Author Topic: Backfiring / Timing / Carb issue CB750 - Help! (Read 25903 times)

harisuluv · « **Reply #100 on:** June 29, 2016, 06:02:20 PM »

Really looks like the brass I pointed out is damaged..

andyvclifford · « **Reply #101 on:** June 29, 2016, 06:06:38 PM »

Hi Harris! I did reinspect that needle jet before reassembly and it was indeed the same as the other 3 - could not see any visible damage or warping :/

andyvclifford · « **Reply #102 on:** June 30, 2016, 05:19:51 PM »

I assume everyone is as stumped as I am but what would be helpful as a next step? Videos / Pictures of?

thanks!

scottly · « **Reply #103 on:** June 30, 2016, 06:12:07 PM »

Quote from: scottly on June 27, 2016, 06:31:40 PM

Quote from: andyvclifford on May 28, 2016, 07:59:45 PM
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!

Ahem..

andyvclifford · « **Reply #104 on:** June 30, 2016, 06:52:04 PM »

Hey Scottly - Yes ufortunately i went back to my previous petcock with no change. I also tried another petcock i had laying around in a parts box that is a few year older model petcock.

scottly · « **Reply #105 on:** June 30, 2016, 07:29:35 PM »

Sorry if you stated you switched back to the stock petcock and I missed it. Hey, I missed the statement that the problem started with the switched petcock, too.

Still, a basic rule of trouble-shooting is if it was working before a change, and then quit working as soon as the change was made, something got messed up at that time. Since your symptoms occur after the bike has been run for a length of time, it still sounds like a restricted fuel delivery, unable to keep up when the bike is going down the road. What else did you do when you changed the petcock, that you haven't already checked? New fuel lines, routing, filters?

andyvclifford · « **Reply #106 on:** June 30, 2016, 10:07:02 PM »

Hey Scottly! Thats ok this is a long thread and sometimes i do something to the bike and forget to mention it also - im a bit all over the place at times.

So since the problem started I have changed quite a few things - some for the better. Here is a Short list of new parts

-Pamco Ignition
-Carb Boots
-Air Box Boots
-Petcock
-Clamps for Air Boots and Carb Boots
-Keihein Slow jets #40
-Air Filter
-Oil, Oil Filter Bolt, Oil Filter
-Spark Plugs

Here is a short list of the things ive done:
-Sprayed Carb Cleaner to check for Air leaks
-Cleaned Carbs 4 times - soaking and mechanical
-Tested Coils 5 Ohms
-Tested Caps around 5k ohms give or take (NGK)
- Cam Chain Tension
- Clear Tube Tests
- Idle Plug Chops
- Set Electronic Ignition timing - dead on at F mark but advance isnt full finishing it at just before the | | marks
-Gapped Valves
-Compression around 120 across all cylinders
- Just recently tried tightening the Exhaust header clamps
-Switched Petcocks around
-Gas cap on/off Running
-Vacuum Sync - although still have to do this for most recent carb rebuild
-Trying different A/F mixtures with air screws
-Oil Change, Oil Filter Change

As a caveat I am a new mechanic - just one year under my belt - and I coud be doing some of these things not entirely correct. Particularly I may be gapping the valves incorrectly, turning the air screws too much - never knew exactly what "one turn" means in particular.

Bike seems to run best when it has brand new spark plugs and then after about 20 minutes it runs like garbage.

scottly · « **Reply #107 on:** June 30, 2016, 10:14:16 PM »

Run the bike 'till it runs like garbage. Pull the plugs and post pic's of 'em.

andyvclifford · « **Reply #108 on:** June 30, 2016, 10:23:45 PM »

Will do this tomorrow! IM going to buy another set of Plugs hopefully autozone has 4

PeWe · « **Reply #109 on:** July 01, 2016, 03:07:50 AM »

Try a new point plate with fresh points, other colis plus new ign wires with good caps.

andyvclifford · « **Reply #110 on:** July 01, 2016, 08:40:41 AM »

Hey Pewe - just replaced the bad daiichi points the PO put in there with a pamco ignition which cleared up some issues but I can safely say the points are no longer the issue

harisuluv · « **Reply #111 on:** July 01, 2016, 07:42:10 PM »

Quote from: andyvclifford on June 30, 2016, 10:07:02 PM

Hey Scottly! Thats ok this is a long thread and sometimes i do something to the bike and forget to mention it also - im a bit all over the place at times.

So since the problem started I have changed quite a few things - some for the better. Here is a Short list of new parts

-Pamco Ignition
-Carb Boots
-Air Box Boots
-Petcock
-Clamps for Air Boots and Carb Boots
-Keihein Slow jets #40
-Air Filter
-Oil, Oil Filter Bolt, Oil Filter
-Spark Plugs

Here is a short list of the things ive done:
-Sprayed Carb Cleaner to check for Air leaks
-Cleaned Carbs 4 times - soaking and mechanical
-Tested Coils 5 Ohms
-Tested Caps around 5k ohms give or take (NGK)
- Cam Chain Tension
- Clear Tube Tests
- Idle Plug Chops
- Set Electronic Ignition timing - dead on at F mark but advance isnt full finishing it at just before the | | marks
-Gapped Valves
-Compression around 120 across all cylinders
- Just recently tried tightening the Exhaust header clamps
-Switched Petcocks around
-Gas cap on/off Running
-Vacuum Sync - although still have to do this for most recent carb rebuild
-Trying different A/F mixtures with air screws
-Oil Change, Oil Filter Change

As a caveat I am a new mechanic - just one year under my belt - and I coud be doing some of these things not entirely correct. Particularly I may be gapping the valves incorrectly, turning the air screws too much - never knew exactly what "one turn" means in particular.

