[Olivier]

I posted this issue in my 'got a new bike!' thread but I feel like I should post it on its own to get more answers.

So basically that is it. I recently bought a NH 250 (from 2007, ~2,500 miles). I think it runs great, starts right up with no hesitation ever, and without clutch (but it's been pretty warm here in MD), frankly I love the thing :-D. Don't know yet about gas mileage because I've only done about 120 miles on it so I haven't gassed up again yet. But when I'm done riding and I park it in the garage, it smells like gas for a while. I do close the petcock when I'm done running. It seems to me that the smell is stronger on the right side compared to the left, and it's definitely stronger around the carb. I think when the bike is cold it doesn't smell anymore, but if you sniff around the carb you can smell it though.

No leak that I can see, including when it's running, I've looked around the carb for bubbles when I rev it on idle and I didn't see anything.

There are two hoses running to the bottom of the bike (crankcase breather is one of them I think?). Seems like one of them should be capped according to what I've seen but neither is.

Somebody suggested the mixture might be too rich. I'm not sure how to adjust it so probably would be best to have it adjusted by a shop (if it turns out it could be the culprit)... does someone have any suggestions? It'd be greatly appreciated, thanks!

 

[dmfcb750]

Stuck float in one of the carbs? Petcock could be it but the smell would be more prevalent on that side if that were the case. Running rich is possible but you should notice performance problems as you increase RPMs. Check petcock first, then floats. With petcock open, run the bike for a couple of minutes. Drain and measure the fuel from each carb. Should be the same amount in both sides.

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[Olivier]

OK so I looked again just now and I lied, there seems to be a leak, I found a drop of gas forming under one screw, see red arrow below...

 

[dmfcb750]

Couple of suggestions, you can try Teflon tape or find a small rubber gasket to fit around the screw. Note: if going the rubber route, make sure it is nitrile based.

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[Olivier]

What's that part? Sorry to be a noob, I don't know anything (yet!) about carbs... float chamber? Maybe the seal is leaking? Is it something I can remove by itself and change the seal of without taking the entire carb off? Tank needs to be dropped to take the carb off, right?

Running rich is possible but you should notice performance problems as you increase RPMs.

Well it looks like it's just a leak now, but now that you mention it, I did notice that the power seems to plateau a bit when I increase RPMs. That might just be normal for that bike though, it's not supposed to be super powerful

 

[dmfcb750]

Looks like the drain for the carb. Find out which carbs go with that engine/year model and you can download a diagram. On my phone or I would.

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[Olivier]

I think this is what I might need to replace...

 

[80cb400t]

My money would be on number 8 in this, specifically the rubber baffle at the top. https://www.cmsnl.com/honda-cb250-nighthawk-2006-6-usa_model33740/partslist/E++18.html#.W76mf_llC9s
It might even be installed wrong because it just looks scrunched up above where I'd expect it to be.

I don't know this carb but it doesn't look like it's coming from the bowl gasket, I can't remember if I've ever seen a post on here about one failing. Those screws also don't leak, it's just where the gas is dripping down to.

Edit: please try what I said in the other thread and see if you can see the gas bubbling out of there when you open and close the throttle.

 

[Olivier]

Edit: please try what I said in the other thread and see if you can see the gas bubbling out of there when you open and close the throttle.

I did and did not notice any bubbling anywhere. I'll try again.

 

[80cb400t]

Hopefully second time's the charm and you don't have to reach the third time

 

[Alan F.]

My money would be on number 8 in this, specifically the rubber baffle at the top. https://www.cmsnl.com/honda-cb250-nighthawk-2006-6-usa_model33740/partslist/E++18.html#.W76mf_llC9s
It might even be installed wrong because it just looks scrunched up above where I'd expect it to be.

I don't know this carb but it doesn't look like it's coming from the bowl gasket, I can't remember if I've ever seen a post on here about one failing. Those screws also don't leak, it's just where the gas is dripping down to.

I agree with the accelerator pump being the source of the leak, at least enough to remove it for inspection. It has some tiny holes that can become clogged and the gasket-like edge of its diaphram could also have been damaged previously and be leaking now.

