[Ashley Nair]

Dear all, I have a 2005 Honda Unicorn which has clocked 50,000 kms till date. The bike is in excellent condition. It was run in with utmost care, serviced religiously at 4000 kms, ridden between 50-60kmph( though with occassional bursts of speed).
However, since the last 10,000kms, I have noticed that there is a considerable drop in low end torque. Earlier, the bike would easily pull from 30kmph in 5th gear but now any such attempt would induce engine knocking. I have checked the drive chain( but not changed chain and sprocket) and ensured that it is of the correct slackness and I have also tried using 20/50 engine oil hoping that it will increase compression but to no avail. The top end perfomance of the bike remains undeteriorated.The engine is as smooth as ever and the bike is still returning an average of 68km/litre( provided she is driven sedately).
What can be the problem? Gurus, please advise

[prajwalkashyap]

Does the idling increase unnecessarily? If you feel that the bike is not pulling sufficiently, downshift and then rev up a bit and observe the rpm level at which u need to upshift to the higher gear. If the upshift level is higher in RPM in comparison to normal usage, my first look would be the clutch plates. They might need replacement. Get the clutch bell checked. I had faced similar problems, and i got the clutch plates and the bell changed and the problem got rectified immediately.

Make sure you get this done by a competent mech. Take the opinion from the A.S.S. but with a pinch of salt. I just picked up a set of the plates and the bell and took the bike to my trusted mech and got it replaced.

Do update us with the progress.

Prajwal

[Ashley Nair]

Idling remains constant between 1300-1400RPM.The clutch disk doesn't seems to be OK since I never felt any harshness in gear change. however, I shall defenitely check the clutch bell and then report.

[drdeepudev]

Dear Ashley, I too have a 2005 model Honda Unicorn, which has clocked 50,000km. Me too, is facing similar problem as yours. In my case, low end torque as well as top speed is getting affected. When I talked with my mechanic, he pointed out two problems. It may be the problem with the valves, which was checked and adjusted but could not rectify the issue. I will be meeting him tomorrow for getting my clutch plates checked. Will be updating you as soon as it gets over.

[Bokky]

Originally Posted by prajwalkashyap

Get the clutch bell checked. I had faced similar problems, and i got the clutch plates and the bell changed and the problem got rectified immediately.


Do update us with the progress.

Prajwal

Excuse my ignorance, what is a clutch bell ?

[ArizonaJim]

Several simple things can cause an engine to lose some of its expected power when it is being asked to work hard, like accelerating while in the higher gears.

The most likely thing is caused by a dirty air filter which is blocking much of the air the engine needs.

Another possible thing is the spark plug gap between the electrodes has worn and it is now larger than it should be.
Such a condition will work fine when the engine is lightly loaded but under heavy loads, the amount of air/fuel mixture in the cylinder is much greater. This raises the compression pressure which increases the voltage required to fire the spark plug. The higher voltage may short thru the center electrode rather than jump the gap which will cause the engine occasionally misfire.
The misfire will create no power so the engine will not have the power it once did.

The high voltage ignition wire running from the ignition coil to the spark plug can break down over time and cause the same problem the excessive spark plug gap caused.

More serious things on engines with high mileage is worn or burned valves leaking and/or excessive wear on the piston rings.
Both of these can allow the high pressure from the burning air/fuel mixture to escape.

Again, when the engine is lightly loaded, the loss of pressure won't be noticed. Under heavy loads, they can cause a loss of power.

While many mechanics will suggest re-boring the cylinder and installing a new piston and rings, often, just replacing the piston rings and lightly honing the cylinder to help the rings break in is all that's needed and a great deal of money can be saved by doing this.

If the piston rings are replaced, re-lapping the valves and valve seats should be done even if they appear to be in good condition.
The increased compression from the new piston rings can cause even a good valve to become damaged unless it is seating tightly against the valve seat.

If the engine has been serviced regularly and has not been stressed by racing, usually the bearings on the crankshaft and in the connecting rod will not need replacing.

If it has been used hard and run at high speeds a lot, the lower bearings may need to be replaced.

As with all things, it is best to start with the simple things like changing the air filter and spark plug.
If that doesn't fix things, more serious things should be looked at.