Topic: Popping noise from carb

[longshanks]

So I'm having a weird kind of "pop" of air coming out of which I think is carb 3. It almost sounds like a compression stroke that isn't igniting and backflowing through the intake. The bike is also missing and backfiring below 3.5k but smoothes out and has good power after that. What also happens too is that when it's plenty warmed up the idle starts to stick at 3k and I can't rev it down. I've pulled the plugs and some look black and a little wet(too rich?) and I have yet to do a proper compression test. Other stuff I've done is adjusted the valves, points, set timing, measure impedance on the ignition wires, and sync the carbs. I pulled a float bowl and it's jetted at 120 and I have aftermarket pod style filters and a aftermarket exhaust. Bike is a 76 CB750K

[longshanks]

Anyone?

[BrianAdair]

It's the pod filters man.. I'm dealing with the exact same issue.

[longshanks]

Almost sounds like a old steam engine. Whats the alternative? stock airbox?

[longshanks]

Are you sure its the pods? a lot of people seem to have them without incident

[Kemp]

 Could be lean on the pilot jet. Jetting with same issues using pods on my CB550. Popping and spitting back through carbs at idle and slow speeds (low revs) around town. Idle sometimes goes way up and hangs there. This is lean condition. Tried adjusting air screws for much richer mixture and this helped (at least idle rpm is consistent) but still pops and spits back through carb(s). Will go at least 2 sizes richer on pilot and also raise needle one notch (this to cure flat spot between 1/4-3/4 throttle).

[BrianAdair]

The pods, have no V stack to them like the stock air box does, also the way the stock air box was designed it restricted the amount of air flow to even things out, with the idle jets/air screws. With just the pod's you are sucking way to much air, and I don't see how the #40 idle jet will work.
The reason it is idling up and staying is it is sucking  air, running lean, and will run too hot, mine you can choke 3/4 of the way, and it will idle down, but if you rev to 3k rpm (no choke) it will stay there. I covered the pods with foil tape completely, except for about a 1" sq on the bottom sides just to test restricting the air flow, and it would slowly climb back down on it's own. Still getting to much air flow. I have an extra set of slides, and I am grinding them down to test closing the air cutaway, and see how this helps, also need to try some 42 idle jets. My engine is not stock, but the problem is the same as you are having. POD's Suck! I don't know how some guys have them, you have to even things out with the amount of air they take in, you have to dump enough fuel.

I am no expert by any means, but this is what I have found with my experience in the past month of dealing with this issue.
Good luck man

[anotherCB]

I have a 77 CB750 myself.  With pods and the 4in2 and 130 main jets everything was fine. I put on a 4in1 last weekend and I had to restrict airflow to get her running well (see photo). Obviously I need some bigger main jets which I still need to get, but I also have the popping noise problem, so would anyone advise to change the pilots as well?

[BrianAdair]

I have a 77 CB750 myself.  With pods and the 4in2 and 130 main jets everything was fine. I put on a 4in1 last weekend and I had to restrict airflow to get her running well (see photo). Obviously I need some bigger main jets which I still need to get, but I also have the popping noise problem, so would anyone advise to change the pilots as well?

It would be the idle jets, mains won't change it, until your at 3/4- WOT. bigger idle jets, filing the slides and restricting air flow.. All trial and error.. lol grinding the slides will sweeten the transition spot between the idle and main, I'm still in the process. found a few places that have the idle jets in #42 #45 but they are $8ea. kinda steep, I have several #40's think i will drill them out with a #42 bit..

Your running 130mains? stock engine? what clip setting on the needle, does it run rich?


edit: I just noticed your running the PD carbs, I don't have those on my 77, they have press in idle jets, not sure if those are available or not.

[camelman]

Popping out the carbs is a sure sign of running lean if you haven't changed anything else other than installing pods.  Putting those pods on will change your pressure drop through the carbs, and you won't flow enough fuel to keep the fuel mixture correct... hence the lean condition.  I believe the popping is actually due to a slow burn in the cylinder from having too lean of a mixture.  The charge in the cylinder is still burning when the intake valve opens back up, and it pops out the intake.  This can possibly set your pods alight depending on what material they are made of.

