Topic: Cylinder not getting hot

[Mark K]

1973 Honda CB750, stock engine rebuilt, bored .50 over, stock airbox w/oiled K&N air filter.

Trying to figure out why cylinder #3 exhaust pipe not getting hot. Cylinders 1, 2, & 4 exhaust pipes very hot, #3 has a little warmth, I can touch it and leave my fingers on it, not warm enough to burn.

Initially had #3 carb float issue (fuel running out of overflow tube and air intake side of carb), fixed that problem by reinstalling original float.

I've uploaded a video on you tube, for anyone who wants to give their educated opinion. Cannot really see it in the video, but engine is burning a little oil, but it's not constant, almost like it's just coming from one cylinder??

Thanks in advance to everyone who can help!

Link to video -   

[Don R]

 A fouled plug on that cylinder? 

[RAFster122s]

Bad spark plug cap can lead to fouled plugs…

[newday777]

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

[seanbarney41]

most often this situation turns out to be plugged pilot jet...if you got a good shorty flathead screwdriver, just unclip the floatbowl unscrew the jet, hold it up to the light to see if it is clear.  I would flush it out whether you see light through it or not before putting it back.

[rotortiller]

Simplest thing is to swap 2 and 3 plugs and wires, then try it to see if the problem moved. (2and 3 fire at the same time) If problem did not move to #2 then I'd look at #3 cylinder compression value as the next simplest thing. If after all that fails I would have a look at the #3 carb.

[willbird]

most often this situation turns out to be plugged pilot jet...if you got a good shorty flathead screwdriver, just unclip the floatbowl unscrew the jet, hold it up to the light to see if it is clear.  I would flush it out whether you see light through it or not before putting it back.

Or you are riding down the road and the bike starts runny crappy. You pull float bowls one at a time to be sure they are properly filled with fuel and you find the pilot jet laying in the bottom of the float bowl :-(. Easy fix anyway :-). Need to get me one of those good shorty flathead screwdrivers.

[ekpent]

I like Don's fouled plug answer followed by Seans check the slow jet and float if problem persists.

[RAFster122s]

most often this situation turns out to be plugged pilot jet...if you got a good shorty flathead screwdriver, just unclip the floatbowl unscrew the jet, hold it up to the light to see if it is clear.  I would flush it out whether you see light through it or not before putting it back.

Or you are riding down the road and the bike starts runny crappy. You pull float bowls one at a time to be sure they are properly filled with fuel and you find the pilot jet laying in the bottom of the float bowl :-(. Easy fix anyway :-). Need to get me one of those good shorty flathead screwdrivers.

Vessel makes some nice short screwdrivers in JIS, or Motion Pro makes screwdriver bit insert in JiS 2 packs that sell for less than $5 a pack. Then a small magnetic driver handle and you are in business...

[willbird]

most often this situation turns out to be plugged pilot jet...if you got a good shorty flathead screwdriver, just unclip the floatbowl unscrew the jet, hold it up to the light to see if it is clear.  I would flush it out whether you see light through it or not before putting it back.

Or you are riding down the road and the bike starts runny crappy. You pull float bowls one at a time to be sure they are properly filled with fuel and you find the pilot jet laying in the bottom of the float bowl :-(. Easy fix anyway :-). Need to get me one of those good shorty flathead screwdrivers.

Vessel makes some nice short screwdrivers in JIS, or Motion Pro makes screwdriver bit insert in JiS 2 packs that sell for less than $5 a pack. Then a small magnetic driver handle and you are in business...

What we are talking about is for the pilot jets so it would be a straight screwdriver.

[MJL]

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

It's a wasted spark system, so if there is no spark on three then there will be no spark on 2.

[RAFster122s]

most often this situation turns out to be plugged pilot jet...if you got a good shorty flathead screwdriver, just unclip the floatbowl unscrew the jet, hold it up to the light to see if it is clear.  I would flush it out whether you see light through it or not before putting it back.

Or you are riding down the road and the bike starts runny crappy. You pull float bowls one at a time to be sure they are properly filled with fuel and you find the pilot jet laying in the bottom of the float bowl :-(. Easy fix anyway :-). Need to get me one of those good shorty flathead screwdrivers.

Vessel makes some nice short screwdrivers in JIS, or Motion Pro makes screwdriver bit insert in JiS 2 packs that sell for less than $5 a pack. Then a small magnetic driver handle and you are in business...

What we are talking about is for the pilot jets so it would be a straight screwdriver.

Sorry, I am a 550 owner, the float bowls are held on by screws on the 550...

[Floshenbarnical]

most often this situation turns out to be plugged pilot jet...if you got a good shorty flathead screwdriver, just unclip the floatbowl unscrew the jet, hold it up to the light to see if it is clear.  I would flush it out whether you see light through it or not before putting it back.

Or you are riding down the road and the bike starts runny crappy. You pull float bowls one at a time to be sure they are properly filled with fuel and you find the pilot jet laying in the bottom of the float bowl :-(. Easy fix anyway :-). Need to get me one of those good shorty flathead screwdrivers.