What an accelerator pump is and how it operates is good to know. Usually located at a low point on the carb, it has a chamber, aluminum on the bottom, capped with a spring loaded diaphram which has a control rod attached to it (#8) the chamber fills with fuel through some tiny passages, and when you whack the throttle open a linkage pushes that rod on the diaphram downward, compressing the spring and squirting a bunch of extra fuel up into the carb throat through another tiny passage. When you back off the throttle a bit the spring is able to relax and the diaphram is pushed back upward, drawing another shot of fuel into the pump reservoir (that cap beneath the spring at #8 on the diagram)
That reservoir can be removed for cleaning, crap can get in there and build up enough to block the exit passage.

I wonder if an obstruction in the exit passage to the carb throat could simply cause enough back pressure when you whack the throttle open, that some fuel is squeezed past the gasket part of the diaphram?

Take the 3 screws out and remove the parts you see above and below #8 in the diagram, take lots of pictures and let us know if you found any crud in there.

 

[Olivier]

Haven't had time to run it today (awful weather) but I did spend some time with it in the garage and at some point I just twisted the throttle (engine off, petcock off position) and all of a sudden a big drop of gas dropped from behind where that diaphragm you're talking about is... what is that for anyway? It's connected to the throttle but I can't really tell what it does. It's not labeled in the service manual either...

 

[J-T]

Looks like the accelerator pump which would also explain your most recent experience. May be as simple as just tightening the screws for #8 as mentioned above.

 

[Olivier]

Does anyone have a picture of how that little boot at the top should look like? I can't find one.

Also, I'm a total noob at this, so just to be clear: I need to take the carb off the bike before I can mess with the accelerator pump, right? Also, I've read in several places that there is a little o-ring between the accelerator pump and the carb body that 1) is hard to find and 2) people often forget to put back on when they mess with it. I wonder if something like that happened...

 

[Alan F.]

You don't need to take the carb off the bike yet, your first move should be to verify the 3 screws at the bottom of that round cover below #8 are tight, be careful not to strip the phillips heads if the screwdriver slips. There should be room for a short screwdriver, or a 90 degree screwdriver. Worst case I'd say unbolt the carb from the intake manifold and hold it so you can reach the screws with a common sized phillips screwdriver.

https://www.partzilla.com/catalog/honda/motorcycle/2002/cb250-ac-nighthawk/cylinder-head

 

[Alan F.]

Does anyone have a picture of how that little boot at the top should look like? I can't find one.

Also, I'm a total noob at this, so just to be clear: I need to take the carb off the bike before I can mess with the accelerator pump, right? Also, I've read in several places that there is a little o-ring between the accelerator pump and the carb body that 1) is hard to find and 2) people often forget to put back on when they mess with it. I wonder if something like that happened...

at about 20-1/2 minutes it's shown.

 

[Olivier]

I agree with the accelerator pump being the source of the leak, at least enough to remove it for inspection. It has some tiny holes that can become clogged and the gasket-like edge of its diaphram could also have been damaged previously and be leaking now.

...

Take the 3 screws out and remove the parts you see above and below #8 in the diagram, take lots of pictures and let us know if you found any crud in there.

I had missed that post somehow, I'm so sorry, thank you so much for all this info! I was wondering about this because when I twist the throttle, the rod doesn't move at all, so I was wondering how that pump mechanism worked...

 

[Alan F.]

No worries, we're all friends here. I enjoy explaining things I can explain, often someone who knows more will correct me or add something new, and everyone learns more. I don't enjoy feeling put down by others about things they think I should know, so I try to be careful not to dish that out myself. Part of my job is to go document technical things in real time, written in a way that non-technical people with big desks can understand. I try to treat my communications here with the same deliberate care, one pursuit helps the other become stronger.

Only here I can enjoy some 'tangential smartassery'! ?

One small tip is not to park a hot bike in the garage, let it cool down outside where the breeze can carry away heat and unpleasant odors for a few hours then wheel it in by hand and park it facing out. Also if you use a bike cover let the bike cool first, never cover a hot bike.

 

[80cb400t]

You can see an even closer version at 14ish minutes in that video. Do not follow his cleaning procedures though, spraying carb clean onto the accelerator pump diaphragm/float bowl gasket is a bad idea.

Also I think the rod moves inside that brown metal sleeve, it should make more sense when you disassemble it. It's different from the old style carbs.

 

[Olivier]

Only here I can enjoy some 'tangential smartassery'! ��

:grin: Well, thanks all! I'll try to take a look over the week-end.