Topic: Will large main jets cause exhaust backfire on decel?

[BC750]

I've looked over everything possible and all ok. Using pods with 4to1 open pipe. Clip set at middle. Main jets are 127.5. CB750K. Bike idles smooth and power is great throughout throttle band. BUT always backfires out the exhaust on deceleration on part throttle. With the needle set at middle and seemingly large main jet result in backfire? Plugs at idle, part throttle are dark/tan (and I'm ok with this) and sooty at WOT.

Your thoughts....thanks in advance!

[jhasewhite]

I have a similar setup, And have been tuning for 2 weeks trying to get the thing not to backfire on decel ...  I'm starting to think it is normal.  127 is pretty rich.

[mrbreeze]

Well....we need a bit more info....where are ya/(altitude,climate)?? What bike?? What mods??? Like you,I run an open 4to1 Kerker...K&N pods...BUTT STOCK 75 K 750 motor. I was runnin' 138.5 mains down at sea level(actually 89ft. above...Modesto,Ca.) Now I live in Vallecito,Ca....elevation 1750 ft. above.Today,me and my best friend from San Jose,Ca (05 Harley Heritage Softtail) rode from my house over Ebbits Pass (hiway 4) came back on 108 Sonora pass. Wadda ride!!!!!! We did 244 miles and got to a top elevation of 9600 ft. at Sonora pass. Before I left...I cleaned my float bowls and changed down to 130 mains. I also packed 120 &125 mains with me for just in case but I made it through OK runn'in the 130's. Anyways dude......just tell us more about your bike and where you ride it.

[eurban]

I've looked over everything possible and all ok. Using pods with 4to1 open pipe. Clip set at middle. Main jets are 127.5. CB750K. Bike idles smooth and power is great throughout throttle band. BUT always backfires out the exhaust on deceleration on part throttle. With the needle set at middle and seemingly large main jet result in backfire? Plugs at idle, part throttle are dark/tan (and I'm ok with this) and sooty at WOT.

Your thoughts....thanks in advance!

Even if you ignore your back firing, having sooty plugs at WOT would indicate that your main jet is too large.  Get yourself some smaller mains and test her out.  You may need to readjust the needle clip position but your end results will be better.  Always start with main jet selection and then work your way down. . . .  Maybe that backfiring will go away too. 

[tonycb650]

could be the exhaust valve .

[754]

If it is a snapping or crackling on decelleration, it is usually a small leak sucking air in where the pipes meet the head flange.

[MoTo-BunnY]

If it is a snapping or crackling on decelleration, it is usually a small leak sucking air in where the pipes meet the head flange.

+1 on that - I have a very small crack near the #4 header end that I have to have soldered because of this - it barely leaks but cause a bit of a pop on decel. Backfired worse (loud) before I put a bit of aircraft temp gasket goop on it (knew it was only temporary). Was kind of fun watching a group of gangbangers standing on the corner duck once, though, when it went *bang* real loud . . . he he he

Might want to try new exhaust gaskets and re-torque to specs? (assuming you don't have a crack or anything)

[DME]

Too big mainjet will not cause popping on decelleration.
A too lean mixture with closed throttle will.

Try turning your pilot air screws in half a turn and see if the popping on decelleration decreaces. It should.

Cheers and good luck
Dan

[BC750]

DME:

That's the answer I was looking for. I just needed to eliminate the big main jets as the cause for backfire. I did adjust the IMS but either lean or rich settings still resulted in poppipng/backfiring on deceleration. Thanks everyone for your input - very much appreciated

As for the cause of the backfiring/popping (per MoTo-BunnY and 754 responses):

After very close inspection of the #2 and #3 headers/head flange, i noticed some black dry soot around the fins of the flange. It was hardly noticeable unless you were really looking for it. I grabbed /wiggled the corresponding manifold pipes and noticed some play (-/+ 1.5 to 2mm) where the pipe and flange mate . Is there suppose to be some freeplay here? Is this freeplay enough to cause a leak? Curious, I tightened the nuts on the flanges almost 1/4 to 1/2 turns each - i guess they were loose. Right away, the backfire was not as loud but still present and not as often.

Is it safe to assume that I'll need to replace the worn out exhaust gaskets and maybe even the flanges if needed?

[MoTo-BunnY]

DME:

That's the answer I was looking for. I just needed to eliminate the big main jets as the cause for backfire. I did adjust the IMS but either lean or rich settings still resulted in poppipng/backfiring on deceleration. Thanks everyone for your input - very much appreciated

As for the cause of the backfiring/popping (per MoTo-BunnY and 754 responses):

After very close inspection of the #2 and #3 headers/head flange, i noticed some black dry soot around the fins of the flange. It was hardly noticeable unless you were really looking for it. I grabbed /wiggled the corresponding manifold pipes and noticed some play (-/+ 1.5 to 2mm) where the pipe and flange mate . Is there suppose to be some freeplay here? Is this freeplay enough to cause a leak? Curious, I tightened the nuts on the flanges almost 1/4 to 1/2 turns each - i guess they were loose. Right away, the backfire was not as loud but still present and not as often.

Is it safe to assume that I'll need to replace the worn out exhaust gaskets and maybe even the flanges if needed?

If you are seeing the soot like that then yeah, there's a leak - dunno if you need new gaskets as the Honda OEM ones are hollow copper and I don't see how they could 'wear out', BUT they only crush down once, so you will have to replace them if you remove the exhaust. There is NO free-play, on my setup - I don't think there should be much, if any?

