Hellbilly, question about idle fixing backfiring

[Lucky 1]

Now we have more info. Not there yet though.

1981
CB750K  Vacuum operated carbs.
Stock air box and filter one year old.
4 into one exhaust with muffler.
idle jet?
Primary jet #68 (wondering if primary is the idle jet?) some confusion here.
Main jet 105 (stock is 102)
needles (meaning slide needles NOT float needles)
Mixture screw position?
Accelerator pump and nozzles working.
Pollution control system could have an air leak, QUOTE "rotten valves"????

Still some gaps and some confusion but we may be closing in on it.
BTW fuel lines do not split from humidity changes.

[newtothis]

iave these same carbs, so i wonder, is there a huge difference between the k and f models?

[Dirty]

I know the 78k and 78f carbs are the same.. Not sure if jetting was the same for both from the factory.

[Lucky 1]

Possible air leak in the pinched hose?
With photos I have discovered problems with
my own bike I did not see with my eyes.

[seestheday]

Wow, good catch. The fuel hose is a new replacement. I must have torn it putting it on, fixed that a while ago, but I do have some weird air/fuel related issues that I'm tracking down. That other hose may be a culprit. I might be putting in all new hoses this winter along with replacing gaskets in the engine...

Thanks Lucky!

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "Lucky 1 [via Honda CB750'S]" <[hidden email]>

Date: Sun, 2 Dec 2012 15:14:52 -0800 (PST)

To: seestheday<[hidden email]>

Subject: Re: Hellbilly, question about idle fixing backfiring

Possible air leak in the pinched hose?

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NAML

[seestheday]

I checked these out again last night and the pic makes the hose on the right look worse than it is. That said, its 31 year old rubber, so I'm sure it could be all replaced.

Anyone have any tips on replacing all the hoses in a Dohc?

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "Lucky 1 [via Honda CB750'S]" <[hidden email]>

Date: Sun, 2 Dec 2012 15:14:52 -0800 (PST)

To: seestheday<[hidden email]>

Subject: Re: Hellbilly, question about idle fixing backfiring

Possible air leak in the pinched hose?

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NAML

[cbNoob]

So did you ever figure out your problem? Because mine does the samne thing

[seestheday]

It's been quite a while since I fixed this and I have rebuilt the carbs and top end since then, but upping the primary jet to a 75 fixed my popping issue at the time.

[bodeane]

Hi, I am new here I came across this post because I am having the exact problem...
I recently just cleaned/Synced the carbs and the bike stated off by lunging around 5K then started working great after about 15 minutes of riding.
THEN. started doing the backfire thing on acceleration and loosing power all the way to the point of not even idling.
I have a few suspect causes maybe if I list what I am thinking someone can point me in the right direction?

1. Air cut off valves. I didnt know about these and then after soaking my carb body in carb cleaner I finally found the port on #1 I took out the rubber and it was not bad but looked like it might have got a bit warped by cleaner. I also did not realize there are 4 of them and the other ones I never bothered to open.
Possibly something to do with these being damaged?

2. Airbox bottom hose broke off.
Tthere is a drain hose on the bottom of the airbox that broke off in the installation and that hole is just exposed now not blocked off. possible cause?

The weird thing is when the bike is working it works amazing then will just totally crap out.

Bike stats:
1981 CB900f
kerker 4-1 exhaust with baffle
main jet 105, secondary 68
OEM airbox

any suggestions welcome.

[seestheday]

So it surges to 5k immediately. Does it ever go down to about 1K?

First things I would rule out are throttle cable being adjusted properly and confirming I didn't have vacuum leaks.

[bodeane]

I looked for Vacuum leaks today by spraying contact cleaner around the carbs, manifold and airbox.
Nothing happened.
I think the cable is fine it definitely seems more like something in the fuel system or something.
I also seems to be having trouble idling the air fuel screws are out 2.5 and i have tried 2 and up to 3.5
Is it even possible for my air cut off valves to cause weirdness like this if they are damaged?

[bodeane]

also it doesnt surge TO 5K it will go to 5 K fine then will start to sort of putter and lunge and loose power.
It seems like it just bogs out at 5k.
Any Ideas?

[TOOLS1]

Does the bike still have the stock airbox, or has it been converted to pods?
TOOLS

[bodeane]

Yes it still has the stock air box, although the hose on the bottom that drains into the black box below the battery is not hooked up it broke off.
Here is a pic of that hose, not sure what it does. http://imgur.com/oq7GHFj
Also I just took a video of it running and stalling out when throttle is applied in a relativity fast matter will upload and send link.

[bodeane]

Here is the video, Im starting to think it might just need the proper idle adjustments and nothing more. i have tried with the air fuel screws out to 4 and in to .5 I think I am missing something in the adjustment process.
http://youtu.be/Tkom4_Q-Ktw