Topic: Popping from exhaust on deceleration 4 into 1 pipe--normal??

[judder]

I just synced my carbs and finally got my exhaust (4 into 1 tiny baffle) to not leak so I took her out for a ride. Wanted to do the ton but thought better of it : ) It pops pretty good on deceleration is that normal with a 4 into 1? My jetting is not perfect yet but the engine seems to run pretty smooth. I put about 30 miles at 55-70 mph and she seemed to run smooth at that speed.

Thanks

[Dimitri13]

Popping on decel means lean.

[lucky]

Popping on decell can be too rich of a mixture.
Popping when giving it throttle is too lean.


But really we would need to know ALL OF THE FACTS.
Do I need to type all of that?

[BrockSamson]

From what I understand is that its a lean condition.  Right now my 1977 750k has 4 into 1 with stock airbox and I get popping on decel.  Literally in a few hours I am going to be putting in larger pilot jets in among other things.  I will probably be riding late tonight or early tomorrow.  I will report back and let you know my findings.

With a 4 into 1 you need to rejet, just the way it is.

Also, it was explained in another thread but I didn't understand and haven't had time to do further research but you would think a rich condition would result excess fuel detonating in the exhaust... but this is not the case.  If someone else knows the thread that explains this can you please post it because I hate giving this guy advice without actually knowing how it works and why the popping occurs in a lean condition.

By the way what kind of bike do you have with what modifications?  Year/displacement/etc.

[BrockSamson]

Did a little reading...

The popping is the result of no detonation of the fuel/air mixture.  This can occur in a rich or lean condition (from what I read).  If the spark can not ignite the mixture it is expelled out the exhaust regardless then is ignited inside the exhaust causing the popping.

Bottom line is that the fuel/air mixture isn't sparking in the cylinder.

[judder]

My bike is a 1974 750K4 with 4 into 1 exhaust, pods, stock clip location and 140 mains I believe. I played with the choke a bit on acceleration and when I choked her a little bit she bogged down. I need to check my plugs to see what is happening. Seemed to cruise just fine and hold speed on the highway.

Thanks

[hondaface75]

I always thought popping from the exhaust on decel was fine, you just dont hear it with normal mufflers, and baffles in. Popping coming from the carbs is bad...

[Tews19]

Popping on decell can be too rich of a mixture.
Popping when giving it throttle is too lean.


But really we would need to know ALL OF THE FACTS.
Do I need to type all of that?

I was waiting for lucky to put that. We all need the info

[BrockSamson]

http://forums.sohc4.net/index.php?topic=4089.0

"The popping on decel is likely from accumulated fuel in the exhaust system lighting off.

Leaks in the exhaust system can provide oxygen to enable the fuel to ignite.

If all the slides don't close evenly some cylinders are getting more fuel than others and that is being passed, unburned into the exhaust.  (Carb sync.)

Leaning the idle mixture (turn screws out), should help abate the fuel buildup in the exhaust when the slides are closed.

You might have to address all three, above.
Cheers,"

That was a post from TT in the above thread.

"So after playing with it for a while, I found that it was a lean condition that was causing the pop. Mixture screws (on airbox side) are now out about 3/8 turn (manual says 1) and the popping has almost completely gone away. Plugs are brown at either setting but the popping is gone. Anecdotally, the engine and the oil tank seem to run a bit cooler as well.

I don't get it - the theory of rich mixure/unburnt fuel in exhaust/pops in the pipes sounds right, but not in this case. Maybe popping could be either too lean or too rich? Ordered a carb synch set, so after I synch perfectly, I'll see how far I can get the screws out before the popping starts again.

For everybody who likes the popping - I don't mind it either, but I ride every day in New York City - pretty close quarters to be making startling noises - especially on my way to work through the Holland Tunnel with the homeland security guys posted at the entrance. They're trained not to like sharp popping noises. "

That was his resolution to the popping.

[lucky]

My bike is a 1974 750K4 with 4 into 1 exhaust, pods, stock clip location and 140 mains I believe. I played with the choke a bit on acceleration and when I choked her a little bit she bogged down. I need to check my plugs to see what is happening. Seemed to cruise just fine and hold speed on the highway.

Thanks

So lets make sense out of all this chaos:

1974 CB750K
4 into 1 exhaust tiny baffle
Pod air filters.
Idle jet size? #40mm stock
Main jet size? stock is #120 or #110mm Which is it?
Needle clip position (from the top)? Stock is position 3 or 4 from the top.
Mixture screw turns out.  Stock is 1 turn out


Float levels set at 26mm??? Correctly.

