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-   -   Honda Unicorn losing Engine Compression - HELP (https://www.team-bhp.com/forum/motorbikes/86443-honda-unicorn-losing-engine-compression-help.html)

I have a Honda Unicorn Nov 2005 make, 49000kms run, always serviced at honda at regular intervals. It never gave me any trouble until..


Exactly one month back while going to work in the morning, the bike stalled in the middle of the road. All the efforts of starting it were proved futile and i had to drag it to the nearest mechanic. He pressed the kick twice and declared the " valves are bent" and have to be replaced. I could not believe it and took the bike to another mechanic. He opened the sparked plug and put some petrol in the engine from the plug opening and the bike started. He said the engine lost the pressure. I was shocked and took it Honda service center. The Workshop manager is a genuine guy, I know him a bit, he checked the bike personally and said their is nothing wrong, may be some carbon had come in the engine which is now flushed away. I trusted him and came back. Everything went fine for a full month and around 900kms but the problem reappeared today.

I dropped my wife to office and realised the bike was running not smooth, i accelerated a bit hard and it flew nicely and then stopped. I was near my home , dragged it ( again, now twice in two months). I opened the spark plug and did the same trick ( putting petrol). It has started but now i am sure something is wrong.

1. Why does the compression is lost?

2. Could it be something related to quality of fuel because I tanked it yesterday only.

3. The FE has dropped significantly over some days. Could damaged Valves be the culprit.

I need HELP as this is my beloved motor and I don't want to lose confidence in it.please:

Compression is lost because of worn out rings or cylinder. Not sure if this can happen at 49K, but still, I wouldn't trust anyone unless I do the compression test by myself. Get a compression meter from the market, should cost you anything hardly & do the simple test by yourself. One more simple technical stuff, if the compression is low, your kicker will go down very easily & might even experience starting trouble. You can then compare it with the kicker of another Unicorn that is relatively new.

Yes, the valves bent can happen sometimes due to poor fuel quality, but may or may not happened only due to previous day's tank up. But consistent poor usage of poor fuel WILL CAUSE valve & cylinder damages.

Again, may or may not be related. Check everything & eliminate all possibilities to arrive at this conclusion. Say, start with carbs, then move to throttle cable, check clutch, then wheels, timing chain.

BTW, how're the tappet noises? Are you feeling a sluggish acceleration or significant drop in top speeds?

I experienced similar problem on my TVS Victor. And I tried all the points that you had tried. Finally, took it to my mechanic. He pointed out that the idling screw is gone. He fit a new one and asked me to keep an eye on it.
I am not sure if Unicorn has this screw, but surely my Victor and brother's Centra both have this and can be used to control idling of the bike.

Yes it does. Every vehicle fitted with carbs has a idle screw to control the idling of motorcycle. But are you saying you've experienced all the symptoms experienced by kkg?

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The tappets noise sound pretty much normal. Their doesn't seem to be any drop in the top speed either. I managed a quick 90kmph two days back and the bike responded well.

And yes the sluggishness was there in the morning before the entire incident but surprisingly after the "petrol trick", the engine is running very very smooth.

BTW is putting PETROL in the spark plug opening Ok? I mean will it damage the engine in any ways. If not, I would have to carry a small bottle of petrol with me always.stupid:

@KKG - No, carrying fuel is not adviced at all. I mean why should you, if required, then there's some problem & get that checked.

I wouldn't dismiss checking the airscrew or jets; for that matter, check the entire carbs too.

Do this simple test when the vehicle stalls on the middle of the road next time. Just open up the tank gas lid & close it & then kick the vehicle. Do keep us posted if the vehicle started. How about using a choke? Does it start when choke is used? I still suspect on any blocks on the fuel lid.

1) IMO, this thread must be in this section : Motorbikes - Team-BHP

2) Are you sure that there is compression loss ? Any bike with compression loss will have loss of power evident from the word go. Also I doubt if an engine having compression loss can hit 90 kmph. If at all the bike manages to reach 90, the bike would still need a long road. Compression loss = power loss. Its that simple. I have used a bike that had compression loss and crossing 70 was tough then.

3) You accelerated and then the bike stopped. Not an idling issue unless the bike stopped when you reverted back the throttle.

I suggest that you do the following :

4) Complete carb service : If I am not wrong, there is an O-ring that can go bad.

5) Replace spark plug.

6) Check out the coils.

7) Clear air filter and fuel filter. There is one very small filter for fuel.

8) Get the valve adjustment done if idling is not stable even with warm engine.

9) If the above works fail to solve issue, get the compression test done. Also if there is compression loss, black smoke will come out from exhaust. There will be an increase of oil consumption.

Add to that one more stuff - check the CDI & the ignition coil in addition. They cannot be dismissed either. I guess that's what is mentioned as
Could you explain this point pls? I like to understand as how this happens.

1) By coils I meant the ignition coil and other coils. In a few bikes I have noticed that these suddenly stops even at speed is an indication to ignition coil failure. Happened with my sister's vehicle also. And at the same time getting other coils checked would be nice. There is also one voltage regulator called RR unit ( mostly ). Good to get it checked it too.

2) About the black smoke from exhaust, if there is compression loss, some oil will end up in combustion chamber which will eventually be burned leading to black smoke being emitted.

Worn out rings, damage to block-piston will lead to the oil entering into combustion chamber. This will result into oil consumption and over time oil level will drop faster.

A view of the spark plug can also confirm if the oil is seeping into combustion chamber.

Right, but its not black smoke, its white smoke on 4 strokes as well as 2 strokes. Black smoke is usually associated with Diesel engines. Also the smoke emission can be caused due to worn out rings, again that is the symptom of compression loss in 4 strokes.

Are you sure about the white smoke ? My Caliber had black smoke, many bikes in my college had issue with engine pickup and power and emitted black smoke. I think when the gasket it gone, its white smoke. White smoke was there in 2 stroke, but most of the times in 4 strokes I have noticed black only. More often than not in all the bikes with black smoke I have helped had rings and block-piston errors.

I'm 101% sure on the white smoke; I'm yet to see a 4 stoke engine emitting black smoke. What happens is, the oil is always sprayed from the bottom of cylinder & then it burns completley. Now, being a 4 stroke, the oil doesn't come to top of the cylinder. But again, if the compression is low, it indicates worn out rings, which means the oil gets on to the top of cylinder through gaps between the rings & the cylinder wall causing improper oil burn & gases escape through the exhaust causing white smoke.

Sorry for the OT @KKG; I'm stopping with this.

I did all the things you mentioned this time. Nothing worked. also, the bike is running as good as new now and yes no smoke ( black or white) is visible off the silencer. Could you please tell me about the Oil filter, I don' remember guys at Honda changing it ever.

No smoke means nothing to worry about the rings; you may want to get checked on the valve bent. I cannot dismiss this thought as I've seen someone ranting on some Bajaj family of motorcycle on valves being bent; the mech was attributing the fault to the petrol pump.

Is it oil filter or air filter? Air filter should be present under one of the side shields at the beginning of the intakes. It is through this, the carbs sucks in the air.

But still, did you do the compression test? Also check if there's any hissing or gas leaking sound through the cylinder gaskets; you may not be able to check this immediately after starting, but probably after riding 4-5 Kms.

What color is your spark plug?

Also check petrol tank for dirt.
With fuel tap off. Disconnect the petrol pipe.
Shake the tank hard. Then collect some petrol in a bottle.

Is it clean?