Topic: What causes popping when throttling down?

[tennesseebreeze]

I've heard it on some bikes but not others- when you let off the throttle even a little on mine, it pops and sputters like it's pissed off you aren't going fast enough. Is this just how these bikes behave?

[KayOne]

On the side of each carb is an diaphragm that enriches pilot circuit on decal. They are likely ruptured. Unfortunately you have to split the banks to replace them and they are costly. Try turning your pilot screws out to 2 1/2 turns, this may richen enough to stop the lean condition on decal. If it does not you can split the banks and plug the vacuum passage to the throttle plates to eliminate the problem

[CBGhia]

Could also be a lean condition on the bike.  Which bike?  Stock airbox and stock exhaust?

[flybox1]

Its not normal, and it can be tuned out.
Popping is a sign of an incorrect (lean) fuel mixture.
Verify fuel level in your bowls, first, and then turn your idle mixture screws RICHER, by 1/2 turn.
Report back....

[tennesseebreeze]

The bike is a 79 cb750F that I just got. I know, I know. It has two cam shafts, but I thought I could sneak the question by on the main page here. It has a stock airbox and a 4 into 1 Kerker exhaust with no baffles. The only things I've done so far are drain the gas and run chemtool through it to get it started. The air filter needs replacing and one of the intake boots is cracked, which I wrapped in electrical tape. That seemed to help the obvious vacuum leak I got at high throttle. It's still sputtering at high throttle, but it's better.

[KayOne]

Check Mike Nixon's website Motorcycleproject.com lots of info on VB series carbs (DOHC, CBX and Goldwing).

[flybox1]

I have been working on one of these bikes, too (VB42A carbs)
The carbs are the root of all eeeeevil with this bike.  I have a thread in the 'Other Bikes' section.

You'll need to do everything listed in this link (except take the carbs apart from eachother) and all will be right in the world.
http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revG.pdf
a thanks to eigenvector for providing the .pdf

[tennesseebreeze]

Thanks guys. I'll tinker and get back to you.

[socal1200r]

Sounds like you definitely have a lean-running condition, especially with your comments about the leaky carb boot and the popping on decel.  I have a 4-1 and stock airbox / filter / no lid on my '79 CB650, and I had an indy shop go thru the carbs when I got it, because it wouldn't run without the bike on full choke, and when I pushed the choke off, it would die.  They rebuilt the carbs, put in new jets, did some adjusting, and it runs great.  No popping on decel, pulls hard on accel.  Like what's already been said, sounds like you need to get the carbs sorted out, then everything that connects to them needs to be good and tight.

 

[ChopSticks]

Is this only for SOHC? I've heard popping of the exhaust in Harley's was due to a rich mixture? Or are you guys talking about a completely different thing?

[flybox1]

As i understand it, popping on decel is the combustion of un-burned fuel.
This does not indicate a rich condition, only the un-burned fuel from the slower/delayed combustion of a lean mixture being expelled into the pipes and detonating there.
Richen the circuit so the mixture can burn more completely, and the popping will cease.

Please correct me if i'm wrong.

[Powderman]

Could be timing related also. Firing a tad late will ignite unburnt fuel in the pipes

[BomberMann650]

my 4wheeler had a gnarly pop like that - found out it was caused by a large split in the carburetor flange.

[eigenvector]

I've read many explanations for why it happens, some are undoubtedly bro-science others different angles on different aspects for why it happens.  The long and short - ignore it unless it's very prominent.  All of my bikes do it, even my computer controlled fuel injected 2015 BMW.

I do notice that the VB42 carbs backfire in a heartbeat - I don't think Honda quite figured out the air cut-off valve yet.

Flybox:  Did you get that sucker running yet?

[tennesseebreeze]

Well I did a little investigating and found a likely culprit for something at least- the carb on the #1 cylinder is totally missing the air/fuel mix screw. Either that, or it's broken off in there. Now I would think that would make it run super rich, but I'm just guessing. I took the plate off the air cut off and the rubber looks perfect. I'm assuming the rest are in a similar condition. Service records for the bike show a carb rebuild as recent as 2010.