The solution is to richen your mixture by turning your idle screw.  If you have to turn it more than a couple rotations from the stock setting, then you should jump up to the next pilot jet.  You also likely need to increase your main jet size in addition to the pilot jet changes, and possibly make changes to the needle settings.  There is plenty of good information in the FAQ section, and some great reading available in a google search to figure this out.

Good luck,
Camelman

[Retro Rocket]

Are you sure its the pods? a lot of people seem to have them without incident

Do they.? Are they on CB750's.? These bikes rarely run well with pods, they have a specifically designed velocity stack that gives smooth acurate airflow to the engine and works extremely well with the stock airbox, pods generally don't have this feature, they are also effected by cross winds and rain, most commonly with pods is the low end "flutter" that you experience, usually this smoothes out as revs rise because pods are designed or work better at higher revs, another good reason not to use them on the street....Some bikes seem to run well with pods and some don't, some bike manufacturers fit stacks from the factory and some don't, one thing that is common is that Honda sohc's don't run very well with pods, at least no with the stock carbs. One member here has developed an "antipod" filter that incororates the stock shaped stacks and works very well. Here's a link..... Worth a look

http://forums.sohc4.net/index.php?topic=79491.525

[longshanks]

Popping out the carbs is a sure sign of running lean if you haven't changed anything else other than installing pods.  Putting those pods on will change your pressure drop through the carbs, and you won't flow enough fuel to keep the fuel mixture correct... hence the lean condition.  I believe the popping is actually due to a slow burn in the cylinder from having too lean of a mixture.  The charge in the cylinder is still burning when the intake valve opens back up, and it pops out the intake.  This can possibly set your pods alight depending on what material they are made of.

The solution is to richen your mixture by turning your idle screw.  If you have to turn it more than a couple rotations from the stock setting, then you should jump up to the next pilot jet.  You also likely need to increase your main jet size in addition to the pilot jet changes, and possibly make changes to the needle settings.  There is plenty of good information in the FAQ section, and some great reading available in a google search to figure this out.

Good luck,
Camelman

I'm not sure how it could be running lean since some of the plugs looked a bit fouled(wet, black, and shiny) and it has an aftermarket exhaust(vance&hines I think!) with 120 main jets. However I tried adjusting the idle screw the other day and nothing happened. I turned it about three rotations and still nothing.

[c(b)hris]

So I'm having a weird kind of "pop" of air coming out of which I think is carb 3. It almost sounds like a compression stroke that isn't igniting and backflowing through the intake. The bike is also missing and backfiring below 3.5k but smoothes out and has good power after that. What also happens too is that when it's plenty warmed up the idle starts to stick at 3k and I can't rev it down. I've pulled the plugs and some look black and a little wet(too rich?) and I have yet to do a proper compression test. Other stuff I've done is adjusted the valves, points, set timing, measure impedance on the ignition wires, and sync the carbs. I pulled a float bowl and it's jetted at 120 and I have aftermarket pod style filters and a aftermarket exhaust. Bike is a 76 CB750K

When it sticks at 3k that's when you're idling right?  The 120 main jet won't have anything to do with that.  When you're idling you have to worry about the pilot jet and the air screw(and the needle position I think a little).  You said it smooths out after 3.5K...what throttle position are you at when this happens?  The way I understand it, the mixtures are set at different THROTTLE POSITIONS, NOT RPMs.  I'm having some problems too so I'm not one to give advice, but mine's starting to smooth out finally.  The carb faq helped me a lot.  Im also running pods and aftermarket 4-1.  Stock airscrew for my bike is I think 1 or 1 1/4 turns out....I'm down to about 5/8 turns out and it's pretty okay.  Stock idle jets still but I don't know for sure.

There are some charts in the carb faq that make it a little easier to troubleshoot.  I never had backfiring problems like yours though.   Good luck.

Also make sure you're making your airscrew richer and not leaner.  I think you turn in to richen it if the screw is on the airbox side of the carbs, turn out to lean it.

