Backfire

[opfermanmotors]

Ok, what could cause a Carb to not start and only backfire?

I know its the carb because if I replace the carb it starts.

So, I have a brand new Bing carb, jetted to stock.  Putting that carb in does nothing much backfire.  If I put a Mikuni TMX Carb that I have which was tuned for the bike, it starts.  Cleaned the carb, checked the jets, everything is fine in the Bing, it should start or even fire, but just backfires with or without the choke on.  The only things I didn't check which could be possible is the intake jet (the hole for the fuel), perhaps that's causing the problem maybe it's too small or too big.

[TMKIWI]

Have you checked the slide.
The only thing I can think of that would make a carb backfire when it is not an timming issue would be the amount of air going into the motor.
Why don't you stick with the Mikuni ?

[opfermanmotors]

The slide is stock.  I bought everything stock for it, slide, needle, vaporizer, pilot, main.  It came jetted a little richer for a Modern Maico 500, so I put it to stock jetting for the bike.  However, both sets of jettings just backfire.

I am sticking to the Mikunini now.  It's just that the Mikuni TMX is some weird carb that's discontinued and doesnt even have a pilot jet.  Also, the rear of that carb is hudge and doesnt fit on the air boot.  I had to rig up something from home depot.  Also, the owner before me jetted it, while seems OK, just would have rather when with what the bike is supposed to have.  Just find it odd that it's back firing.  However, it's not kicking back when it back fires so it could possibly be a lean condition.  Sometimes it just does poof, not really a loud backfire.  donno, the carb is brand new and I even re cleaned it and used an air compression on it and still has the problem.

Even tho I stuck the mikuni on, would still like tofigure out what's wrong that that carb setup.

[pushdogg]

Have you checked the slide.
The only thing I can think of that would make a carb backfire when it is not an timming issue would be the amount of air going into the motor.
Why don't you stick with the Mikuni ?

I was going to ask the same question about sticking with the mikuni. My not so vast experience tells me an overly rich/lean mix makes for backfires.....and usually your first instinct is right.....I would check the intake jet.

[TMKIWI]

Are you sure the reeds are ok ?

[opfermanmotors]

Reeds are new and it starts fine with the Mikuni so seems to me its definately something with the Bing carb setup. 

[2T Institute]

Ok, what could cause a Carb to not start and only backfire?

I know its the carb because if I replace the carb it starts.

So, I have a brand new Bing carb, jetted to stock.  Putting that carb in does nothing much backfire.  If I put a Mikuni TMX Carb that I have which was tuned for the bike, it starts.  Cleaned the carb, checked the jets, everything is fine in the Bing, it should start or even fire, but just backfires with or without the choke on.  The only things I didn't check which could be possible is the intake jet (the hole for the fuel), perhaps that's causing the problem maybe it's too small or too big.

BING that's your answer. Sure you have no ignition problems?

[opfermanmotors]

Ignition is brand new, just timed and Mikuni works.  I had the same problem on the old ignition with the Bing, which is why I thought I would switch since its an old motoplate anyway.  However, backfires with the Bing and works with the Mikuni. 

You can see it in this video.  It doesn't kick back when it backfires tho, it just goes poof.

1986 MStar (Maico) 500 Rebuild Part 5

[riffraff]

Have you tried swapping carbs while the engine is still warm and see what it does? Can it be push started with the Bing on? not that either would fix the problem I'm just curious 

[opfermanmotors]

Swapped back to Bing, didn't work, now today I swapped back to Minkuni and guess what, didn't work today and I got the same thing.

So now I no longer thing its a carb problem.  Must have somehow got lucky the first time I swapped to Minkuni and whatever the issue is dissopeared for a bit.

Checked timing, timing is the same, so possible ignition but would have to be a short or something somewhere that somehow at one time stopped shorting.

Could be the reeds.  I had taken out the reeds and reseated them before I started the Minkuni the time it worked.  So, perhaps might have to do that again and see if its the reeds.

[SachsGS]

The starting ritual I use for my newer Bing carbed big bore Maicos is as follows:
-choke on
-no throttle
-decompressor pulled in
-kick thru 5 or 6 times to prime engine
-one swift kick to start engine

If the engine backfires repeat.

[ACMX]

The only time my bikes have ever poofed at me like that was right after i'd blown it up. 
But that video reminded me of warmup technique for the bike... Isn't it back to ram the throttle like that right after it's started? My dad has always told me to just slowly roll on the throttle to clear it out right after it's been started (but frankly i dont believe in that, i feel like im hurting the bike) so what I do is give it about 30 seconds to idle (depending on how cold it is outside) Then turn the choke off, and give it a small bit of throttle and if it bogs give it 30 more seconds, and then slowly roll on the throttle to about 1/4 till it is clear then again to 1/2 then again to 3/4 then finally roll it on till it is totally clear with no bog. Am I hurting my bike?

[ford832]

It sounds ignition to me.On the other hand,a friends fourstroke once did exactly the same thing.By about the tenth kick the exhaust plug he'd forgotten to take out went whistling by the side of my head and it started right up 

[opfermanmotors]

Put in yet another new reeds  (VForce), same thing.  But I did get it running again.  It seems to just be a problem with getting the right amount of fuel into the engine.  I've been able to kick it a few times throttle closed, then kick it wide open a few times and then it starts. I think part of the problem may be that the kicker wants to start half way down and not allow me to get a full kick.  

I will say one thing that the bike screams, very crisp and just revs to the moon with barely touching the throttle.  Afriad it will blow up if I open the throttle wide open!  Bike is prety light, need to get back brakes working now.

[SachsGS]

A friend of mine had a 90's Maico 440 that he had problems starting cold so he put a snowmobile primer button on the bike - after that one kick and Big Blue was running!