 pm
My Bike: Zx4 h1

Re: Low compression

Post by Zx4 h1 »

One was always low since i had the bike put back together but they have said all are low now and did give amounts over the phone from what i can remember. Ive ordered a compression tester and me and a friend will test it again when it comes aswell as putting some oil into the cylinders after and leaving 24 hours which is supposed to improve compression
Zx4 h1
Posts: 13
Joined: Wed Sep 02, 2020 12:14 pm
My Bike: Zx4 h1

Re: Low compression

Post by Zx4 h1 »

Thanks banner001 and if you were closer id probably take you up on that but in boris’s world its a nightmare and also getting the bike to you. Thing is after working on the resto for 2 years and all the money to finally get it done resprayed and everything and running right i had an uninsured driver crash into it in front of my eyes and it seems as if ive had constant bad luck and expense with it to be told this time i could spend upto 1000 and still not diagnose the issue is very disheartening
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banner001
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Re: Low compression

Post by banner001 »

lockdowns wont be in place forever, and its not gonna cost £1000 to look into it - thats just a garages way of saying "dont want it mate", as they would actually have to do some investigating rather than just looking at what the PC says...

although yes if the engine required a full strip down depending on what was wrong/other bits to look at you could be looking anywhere from £50 to £1000 depending on whats wrong, whats worthwile to change etc.

If you cant remember the psi that the garage said for each cylinder i would wait to get your compression tester, then test every cylinder cold and with no additional oil, then see if oil makes a difference.

i would also remove the timing cover just to check the condition of the timing chain, and if you are feeling adventurous, grab some feeler gauges and check the valve clearances too.

If they all check out and the compression across all 4 cylinders is low i would stick a new head gasket on it, and inspect the cylinder walls at the same time, then retest the pressures, and if its still bad its either valve seating or rings
UK ZXR400 L3 (1993) - Fully restored and on the roads, my green beast!
JPN ZXR250 A2 (1990) - Revs to 19,200rpm... 'nuff said :smt003
Zx4 h1
Posts: 13
Joined: Wed Sep 02, 2020 12:14 pm
My Bike: Zx4 h1

Re: Low compression

Post by Zx4 h1 »

They checked the valve clearances and they were fine and compressions were 1, 80. 2, 70. 3, 70. 4, 80. They said one of the O rings were leaking and the petcock wouldn’t turn off and when they took one of the spark plugs out it squirted fuel out and they think theres fuel in the engine so no wonder it didn’t want to start
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banner001
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Re: Low compression

Post by banner001 »

So yeah they are very low, I would do the checks I me tioned in the previous post as they can be done for only a few quid and rule out a lot of potential issues
UK ZXR400 L3 (1993) - Fully restored and on the roads, my green beast!
JPN ZXR250 A2 (1990) - Revs to 19,200rpm... 'nuff said :smt003
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