Weird "knock" sound from engine...
 

Hi all,

Touring around south America on a newish 250XR Tornado Honda, it has around 14,000k on the clock. I recently had it serviced by recommended mechanic in Sucre, they checked the valves. I have done around 1,000k since. PS they changed the sparkplug as well.

In the past two days I noticed quite a distinct "knock" sound, this apparently only happens 1, when I start the bike, it sounds like it is comnig from the top part of the engine. And it doesnt always happen, and if I give it revs it sounds fine, however 2, if the bike is idling and I just give rev it a little bit, i.e. pull and let go off the throttle just a little, every so often I hear quite a loud knock. Sometimes it cuts out afterward.

It only seems to happen when I give it gas when the bike is idling. Lately I messed around with the pilot fuel screw on the carb to try to readjust for the decrease in altitude, so I screwed it out. But other than that, no one has changed anything on the bike since its last service.

Having read up a little, I see that a too-lean mix can cause both engine knock (which I presume this is what it is?) and pre-ingition. I am hoping it is this and nothing else, any of this sound familiar?

One thing I am rather concerned about is maybe they didnt tighten something when they checked the values. From memory the values were okay and did not need adjustment. I mean, if there was something wrong internally I assume the engine would either be ****ed by now and not running at all?

Any ideas? Driving around the bike runs just fine, and like I say, seems to be only happening when giving it a little gas from idle or when starting. As usual, any suggestions would be great, as I am feeling a bit worried.

Many thanks,
Rtw

EDIT; I just tried to it again and now can't get it to happen, difference obviously being before it was hot and now its cold... Any help? :confused

Might be running a bit weak, might be too far advanced but most likely crap fuel. Treat it to the dearest and see if that makes a difference.

Part of the problem with coming up with a suggestion as to what it might be is that one man's "knock" is another man's "tinkle".

Running weak / crap fuel tends to be more at the tinkling end of the spectrum - a bit like ringing a small toy bell (better analogies welcomed).

A heavy knock - like knocking on a heavy wooden door, could be something mechanical but your other symptoms don't suggest that.

Excess valve clearances are somewhere in the middle - tapping a screwdriver on the engine sort of sound.

If it's a "heavy" sound and only happens for a short while when riding on a cold somewhat worn engine it might be piston slap - that's just God's way of telling you to warm the engine up first. I've had that on my XR600 for many years (and just ignore it) but it tends to be something that bigger pistoned engines do more easily. I wouldn't have expected it on a 250.

Tough to diagnose without hearing it. But it's good you're "tuned in" to your bike's "normal" sounds ... and I'm glad it went away ... for now.

Could be a number of things. If it's only happening at idle then not much to worry about. If you hear/or feel bad noises under a load that can be more worrying.

Could be:

1. A different weight oil put in the bike? ... or oil level may not be FULL ...?
(causing the top end to be a bit noisy when cold?) Keep oil level at MAX and use the right weight. (probably 10/40 for your bike) Use BEST oil you can find.

Could also be the mech set the valves and didn't tell you? You watched him work on it ... right? :innocent: If it's more a "ticking" sound than a "knock" at idle then I say valve clearance is a bit loose on one or more valves. Maybe the Mech was lazy ? ... or set them too loose?

I set ALL my Japanese bikes close to the minimum setting of the range (IE: .8 mm to .12 mm range ... I set at 8 or 10mm) That is what they do at the factory, so I copy them. The Japanese LOVE quiet motors. Loose valves are noisy.

As wear happens some valves may loosen, some may tighten. loose is better than tight. Tight will ruin the engine. Loose just make noise. But as long as you have SOME CLEARANCE ... then you are OK. Honda's are very stable in this regard so don't be afraid to run near minimum clearance.

Fuel can have a big influence too ... but not so much at idle. If you "lug" the engine with low octane fuel it may make all kinds of bad sounds. Normal ... but not good. Try to always use the BEST gas you can find. And maybe consider using some 'top oil" additives used in Diesel engines and trucks in your fuel.

At such low mileage piston slap is unlikely and in any case would NOT present itself at idle unless revved mercilessly when cold. would mostly present under a load at full throttle at lower revs. (lugging)
That is when you would hear/feel piston slap. (I know cause my trusty DR now has it ... but at 55,000 miles)

The above advice about warming is GOOD! Warm her up in cold weather for a good 2 to 3 minutes, first at idle, then gentle revving. Use the VERY BEST OIL you can find. If possible imported from UK, EU or USA. Most local oil is CRAP. The big oil companies DO NOT share proprietary oil additive formula packages with small local companies. BUY THE BEST. Worth it, IMHO.

In Mexico I paid $15 usd for ONE LITER of US made Mobil One synthetic. (imported from USA to mexico)
In the USA I can buy a 5 liter jug of the same oil for $24. :eek3:

Good luck!

PS: the pilot fuel screw ONLY affects mixture at IDLE. Has NO AFFECT during normal running so NO RISK of running too lean there.

Thanks all - seems to have gone away completely now, only thing that changed is I've risen in altitude. Could have been just a bad tank of really bad fuel, as if it could get much worse in Bolivia!

Back of Beyond, the ringing bell analogy is very apt - would definitely agree it sounded like that!

Mollydog, if the pilot fuel screw only effects idle, why do people recommend turning in inwards when rising in altitude?

Cheers!

Good to hear it's cleared up!
Oil weight and level are important. A heavier oil in very cold weather may allow the top end/valves to be noisy until fully warm.

Good question regards fuel screw and altitude. :thumbup1:

Several things can help the engine run better at high altitude. You may notice a low idle, stalling and hard starting as you ride higher. Engine won't take throttle, and of course will be way down on power. Read on:

You are correct ... going "IN" on the fuel screw does lean out the mixture but the Pilot fuel screw only affects idle to 1/8 throttle. Running lean in this circuit will save fuel on the overrun (coasting with throttle shut) but YES you will get back firing.

Keep in mind it's likely your new Honda is already super lean in stock form. (as most ALL bikes are these days) Should run OK up to about 2500 mtrs.

But going "IN" on the fuel screw will help with a smoother idle and starting once up over 2500 mtrs or so. Also will help initial throttle response. But in conjunction with leaning the fuel screw, you should raise the idle speed as well ... and there is more ....

Two more steps for VERY high altitude riding:
Open air box, either remove side cover or cut open a hole, remove snorkel or do whatever to get MORE AIR into air box and engine. Motor will run smoother, idle better and run more normally with a bit more power.

The last, and most effective thing ... but also the most work, is to change your main jet to a smaller main jet. One or two sizes DOWN (smaller number) will help a lot at high altitude ... and save fuel too.

Problem with that is, once down at lower altitude, you really should change back to original main jet. Running TOO LEAN (smaller main jet) at sea level in hot weather is NOT good for the engine.

When riding at high altitude I do steps above in the order I listed them. However, on my bike, the air box is already opened up. An aftermarket header and exhaust can also lean out mixture and help performance at high altitude.

All these elements work together and every change affects outcome, all are integral and related.

Also keep in mind ALL engines lose power at altitude. F.I. (fuel injection) keeps mixture right, which means even at 5K mtrs. engine will start and idle fine. But it will be WAY down on power, same as a bike with a Carb.

The fiddling I describe above will get your bike starting, idling and running a bit better, but it will STILL be down on power, especially higher than about 2500 meters.

At 4000 meters my DR650 felt like it had lost half it's power. So don't think anything is wrong .... it's physics of internal combustions engines is all.

bier