Topic: decel popping

[M.Henry]

My bike is finally back together from the winter season and she is acting up. I did an engine swap of a k3 into my k7 bike. I kept the pd carbs I originally had fro the k7 motor. The k3 is stock other then using some f2 or f3 pistons (Honda 410). I also run a set of 4 into 4 drag pipes while I am restoring my original 4 into 4 mufflered pipes.

So my problem is that I am having sever decel popping problems. Like whenever I let off the throttle it pops at least 2 or 3 times minimum. If I engine brake any then it gets a crazy 6 or 7 pops. With the drag pipes this means very loud pops and since I commute onto my universities campus I'm always concerned that I am way to obnoxiously loud.

So I have done some reading around on this forum about this popping and from the sound of it I am experiencing decel lean popping. This happens when an open throttle snaps closed, the engine will find a place to breathe air and fuel from even if it means from the slow or pilot jets, or even pulling from the exhaust during the overlap phase of the cams. This leads to a very lean amount of fuel coming form the usual carbeuration, but primarily a low oxygen environment. The chamber then burns as much as it can, but then when the gasses are pushed into the exhaust it finds a more oxygen dense environment and can combust a second time with a small quick pop.

Is my understanding correct in this? and more importantly, What Direction should I go with tuning my set-up so that I can reduce this popping?

I am unsure if my pd carbs have a vacuum cutoff valve which would help stop this issue.

[M.Henry]

Oh, I forgot to mention something that could be huge on this concern. I am using pipes set for a k7, but the head it is mated to is a k3. I noticed that the stud towers from the head are much longer on the k3 head, so the header gasket copper rings were not quite sealing the head to the headers. This left a the headers with room to wiggle some from the head mounting end. I have a second set of the copper crush gasket rings that I will double up in each stack up. I am sure that this loose sealing probably does not help the popping because it would really allow for a oxygen source right at that lean condition popping.

[evanphi]

You're exactly right. Popping on decel means a lean condition, and that exhaust leak is contributing to the issue. Fix that first, then look at enriching your pilot circuit. Can usually be tuned with the air screws. A little bit of popping is fine, though. It's not going to kill your engine. A freer flowing exhaust will allow you to hear more pops anyway.

Do a plug chop once you have fixed your exhaust leak to determine your fuel mixture at each important position, adjust based on those results, and see if you still pop. If you do, don't worry about it.

[Bankerdanny]

The PD carbs are also leaner from the factory than the prior carbs, so if you are running stock jets my guess is that you are running very lean.

[M.Henry]

You're exactly right. Popping on decel means a lean condition, and that exhaust leak is contributing to the issue. Fix that first, then look at enriching your pilot circuit. Can usually be tuned with the air screws. A little bit of popping is fine, though. It's not going to kill your engine. A freer flowing exhaust will allow you to hear more pops anyway.

Do a plug chop once you have fixed your exhaust leak to determine your fuel mixture at each important position, adjust based on those results, and see if you still pop. If you do, don't worry about it.

Thanks. I hope that the exhaust leak fix will cure the problem some. I was mainly just afraid that there might be an issue of burning the valves. I have adjusted the carbs around in the past so It is not exactly the from the factory conditions of the PD carbs. I have not put a larger jet in it though.

[M.Henry]

On a related note, I have some idle issues. When it revs down initially it will hold steady at 2k for a few seconds, then it will drop down to about 1.3k (depends on how I have the idle screw set). This drop will sometimes go low enough to kill the engine if I do not keep it alive with some idle action. What this equates to is me usually just keeping her around the more stable 2k. which is much louder. Where should I adjust for that? the idle screw? Should I make here leaner or richer? Would more information need to be known?

[cb_n00b]

My bike did something similar when I put the MotoGP Werks pipe on it...It was pretty easily fixed by very small tweaks to the idle air/fuel screws...I just enriched them maybe a 1/4 turn and it pretty much fixed the issue.

You're going to be chasing your tail if you don't get the exhaust leak fixed first though.

[M.Henry]

oh yeah. fix the big things before you fix the fine things. I've learned that lesson before. I'm just trying to get ahead on the learning and preparing for when I do the work.

[evanphi]

Don't get too caught up in adjustments just yet, though. Solving that leak could fix all of your issues! You're correct in all your assumptions, though.

