smart51
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| posted on 22/10/06 at 07:25 PM |
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engine problems
I want to open this up to a wider audience. My engine has just come back from the shop where they have fixed 2nd gear. The gearbox is integral to
the crank case and they've had the bottom half off.
The engine won't start. It turns over faster than it should and I've tracked it down to low compression - 30 to 50 PSI in each cylinder.
I have spark, fuel and airflow. The cam chain has not been moved as all the timing marks line up properly. The carbs haven't been touched as
they stayed here with me. I doubt very much that the head's been off.
What could cause low compression in all cylinders that could be done when working on the bottom end of the engine?
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Jon Ison
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| posted on 22/10/06 at 07:31 PM |
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I cant answer your questions but have you tried a compression test after dropping a bit of oil down each cylinder and did you have the throttle fully
open when you tested it ?
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smart51
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| posted on 22/10/06 at 07:34 PM |
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I didnt' try oil. The throttle was fully open and I had the carb slides lifted up as well.
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Jon Ison
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| posted on 22/10/06 at 07:42 PM |
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TBH if it spins over fast it does point too ether low compression, ignition timing probs or both, you ain't having no luck are you ? Have you
spoke with the repair people ? I assume your pulling your hair out, I would be, sorry cant offer any help.
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ruskino80
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| posted on 22/10/06 at 07:58 PM |
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probably way to obvious-but have you established if all pistons are moving?
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smart51
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| posted on 22/10/06 at 08:02 PM |
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Ooo, I hadn't thought to do that. There is compression in each cylinder, though not much. For one of them to be stopped would be very bad.
How could I tell that they're all moving?
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ruskino80
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| posted on 22/10/06 at 08:11 PM |
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remove all plugs-insert say a pencil into each pot one by one whilst rotating engine you should feel each piston come up to tdc and then fall---do it
slowly or you will snap the pencil off(also coathanger or similar)
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yorkshire-engines
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| posted on 22/10/06 at 09:21 PM |
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Hi when the crankcases are split to replace gearbox there is no need to disturb the crank or cams you can leave them in situ it sounds like
they have not done this and although the cam marks line up have you checked the crank mark as it looks like your valve timing is out and causing low
comp
iv done loads of these and there is no reason why it shouldnt be the same as it went in
cheers malc
ps ring me if you cant find fault
07960011585
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smart51
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| posted on 22/10/06 at 09:55 PM |
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Malc,
With the T mark on the on the timing rotor pointing at the mark on the crank case, the dots on the cam shafts line up with the marks on the cam
holders. The leading edge of the large tooth on the timing rotor is right in the centre of the timing pickup. Could the timing still be out?
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thomas4age
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| posted on 23/10/06 at 12:19 AM |
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Just a thought.
I did to 4age engines, one with the toyota desinged and cast 16v head, and one with the yamaha cast 20v silvertop head.
when replacing the timingbelt on the 16v the marks on the pully needed be alainged with the timing marks on the camcovers, but when I did it like that
on the yamaha head I had the same problem asl you No compression whatsover.
although my 20v velve cover has something that can be seen as timingmarks, it turned out that on the 20v head you need to set the intake cam with the
left timing mark to the cover, but the exhaust cam is timed not on the cover mark but on a second timing mark on the intake-cam-pully. although the
timing cover made me think otherwise.......
looks like the toy folks cast the timing marks and yamah designed the cams and pullys with different idea of marking...
here's piccie
[img][/img]
I can't believe that the engine is so bad that there's no compression at all, so the must be a fault in the build-up, if not you have on
really bad engine!
regards Thomas
[Edited on 23/10/06 by thomas4age]
If Lucas made guns, Wars wouldn't start either.
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britishtrident
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| posted on 23/10/06 at 07:25 AM |
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If all cylnders are affected its either valve timing way way out or much more likely bent valves.
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smart51
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| posted on 23/10/06 at 07:48 AM |
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I was worried that someone would say that. My train of thought was that the pressure is held in by the valves, the head gasket and the piston rings.
