Topic: Dreaded Popping During Deceleration

[vames]

Hi - I'm having a hell of a time tracking down the cause of this rolling popping backfire I get on engine-engaged deceleration, and, at a much lower frequency and volume (like once every 4 seconds) at idle on my 76 cb750f. My carbs are clean with rubber bits replaced and bench-synched, valves are spot on, air box is stock, after market hooker muffler (a little loose where it mounts to the stock header), timed perfectly with electronic dyna ignition, all plugs  a medium brown, air screws out one turn each. This bike is fast and smooth and the engine works like it should, but still the popping.

It can be avoided by staying on the throttle a bit when decelerating. Does that suggest I'm running too lean? Too rich? Could it be the slight loose fit of the muffler to the header? Would close but not spot-on carb synch cause it?

What are the top three or five reasons that these bikes do this? 

[dusterdude]

you are pulling air in from the exhausts some how,either a leak or baffleless pipes i`d say.

[ProTeal55]

I like popping on de-cell , but thats just me 

[Harry]

SOunds lean at 0 to 1/3 throttle.

Try taking your "air" screws *(actually fuel screw on 750, I think) out one more turn.

[vames]

I wouldn't mind the sound that much if I rode on open roads in the country, but I ride this thing around New York city and there is lots of echo and proximity to people and it just isn't cool.

My mixture screws are on the airbox side of the carb - I'm under the impression that on these, out means leaner and in means richer. Maybe I should try to richen it a bit by trying 1/2 turn or all the way seated. The only place I could see it sucking air in the exhaust is where the muffler meets the header, but would it still cause popping that far downstream from the engine?

Is there what if I advance or retard the timing ever so slightly?

 

[Bob Wessner]

"Maybe I should try to richen it a bit by trying 1/2 turn or all the way seated."

A half turn is quite a bit. On my earlier 750, I was quite suprised to find out just how small and adjustment to the air screw it required to alter the lean/rich ratio. So far, I have found that +/- 1/8th turn relative to the manual specs seems to be just about right, assuming all else is stock.

[TwoTired]

The popping on decel is likely from accumulated fuel in the exhaust system lighting off.  Which can occur from multiple stimulus.

- Leaks in the exhaust system can provide oxygen to enable the unburnt fuel to ignite with an ember from the piston chamber.

- If all the slides don't close evenly, some cylinders may get more fuel than others and pass that charge unburned into the exhaust.  (Carb sync is needed.)
Leaning the idle mixture (turn screw out on Non-PD carbs) of the offending out of sync carb, should help abate the fuel buildup in the exhaust when the slide is in it's closed position.  This is a diagnostic, not a fix of the source problem of out of sync carbs.

If carbs are in sync and there are no exhaust leaks, popping on decell is caused by a too lean mixture (normally caused by altering the inlet duct).

If the idle mixture is too lean, the cylinder will not fire the mixture at all and shove the lean mixture out the exhaust port unburnt.  The rider can't tell, because he isn't asking for ANY power.  However with enough exhaust cycles, there is enough fuel to ignite in a 4 to1 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.
Cheers,

Popping can have more than one cause and the correction must fit the cause.

Cheers,

[dusterdude]

listen to bob,vames.he`s the carb guru here.

[bike54]

hi
i had the same after refitting my exhausts with new gaskets every thing fine for a couple of days then she started popping on the over run turned out the exhaust bolts at the head needed re tourqueing must have settlled with the heat that fixed it the old girls a cb550 k3

merv 

[mcpuffett]

my 750 does it no probs for me i like it  check how your pipes are sealed where they go in the head    mick.

[kghost]

 Could it be the slight loose fit of the muffler to the header?

Theres your answer, Fix the joint before anything else......

[carl550k]

had the same prob with my 76 550k. turned out my points were shot which was making my timing way off. another set of points moved the timing back where it should be. no more popping.

[Zeke]

I have to agree with leaning the idle mixture.

Everything I've read suggests that popping when decelerating is caused by a build-up of unburned fuel which then sporadically ignites in the hot pipes.

A week ago I cured this prob on my bike by leaning all the pilots an 1/8 of a turn.  Actually, still pops a little little bit so I'll just nudge them lean a bit more.

It idles better, and the only down side is that it's a bit hard to start when the weather is cold (like 45-50degF) then I need to twist the throttle slightly when cranking.

Zeke

[mrbreeze]

My bike quit popping dramatically after I used my carb tune for the first time. Bench synching is good enough to get it running but you will truly be amazed at the difference it makes using the Morgan carb tune(or I imagine any manometer set up)

[vames]

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. 

[Robert]

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. 

Yup. those "you're shot if you pop" signs at the beginning of the tunnel are quite scary.