Exhaust like a shotgun

[Fire-medic]

Found split rubbers on inlets so they were also replaced.

Not uncommon to have surface splits not causing leaks. However, since you replaced 'em, one less thing to worry about.

Have you used starting fluid to spray around the airbox to carburetor bellmouths, the carburetor to VBoost rubbers, and lastly, the VBoost to cyl head connections, where live thin O-rings? I've had the cyl head to VBoost O-ring leak, causing issues. Be sure to replace all four of the O-rings! If the VBoost is coming off, it's a false economy not to replace all four skinny O-rings.

I assume you replaced the sparkplugs? Did you remove the sparkplug caps (they unscrew) from the high-tension wires to check for green corrosion on the wire ends at the sparkplug caps? Trim a bit back on the wires to remove the green corrosion and add a small schmear of dielectric grease on the wire end, and replace the sparkplug cap.

You should be able to let the engine carburetor intakes 'sniff' some starting fluid (you don't need a lot) and it should start. Then you can give the engine short sprays as it threatens to die, to keep the engine running, and hopefully it will begin to run on the fuel pump's supply to the carbs.

Have you tried to see how-much gas comes out of the float bowl drain hoses? The carbs should all pass a similar amount of fuel with the float drain screws open, one at a time. Loosen the screw, turn on the ignition, and see what the fuel flow out is like, Repeat the same trial for each float bowl drain. You're using fresh gas, yes? Replaced the fuel filter? You can find a generic one at your local parts house, bring your old one to compare.

You want to have removed spark plugs in the plug caps and grounded because ignition systems don't like their energy not discharged when it's generated. People usually stop the gas flow and remove and test plugs grounded one at a time to check for spark output.

 

[VJohn]

Not uncommon to have surface splits not causing leaks. However, since you replaced 'em, one less thing to worry about.

Have you used starting fluid to spray around the airbox to carburetor bellmouths, the carburetor to VBoost rubbers, and lastly, the VBoost to cyl head connections, where live thin O-rings? I've had the cyl head to VBoost O-ring leak, causing issues. Be sure to replace all four of the O-rings! If the VBoost is coming off, it's a false economy not to replace all four skinny O-rings.

I assume you replaced the sparkplugs? Did you remove the sparkplug caps (they unscrew) from the high-tension wires to check for green corrosion on the wire ends at the sparkplug caps? Trim a bit back on the wires to remove the green corrosion and add a small schmear of dielectric grease on the wire end, and replace the sparkplug cap.

You should be able to let the engine carburetor intakes 'sniff' some starting fluid (you don't need a lot) and it should start. Then you can give the engine short sprays as it threatens to die, to keep the engine running, and hopefully it will begin to run on the fuel pump's supply to the carbs.

Have you tried to see how-much gas comes out of the float bowl drain hoses? The carbs should all pass a similar amount of fuel with the float drain screws open, one at a time. Loosen the screw, turn on the ignition, and see what the fuel flow out is like, Repeat the same trial for each float bowl drain. You're using fresh gas, yes? Replaced the fuel filter? You can find a generic one at your local parts house, bring your old one to compare.

You want to have removed spark plugs in the plug caps and grounded because ignition systems don't like their energy not discharged when it's generated. People usually stop the gas flow and remove and test plugs grounded one at a time to check for spark output.

Pretty much a tick to all of that. We did find the rollers on the starter clutch had gone square shaped so changed them and now the sparks are good and in sync as the engine turns over beautifully.

However we've tried starting the machine and we're getting explosive blowback out of the carbs including flame.

 

[02GF74]

^^^^ that can happen when the spark is missing or occurs at the wrong time - there is air/fuel mixture hanging around from not being ignited, along comes a spark and boom.

I would suggest checking the wiring to the ignition coils and leads from the coils to the plugs are not crossed over.

If you remove all plugs and have them lying on the heads, when turning the engine over, are the sparks regular and in sequence?

Are you able to identify which cylinders do this?

You could remove 2 plugs (probably best choose cylinders that share the same vboost connection) and try to start - engine can run but not as smoothly as on 4 cylinders in order to locate the bad ones(s).

Note I had similar when I bought mine, clearing the pilot jets cured it.... but I believe you have done full service on the carbs).

 

[VJohn]

^^^^ that can happen when the spark is missing or occurs at the wrong time - there is air/fuel mixture hanging around from not being ignited, along comes a spark and boom.

I would suggest checking the wiring to the ignition coils and leads from the coils to the plugs are not crossed over.

If you remove all plugs and have them lying on the heads, when turning the engine over, are the sparks regular and in sequence?

Are you able to identify which cylinders do this?

You could remove 2 plugs (probably best choose cylinders that share the same vboost connection) and try to start - engine can run but not as smoothly as on 4 cylinders in order to locate the bad ones(s).

Note I had similar when I bought mine, clearing the pilot jets cured it.... but I believe you have done full service on the carbs).

So I've literally just walked away from the bike just now. It looks like all 4 are doing it when they feel like.

Next I was also thinking was the coils. Funny thing is I've never touched the front ones so be suprised if crossed over. But will have to check to be sure!

 

[Fire-medic]

So I've literally just walked away from the bike just now. It looks like all 4 are doing it when they feel like.

Next I was also thinking was the coils. Funny thing is I've never touched the front ones so be suprised if crossed over. But will have to check to be sure!

They are, in-fact, "crossed-over," the left front coil fires the right front plug, and the right front coil fires the left front plug. Look carefully.