Hellbilly, question about idle fixing backfiring

[seestheday]

I thought it would be best to start a new thread instead of hijacking the other thread.

I tried upping the idle to just under 2000 and it cured my backfiring when accelerating to 6K rpm, and the decelerating backfires.

Now my only question is why did this work?  I'm also guessing that there is an underlying problem that I also need to address because an idle set at 2K is pretty far from factory spec.

[shinyribs]

I know this is for Hellbilly,but what mufflers are you running? If your exhaust is way open it will spit and pop thru the pipes due to lack of back pressure.                     Sorry for jumping in,just putting it out there.

Hellbilly,your public awaits.....

[Lucky 1]

You need to give us the whole story all at once. Not in pieces.

Year of motorcycle.
Size of motorcycle.
Type of intake system?
Type of exhaust system?
idle jet?
Main jet?
needles ,position,shims????
Mixture screw position?
If it has accelerator pump and nozzles are they working???

THEN we could help

[seestheday]

Here is all the info.

1981 cb750K
DOHC carbs (recently cleaned thoroughly including gently putting e string through all holes)
stock airbox
auto-petcock removed
I believe the main jet was up one size, everything else was stock.  This was done by the PO.
its a dohc so floats can't be adjusted and they seal great.
Mixture screw position is at 3 turns out.  This is where it idle'd highest when i adjusted them.
I have an aftermarket exhaust.  It's a cheap 4 into 1 slip-on.  This was put on by the PO.
Accelerator pump works great

Air cut off valves are not blocked and there is a possibility they have rotted

Compression is 135, 125, 137, 140 (cold).

[Hellbilly429]

I gotta say I never expected anyone to ask me a question, lol. Have you done anything recently with the ignition at all? If it is retarded at all it will make it run rough and pop and spit and sputter like crazy, I am not sure if you have been down that route already tho. If you got any water in the ignition it will run like crap too, I washed my bike and got water in there pretty bad and the bike wouldn't fire...

As for the idle screw we just warmed the bike up, as it revved up on its own we turned it down some more, then it would rev again and turned it down until it was totally warmed up and set the idle at 1k rpms.

Have you checked the plugs at all? I have never had anything before that was so finicy about the plugs, and since these bikes didn't come with 4-1 exhaust and mine has one on it too, I am sure that the bike is running lean and will foul the plugs out pretty badly. Once I got everything adjusted on the carbs just fine and replaced the megaphone muffler with a 2" glasspack they stopped fouling out, except for when I ran the battery dead cause my new regulator/rectifier took a crap, these bikes don't run well with low voltage!

Hopefully I have helped out, I guess it would be easier to diagnose if you could post a video up of what it is doing. Good Luck!

[shinyribs]

Sees' , when I first got my bike going it popped and cracked thru the tail pipe real bad. It was also running very lean with the pods,100% open exhaust and factory jetting. Once I got it jetted correctly it was much better,but it would still pop and crack when decelerating. If i gave it just a tad of throttle it would stop completely. I imagine it would be the same as you raising your idle speed. Now that I do have a muffler of sorts it is all be gone now,but if I am going pretty fast,say 5000 rpm+ and let off it will still pop and crack.Again,just a touch of throttle and it smooths out. I know mine is back pressure related. Just something to consider.

But you say yours does it under acceleration,too? That would not be back pressure related,as far as I know,since you would be filling the exhaust system with hot,freshly spent gasses,it should be good and full.

Not to nit-pick,but when you say "backfire",is it an actual back fire *BANG!* or just popples and crackles?

[Lucky 1]

What size is the idle jet?
What size is the main jet?


1981 cb750K.
DOHC carbs. (vacuum operated slide)
stock airbox.

"I believe the main jet was up one size"?????? (still unknown, although the carb was cleaned)???
 
Mixture screw position is at 3 turns out.  
Exhaust 4 into 1 slip-on (muffler).  
Accelerator pump works great. Do the nozzles work???

I just keep digging for actual facts.-LUCKY

[seestheday]

I haven't done anything to the ignition lately.

Changes I made since last year are removing the auto-petcock, cleaning the carbs, syncing the carbs, dialing in the air/fuel mix and getting the idle down to 1K (it was previously running around 1.6K, so I suspect that this problem was there before - I have also heard that an auto-petcock will let you run leaner than you otherwise would be able to).

The backfiring was actually probably more of a popping when it rev'd to 6K.  It is a series of very rapid bangs/pops.  Is a backfire usually a single bang?

I didn't write down the jet sizes that I saw when I cleaned them all, but I remember noting that the main jet was 1 size up, and the same size as the cb900 jet.  All other jets were stock.  I'll have to look up what the exact sizes are tonight in the FSM and my Haynes manual.

Lucky - accelerator pump sprayed gas nicely through the accelerator pump nozzle when I had them on the bench and tested it by filling it with gas and quickly turning the throttle.

