Topic: Backfiring / Timing / Carb issue CB750 - Help!

[harisuluv]

Are you using the same plugs?  What are you doing to the plugs between each chop?

[harisuluv]

Lack of pictures are pretty much making you hit a wall. 

2-3 pictures of just your bike would be something to point to ruling out basic stuff.  All I see is fuel level and some plug chops.  I mean I know you're frustrated, but I don't have anything to go on.  For all I know you could be using the choke backwards.

[andyvclifford]

Each Chop was a new set of plugs, each gapped the same before i put them in.  I am not using the choke backwards.  When i get the time to pull the carbs off again ill take some more pictures

[scottly]

shot in the dark here but take the gas cap off and run it, if the cap got rusted its not venting enough to get the proper fuel (vaper locking)

1+ Or just loosen the cap to reduce the fire hazard.

[andyvclifford]

Tried the cap - to no avail .  Tearing down carbs and airbox again tonight (friday night yay).  I will take pictures of everything and post.  Tomorrow new Carb Boots and air filter boots should be arriving along with new clamps.  Ill do another clear tube test to validate the fuel levels and post up documentation.  Anything else that people would like to see to help diagnose please let me know!

Thanks

[andyvclifford]

New Carb Boots, Airbox Boots, and Clamps just arrived!  So as promised I am posting a large ammount of photos of the carbs assembled, disassembled, and some pictures of the bike.  If there is anything missing please note and i will try to provide.  I have included the link for the photobucket album below, everything should be labeled clearly, please ask if not!

http://s1339.photobucket.com/user/andyvclifford/embed/slideshow/Carburetors  Please Click on "view album" and then click on the images to scroll through for titles, for some reason photo bucket wont show the titles in the slideshow view

[andyvclifford]

Also have listed them below in this post-

Carb Engine Side (1 - 2 - 3 - 4)



Carb Rack Assembled Air Side Choke Open (1 - 2 - 3 - 4)



Carb Rack Assembled Air Side Choke Closed (1 - 2 - 3 - 4)




Carb Rack Air Side Top (1 - 2 - 3 - 4)




Old Carb Boots Carb Side (4 - 3 - 2 - 1)



Old Carb Boots Engine Side (4 - 3 - 2 - 1)



Air Box Front Top



Air Box Front Top #2



Air Filter



Air Box Inside Bottom



Old Air Boots Bottom (4 - 3 - 2 - 1)



Old Air Boots Top ( 4 - 3 - 2 - 1)





Float Bowls 4 - 3 - 2 - 1



Fuel Drain Screws



Carb Jets Installed (1st picture)



Carb Jets Installed (2nd picture)



Carb Jets / Floats installed 1 - 2



Carb Jets / Floats installed 3 - 4



Carb Jets Removed Top down 1 - 2



Carb Jets Removed Top Down 3-4



Carbs Dissassembly Full View (Carb Order 4 - 3 - 2  -1 )



Carb Manifold, Fuel Bowl Dissembly



Carb Bodies, Air Box, and New Boots (Order 4 - 3 - 2 - 1)


Carburetor Slides Full (4 - 3 - 2 - 1)



Carb Slide Assembly Up Close Side


Carb Slide Assembly Bottom


Carb Slide Assembly Up Close (Needle in Middle Position)


Floats, Needles, Dowels, Jets, Air Screws (4 - 3 - 2 - 1)


Float Dowel And Needle Up Close


Main Jet Up close (#110)


Main Jets (#110) Hole Comparison (Angle Blocking some light in Right Jet)


Pilot Jet #40 Up Close


PIlot Jet #40 Aperture


Air Screw Up close


Carb Up Close Top #4


Carb Up Close Top #3


Carb Up Close Top #2


Carb Up Close Top #1



Carb Up Close Air Side #4


Carb Up Close Air Side #3



Carb Up Close Air Side #2


Carb Up Close Air Side #1


Ignition Coil 1-4



Ignition Coil 2 - 3



Cylinder Intake Ports (1 - 2 - 3 - 4)



Intake Port #4


Intake Port #3


Intake Port #2


Intake #1

[harisuluv]

Looks like you have an aftermarket carb kit installed at some time.  What is OEM and what is not?

Also this looks buggered.

[andyvclifford]

As Far as I can tell the PO (bought the bike in October of 2015) replaced the Needles, the Main Jets, Pilot Jets, Fuel Needles, The Fuel needle holders and the fuel bowl gasket along with the gasket at the top of the carb. 

The Air screws are stock and it looks like 3 of the 4 emulsion tubes are stock. 

When you say it looks messed up are your referring to the needle jet specifically?  Should i remove it and take some photos of it?

thanks!