Its unlikely that the gasket or rings have gone on all cylinders equally, the timing seems to be right so one or other set of valves may be damaged.
Or both sets.
How do I check this? Is it a head off job?
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Coose
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| posted on 23/10/06 at 08:05 AM |
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quote:
Originally posted by smart51
Malc,
With the T mark on the on the timing rotor pointing at the mark on the crank case, the dots on the cam shafts line up with the marks on the cam
holders. The leading edge of the large tooth on the timing rotor is right in the centre of the timing pickup. Could the timing still be out?
Yep, referring to the Yamaha manual, that looks right. The timing mark for the rotor on the crankcase is on the left - you're not using any
other mark are you?
It still seems very strange that you suddenly have low comp on ALL cylinders when they should not have been disturbed. I really would check your
compression tester on another motor as it may be damaged and leading you up the garden path!
Spin 'er off Well...
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smart51
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| posted on 23/10/06 at 08:20 AM |
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What's the chance of the timing rotor slipping on the crank (or being "slipped" by a mechanic)? is is splined? My boss'
suggestion was that it might have been tampered with.
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02GF74
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| posted on 23/10/06 at 08:22 AM |
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dunno why noone hasd mentioned it but have you rang the company that did the work about this? To find out what they did exactly?
Does it look like the head was removed (thinking could the company done a drity and swapped a head over? )
It does sound like timing. did you set this yourself in the past?
Go back to first principles and remove the top coer to see the valves. Do the pencil thing and turn the engine over to see the valves move in
realtion to cylinder one - the valve opening sequence is well documented and yours weill be similar nough to show if it is totally out.
Also when both valves are closed (compession sroke), try to blow into the inlet/exhasut port if you can to seek a leak.
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Coose
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| posted on 23/10/06 at 08:52 AM |
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quote:
Originally posted by smart51
What's the chance of the timing rotor slipping on the crank (or being "slipped" by a mechanic)? is is splined? My boss'
suggestion was that it might have been tampered with.
If you remove the rotor fixing bolt, under the large washer you should see a pin in the end of the crank that sits in a slot in the rotor. That could
have been snapped, but it's doubtful.
I would do as someone has just said and check the engine number, and even if it's ok get back on to the people who looked at your box to see
what they've done.
Who was it that looked at it?
Spin 'er off Well...
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smart51
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| posted on 23/10/06 at 09:13 AM |
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The work was done by Bruce Cribb of Cribby Tuning in Oldbury. He was recommended by the local Yamaha dealer as he used to work for them. I spoke to
him on Saturday and has agreed to come to my house tomorrow to look at it. He assured me that he wouldn't leave me with the problem. I'm
quite confident that he won't. I'm just trying to get it fixed, partly to make sure its nothing I've done before I get him to leave
his shop for the day.
As for the engine number, it is drilled off, just as it was before, and the sealer on the rocker cover gasket was the same colour as the sealer I used
last year. There seems to be less paint on the block round the exhaust ports than I remember but it seems to be the same engine.
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Coose
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| posted on 23/10/06 at 09:17 AM |
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Right-ho, the best thing you can do is to see what he says then. It's always hard to make suggestions without being stood in front of it.....
I'm sure you'll keep us posted! 
Spin 'er off Well...
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smart51
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| posted on 23/10/06 at 10:06 AM |
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The engine guy has just phoned me with one last thing to try. He suggests a squirt of oil in each cylinder, plugs back in and then try to start the
engine. The oil might raise the compression enough for it to start.
Is this going to cure anything? Will starting the engine once clear something? Or is is just to prove that it is only low compression that is at
fault? I guess that oil will only seal the piston rings to see if they are the cause of the low pressure.