[PeWe]

My bike have had popping at decel due to rich condition. After Hondaman explained it I understood how my Mikuni TMR had to be jetted. Dyno with A/F meter verified it. It can pop sometimes after some revving which is caused by acc pump and the extra fuel will pop which is OK.
The std carbs for old CB750 get more fuel when open the air screws more, like a paint spray gun. Less air, less paint will flow.
Back in the 70-80's I thought that the decel popping was too lean and closed the air screws a little and problem solved. The only thing is that it is completely the other way.  Popping due to rich, close air screws and less fuel will flow in the pilot circuit.

My Mikuni TMR I have now had horrible popping when needles were 2 steps too rich. An hour on the dyno showed the needles were too rich and I could make it right to next dyno session a few weeks. Last thing was the fuel screws I could make leaner until the rich smell at idle disappeared.

It runs well now. I had my first ride of the year today, a few hours ago.

[dave500]

a little popping is fine,even new they would pop but very muffled in the stock exhaust youd hardly notice.

[Bankerdanny]

I like the decal pop on my 550. It gives it character.

[dave500]

ha ha mine too banker,it sounds tuned if its not like a machine gun but just a few random pops,i can free rev mine quite high and it wont pop only when ridden,if its popping when free revving it needs to be looked at,i hate the bikes that pop and bang with each gearshift!

[M.Henry]

the popping does give some great character to the bike, aside form the classic bike sitting next to all the newer bikes at my school.

[Paintedseat]

If you are running the stock jets and needle position you are lean. You gave adjustable needles. They have 5 notches. I put mine on the 4th or the 2nd from the bottom. You should have 115 mains and they are bigger than the 110 that they used in 78. If you have stock air box you can get away with them. The pilot jets are 35 and the hanging idle is probably because of that. I tested 38 and 40 and found with the needles adjusted the 40 was too rich. I use the 38 and it starts first hit and choke is hardly needed. Also check your accelerator pump arm and make sure it is pumping fuel. Also the rid should be in contact with the arm. The accelerator pump really helps the lean condition. The extra shot of fuel is very important to the function of the PD carbs.

[HondaMan]

If the carbs are not vacuum-balanced, one cylinder that is the leanest will pop the most. It will be tough to figure which one unless you have gages or else 4 pipes, though! After a while of running, look for the lightest-color plug: that will be the leanest one. Adjust it's slide open slightly more at the linkage (PD carbs), like 1/8 turn at a time, and test ride until you find it.

You might also find that the tiny O-rings above the idle mix screws are hard, cracked, or even missing: this will make a PD carb or two "hang" in high-idle mode for a while, too. The O-ring also sits ABOVE the tiny washer in there: if someone installed these upside-down, it will idle all over the place.

And, if the compression between cylinders is more than 10% difference in PSI, it will have struggles to idle well.

[M.Henry]

I do have a set of vacuum gauges and luckily I do have the 4 into 4 exhaust system, So I can tell which cylinder is acting up sometimes.

I will get the header gasket rings in this week so I intend to pull the exhaust and intake systems apart some next weekend. So y'all think a full carb tear down to see the linkage settings, and where the O-Rings are. I will also clean the spark plugs off, and give them a look after some driving. I will also check the float level since I haven't done that in quite a while and tons of small work has been done to the bike here and there

[evanphi]

I do have a set of vacuum gauges and luckily I do have the 4 into 4 exhaust system, So I can tell which cylinder is acting up sometimes.

I will get the header gasket rings in this week so I intend to pull the exhaust and intake systems apart some next weekend. So y'all think a full carb tear down to see the linkage settings, and where the O-Rings are. I will also clean the spark plugs off, and give them a look after some driving. I will also check the float level since I haven't done that in quite a while and tons of small work has been done to the bike here and there

Be sure to use the clear tube method for checking float height.

[TwoTired]

Decel popping is likely too lean.
If the ratio is too lean to burn, the charge gets pushed into the exhaust, repeat cycles eventually builds up enough fuel in the exhaust to support ignition and due to wasted spark, and an open exhaust valve...pop, igniting the unburned mixture in the exhaust header pipe.

PD carbs were designed for lean burn engines.  The idle circuit was tuneable for absolute minimum fuel delivery per EPA idle emissions requirements.  That's why they had to add accelerator pumps to these carbs, or they would stumble big time upon any throttle advance.  The engines these carbs were paired with also had high pressure exhaust systems which retained spent gas in the exhaust to prevent the decel popping.

I do find it humorous of you're expressed concerned about noise while still running obnoxious drag pipes.  I suspect there is more worry about police intervention than annoying others.  But, that's just my theory...

