Topic: Left side outlet fuel line routing? Jetting?

[Dunk]

I've dug through a few older threads and found no solid answer on fuel line routing for a CB750 K5 with left side single outlet petcock. I'm wondering if mine may be a bit restrictive at higher RPM, with a sharp 90* bend off the petcock that doesn't quite kink the line but surely is restrictive. The carb plate has two holes on the right for fuel lines, like an early carb plate. If there was one hole in the right side I could make a gradual bend, but that's not the plate on the bike and my three spare racks of carbs all have the same plate with two holes on the right. I don't know if this is an original K5 carb plate? How were these fuel lines routed to avoid kinking or pinching?

The reason I ask is this bike had a 40 pilot jet and 115 main jets. It seems a little lean at idle, popping and backfiring slightly on decel no matter how I set the mixture screws (1/2 - 3/4 turn out seemed in the ballpark). At 3/4 or higher throttle it felt lean, it would slow down above 3/4 throttle and accelerate quicker if letting off a bit.

I swapped 42 pilot jets in today. Idle is now not right. Clutch rattle is worse, it doesn't consistently return to idle, hangs at higher RPM. I went as far out as 1.5 turns on mixture screws and the exhaust still smells rich at idle. Clearly I should go back to the 40 pilot jets, though I will check my linkage and sync carbs. Boots look horrible, but are soft. I'm ordering a new set and will sync carbs.

I also was going to put a set of 120 jets in, but instead put 125 jets in because it felt that lean to me. Now it has a lot more power at low to mid RPM, much more torque when getting moving and power comes on quick. I still have an issue at 3/4 or more throttle where it makes less power. It feels different though, it takes off for a moment then falls flat until backing off. It doesn't quite feel like a rich blubber, but rather almost like it's lean/ Back off below 3/4 and it accelerates quick though. This is what got me thinking about fuel line routing, maybe the line is not flowing enough, though I suspect it still needs to go down to 120 jets.

[Alan F.]

On my k3 with right hand side petcock I remember fitting lines through those RH side holes in the carb plate then fishing them out to measure them at 6" and 12" which to me was inline with the silly mnemonic patterns that I'd seen on the rest of the parts I'd personally measured. For instance the OD of the lower steering bearing race at 1.969"..... 1969, memorable for a CB750 eh? My k8's race measured the same.

I'm not sure how 6 and 12 inches will play for your LH side petcock but thought I'd offer what I can, good luck.

[Dunk]

I went back to the 40 pilot jets and down to 120 mains (originally 115, tried 125 the other day). I ended up at 1/2-5/8 turn out on the air screws, in a bit from 3/4 originally.Synced carbs. They were close, but a little off (last synced a year ago). Clutch rattle is less, but still present unless idle is higher than I like, I'll check actual RPM with my dial back light one of these days. End result is it runs much better. Appreciably more power than 115 jets. Still feels like it loses power slightly when going WOT at a RPM it should be ready for it (5000-6000) vs backing off a hair from WOT. 125 must be too much, not sure it's worth trying 122.

Either way, I'd like to get the fuel line routing better if possible. Did they ever may a carb plate with holes for the fuel line on the left side? I'm tempted to drill a hole in this one when I have the carbs off to replace the cracked boots.

[Maltboy]

If you have the original K5 carb holder, there should be a hole between the 1 and 2 carbs and another one between the 3 and 4 carbs to route the fuel line.  There's a short piece (40mm) of 5.5mm fuel line that comes off the petcock and makes a 90
into a T fitting. Then there are two more hoses off the T.  One makes a 90 into the 1-2 hole in the carb holder, the other goes straight across the intake boots and makes a 90 into a hole between 3-4.  If you use original Honda 5.5mm fuel line it will make a sharp 90 without kinking.

[Dunk]

Thanks, this is super helpful. My carb plate must be an early model as it has two holes on the right. Same with all my spare racks. Drilling a hole on the left like yours would make the routing much smoother.

[PeWe]

Which exhaust system used?
My K2 using my old K6 carbs runs fine with stock air box, Yamiya no number 4-4 (good flow):
Air screws 1 turn -1/8. 1 work too, - 1/8 to fix a little hesitation at low lift.
Pilot: 40
Needle heights: 4th from top.
Main: 115

If exhaust is stock 4-4 HM341, I doubt needles should be 4, probably 3
Mains probably 105.

[Dunk]

Winning Performance (Kerker) with 1.5" "race" baffle. Stock air box and filter. Engine has been apart from PO, but no idea what was done. There was a significant and noticeable improvement in acceleration with the larger main jets. No idea on needle position, haven't checked.

I also have a K1 that has a Yamiya exhaust and ported head. That has, I think, 125 or 130 jets and could probably use a hair larger. I don't recall what pilot jets it has.

[PeWe]

Larger mains on my K2 helped me to realize that needles were too lean.
I tested 120 from115 and earlier 110, suddenly the acceleration from 70kmh woke up. But, it ran way too rich at full throttle, 160kmh --->
So I raised needles 1 notch, reduced mains back to 115 and all happy.

[Don R]

 My brother's K4 has the fuel lines routed straight down below the carbs, through a filter and back up into the fuel tees which are pointed down. He did that for decades. Experts will tell you that cannot possibly work and yet it has.

