Good compression, but really bad leak-down test?

[Scrallar]

Hi everyone.

I bought a Yamaha vmax 1200 from 1992 which was hard to start and when it started it was 4000rpm idle. (It was bad o-rings at the intake manifold which is fixed now).

I did a compression and leak-down test that showed.
Cylinder 1 -180psi (under 10%)
Cylinder 2 - 25ps (Aorund 30% And a lot of air could be heard in the intake)
Cylinder 3 -130psi (Under 10%)
Cylinder 4 -140psi (Under 10%)
I can't find the paper with leak-down notes, so that's what I can remember they were

I took the engine out and the cylinder heads were sent to the workshop so that they could be rebuild.
(They grinded the valve seats, and installed new valve steam seal).

I installed it all again with the newly renovated cylinder heads and ran it warm and made a compression test and leak-down test again
Cylinder 1 -170psi ( 90%)
Cylinder 2 - 170ps (under 10%)
Cylinder 3 -170psi ( 90%)
Cylinder 4 -170psi (Under 10%)
I can clearly hear the air out of intake and exhaust on cylinders 1 and 3.

Can some explain to me the good compression test, but very poor leak-down test on the rear cylinders?
Should I hand over the cylinder head to the workshop again, or should I leave it now where the compression is good enough

It drives fine (I have only driven 10miles on it as it is cold in Denmark right now) , and the vboost kicks in as i should. But it is hard to start (If I use the choke it wont start), and the idle is rough. Sometimes it backfiring/popping out of carburetor 1 when it is around 1500rpm


Carburettors have been separated, cleaned and synchronized.
New spark plugs
New spark plug caps
New gasket kit

 

[Fire-medic]

Since you have good compression, where would it be leaking down from? The valve stems? Assuming the valves are properly-seated/sealed. I suppose I would check the valve clearances again, and the timing chain/timing, to ensure that the valves are fully-seated when they are supposed to be.What are your air bleed screws below the diaphragms set-at? Are their rubber -rings in good shape? Did you do the carbs yourself? Can you see through the pilot jet? It doesn't take much to plug it. Deteriorating internally fuel lines could easily pass-along debris which could clog the pilot jets, besides the usual problem of a rusty gas tank shedding material into the gas supply.

 

[sdt354]

Cam timing?

 

[Scrallar]

When I do leak-down, I can clearly feel/hear the air in the intake and exhaust. So my guess is the air passing the valves...
But they should be properly-seated/sealed after the workshop has rebuild them. But maybe they have done a bad job. I just don't understand why there is such good compression then..

Valve clearances was set to
Exhaust 0,29-0,30mm
Intake 0,14-0,15mm

The air screw was set 2,5turns out, and the rubber o-ring looks in good shape.
I did the carbs myself, and they actually looked fine when I separated them (no dirt in them). Jets were not clogged, and the float was set to the right height.
The diaphragms on the sulides has no damage or holes, one has softer rubber than the other three, oo I think one of them has been changed at some point, but it should not mean anything for rough idle..

Next week I will try
Check valve clearance.
Check the chain/timing
Check that the camshaft dont push on the valves when the piston is in TDC.

 

[MaxMidnight]

I assume you are doing the tests with the motor at operating temperature?

 

[Scrallar]

Yes sorry I forgot to write. Compression test and leak-down test were made when it was operating temperature

 

[SpecOps13]

I've had 2 Vmax's. One in immaculate condition. Under 20 k Miles.. One Modified with 37 k Miles..
The first read 160 on all 4 cylinders with a slight variation of about 5 Pounds Max. Minimal leak down.
The one with Engine Mods reads 220 + or - about the same with minimal leak down. Both run perfect.
If you are running 170 lb that sounds pretty good. You say that the bike runs fine. I'm really wondering
where the problem is. Are the heads torqued correctly and are you sure the cams are timed correctly?
Would almost have to see the bike to say that there is a problem..Any chance of doing a video? Need more info...

 

[sdt354]

Can you do the leak down test again? Anybody can make a mistake. Building good compression, but not holding on the leak down test. I'd redo it just to eliminate the possibility. Then a recheck on cam timing and clearance. After that I'd pull the head off again for inspection.

 

[Scrallar]

SpecOps13:
Yes the heads torqued correctly, and in the right order. I will check timing when I get the opportunity next week. But I'm 99% sure it is right.
I will take some pictures/videos next week.


sdt354
I actually made compression and leak-down test three times to be sure, and every time it is cylinder 1 and 3 where the air escapes out of intake and exhaust when I do the leak-down test.

 

[sdt354]

removed

 

[drum4fun]

I assume you have them at tdc when doing the leakdown

 

[Scrallar]

Yes I turn the piston in tdc on the cylinder I leak-down test.

 

[El Canadiense]

Just wondering if the bike was in 5th gear, rear brakes locked on and rear wheel locked to prevent movement? If not, it is possible the wheel turns while doing the leakdown test and move the valves into an open position.

 

[sdt354]

Yes I turn the piston in tdc on the cylinder I leak-down test.

