Topic: Runs pretty good on 3 out of 4 cylinders!

[ZTatZAU]

I guess you could say it's a testament to these motors that it wasn't apparent that my K1 750 is running on only 3 cylinders. 

I thought I had the engine all sorted last year, early on in this recent resurrection, when I set the valve clearances, ignition timing, and went through the carbs.  It all seemed to be running good.  Fast forward to my firts trip out on the road in the last few days...

Number 4 is not missing, spitting, or popping but an IR temp sensor shows the #4 exhaust pipe at the first bend is much colder than the others.  I noticed an occasional backfire type pop on deceleration but can't be sure where that's coming from.  Here's what I've found so far...

Good spark from #4 high tension lead to engine cylinder
Good spark from #4 spark plug grounded to cylinder
     (Spark plug anode & electrode looked slightly dark and damp.)
#4 Compression = 135 psi
Clear tube from #4 carb drain showed fuel level about 1/4" below carb body
Clear tube from #1 carb drain showed fuel level about 3/16" below carb body
Pulled bowl and float did appear to be stopping fuel flow at needle valve...

However, upon reassembly I'm noticing a drip from #4 overflow tube once every thirty seconds or so.
Engine starts, runs smooth and idles well, but #4 STILL NOT FIRING.

What's next to go after here?  Should I be resetting the #4 float tab?  Inspecting the overflow tube for cracks?  Pulling main jet to look for obstructions?

Suggestions welcome!
ZT

[seanbarney41]

clear out that pilot jet

[ZTatZAU]

Cooler temps indicates a lack of combustion. Spark is there, but nothing to ignite. Focus on the carb and check the emulsion tubes and jet passages.

Thanks calj737, I agree and will be focusing on the #4 carb.  I went through the carbs last winter when I began this resurrection and felt pretty comfortable following the service manual to R&R the jets with the carbs removed and upside down on the bench.  In this case, is this something you more experienced guys would do "upside down" with the carb still installed on the bike?  Or do you always remove the carb to work on it as a matter of course?

I remember paying particular attention to the holes in the emulsion tube, as I considered drilling some of the holes out as per Honda Man's recommendation re: modern fuels.  I didn't drill anything but made sure all the holes were open.  That's not to say the #4 emulsion tube didn't get plugged up when I first ran the bike with the old gas tank which did have some rust in it.  I'll be checking that out!

clear out that pilot jet

Thanks Sean!  I'm not too familiar with the vernacular here.  When you say "pilot jet", are you referring to what the service manual calls the main jet or the slow jet?  And would you mind explaining why you would start there, in this scenario?

ZT

[Stev-o]

Pilot jet = slow jet.

[ZTatZAU]

Pilot jet = slow jet.

Thanks Stev-o!

[ZTatZAU]

If you don’t mind some contortion, service your carb on the bike.

Thanks calj!  I'll do my best!
ZT

[Don R]

 An end carb is easier but they all can be cleaned in place. Be sure to blow carb cleaner through the air bleed in the front, then remove the screw and cover the air bleed with a finger and spray through the adjustment screw hole. Observe where it comes out. don't forget your safety glasses. Remove the pilot jet and spray through it too.
 Hondaman mentioned white chalky stuff from the 90's fuel in the small air bleed passages. He had to poke a curved wire. I suspect that in my brothers bike, carbs appear clean but just don't keep working. Replace the plug or swap it to an end cylinder (for ease).

[seanbarney41]

yup, Don just nailed it above.  I will add further...even after a thorough initial cleaning, the pilot/slow jet will be more likely to re-plug itself because it's orifice is so tiny.  The main jet requires a relatively sizeable chunk to re-plug.  In fact, the main jet orifice is big enough to be capable of cleaning itself with the flow of fresh gasoline, to a certain extent.  The slow jet? can be blocked by a mere droplet of water, as gasoline and water won't mix, no gas will flow through this tiny orifice until some other force manages to clear the water droplet.

[BenelliSEI]

ZT….. don’t be tempted to poke anything through the holes on the jets. Any damage, scrapes or scratches will change their spray patterns and destroy performance. Soak in carb cleaner and blow with compressed air.

[RAFster122s]

The white deposits often collect just past the jets too, seen it in CV carbs off 78 CX500. It is fairly firm deposits too. I had to use a good ultrasonic cleaner to get it to break up.  CLR can help   clean your brass, immerse for about a minute.  But, if you have an ultrasonic cleaner it works even better.

[ZTatZAU]

Thanks everyone!  I appreciate your replies!

An end carb is easier but they all can be cleaned in place. Be sure to blow carb cleaner through the air bleed in the front, then remove the screw and cover the air bleed with a finger and spray through the adjustment screw hole. Observe where it comes out. don't forget your safety glasses. Remove the pilot jet and spray through it too.
 Hondaman mentioned white chalky stuff from the 90's fuel in the small air bleed passages. He had to poke a curved wire. I suspect that in my brothers bike, carbs appear clean but just don't keep working. Replace the plug or swap it to an end cylinder (for ease).

Yes, at this stage for me anyway, being an end carb is the only reason I thought it possible to work on the carb "in situ".  Glad I asked.

RE: Be sure to blow carb cleaner through the air bleed in the front, then remove the screw and cover the air bleed with a finger and spray through the adjustment screw hole. Observe where it comes out. don't forget your safety glasses.


I'll be sure to do that!  Pardon my asking, Don, but where should it come out?

yup, Don just nailed it above.  I will add further...even after a thorough initial cleaning, the pilot/slow jet will be more likely to re-plug itself because it's orifice is so tiny.  The main jet requires a relatively sizeable chunk to re-plug.  In fact, the main jet orifice is big enough to be capable of cleaning itself with the flow of fresh gasoline, to a certain extent.  The slow jet? can be blocked by a mere droplet of water, as gasoline and water won't mix, no gas will flow through this tiny orifice until some other force manages to clear the water droplet.

Thanks for the additional explanation on your slow jet suspicion and diagnosis.

ZT….. don’t be tempted to poke anything through the holes on the jets. Any damage, scrapes or scratches will change their spray patterns and destroy performance. Soak in carb cleaner and blow with compressed air.

The white deposits often collect just past the jets too, seen it in CV carbs off 78 CX500. It is fairly firm deposits too. I had to use a good ultrasonic cleaner to get it to break up.  CLR can help   clean your brass, immerse for about a minute.  But, if you have an ultrasonic cleaner it works even better.

Thanks John!  Thanks David!

I don't expect everyone to remember all the past threads on a project but I did dis-assemble and go through all the carbs last winter/spring, including a good cleaning in an ultrasonic cleaner.  All the original brass looked to be in good shape and were cleaned with carb cleaner and compressed air.  No wires, as I was advised back then.
 


So I really don't expect to find any heavy white deposits but rather hope to find something obvious in the #4 slow jet.  If not, I'll go from there.

Thanks again for all the advice and suggestions.
ZT

[BenelliSEI]

ZT….. it takes nothing to plug up the slow jet. If you can’t see through it when you pull it out, problem solved.

[ZTatZAU]

Thanks everyone for your replies and helpful info!  Bike is now running on all 4 cylinders!

But I wish I could take and post a picture of what I see looking through the bore of the #4 slow jet.  Granted my eyes ain't what they used to be, but even with an opti-visor and a round lighted magnifier viewer, I never did see what looked like a clean bore.  It always looked like there was something like a shadow of a spider web down in the bore just about where the jet is threaded on the outside.  I thought it might be shadows from light coming in through the cross drilled holes but covering those holes, best I could with thumb and finger, didn't seem to change the view.

I let the jet soak overnight in carb cleaner and then blew it out again with compressed air.  My air nozzle fit nicely over the cross drilled holes and when held securely, I could feel a strong, but very narrow, continuous stream of air coming from the screw driver end of the jet.  But I still couldn't see what looked like a clean bore when I held the jet up to a light.

I put it back together anyway, started up, warmed up a little idling, checked header temps and all seemed to be in the same ball park around 100°F.  I then took it for a ride, up and down the block a few times.  (Ran good and strong as it seemed to before!)

Came back in to the shop and with everything now warmed up, checked the header pipe temps at idle again...

#1 - 190°, #2 - 176°, #3 - 189°, #4 - 174°

So I'm pretty sure all cylinders are running now.  But I did find the temperature pairs interesting, with cylinder pairs 1 & 3 and 2 & 4 sporting similar temps but about 15° difference between the pairs.

Could this 15° difference between cylinder pairs 1&3 and 2&4 be the result of slightly different ignition timing, which I know affects exhaust gas temps (EGT)?  Or more likely the result of slightly different mixtures in the two cylinder pairs due to float heights or mixture screw settings?

Thoughts?  Comments?
ZT

[Don R]

 Recently I had a problem K2 that I finally realized had an enlarged hole through the pilot jet. That said I still do use a guitar string to carefully poke through the jet.
 I haven't gotten good repeatable results with my heat gun, I chalk it up to the chrome.

