Intermittent Ignition cyl 2 - No sign of vacuum leak 1978 CB750 K8

[tdubsy]

Hey Everyone,

Noticed a bit of an issue today.

I was using a colortune system to adjust my mixture and I noticed that cylinder 2 was firing intermittently below 2000 RPM. You are able to see the spark but there is only ignition every few cycles.
There wasn't any obvious signs of a vacuum leak as I sprayed both engine and intake boots with WD-40 and the RPM's didn't even flinch.


I did notice a popping whenever it would ignite and I could see the carb boot expand when this happens (Had to set the pilot screw a touch rich to prevent this)

I have removed the carbs and cleaned them a few times also done some re jetting.

Is there anything I should triple check?


Videos below

Cyl #4 Acting Normal at idle https://www.youtube.com/watch?v=Ek-AYEcIvQo
Cyl #2 Igniting intermittently https://www.youtube.com/watch?v=LTz0KHjlfZo
List of things I have done so far
- Rebuilt Carbs with new floats, gaskets,seals
- New Carb boots
- Installed dyna electronic ignition
- New ignition coils
- New spark plugs and wiring
- static and dynamic timed ignition
- adjusted valve spacing to OEM specs
- Synced carbs
- 115 Main jet (up from stock 105)
- 40 Slow jet (up from stock 30)
- Checked cylinder pressure (within 5 psi of stock value)

Thanks


Thomas

 

[amc49]

...................my mistake.

 

[tdubsy]

I will give it a try today with carb cleaner. brand new ignition system and coils so I'm assuming the spark is good (you can also easily see the spark through the colortune as it performed perfectly on the other cylinders especially #3 with the shared coil.

Thanks


THomas

 

[amc49]

I apologize for wiping the post; I decided I was somewhat confusing there.

Never assume new parts are good, too many of them now from China and not made as well as even 15 years ago. The Chinese are slowly turning the tables on us, I saw it first hand in the parts industry, they now give us bad parts intentionally. One has to recognize that now it is China or us.

Why have you jetted up? There is no need if bike is still original.

 

[tdubsy]

I'm still learning tons so by no means am I even close to an expert so I may have done a few things wrong.

Originally the bike was barely running and had to idle at 2500 RPM. So every time I've tried a new potential fix for its many symptoms its seemed to get better and better.

I believe I saw a few people recommending going up a couple sizes with some of the symptoms I was experiencing. As I had to have the pilot screw turned out a few extra turns and I live close to sea level. So it seemed to make sense. (But its also the internet so it could all be wrong)

With the current jetting I get a nice blue ignition throughout using the colortune and the pilot screws are around 2 turns out and closer to OEM reccomendations.


Thomas

 

[amc49]

The jetting itself is to OEM recommendations. I'd like to see you checking colortune on the mainjet as that would be at 80 mph and faster, and ideally at over 100 mph with normal road gearing. The color blue depending on intensity and hue can mean too lean as in 'lean best horsepower setting' (look it up) which makes the max amount of power but also damages the motor easier.

It is not important where the pilot screws end up at so long as you are at best power point and either way in or out make the idle worse.

Just some thoughts and yours and do as you will there.

 

[tdubsy]

Yeah Ideally I would like to get to that point,

But I need to figure out why cyl #2 isnt igniting consistently below 2000RPM first.

I realized I've checked all valve spacing apart from #2 intake as I cant get it with a socket and a the crescent wrench seemed like it wanted to strip it. (17mm wrench in the mail)

Could this be an intake valve spacing issue? It seems to be the last thing I can think of


Also I double checked all float heights I set all with calipers to 14.5 but on the clear tube test it was about .75cm away from the top of the bowl (the other three carbs were perfect). I re set the float height to about 11mm and It just barely reached the bottom lip of the bowl. I tried a new float needle as well but no luck. Any ideas what could cause this?

 

[dirtdigger]

Have you ever checked compression?

 

[tdubsy]

Yes all cylinders were within 5 psi of spec

 

[amc49]

I saw that earlier but said nothing. The odds of all four being the same and like new are astronomical if the bike has any miles on it. Making the test inconclusive at best. Not impossible but highly unlikely. And if the spec (I for one certainly don't know it) is only 50 psi wrong anyway. The answer is therefore vague and means nothing.

Not trying to be insulting at all but compression testing is one of the things that comes across with misinterpretations more than any other aspect of tuning. The test needs to be run several times on each cylinder just to make sure the numbers are real and not doing so then makes for whopping effort expended on tuning that does not do anything at all because the basic engine was flawed to begin with. The gauges themselves are junk more often than not and many do not read correctly at all. I immediately question one that says all 4 cylinders are exactly the same, it's the same as saying that 4 blonde women are the same. I question the gauge used there instantly, although it is possible but a longshot.

