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Compression


sonofdennis

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sonofdennis

Did my operating temperature compression check last weekend. My gauge is in 5lb increments. As best as I can figure(I did it twice each cylinder), the compression is Left 160 psi and Right 161 psi. Manual says over 145 hot is good and that normal is 125 - 145.

Procedure:

Heat bike to operating temperature (my case a 45 minute ride on 100 degree day), pull plugs then test for compression with full throttle applied.

Bike likes to ping and rattle on a hot day in a 6th gear 3.5k to 4.0k roll on.

Carbon? Read that high compression could be carbon build up and causing pre ignition. Bike now has 42k miles. Been running techron through it last three tank fulls.

Opinions?

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Did my operating temperature compression check last weekend. My gauge is in 5lb increments. As best as I can figure(I did it twice each cylinder), the compression is Left 160 psi and Right 161 psi. Manual says over 145 hot is good and that normal is 125 - 145.

Procedure:

Heat bike to operating temperature (my case a 45 minute ride on 100 degree day), pull plugs then test for compression with full throttle applied.

Bike likes to ping and rattle on a hot day in a 6th gear 3.5k to 4.0k roll on.

Carbon? Read that high compression could be carbon build up and causing pre ignition. Bike now has 42k miles. Been running techron through it last three tank fulls.

Opinions?

 

Alan, first make darn sure your compression tester is reading correctly.. There is probably a certain amount of error in your gauge (how much probably depends on how precision it was to begin with & how it’s been treated in storage & in past usage)..

 

If in fact your compression tester is calibrated fairly close to it’s dial readings then you probably do have a carbon build-up problem.. Pull a spark plug & look inside the cylinder at the top of the piston… A small dash light bulb soldered to a couple of wires then passed in through the spark plug hole will light up the entire inside of that cylinder..

 

Tecron would not be at the top of my list for heavy carbon removal.. Probably be better off using a commercial carbon softener (personally I like the older GM top end cleaner)..

 

If you want to run something as you ride there are other types of carbon softeners available like Sea Foam or the like..

 

 

Plain old water works about as good as anything but you best know what you are doing to use water as it can stick piston rings, score cylinder walls , or work it’s way into your engine oil system if used incorrectly..

 

Twisty

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sonofdennis

Compression gauge is 8 weeks old and has never left the case except for one time I checked compression cold. I don't think the gauge is off by much if at all. Top of piston is black with what appears a thin crust. Intake valves when I had the TB off looked like a lot of old oil buildup on the stem. Bike uses practically no oil. I don't want to pull the heads unless I absolutely have to. I'll look into sea foam. The old timer I bought the bike from I think lugged it a lot from talking to him. He would shift at 3k RPM as his limit.

Carbon may also explain why some times when I start the RT after fully warm it will detonate. Sounds like a hammer on the piston, only once was it hard the other times it's more slight.

 

Alan

 

Thanks

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Compression gauge is 8 weeks old and has never left the case except for one time I checked compression cold. I don't think the gauge is off by much if at all. Top of piston is black with what appears a thin crust. Intake valves when I had the TB off looked like a lot of old oil buildup on the stem. Bike uses practically no oil. I don't want to pull the heads unless I absolutely have to. I'll look into sea foam. The old timer I bought the bike from I think lugged it a lot from talking to him. He would shift at 3k RPM as his limit.

Carbon may also explain why some times when I start the RT after fully warm it will detonate. Sounds like a hammer on the piston, only once was it hard the other times it's more slight.

 

Alan

 

Thanks

 

Knock on hot startup, especially if it sounds more on the Left side, is almost certainly the camchain tensioner. My cure for your bike would be to find a long empty cop-free road somewhere and ride the bike long, hard to the redline, and in high gear through a couple of tankfulls of fuel. That will burn off the carbon without chemicals and is much more fun.

 

Andy

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Bike likes to ping and rattle on a hot day in a 6th gear 3.5k to 4.0k roll on.
Not sure that's all that unusual. Is it really bad or just barely noticeable?