Cheers,

[M.Henry]

I do find it humorous of you're expressed concerned about noise while still running obnoxious drag pipes.  I suspect there is more worry about police intervention than annoying others.  But, that's just my theory...

The drag pipes were a replacement mod for my very broken original exhaust system. a proper exhaust system is on my list of projects to do this summer or over the winter break. I may go so far as getting my buddy to bust out his expert TIG skills and make me a Titanium exhaust system with those beautiful coloration of teh welds in the turns ( the usual bend locations). Why he keeps telling me he will do it for free? Idk. maybe it is because he hasn't gotten to play with welding titanium in a year or so. If not I might just work with him on bending some pipes adn getting mufflers.

tl;dr Its a temporary mod until I can get a fresh set of 4 into 4 fabbed up again.

[M.Henry]

I suspect there is more worry about police intervention than annoying others.  But, that's just my theory...

Funny story there. I actually got pulled over last evening by the police when limping my bike on the 0.5 mile trip home from studying on campus till midnight. I walked outside after studying to a bike that had appeared to have no electrical system working. I kickstarted her for a few minutes then started limping back. Luckily I had 2 friends escort me home, one  was me very experienced motorcyclist room mate, and the other was a friend in her honda CRV. Though when I passed the one gas station on the trip the three cop cars sitting outside jumped in their cars and had to come ask me why I was going 10 MPH and was on teh road with no headlight. Luckily they were understanding of me when I said I was just limping it home and that it had just broken on me.

I think it is that my stator is not charging the battery up. I noticed all my lights being dim/weak the other night when i was riding around dusk. I just read the battery at 11.6V. Guess I will have to work on troubleshooting that next.

[TwoTired]

Unless the stator was attacked by road rash, hatchets or bullets, it more than likely ok.  Very robust, that.  Slightly less so is the field coil.  Both can be checked with an ohmmeter.

Charging system methodical verification checks, CB750, CB550, CB500, CB400, and CB350.

Begin with problem verification and characterization with recorded data.
A -- Fully charge a known good battery.  Let it rest for 2 hours, off the charger, and measure the battery voltage.  (Target is 12.6-12.8V.)
B -- Start the bike and measure the battery voltage at idle, 2000, 3000, 4000, and 5000 rpm.
C -- Repeat the measurements of B with lighting off.

The above tests identify charging system success, failure, or degree of "faulty".  The success voltages are listed in the Shop manual.

D -- Assuming the above indicates faulty, do check the RECTIFIER diodes with a diode tester or ohmmeter capable of testing diodes and uses more than .7 volts to make the test(s).
Of the twelve test made in D, six must read low ohms and six must read very high ohms.

E- assuming no faults were found in D,  Measure the white and green wires disconnected from the REGULATOR.  CB750s should 6.8 ohms - ish,  CB550s/350s/ and 400s should read 4.9 ohms- ish.

F- If there are no bullet holes or road rash/divots on the alternator case, the stator is probably good.  But, you can check for yellow to yellow continuity (.35 ohms) among all the wires, and that no yellow wire has continuity to the engine case.

G- Assuming no faults found in D, E, and F, measure the disconnected terminals of the REGULATOR.  The black and white terminals should measure zero ohms (subtract meter error if there is any).  Higher than Zero ohms, indicates internal contact contamination needing cleaning and attention per shop manual.

H - Assuming D, E, F, and G have not found faults. We can verify all the of the charging system minus the regulator is functioning correctly, by using a temporary jumper to connect the disconnected white wire (normally attached to the REGULATOR) and connecting the White directly to the the battery POS terminal.  Repeat the B and C tests.  However, if at any time the battery voltage rises above 15V, stop the test.  Such an indication would prove the charging system capable of maintaining a known good battery.  If this test never achieves 15V, then there is a wire/connector issue in either the ground path leading back to the battery NEG terminal, a wiring/ connector issue withe the rectifier RED path to the battery POS terminal, or you made a mistake in D through G.

I - (not used, can be confused with L)

J -  The only parts that remain to prove or expose are the REGULATOR (in active mode) and the electrical path between the battery POS terminal and the black wire that connects to the REGULATOR.