[longshanks]

So I'm having a weird kind of "pop" of air coming out of which I think is carb 3. It almost sounds like a compression stroke that isn't igniting and backflowing through the intake. The bike is also missing and backfiring below 3.5k but smoothes out and has good power after that. What also happens too is that when it's plenty warmed up the idle starts to stick at 3k and I can't rev it down. I've pulled the plugs and some look black and a little wet(too rich?) and I have yet to do a proper compression test. Other stuff I've done is adjusted the valves, points, set timing, measure impedance on the ignition wires, and sync the carbs. I pulled a float bowl and it's jetted at 120 and I have aftermarket pod style filters and a aftermarket exhaust. Bike is a 76 CB750K

When it sticks at 3k that's when you're idling right?  The 120 main jet won't have anything to do with that.  When you're idling you have to worry about the pilot jet and the air screw(and the needle position I think a little).  You said it smooths out after 3.5K...what throttle position are you at when this happens?  The way I understand it, the mixtures are set at different THROTTLE POSITIONS, NOT RPMs.  I'm having some problems too so I'm not one to give advice, but mine's starting to smooth out finally.  The carb faq helped me a lot.  Im also running pods and aftermarket 4-1.  Stock airscrew for my bike is I think 1 or 1 1/4 turns out....I'm down to about 5/8 turns out and it's pretty okay.  Stock idle jets still but I don't know for sure.

There are some charts in the carb faq that make it a little easier to troubleshoot.  I never had backfiring problems like yours though.   Good luck.

Also make sure you're making your airscrew richer and not leaner.  I think you turn in to richen it if the screw is on the airbox side of the carbs, turn out to lean it.

Yes, it takes around 30min to get really warm and then the high idle problem surfaces. It's pretty consistent at doing this too. The bike smoothes out under acceleration(1/4 to 1/2 open on the throttle?)

[c(b)hris]

So I'm having a weird kind of "pop" of air coming out of which I think is carb 3. It almost sounds like a compression stroke that isn't igniting and backflowing through the intake. The bike is also missing and backfiring below 3.5k but smoothes out and has good power after that. What also happens too is that when it's plenty warmed up the idle starts to stick at 3k and I can't rev it down. I've pulled the plugs and some look black and a little wet(too rich?) and I have yet to do a proper compression test. Other stuff I've done is adjusted the valves, points, set timing, measure impedance on the ignition wires, and sync the carbs. I pulled a float bowl and it's jetted at 120 and I have aftermarket pod style filters and a aftermarket exhaust. Bike is a 76 CB750K

When it sticks at 3k that's when you're idling right?  The 120 main jet won't have anything to do with that.  When you're idling you have to worry about the pilot jet and the air screw(and the needle position I think a little).  You said it smooths out after 3.5K...what throttle position are you at when this happens?  The way I understand it, the mixtures are set at different THROTTLE POSITIONS, NOT RPMs.  I'm having some problems too so I'm not one to give advice, but mine's starting to smooth out finally.  The carb faq helped me a lot.  Im also running pods and aftermarket 4-1.  Stock airscrew for my bike is I think 1 or 1 1/4 turns out....I'm down to about 5/8 turns out and it's pretty okay.  Stock idle jets still but I don't know for sure.

There are some charts in the carb faq that make it a little easier to troubleshoot.  I never had backfiring problems like yours though.   Good luck.

Also make sure you're making your airscrew richer and not leaner.  I think you turn in to richen it if the screw is on the airbox side of the carbs, turn out to lean it.

Yes, it takes around 30min to get really warm and then the high idle problem surfaces. It's pretty consistent at doing this too. The bike smoothes out under acceleration(1/4 to 1/2 open on the throttle?)

sounds like mine too man.  mine was lean.  I ended up trying to move the air screws in 1/4 turn increments, and then 1/8 turn increments.  Mine would do that hanging on high rpms thing, and then when I turned it 1/8 turn richer it went away.  It was a pretty fine adjustment on mine, and I had to experiment, but eventually it stopped doing it.
 
another issue i had that i think contributed was a clogged petcock.  the plastic tube in mine was actually broken, and there was a little stray piece that would randomly block one of the passages, causing i think 2 of my carbs to starve.

idk if the airscrews will fix your issue, but I can turn my airscrews 1/8 turn leaner from where they are and the behavior with the hanging rpms will come back.

goodluck.

