Compression lowered in cylinder #3

[Erga]

Reposted here as I managed to put this to Gen 2 board

Got myself my first V-max (2000 Carbon) last summer and I've been so far really happy with the thing. However seems that I've run out of luck as I started feeling momentary power loss and exhaust popping when driving on stable revs (2000-4000) in the beginning of summer. After some investigation the spark plug in cyl #3 was pitch black. Swapped the plugs without any improvement so I decided to take it to a shop. They confirmed that cyl #3 is not having proper pressure so it is leaking it somewhere. They checked coils, valves, cleaned carburetors and changed a load of gaskets but nothing helped. So they gave up...

Before I take it to another shop and pay another 1000 I figured that I might as well seek some help from forums if people have had similar issues. Any advice where to start looking for the culprit would be highly appreciated.

Some facts as requested by @Fire-medic from the shop:
- Pressure difference was around 2% between the problem cylinder and the rest of the cylinders (88psi vs 90psi). With added oil drop it was even 95psi. They use a meter that doesn't seal the hole fully, hence the readings are lower that with some other meters.
- The shop changed gaskets were basically all carburetor gaskets
- The shop didn't remove the engine (because here in Finland the driving season is short and all shops 100% loaded with service) so they couldn't rule out the fact that there wouldn't be a micro leak in the valves due to deposit
- The shop also suggested that there could be an issue with the idling circuit and modern fuels that can cause deposits there although the carburetors were cleaned

 

[MaxMidnight]

First thing, are the deposits sooty or oily and is only one cylinder that has this?
When you say 'swapped the plugs' was this with new ones or those in the bike.

The pressure readings are low, they should be around 142 p.s.i. so my best guess is that the check was done with the throttle closed and/ or the engine wasn't at operating temperature.
A 2% difference between cylinders is well within spec, Yamaha specify a 14 p.s.i. difference is acceptable.

Invest in a compression tester and do the job properly - the only extra kit you need is a plug spanner. I suspect you won't find an issue there but if you still get reading of around 90 p.s.i. try seeing if some oil in the bores brings this up. If not you need to check valve clearances.

However, before you do that you need to find out why the plug is black. If sooty then there excess fuel being burnt which can be down to several issues.
Start with a new set of plugs and do a plug chop at (say) 3K where you are getting the problem.

Depending on what you find will determine ther next step.

 

[Fire-medic]

They use a meter that doesn't seal the hole fully, hence the readings are lower that with some other meters.

I'm not familiar with this type of equipment, it seems such a device would be circumventing the purpose of a compression check altogether.

Invest in a compression tester and do the job properly - the only extra kit you need is a plug spanner. I suspect you won't find an issue there but if you still get reading of around 90 p.s.i. try seeing if some oil in the bores brings this up. If not you need to check valve clearances.

I suspect they checked/set the valves? As MaxMidnight says, oil not increasing thins, something's probably going on with the valves. That was the point of my next test, the leak-down test.

 

[Erga]

Thank you for your advice, I will get a tester and will take a closer look again.

At first I bought new spark plugs and swapped those. Based on the attached document, the old plug of the failing cylinder was fouled (attached a picture of the old plugs).

The shop I used was official Yamaha dealer so I would have expected them to have correct compression measuring equipment but they explained that there are two kinds (don't know about those).

I was also suspecting the valves but the mechanic told me he checked them to be ok.

 

[Fire-medic]

Well if it's a Yamaha dealership, they should have the equipment, tools, and skills to repair anything on that bike. Unfortunately doing a dealership tear-down to over-bore the cylinders would be cost-prohibitive. It would probably be less-expensive to buy a good used engine. I don't know what the availability is of those where you are.

 

[MaxMidnight]

Can we assume that apart from the issue between 2 & 4K the motor starts, runs and pulls well and that you don't have excessive oil consumption or smoke from the exhaust?
If yes then any talk of boring or replacing the engine is, IMO, premature.
Hopefully your compression test will settle if there are any problems with the bores or valves and will eliminate one potential problem area.

 

[Erga]

Yes, the engine works well and revs nicely the only way this occurs is the popping and at the same time small losses of power on stable revs when driving. I couldn't get my hands to pressure meter yet but I did find an endoscope and checked the valves through carbs. The cyl 3 valves have quite a lot of deposits compared to other cylinders. maybe I'll do a cleanup for those first.

 

[Fire-medic]

I'm not suggesting you try this! However in the interest of posting a 'shade tree mechanic' historical maintenance artifact, I'll post it.

If you're anywhere close to Social Security age, and have been driving/riding (and wrenching) for 50+ years, you will recall that 'back in the good-old days,' when fuel injection was something only a handful of USA manufactured cars had (the mechanical Rochester fuel injection first offered in 1957) cars were much-more maintenance-intensive, and getting 100,000 miles out of a domestic car without cylinder head removal, and possibly re-ringing/over-bore/crankshaft removal & work was unusual. Tolerances and material were not what they are today, when cars and trucks can often reach 200,000 miles with only oil changes and timing belt replacement.

It wasn't uncommon to have your cyl head(s) removed to be 'de-carboned' and valves 'lapped.' Maintenance intervals were less.

I recall reading about an old-timer's method of cleaning the carbon deposits by slowly putting a bit of water down the carburetor throat. Since water turning to steam expands to 1,700 times its volume, it would work to remove the carbon deposits by the action of water becoming steam. The engine would make alarming noise unheard during normal operation, and the exhaust would pour out deposits off the cylinder head and valves in a sooty cloud.

I am not suggesting that you do this with your VMax! It's only posted as a historic context concerning one method used by some in a bygone era.



Seafoam vs. water

 

[MaxMidnight]

I've no comment on Mr Medics suggestion but you still need to establish a) what caused the build-up and b) if it originates from oil or fuel.
You have no way of knowing how long it took for the carbon to build up but unless you fix the cause it will reoccur.

 

[Fire-medic]

I've no comment on Mr Medics suggestion

Please note I said, "I'm not suggesting that you try this!"

The information was presented from a historic perspective only.