Bike seems to run best when it has brand new spark plugs and then after about 20 minutes it runs like garbage.

That's great that you know what you have done, but I think the relevant information is what changed immediately before the symptoms.

andyvclifford · « **Reply #112 on:** July 01, 2016, 07:59:25 PM »

Hey harris,

I think it was a slow degredation to this point. The only things I remember changing was the petcock - which I have since changed back. Once it got to a point where I felt like it was getting too bad to ride I started with carb rebuild, ect, ect. The only update I have for today that i had time to do was check again for some leaks on the carbs after theyve been sitting with their new rubber for a week. I noticed that when I started to spray carb cleaner around the T-connector that sends fuel between the carb bodies there was a significant idle drop - meaning most likely those O-rings are shot. Will try to replace those tomorrow when i get spark plugs, does anyone know what size those are ?

harisuluv · « **Reply #113 on:** July 01, 2016, 10:06:55 PM »

That hypothesis doesn't seem to make sense if you consider that air is able to leak in but yet the fuel is somehow not leaking out at the same time. (Unless you're saying you have a fuel leak there).

Sounds like you have some other leak if carb cleaner is affecting your RPMs. I guess if you running lean it would make sense that introducing a richer mixture (carb cleaner) would make the RPMs drop..

andyvclifford · « **Reply #114 on:** July 01, 2016, 11:51:23 PM »

Ok I apologize i wasnt stating that was for sure the case, but I am trying to find anything that can lead me to whats actually wrong.

harisuluv · « **Reply #115 on:** July 02, 2016, 12:12:06 AM »

No need to apologize, I'm just talking through the thought process. I'm not sure what's going on either.

Just be careful cause your reasoning must be sound or else you will just be throwing darts at a board.

scottly · « **Reply #116 on:** July 03, 2016, 09:47:13 PM »

Andy, why did you change the petcock in the first place? Did you suspect a lack of fuel flow? Measure the fuel flow out of the open petcock into a suitable container over a period of time.

andyvclifford · « **Reply #117 on:** July 04, 2016, 06:43:24 AM »

Good Question - I replaced it because it had become increasingly difficult to turn the knob on it until it pretty much just seized in place so i figured - time for a new petcock!

Mitchell · « **Reply #118 on:** July 05, 2016, 04:23:58 PM »

Just throwing this out there but how are your fuel lines routed? They could be getting to hot and vapor locking.

andyvclifford · « **Reply #119 on:** July 05, 2016, 04:34:58 PM »

Hey Mitchel - What is vapor locking? If you go back 1 page you can see how the fuel lines are routed on the carbs off the bike. Imagine that going up to the petcock. I can take a picture of it on the bike later this week.

Also I was wondering if i turn the air screws all the way closed (as rich as possible) - should the bike stumble and stall?

flybox1 · « **Reply #120 on:** July 05, 2016, 05:49:56 PM »

Quote from: andyvclifford on July 05, 2016, 04:34:58 PM

Hey Mitchel - What is vapor locking? as the gas drains into the carbs, it creates a vacuum in the empty space of the tank, preventing additional flow of gas to the carbs. If you go back 1 page you can see how the fuel lines are routed on the carbs off the bike. Imagine that going up to the petcock. I can take a picture of it on the bike later this week.

Also I was wondering if i turn the air screws all the way closed (as rich as possible) - should the bike stumble and stall? Maybe.... gas will still be supplied, at a minimum, based on the size of the pilot jet. If that makes it rich enough, then yes, it will stall

andyvclifford · « **Reply #121 on:** July 05, 2016, 06:13:51 PM »

When i vacuum synced my carbs again this weekend i was also working with the air screws. It seems that when closed them (rich) they didnt stutter or really respond at all. When i started to turn them out past 2 turns the idle actually sped up. Pilot jet #40 Keihein - clean. Does this help indicate a lean circuit?

scottly · « **Reply #122 on:** July 05, 2016, 07:54:22 PM »

Andy, your symptoms are sometimes contradictory; sometimes it acts rich, and sometimes lean. Here's what I would do:
#1. Set the idle mixture screws to the standard setting.
#2. Base your testing on actual riding, not just idle.
#3. Check the flow out of the tank through the petcock; there might be an obstruction inside the tank above the petcock. I haven't seen any mention of the in-tank filter screen.
#4. Your last clear tube float levels look low to me, even the highest one. Your reference line looks to be 2mm low?
#5. Is your charging system operating properly? If not, the battery voltage will drop over time, which will affect the ignition.

andyvclifford · « **Reply #123 on:** July 06, 2016, 08:20:51 AM »

Hey Scottly - thanks for your indepth reply!

I have set them to factory and ridden - the main symptons are described in previous posts - basically with new plugs it rides "OK" for about 10 minutes or so and then starts backfiring, popping, and stuttering on both deceleration and acceleration off of idle until it will barely go, and lurches a lot until its up in the 4k rpm range.

I will take some pictures of the petcocks when i get back home to see!

I will also check the charging system and post results.

Finally on the float bowls - thank you - Where exactly should my line be? I asked before in the thread, if i use my calipers to measure down from the gasket how many mm would it be?

ok! Lots of things to try

flybox1 · « **Reply #124 on:** July 06, 2016, 08:24:41 AM »

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Author Topic: Backfiring / Timing / Carb issue CB750 - Help! (Read 25903 times)