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Discussion Starter · #1 ·
Picked up this sled for my wife and ice fishing! Has 5300 miles. Di a compression test
and got approx 120 per side. Seems low. My 500ss is 136 psi .
I realize that the numbers being close are just as important but like to know what these motors were
suppost to put out new.Thx for any help
 

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On my 2002 380 and my 1988 377 I have about 135 (fresh rebuild), but my gauge is a little on the low side.
120 even for me is low, should still run but would good candidate for new properly sized rings and a fast stone honing.

Better question is is it hard to start and will it restart when hot?
 

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So much variance in gauges leads to a vast difference in readings. If you you are seeing 120 psi on a gauge that reads 136 on a liquid I think the fan engine is in as good of shape as the liquid. Just my opinion.
 

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Discussion Starter · #7 ·
Thx Guys. Will hold off on the new top end then. It does start and run when hot or cold,no issues there. My test numbers were taken with the motor started cold and run for 8 minutes, reved on stand to 5 grand a few times to warm it up good.
Worth mentioning ,i have test ridden a few of these 05/06 Revs with the 380 fan and there really down on power
compared to the older 380,s from say 03 and back into the 90,s. A friend has a 99 380 totally stock and it will totally smoke these newer 380 Rev,s. Anyone else notice this?
 

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Discussion Starter · #9 ·
Never take compression after running the engine, it should be done only after at least 12h of rest, that would be a good reason the numbers are low.
One thing I do know is that u dont drive the sled with a cold motor. Reason for taking a cold reading is only to compare to the hot reading. If seal is good( rings/pistons in cylinders) both readings should be very similar.(Hot vs cold) If the hot reading is less than 5 psi from cold it could use rings, if less than 10 psi then pistons also and possibly cylinders are oval. The more blow bye the less seal and therefore less power. This is strait from Bill Cudney who builds engines and raced for many years. I do appreciate your advise. Compression on a 2 stroke is critical not so much on a 4 stroke.
Obviously it will still run and operate ok with less compression but your losing power and fuel mileage will suffer slightly also. It will also Start easier with good comp especially when engine is hot!
 

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On my 377 and the 380 I have taken compression reading hot and it would be any where from 90-120, when taken cold it was always 132-135, so hot is worthless for me, the numbers are not constant enough. If it works for you then use it.
Are the hot numbers varying so much on a fan engine becose the block temp varies more compared to a liquid cooled engine I have no idea.
Also when taking the compression the throttle should be fully open.
 

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On my 377 and the 380 I have taken compression reading hot and it would be any where from 90-120, when taken cold it was always 132-135, so hot is worthless for me, the numbers are not constant enough. If it works for you then use it.
Are the hot numbers varying so much on a fan engine becose the block temp varies more compared to a liquid cooled engine I have no idea.
Also when taking the compression the throttle should be fully open.
12hrs of rest? Kinda an arbitrary number, no?

That's just to marinate steaks.

Throttle open or closed is irrelevant, open just speeds up the process as more air can pass through the carbs quicker. The end result will be the same eventually. The compression is what it is.

Just ride the thing. It's a low power fanner, it's fine.

Sent from my Pixel 3 XL using Tapatalk
 

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All I am saying is from past experiance that works for me and from information I have picked up for more experianeced mechanics.
On the internet it is easy to get information from second/thired hand people that have never even tried what they are quoting on the other hand I am 55 and have been taking apart engines since I am 12, no I am not a professional mechanic I am more of a shady tree one, but in the past I have learned from professionals and have asked questions to actual mechanics that have dedicated their lives to the job.

So a 12h is arbitrary, most recommendations is for over night, but 12h has always worked for me.

When I take the compression I pull the cord 4 times and every time I look at what the gague is doing ex: 1st pull 40lb 2nd pull 100lb 3rd pull 133lb 4th pull 133lb, next cylinder if the numbers are off I know somthing is not looking to good.

Riding a low power fanner could be much more fun than riding a brand new MXZ XRS competition package... or what ever.

It is important to verify the compression on a regular basis, when I rid with my 1988 safari sometimes I am 50-75 km's away from home, I don't feel like walking back.

If you don't open the throttle wide open you don't know how many pulls will be required to fill the cylinders with air, again this was recommended by actual mechanics, this is not something that was read on the net. Some carbs have less than 1mm opening when the throttle is closed as compered to 30+mm.
Try sucking on a 1mm straw and after that suck on a 1" pipe and tell me if you see a difference.
 

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I’m not trying to get into an argument. It’s been a source of differing opinions over the years. My opinion is it makes very little difference what you do with the throttle on a two stroke. Heck it doesn’t even matter it there’s a reed valve let alone a carburetor. While cranking with the recoil or electric starter, until that piston covers the exhaust port not much is happening in that engine except atmospheric pressure.
 

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Discussion Starter · #14 ·
12hrs of rest? Kinda an arbitrary number, no?

That's just to marinate steaks.

Throttle open or closed is irrelevant, open just speeds up the process as more air can pass through the carbs quicker. The end result will be the same eventually. The compression is what it is.

Just ride the thing. It's a low power fanner, it's fine.

Sent from my Pixel 3 XL using Tapatalk
Mill,guys like u crack me up! Read my first post, again. I just purchased the sled.I always do a comp test on a new purchase of a used sled to see if motors ok, Its common sense and good practice. Im not looking for a rocket ,I already have 500ss and an 800x . I was planning on JUST RIDIN THE THING. Lighten up a lilt was a simple request for a starting number, never had one of these before.
 

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in the past I have learned from professionals and have asked questions to actual mechanics that have dedicated their lives to the job.
They are good and there is much to learn from them. Participating on DooTalk holds the advantage of having this experience to work from and to push the subject beyond the status quo. On a 600 that I tested and posted the results, the mag side first showed 136psi, and 15min later 148psi, all with no oil added through the plug holes. This is one procedure that I would like to see evolve.
 

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Thx Guys. Will hold off on the new top end then. It does start and run when hot or cold,no issues there. My test numbers were taken with the motor started cold and run for 8 minutes, reved on stand to 5 grand a few times to warm it up good.
Worth mentioning ,i have test ridden a few of these 05/06 Revs with the 380 fan and there really down on power
compared to the older 380,s from say 03 and back into the 90,s. A friend has a 99 380 totally stock and it will totally smoke these newer 380 Rev,s. Anyone else notice this?
They restriction plates in the carb boot. You can either buy the older boots without them or you can open them up by grinding them out or cutting them out. After that you have to re jet it and it will wake that motor right up.
 

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Whey would they ever use restriction plate or what ever in these carbs, the 380 has about 65hp, restricting it would probably create worst gas millage by having to openning the throttle more.
 

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Discussion Starter · #19 ·
Yes u are correct ,there's restrictor carb boots on the Rev 380 (as per Doo dealer) on the Rev 380 fan from 2004 thru 2006,the only years the Rev 380 existed. Apparently as per Skidoo the motor wasn't spa compliant and that was the cheap way of getting it thru. Compression is down also. 125 per on 04 thru 06. were as the older 380 sleds had way more power and compression.Friend has an stock 03 and its much quicker. I would like to know of what carb boot off older sleds that work inlace of these restrictor boots currently on my sled. I removed them and can cut out a bigger hole but not much material left and boot will be flimsy and may split. Anyone change the boots?
 
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