Topic: Cylinder 1 blocked pilot circuit or??

[zabzob]

1975 CB750K5 with 657Bs.

Bike was running pretty well last year, decided to the pull the carbs this year because I think it's been 5 years since they were last cleaned. Ran them through and ultrasonic cleaner. They've never looked better. Everything hosed out after and sprayed through. Floats set to 26mm, and all original Honda/Keihin jets are in the bike.

Back on the bike and #3 started dumping fuel everywhere. I pulled the bowl off, moved the float up and down a few times, sorted itself out. Phew!

But #1 does not appear to be doing anything at idle. Other pipes are heating up, #1 not so. Decel popping as well, and sometimes when idling I get pops out of the left side of the bike (I have 4 into 2 exhaust). Seems pretty okay at more open throttle, but boggy... carbs aren't fully synced yet either, so that could account for bogginess.

I pulled out the pilot jet to blow it out, seems clear. Check the spark plug, strong spark consistently. It got a little better after this but not great.

I am pretty sure it's a blocked pilot circuit, not in the jet but elsewhere in the circuit. What can I do on the bike to clear this? I can remove the jet again and spray carb cleaner up there. Where should this come out? What about the air screw, should I send some carb clean down there too?

Or, do you think I'm barking up the wrong tree?

[M 750K6]

Does have symptoms of low speed circuit being blocked still, or float level too low on no.1. Are you using choke? Are you sure your choke slide on no.1 is closing fully?

Might just be carb synch. My no.1 was well out of synch when I put mine back together.

Does it chime back in, once the bike is warmed through?

[PeWe]

Check plug if it will create a spark.
Cap on, contact with head or valve cover. Start and see.
An extra plug good to have for this test.
Try starting fluid spray. Upon starting, a good cleaner.

[zabzob]

@PeWe spark is strong and consistent on number #1. I can see about starter spray.

@M 750K6 chokes all open and close together perfectly. I spent some time with the choke linkage to ensure this. I did a bench sync before I put them back on the bike so they shouldn't be too far out.

Carbs are going to have to come out because the needle clips are in the wrong position, so I'll run through the pilot passage on #1 and see what happens...

[BenelliSEI]

I’ve had plugs that fire outside the cylinder, but fail under compression. Try a new one in that cyclinder before tearing everything else apart again. Have you checked the caps are screwed tightly and cleanly on the wire ends?

[HondaMan]

You removed the carbs: chances are pretty good that their hoses don't seal very well on the carbs anymore. They are hard, hard rubber now...

You can try to improve the flatness of the clamps where they close together with the little flat-faced screw mounts to make more clamping tension: I have [more than once] made flat-sided washers to fit inside those little screw-mount brackets, then flattened out the clamp stops and reassembled, and been rewarded with better sealing, at least for a while. I have also used the Oetiker clamps (#050-9, 50mm size, available at Aircraft Spruce) but be advised that it is possible with these aircraft-quality clamps to cut clear thru the rubber if you get carried away with tightening things too much (don't ask, I won't admit it in public...    )
Use these clamps ate the head side of the hoses, they will not fit at the carb side.

There are also some good clamps with worm-gear drive that fit the little hose grooves and allow for tighter grip on the hoses, but I can't remember who in these forums found those, nor where they are? They cost less than the aircraft clamps, too.

[willbird]

Oetiker clamps fit both ends of the rubber on my K2. They are kinda expensive but should only need to buy one

Bills

[PeWe]

I added cable ties under my clamps to ensure an even clamp force saving the soft rubber.

Used an old bad carb boot for shaping and cut in correct lenght.
Shaped it with a heat gun.

Maybe possible with stock clamps too.

[Don R]

 Spray carb cleaner through the air bleeds at the entry to the carbs, observe where it comes out, you can also spray it through the pilot screw hole while blocking that air bleed with your finger.

[HondaMan]

I added cable ties under my clamps to ensure an even clamp force saving the soft rubber.

Used an old bad carb boot for shaping and cut in correct lenght.
Shaped it with a heat gun.

Maybe possible with stock clamps too.

Yep, me too, Per: on a 'show' bike 750 I once built up for someone I made thin strips of aluminum sheet to lay into those grooves so the OEM clamps could still be used (show bike rules, you know...) and it worked fine, but BOY was that ever tedious work!

[BenelliSEI]

I added cable ties under my clamps to ensure an even clamp force saving the soft rubber.

Used an old bad carb boot for shaping and cut in correct lenght.
Shaped it with a heat gun.

Maybe possible with stock clamps too.

Yep, me too, Per: on a 'show' bike 750 I once built up for someone I made thin strips of aluminum sheet to lay into those grooves so the OEM clamps could still be used (show bike rules, you know...) and it worked fine, but BOY was that ever tedious work!

I’ve done some pretty bizarre stuff, but that one might be a prize winner!

[zabzob]

This turned out to be a blocked pilot jet. Ran okay the first time I had it out, something must have dislodged and gotten jammed in there. Visibly smaller opening than the other pilot jets when I looked through it.

All pipes now equally hot after idling.

Confirmed the needle clips are in the right position as well.

[newday777]

Good to hear you got it fixed

[HondaMan]

Good sleuthing!