[anotherCB]

Brian77cb750: Yes, 130 mains, needle setting is one richer than stock, engine is stock, only mods are the pods and the 4in1.

[nancy]

Is it possible u have a vacuum leak at no 3 from the carb to motor rubber? Mine was - and symptom was mainly a "pphhhsst" sound and a visible puff of vapour out of the carb area...at idle and the noise would occur during a ride..but didn't seem to cause performance issue. Turned out I had a split in the boot right where the securing band was biting in..
Mark

[anotherCB]

I don't think a vacuum leak is an issue. I had everything apart and triple checked but will surely check again when I take the carbs off to change the needle setting and the mains.

[longshanks]

Is it possible u have a vacuum leak at no 3 from the carb to motor rubber? Mine was - and symptom was mainly a "pphhhsst" sound and a visible puff of vapour out of the carb area...at idle and the noise would occur during a ride..but didn't seem to cause performance issue. Turned out I had a split in the boot right where the securing band was biting in..
Mark

Yes, I'm pretty positive there is some kind of vacuum leak at the no. 3 carb boot. I sprayed some ether around it and the rpms climbed. No visible cracks though.

I'm more concerned with the missing at rpms below 3.5k though and my high idle.

[longshanks]

Ok, so a bit of an update, the fast idle issue is resolved. Pulled all of the float bowls and the emulsion tubes were pitted and the PO must have taken a wire brush to them because they were scored pretty well. 

Put new emulsion tubes in and cleaned out the main jets(120) and pilot jets and synced the carbs as well as adjusted the return throttle cable.(this may have fixed my high RPM problem). I then backed out the air/fuel mixture screws 360 degrees and took it for a spin. The hesitation and misfiring was still there and some backfiring from the exhaust. Then backed out the screws a 1/4 turn and the backfiring cleaned up a bit. However, the hesitation is still there. I should note that the no. 1 cylinder is running rich and the plug is black and sooty while all the others are a nice light brown color. So if I keep backing out the screws am I making a richer mixture yes? (I don't have this backwards right?)

I should also note that its been a little coldblooded lately and I have to hold the throttle open for a bit or adjust the idle screw to keep it running.

To re-iterate my main problems are backfiring or "farting" out of carb 3, hesitation and stumbling at any rpm below 3.5k and backfiring out the exhaust on de-acceleration. Ignition problem?, needle position? vacuum leak? pods?

[c(b)hris]

I think the way it works is if the air screws are on the intake side of the carbs, then turning them out=lean, thurn them in=rich.  Opposite if on engine side.  Might want to verify that though.

[dave500]

do you still have pods on it?if its the one carb it might be the inlet valve,checked the tappets?rig up a leak down test and listen in that carb,or with the ignition of and all but that cylinders plug out try cranking it by hand on the kicker and try and listen if it hisses on compression maybe?

[longshanks]

do you still have pods on it?if its the one carb it might be the inlet valve,checked the tappets?rig up a leak down test and listen in that carb,or with the ignition of and all but that cylinders plug out try cranking it by hand on the kicker and try and listen if it hisses on compression maybe?

Yes valves have been checked twice. I'll try that kickstart method you mentioned, thanks. (Do you mean with the no. 3 plug in and the rest out or vice versa?)

[gnarlycharlie4u]

idk if it's been mentioned but have have you tried syncing the carbs? what about adjusting your air screws and needle clip positions?

what carbs are on this bike?

also, are you ABSOLUTELY sure they're clean and that the float heights are correct?

Yes, I'm pretty positive there is some kind of vacuum leak at the no. 3 carb boot. I sprayed some ether around it and the rpms climbed. No visible cracks though.

I'm more concerned with the missing at rpms below 3.5k though and my high idle.

oh... well that explains a lot actually.
it's been mentioned but it's not the rpm's so much as the throttle position that matters with the carbs.
fix that vacuum leak before you go changing anything else.

[gnarlycharlie4u]

edit: I just noticed your running the PD carbs, I don't have those on my 77, they have press in idle jets, not sure if those are available or not.

http://www.siriusconinc.com/pro-detail.php?pid=&product_id=1805
the #35's are available too.