Compression drop.

Originally - Too much oil? (from RS general talk)

Hi All
2007 RS Venture back at the dealer. 20p.s.i., 220p.s.i., 60p.s.i., respectively. I've had it in before and complained about hard starting, and poor mileage. The first time they synced the carbs. Carb sync about 2000 miles after oil change. All seemed o.k. for a couple of days. The sled never ran right since it's last oil change. I think the dealer put in too much oil.
Since the oil change at Arnprior Sportland, performance has been way down, mileage way down, hard starting etc. I have also put 3500miles on it since the oil change. It had been getting worse every time I used it. Can too much oil cause it to carbon-up enough to drop my compression as stated above, over this period of time?
Maybe the carbs being out of sync was caused by carbon build-up in the first place?

The oil or carbon has nothing to do with compression. The rings & valves seating correctly is what makes compression in a 4-stroke.

Sounds more like the valve lash has tightened up enough on a few of them to cause the valves to hang open slightly & cause a lack of compression, & thus a lack of performance & high fuel consumption.

These engines are supposed to go 20,000+ miles before a valve clearance check, but yours may be the exception.

One more thing that certainly isn't doing the engine any good is not getting the oil changed at the proper intervals. 3,500 miles is way, way over due for the oil to be changed. Book states 2,500 miles, I change mine every 1,000. I personally don't care what the book says, or if the engines aren't run in a dirty environment, they are still being run at high rpms & high loads. Therefore, in my opinion, the few extra bucks for oil is worth every penny in terms of engine longevity.

I think you mentioned in another thread that some of the valves were now too tight. If so, that could cause all kind of problems including low compression, carbon build up on the valve heads, and possible burned valves. Did Arnprior Sportland mess with the valve shims? CV carbs out of sync will mostly cause rough idle and maybe poor starting, but shouldn't contribute to carbon build up. If your valve clearance is really too tight, either it came that way from the factory (hard to believe), or someone that didn't know what they were doing messed them up. The latter seems likely if you saw an immediate performance drop after having it serviced. Sounds like you might have a warranty claim or a law suit on your hands. Good luck!

The Genesis 120 is one of the most reliable motors ever built.

A valve check was done on my motor at 22,500 miles. 3 intake valves were slightly out of spec tolerance. The remaining valves were perfect.

I should pass 35,000 miles today, once I get on the trail.

I did...


Your Dealer and Yamaha needs to handle this problem.

How competant is your dealer and mechanic?

Wishing you the all best and a timely solution to your motor problem.

Sounds like you have bad rings..

You are burning more oil because the rings are letting oil into the cylinders

Time for a rebuild.

THere has been several 4 strokes at my deale for rebuilds, a couple blew up and others were low on compression

20p.s.i., 220p.s.i., 60p.s.i

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Thats more of a variance than you can get from just plugged up rings. Thats either catastrophic failure (which is EXTREMELY unlikely since it still runs), or it must be valves.

The lubrication system on these things is such that they are insensitive to having too much or too little oil. The amount of oil in the engine levels out almost immediately after starting.

Regarding mileage between changes; again contrary to what some have stated, NOT a big deal. Oil doesn't really wear out, mainly it gets contaminated, so the ability to lubricate doesn't go away. You mainly want to change the oil before it gets completely saturated with contaminants, otherwise you can get sludge. The other time you want to change the oil is before long term storage, otherwise you can get internal corrosion. A thousand miles isn't going to push you over, the maintenance schedule is written VERY conservatively and takes into consideration VERY poor oils with very little capacity to suspend contaminants.

The first step here is definitely a top end inspection - valves, upper cylinders, top of pistons.

theCATman said:

The oil or carbon has nothing to do with compression. The rings & valves seating correctly is what makes compression in a 4-stroke.

Sounds more like the valve lash has tightened up enough on a few of them to cause the valves to hang open slightly & cause a lack of compression, & thus a lack of performance & high fuel consumption.

These engines are supposed to go 20,000+ miles before a valve clearance check, but yours may be the exception.

One more thing that certainly isn't doing the engine any good is not getting the oil changed at the proper intervals. 3,500 miles is way, way over due for the oil to be changed. Book states 2,500 miles, I change mine every 1,000. I personally don't care what the book says, or if the engines aren't run in a dirty environment, they are still being run at high rpms & high loads. Therefore, in my opinion, the few extra bucks for oil is worth every penny in terms of engine longevity.

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If I knew I had to change the oil every 1000 miles, I sure wouldn't have gotten a 4-stroke. $136.95 to change the oil at Arnprior Sportland. I can buy a lot of 2-stroke oil for that king of money. Ski-Doo 2-strokes are better on fuel than Yamaha 4-strokes also. I run 4000-5000 miles a season. The cost of running 4-strokes is a lot higher than expected.

Well then sir, you're getting raped up the #*$&@ really bad for a simple oil change. I hope you ask for lube on you too, along with lubing the sled.