Also, Be careful about over-torquing, in any case, as the official Honda manual says only 5.7-8.6 lbs-ft / .8-1.2 Kg-m which is pretty low. (specs are for the CB500/550 though - they may be different for the CB750)

[754]

Headers only go over the outer of the spigot, stock goes in and out & is very hard to leak.

Headers however are hard to get on if too tight, so they tend to be loose, then they require a slit or two so they can close up(this is often where the leak is)

Also the header material is often thinner than stock material, so clamps dont pull tight as easy, often you have to hacksaw out a slice to allow it to close up more (zip disc will work too).

 Leave the spigots alone just check that screws are tight, also dont worry about 500 torque spec , they are studs, 750s have a collar that is a different deal.

[jeanhank]

I'd like to second the opinion that it's a lean condition at closed throttle and to turn in your air screws a half turn or so...  this however accounts for "popping"... "backfiring" as i understand it is like a sneeze out of your carbs, so if it is in fact that, you might also want to check your ignition timing, because timing issues can cause backfiring especially when the motor is cold.

[Tower]

I'd deal with the leaking exhaust manifold first, before resizing and tuning the jets.

Test your carb setting this way:
Replace plugs and do a plug chop after a 2 mile high rpm run.  Report the plug colour - they should be either whitish, tan coloured or blackish-sooty.
 
Then do the standard main jet test and report when in the sequence the bike stumbles, i.e. stumbles then accelerates, accelerates then stumbles, or smoothly accelerates with no stumble:
Main Jet test  - Blip throttle at high (6000+) rpm

That is all the info you'll need to analyze whether or not your main jets need resizing.

Once you figure that out, move to the next test: Mid (3500-4500) rpm - Needle jet.

[fmctm1sw]

I'd like to second the opinion that it's a lean condition at closed throttle and to turn in your air screws a half turn or so... 

I will agree with that.  Mine popped upon deceleration until I properly set my air mixture screws (which were only set the way they were because I was too lazy to determine why my plugs kept carbon fouling in the first place)... 

[BC750]

Removed the exhaust header today and discovered the PO didn't remove the old copper gaskets (all 4!) He basically just sandwiched two gaskets for each pipe. I wouldn't have known as well if it wasn't pointed out by this forum that those gaskets are almost impossible to see. See pictures attached.

As well, since these are aftermarket pipes, the union fitting to the #1 and #2 pipes are loose or seperated as the tack/weld went loose. It's obvious from the pictures that each pipe is leaking. I just dropped off my header to a local muffler shop that will be rewelding/tacking each pipe (flange?) and squaring each pipe end so they'll seal properly.

You can see the brand new copper gasket as well. I think we solved the popping/backfiring issue?

To be continued....

[MoTo-BunnY]

kewl!  glad you got it figured out    

I have to get motivated and fix my very small exhaust leak, soon, its annoying me - it just does a real light pop sometimes, if you let off of the throttle quick.    

[BC750]

NO MORE BACKFIRING, POPPING, CRACKLING - PROBLEM SOLVED!

Thanks everyone for your input. The exhaust leaks were due to sandwiched, worn gaskets and weak weld seams from the header adapters. The muffler shop suggested not rewelding or tacking as it would make the pipe weaker. However, they suggested to use PYRO-PUTTY - a high heat ceramic paste to fill the seams and weak joints. More info about the product here http://www.aremco.com/PDFs/A3.pdf

This stuff works well (better than cold weld) I smeared the paste inside and outside the pipe seams/fittings. Waited for it to cure, smoothed it out with some sandpaper, installed new gaskets and voila! Healthy sounding exhaust!

Thanks everyone for your input!!

[MoTo-BunnY]

NO MORE BACKFIRING, POPPING, CRACKLING - PROBLEM SOLVED!

Thanks everyone for your input. The exhaust leaks were due to sandwiched, worn gaskets and weak weld seams from the header adapters. The muffler shop suggested not rewelding or tacking as it would make the pipe weaker. However, they suggested to use PYRO-PUTTY - a high heat ceramic paste to fill the seams and weak joints. More info about the product here http://www.aremco.com/PDFs/A3.pdf

This stuff works well (better than cold weld) I smeared the paste inside and outside the pipe seams/fittings. Waited for it to cure, smoothed it out with some sandpaper, installed new gaskets and voila! Healthy sounding exhaust!

Thanks everyone for your input!!

That's great! This forums sooooo R-O-C-K-S!   



So what type of the Aremco pyro-putty (their product number) did you use? They have many different variants - maybe the #552? (you got me thinking that this is the route I should go, too)


Also - anyone know what temperature a repair would have to stand, if it was right near the exhaust studs? (my exhaust crack is on the trumpet like flare, on the very end, that presses against the gasket - on a CB500)

[BC750]

So what type of the Aremco pyro-putty (their product number) did you use? They have many different variants - maybe the #552? (you got me thinking that this is the route I should go, too)

I used the #2400 (what the shop had laying around). They had a quart container and let me have at her in the shop. The muffler guy told me it would be around 900 degrees F near that exhaust flare but said the pyro putty is good to 2000 degree F. So more than adequate.

I rode last night and everything looks solid. Some techs I've talk to say the putty is temporary and usually breaks down but I think they might have been referring to cold weld. In any case, I'll report the status in a couple weeks.