Topic: Backfiring when decelerating?

[wildrum88]

Hi all, just wanted to run something by anyone who is interested but i just did a rejet on my 1982 cb650 because she was running pretty lean and kept backfiring on the decel. just a little background: she has pods/open 4-1 headers. the stock jets were 120 mains, 35 idle. i upped them to 130 mains, 37.5 idle and threw one shim on the needle. She is running just fine and does a lot better in the mid-rpm range but is still backfiring/popping quite a bit when a decelerate going down hills, etc. any idea why this might be? the idle screws are at 1.5 turns. Thanks!

[2wheels]

Open 4 - 1 header ?
Try putting the baffle in and see what happens.

[TwoTired]

Almost certainly a lean mixture due to pods and open exhaust.

Have you checked the air cutoff valves?  They enrich the mixture during decel (Throttle closed and high vacuum present).  But, are unlikely to be calibrated for the changes you've made, so even if they are working to capacity it may not be providing enough enrichment.  But, it begins with seeing if they are working at all on each and every carburetor.

[octagon]

never hurts to check your exhaust flange bolts and make sure they're tight either

[bwaller]

never hurts to check your exhaust flange bolts and make sure they're tight either

Or that there are actually exhaust gaskets in place and are sealing.

[Mainerider]

I have a fair amount of experience w the 650. The decel popping is a typical lean burn condition. The mods that have been done to the bike  have created this. The air cutoffs are designed to bleed off engine vacuum but your mods are probably overwhelming their capacity. Pods look cool but an airbox serves multiple roles beyond being just a happy home for the filter element; without it in place that carefully calibrated fuel/air delivery compromise is well, compromised. Then, add open exhaust to throw off the exhaust wave pulses, too... Be thankful it even runs...

[BobbyR]

I had the problem and the float bowl level was too low in one carb.

[grcamna2]

Almost certainly a lean mixture due to pods and open exhaust.

Have you checked the air cutoff valves?  They enrich the mixture during decel (Throttle closed and high vacuum present).  But, are unlikely to be calibrated for the changes you've made, so even if they are working to capacity it may not be providing enough enrichment.  But, it begins with seeing if they are working at all on each and every carburetor.

+1 on the air cut valves...and an airbox.

[stampederunner]

It's just your bike telling you don't slow down. When she starts back talking to you just give her a fistfull of throtle and that will shut her up!

[70CB750]

So where and how do you fix the lean condition?  Needle up?

[TwoTired]

So where and how do you fix the lean condition?  Needle up?

First of all, we don't have a current fuel map to determine if the fuel metering is now correct for pilot, throttle valve, and main.
So, any or all of these could be lean and contributing to the decell popping.

All we have is:
 

She is running just fine and does a lot better in the mid-rpm range

>IF< the current fuel map is indeed now correct for the modded set up, THEN perhaps all is need is to make a mod to the air cutoff valves.
  These are supposed to enable fuel enrichmentt when a high vacuum is placed on its associated diaphragm.  The extra fuel prevents missed firing cycles that would allow unburnt fuel to accumulate in the exhaust.
The delivery circuit was originally "tuned" to provide just enough extra to prevent the popping without wasting lots of fuel.

Depending on the current fuel map, it may not provide enough extra fuel now with the mods.  Or, it may have one or more failed air cutoff valve circuits.

So, first check the air cutoff valve operation.  Then decide to modify the fuel map or the valves to correct the objectionable symptom.

Cheers,

[Mainerider]

I'm fairly certain that the air cut-off diaphrams don't actually add fuel to the mixture .  A sudden increase in engine vaccum (as on a quick closing of throttle) creates low pressure in the air cutoff housing, which causes the diaphram to open momentarily and partially block the pilot air circuit passageway.  This creates less aeration, which effectively enrichens the mixture.

There is an accelerator pump on both the earlier 650 mechanical-slide carbs and the '81 onward CVs which does deliver a shot of fuel during abrupt throttle roll-ons.

[TwoTired]

Is the vacuum source for the air cut off diaphragm on the engine side of the slides or the filter side of the slides?

[Mainerider]

It operates on vacuum created downstream of the throttle. A true Coast Enrichener version works similarly but in that case vacuum lifts the diaphram against spring pressure, which in turn releases a sprung valve in the carb body. That sequence increases flow in the pilot circuit and does supply extra fuel. A matter of semantics, really;the terms are used interchangeably and generically, like fork legs, tubes and sliders.  End result is the same, though; richer mix...with an airbox and proper exhaust of course...

[jessezm]

Sorry to hijack, but still on topic:  Besides the annoyance of the popping sound and the funny looks you might get from your riding buddies, are there any real negative effects resulting from backfiring under these conditions?