[Dunk]

Thanks, I will check needle position and raise them when I get the new carb boots. It pulled harder down low and midrange with the 125 jets, though was too big at WOT. I may try going back to 115 as well or even 110 for comparison.

Don, I've seen people run them that way, but always with the warnings of poor flow or air locking. I may drill another hole in the carb plate as in Maltboy's picture to run them that way. Easier access to bowls and jets that way.

[Dunk]

The inline filter was causing starvation. It was melting and the inlet was restricted with melted plastic. This was surely causing a false impression of being lean and explains why with larger jets it pulls hard then tapers off (as the bowls drain). It got bad enough that it would stumble at extended 60+ MPH, and stall when I slowed.

I replaced the filter with a straight section of 5.5mm hose and no more problems, it might be a tad rich at WOT... I think 115 jets and raising the needles might be the hot ticket. Still waiting on new carb boots from BikeBandit (ordered 4/20/2021) and I'll try that when the carbs are off.

[ekpent]

 Are those rubber intake manifolds stock or aftermarket ? Notice a lot of cracks etc. on the one predominately pictured.

[70CB750]

The inline filter was causing starvation. It was melting and the inlet was restricted with melted plastic. This was surely causing a false impression of being lean and explains why with larger jets it pulls hard then tapers off (as the bowls drain). It got bad enough that it would stumble at extended 60+ MPH, and stall when I slowed.

I replaced the filter with a straight section of 5.5mm hose and no more problems, it might be a tad rich at WOT... I think 115 jets and raising the needles might be the hot ticket. Still waiting on new carb boots from BikeBandit (ordered 4/20/2020) and I'll try that when the carbs are off.

Good show!  Inline filters really are not needed, the sediment bowl is enough.

[Dunk]

I forgot to attach the picture. The filter was very melty and wavy, restricted on inlet. I have another of the same filter on an XT200 that stalled on me after extended high speed and was hard to start, but once started ran fine at lower speed. I bet the filter is the problem on that too. I'm sure with a quality filter it would be fine and prevent some sediment from getting to the carbs, but these cheap filters are more suitable for small engines even though advertised as suitable for motorcycles.

No idea on age or make of the boots. I got this bike from a forum member back in 10/2015 and it has been my daily rider since then. About 140 miles on the bike today, 100 in the rain, and it ran well with just the rubber fuel line even with the sharp bend.

[Dunk]

Finally got around to rebuilding these carbs, drilling an extra hole in the early style carb bracket, and replacing the boots.

Drilled a hole in the left side of the plate to match the right side. 1/8 to start, up to 7/16", then chamferred one side with a 5/8" bit and the other by hand with a file. Fuel line routing in much more tidy and generally downhill. Might need slight adjustment on length.

New Honda boots didn't fit as nice as I would have liked, but they're always a pain. I had to rotate the inner two more than the outers to get the rack aligned approximately level to the air box.

Needles were on 4th from the top already. I changed the 120 jets for 115. Still need to vacuum sync carbs and dial it all in, but it starts and runs pretty smooth with a bench sync. I've not rode it yet but I think it will perform better. 120 was too rich at WOT, plugs were sooty and I was only getting ~100 miles to reserve rather than the 130 it used to get.

[Dunk]

Synced carbs, still has a clutch rattle up to 1500+ RPM. Not happy about that. I'd like a quiet clutch. New steels, frictions, and springs on the way as it'll slip a bit at high RPM. Maybe that will help.

Runs good with 115 jets and needle on 4th clip. Definitely runs smoother and pulls stronger all around. I think my mistaking  lean primary jet condition was a result of poor fuel line routing starving for fuel.

Below 3000 RPM when rolling into the throttle easy it has a slight stumble and misfire. 40 slow and 115 mains. Idle air screws are at about 1/2 turn out for best idle This tells me maybe it wants larger slow jets to stay in the 3/14 - 1-1/4 range on the air screws. I have a set of 42 slow jets, maybe worth trying.

[PeWe]

"Idle air screws" are air screws?
They have a bigger range than idle only.
You will notice that when cruising around on 5th gear on even throttle.
Bring a screwdriver with you.

I did that and started with way too much open just to verify the range the air screws affect.
I got fuel starvation like time for reserve in 100kmh (60mph).
Air screws opened 1 1/2 turns.

Reduced to 1 turn in steps which fix it. Probably 1/4 steps.
Then reduced 1/8 further.  I have changed to KN filter in stock box and changed back to 1 turn. Earlier might have been rich.
I'll see final when riding. Waiting for spring, got snow yesterday....

Test with 1 turn open.

[Dunk]

I checked this morning before heading to work and they were set at 3/4 out. Changed to 1/2 out and the stumble rolling on at lower RPM is reduced. Not much change with the screws further in when I've tried before.

The air screws are the newer style with hole through them. Should I change to the early solid style or try changing to 40 slow jets to 42?

[Don R]

 That fuel filter reminded me of the one on my Dad's lawn tractor back when alcohol was first introduced to gas.
    I also ran the ones with glass bowls and plastic screens that you could clean on our jr dragsters. It took a couple weeks but the methanol swelled the filter screens closed.