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