'96 Techno 250 "popping" (backfire)

[russd]

I recently picked up a 96 Techno 250. Being a 4-stroke guy, I thought nothing much of the fact that the bike popped a bit at idle and/or slightly cracked throttle. Now that my neighbors are complaining, I decided to investigate and have found that this, in fact, is not normal...

I have completely cleaned the carb, installed a new plug (BP5ES), gas is 92 (R+M)/2 (95 RON) with 75:1 mix. Starts easy, runs good, plug doesn't foul, no excessive smoke. It pops whether the engine is cold or hot (which tells me the pipe isn't igniting unburned fuel). What could be causing the backfiring??? Any help appreciated.

thanks.

-RussD

[tamnortheast]

Hi Russ

it's probably just your rear silencer that needs repacking, mine was rather noisy until i sorted that. all you need to do is remove the black plastic end piece (a couple of screws from memory) and pull out the centre baffle tube. you will find it's wrapped in a sort of fibre wadding that will be black with oil. remove this wadding and re wrap with silencer packing (loft insulation seems to work just fine) and reassemble.

hey presto .....much more neighbour friendly.

oh by the way give the baffle tube a good clean while its out, use a blow torch to burn off the oily mess.

all the best Tim

[brian r]

Russ,

A local bike shop should sell the proper packing.

[russd]

Thanks for the tips. I repacked the muffler a couple weeks ago - still loud pop bang at low rpm (about 1 pop every second or two). I also have checked the flywheel shaft key and it's fine. Can't pull the flywheel to check the timing 'cause I don't have the pulling tool. I do have a timing light - are there any reference marks I can use?

Someone mentioned that I should check the grounding on the bike, but I don't know where to start. I suppose it could also be the center muffler that's toast, but can't really fix that. Other threads have mentioned the ignition module ... but that's US$300 for a replacement so I need to make absolutely sure it's that before I even consider going that route.

Any other suggestions and/or tips would be awesome. Thanks.

-RussD

[bob]

Hiya,

It is possible that it could be the CDI, it is difficult to give a solution to an unusual noise when you haven't actually heard it.

I had stator problems with a previous rev3, and by asking very nicely I managed to get a stator and CDI unit to try on my bike and return whatever was not needed (Did this through the UK importer JLI).

I would suggest that you check the air intake thoroughly, and following that try adjusting the idle mixture screw to see if that changes anything. You can always turn it back to where it was before provided you remember how many turns you have given it either way.

Failing that then try contating your local dealer or importer and see if you could buy some parts to test on your bike and return whatever wasn't needed.

Good Luck,

Bob

[veggiedog]

Uh, I know this is an old thread, but I felt compelled to comment since I am looking to quieten up my 2004 Rev3 a bit.

All the larger Betas that I have heard (maybe 5 - 10) 'pop pop' every second or so. The pop is so significant that it will produce little dust clouds behind the bike on every pop as the exhaust stream hits the ground on a dusty trail. The 280s are louder than the 250s, and the 125s don't seem to pop much at all. I cut my beta muffler in half and had a look. I welded up the inlet pipe seam (from the factory it is not air tight) to make sure no air was getting into the muffler, and I welded the muffler back together. I repacked the pipe, and all to no avail. I am confident it has something to do with the motor/exhaust design rather than a maintenance issue. My Scorpa 250 is hush quite compared to my Beta Rev3 250. I would love to see the inside of the scorpa muffler to see if I can make a change to the beta muffler to eliminate the 'pop pop' but I am unwilling to cut open my fully functional scorpa muffler, and I am not confident it will make any difference as I suspect this may be an engine design issue. I don't believe it is a rich fuel problem as most rich fuel problems will not result in exhaust popping unless outside air makes it into the exhaust through a leak of some sort: it requires fresh air to ignite the unburned fuel inside the exhaust system. Additionally, the exhaust does not smell of unburned fuel, nor does the spark plug indicate that the engine is running rich.

Has anyone else had any success quietening up a Beta Rev3. And please, don't tell me your Beta Rev3 is quiet, I know better: they make a very distinctive 'pop pop' unlike the smooth sound of a GasGas or Sherco, or my Scorpa's yamaha motor.

[prxeight]

Dont have a solution for this, but I'm relieved to hear other bikes have the same popping going on!

[b40rt]

Check your plug gap, if its ok try widening it and see it this makes it any better.

[sammyd173]

I think I found the solution to the popping today!!

I have a '97 Beta Techno I have desperately been trying to quiet down so I can ride around my local mountain trails here in San Diego.

I put in a new plug, gapped it as suggested, and re-packed the silencer 5 ways, no joy.

Finally took someone's suggestion to lean out the bike with the pilot screw. The pilot is on the carb on the right side of the bike. It is to the left of the pilot air screw that adjusts the idle using your fingers.

Backing the pilot screw (not the idle screw) out (counter-clockwise) lets in more AIR (not fuel, as I had supposed). More air makes the bike lean. When the bike is running rich (too much gas) it causes the popping.

Here's how someone suggested you set it. Get the bike fully warm - actually go ride it around. Get off the bike with it idling. Turn the pilot screw - the one you have to use a screwdriver for - all the way in (clockwise).

Next back the screw out somewhat slowly and listen to for the idle to speed up. I think mine backed out two or three turns from all the way in. More than this and you need a new pilot jet, apparently. When the idle speed is no longer increasing, that's where you are supposed to stop.

Your bike may be idling a little faster now, so adjust accordingly with the idle control (you use your fingers on this one).

Incredibly my bike no longer pops at idle (at least while hot - have not tested from cold start up). I'm thrilled. Even coming back down from half throttle - no pops.

Another tip - if you really need to quiet the bike down to get to a riding spot past some houses, take an old grip (dirt bike, throttle side as it has a larger diameter). Cut a 1" slit in the grip from mid way back to the end of the grip. Start the bike up then put the grip on, the slit should point down to the ground.

This will not cause any problems as far a keeping the bike in the lower revs to get to where you need to be. It has never over heated any of my bikes and I've done this on two and four strokes. Once you get to your spot, pull the grip off and stick it in your pocket. It makes a remarkable difference to exhaust volume. Sorry for the long post!

[goudrons]

Usual suspect for a popping exhaust is a leak somewhere drawning in air and leaning it right out (does the popping go if you open the choke a little?)

But as we're talking about a Beta I would say it's the Mikuni carb being a Mikuni carb!

You can check it has the right jets and the float height is set correct (important due to the angle it's fitted at), this may help.

I write "may" help, but there are endless threads on the subject of these carbs and Beta's, after years of fiddling and resetting float heights myself to cure various pops, bangs and pings on a succession of different Betas, including some terrible pre detonation on several Rev 3's, I came to the same conclusion Beta did in 2008 and binned the fecking thing.

A Dellorto 26mm VHST bought cheap from a Scooter spares place in Holland for a hundred Euros has transformed whatever Beta I've fitted it too, along with a Ossa Mar which runs perfect on it without any adjustments.