[harisuluv]

Well often times carbs can be "fixed" by replacing aftermarket parts with OEM brass.  Specifically the jets.

The part I circled does not look round to me, but it's supposed to be round.  You don't have to take it out, you just have to inspect it.  It's out of focus so I can't say for sure.

[andyvclifford]

Great, I will take a look at that needle jet again before i try re-installing on the bike.  It might just be the angle because i seem to remember them being all around and OK.  Where is the best place to buy OEM jets?

[flybox1]

Jetsrus.com

[andyvclifford]

Ok Good news is I fished through the box random parts that the PO gave me and found in an unlabelled little box *most* of the original carb parts.  As expected everything is gummed up with old gas so im cleaning the pilots out now and soaking the jets.  Ill replace everything I can that looks to be aftermarket with these once they are clean along with the new carb boots, air boots, and clamps and try again.

thanks!

[flybox1]

all OEM main and pilot jets should be stamped with the Keihin K

[andyvclifford]

These most definitely are marked with a "K" the only weird thing is that the main jets are two different sizes - I have two keihein 110 and two keihein 125.  The aftermarket Main jets are all 110.

Is there any reason or history of a CB750 coming from factory with 2 different size main jets ?  I am going to stick with the aftermarket 110s.

[flybox1]

No.  All main jets should be the same size, and all pilots should be the same size.
Stick with the Keihin 110's.  You'll have to order two more.

[andyvclifford]

I have 6 110 jets - so i should be ok however i will order 4 keihein 110s to replace the aftermarket ones.

[harisuluv]

Did you find the original needles?

[andyvclifford]

Yes!  All 4 needles they have a "017" stamped on them?

[harisuluv]

What you pictured is an aftermarket part.  Looks like they didn't install the needles from the kit.  D17 is a keyster or derivative part.  You should check the needles currently installed for the # on them and also to verify the are in the right spot.

[andyvclifford]

Good to know. The clips are in the middle position on the needles as currently installed. What would the needles be stamped with if stock ?

[scottly]

I am trying To wrack my brain about what has changed from when it ran fine and the only new thing is the petcock. Is it possible this could be part of the problem ?  It is a brand new petcock but not the OEM one.

If that's when the problem started, it's certainly worth a look!

[jodangler]

it looks like we have the same carbs and jet sizes and I have a cb750. Anyways when I was rebuilding my bike I had the same issues. After gapping my valves, adjusting my chain, bench sync carbs, gapping points and timing the f mark. I was getting the same popping and stumbling, bike was running lean.

A few things That I had to go through was that my exhaust headers weren't sealed right, exhaust fumes were coming out after it would warm up, had to retorque. A couple of rubber boots had a hairline crack, bought new boots. Carb bench sync were wrong and had to redo and lower the slides to the point of almost closed. I had diachii points, needed to get Tec points and new condensers. I also checked my spark plug caps and wires and trimmed the ends to make sure my plug cap had fresh copper/contact and that caps weren't cracked because those thing get really hot and I heard they can cause funky issues. I feel like I'm missing one more step but can't remember.

I read ur thread and the only thing that stuck out was that you never mention if you gapped ur valves and in ur picture the carbs slide seemed a little higher than what I got but I can't really tell, for all I know they could b all the way down. One thing I do know u r running lean. My plugs are light chocolate brown, the colors change during spring and fall, I think they change the recipe in the gas up here when winter moves in..
Good luck

[andyvclifford]

Thanks!  So I will go over the stuff I have done so far -

Brand New OEM Honda Carb Boots, Clamps, and Airbox Boots
Brand new Pamco Electronic Ignition
Cut back Spark plug wires and reconnected and tested all Caps they are roughly 5k Ohms give or take some (NGK caps)
I have re bench synced but I will do another vacuum sync tomorrow
Replaced the slow jets with the original jets that came with the bike #40s.
Fiddled around with air screw settings over and over with many idle plug chops and just cant get it right.
Tested coils and they both test for 5 ohms. 
Timing is dead on at the F mark but the advance seems to not fully advance anymore it sits just a little before the first of the | |  marks. 

Popping at Idle still persists and on decel - ONLY WHEN BIKE IS WARMED UP and has been ridden for about 10-15 minutes.  The only things I see i could check again would be the carb sync and the exhaust headers - by torque them down you mean where they connect to the engine correct?  The problem seems to really kick in once the bike is hot.  WHen i pull the plugs after riding for about 15 minutes 2-3 are usually charcoal black and 1-2 are kind of just brown ish/ whitish

[andyvclifford]

Also - Gapped my valves they are all set to the standard gaps