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Discussion Starter #1
Hi, i repeated a compresion test:
105psi front
95psi rear
Same hot or cold, wide open throttle.
44000km
On previous test at 29000km i had exactely the same numbers.
Almost no oil consumption over any change interval (6000km)
Valve clearance always 0 for closer and 0.10mm opener.
Cam timing not checked yet.
I have to clean tb every 6000km because it became dirty.

It is common? Any advice?

Thanks
 

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Hi, i repeated a compresion test:
105psi front
95psi rear
Same hot or cold, wide open throttle.
44000km
On previous test at 29000km i had exactely the same numbers.
Almost no oil consumption over any change interval (6000km)
Valve clearance always 0 for closer and 0.10mm opener.
Cam timing not checked yet.
I have to clean tb every 6000km because it became dirty.

It is common? Any advice?

Thanks
What model and year is the bike?
 

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Keep in mind without a reading with your gauge at a very early mileage it can be hard to tell. The difference between the two could be considered not ideal but in the useable range. If concerned do 2 things.

Test again with a quality gauge that is not yours to back up the results.

do a leakdown test to measure percentage of loss piston to cylinder as well as valve seal.

I did have a 900ie go through the shop that had burnt valves due to them not being adjusted so this is your main concern and a leakdown will answer the question. You could also simply check that the valves are closing and have clearance on the openers at TDC.

On the gauge a simple Schrader valve of the wrong pressure rating will cause different readings so if another gauge reads different but similar between the different cylinders you can use yours as you have to compare .
 

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Gauges on compression testers are notorious for being inaccurate. That being said, if you have 2 consistently different numbers you can safely assume that rear is 10ish psi lower than the front, but not that your front is actually 105 and rear 95. I don't know the specs for your bike but I would think a 10psi difference between cylinders is within spec, as it is for many vehicles, and keep in mind that no 2 cylinders will have exactly the same compression. Having a very low oil consumption is a good sign and for me could reinforce the notion that your bike is in fine health for its mileage. Common causes for low compression are piston rings and poorly seating or burnt valves, but with a bike that is running within spec and otherwise fine I would not try to chase a problem that might not exist by dissembling your heads. Normal wear on high-mileage engines include piston rings tolerances becoming wider and pitting and deposits on valve seats and valve mating surfaces, both resulting in lowering compression over time.

Another thing you could look at is shining a flashlight down the spark plug hole of both cylinders and seeing if your rear cylinder head looks substantially cleaner than the front. This can indicate that there is a hole in your head gasket and coolant is steam cleaning the cylinder. Past some simple checks like that I would have your engine looked at by a professional if you are adamant about getting to the bottom if the psi differences, and you are not confident about working on engines.
 

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dadouzzu gave us all the clue.
I have to clean tb every 6000km because it became dirty.
No oil use is suggesting good rings. If the throttle body needs cleaning that regularly combustion gasses are coming back into the inlet track. Either massively wrong inlet timing or valves leak.
 

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Discussion Starter #7
Thanks forr all the comment

I know, most compression tester are chinese junk.
There are 3 type of tool on ebay/ amazon
1 cheap cinese crap
2 a bit more expensive european remarked chinese crap (mine)(same object as 1 with eu tag)
3 superexpensiveand unavailable tools. (Like 500euro bgs that is a german brand that also buy from china and apply eu tag)

I know that my flexible adapter has a crappy valve that loose about 1bar.

The universal adapters (with rubber cone at the end) are better.
I check them measuring a good pressure regulator output.
Anyway i will try to do some other check on the tool.
Asap i will also buy a leakage tester (try to not buy crappy china junk :)

@ducvet
All the valve were at 0 closing and 0.1 opening except for horiz intake that has a broken half ring and 0.08mm clearance.
I replaced the half rings on this and the clearance became less than 0.03.
Anyway i don't know how it was prior to 29kkm

@gws and @Stillrollinalong

I will update as soon i can do a leakdown test.
I had disconnected the crankase breathing hose from the breather tank and put it in a white rag to see if the dirt come from crankase.
There is a little smell of burnt oil when i heat up the bike but no smoke or oil residue in the last 100km.

The bike is not liquid cooled and has no leakage outside the engine.
981971


Maybe the intake timing is not perfect but not massevely wrong
 

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Discussion Starter #9 (Edited)
Waiting for the end of lockdown I've done some investigation on my compression tester.
I know the flexible hose adapter is crap. Instead of a proper schrader valve they used a superstrong spring that loose about 1bar, copared with my non chinese pressure regulator gauge.
Rubber cone adapter have a proper valve, but installed in the wrong position. Instead of putting it under the rubber cone they put it at the end of the tube. Also the rubber cone can eventually leak if not sqare and properly forced on spark plug hole.
To partially correct this i cut the adapter and jb welded it to an old spak plug:
984214


Now the reading are more consistent and high:
984215


This measure were done with inlet cam centerline @ 113° and cold engine.
More acceptable?

Edit: timing was 120°v and 117°h
 

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I know that there’s a threshold where compression becomes too low for engine to operate, and generally it seems to be somewhere around 100-ish? I think 125 is very acceptable for those engines, or at least that’s what I was told once.

I have a compression tester that reads 75psi no matter what I use it on. The first time I used it on an engine, I blindly believed the gauge and went ahead and rebuilt the Moto Guzzi engine (which was on the bench anyway). When the rebuilt engine show the same 75psi, I knew the gauge was shite. Ever since, I just keep it so occasionally I can lend it to friends to screw with them.
 

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Discussion Starter #11
I know this things are crap, i checked this up to about 130psi connecting it to a good pressure regulator and comparing the gauges at various pressure points...
After the mod they tracks each other pretty good..

Still remain some empty space between valve and spark plug hole. From the math it can still cause 5 or 10psi lower readings
 

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You don’t race it ? The valve adjustment is good, it doesn’t burn oil, it runs well ?
You are bored. Go ride .
 

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Discussion Starter #13
You don’t race it ? The valve adjustment is good, it doesn’t burn oil, it runs well ?
You are bored. Go ride .
No race, just doing maintenance like everyone here that can't ride it because of the lockdown:(. And to just hoping for no downtime after that.

No big issue apart some roughness/misfire around 4000rpm, 15°, no idle under 1000rpm, require fast idle lever up to 40°C and frequent throttle body cleaning.
 

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You can blow some air into the combustion chamber and see if it leaks when each valve is closed. You can usually hear or feel it and compare the leakage, or know you have a really bad one.
 
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