Too many unknowns right now to diagnose.
Have to fill in some of these gaps
Need to know which carbs you have. What number?

[phil71]

Lucky .. "Popping on decell can be too rich of a mixture.
Popping when giving it throttle is too lean." is backwards.


what are mixture screws on the airbox side?

[lucky]

Lucky .. "Popping on decell can be too rich of a mixture.
Popping when giving it throttle is too lean." is backwards.


what are mixture screws on the airbox side?

That is what I said on REPLY#2  I am pretty sure.
Mixture screws on the airbox??
3/8 turn?? very strange.

[phil71]

You still have it backwards. Decel pops are caused by lean condition.

[TwoTired]

Popping on decell can be too rich of a mixture.

.... If you have an exhaust leak.  Otherwise, its a lean condition. Caused by the pods.   The air screws turned inward (non-PD carbs) will enrich the mixture for throttle closed deceleration.

If it is too lean for a given cycle, the cylinder will not fire the mixture at all and shove the lean mixture out the exhaust port unburnt.  Rider can't tell, 'cause he ain't asking for ANY power.  However with enough exhaust cycles, there is enough fuel to ignite when combined in a 4 to 1 collector with an ember from any of the cylinders.  Pop.  The longer the deceleration time and the leaner the mixture, determines just how much popping occurs.
So, if the mixture is kept rich enough that the mixture fires even with no power being produced, no fuel collects and no popping occurs.

[BrockSamson]

Well my issue was definitely an exhaust leak.  My pipes didn't even have the copper gaskets  .  I changed the oil, spark plugs, wrapped my header and bolted on the header with the new gaskets... no more popping. 

The bike operated beautifully until I got in it pretty hard and I started hearing a loud ticking the bike died right outside my apt complex.  I had to push it across the street and diagnose the problem.  I found nothing obvious wrong so I am thinking I didn't tighten down the studs enough and a nut backed out and I have a very bad exhaust leak now.  The ticking is quite loud for as long as the bike actually stay running.  I am waiting for the bike to cool down before I walk out there and start poking around and getting a wrench on those studs.

Either way, I would double check for an exhaust leak before doing anything else.

[TwoTired]

http://forums.sohc4.net/index.php?topic=4089.0

"The popping on decel is likely from accumulated fuel in the exhaust system lighting off.

Leaks in the exhaust system can provide oxygen to enable the fuel to ignite.

If all the slides don't close evenly some cylinders are getting more fuel than others and that is being passed, unburned into the exhaust.  (Carb sync.)

Leaning the idle mixture (turn screws out), should help abate the fuel buildup in the exhaust when the slides are closed.

You might have to address all three, above.
Cheers,"

That was a post from TT in the above thread.

I have amended and expanded the quoted text in the referenced quote

[lucky]

Lucky .. "Popping on decell can be too rich of a mixture.
Popping when giving it throttle is too lean." is backwards.


what are mixture screws on the airbox side?

I agree.

About the exhausts having no copper gaskets...well usually when talking about carb tuning information it is assumed that ALL OTHER ISSUES have been rectified BEFORE any talk about mixture setting is discussed.

[Romhog]

hmmm... this is an interesting post to read in my current state. Among other things I was dealing with popping on decel.  as well. I have a strait out pipe design from my stock 4 into 1 pipes. No leaks up top but I'm starting to think I just don't have enough back pressure after all.  Time for a potato test.... JUST Kidding!

[TwoTired]

Among other things I was dealing with popping on decel.  as well. I have a strait out pipe design from my stock 4 into 1 pipes. No leaks up top but I'm starting to think I just don't have enough back pressure after all.

That is exactly right.

You will need pilot circuit enrichment to reduce the popping.

[phil71]

Loud pipes waste time

[killersoundz]

Don't expect to get rid of all the decel popping with straight pipes on bikes like this. If you do manage to, it will likely end up too rich one way or another from what I've been reading. Just find a happy medium

[PeWe]

Nice discussion here

I have always thought that popping during deceleration into low speed was too lean fuel mixture. Larger low speed jets or less air depending on carburator. 

The popping sound with CB750 4-4 was really wonderful sound! A good reason to buy an extra CB750 OEM style, CR750 style or a mix between them.

[Romhog]

Not for me. its a personal journey. one worth perusing. lets just say, I want people to hear me coming. and not for the showoff reasons.

it is eating up a bit of time though but not a waste.

Loud pipes waste time

[Romhog]

That is exactly right.

You will need pilot circuit enrichment to reduce the popping.

TwoTired,I will try that first! Thanks for the tip.