Pegaso 650 backfires into the intake... SOLVED!

[RhynoCZ]

What to look for? Valve clearances adjustment done, electrics sorted, carburetors cleaned and having stock jets and setup, no ''visible'' air leaks as far as I can tell, fresh fuel and spark plug...

I'm clueless.

It starts alright but in just a few seconds after that it starts to backfire into the airbox. Then, the engine stalls. Cold/hot does not matter, the engine does run, roughly, if held in high revs.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Easy-X]

Backfiring into the airbox implies the intake valve was open when the boom came...

Timing or sticky valve?
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[WD Forte]

I had to google a pegaso 650.........
(you can expect a strongly worded letter in green ink soon)

I assume it's an early twin mikuni version?
Phuctyfino of course but guesses

As backfires are usually caused by lean mixture or pre ignition and it cuts out
The Tarot cards tell me to looK at the fuel taps first to check they give a good steady flow.
If they're just dribbling they'd probably fill the float bowl enough to start but go dry when
asked for more.
this would lean out the mixture and possibly cause backfires and such.
vac taps are notorious for this.

Exhaust header leaks can make it pop back a bit but probably not stop the motor

Dont know what system it uses, but an ignition fault like a component breaking down
as it heats up then causing false triggering might give it conniptions
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bikers smell of wee

[RhynoCZ]

Yes, Mikuni carburetors. It's an Italian - German (BMW F650GS) - Japanese parts bin mashup.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[steve the grease]

The reason it backfires into the inlet is because the combustion process is still taking place when the inlet valve(s) open. So why is that? Either the timing is so retarded that the combustion process is starting so late that it's still burning when the inlet opens, OR the mixture is so far off ( generally weak) that the combustion process takes all day, so that its still burning , blah bla inlet , blah blah. Headers hot are they?

[Robby]

Check the timing.

[sickpup]

The blanking olug is missing off one of the balancing points. Really it is.

[RhynoCZ]

The timing is perfect, and there is no such a thing as balancing points. The mixture is lean, though.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[sickpup]

[RhynoCZ]

It's the year 2000, the last gen. with carburetors, and it's just not there. A friend of mine just obtained a ''new'' ECU for the bike, so will try that as well.

If the ECU is faulty and the ignition happens too soon or late, it could also, theoretically, cause the issue.

I mean, it's Italian, so it is expected for the electrics to fail, at some point at least.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[sickpup]

[ADSrox0r]

[RhynoCZ]

And the ''new'' ECU did not help, back to the drawing board.

EDIT: I googled a bit more and found some people use acetone and compressed air to clean the idle jet in order to fix the issue. Well, all the carb bits were in the ultrasonic cleaner, but I guess it's time to try the acetone and air method now.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[ADSrox0r]

You said the carbs are cleaned, are the seals and diaphragm sound?

It's a pain with a stamped in idle jet but you can remove it if you're careful (I haven't as yet, I'm told you can)

Oh and for reference, what are your mix screws set at?
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Current bikes: '08 VFR 800 VTEC(yo) , '07 ZZR1400 Winter hack: '95 Aprilia 650 Pegaso Currently lusting after: RC30

[RhynoCZ]

Cleaned and taken apart again for the acetone + compressed air thing. Did not help. It still backfires into the left carburetor. *the left one only

The diaphragms are like brand new, all the seals as well. However there might be one issue, that wouldn't cause this. Many of the o-rings had to be replaced due to deterioration and the new o-rings, although being up to spec, are a bit thicker, as they are not damaged/compressed yet, so when the manual says give it 3,5 turns, the idle jet might not get into the exact same position as it used to. But that alone would not cause such horrid engine running.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Riejufixing]

Still spitting back? Inlet valve problem (sticky, bent, seat damaged...)?

[jaffa90]

Long shot, try a denser air filter (rubber foam) .

[RhynoCZ]

The valve clearances are up to spec (checked again) and the crank rotates with no excessive resistance. Maybe, just maybe the intake valve seat on the left hand side is damaged or even the valve itself missing some bits, however the owner refuses to take the head off, at this point.

@Jaffa: It runs an airbox, doing that would not solve the issue, as it's always the left carburetor, that runs like crap.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[jaffa90]

@Jaffa: It runs an airbox, doing that would not solve the issue, as it's always the left carburetor, that runs like crap.

Not to solve it but does it make it a little better or worse.

My brandnew KLV 1000 was spitting back (very weak mixture)at a certain rev range until a power commander was fitted.

[Sister Sledge]

Is this bike running one intake valve for one carb and another intake valve for another carb?
If it is then surely a damaged valve is a likely cause?
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CCM 404 DS

[jimspeed]

I think I would do a compression test followed by a leak down test at this point.
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[RhynoCZ]

It's fixed now and it was the pressed in jets! More acetone, more cleaning, runs alright now. What a stupid design.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor