Topic: Motor has extreme sputtering after highway travel today. Halp?

[Obsidus]

So I went to ride up to Lyons today. After about twenty minutes at 65 mph, the speed changed to 75 and I brought it up....or tried to. Right after about 60-65 mph, the engine would sputter and give me fits while gaining little to no speed. So I pulled over to check my timing and all that. I adjusted the 2-3 points and everything seemed fine..at idle. Now it sputters so bad out 1st, 2nd and 3rd gear if I twist more than 1/4 throttle. I replaced the plugs, re-checked my timing, cleaned my float bowls and jets and even adjusted my air screws. It's sputtering and kicking me around in the seat terribly. This problem only started after the attempt at greater highway speeds.

I've talked to Uriah (you Colorado guys know him) and he said the shoddy top end response is because my jets are too small (I'm running size 100s'). When I described the sputtering, he told me it could be valves, vacuum leak at boots, or something got was loosened as a result of vibrations.

Specs:
76' cb750k
open headers (shorty pipes)
pod filters
Keihin carbs
100 size mains
40 size pilots
air screws were at about 1-1/8 turns out


I should also mention that when I pulled my plugs, the ones in 1 and 4 had a gray/tan color to them whereas 2 and 3 had more of a sooty look.

[lostmykeys]

 Is this bike the same one in your avatar?
 I have run bikes with pods,and I have to say if the bike likes them I am happy too!
 Are you running K&N pods or those cheep foam types.The reason I ask is the that those cheep foam type pods fall appart and when they do guess where the foam goes? Thats right, into your jets.

[DJ_AX]

Was there enough gas in the tank?

Had to ask... others have made this error before.

Not me

[Spanner 1]

Ignition condenser IMO.

[lucky]

Main jets way too small. Stock was #120mm.

I would put #120's in it . Hope you did not do damage to it by running it so lean.

[BobbyR]

Specs:
76' cb750k
open headers (shorty pipes)
pod filters
Keihin carbs
100 size mains
40 size pilots
air screws were at about 1-1/8 turns out

You are probably running lean. Your shorty pipes will lean out the motor. The Pods will lean out the motor. I would also adjust your float levels when you swap out the jets. I would also invest in some new plugs. 

[seanbarney41]

there is no way your bike was ever running good with that combo...and Lucky...105 mains were stock for '76...

[Bankerdanny]

I second the condenser. If it ran fine at highway speeds before then started acting up after an extended highway speed run check the small nut that connects the condenser wire to the points. A loose connection will kill the power at higher rpms

[lucky]

there is no way your bike was ever running good with that combo...and Lucky...105 mains were stock for '76...

Wasn't the 105 mains in the F model?

I would still put #20's in it.

[singedebile]

curious about the condenser, if you get a good spark on both sets of cylinders could that still be an issue? (I'm having trouble with what seems like some of my cylinders not firing)

[seanbarney41]

there is no way your bike was ever running good with that combo...and Lucky...105 mains were stock for '76...

Wasn't the 105 mains in the F model?

I would still put #120's in it.

me too...at the very least with that combo

[Spanner 1]

yes.. to condenser Q's.... they can act-up with heat and higher rpms. ( corresponding to higher voltage in the ign. system too )... change-out 2-3 condenser, por favor 

[Obsidus]

yes.. to condenser Q's.... they can act-up with heat and higher rpms. ( corresponding to higher voltage in the ign. system too )... change-out 2-3 condenser, por favor 

Ignition condenser IMO.

Now that I think about it, I did have a questionable amount of spark on the 2-3 points when I checked them so that seems more than reasonable. I pray, pray and PRAY it's that cheap of a fix. haha

Is this bike the same one in your avatar?
 I have run bikes with pods,and I have to say if the bike likes them I am happy too!
 Are you running K&N pods or those cheep foam types.The reason I ask is the that those cheep foam type pods fall appart and when they do guess where the foam goes? Thats right, into your jets.

Actually, that was the first  motor in this bike. That motor threw a ring not too long after that pic was taken. It was a 78' K model.

I'm running those knock-off pods, yeah. I haven't checked the carb throats but I will be sure to do that on Thursday. I think it is worth noting that I did not see much obstruction in my mains, the most I found was a speck of gunk in one of them. Same with pilot jets.













PLEASE keep posting possibilities as it is extremely helpful in my method of troubleshooting. Though I did build a chopper, I'm still a green horn here.


Thursday is when I take the bike to Uriah's and get it sorted.

Was there enough gas in the tank?

Had to ask... others have made this error before.

Not me

Ha! Yes, I topped off just before the jaunt.

Main jets way too small. Stock was #120mm.

I would put #120's in it . Hope you did not do damage to it by running it so lean.

So do I, sir. It is a fine idle still, all cylinders fire and warm up, so I'm in the hopes that the worst possible damage could be the rings.

[scottly]

 Are you running K&N pods or those cheep foam types.The reason I ask is the that those cheep foam type pods fall appart and when they do guess where the foam goes? Thats right, into your jets.

That's wrong. In order for the foam to get into the jets, it would have to get pulled into the float bowls first. It *might* get pulled into the air-bleeds, worst case, but most will be sucked into the combustion chamber and burned. Granted, I replace them before they deteriorate to that point. I've been running foam filters for over 30 years on my bike...

[Obsidus]

worst case, but most will be sucked into the combustion chamber and burned.

That's hardcore.

[Shane72]

Curious here...


Is this the first time you've attempted that rate of speed with this current combination of intake, carburetion, and exhaust?

I'm thinking jetting as well.  If idle and low speed are good but you can't boogie down the road, main jets are where I'd look.

If, however, you've successfully run at these speeds before without issue, then something else may be to blame.

All the best,

Shane


Edited to add...I just re-read your OP more carefully.  Sounds like idle and low speed are now affected after the higher speed attempt.  My main-jet diagnosis does not address the idle and low speed issues.  Something else may be causing that, but without more information I'd just be guessing.  Lean-condition damage may have been done.

[Obsidus]

Curious here...


Is this the first time you've attempted that rate of speed with this current combination of intake, carburetion, and exhaust?

I'm thinking jetting as well.  If idle and low speed are good but you can't boogie down the road, main jets are where I'd look.

If, however, you've successfully run at these speeds before without issue, then something else may be to blame.

All the best,

Shane


Edited to add...I just re-read your OP more carefully.  Sounds like idle and low speed are now affected after the higher speed attempt.  My main-jet diagnosis does not address the idle and low speed issues.  Something else may be causing that, but without more information I'd just be guessing.  Lean-condition damage may have been done.

Yes, this is the first time I've pushed +65 mph on this particular motor.


What is the possible extent of damage from a lean condition? Seeing other bikes with lean damage, I've only ever seen a ring or two get thrown.

[scottly]

Please post a pic of your spark plugs.

[Obsidus]

Please post a pic of your spark plugs.

The new ones are still in there but I have the old ones in the garage. I'll try to remember to do that on Thursday.

[scottly]

While you're at it, please post pics of the new plugs, also.

[seanbarney41]

Curious here...


Is this the first time you've attempted that rate of speed with this current combination of intake, carburetion, and exhaust?

I'm thinking jetting as well.  If idle and low speed are good but you can't boogie down the road, main jets are where I'd look.

If, however, you've successfully run at these speeds before without issue, then something else may be to blame.

All the best,

Shane


Edited to add...I just re-read your OP more carefully.  Sounds like idle and low speed are now affected after the higher speed attempt.  My main-jet diagnosis does not address the idle and low speed issues.  Something else may be causing that, but without more information I'd just be guessing.  Lean-condition damage may have been done.

Yes, this is the first time I've pushed +65 mph on this particular motor.


What is the possible extent of damage from a lean condition? Seeing other bikes with lean damage, I've only ever seen a ring or two get thrown.

quite likely a hole in the top of a piston

[Obsidus]

Wouldn't that turn one of my pipes into an oil spout?

[ekpent]

On top of the lean jetting etc., 100's way small, also make sure that your tank vent and all the drain tubes etc are clear so your not running your bowls low on fuel.Easy to pop off a bowl on these to check the level after a ride and the fuel tap turned off.

[lucky]

yes.. to condenser Q's.... they can act-up with heat and higher rpms. ( corresponding to higher voltage in the ign. system too )... change-out 2-3 condenser, por favor 

Ignition condenser IMO.

"Threw a ring"??

Now that I think about it, I did have a questionable amount of spark on the 2-3 points when I checked them so that seems more than reasonable. I pray, pray and PRAY it's that cheap of a fix. haha

Is this bike the same one in your avatar?
 I have run bikes with pods,and I have to say if the bike likes them I am happy too!
 Are you running K&N pods or those cheep foam types.The reason I ask is the that those cheep foam type pods fall appart and when they do guess where the foam goes? Thats right, into your jets.

Actually, that was the first  motor in this bike. That motor threw a ring not too long after that pic was taken. It was a 78' K model.

I'm running those knock-off pods, yeah. I haven't checked the carb throats but I will be sure to do that on Thursday. I think it is worth noting that I did not see much obstruction in my mains, the most I found was a speck of gunk in one of them. Same with pilot jets.













PLEASE keep posting possibilities as it is extremely helpful in my method of troubleshooting. Though I did build a chopper, I'm still a green horn here.


Thursday is when I take the bike to Uriah's and get it sorted.

Was there enough gas in the tank?

Had to ask... others have made this error before.

Not me

Ha! Yes, I topped off just before the jaunt.

Main jets way too small. Stock was #120mm.

I would put #120's in it . Hope you did not do damage to it by running it so lean.

So do I, sir. It is a fine idle still, all cylinders fire and warm up, so I'm in the hopes that the worst possible damage could be the rings.

[BobbyR]

Maybe there is no worst possible damage just need to sort some things out.