 

[tdubsy]

I saw that earlier but said nothing. The odds of all four being the same and like new are astronomical if the bike has any miles on it. Making the test inconclusive at best. Not impossible but highly unlikely. And if the spec (I for one certainly don't know it) is only 50 psi wrong anyway. The answer is therefore vague and means nothing.

Not trying to be insulting at all but compression testing is one of the things that comes across with misinterpretations more than any other aspect of tuning. The test needs to be run several times on each cylinder just to make sure the numbers are real and not doing so then makes for whopping effort expended on tuning that does not do anything at all because the basic engine was flawed to begin with. The gauges themselves are junk more often than not and many do not read correctly at all. I immediately question one that says all 4 cylinders are exactly the same, it's the same as saying that 4 blonde women are the same. I question the gauge used there instantly, although it is possible but a longshot.

Sorry, wrote things down wrong everything is around 135 psi +/-5 psi

Even if I did have a pump that wasn't accurate if there was a compression issue on just #2 wouldn't it be getting a much different reading than the other 3?

 

[dirtdigger]

If you spray carb clean into just the #2 carb does it respond? What brand coils and wires and spark plugs are you using. So does it actually not run well or just has the problem with your colortune thing?

 

[tdubsy]

Yes it will respond,

Nothing wrong with the colortune it just made the issue more obvious

I'm using
Magna dual output coils with whatever wire they come with
Dynatek ignition
NGKD8EA plugs and boots


The color tune works great on all other cylinders it just made it very obvious to me that #2 wasn't ingniting. You could see every couple cycles it would ignite and the carb would spit. If I brought the revs up a bit to about 1800RPM it seemed to ignite with every spark.

#3 has no issues so I doubt it's the coil

With the regular plug I could hear the spitting as well.

It just wasn't obvious to me what was happening before the color tune as the occasional ingnition or stable ignition at higher rpms caused the pipes to hear up so I never noticed a cold exhaust pipe

 

[dirtdigger]

If that cylinder works when you spray carb clean in it then you have the pilot jet or passage still plugged.

 

[tdubsy]

Is there a possibility of a bad pilot jet? I'm using #40 I purchased off cb750supply. I've probably cleaned these 12 times trying to figure out what's going on. I have them on my bench right now and air blows through all jets easily

 

[dirtdigger]

but are the passages in the carb clean? If it responds to carb clean it is not getting fuel....or not enough...it either has a big vacuum leak or no fuel through the pilot jet. Either passage, jet restriction or float level too low.

 

[amc49]

X2 that. You have a whole path past that pilot with at least 2-3 other holes past it too. I've seen both a damaged mixture needle seat and an improperly installed mixture needle o-ring cut to let a piece block off the idle mixture feed knock out a cylinder to where it barely hits like yours.

 

[tdubsy]

Found something interesting today.

I switched the ignition cables between #2 and #3 and now #3 is the one not firing correctly.

these are brand new coils and cables. does it make sense something is up with the cable as the coil still fires one of them fine? There is spark from both. I will replace what seems to be the bad cable and hopefully that brings me luck

I also went back to #30 Idle jets and stock mixture screw setting just for kicks and re synced the carbs to see what happens and the bike runs terrible. Wants to die on a slow blip of the throttle I have to slowly bring it up to get to high rpms.


I also double checked float heights. Interestingly I had to set #2 to about 12.5mm to get a good result with the clear tube method while the others were closer to 14.5

 

[amc49]

I for one have never seen a 750 that used a #30 pilot, they were always 35, 38, or 40.

http://www.globalarray.net/user/mike_in_phoenix/motorcycle/text/specs_revised.html

There is no 30 listed in there anywhere.

If the new plug wire is screwed in off center it can do that. If they are crap carbon core wires they can too. Check the cable for ohms, no more than 5000 ohms per 12 inches of wire length. And if you have resistance wire and resistor caps mixed that is bad too.

 

[dirtdigger]

If your miss follows the wire then its a wire issue. Fix this first before doing anything else.

Sounds to me if you have one that needs a different float level then that float isnt right with the other 3

35 is the stock pilot size for the keyhole carb, 40 with the roundtop With the keyhole carb the accel pump usually makes it pretty easy to get off throttle to work well. Even on my big motors I only went up to 40 with the keyhole. Though I dont run stock carbs much anymore on any of my motors.