Vessel makes some nice short screwdrivers in JIS, or Motion Pro makes screwdriver bit insert in JiS 2 packs that sell for less than $5 a pack. Then a small magnetic driver handle and you are in business...

What we are talking about is for the pilot jets so it would be a straight screwdriver.

Sorry, I am a 550 owner, the float bowls are held on by screws on the 550...

https://www.amazon.com/I-MART-Offset-Screwdriver-Phillips-Driver/dp/B07HDJGV92/ref=asc_df_B07HDJGV92?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80333185695376&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583932713475491&psc=1

[TwoTired]

It's a wasted spark system, so if there is no spark on three then there will be no spark on 2.

… unless the spark lead or spark plug tip is grounding out.  Then spark on one cylinder.

For the OP.  I would suspect a pilot jet issue.   But spark plug deposits would be an easy indicator for aid in diagnosis.

[willbird]

most often this situation turns out to be plugged pilot jet...if you got a good shorty flathead screwdriver, just unclip the floatbowl unscrew the jet, hold it up to the light to see if it is clear.  I would flush it out whether you see light through it or not before putting it back.

Or you are riding down the road and the bike starts runny crappy. You pull float bowls one at a time to be sure they are properly filled with fuel and you find the pilot jet laying in the bottom of the float bowl :-(. Easy fix anyway :-). Need to get me one of those good shorty flathead screwdrivers.

Vessel makes some nice short screwdrivers in JIS, or Motion Pro makes screwdriver bit insert in JiS 2 packs that sell for less than $5 a pack. Then a small magnetic driver handle and you are in business...

What we are talking about is for the pilot jets so it would be a straight screwdriver.

Sorry, I am a 550 owner, the float bowls are held on by screws on the 550...

https://www.amazon.com/I-MART-Offset-Screwdriver-Phillips-Driver/dp/B07HDJGV92/ref=asc_df_B07HDJGV92?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80333185695376&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583932713475491&psc=1

There is room to use a normal type screwdriver (rather than offset) but it needs to be on the short side.The slot in the pilot jet is .032" and the pilot jet is .230" or so dia. Everything I have the proper slot size is too long and on the narrow side dia wise.

https://www.amazon.com/336-Stubby-Carburetor-Screwdriver-24-5/dp/B085CS9B8V

This one is the correct width for the slot but could be 2mm bigger dia.

These right here might work. Ordered a couple of the 50mm long ones intending to make a handle for them on the lathe. Might get fancy and make a tube to press over the end to fit the OD of the jet.

https://www.amazon.com/Genius-Tools-203055-1155-Slotted/dp/B08FFCNPLQ

[newday777]

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

It's a wasted spark system, so if there is no spark on three then there will be no spark on 2.

One plug cap might not be making good connection with the plug wire and weakly jumping the distance from the worn wire to the cap

[Mark K]

A fouled plug on that cylinder?

Thanks Don,
I'll double check the plug. Plugs and caps are new and both 2 & 3 plugs were showing good spark last I pulled them when I was trying to track coil/points issue. Engine has NOT run very much (fuel tank issue I've been trying to work thru).

[Mark K]

Bad spark plug cap can lead to fouled plugs…

Thanks RAFster,

All plug caps are brand new. All checked out w/FSM numbers when I checked them before installing them on the bike.

[Mark K]

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

Thanks Newday,
Had to replace one of the coils, I did clip 1/4" off the plug wires to ensue good connnection. I tested the resistance of the new plug caps and the numbers were in line w/FSM.

[Mark K]

most often this situation turns out to be plugged pilot jet...if you got a good shorty flathead screwdriver, just unclip the floatbowl unscrew the jet, hold it up to the light to see if it is clear.  I would flush it out whether you see light through it or not before putting it back.

Thanks SeanBarney,
 I'll be sure to check the pilot jet, and flush it before I reinstall it.

[Mark K]

It's a wasted spark system, so if there is no spark on three then there will be no spark on 2.

… unless the spark lead or spark plug tip is grounding out.  Then spark on one cylinder.

For the OP.  I would suspect a pilot jet issue.   But spark plug deposits would be an easy indicator for aid in diagnosis.

Thanks TwoTired,

I'm going to pull the plugs, check the deposits as well as checking the pilot jet.

[Mark K]

Thanks to everyone who responded, I really appreciate the suggestions. Hope to get back to work on it this weekend.

[willbird]

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

Thanks Newday,
Had to replace one of the coils, I did clip 1/4" off the plug wires to ensue good connnection. I tested the resistance of the new plug caps and the numbers were in line w/FSM.

Not intending to insult your intelligence :-). But did you SCREW the wires caps onto the wires ?? Just saw a similar connection on a gas burner igniter at work where somebody just shoved the wires in which did not work so well ;-). The igniter transformer for our gas burner works exactly like a SOHC spark plug cap.

Bill

[Mark K]

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

Thanks Newday,
Had to replace one of the coils, I did clip 1/4" off the plug wires to ensue good connnection. I tested the resistance of the new plug caps and the numbers were in line w/FSM.

