Author Topic: Backfiring / Timing / Carb issue CB750 - Help! (Read 26380 times)

PeWe · « **Reply #100 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 #101 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 #102 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 #103 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 #104 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 #105 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 #106 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 #107 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 #108 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 #109 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 #110 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 #111 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 #112 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 #113 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 #114 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 #115 on:** July 06, 2016, 08:24:41 AM »

jodangler · « **Reply #116 on:** July 14, 2016, 09:18:39 PM »

if you are sync'ng your carb with gauges you might want to calibrate your gauges to make sure they are all measuring the same. another thing that comes to mind that you could check would be the float plungers with the little filter/screens, maybe the little screens are blocked or the plunger are stuck or wrong size and check to c if u have a clean fuel tank it could be dirty/rusted/kremed-lined. its all a guess.

if your rubber boots are new maybe you dont have them clamped down if not you should really inspect them for fine hair cracks and if they are old maybe you need to re-soften to get a better seal...

g8ts · « **Reply #117 on:** July 26, 2016, 10:07:31 AM »

Update? 6 pages of replies, I need an update of how this progressed?
Thanks

jodangler · « **Reply #118 on:** July 27, 2016, 07:27:29 AM »

I was tuning my carbs yesterday to give my bike a tuneup and noticed that my gas wasn't flowing properly down to my carbs and thought this could be something for you to look at so what I have read in other post is that if your carb vent tubes are clogged you can get a vapor lock symptom which would give you the problems you r experiencing. I don't remember if it was covered already but you could also do a clear tube check to c where your gas sits inside your carbs to make sure you have enough gas to run your bike. I attached some pics for ref... You might also want to check this website for more info it helped me out and it might help you to... http://www.hondachopper.com/garage/carb_info/carb_info.html

PeWe · « **Reply #119 on:** July 27, 2016, 11:52:38 AM »

I had a long lasting problem due to one bad ignition wire. Sometimes it acted as too rich, sometimes as too lean. I tested a lot of carb jetting until the intermittent wire problem became less intermittent and I could not miss it.
The ign wire was new and I could not dream about a faulty wire.
Thats why I earlier recommended to swap the entire ignition set, only as a fault tracing routine.

flybox1 · « **Reply #120 on:** October 11, 2016, 02:54:18 PM »

Quote from: jodangler on July 27, 2016, 07:27:29 AM

I don't remember if it was covered already but you could also do a clear tube check to c where your gas sits inside your carbs to make sure you have enough gas to run your bike. I attached some pics for ref..http://www.hondachopper.com/garage/carb_info/carb_info.html

Waaaay to low in your bowl there jodangler

flybox1 · « **Reply #121 on:** October 11, 2016, 02:59:45 PM »

Quote from: andyvclifford 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?

These air screws need to be set and then left alone.
They are set based on Air/fuel ratio....or plug color, if you dont have access to an O2 sensor.
get them dialed in, and then leave them alone. the only thing to adjust in a vacuum sync are the slide adjusters and their lock nuts.

Please state again what the main symptom or issue you continue to have....
Thanks....

gypsywind · « **Reply #122 on:** October 18, 2016, 04:21:50 PM »

make sure the advanse weights are not sticking or wore out its happened to me before

BestBoy · « **Reply #123 on:** August 26, 2019, 03:46:06 PM »

So... after 6 pages... no solution after 3 years?

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Author Topic: Backfiring / Timing / Carb issue CB750 - Help! (Read 26380 times)