I do pretty much all the maintainance on my toys myself. I'd suggest you do the same especially the oil changes. Know how much my oil changes cost me on my sled?? $25 TOPS!! 4 quarts of Mobil 1@ $4.59 each & 1 filter @$5.99 plus tax.

You state you average 4-5k per season. The average 2 stroke sleds burn a gallon of oil every 500-600 miles @$40/gallon for good oil, not Wal-Mart $hit. So.... you'd need at least 10 gallons to get you thru your average season. My math comes out to $400+ 2-stroke oil VS $50-70 assuming an oil change every 2,500k. This is doing it yourself.

I never said you had to change your oil every 1,000 miles. I said I do!! You get more miles per season than I do. I get 1-1.5k per season, so I change mine once a year. Yamaha reccomends an oil change every 2,500 miles or once a season, which ever comes first. I did state that running it way over on service intervals certainly isn't doing the engine any good.

So if you want to call names & throw insults from behind a computer screen tough guy, I guess I am "THE KING OF MONEY"

Lazybastard.... oil certainly does break down & lose its lubricating abilities. You 100% right about forming sludge. That's when the oil is breaking down & not lubing properly.

I have seen many, many engines that have been neglected as far as regular maintainance is concerned, but I'm sure you're not interested anyhow. You guys just keep running your high revving, high load engines overdue on services & cry me a river when it wears out prematurely.

Now that I hope I'm done defending my opinions & facts, back to the original issue. Find a better mechanic & get the valves relashed.

Roger, a question and an observation:

How many miles are currently on your sled?

Tight valves will make an engine hard to start.

Have the engine leaked down by a capable mechanic and you'll know if the problem is valves or rings.

theCATman said:

oil certainly does break down & lose its lubricating abilities.

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Except that it would take you about a MILLION miles for it to be significantly affected, therefore it's not a relevant consideration. Ever heard of oil recycling? The same old oil that comes out of your engine can get re-processed into like-new or better than new oil for use again. The reason for changing your oil is to get rid of the various contaminants, and as long as it isn't saturated, its still good to go, just not necessarily good to sit.

With respect to valve adjustment, here is what I'm seeing after the 50 or so RX/Apex engines I've done.

Ride your sled primarily on the trails, presumably keeping the RPM in the 50-70% range of redline and you'll probably come in close to the Yamaha interval for service. Spend a lot of time lake running, upeer third to WOT and your valves will be close to out of spec. by as little as 5K.

The attached .xls is how a customers engine left the shop. Most of the shim values were 15-25 higher when the engine was broken down for rings. Wish I could post the 'before' .xls, but I had a hard drive failure back in Feb and lost some data.

You might notice that both intake and exhaust are set near the max. spec. This is a boosted motor, and closer to max = less overlap, which is a good thing on a boosted engine.

No real comment on the oil thing other than to say oil is cheap, motors are expensive (so is my labor)...

First off, nice read there lakercr. Nice to see another person, (and a PAID MECHANIC) on here who has a clue on what is most likely wrong with this guys engine. BUT!!!!..... judging by the slam I've gotton on my posts by the "know it alls" you'll prolly be next for the punches & kicks. Good luck is all I can say.

I have seen many, many engines that have been neglected as far as regular maintainance is concerned, but I'm sure you're not interested anyhow. You guys just keep running your high revving, high load engines overdue on services & cry me a river when it wears out prematurely.


Just a regurgitation from my above post. Think whatever you want, but to get jerky like you, ever heard of thermal breakdown??? When oil starts to break down its lubricating properties are no longer effective & surprise, surprise ..... engine wear!!! Your arguing about f'n oil isn't even what the issue is with this guys engine. I originally posted that most likely some valve clearances have tightened up that's what I'd put money on is wrong. However like another person posted a leakdown test will tell for sure. If air is heard coming from the breather then it's rings, if air is heard coming out the exhaust or intake it's valves.

I'm done with this thread. First, I get insulted by the guy who I'm trying to help, because apparently he knows what he's doing with sleds & doesn't need someones opinion & help, but yet posts a topic on why his engine isn't running right and hasn't the slightest clue what it could be. Then when I post my opinions & facts about engine maintainance, I get a raft of $hit again from someone else that i don't know what I'm talking about.

Man, I've learned alot on this forum & I thank the people who know WTF they are doing & talking about, but I tell 'ya...... weed thru the piles & piles of garbage on here, there's only about 40-50% of the population on here who are credible.

To the original poster - I'd encourage you to take my advice and have a leak-down test done. There is something very wrong with that mill to be putting up those compression numbers. Frankly, I'm amazed you can even get it to start.

If you're unfamiliar with this procedure, the quick description is that you're putting shop air (air from a compressor) into the combustion chamber through the spark plug hole. The shop air is attached to a gauge (two actually) that reads % of air leaking out of the combustion chamber. The benefit to doing this as opposed (or in addition to) a simple compression test is you get an indication of where your combustion pressure is going (as another poster has already explained). If you can have this done and post the results there's a pretty good chance I'll be able to tell you what the problem is.