Topic: 1981 cb650c spark, backfire, missfire issue

[levimundt]

hey guys, new to the site as a member but have learned and fixed alot of stuff with all of your help. my issue here is that i cant seem to find an answer about whats goin on with my bike. it will start up and run but number one cylinder backfires like a bast*rd, 2 and 3 cylinders are firing and getting hot, and number 4 cylinder is cold but no backfire. i have swapped coils and it still remains that 1 and 4 arnt firing. i swapped spark ignitors and nothing changed. replaced plugs, wires, and caps today, still no luck. what would be my next option to check? heres a pic of the bike just to show you guys my build progress also.

[levimundt]

bowls are all full and float height was addressed last season, possibly need to be checked again? i figured it was electrical being number 1 and 4 cylinder

[levimundt]

i was gunna ask if maybe its a bad pulsar generator?

[levimundt]

i will give that a try today i suppose.

[levimundt]

is there a possibility that it is out of time? i did just get done putting a gasket kit and rings in it. i followed the clymer for timing specs but there could always be an error on my end

[levimundt]

bump. still having the issue after switch coil leads

[levimundt]

tinkered again with it today. spark on #1 cylinder is very week and pretty good on #4...leading me to believe that 4 has a fuel issue. but number 1 is electrical issue. im stumped

[MiGhost]

The 650 uses electronic ign. The good part is that it is two separate systems. one for 1-4, and the other for 2-3. This will let you swap from one system to the other for trouble shooting.

Starting with the weak spark on #1. This would indicate a problem with the plug, cap, or wire for that cylinder. Swap the plugs between 1, and 4. If the weak spark moves to #4 the problem is the plug. Otherwise check the cap for correct resistance (should be 10K ohms), and make sure that the cap is seated securely on the plug wire with the wire seated securely in the coil. They are attached by little threaded barbs that the wire screws on to. you can trim approx 1/4" of the ends of the wire to allow the barbs a fresh bite in to the wire.

If nothing changes. You can work backwards through the system swapping the coils, ignitors, and pulsers. But the fact that you have spark on 1-4 would indicate that everything is working. This would leave the no fire on #4 to be a fuel issue.

Check the slow jets to make sure they are not plugged up from sitting. Also try to get the bike sitting straight up. Fuel may not be flowing good enough to the #4 carb while on the side stand.

[levimundt]

Alright update. Rechecked spark in number one and it turns out that my first spark tester I used was a piece of sh!t. I have great spark on number which tells me this whole issue I am having is fuel. So I removed arbs yesterday and cleaned the jets best I could with a wire burch wire and carb cleaner/compressed air. Put it back together and number for is no longer backfiring but still not running even close to as hot as the rest. 4 is now firing. But 3 now isn't. Here's the kicker. I have pod filters so I screwed in the air/fuel screws on all four carbs so I didn't run lean. Could those screws cause 1 and 3 to not burn as hot? Or ignite on every stroke?

[iron_worker]

Are you sure in is lean on your carbs? I'm not familiar with the 650 but I know on some models of carbs it's an "air screw" and turning out is lean and turning in is rich ... on others it's the opposite. Other carbs meter the fuel to the pilot circuit and not the air thus out is richer and in is leaner.

IW

[levimundt]

yes i turned them in so it gave it less air. since i have pod filters

[MiGhost]

yes i turned them in so it gave it less air. since i have pod filters

The 81 650 should have the VB44 vacuum slide (CV) carbs. The idle mixture screw controls both fuel, and air to the idle/slow circuit. Turning the screw in makes the bike run leaner by reducing the amount of air/fuel mixture delivered. Correct mixture screw setting should be 1 1/2 to 1 3/4 turns.

Did you pull out the pressed in sow jet while you were cleaning to make sure that they were not plugged?

[levimundt]

should ech carb be at the same exact setting in regards to the air/fuel screw? and no i used a torch head cleaner. how do you pull them out? also are the cv carbs supposed to have the rubber plugs that cover the idle jet?

[grasscutter]

yes i turned them in so it gave it less air. since i have pod filters

Pods are a huge issue for tuning.
Multiply that by 100 on a CB650.  CB650's are extremely lean!!  Add pods, and its literally off the charts.

You still have a fuel issue. 
Clean carbs again. 
Soak them overnight. 
Do not poke wires through any jets.
Go slow. 
Take your time.

And find a way to get the stock air box back on.  Unless you plan on leaning how to shim needles, and know differences in needle jets.
CB650 carbs especially the CV's need the vacuum created by the airbox.

(adding larger main jets rarely, and I mean rarely does anything, when people say they 're-jetted' for pods.  Its the low to mid-range that needs attention with pods.  CB650 carbs have very little adjustment possible in that range, unless you learn about tuning jet needles and needle jets, which I've already stated).

Either way, your carbs are still dirty.

-Also: each carb is not necessarily the 'exact' same screw setting.  You do set them the same when bench syncing. 
Then when using a manometer or your any vacuum carb sync gauge, you adjust each one per individual cylinder vacuum.

-And:  There should not be any rubber plugs in in or around the pilot or main's in the CB650 carbs.  Assuming you have the original factory set, and they have not been changed.

There are numerous o-rings and gaskets in that need replaced.  Vacuum issues will plague you if you think that a simple 'blowing them out' will do the trick.  Honda part # is 16010-460-305 for carb kits per carb.  My cost where I'm at is about $29 per carb.

