Hi,

The past two times I have started my 250 in colder weather (55F) there has been a loud pop from the exhaust. The bike seems to run normally. Is this something to be worried about?


Thanks
Ben

If it sounds like a misfire, the bike may need some valve adjustment and some carb cleaning, because those are caused by lean mix of air-fuel or poor sparks.

Ok, I was planning on shimming the carb needles but it got put off. Would that help?


Thanks
Ben

Yes, it could, but only if fuel is not restricted to flow through the several jets and passages.
The needless don't do much for start ups, they are more influential at middle range.
It seems that your fuel restrictions are for the idle and choke circuits.

Checking the flow through the petcock won't hurt, just in case you have restriction due to filters or poor vacuum.

When the air is cooler, the mix gets more air than fuel and burns slowly, producing that boom while the exhaust valves start opening.

ok awesome, thanks!

Where are you setting the choke when you start it?

I'm thinking you may have more choke than you need for the temp, and backing it off some may cure the problem.

http://www.ninjette.org/wiki/My_Bike_Backfires

Wasn't there a thread that said this was normal for newgens?

http://www.ninjette.org/forums/showthread.php?t=15025

Normal thing. Happened just the other day to me; this time I was ready for it :)

i learned recently that coils that are going out, when cold, don't produce a good spark. so when i started getting some backfire on cold starts, i first wrote it off as nothing, maybe old plugs at worst. turns out one of the connectors on one of the coils was broken. replaced the coil and haven't had the issue since.

i learned recently that coils that are going out, when cold, don't produce a good spark. so when i started getting some backfire on cold starts, i first wrote it off as nothing, maybe old plugs at worst. turns out one of the connectors on one of the coils was broken. replaced the coil and haven't had the issue since.

So this implies that a bad spark wouldn't ignite the fuel but when it finally produces a good spark, it'd cause a backfire because of the fuel that wasn't ignited?

But can that explain the pop right at the moment the button's pushed to start the bike?

...........But can that explain the pop right at the moment the button's pushed to start the bike?

Even at the low speeds of cranking up and idle, there are more than 20 explosions per second.
It only takes one of those happening late to produce a loud boom.

I have seen mufflers completely opened by the instantaneous increment in pressure generated by one of those (that is why cylinder's walls and piston's heads must be thick and strong).

I don't believe that any exhaust or intake pop is normal for pre or new gens; they are symptoms of something out of whack and I wouldn't just ignore it.
If the mix is too lean or too rich at start ups, or if the sparks are too weak, then they cannot be optimal at normal operation and high rpms'.

Learn more here:
http://en.wikipedia.org/wiki/Back-fire

http://www-mat.ee.tu-berlin.de/research/sic_sens/motor2.gif

Ok thanks for all the replies. I ended up putting in the two washers in the carb needles anyways. Now Im having a problem keeping a steady speed. Acceleration is more flat. When i try to maintain a speed with minimal throttle input, It seems like the engine is missing a LOT of combustion cycles and I get this super low pitch bass sound. It almost sounds like someone humming a really low note.

I was thinking that 2 washers was too much and I either need to take one out, or lean out the fuel mixture via the carb screws.

any suggestions?

Thanks
Ben

How does it run wide-open?

Set your idle mixture screws to high highest idle speed, then turn out 1/4. That should put you around 2 1/2 turns out usually.

If wide-open isn't good, your carb (main jet) is partially plugged and needs to be removed and cleaned.

Thanks for all the help.

I am removing the carbs and cleaning them (its going to be a wet and cold weekend) and I have all the screws removed and the boots going to the engine and the airbox loosened. I removed the screws for the airbox to give it some movement but I still can't back it up far enough to get the carbs out. Is it suppose to be this difficult or is there something I am missing?

Thanks
Ben

well, I got it in. Everything is running 1000x better.

thanks again for all the suggestions

well, I got it in. Everything is running 1000x better.

thanks again for all the suggestions

What did you find, and what fixed the problem?

Well most everything inside the carb was clean but I sprayed out the jets with carb cleaner just to be sure. Otherwise I readjusted both of the mixture screws to 2.5 turns out from full lock, put some slightly less thick washers under the carb needles, re installed, and kaablamo! Low end power delivery used to be rough and hesitant, but now it is smooth and consistent.

I think I am still running a little rich because it takes a few seconds the tach needle to return to idle from a rev (that means its too rich, right?) and sometimes it will stick 1k to 1.5k rpm above idle.

Well most everything inside the carb was clean but I sprayed out the jets with carb cleaner just to be sure. Otherwise I readjusted both of the mixture screws to 2.5 turns out from full lock, put some slightly less thick washers under the carb needles, re installed, and kaablamo! Low end power delivery used to be rough and hesitant, but now it is smooth and consistent.

I think I am still running a little rich because it takes a few seconds the tach needle to return to idle from a rev (that means its too rich, right?) and sometimes it will stick 1k to 1.5k rpm above idle.

I too have that kind of issue with the tach not jumping into idle the moment I let go. But that wouldn't make sense since I still have unshimmed needles and I've got a slipon which is supposed to make it run slightly undesirably lean.

Well most everything inside the carb was clean but I sprayed out the jets with carb cleaner just to be sure. Otherwise I readjusted both of the mixture screws to 2.5 turns out from full lock, put some slightly less thick washers under the carb needles, re installed, and kaablamo! Low end power delivery used to be rough and hesitant, but now it is smooth and consistent.

I think I am still running a little rich because it takes a few seconds the tach needle to return to idle from a rev (that means its too rich, right?) and sometimes it will stick 1k to 1.5k rpm above idle.

That's actually a sign of being too lean or having a vacuum leak.

Check for leaks with carb cleaner around the manifolds. If that's fine, turn the idle mixture screw out another 1/4 and see if it gets better.

:whathesaid:

It is too lean.

Ahh, thanks guys, you learn something new everyday :D

Oh okay so mine makes sense.


Sorry, OP, guess it's either down to the aforementioned vacuum leak or some other issue.

I am going to see what turning the needles a quarter turn out does. Now, I just need a really small flat head...

I am going to see what turning the needles a quarter turn out does. Now, I just need a really small flat head...

Good time to be an IT specialist, I have plenty of those laying around. Anyways, I hope you figure it out and if this is just what it needs. :thumbup: