Low engine compression.

[Yamaha_Fan]

I was changing out my sparkplugs today and decided to check my compression while I was at it.

The numbers I got seem pretty low, 125-130psi. This is 97 vmax with 29k miles. The minimum according to the manual I have is 170psi. Wondering what I can expect on an engine with this mileage, any reason for concern?

The good thing is all the cylinders are pretty close together. I did not try pouring a bit of oil in the cylinder and seeing if the numbers improved, but maybe I should go back and do that. The next thing I'm going to do is check my valve clearances.

 

[MaxMidnight]

According to the Yameringha Service manual standard is 142 psi, the minimum 128 and the maximum 171 and the maximum variance between cylinders is 14 psi. On that basis you are just on the minimum.
Not that this is lower that the value in the Specification section where the compression pressure is given as 206 psi. I suspect this is due to the CV slides(?)...I stand to be corrected on this!

Can we assume that you had the motor at operating temperature, the throttle open, plugs removed (and grounded)?

If the answer to the above is 'yes' then the next step as you say is to check the valve clearances and re do the test with squirt of oil down the bores.

Whilst a compression check is a quick and easy thing to do but to fix any 'problems' could potentially be an expensive thing to fix. :bang head:

Is the bike new to you or have you had it for some time? If the latter do you feel that the performance is down or is it using a lot of oil?

If no then I would be inclined to leave well alone.

 

[davesax36]

I think as long as they're within 10% of each other, and you're not burning oil you should be fine.

 

[Damon]

Did you perform the compression test with the carburetor rack removed or with the slides & butterfly valves fully open?

 

[one2dmax]

I would not personally worry much. The biggest thing I look for is that they are within 10% from the highest to lowest which you are well within that. As the others have asked there are specific ways to measure the compression to achieve valid results to compare with the manual requirements.

 

[Yamaha_Fan]

According to the Yameringha Service manual standard is 142 psi, the minimum 128 and the maximum 171 and the maximum variance between cylinders is 14 psi. On that basis you are just on the minimum.
Not that this is lower that the value in the Specification section where the compression pressure is given as 206 psi. I suspect this is due to the CV slides(?)...I stand to be corrected on this!

Can we assume that you had the motor at operating temperature, the throttle open, plugs removed (and grounded)?

If the answer to the above is 'yes' then the next step as you say is to check the valve clearances and re do the test with squirt of oil down the bores.

Whilst a compression check is a quick and easy thing to do but to fix any 'problems' could potentially be an expensive thing to fix. :bang head:

Is the bike new to you or have you had it for some time? If the latter do you feel that the performance is down or is it using a lot of oil?

If no then I would be inclined to leave well alone.

I had the engine warm, throttle open, but I had the other 3 spark pkugs unplugged, not removed and grounded. This bike is new to me.

I wonder why the clymer manual I have has higher numbers for min/max compression.

 

[sdt354]

Compression test can be misleading. Like mentioned already, the test needs to be done as stated.
A leak- down test gives you more information and is more accurate, imho. +1 to equal over high numbers. And if it doesn't burn oil and runs well, check the valve clearance and enjoy. Lining up with another good running, stock vmax in a roll on is a good way to judge your engine performance. Crack those throttles open at the same time see how you do. I'm not suggesting you exceed the speed limit or anything......

 

[blaxmax]

I would worry more about the one lower down that says (3.6 psi).

 

[MaxMidnight]

I had the engine warm, throttle open, but I had the other 3 spark pkugs unplugged, not removed and grounded. This bike is new to me.

I wonder why the clymer manual I have has higher numbers for min/max compression.

Try it again with all of the plugs removed.

Don't know where Clymer got those numbers; possibly with the carbs or throttle slides removed?
Haynes quotes the same values as the Yameringha Service Manual.

 

[Yamaha_Fan]

Try it again with all of the plugs removed.

Don't know where Clymer got those numbers; possibly with the carbs or throttle slides removed?
Haynes quotes the same values as the Yameringha Service Manual.