K - Lying to and starving the regulator
The regulator can only do its job correctly if it gets a proper voltage report of true battery voltage status.  The Vreg monitors the Black wire for this status.  Measuring the voltage lost between the Battery terminals and the Vreg connections identifies problems that are not really the charging system's fault.
Two connection paths must be checked, the Battery POS terminal to the Black wire connection at the VReg, and the Battery NEG terminal to the Green wire connection to the Vreg.  A volt meter can measure these losses directly by placing a probe between the two identified points, Black path and then the green path.  The numbers are summed and the error seen by the Vreg quantified.  Anything over .5V loss is cause for concern and anything over 1V is a certain issue to be corrected.  Each connector, terminal, fuse clip, or switch in the pathway can cause voltage reporting loss.

The regulator also passes the received voltage on to to the Alternator field coil to create a magnetic field within the alternator,  The voltage level determines the strength of the magnetic field and the maximum output capability of the alternator.  Therefore, starving  the Vreg of true battery voltage leads to reduced max output capability of the alternator.

L - regulator operation/verification.
  The Vreg sends voltage to the alternator field in response to measured voltage which is battery state of charge.  Any voltage at the battery of less than 13.5V sends full black wire voltage to the alternator's white wire.  The alternator output will vary with RPM, even if "told" to produce max power by the Vreg.  If the alternator has enough RPM to overcome system load, any excess power is routed to the battery which will raise the battery voltage (slowly if depleted and rapidly if nearly full).   When the battery reaches 14.5V, the regulator reduces the voltage to the alternator, reducing output strength and preventing battery overcharge.  If the battery exceeds 14.7V, the regulator clamps the alternator field coil power to zero (0V), effective shutting off the alternator.

Because, there is electrical load from the system, an alternator that is not producing power allows the battery to deplete and the voltage falls.  The Vreg responds by turning the alternator back on in accordance with battery state/ charge level.

The Vreg state changes can be monitored/verified by observing the battery voltage state, and the White wire to the alternator field.  (Two meters are handy for this.)  The "trip" voltages can be adjusted with the adjust screw, while changing engine RPM and electrical load that the bike presents to the battery/charging system to "make" the battery reach the voltage levels need for the set trip points.  IE. with load reduced (lighting off) and the engine above 2500 RPM, a charged battery will attain 14.5 V.  Anything above that and the adjust screw needs to be backed out to keep the battery safe from harm.
The shop manual outlines bench set up mechanical adjustments that should be performed on unknown or tampered units.  These should be resolved before final trip point adjust tuning.

Note that while the system is working, the Vreg can change states rapidly before your very eyes., changing 5 times or more while you blink.  Therefore, you may have to mentally average values measured on the White wire if your selected meter doesn't do that for you.

[M.Henry]

oh wow. Thank you for that post. I hadn't even put any thought into learning what to troubleshoot yet. Very busy part of the semester right now. 

I was thinking that the it was likely the charging system on the engine more then likely. The engine ran fine electrically before all of my engine swap work this winter. the original motor grenaded itself so this is a k3 motor in my k7 frame. The motor does not seams to be the best maintained one. very rusty around the edges, and it took me lots of care to assure it wouldn't give me trouble. I figured it just had a bad charging system attached and I was going to try to throw the field coil panel off of the original k7 motor on this one to resolve the trouble.

I would much rather play with the multi-meter and properly knwo what is going on. Thanks.

[M.Henry]

Well I charged the battery yesterday and this morning it was sitting at 12.6V. Granted I just threw it on the charger and a whiles later I pulled it off. so I don't know for certain if it did full charge. 

[RJ CB450]

It really is a tricky one, the popping side.  Lots of higher detailed answers here, so take this in context with them.  Popping vs backfiring.  Crisp and loud bangbangbang are the bad ones.  This is when you are getting the hard fuel in and it is igniting hard right in pipes.  The valve killers.

The gurgly/rice crispies poppy is more common with open exhausts.  Lack of backpressure and volume means you will hear the gurgling more often and not much can be done.  Like others say, just the leaning out from throttles fully closed.

To put into some context, The second gen (as I call em) CV carbs with the metal pucks and full metal dome.  Most have an air bypass diaphram on the one side.  A common cause of grief, but it is so you dont lean out when you snap off throttle.

However, this isnt the case for the CB650.  They do not have this air cutoff.  Stock exhausts it isnt noticable, but on my 650, there are glasspacks on the oem downpipes.  The bike runs well, but snap that throttle or engine brake, all four with crackle away.  I could tune it to not do it, and for a while I did, but it was so rich on idle, it gave problems cold start and really mucked the idle.