Compression/Power loss Left Side

Tornado

Tornado

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Bike is 850 mk2, rebuild head and first overbore done two yrs ago. Since sometime last season been having a power dip on what seems like every other engine revolution.noted at pretty much all throttle openings and rpm. Not huge dips, but enough to notice. Pressure pulses out exhaust while idling are weaker on left. Compression test when hot show 120 psi left, 145 psi right.
Head hasket is copper but no idea if shop that did the assembly annealed it. I did do a retorque on head bolts some 500 miles following the fitting.

So compression diff is the big clue. What testing can i do to further pin down if its HG leaking, a valve (new kibblewhites, guides, seat grinds done 3 seasons ago) or piston rings?
 
+1 on the leak down test
Do you have a steel tank?
Or any sealant in the tank?
When my fiberglass tank started to melt I had the symptoms you describe
Resin had found it's way onto the inlet valves and was stopping them closing very slightly and losing compression
 
Squirt some oil into the spark plug hole.

Turn the engine over several times to distribute the oil then redo compression test. If cylinder pressure rises, the rings/cylinder are at fault. If no increase, the valve train is at fault.
 
Q-Tip around head gasket - if clean, not head gasket (probably best to wash, then ride, then check). Then what Dan1950 said.
 
baz said:
+1 on the leak down test
Do you have a steel tank?
Or any sealant in the tank?
When my fiberglass tank started to melt I had the symptoms you describe
Resin had found it's way onto the inlet valves and was stopping them closing very slightly and losing compression
Click to expand...
Steel un-lined tank (re-pro likely from Pakistan).
 
Don't have access to a compressor so I guess no way to do a leakdown myself?

Will try the oil in plug hole to see what it does to compression.
 
marshg246 said:
Q-Tip around head gasket - if clean, not head gasket (probably best to wash, then ride, then check). Then what Dan1950 said.
Click to expand...
If HG leaking at from cyl to pushrod tunnel or betwen cylinders, I'd guess maybe no outward signs of oil/carbon at joint?
 
Tornado said:
If HG leaking at from cyl to pushrod tunnel or betwen cylinders, I'd guess maybe no outward signs of oil/carbon at joint?
Click to expand...
Between cylinders unlikely since he has different compression between them. To pushrod may well not show - still a valid test.
 
Clearances ?

Broken rods must be getting in touch with their feminine side in this more modern age, back in the day I remember them being way more vocal as they oozed testosterone
 
Did the oil down plug holes test today, all values on cold engine:

Dry: Left 120 psi Right 125 psl

Wet: Left 130 psi Right 135

So oil made 10 psi more both sides.

Significant? Needs to be hot to be valid?
 
A few things:

Did it run strongly before you re-torqued the head? That might suggest the cause was the head gasket not sealing well.

Did you remove the valve covers to inspect the valves going up and down, and check the clearances as well?

What do the spark plugs look like when the bike runs?

Do you have a way to look into the bores through the spark plug holes with the cylinders down? (bore scope) I had a circlip pop out of a wristpin at 65mph with a loud POP, so I shut the engine off immediately and trucked the bike home. Many people have lost piston circlips and they scratch the hell out of the bore while they eat up the side of the piston. If you can lower the pistons and look into the bore with a scope you may see evidence that might help you chose a direction to go
 
Last edited:
I wouldn't be too worried about the 10 psi difference between the cylinders, yet. It is not great, but less than 10%. How many miles since the engine work ?

Worn cam lobe ?
 
o0norton0o said:
A few things:

Did it run strongly before you re-torqued the head? That might suggest the cause was the head gasket not sealing well.

Did you remove the valve covers to inspect the valves going up and down, and check the clearances as well?

What do the spark plugs look like when the bike runs?

Do you have a way to look into the bores through the spark plug holes with the cylinders down? (bore scope) I had a circlip pop out of a wristpin at 65mph with a loud POP, so I shut the engine off immediately and trucked the bike home. Many people have lost piston circlips and they scratch the hell out of the bore while they eat up the side of the piston. If you can lower the pistons and look into the bore with a scope you may see evidence that might help you chose a direction to go
Click to expand...
No bore scope presently.

Following rebore, bike ran well and continued to after i re-torqued head and set valve clearances....must have done 6k or 7k miles since then. Some point towards end of last seaon noted the power dips. Continues now. Became aware of bit of tappet noise on left side prior to noticing power dips. Did another clearance check/adjustment but never got tapping cured.

Plugs have been quite good, brown tan, not oily or sooty. Running needles on richest/ highest position to cure some exhaust popping.
 
gjr said:
I wouldn't be too worried about the 10 psi difference between the cylinders, yet. It is not great, but less than 10%. How many miles since the engine work ?

Worn cam lobe ?
Click to expand...
About 8k miles since rebore. Head/valve work done about 10k ago.
 

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