The Clymer book also doesn't say anything about removing the carbs/throttle slides or the other spark plugs for that matter, and it says to turn the kill switch to off, and then crank the starter, which makes no sense to me because with the switch on off the engine won't crank. I may try it again at some point but for now I'm not really worried.

 

[Yamaha_Fan]

I would not personally worry much. The biggest thing I look for is that they are within 10% from the highest to lowest which you are well within that. As the others have asked there are specific ways to measure the compression to achieve valid results to compare with the manual requirements.

Thanks for the input. I actually was trying to buy some spark plug boots and wires from your website but couldn't get that page to load. I sent you an email last night.

 

[blaxmax]

Thanks for the input. I actually was trying to buy some spark plug boots and wires from your website but couldn't get that page to load. I sent you an email last night.

Have you considered a set of "cops"?

 

[sdt354]

When cranking your engine over, leave the key off and ground the blue wire with the white stripe(on early gen 1's, it may have changed later to another color) The wire runs behind the battery on the right side. Unplug the bullet connector.Use a small screwdriver or any conductor, and ground it. The engine will crank now without high voltage sparks arching to ground. The engine will crank in gear, so put it in neutral first. You also won't have your headlight on draining the battery during the compression test. Remote starters are dirt cheap and worth buying or making for working on your bike or anything else.
Steve-o

 

[Damon]

The method I use maybe correct and/or incorrect but provides me with consistent data. First remove the carburetors & all the sparkplugs. I use what we will call a click #1 reading then a click #5 reading for each cylinder. I generally perform this on my engine test stand which allows for no ignition power (so I don't have to ground the plugs). Click #1 is the first compression stroke while the meter is installed to a given cylinder. I look for that reading to be at least 50% of the total, The total reading is the value once I reach click #5 (the 5th compression stroke) An example would be click #1 = 100PSI. then click #5 = 183PSI. I would consider this to be a good healthy cylinder. With a fresh rebuild I will generally see all four cylinders read 98lbs.~110lbs. during click #1 and click #5 readings of 180PSi ~ 186PSI. As mentioned the 10% rule is also very important as to the balance across all 4 cylinders. Hope this is some help!

 

[Yamaha_Fan]

Have you considered a set of "cops"?

I haven't, mainly because I don't know what they are. I will look into it.

 

[blaxmax]

I haven't, mainly because I don't know what they are. I will look into it.

http://www.vmaxforum.net/showthread.php?t=40354

 

[Traumahawk]

I haven't, mainly because I don't know what they are. I will look into it.

COPS stands for Coil over plugs.....kinda like whats found in the newer cars....and bikes. You do away with the coils and the spark plug wires of the Vmax, and you adapt the COPS from later model sports bikes to the Vmax. The down side is no hp increase....the upside, increased reliability, less parts to have issues with, and if you have a COP go bad, you can pick one up at the local parts store. You cant do that with vmax coils.

 

[Traumahawk]

If your mechanically inclined, several people have bought COPS from donor bikes on ebay, and the wiring harness, and made their own. One person posted that it cost them 25 dollars to make. If your not so inclined, there are several people that make them. I personally have the kit put together by Gannon, and sold thru Sean. It is a top notch product.

 

[blaxmax]

If your mechanically inclined, several people have bought COPS from donor bikes on ebay, and the wiring harness, and made their own. One person posted that it cost them 25 dollars to make. If your not so inclined, there are several people that make them. I personally have the kit put together by Gannon, and sold thru Sean. It is a top notch product.

I appreciate your help, Eric.

 

[Yamaha_Fan]

If your mechanically inclined, several people have bought COPS from donor bikes on ebay, and the wiring harness, and made their own. One person posted that it cost them 25 dollars to make. If your not so inclined, there are several people that make them. I personally have the kit put together by Gannon, and sold thru Sean. It is a top notch product.

I will have to pick up a set. Seems like a good upgrade. I was looking to replace mine anyways because when I was changing my plugs I noticed the wires were all cracked and dry rotted, especially around the area on the plugs.