Ok Mechanics/Wrenches, answer this one...

[grasshopper]

First and foremost:

-Leak down test 98% front and rear cylinders
-Cam timing with a degree wheel rite on the money

When I cover up the front carb with the palm of my hand on my SV the motorcycle dies. When I cover up the rear carb with the palm of my hand the rear carb leans out and the bike stays running. We actually had an O2 sensor in each individual header pipe to observe the air/fuel ratio.

I observed the phenomenon first on the race bike with the flat slide carbs. So we chased all kinds of shit. We put the front carb on the back and the back carb on the front. Same thing happened. Then I thought maybe the rear intake manifold had a hole and was sucking air so I swapped that from the front to rear. Same exact thing happened.

So I put I put the front carb and intake manifold on the front, rear carb and rear intake manifold on the rear, took a swig out of my frosty beer with complete fucking confusion.

So I pushed my street SV that runs perfectly into the shop. It has stock carbs and a stock air box on it. I popped the tank up, pulled the air filter out and held my hand over the rear carb. The motorcycle won't die. The rear carb leans out and the bike keeps running. I hold my hand over the front carb and the motorcycle goes fat and dies. SAME EXACT FUCKING THING!!!!

Can someone please tell me why an SV650 does this? I'm flabbergasted by this and so are other people who know motorcycle mechanics extremely well. In theory the rear carb should go fat with lack of air and fuel flowing into it.

[grasshopper]

Somebody get Damico or Key to get on line and give the for real answer.

The only thing it could be is the Top Dead Centers are 230 degrees from each other, instead of 180 degrees like an I4. if this is the case, the front cylinder's inertia is carrying the motor past, since the front cylinder fire's closer to the rear's TDC.

A friend of mine suggested this.

I hope I'm just not talking to myself in here. 20 views and no replies. HELLO HELLO HELLO, is there anyone out there? 

[HAWK]

If the engine won't die when you cover the rear carb then it must be getting a fuel air mix from somewhere else. I don't know the SV top end yet so I can't make a guess but look for balance or crossover passages between the two cylinders.

BTW O2 sensors really only sense change not absolute mixture, when you cover a carb and really whack out the mixture I wouldn't necessarily trust the O2 sensor.

[benprobst]

why not just stop covering the carbs with your hands and ride the thing? 

[Ducmarc]

is there an exhaust gas recirculation system that might run through the rear head? if you checked compression with throttle closed and had same compression as open then your getting air  from somewhere .don't  know much about sv's just spaghetti burners

[grasshopper]

This was taken off Svrider.com

The 90 degree V makes the firing angles look like this:
0 deg. - rear cylinder fires
450 deg. later - front cylinder fires
270 deg. after that - rear cylinder fires again
450 deg. after that - front cylinder fires again

The engine has a long wait for the next firing of the front cylinder.  There isn't enough inertia at low RPM to carry the engine past the next compression cycle of the non-firing front cylinder so it dies.  When you cover the rear carb the engine has to wait only 270 degrees for the next rear cylinder compression cycle, so it has enough inertia to carry it over.

A bigger puzzle is why covering the rear carb causes that cylinder to lean out where covering the front causes that cylinder to go rich.  I haven't figured that one out yet.  As mentioned above, covering the carb should make that cylinder go rich, like a butterfly choke.  The only difference between the front and rear intakes is that the rear provides vacuum to the petcock and the fuel pump.  I can't see why that would make a difference.

Also, depending on how lean the rear cylinder gets it could be that the rear cylinder is still firing (although weakly) with that carb covered.  The only difference between the front and rear intakes is that the rear provides vacuum to the petcock and the fuel pump.  I can't see why that would make a difference.

[EX_#76]

why not just stop covering the carbs with your hands and ride the thing? 

LOL!!  Good one Ben!! GHopper if the bike runs ok what does it matter.  The next thing you know your going to be concerned about why the sky is blue. 

Seriously I can not think of a reason for the phenomena you are experiencing.  There are no exhaust gas recirculation passages in the heads.  The stock bike could get air and fuel through the choke system by pulling a vacuum when your hand is on the carb.  I don't know what the mechanism for getting air and fuel through the flat slides, maybe it is being pulled through the accelerator pump.  I don't think it is the firing order or the timing because if you taking one cylinder away in both cases.  You have the same rotating mass and each cylinder makes very close to the same amount of power.

Key has brought up something that might make a difference.  Perhaps the problem is your hand does not seal the carbs equally due to gaps around you fingers and how you place your hands on the carb intake.  l think you should try your ass cheek to get a better seal on both of the carbs and report back with the results of the experiment.   WHAAAA HAHAHAHAHAHAH we are hoping for a backfire

ROFLOL  I need air!  HAHAHAHA  WHOOO HOOOO!!!!

[Ducmarc]

is this a test?????????????????? will it idle with the rear plug wire off

[Jason748]

What about with the fuel line disconnected...  how long will it run then.   Wait we already know the answer to that. 

Forgive me but I'm not real familiar with SV, but as stock isn't the needle controlled by airflow?  So with no air flow coming past there is no venturi effect and therefore you only get a dribble of fuel - causing a lean condition?

[SV88]

LOL!!  Good one Ben!! GHopper if the bike runs ok what does it matter.  The next thing you know your going to be concerned about why the sky is blue. 

Seriously I can not think of a reason for the phenomena you are experiencing.  There are no exhaust gas recirculation passages in the heads.  The stock bike could get air and fuel through the choke system by pulling a vacuum when your hand is on the carb.  I don't know what the mechanism for getting air and fuel through the flat slides, maybe it is being pulled through the accelerator pump.  I don't think it is the firing order or the timing because if you taking one cylinder away in both cases.  You have the same rotating mass and each cylinder makes very close to the same amount of power.

Key has brought up something that might make a difference.  Perhaps the problem is your hand does not seal the carbs equally due to gaps around you fingers and how you place your hands on the carb intake.  l think you should try your ass cheek to get a better seal on both of the carbs and report back with the results of the experiment.   WHAAAA HAHAHAHAHAHAH we are hoping for a backfire

ROFLOL  I need air!  HAHAHAHA  WHOOO HOOOO!!!!

Great answer Bartz - but this is clearly a case of "impropriety"... Let me explain: you suggest a potential solution which will insure that you won't have to wear the potato head patch next year!

[EX_#76]

  Great answer Bartz - but this is clearly a case of "impropriety"... Let me explain: you suggest a potential solution which will insure that you won't have to wear the potato head patch next year!

You caught me!!!

[grasshopper]

Hey Guy, go out in the garage and fire up your bike and try it. Hold your hand over the front, the bike will die. Hold your hand over the rear carb the bike will keep running.

Get back to me.

I'm going to strap a quarter stick of dynamite to that Potatoe head of yours.