Low Compression

[Kram]

I had my bike booked in today to be dyno'd and get the carbs balanced, fuelling done, etc. And have valve clearances checked.

The first dyno run showed my FZR600 pulling 66.5BHP (really low) and the guy who did it told me that i have low compression on 2 off the cylinders.

What would be the cause of low compression? the dude said it could be a valve, but i'm unsure.

any help would be appreciated
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[edd]

low compression can be a number of things, valves (most commonly) piston rings (seemingly unlikely in your case), head gasket failure a number of things but most likely a valve id have said. Take off your valve cover and check you havent got a tight valve (this would make a valve stay open a bit all the time) other than that id suspect you may have got a slightly bent valve. This isnt good, and id imagien your 20v head being a pain in the arse to strip. If you have no luck with the tight valve situation, take the head off, and sit the head sideways, pour antifreeze into the intake ports, and see if it drains out through the valves. Do the same thing for your exhaust ports and see if those valves leak too. If any valves are leaking, mark them, remove them and check them for straightness. If they arent straight then they will need to be replaced, while you are on you may as well do your valve guide oil seals as well. Good luck.
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Bluespark Automotive - Diesel Tuning for Performance and Economy

[0ddball]

Try the simple things first. Take it to a garage and let them do a leakdown test on it. They pressurise the cylinder with compressed air and see how much it drops (some loss is normal) and where the air is leaking.

Which cylinders were the pressures low on? If it's 2 adjacent ones it points to a head gasket problem.

[Kram]

he said there could be compression issues on more then one, but wasn't very detailed.

how much would a leakdown test cost? and how long does it take?
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[edd]

It shouldnt cost you much or take very long at all, to be honest It all depends on the kind of garage you visit. If you visit a nice clean dealership who want to book you in for a timein the next week and have their mechanics do it, then it you will pay much more than if you take it to a small friendly place when they arent busy, and get talking to a mechanic/the owner, one of these places would probably do it for a couple of quid or even for free.
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Bluespark Automotive - Diesel Tuning for Performance and Economy

[Kram]

ok mate cheers.

will have a lookin tomorro when i'm off
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[edd]

did you get a test done? if so what were the results?
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Bluespark Automotive - Diesel Tuning for Performance and Economy

[Kram]

not yet, i will phone around tomorrow, and see if i can get a free one.

but i have a royal marine 5 day audition next week, so don't have to much free time till after that
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[Heckthusula]

Mate, that HP reading isn't that low for the back wheel. Get your shims done if you haven't already. Oh, I'm in the same boat by the way!

[Kram]

marine audition next week? or the low compression?
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[Heckthusula]

Sorry - not the marine audition! This post: https://www.bikechatforums.com/viewtopic.php?t=87834

Bit punier bike though

[Kram]

i got a nakered chain and sprockets on my bike, and a bit of jerkyness if i'm not careful with the throttle. (if that helps)

also if i have a pillion on (thus making the chain tighter) it won't jerk, so that might be your problem.

shame a new chain and sprockets for my bike is £110
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[Heckthusula]

Yup, same here.. A little extortionate! Then they try and persuade you to get a Z ring chain or whatever the latest and greatest is, which is even more expensive. I reckon my carbs need cleaning out though to be honest, because it even jerks on the overrun -definitely feels like a fueling problem. A new airfilter wouldn't go amiss either.. Spark plugs.. God, the expense!

[Kram]

just rang up my local mechanic chap, and he said a leakdown test would not show anything, as they only really work on high CC cars (2L up)

A compression test will also only prove that there are compression issues. And the only way i can find the problem is to check valves, then work down the engine
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[0ddball]

He's yanking your chain (and probably doesn't even have the equipment).

Find one that knows what they are talking about.

