Air Intake "Pops" Randomly

[Mightymouse]

Hey y'all,

This started happening fairly recently and it's got me scratching my head. What's causing this and how to fix?



Thanks guys.

 

[Fire-medic]

Prolly, a lean condition, due-to an intake leak, spray some starter fluid around the airbox to carb connections, 1 at a time; same thing to carbs to VBoost manifold, and then VBoost manifold to cyl heads. Don't stop at the first time that you catch the revs increase, you may have > 1 leak.

 

[Mightymouse]

Prolly, a lean condition, due-to an intake leak, spray some starter fluid around the airbox to carb connections, 1 at a time; same thing to carbs to VBoost manifold, and then VBoost manifold to cyl heads. Don't stop at the first time that you catch the revs increase, you may have > 1 leak.

Thanks Fire-medic,

I'll check that out. How would a lean condition cause that pop to happen?

 

[Mightymouse]

Collected some additional info in case it helps.

Found this video:

The guy describes a similar popping noise and said the "main jet fell out." I think he ment the needle jet was out of position. You can just make it out from the video he took...So I took a quick look at the "top" part of my needle jets where the the vacuum pistons are. All of them look good I think.

I slid the vacuum pistons a few times on each carb and noticed the ones on carb 1 and 3 slid easier than the one on carbs 2 and 4. The vacuum piston on carb 2 slid back but not as easily as the others. This was also the same carb that the popping noise came from.

Could this rough sliding vacuum piston cause that popping noise?

I'll also check that leak Fire-medic. Thanks.

 

[Mightymouse]

I took some pictures of the intake above each carb in case that helps. Maybe there's a problem I can't see.

 

[Fire-medic]

The jet block in a VMax holds a pilot jet (#42) and a main bleed jet (#44). The main jet (#46) also screws into the jet block, but is not in the same configuration as the pilot jet and main bleed jet, which lie parallel to one-another. There are two rubber plugs at the end of the holes of the jet block into-which the pilot jet and the main bleed jet screw-into. The rubber plugs rest against the body of the carburetor and keep the passageway for each of those jets, plugged.

https://www.ronayers.com/oemparts/a/yam/500452a9f8700209bc792425/carburetor

Maybe one time in 26 years of working on these bikes, and owning them, I've seen exactly one of the jet blocks, upon carburetor teardown, have a loose brass jet. Never has it been a main jet.

A plugged jet or a jet no-longer properly screwed-into its port will certainly affect the normal operation and proper carburetion of the cylinder it serves.

 

[sdt354]

Shotgun the carbs then sync them up. And retest. They won't idle/run the same with the lid off, too much air.

 

[Traumahawk]

Thanks Fire-medic,

I'll check that out. How would a lean condition cause that pop to happen?

Mine used to "pop" or sneeze back thru the carbs when it was too lean, before it had properly warmed up. Now you have to find the cause of why it would be too lean. It can be A/F screws set too lean, a leak in the intake, or all/any of the reasons that FM said above.

 

[Mightymouse]

The jet block in a VMax holds a pilot jet (#42) and a main bleed jet (#44). The main jet (#46) also screws into the jet block, but is not in the same configuration as the pilot jet and main bleed jet, which lie parallel to one-another. There are two rubber plugs at the end of the holes of the jet block into-which the pilot jet and the main bleed jet screw-into. The rubber plugs rest against the body of the carburetor and keep the passageway for each of those jets, plugged.

https://www.ronayers.com/oemparts/a/yam/500452a9f8700209bc792425/carburetor

Maybe one time in 26 years of working on these bikes, and owning them, I've seen exactly one of the jet blocks, upon carburetor teardown, have a loose brass jet. Never has it been a main jet.

A plugged jet or a jet no-longer properly screwed-into its port will certainly affect the normal operation and proper carburetion of the cylinder it serves.

Awesome. Thanks for the info Fire-medic. Guess I can rule that possibility out. I’ll do the leak down test first as you suggested.

 

[Mightymouse]

Mine used to "pop" or sneeze back thru the carbs when it was too lean, before it had properly warmed up. Now you have to find the cause of why it would be too lean. It can be A/F screws set too lean, a leak in the intake, or all/any of the reasons that FM said above.

Thanks Traumahawk, it seems to be a lean issue. It does seem to do it more frequently after a cold start. I’ll be doin a leak down test as Fire-medic suggests.

 

[Mightymouse]

Shotgun the carbs then sync them up. And retest. They won't idle/run the same with the lid off, too much air.

Thanks sdt354. I had the carbs cleaned and synced about 1,500 miles ago (10 months).

Would it make sense to do a leak test first and then possibly perform the carb sync afterwards?

 

[Fire-medic]

Thanks sdt354. I had the carbs cleaned and synced about 1,500 miles ago (10 months).

Would it make sense to do a leak test first and then possibly perform the carb sync afterwards?

No difference, carbs probably aren't out of adjustment so soon. Take your time and be thorough.

 

[Mightymouse]

Prolly, a lean condition, due-to an intake leak, spray some starter fluid around the airbox to carb connections, 1 at a time; same thing to carbs to VBoost manifold, and then VBoost manifold to cyl heads. Don't stop at the first time that you catch the revs increase, you may have > 1 leak.

Sprayed starter fluid around those connections tonight and didn't hear any noticeable rev increases. I'll try again tomorrow because I don't think I was being as thorough as I could have been.

I took some slow motion footage of the needle jet diaphragms as I blipped the throttle a few times. I was trying to catch the pop/backfire "in the act" but unfortunately it didn't happen. I'll try to catch it tomorrow when I have more time and the light's better. Maybe it could help diagnose the issue?

Here's some slow mo footage of carbs 1 & 2:

...and here's the footage of carbs 3 & 4:

Anything look funny? Do you want to see higher revs? Please advise.

 

[Traumahawk]

Since your not finding an air leak, you could try and go a little bit richer on the A/F screws. You could try 1/4 or a turn out (richer....to the left) or maybe even 1/2 a turn. The A/F screws are set up a little bit lean from the factory.

 

[sdt354]

Since your not finding an air leak, you could try and go a little bit richer on the A/F screws. You could try 1/4 or a turn out (richer....to the left) or maybe even 1/2 a turn. The A/F screws are set up a little bit lean from the factory.

Ditto T.

 

[chickshim]

Check your slide diaphrams. It was not uncommon for them to twist on the slide. Pull the carburetor caps and make sure that the diaphrams are straight and do not look twisted like a candy cane. If they are twisted they will not respond to the throttle plates opening the way they should. If they are twisted replace the slide assembly.