Too much airflow causing idling issues?

[mistermentali...]

I have a hn125-8 and it has a 42mm cone air filter. Until recently the filter had a splashproof cover on and the bike ran fine and idled just fine.

The filter broke so I ordered another which was identical but has no splashproof cover. The old cover cant be swapped over to the new filter.

It ran fine for about a week but now is stalling, even when warmed up and idling fine it sometimes just dies and it can take a few minutes to restart and still can die again seconds later. It gets better after about twenty minutes of use and then stalls much less. It runs fine at high speeds but has an occasional hiccup going through the gears whereby it lurches as if needing more fuel or air.

I have had to adjust idle revs to just over 2000 to keep it from stalliing or it dies again. It seems to just randomly drop revs and about half the time this causes a stall.

Nothing has changed and it was fine for about a week so I am baffled as to what could be the problem but could it be the filter provides too much air because it has no cover on it which would have restricted airflow a little?

The carb is a basic one I think its a pz26 or pz27 and I have tried adjusting the idle and also the mixture screw but cant seem to solve the problem. The spark plug is a tan colour and not fouled in any way whenever I check it.

Any ideas on what could be causing stalling and why the bike can be idling fine when started or at lights and then just die and take a while to get restarted?

Any help much appreciated thanks,

Dave

[RhynoCZ]

The spark plug is not fouled because you are running a very lean mixture.

Try applying the choke while riding. You'll feel a very distinctive improvement.

This can not be solved by tuning the carburetor. The mixture screw is for fine tuning only. What you need is to get the ''splashproof cover'' again or change the jet to compensate the higher airflow (give it more fuel).

*EDIT: You can also tape over the filter, covering the same amount the ''splashproof cover'' would. Not perfect, but should do the trick, for a while at least.

**EDIT: Do you still have the original airbox?

https://www.tuningmatters.com/wp-content/uploads/2014/07/spark-plug-lean-rich-optimal-640x360.jpg
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[mistermentali...]

Thanks for the quick reply. I don't have the whole airbox assembly but I did put some tape over the top end of the filter and it seems to idle better so as soon as I get paid I shall buy the same filter I had before with the cover on. That will mean riding with tape on filter for about a week and a half but your advice about the cause seems spot on.

The cover had holes in the top so I cant be sure just how much air it restricted so I taped just about the top third of the filter. I can't say how it will run until I go work tomorrow but the idle definitely improved so when I get to work I can re adjust the mixture screw if needed.

It may be a temporary fix until new filter arrives but your advice really helped, and you've shown me the cause and how to fix it (new filter or jet) which I shall be doing so thank you again

Dave

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To give an idea on the effect of pissing about with air filters on jetting, removing the inner “trumpet” tube (just a spout which fits on the inside of the air box) on a TZR250, means main jets have to go up from 230 to around 320/330 to compensate.
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TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[Bikeless]

My advice is don't fart around with the standard set-up ,leave it alone and buy a faster bike if that's what your trying to achieve.

[RhynoCZ]

You should also know, that running the engine with (very) lean mixture leads to overheating. I would advise you to not ride it. If you decide to ride it regardles of that, bear in mind you shouldn't really push it, so at least avoid constant high speed riding.

To get the best performance, you're going to have to either put it back to the factory specs or pay a specialist for the tune up. You may experiment with various jets and filters yourself, but the question is do you want to?

By the way, your spark plug is already worn by the lean mixture, it won't turn white again. So after you get the ''proper'' filter, don't take the colour of your current spark plug as a indicator of anything, unless you go too far and make it run too rich. In which case the plug will turn black, as seen in the picture above.

[temeluchus]

In cold, wet weather, you can suffer from condensation causing the float bow to fill with water. It will also be far more sensitive to changes in the weather. I'd say drain the float bowl first and see if it improves.

You'll also usually have to fit a much larger main jet and possibly a larger pilot jet.
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Some shite cruiser. Now with guns and FREEDOM!

[mistermentali...]

Hopefully it wont be running lean now, I have a new plug I can put in tomorrow and check to be sure and can buy more plugs when I get my wages and get the filter.

I didnt originally put the filter on for any particular reason other than the airbox was damaged and on the bikes forums, which have shut down now, people said uaing a filter and bigger jet gave it a little more poke which I wanted as before that it only did about 45 to 50 top speed.

