Leaking valves?

[fosdyke]

Hi

Just want to float this around the workshop - sorry not a Yamaha - but the principles should be the same.

I suspect leaking inlet valves as compression on 1 & 4 are lower than 2 & 3 on a compression test. The engine runs reasonably smoothly but their is a slight amount of 'blowback' particularly through no1 carb intake. I have checked valve and ignition timing and these are OK, so I am looking for a way of testing for leaking valves without:

1. doing a leak-down test - i don't have the tools

2. tearing the engine apart - will do if I have to, but will be gutted if after, this is not the problem!!

any ideas???

[drewpy]

Hi

Just want to float this around the workshop - sorry not a Yamaha - but the principles should be the same.

I suspect leaking inlet valves as compression on 1 & 4 are lower than 2 & 3 on a compression test. The engine runs reasonably smoothly but their is a slight amount of 'blowback' particularly through no1 carb intake. I have checked valve and ignition timing and these are OK, so I am looking for a way of testing for leaking valves without:

1. doing a leak-down test - i don't have the tools

2. tearing the engine apart - will do if I have to, but will be gutted if after, this is not the problem!!

any ideas???

are the valve gaps adjusted ie shims etc?

if the compression is more than 10% down I suspect you would have to tear down to investigate further.

[fosdyke]

are the valve gaps adjusted ie shims etc?

if the compression is more than 10% down I suspect you would have to tear down to investigate further.

Thanks for the reply - this is a rebuild so I don't mind taking apart to get it right - I checked the ring gaps and replaced 1 & 4 rings so I don't think it is a ring problem which is why I think it is more likely to be leaky valves. That and the fact (confession time here), that first time I put the engine back together I got the inlet and exhaust cams the wrong way around and there could be valve damage although I would have thought the valves would have worked in reverse order thus avoideing this?

Just looking for a way of checking without resorting to the spanner!

[drewpy]

Thanks for the reply - this is a rebuild so I don't mind taking apart to get it right - I checked the ring gaps and replaced 1 & 4 rings so I don't think it is a ring problem which is why I think it is more likely to be leaky valves. That and the fact (confession time here), that first time I put the engine back together I got the inlet and exhaust cams the wrong way around and there could be valve damage although I would have thought the valves would have worked in reverse order thus avoideing this?

Just looking for a way of checking without resorting to the spanner!

if you have just rebuilt the motor it could be that it just needs running a bit. The valves need to bed in, I don't think that swopping the camshaft over (temorarily) would do damage unless you miss-timed the valves thus hitting the piston crown.

did you use new valves and ground them in or reuse the old but didn't put back into their original place?

[fosdyke]

if you have just rebuilt the motor it could be that it just needs running a bit. The valves need to bed in, I don't think that swopping the camshaft over (temorarily) would do damage unless you miss-timed the valves thus hitting the piston crown.

did you use new valves and ground them in or reuse the old but didn't put back into their original place?

Re-used old in same place - the valve seats looked good so I didn't feel the need to lap them in.

[Airhead]

IMHO Looks to me like they need lapping in then

[yoda]

I suspect that you have done a little damage with the cams. it is a question of overlap that can cause the problem.

You must lap the valves in properly. Do not get any paste on the valve guides. To test the valves for leakage, after lapping, reassemble the cylinderhead that you are lapping, just the springs etc not cams and fill the head with white spirit. If there is a trace of the spirit in the inlet or exhaust passages after say five minutes, then you need a bit more lapping.

When you have reassembled the engine, run it normally for a few hours and the retest the compression.

You did remember to roughen the bores when you put the new rings in?

[fosdyke]

I suspect that you have done a little damage with the cams. it is a question of overlap that can cause the problem.

You must lap the valves in properly. Do not get any paste on the valve guides. To test the valves for leakage, after lapping, reassemble the cylinderhead that you are lapping, just the springs etc not cams and fill the head with white spirit. If there is a trace of the spirit in the inlet or exhaust passages after say five minutes, then you need a bit more lapping.

When you have reassembled the engine, run it normally for a few hours and the retest the compression.

You did remember to roughen the bores when you put the new rings in?

Thanks Yoda - I rather thought I would have to get the spanners out again just to make sure - I was concerned about a possible overlap. Off to the factors to get some carborundum paste tomorrow! And yes I roughed up the bores prior to reassembly.

[fosdyke]

Thanks Yoda - I rather thought I would have to get the spanners out again just to make sure - I was concerned about a possible overlap. Off to the factors to get some carborundum paste tomorrow! And yes I roughed up the bores prior to reassembly.

OK - update time.....

Engine now in rather more bits than when I started - valves are all OK - NO LEAKS WHATSOEVER!! so just a little annoyed shall we say - back to more head-scratching, and a v careful reassembly tomorrow, can only think that blowback has something to do with the valves or valve timing.......

:huh: :huh:

[beginbush]

G`day,

Before doing all the hard stuff like ripping ya motor appart, did you do all the simple checks first.

i.e;

I don`t know what bike you have, or how many carby`s, but have you checked those out fully and sync`d them?

Is there no air sucking where it should not with the carb or carby`s?