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02GF74
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| posted on 23/10/06 at 10:13 AM |
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what tappets has it? hydraulic? most likley to be shims on bikes but possibly hyrdaulic?
just wondering if the engine was upside down or on its, they could have drained or something? (although that should cause them to be open less so
maybe less air is getting in so lower CR?)
do you have oil pressure?
a quick rule of thumb shows you current CR is less than 4:1; I doubt the engine would run well, if at all with it so low.
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Hellfire
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| posted on 23/10/06 at 10:15 AM |
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Hope you get it sorted... sorry we can't help!
ATB Steve
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Coose
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| posted on 23/10/06 at 10:28 AM |
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The tappets are bucket & shim. There is absolutely nothing that should have caused a complete lack of compression. As has already been said,
he's either disturbed the cam timing (but that has been checked) or your compression tester is at fault.
I can't see you having bent valves as this would have been evident before you took the engine out, and I can't see any of your valves
sticking and they wouldn't all stick (or at least one valve on every cylinder)!
One way of checking for bent valves is to check the valve clearances - if they've opened up you could have a bent valve. This is properly
clutching at straws though!
[Edited on 23/10/06 by Coose]
Spin 'er off Well...
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smart51
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| posted on 23/10/06 at 10:31 AM |
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The cam lobes push directly onto buckets with shims underneath them. The shims push the valves. I reshimmed the engine last year. I guess its
possible that when the engine was upside down, the buckets could have moved and the shims slid out.
I could lift the lid again tonight and check the valve clearances.
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Catpuss
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| posted on 23/10/06 at 10:58 AM |
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Can't remember if I posted this before but...
Off the wall suggestion, with bike engines there is an ignition lock out if the bike is in gear with the clutch out and side stand down. Could it be
during rebuild of the gear box the sensor for the neutral position has been disturbed. This can easily be incorrect if the gear box has been in and
out? If you are using the origional bike loom this could be a problem. Have you got a neutral light indicator and is the engine *really* in neutral
when trying to crank it over?
I spent hours by the road side when this circuit failed on my firestorm. It would spin up but not fire.
Long shot, but have you checked to see if the cam chain tensioner has been disturbed. If that was disturbed then the cam chain would almost certainly
have been disturbed. But as others said, no reason for this.
Also if the timing was out significantly the values would be hitting the pistons, hence a nadgered engine.
I would get the rocker box off and see what his happening at the top end and crank it over by hand with a socket on the cranck nut.
It *could* be one of the cams has slipped its chain of the tensioner was loosened (some bike engines do this), but that could also fsk the engine. But
as you say the align marks seem OK.
Long shot but if the cylinders are all reading consistently low it may even be that its always been low but coking has stopped blow by. The squirt of
oil will tell you if its the piston rings that are causing low compression as the oil will temporarily seal them. Normally you get combusion
carbonisation fill in some of the gap (hence haynes manuals say don't clean around the top rings if you are just doing a clean with no rebore to
avoid oil consumption). If the engine was upside down/bashed about splitting cases it could be some of coking loosened and hence blow by. Even then I
would expect inconsistent pressure.
As others said it may be a red herring and the compression tester may be mis reading.
Are the plugs wet (with petrol) after trying to start? If so, bad spark/timing.
[Edited on 23/10/06 by Catpuss]
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smart51
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| posted on 23/10/06 at 04:47 PM |
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A friend at work suggested removing the carbs and turning the engine over with the palm of my hand over the inlet ports. This is to guage whether one
or more are leaking.
Three of them go: suck, pause, suck, pause
No2 goes suck blow, pause, suck blow, pause
I've turned the crank with a socket, with the rocker cover removed and the exhaust valve buckets move down well enough. Also, none of the valve
clearances are too big, suggesting that the 3mm thick or so shims are still in place.
What could be the cause of this? I did remove all the paper towels from all the ports before fitting manifolds.
edit I took off the exhaust manifold and it works properly now. Better check the exhaust when the engine's out. So the inlet valves
don't leak, which is a good sign. The engine still doesn't start even with the exhaust off.
[Edited on 23-10-2006 by smart51]
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