Check out # 1 in the diagram. Vacuum balance tubes and fuel balance tubes. This is a common place people skimp when rebuilding CB650 carbs. 

[levimundt]

putting the stock airbox on is next to impossible with the frame mods that i did. you can see in the picture i attached to my first post. is there any way i could keep pods but restrict the air going in them? or maybe adjust the carbs so that my choke is partially on cutting of some air thats coming in? ill pop the carbs back off and soak them while i paint my tank. im told that using pine sol is the way to go because it doesnt ruin rubber. currently the bike runs, but the plugs are WHITE. like you said its extremely lean, what should i do with the air fuel screws to compensate a little bit?
 

[grasscutter]

Sorry.  What you are asking for is a common issue when doing frame mods and pods. 
Most folks who do this, are going for a 'look', rather that having a bike that runs well.
Which is exactly what you now have.  Cart before the horse.

Pods don't make it lean just because of more air flow, the whole operation of the CV carbs absolutely has to have the vacuum the airbox provides, to have clean throttle response from idle all the way through top end.
You can't 'adjust' it out with air screw adjustments. 
You have to do what has been mentioned:  Learn jet needle and needle jet tuning.  Then and only then can you start to fix the lean condition you currently have created.
There are no shortcuts.  Sorry.

-Pine sol?  Don't over think this stuff.  People recommend use of other cleaners so they don't have to purchase new o-rings, gaskets, etc.  Which is just another shortcut that causes frustration.

-Keep running white plugs and you'll cook that engine.

-Are you soaking the carbs as one whole unit??  Gulp. 
Soaking them, means: dis-assembly first, then soaking them in a 1-gallon type can of carb dip. 
Then blowing out with carb cleaners, and compressed air.
Then re-assembling with correct new gaskets, o-rings, etc.

Doing anything less than this will be met with different levels of 'success'.

**If you could fabricate an airbox to fit in the space you have... That would be the best place to start with the lean condition.
***Carbs cleaned and rebuilt first of course.

[levimundt]

ive ridin the bike once because i just finished it. came back from my 3 mile ride and plugs were white, im not planning on riding it till its fixed. why dont i just chsnge the main jets so while im cruising at 55mph its not lean?

[iron_worker]

The main jet is only one part of the equation. At cruising speed you're probably partially on the needle jet circuit as well. Pilot jet size will also need to be tuned to get it to idle properly and come off the line like it should.

IW

[levimundt]

can i drill out the idle jets to the correct size?

[grasscutter]

Obviously the answers we've posted are not what you want to hear.
What else can we do for you?

(and to answer the question... NO, you do not drill out jets.  Jets are cheap, just go buy the next size up.)

And yes, I actually do think you should take the time to change the main jets. 
It's clear you are a man who learns from experience. 
Put some different mains in, and see how it runs.

We've already told you what you need to do, but what the heck, give some bigger mains a try.
Maybe start @ 130.  120 is stock, and you need lots of fuel, so let's go BIG, right? 
Go big or go home!!!
Report back on how your 'off the line' throttle response is.

[levimundt]

how do you figure its not what i want to hear? im coming up with other alternatives because you cant just buy the next size up for this style carb, you must know that since you seem to think your a cb650 god. the jet is pressed in, making it non removable. ive seen plenty of these bikes with pod filters, your trying to tell me i need to be an carb guru to figure it out. so if you want to be a smartass about what i should or should not do then dont bother posting on my thread. if you believe there is no other option then running the stock airbox then your wrong, beacause plenty of guys do this. 

[grasscutter]

Fair enough.  I'll step aside and let others take over your carb tuning pod issue.
Although maybe this forum would be better suited to your style. ---> (www.dotheton.com)

(and NO, I've never seen any CB650 main jets that are pressed in).

[MiGhost]

The only jet that is pressed in is the slow jet. It is absolutely necessary to pull these to get them cleaned properly. Running a tip cleaner through the jet will only make sure the main opening is clear. There are several tiny holes on the sides that allow the air to mix with the fuel to create the correct air/fuel mixture.

To pull the slow jet you will need to fashion a puller from a screw/bolt, nut, and piece of tubing. Tap threads into the jet to thread the bolt/screw into it through the tubing. Using the nut on the screw/bolt will pull the jet up to the tubing.

If you soaked the rack complete. It is possible that the chemicals could have damaged the rubber pieces (o-rings, diaphrams). You will need to check every piece, and replace what has been damaged.

In order to make the carbs with pods. It is necessary to recreate the action of the airbox boots. The airbox boots are used to direct the airflow into the carb throat for proper operation of the air jets.

Understanding how pods affect the airflow through the carbs will go a long way toward being able to set them up.

[levimundt]

grasscutter, if you actually read my previous posts you would know that the pilot is my main concern at the moment and it IS pressed in.

Now that was worth my time to read Ghost, i truly appreciate the response. now with my bike the way its built now, i can still fit the piece of the stock airbox that connects to the carbs. do you think it would run alright with stock jets if i made my own filter assembly to fit that portion of the stock airbox?

[MiGhost]

now with my bike the way its built now, i can still fit the piece of the stock airbox that connects to the carbs. do you think it would run alright with stock jets if i made my own filter assembly to fit that portion of the stock airbox?

No. You will still need to do the re-jetting work, but the airbox boots will eliminate most of the problems associated with running pods. The airbox boots should replace the rubber adapter on the pod base.