Note: You did say "I suspect leaking inlet valves as compression on 1 & 4" there may be something in that, but not valves.

You said the timming was ok. It has to be not OK, but perfect.

Is the exhaust bedding in properly without sucking or leaking?

*Its worth checking little things like that first.

One more thing, I know around the world money is a bit tight at the moment, however, it is much better practice to replace a full set of rings on a multi cylinder bike, not in only half the motor.

Cheers.

[fosdyke]

G`day,

Before doing all the hard stuff like ripping ya motor appart, did you do all the simple checks first.

i.e;

I don`t know what bike you have, or how many carby`s, but have you checked those out fully and sync`d them?

Is there no air sucking where it should not with the carb or carby`s?

Note: You did say "I suspect leaking inlet valves as compression on 1 & 4" there may be something in that, but not valves.

You said the timming was ok. It has to be not OK, but perfect.

Is the exhaust bedding in properly without sucking or leaking?

*Its worth checking little things like that first.

One more thing, I know around the world money is a bit tight at the moment, however, it is much better practice to replace a full set of rings on a multi cylinder bike, not in only half the motor.

Cheers.

Thanks for the reply.......................too late the things in bits now! Not too bothered this is about the third time I've had the head off during the rebuild - do want to get it right - that being said however I am on a reasonably tight budget, and on the basis that the bike only cost me £50 in the first place replacing all the rings @ £30 a set would have more than doubled the value (lol) !

The bike is a 92 Kwak GT550 straight 4 - so reasonably bullet proof (the other reason I only did half the rings). As to testing other things - you could have something about valve timing, it is a bit of a b***ger to set up 'exactly' and when you check the marks the degree or so it seems to be out could make all the difference - although why only on 1 & 4? I would have thought if the timing was out enough to cause a blowback in 1 & 4 it would do the same in 2&3??

It is the fact that I get a distinct blowback 'apparantly' on the compression stroke that seems to suggest valves or valve timing - I intend to fully check the clearances and shims on reassembly - maybe the valves aren't closing completely on these two cylinders for some reason - I did have them all out to replace the valve stem oil seals and examine the faces.

Bit intrigued about your comments about the exhaust - certainly isn't (wasn't) leaking but not sure how it could affect blowback - by putting the heel of my hand over the inlet manifold you get a good vacuum - but on 1 & 4 it is followed by a little backward pressure that I don't get on 2 & 3.

Ahhh well back to the spanners.....

[drewpy]

OK - update time.....

Engine now in rather more bits than when I started - valves are all OK - NO LEAKS WHATSOEVER!! so just a little annoyed shall we say - back to more head-scratching, and a v careful reassembly tomorrow, can only think that blowback has something to do with the valves or valve timing.......

:huh: :huh:

did you lap them in this time?

just a thought, is the head and barrel flat? use an engineers steel ruler and check for warpage, could be gas escaping to next cylinder

[yoda]

After checking the cylinder head as per Drewpy, re-assenble the engine and check the valve clearences. When they are OK try and run the engine. You need to do a few hours to rebed it in and then check the compression. The blowback might be caused by a crack in the head which opens up under pressure. However, you must have equal spark plug gaps and ensure that they are all firing.

These engines are bullit proof so they say, so it will not be difficult to find the problem.

[fosdyke]

After checking the cylinder head as per Drewpy, re-assenble the engine and check the valve clearences. When they are OK try and run the engine. You need to do a few hours to rebed it in and then check the compression. The blowback might be caused by a crack in the head which opens up under pressure. However, you must have equal spark plug gaps and ensure that they are all firing.

These engines are bullit proof so they say, so it will not be difficult to find the problem.

Thanks guys but I think I have tracked down the culprit, though I am not sure why.......

before reassembling engine as per Yoda's instructions I carefully checked all the valve clearances - sure enough 0 clearance on 1 & 4 inlet valves - infact the valves weren't even closing when they should - hence the low compression and blowback! I have now ordered 'undersized' shims from the dealer (the valves close with the shims removed) - the mechanic there seemed to think the valves might have stretched, and said he had come across it once before.

What I am bemused by is why ALL of the valve clearances are less than I would have expected from an engine with nearly 50k on it - any ideas?

The bike was a 'basket case' when I got it so I assuming all of the various bits came from the same engine - could be that the guy who abandoned it as a 'project' got 'new' cams which might explain the clearances issue.

Will report back at the end of the week when I expect the shims to come in............in the meantime back to the w&d on the tank prior to spraying!!!

[drewpy]

its called valve seat recession. the valve seats wear and the valve starts to seat deeper into the head closing up the valve clearance

[yoda]

its called valve seat recession. the valve seats wear and the valve starts to seat deeper into the head closing up the valve clearance

Agreed. I only think that they might not be standard cams or valves. However you said it was a basket case.

[fosdyke]

Agreed. I only think that they might not be standard cams or valves. However you said it was a basket case.

Not sure that it is recession - valves and seats looked normal to me when I took them out, and that would be quite a large amount of wear if it was - and I would expect it more on the exhaust valve than the inlet. I am leaning more to the non standard cams possibility, or cams from a bike with fewer miles and not matched to the shims.

Will keep you posted.