Now, a little more detail about how this bike runs. It starts up right away, with choke needed when it's cold. I've set the idle to about 1500 rpm for a warm engine. When I first start it I have to hold the throttle to this or it will stall (the cam chain needs tightening as well, which doesn't help) After riding it for a bit it sits at the rpms I set. Of course, none of this matters until I get a mix screw in there. I've been searching all morning and haven't found a place to buy just the screw. Anyone have a source for these or an extra laying around? I'd rather not have to buy a whole rebuild kit if I can avoid it.

[lrutt]

On the side of each carb is an diaphragm that enriches pilot circuit on decal. They are likely ruptured. Unfortunately you have to split the banks to replace them and they are costly. Try turning your pilot screws out to 2 1/2 turns, this may richen enough to stop the lean condition on decal. If it does not you can split the banks and plug the vacuum passage to the throttle plates to eliminate the problem

Only on newer bikes, not the old SOHC's.

[lrutt]

intake boot leak, exhaust header leak, too lean. Start with the easy things first. check boots and exhaust then go from there.

It's what's called 'after burn' unburnt gasses collect in the exhaust then pop a little. Not the same as a 'backfire', that's on the intake side.

[tennesseebreeze]

I have a boot leak for sure. My temporary fix was electrical tape.

[flybox1]

On the side of each carb is an diaphragm that enriches pilot circuit on decal. They are likely ruptured. Unfortunately you have to split the banks to replace them and they are costly. Try turning your pilot screws out to 2 1/2 turns, this may richen enough to stop the lean condition on decal. If it does not you can split the banks and plug the vacuum passage to the throttle plates to eliminate the problem

Only on newer bikes, not the old SOHC's.

lrutt....this is a 79 CB750 DOHC. 
There are cut-off diaphragm on the outsides of VB carbs

Tennessee...did you find a replacement fuel screw yet?
You need to go through the carb rebuild .pdf i posted above.  Once you do, your carbs will be right.

[tennesseebreeze]

On the side of each carb is an diaphragm that enriches pilot circuit on decal. They are likely ruptured. Unfortunately you have to split the banks to replace them and they are costly. Try turning your pilot screws out to 2 1/2 turns, this may richen enough to stop the lean condition on decal. If it does not you can split the banks and plug the vacuum passage to the throttle plates to eliminate the problem

Only on newer bikes, not the old SOHC's.

lrutt....this is a 79 CB750 DOHC. 


There are cut-off diaphragm on the outsides of VB carbs

Tennessee...did you find a replacement fuel screw yet?
You need to go through the carb rebuild .pdf i posted above.  Once you do, your carbs will be right.

I just ordered one from California. That carb rebuild PDF was chocked full of information. Thanks for that. Aside from the missing screw, a cursory exam of the carbs look to be good. The cut off diaphragms, for instance, are in great condition.

[flybox1]

good news to have those cutoffs usable.  the cheapest i found them is $30 each at jetsrus. 

[tennesseebreeze]

good news to have those cutoffs usable.  the cheapest i found them is $30 each at jetsrus. 

I know. Friggin' pricey.

[PeWe]

High rpm, throttle off. No problem if it pops while the rpm is high.
Popping during low rpm throttle off, air screws too much opened, small pilot jets, lack of fuel.

[evanphi]

High rpm, throttle off. No problem if it pops while the rpm is high (and gradually decelerating in gear with throttle closed)
Popping during low rpm throttle off, air screws too much opened, small pilot jets, lack of fuel.

This is how my bike performs. Perfectly normal fuel levels across the range.

If you're getting popping at idle, especially, enrichen your air screws.

[tennesseebreeze]

Getting quite a bit of popping coming off high rpm, but none while at idle.