[Kram]

Just some helpful info:

Went to start bike this morning to goto work (after rain last night + this morning, was only spitting when i started bike) amd after about 5 minutes of turning the bike over (with no sounds of it firing, was just turning over) it started firing one cylinder (could tell by noise)

Then after about a minute, a second cylinder kicked in (would now run on fully open throttle, without starter and rev counter at just below 1k)

Then about a minute later, a third cylinder kicked in, and now i could rev the bike (was struggling but did rev) then the fourth cylinder kicked in, and it was fine.

For the first few minutes the engine would bog down over 6k revs, but after it was warm, it was fine.


Hope this helps in trying to diagnose my problem.

Thanks

Drummer
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[Kickstart]

Hi

Shimmed head on a Yamaha so the cams have to come out, means that most people do not bother to do the shims. Only 16 valves on the 600 though.

Checking the clearances is time consuming and replacing the shims is a pain.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Kram]

do you know a rough price guide to the cost of valves/shim etc. if i do need new ones?
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[Kickstart]

Hi

Shims are a couple of quid each, and you can probably shuffle a few between different valves. Trouble is that you need to measure the clearances, pull the cams out, check the size of the existing shims and order new ones. Then once they have arrived put it all back together again.

Valves I am not sure on. As an example on the 1200 Bandit they were £12 each inlet valve and £20 each exhaust valve. However this will mean taking the head off, so you can add the cost of gaskets to that. You can get the head off with the engine in the frame.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Kram]

i'm hoping to take the top off, check valves (hope i can identify problem)

What do you do to check the valves? and how do you check the shims?
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[Kickstart]

Hi

You can visually check the valves, check for leakage page them by pouring a thin liquid down the ports and see if any seeps past the valve and you can take them out and rotate them to check carefully.

Need feeler gauges to check the clearances. You then compare that clearance with what it should be and with what the current shim is to tell you what the new size shim is you need. Note that shimmed valves normally have the clearance close up with age (so they get quieter as the valves clearances go out).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Kram]

well the bike just hit 40k miles, so i think they deserve a check. do i need any special tools to get to the valves/shims?
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FS1E -> SR125 -> CB400N -> CB650 -> FZR600 Genises -> VFR400 NC21 -> CBR900RR Fireblade -> Renault Clio 1.4

[Kickstart]

Hi

No special tools. Fairing off, radiator off, spark plugs out, rocker cover off. Check clearances. Cam chain tensioner out, cam caps off, cams out, buckets off, remove shims and check size.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Heckthusula]

I don't want to rain on anyone's parade, but if you're unfamiliar with the workings of a DOHC sportsbike engine, I wouldn't go performing major surgery yourself unless you're prepared to end up with an expensive paperweight. Take it to a mechanic, it'll probably be about £150 for the full works. And besides, I doubt you'll need valves replacing?! Plus it doesn't seem to be that ill at all, nothing a new airfilter and reshimming (That's the valve clearances on an OHC engine - oh for the days of rockers..) wouldn't fix. Power at the back wheel is never going to be spec power anyway.

[edd]

So long as you follow the instructions in your haynes (or similar manual, ill assume you have one) you should be ok. Get the valve cover off, check the clearances. Write down what needs changing (if your clearance is 0 you will have to guess what the shim size needs changing to) After you know what your clearances are, and what they need to be, roll your engine round to TDC (there should be marks on the ignition rotor, again this will be in the Haynes) To prevent your camchain slipping, before you even take the CCT out you can cable tie your camchain to the sprockets so they wont change timing. Then take out one camshaft at a time and just move it out of the way of the buckets, allowing you access to the shims. A vernier caliper is helpful at this stage because the shims size marking is often worn off the outside so you can use the caliper to find out the shim size. Write down the sizes of all the shims in the head, and work out where you can put what you have in order to need the least number of new shims. Go to your shop of choice, get your new shims, put them in, torque the cam caps back up and check the clearances again. Do make sure you don't overtighten cam cap bolts as they are a bitch to helicoil a lot of the time. hopefully your clearances should be sweet and your compression should be back to normal.

good luck

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Bluespark Automotive - Diesel Tuning for Performance and Economy