Not intending to insult your intelligence :-). But did you SCREW the wires caps onto the wires ?? Just saw a similar connection on a gas burner igniter at work where somebody just shoved the wires in which did not work so well ;-). The igniter transformer for our gas burner works exactly like a SOHC spark plug cap.

Bill

Bill, no insult at all. If you've read any of my forum questions, you'll figure out pretty quickly I'm not the sharpest tack in the box. But, I did know to screw the caps onto the wires! Thanks!

[willbird]

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

Thanks Newday,
Had to replace one of the coils, I did clip 1/4" off the plug wires to ensue good connnection. I tested the resistance of the new plug caps and the numbers were in line w/FSM.

Not intending to insult your intelligence :-). But did you SCREW the wires caps onto the wires ?? Just saw a similar connection on a gas burner igniter at work where somebody just shoved the wires in which did not work so well ;-). The igniter transformer for our gas burner works exactly like a SOHC spark plug cap.

Bill

Bill, no insult at all. If you've read any of my forum questions, you'll figure out pretty quickly I'm not the sharpest tack in the box. But, I did know to screw the caps onto the wires! Thanks!

Ignorant does not equal dumb :-). The instructions for our system at work show the wire twisted up like a Licorice stick to illustrate screwing it into where it goes :-). The mfg is German I think, "Kromschröder".



Bill

[Little_Phil]

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

If original coils, did you clip off 1/4" of the plug wires to give good connection to the plug caps? Did you test the plug caps resistance?

It's a wasted spark system, so if there is no spark on three then there will be no spark on 2.

Going back to this, you can get a spark on one cylinder if the other line of the HT is open circuit. Just had mine running on three with one HT lead well clear of earth.

[rotortiller]

It's a wasted spark system, so if there is no spark on three then there will be no spark on 2.

Curiosity got the better of me so I removed No1 spark plug cap, then removed No4 cap and hooked it to a new spare plug grounded to the engine due to easy access.  It sparked fine on my 750K7.

[willbird]

It's a wasted spark system, so if there is no spark on three then there will be no spark on 2.

Curiosity got the better of me so I removed No1 spark plug cap, then removed No4 cap and hooked it to a new spare plug grounded to the engine due to easy access.  It sparked fine on my 750K7.

It has to be completing the circuit SOMEWHERE :-), maybe not a good place :-).

[rotortiller]

I know what the diagram says which kinda says it shouldn't do that. My bike works fine and it does what it does, just like others have observed. It's enough that it works lol.

[willbird]

I know what the diagram says which kinda says it shouldn't do that. My bike works fine and it does what it does, just like others have observed. It's enough that it works lol.

The "lose two  with ignition problem" is still a good general rule though.

One pipe cold for me so far has always been a carb issue. No fuel in the float bowl, pilot jet vibrated loose and fell into the bottom of the float bowl, etc. One plug gap totally closed can give you one cold cyl tho :-).

This situation right here can cause some weird stuff until you find it and fix it :-).





Bill

[TwoTired]

Current flows in a loop. Once voltage rises high enough to jump a spark gap(s) and form a plasma channel, it requires current to sustain that spark event.  Insulation is not absolute.  If the voltage is high enough, it will simply pass through it at the lowest resistance point (crack or thinned portion), finding the lowest resistance path back to complete the current loop. 

If you disconnect a lead and still get spark on one of the pair of plugs, the spark current path found an insulation weak point.   Broad daylight can hide where that is.  And if it is passing though the insulation, it "remembers" that point for future escape and weakens the insulation at the point of escape, usually by vaporization in the arc channel it formed.

There is also the possibility that as the spark lead insulation ages and hardens, it loses some insulation value and is less able to contain a voltage and current that normally finds an easier path to complete the current loop on it's center conductor.

While High Voltage may appear to bend the rules, it actually has defined physics to which it adheres.

[Mark K]

Pulled #3 carb pilot jet, completely clogged, unable to get it opened up. Replaced pilot jet w/same size from rebuild kit. Replacing the pilot jet fixed the cold #3 cylinder issue.

New problem, #2 carb overflow pouring fuel, just like #3 carb did, will reinstall old float, that fixed the same issue on #3 carb.

Ran the bike for about 15 minutes (w/2 fans blowing on the engine), engine idling between 1100-1300rpm. Used an IR thermometer to 'read' the pipe temps, 2-3" from the exhaust port.

Exhaust pipe temps
#1 - 182*F
#2 - 179*F
#3 - 177*F
#4 - 108*F
Checked each one numerous times and kept getting 108*F on #4 pipe.

Pulled the plugs, gap still set at .026" All plugs looked black and sooty. I cleaned the plugs after pictures and reinstalled. Plugs are basically brand new NGK D8EA, engine has only run for about an hour total time since rebuild, have gas tank issue trying to work thru, as well as the other issues.

Suggestions as to what's going on are highly appreciated.

Pictures of the plugs