[MCRider]

Sorry to hijack, but still on topic:  Besides the annoyance of the popping sound and the funny looks you might get from your riding buddies, are there any real negative effects resulting from backfiring under these conditions?

A backfire means a flame has entered the exhaust pipe thru the exhaust valve. A hot flame over a valve seat is what cause a valve to "burn". If its bad enough and goes on long enough. you'll burn a or several valves. This results in a loss of compression as those valves don't seal as well.

And so on.

[jessezm]

Yeesh, that does sound bad...

[TwoTired]

The "backfires" are explosions in the exhaust system, hammering the exhaust components from the inside.  Bigger hammers make metal bend and stress weld points.
It can expose weaknesses in the exhaust.  But, it really depends on how violent the backfires are or how much fuel has built up inside the exhaust system.
Sometimes the silencer can separate from the header as a "safety pop-off valve" would?

Also the pressure buildup in the pipe during the explosions puts pressure on the exhaust valve against seating pressure of the spring.  Perhaps some hold open or delay of closure can occur which might pass hot gasses over the valve seat and erode metal there.

[MCRider]

Where the actual damage occurs can depend on if its a lean backfire or a rich backfire, and if its accompanied with advanced or retarded timing.

Simply said, it shouldn't be allowed to continue.

[jessezm]

Again, not to threadjack, but since you asked...  This is on a CB400F with the following mods:
466cc kit
Web Cam (their "hot street" cam)
Stock carbs bored to 24mm
K&N pods
ported heads (JMR stage 1)
yosh-style open exhaust
Carbs synchronized, timing spot-on (dyna-s and dyna coils)
Carb settings: 98 mains, 38 pilots, stock needles shimmed 1/2 step rich from center, air screw: 1.75 or 2 turns, float level: 24mm

The popping is generally on a warm engine when I close the throttle just as described in the first post...

I need to play with the air screws again, but from memory, I could get the popping to stop by turning in the air screws more, but it would also produce a pretty soggy feel off idle.  I have a reproduction air chamber on the way that I am going to experiment with, but not the air filter housing.  I'll need to come up with my own adapter and air filter set up.

I know that's a lot of variables, but does anything jump out as an obvious fix for the backfiring?

[Mainerider]

If the bike is not overly lean and surging, say on acceleration or at steady rpms, then no not really. My VFR lean pops on decel due to a Two Brothers can;

[jessezm]

Well it's not lean surging at low-RPM--it's actually a tad rich at low throttle openings.  It's only on decel with the throttle valve closed.

[grcamna2]

Well it's not lean surging at low-RPM--it's actually a tad rich at low throttle openings.  It's only on decel with the throttle valve closed.

Jesse, did you get you're airbox from DSS yet and hook it up ?

[jessezm]

Well I ordered one off ebay from Japan, it looks to be the exact same unit, but was cheaper with free shipping...  It hasn't come in yet, though--probably another week.

[lucky]

Hi all, just wanted to run something by anyone who is interested but i just did a rejet on my 1982 cb650 because she was running pretty lean and kept backfiring on the decel. just a little background: she has pods/open 4-1 headers. the stock jets were 120 mains, 35 idle. i upped them to 130 mains, 37.5 idle and threw one shim on the needle. She is running just fine and does a lot better in the mid-rpm range but is still backfiring/popping quite a bit when a decelerate going down hills, etc. any idea why this might be? the idle screws are at 1.5 turns. Thanks!

QUOTE..."Hi all, just wanted to run something by anyone who is interested but i just did a rejet on my 1982 cb650 because she was running pretty lean and kept backfiring on the decel. just a little background: she has pods/open 4-1 headers."


 Adjust your mixture screws leaner until it goes away. But with 37.5 idle jets it probably won't help.

Backfiring or popping or bogging down sucking sound on accelerating is a lean.

You said "I threw some shims on the needles" You did not say how thick the were???
Very important fact.

You have pods and 4 into one exhaust so a #37.5 idle jet is not big enough.
Try at least some #40"s.

I would take the shims off of the needles for now.
Just increase the idle jets to #40's


Only make one change at a time.

[wildrum88]

Thanks Lucky, i just read your other post about the 78 cb750 and i am making the mistake of doing too many things at once. I will be ordering some #42 and #45 idles soon and reporting back. do you think #45s are too big? The bike seems to run great, no bogging down or lack of acceleration whatsoever so i think I've got the mains (130s now) right. as of right now i have the idle screws out 2 full turns and 2 shims (.037 thousands i believe) on each needle. The bike takes forever to cold start, but once she's running, she's runnin and will start right back up first try when warm. I'm sure this has to do with the idle jets. thanks again for the advice!