[ZTatZAU]

Recently I had a problem K2 that I finally realized had an enlarged hole through the pilot jet. That said I still do use a guitar string to carefully poke through the jet.
 I haven't gotten good repeatable results with my heat gun, I chalk it up to the chrome.

Interesting, but I don't think my hole is enlarged; unless it came like that from the factory.  My carb brass is all OEM Honda/Keihn and I know I've been the only one into the carbs.

Not that I would try it, but can you tell me what gauge guitar string you've used?

I must confess, I resisted the urge to try some aviation safety wire (0.032", I think) but I couldn't resist cutting a single nylon bristle (measured 0.029") from a broom and I never could push it all the way through.  From either end, the nylon bristle got hung up right about at the shoulder of the jet, just above the treads.  I assume this is where a measured orifice is located but haven't a clue as to what it looks like inside the bore of the jet.

Maybe someone can enlighten me on that.  Does a #40 jet have a 0.040" orifice?

As for the heat gun and "chrome" not always working, I'm not sure what you mean or what is chrome.
Can you explain?
ZT

[Don R]

 My header pipes are chromed, I think it reflects the true heat reading somehow. It might just be my cheapo I/R heat gun, but I've read that I'm not the only one confused by infra-red heat gun readings.

[ZTatZAU]

My header pipes are chromed, I think it reflects the true heat reading somehow. It might just be my cheapo I/R heat gun, but I've read that I'm not the only one confused by infra-red heat gun readings.

Oh!  Now I get it.  The heat gun is really the IR Temp sensor gun reflecting off the "chrome" pipes.  Duh!
I thought you were using a bonafide "heat gun" on the pilot jet to help clean out junk and couldn't figure out what was chrome.
My bad! But glad I asked. 

And I don't think they come any cheaper than my Harbor Freight Temp Gun.  So far it seems to work for me.  I recall the instructions spoke of a sensing cone, expressed as a ratio, that determines the area of the temperature sample as a function of the distance to the area being scanned.

It seems my readings, on chrome pipes were reasonable.
ZT

ZT

[jonda500]

The jet numbers are expressed in hundredths of a millimeter. #40 jet has a 0.40mm hole.
John

[ZTatZAU]

The jet numbers are expressed in hundredths of a millimeter. #40 jet has a 0.40mm hole.
John

Thanks John!  I appreciate the info.
That's only about 0.015". 
No wonder the 0.029" nylon broom bristle wouldn't pass through.
ZT

[newday777]

"From either end, the nylon bristle got hung up right about at the shoulder of the jet, just above the treads."

The jets center hole is not straight through 1 size. It is tapered. I have a picture and will look for it that has a jet cut in a section slice to show it that someone posted a while ago.

[newday777]

I don't remember who posted this but here you go so you can see the center hole of the idle jet and why you can't poke straight through a jet nor do you ever want to drill a jet through.

[BenelliSEI]

Interesting cutaway! I notice when you look through a clean one there always appears to be something (tiny hair) in the smallest portion. Apparently that is an optical illusion.

[MauiK3]

It sure takes a tiny drill to make that hole!

[ZTatZAU]

"From either end, the nylon bristle got hung up right about at the shoulder of the jet, just above the treads."

The jets center hole is not straight through 1 size. It is tapered. I have a picture and will look for it that has a jet cut in a section slice to show it that someone posted a while ago.

I don't remember who posted this but here you go so you can see the center hole of the idle jet and why you can't poke straight through a jet nor do you ever want to drill a jet through.

Outstanding, Stu!  Thanks so much for digging up and posting that cross section picture.  As they say, a picture IS worth a thousand words.

Interesting cutaway! I notice when you look through a clean one there always appears to be something (tiny hair) in the smallest portion. Apparently that is an optical illusion.

Uhhh, you must have missed this above, John... Granted my eyes ain't what they used to be, but even with an opti-visor and a round lighted magnifier viewer, I never did see what looked like a clean bore.  It always looked like there was something like a shadow of a spider web down in the bore just about where the jet is threaded on the outside...  and nobody commented.  After several cleanings, this had me stumped for a while.  So, I guess I was seeing what I was seeing when looking down what turned out to be a clean bore.

It sure takes a tiny drill to make that hole!

Yep!  Less than 1/2 mm.  And it's got to be fairly long too; to go all the way through the length of the jet when the drilling begins.

With all that said, thanks again to everyone who replied here and who have helped me along the way!  I've learned a lot with your info, advice and suggestions.  The bike is running great on all four now!  Here's another pic I took yesterday...



ZT

[BenelliSEI]

Looking GREAT! enjoy.