Doing that gave me a few mph more and then using a lithium ion battery and changing the sprocket to have a different number of teeth got me cruising at 50 but able to pop up to and hold 56 to 60 if I need to (the manufacturer sells the alternate sprockets for people who want either more speed or better fuel economy, they sell two different sets).

I should clarify the bikes had an air filter for about three years, it isnt a recent change so it already has a larger jet. At the time people on the bikes then forum recommended a 95 jet and that worked great so I just stuck with that.

I will possibly go up to another larger jet once I have the new filter, I thought tan was a good spark plug colour as the ngk website says "In general, a light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition" so I've always run the bike so the plug is an off white to light tan colour but will have to keep it as white as possible, the picture on their site does show a white plug even though the writing says light tan so I seem to have been running a little lean.

In regards to the reply about water in the carb bowl, I had an issue with that a couple of times with previous filters which caused me to get one with a cover so I do check that if I have a problem but its a very good point so thank you, and thanks to all the replies.

I shall run with new plug and see if its running rich or lean, if its still lean I may have to get bus in for a week but will get new filter and if I find it running lean still then a larger jet.

Thanks again,

Dave

[Rogerborg]

[RhynoCZ]

I know this isn't a popular answer, but keeping it stock and in a well working order is the way to go here. You were not wrong by thinking the final drive gear ratio would change things up a bit, but you certainly shouldn't put a cone filter there just like that and definitely shouldn't waste money on a fance battery.

Although at the same time, some people are just adventurous. My 67 y.o. neighbor put a quickshifter, power commander and even a turbo on his Ninja 250. All that to get about 60hp (from 29.91hp). But, he does all that because he's an bored engineer in pension. He put a lot of effort, studying and money into his project. He doesn't feck about just to gain extra 5mph, he does that to have some fun. If he wanted to just go faster, he'd buy a faster machine, that doesn't require short service intervals and a lot of fecking around to make it run well.

I say try to put it back to stock (ebay for parts) or pay a specialist to tune it up for you. OR even better, sell it as is and buy something a bit quicker.
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[mistermentali...]

Thanks for all the replies, I shall try to answer them all.

The cone air filter broke at the hose side, the hose split. The filter itself is ok but I couldnt get a hose that fitted it so cant use it.

Regarding batteries, I like the lithium batteries, only five pound more than a standard gel one so for me its worth the small difference and for 35 pound I get a super light battery from jmt with two year warranty.

How do you de cat the bike, does that involve the exhausts or is that what its called when you remove the egr? I would be happy to do that and use airbox with filter instead of foam. Unfortunately I cant sell the bike and get a better one as I have a low income and my only option is to keep what I have in running order as best I can.

I'm not trying to make the bike go as fast as it can, I just wanted to get it to cruise at fifty so am happy it does that, but I do understand that stock is safest and most reliable option.

Thanks again,

Dave

[Rogerborg]

[RhynoCZ]

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[RhynoCZ]

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[RhynoCZ]

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[mistermentali...]

I looked into how to de cat after it was mentioned but decided to avoid that. I managed to get my filter with the splash cover on by using the hose from a different filter. It held in place on the filter fine so is now in use again.

I put the new spark plug in to see if my engine runs lean or not and if it does then I will get a bigger jet, all the help here has been great so thank you (again) as although I have learned to do things like cylinder head replacement and valve clearances I still am unsure of the dark art of carburettors so I am reading up a lot.

If I do have any major issues I may revert to stock with an airbox, but hopefully its all ok again now as I do like the little extra poke and the louder sound.....when I filter through traffic people move to one side more than when the bike had an airbox, back then they didnt hear me and with a filter it is a little louder which I like.

Thanks,

Dave

[mistermentali...]

I ended up changing piston rings piston barrel and cylinder head. Compression is now 150 psi, thats before running piston rings in, but still having issues with very black spark plug and bike bogging down at top end so think its now just carb related. But the lack of compression is fixed.

I had one issue as I had to helicoil one of the piston barrel threads and despite going very slowly the drill bit went right through. Most swarf came out with drill and helicoil bits but the tang from coil is still there in wherever the hole leads to. I'm hoping that its a hollow area under the bolt as its outside of the piston and tappets enclosed area and the engine seems fine.

So, compression fixed and now just got to sort issue with running rich.

Dave