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

[flybox1]

methinks its more a fuel supply/delivery issue.
your #40 pilot jets should be providing enough fuel for the plugs to color 

[andyvclifford]

Maybe before I waste yet another set of brand new plugs I should pull the carbs and just triple check the pilots ?

[flybox1]

if they are white, you can still use them for idle plug chops 
also, please, verify fuel level with the clear tube test.

[flybox1]

if they are white, you can still use them for idle plug chops 
also, please, verify fuel level with the clear tube test.

He did that on Reply #28, Fly. Look back several days.

Days?!? I can barely remember yesterday! 
smh

[andyvclifford]

Fuel Levels are solid in the bowls.  *hopefully* i will have time after work to do a last carb clean and plug chop and then from there i really dont know where to go.

[andyvclifford]

Update -

Another full 3000 mile tune up - Oil Change, Oil Filter, Tappet Clearance, Points Adjustment, Cam chain tension, Another carb clean with extra attention the idle circuit / slow jets, made sure clamps were on tight. 

I also thought after reading a lot of responses and looking at more things online to try swapping out the condensers on the points with the original condensers that the guy who sold the bike to me had taken off.  The bike seems to run a lot better when its heated up which leads me to believe the condensers were failing, especially when hot.  There is still a noticable ammount of backfiring coming from the 1-2 cylinder exhaust pipe - unfortunately i dont have the 4 into 4 exhaust so i cant quite figure out which cylinder its coming from.  Any suggestions?

I will do an idle plug chop tomorrow after work, but doing all this took a considerable ammount of time so i have not had a chance to do that yet!

[flybox1]

Keep at it, and enjoy what you are doing.  Itll all be worth it!

[andyvclifford]

If its not a carb or electrical issue on that cylinder what would be the next culprit?

[flybox1]

OK...so I just re-read this thread to get everything organized in my mind.
Do you have a picture of your most recent idle plug chop?  Please post it if you do.

[andyvclifford]

I do not - partially reason why is today on my way to work the bike started to run horribly again after riding all night before just fine.  Loud backfiring on idle and decel and then a couple cylinders stopped firing and i barely made it home.  I also think there might be a pinging sound from the engine which i know is very bad.  Im worried to even do an idle plug chop at this point - should i not even run this bike ?  Im really lost on this, it just seems to get worse the more i try to fix it!

[flybox1]

Another full 3000 mile tune up - Oil Change, Oil Filter, Tappet Clearance, Points Adjustment, Cam chain tension, Another carb clean with extra attention the idle circuit / slow jets, made sure clamps were on tight. 

Something is being missed in this process ^^^

Please take us some pictures of your bike and post them.
Carbs, boots, air filter, ignition plate close up, exhaust.  Exhaust flanges at the head.....etc.

[andyvclifford]

Update:  Replaced failing points with a pamco ignition which did help a significant ammount - can now rule out the points as part of the problem which they definitely were.  However - cylinders 1 + 4 are still running very lean and performance on a whole gets horrible as the bike warms up, a lot of stuttering, blubbering, and almost stalling until i get up higher in RPMs.  Today, after replacing the #1 carb body with a new carb body to see if that was part of the issue i performed idle plug chops to try and get the idle circuit tuned properly.  The results were not great.  I ended up having to have the air screws almost completely closed (rich) to even get a little bit of color on 1 + 4 as you can see below.  I am SO lost right now and at the point where i feel like i should just give up and sell the bike.  I cant seem to bring this thing back to life and dont know what to do.  Any help would be much appreciated.

For purposes of testing I did all plug chops for 7 minutes running idle with the fan on the bike.  Plug order in Picture from Left to Right is 4 - 3 - 2 - 1

Plug Chop Results -

#1 Factory screw settings 1 + 1/8 turn out -  All completely white
#2 1 Turn out - Mostly still white, #2 showing some color
#3 3/4 Turn out - 2 + 3 Coming into a little more color, 1 + 4 still quite lean
#4 1/2 Turn Out - 2 + 3 seem ok,  1 + 4 starting to show some more color but still lean
#5 1/4 Turn out - Finally some color on 1 + 4 but at the risk of almost completely cutting off air to the idle mix - Only pulled 1 + 4 for this run

I dont think having to shut off the air mixture in the idle circuit is probably sign of a healthy system.  Since 1 + 4 are showing quite a bit of trouble is it possible there is something wrong with that Ignition coil?

Thanks
Andy

[DWS]

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) and check the temps on each of the cylinders by using a temp gauge right as the pipes come out of the cylinders see how close the temps our you can  tell if they are firing this way also.
and believe it or not i had a piece of the petcock seal inside the petcock that was restricting the flow took me four days to figure that one out.

[flybox1]

Post pictures of your carbs.  Jets as installed, slides, idle adjuster screw, floats. Intake side, engine side......
All of it.

[andyvclifford]

Im Looking for feedback regarding what i just posted.  Pictures of carbs will have to be another reply

[andyvclifford]

As I said before the carbs have been rebuilt and cleaned 3 times during this thread - every single part of it is stock settings for the year of this bike.  If it comes to taking them off again and dismantling i will provide pictures but I have provided quite a bit of information up to this point - there must be something in there that points to what might be wrong after ruling out many of the basics?

[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!