Here are the links to the threads where I discussed cleaning my carbs:
http://honda-cb750-s.456789.n3.nabble.com/DOHC-carb-cleaning-procedure-td867809.html
http://honda-cb750-s.456789.n3.nabble.com/Carb-cleaning-questions-td3485857.html

[Re-run]

I dont know about the dohc but on the sohc, when you tune the carbs, you get the idle to its highest and then back it off a few rpm, about 50 or so.

[seestheday]

By backing off, do you mean turning the air/fuel screws in or out?

[Hellbilly429]

I meant the one idle screw, not fuel air screws, I turned them in to drop the rpms. Have u pulled the plugs to make sure its not just running rich? It sounds like that's what the problem is as mine was doing about the same thing. Pull the plugs and make sure they are clean first, then mess with the idle screw

Sent from my HTC Inspire™ 4G on AT&T

----- Reply message -----
From: "seestheday [via Honda CB750&apos;S]" <[hidden email]>
To: "Hellbilly429" <[hidden email]>
Subject: Hellbilly, question about idle fixing backfiring
Date: Thu, May 3, 2012 12:01 pm

By backing off, do you mean turning the air/fuel screws in or out?

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[Re-run]

Backing them out. On the sohc, out is richer and I think yours works the same. So you tune it for max rpm, and then richen the mix just a hair more and the rpms drop about 50 or so. Thats how it works on my 78 anyways. But you have different carbs so it may work slightly different.

[shinyribs]

OK. I understand you complety with what you are hearing.  Whenever I ran into this on a car we would call it breaking up. Lots of things there that don't apply to bikes, but is could be caused by crossed or shorting plug wires. Spark jump between wires. Or a lean condition. But the lean conditions will really only show at sustained wot.. not just on acceleration

[Lucky 1]

This is what know now.


1981
CB750K
Stock air box /Filter??
Slip on muffler
idle jet?
Main jet?
needles ,position,shims????
Mixture screw position?
Accelerator pump and nozzles working.
Pollution control system could have an air leak, QUOTE "rotten valves"????

We will keep digging.
Once we have all of the actual facts a diagnosis is possible.-LUCKY

[Hellbilly429]

Oh whoops, my bad guys, was replying by email and of course never got re-run's message so I thought the backing off thing was aimed towards me. Now that I try to remember it probably is out on the idle screw to drop rpms, slight dislexia there, was trying to envision what my hand was doing when I was reaching under the carbs and looking at the gauges.
When mine was doing that it was also stumbling pretty badly and missfiring, so I am not totally sure you have the same issue as mine.

[seestheday]

1981
CB750K
Stock air box /Filter is OEM (1 year old)
Slip on muffler - link to pics and discussion trying to figure out what it is: http://honda-cb750-s.456789.n3.nabble.com/Exhaust-what-should-I-look-for-to-identify-td3422798.html
idle jet?
Primary Main jet - 68 (stock)
Secondary Main Jet - 105 (stock is 102)
needles ,position,shims - needles in good condition and working well.  position is 15.5mm per the Haynes manual and do not have an adjustable tang like the SOHC's.  No shims/washers are installed.
Mixture screw position - 3 turns out (this was the fastest idle point)
Accelerator pump and nozzles working.
Pollution control system could have an air leak, QUOTE "rotten valves"????

Regarding the idle jet (a.k.a. slow speed jets) I didn't think they were removable when I cleaned the carbs, but after reading this (http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revE.pdf) again more carefully I see that they are in fact removable for my year of bike (pre 1981 they were pressed in).  I know that I cleaned them, but I believe that I did it while they were in bike.

All this said, I'm going to have to pull the carbs and replace the float bowl gaskets because this morning the gasket on the #1 bowl gasket failed and started pissing fuel out the side when I turned the petcock valve to turn the gas on.  We had a big temperature/humidity swing last night, and I'm guessing it was on its last legs.

I already have a new set of gaskets so I'll be pulling the carbs off soon to put them on.  In addition to cleaning them again and pulling the slow jet to clean it well, I think I'll do the air cut off bypass to eliminate that issue.  If that doesn't fix my popping problem then I'll start trying some new jet sizes.

[cdaiscool]

I'm cleaning my '81 carbs right now. The early years are pressed in as well. Late '81 and beyond were screw.

Mine are pressed in. So yours might be as well.

[seestheday]

oh, good to know

[seestheday]

Sigh, so I feel pretty silly.  It wasn't my float bowl gaskets that were leaking.  It was my fuel hose that split.

I put it on when it was cold out and must have damaged it a bit getting it on.  The huge swing in temperature/humidity must have done it in.

Here's a pic.  Didn't notice it until I lifted the tank to start to pull the carbs off.



I still plan on paying with the jetting and putting the new gaskets in (I have them already), but it's good to know it's not as urgent as I thought.

[seestheday]

Sigh, so I feel pretty silly.  It wasn't my float bowl gaskets that were leaking.  It was my fuel hose that split.

I put it on when it was cold out and must have damaged it a bit getting it on.  The huge swing in temperature/humidity must have done it in.

Here's a pic.  Didn't notice it until I lifted the tank to start to pull the carbs off.



I still plan on paying with the jetting and putting the new gaskets in (I have them already), but it's good to know it's not as urgent as I thought.