 

FWIW I have always had a difficult time getting compression pressure readings off of a gauge correlate very well to manufacturer's specs. More important than the absolute reading is variation between cylinders, and if the variation is low (and there are no obvious running problems such as low power, oil consumption, excessive pinging, etc.) then I certainly wouldn't tear into an engine based on some high compression readings alone.

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Don't worry about it. If the difference between 145 and 160 was caused only by carbon buildup, you would be experiencing the worst case of detonation or preignition mankind has ever seen!

 

My K100RT also shows about 160, which is a lot higher than the stated "normal" value, and it runs great.

 

What matters above all is that the values are more or less equal between cylinders. One thing that really affects the readings is the amount of air volume trapped in the compression tester before its valve. If your tester has less extra volume, than the one BMW uses, then your values will naturally be higher.

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sonofdennis

Pinging seems to follow fuel source to some extent but not totally. Pinging also seems to happen first roll on, after I back off and load her up again it doesn't ping as bad....pure FM as far as I can tell. Detonation at start up does tend to be left cyl. but I have already replaced the tensioner with the newer version. I do have a long multi day trip ahead of me so maybe I'll run her out. Is it worth while to put some sea foam into gas for a tank or two? Then run it out?

 

Alan

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Compression gauge is 8 weeks old and has never left the case except for one time I checked compression cold. I don't think the gauge is off by much if at all. Top of piston is black with what appears a thin crust. Intake valves when I had the TB off looked like a lot of old oil buildup on the stem. Bike uses practically no oil. I don't want to pull the heads unless I absolutely have to. I'll look into sea foam. The old timer I bought the bike from I think lugged it a lot from talking to him. He would shift at 3k RPM as his limit.

Carbon may also explain why some times when I start the RT after fully warm it will detonate. Sounds like a hammer on the piston, only once was it hard the other times it's more slight.

 

Alan

 

Thanks

 

Alan, it really doesn’t take much carbon to boost compression a few pounds.. Being within 10 psi of specs is usually considered OK & you are only a little above that..

 

I would imagine that you have a little carbon build up on both the cyl heads & piston tops so added together that could easily increase the compression slightly..

 

If your valves are adjusted on the loose side of spec that can increase it slightly more if a slightly earlier closing valve..

 

That carbon on the back of the intake valve is from a bit of oil running down the valve stem after engine shut down.. A little won’t hurt but a lot will impede air flow into the cylinder at high RPM’s..

 

On that hot restart knock? If only on one side (the L/H side) & it goes away right after start you probably have the L/H cam chain tensioner bleeding down (there are service parts to remedy that).. If you are talking about drive away spark knock then that is either a compression issue or spark timing issue.. (some burst knock on heavy throttle up seems to be present on SOME BMW R bikes)..

 

You might pop off the timing hole plug & shine a timing light into the timing hole to see if your ign timing is close to where it belongs.. I know it is supposed to be non adjustable but there is enough slop in the HES attaching holes to allow it to be off enough to make a difference.. (won’t hurt to check it anyhow)

 

Twisty

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When you do the compression test also leave the throttle at least part way open when cranking . I have seen it on vehicles cause a higher reading increasing as each cyl is tested . Cannot give an explanation just my experience . Dave

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sonofdennis

Great idea on timing light in timing hole. I love using my tool box toys. On the RT 1150 which plug wire? I guess kind of a stupid question since my light is induction and I have a 50/50 chance making trial and error simple

 

Thanks,

Alan

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Are you running premium gas? If not, you can reduce knock by using higher octane gas.

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sonofdennis

Both plugs fire each revolution?!!! Say it's not so

My Harley was the same way :-) I guess no cost to wasting a spark.

I use premium name brand fuels to try to keep the quality up but so far not enough to stop all pinging. Has any one had good luck with an octane boost?

Thanks,

Alan

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