On the compression stroke hopefully. I realize this is very basic. No disrespect intended.

 

[Scrallar]

Just wondering if the bike was in 5th gear, rear brakes locked on and rear wheel locked to prevent movement? If not, it is possible the wheel turns while doing the leakdown test and move the valves into an open position.

If the piston is not 100% in TDC, then it will skip a stroke as soon as I add air. And it's clear to hear if it does.
So it is not necessary to keep the engine locked if you have got the piston 100% in TDC

On the compression stroke hopefully. I realize this is very basic. No disrespect intended.

I must admit that I have not noticed whether it was compression stroke or the exhaust stroke
I thought that as long as the piston was in TDC, the valves were closed.
But now I've seen a lot of youtube videos and can see they all say it should be compression strokes. (I am new to leak-down test equipment)

I'm glad you wrote it so I can do it correctly

This weekend I will do a new leak-down test where I am sure it is in compression stroke. And then I make a little movie too, and check timing

 

[sdt354]

That would explain the good compression and bad leak-down results. The exhaust valve is fully open on the exhaust stroke and the intake valve is starting to open even before TDC on the exhaust stroke. If I recollect?

 

[El Canadiense]

http://www.dansmc.com/leakdown.htm

 

[VmaxKen]

Don't feel bad. Not your fault. It's the changing of the times. use to be almost every 'self respecting' high school had 'Auto Shop' classes. My high school's class was considered to be one of the best in all of San Francisco. But 'something happened on the way from that place'. I own and operate the oldest 'Motorcycle Tire, and Modification Center' in California - (if not ((THE)) entire country. In the last few years we have been having problems hiring 'mechanics',
I refuse to use the modern term 'techs'. (I'm old school, and funny that way). I went to my old high school to see if any one there would like to work as a 'mechanic'. I didn't even have to go inside. Driving by, (through the windows)
, I saw all our old automotive equipment, workbenches and 'lifts' had been replaced by desks, with computers on them.

I guess it made sense, as "the times,they are - a- changin", (a 1964 Bob Dylan song) Sure I get that - but we can not find anyone, (man or woman), willing to even get their hands dirty - Let alone knowing the basics of 'internal combustion' engines.

So, no - there is no such thing as a 'stupid question'. And you are not stupid, ignorant, or 'dumb'. We are only products of our our own 'life experiences'.

I do not know what a 'future' world will be like, with millions of 'computer techs', but very few ACTUAL mechanics.

I guess (for some) chang is not always a good thing?

 

[VmaxKen]

Don't feel bad. Not your fault. It's the changing of the times. use to be almost every 'self respecting' high school had 'Auto Shop' classes. My high school's class was considered to be one of the best in all of San Francisco. But 'something happened on the way from that place'. I own and operate the oldest 'Motorcycle Tire, and Modification Center' in California - (if not ((THE)) entire country. In the last few years we have been having problems hiring 'mechanics',
I refuse to use the modern term 'techs'. (I'm old school, and funny that way). I went to my old high school to see if any one there would like to work as a 'mechanic'. I didn't even have to go inside. Driving by, (through the windows)
, I saw all our old automotive equipment, workbenches and 'lifts' had been replaced by desks, with computers on them.

I guess it made sense, as "the times,they are - a- changin", (a 1964 Bob Dylan song) Sure I get that - but we can not find anyone, (man or woman), willing to even get their hands dirty - Let alone knowing the basics of 'internal combustion' engines.

So, no - there is no such thing as a 'stupid question'. And you are not stupid, ignorant, or 'dumb'. We are only products of our our own 'life experiences'.

I do not know what a 'future' world will be like, with millions of 'computer techs', but very few ACTUAL mechanics.

I guess (for some) chang is not always a good thing?

 

[VmaxKen]

Don't feel bad. Not your fault. It's the changing of the times. use to be almost every 'self respecting' high school had 'Auto Shop' classes. My high school's class was considered to be one of the best in all of San Francisco. But 'something happened on the way from that place'. I own and operate the oldest 'Motorcycle Tire, and Modification Center' in California - (if not ((THE)) entire country. In the last few years we have been having problems hiring 'mechanics',
I refuse to use the modern term 'techs'. (I'm old school, and funny that way). I went to my old high school to see if any one there would like to work as a 'mechanic'. I didn't even have to go inside. Driving by, (through the windows)
, I saw all our old automotive equipment, workbenches and 'lifts' had been replaced by desks, with computers on them.

I guess it made sense, as "the times,they are - a- changin", (a 1964 Bob Dylan song) Sure I get that - but we can not find anyone, (man or woman), willing to even get their hands dirty - Let alone knowing the basics of 'internal combustion' engines.

So, no - there is no such thing as a 'stupid question'. And you are not stupid, ignorant, or 'dumb'. We are only products of our our own 'life experiences'.

I do not know what a 'future' world will be like, with millions of 'computer techs', but very few ACTUAL mechanics.

I guess (for some) chang is not always a good thing?