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Slight loss of compression and not idling nicely

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mattwilkinson

Joined Jun 2014
123 Posts | 2+
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Hi Everyone,

My 2007 FE550 had a leaking intake boot and a leaking water pump seal...
I have fixed the water pump seal and thought I had fixed the intake boot, But I have experienced a slight drop in compression


I never used to be able to kick over the bike this easily, and the electric start used to struggle.... It seems like the intake boot still isn't sealed, or something else isn't right because when you rev it, it doesn't sound clean


Can a leaking intake boot cause loss in compression?
 
No sir....

A leaking boot will make the engine run more lean, as it is now not controlled by the carburetor. (You getting extra air in the chamber)

The rough idle could be from that...

I don't want to spook you, but what do you do with the bike? If you were on dirt/mud trails with a whole in the boot, you could have gotten dirt in the combustion chamber and scratched things up? Worn out Piston rings?
I hope its not that...

Loose spark plug can cause low compression

Get a compression tester and give us some numbers...

Also tell us more about the bike.
Hours, Mileage, usage, previous maintenance?

It could be getting old?
 
Last edited:
intake boot, cheap and easy fix. My rule, start with a known defect that is cheap and easy and work your way to difficult and expensive. It needed to be fixed anyway, so it's no loss even if it's not the source of the problem. One more thing off the "could it be this?" list.
 
Might be a valve spring, the valves were adjusted maybe 30 mins or an hour ago.... But the bike had been sitting for a couple of years, would that weaken anything?

When I tightened up the intake boot, I did have more compression... coincidence?
But now, even though I haven't done anything I have lost that compression again... I have done up the intake boot very tight and it hasn't made any difference


How hard is it to check the valve springs?
 
might be able to see enough through the lash inspection cap (2 bolts) with a dentists size mirror, look at the RHS exhaust valve spring first

if not, removing the rocker cover is easy, use plenty of bog refitting it to avoid leaks. axial play of the rocker arms rec spec is IIRC 0.1mm

specs part numbers etc of the different types of duals here http://husaberg.org/mechanical/10680-all-about-valve-springs-nice-pics.html
 
Whereabouts is that lash inspection bit?

Also would I need to check both rockers or just the exhaust ones?
 
How many total hours on your bike?

It could be worn out cam follower bearings. Like needles missing worn out. Stacked up and wrecking schist.

I hope it is something eaisier and cheaper though.
 
209 Hours/8000KM/about 5000 miles

At some point it magically fixed itself, and had high compression again...
What kind of thing could be doing this?
eg (A valve spring is probably unlikely)

Maybe a valve or something is heating up when warm and sealing properly? so next time I kick it over it's sealed and has good compression?


Edit:
Just checked the valves, And I couldn't see anything wrong with them.... But honestly when I got the valves done the mechanic wasn't the best, And i think i'll check them myself properly tomorrow... whats the best way to make sure it's at TDC?



Problems happening:

Low Compression (at one point it magically fixed itself)
Backfiring at idle, haven't got it really hot though
Doesn't start straight up

I previously had an intake leak but I have tightened it up a lot since then, And i'm pretty sure there's no way it would be leaking now


Intake:


Exhaust:







Does this look right?
 
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the decomp spring is correct. valve springs look OK but its hard to tell from the pics

to lock TDC there is a bolt on the front of the 550 cases that you remove and replace with a longer one .. that won't work unless already nearly already located spot on

this may not actually be true TDC either but its close enough for doing the valve lash, a tight valve could do what you describe, or like idle suggested maybe a cam follower brg has gone probably sometime before the last lash adjustment

If you're still concerned the next easy place to look is to take the rocker cover off
 
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Okay thanks, I might have to take it off and have a look

To get tdc should I just take off the flywheel cover and turn it until it is at tdc on the mark? (This is how I did it on my pit bike) would that be the easiest way?


I want to warm the bike up a fair bit, to see if the compression comes back again, but am worried something will get damaged internally
 
I wouldn't run it. That's the upper limit of the rocker bearings.
If you pull the magnetic drain plug you prolly will find some treasure on it.

I think it's as I said, you've lost a few needles out of the bearing, it was adjusted to take up that slack, then they stacked up in the roller and are not letting the valve close.

Next is it binds up and starts wrecking the cam.

I never noticed s lack of compression, but got different results when cranking the engine over after setting the clearances.

I'd set them in spec, check by wiggling up and down. Then used the button to spin it over a dozen times.

Then at tdc, wiggle them and they would be tight.

Opened it up and found most of the needles still in the bearings.

3741DA7F-4BD1-4FEA-B9AC-DCB69635A013-944-000001707437EB94_zpsbf84195a.jpg


This is how I installed the new bearings. The old ones can be pressed out with a chain tool.

F4BC79DE-F43C-41AA-897F-51AE2218B287-501-000001D0854025EE.jpg
 
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To take off the rocker cover, do the valve covers hold it down or is it just the 4 or 6 outer bolts holding it on?
 
torque wrench is always good, can use extensions or a long socket if hard to reach

page C6 in the 05 manual has the pic
 
Got the cover off and got some more shots
Anything else you'd like me to check?

After that i'll put it back on and check the clearances.





 
When adjusting your valve clearance, use the 1/6 turn, not feeler gauges.
Your tappets are domed.
 
Do I need to replace the tappets? Will it affect anything?

It doesn't look like much is wrong with the valves, But i'll put it back on and then check the clearance....

Is it possible that it's to do with oil level, or not warming it up enough?
 

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