Topic: Stock bike, stock jetting, still rich?!

[Dirtdevil]

1975 cb750K
Okay, so, here's the scoop! I just go this thing back together after a full blown overhaul of the entire bike. Split the cases, had everything machined, fresh hone with new rings, head machined... Everything is stock! Stock exhaust, stock airbox, stock jetting. The only aftermarket item in play here is a hondaman ignition box.
It fired up right away and stopped smoking fairly quick. I checked the timing with a light, sync'd the carbs, and took it for a ride.
Problem is this: The idle keeps creeping up on me. Ill set it to 900 and then after about 15min of riding, its at 1200. Also, it started to act as though it had a plugged pilot jet, or maybe as though the carbs went out of sync again. So I pulled the plugs and discovered that #1 looked just as it should (golden brown), but #2, 3, and 4 were all sooted up like it was way too rich!
I initially thought that maybe it was an ignition issue, but then why would #1 be burning correct?
Points are brand new Honda OEM.
Running the 5K caps and a non resistor plug. But again, why would only #1 be burning correct?!
I checked for air leaks, couldn't find any.
Float levels are all where they should be.
I am baffled!
HELP!

[tlbranth]

On mine the needles had to be dropped a notch to help with rich running. I think the needle or the thingy it sinks into get worn and so richen up the mixture.

[Patrick]

This is strictly a shot in the dark, but I don't think No. 1 is burning correctly. I think it is burning lean. Because it is burning lean it runs hotter and faster. The engine will run at the speed of the fastest cylinder, so as No. 1 burns leaner and hotter the idle rises.

Meanwhile, the rest of your cylinders are not running lean, but they are running as fast as the lean cylinder. This is pumping in gas they are not capable of handling, since they aren't burning lean. They work fine at first, but start fuel fouling soon. Ir starts with the cylinder dropping out as the plugs start to sputter. Check for an air leak on No. 1 and synch them again.

At least, that's my armchair shot in the dark.

Patrick

[scottly]

Float levels are all where they should be.

How did you measure float levels? Have you done a "clear tube" check of the actual fuel levels in the float bowls?

[Dirtdevil]

That all makes sense. I have come  to accept that I'm going to have to pull them back off and check it out. I checked for air leaks today and couldn't find any. I will check again tomorrow. But if everything is the same on all 4 carbs, I am baffled as to why the #1 would be burning right where it should, while 2, 3, and 4 are so darn rich! The bike only has 13K on it and it was sitting in an Airplane hangar for most of its life. It was all original when I got my hands on it. I was hoping ot have it tuned by Saturday. looks like i have my work cut out for me!
I measured the levels as the manual says to. I tried the clear tube method today but when I unscrew the drain plug, nothing comes out the drain. It only comes out of the screw. Is there a special adapter for this method?

[scottly]

On your bike, a quick and dirty way to do the clear-tube test is to heat a piece of vinyl tubing with a lighter and stretch it slightly, to make it an hour glass shape. Snip it off in the middle, and thread the tapered end into the drain plug hole. You should be able to do this with the carbs mounted on the engine. 

[pmanning]

My K5 with stock air box/filter and stock exhaust system chronically ran rich periodically fouling the plugs.  I spent a lot of effort ensuring that all things carb and tune related were set to specification without solving the problem.  I ran across a post on this site that recommended changing the pilot jets to #38 instead of the stock #40s.  That seems to have solved the problem with my bike.

[Dirtdevil]

Okay. I had thought about this, but wasn't sure it it would actually work. This may be the First thing that I do in the AM. I guess that if #2, 3, 4 are excessively high, it would explain the rich mixture, right? If my memory serves me, it is supposed to be 3-4mm below the split line where the bowl meets the body, correct?

[scottly]

I guess that if #2, 3, 4 are excessively high, it would explain the rich mixture, right? If my memory serves me, it is supposed to be 3-4mm below the split line where the bowl meets the body, correct?

Correct. If all levels on all 4 carbs are the same, the next suspect would be plugged air holes in the sides of the emulsion tubes. Have the tubes been removed and cleaned?

[Dirtdevil]

Yes. I made sure that I got clean and uniform spray through all ports. I pulled the carbs completely apart. Air screws were 1 turn out. i usually would turn each one in until it stumbles and then out till it speeds, but on these i guess its opposite? Meaning that turning it out will actually lean the mixture, right?

[scottly]

Idle mixture screws have the most effect at idle. If all other settings are the same, the mixture screws should all be nearly the same. Verify the fuel levels first. Also, check that the choke blades are all synchronized and opening the same amount. It is normal for RPMs to climb some as the motor warms up; final adjustment should only be done on a warm motor. 

[TwoTired]

The 75 K model does not have accelerator pump carbs.  So, the idle mixture must be rich to allow lifting the slides and still have the engine pick up.  You don't tune these pilot screws for fastest idle speed, or the engine will fall flat on its face when you do a throttle twist.  You tune them just lean enough to keep from fouling spark plugs during long idle periods.

The pilot screws are air restriction screws, outward leans and inward enriches.  There are limits in each direction where they become ineffective at changing things.

You are right to look for a difference if just one cylinder is misbehaving.  Have you done a compression check to verify all cylinders are mechanically equal?  Be a shame to focus on carbs if a valve isn't seating properly.  Like a tappet adjustment creep?

Cheers,

[Dirtdevil]

OKAY! Here's the scoop! Talked to the fellow at Cycle X today ( hell of a nice guy by the way) and he gave me a rundown of some things to try.
Pulled the carbs and re-checked every single orifice- all passed cleaner consistently
Float levels are all between 3- 4 mm
No air leaks at ALL!
Re-synchronized the carbs
Checked all of the jets and needle positions- 40 pilots, 105 mains, 4th clip from the  top.
Re-tensioned the cam chain
checked the valve clearances
Put in a set of new plugs
Made sure that the timing was advancing correctly

AND---- Still WAY fat on the 2, 3, and 4 cylinders!!!!

So what i'm thinking is this; the head had a bad leak prior to me taking ownership. So, perhaps the previous owner, who is a sort of mechanic himself, opened up the jets with a jet-drill in an attempt to compensate for any lean conditions that may have existed as a result of said head leak.
I don't have jet gauges, its too small to eyeball, I am going to order new 40 pilots and 105 mains and see if that works.
I can absolutely not think of anything that would cause such a drastic difference in mixtures, aside from some wacky stuff going on with the jets.





 

[Patrick]

Reading through this, I don't know that you ever tried a clear tube test on your float bowls. Since you are going to have the carbs off and open again, make sure your floats move and down freely within the specified range. Sometimes they can stick if you let them open too much. Sometimes they can stick of you leave a tiny hitch in the swing. And if the float doesn't move freely, no. 1 cylinder may be getting starved. Don't let the happen. I will send you a picture of what a lean-condition F3 melted piston looks like. What's left if it.

Also, make sure your fuel needles are not leaking and are sealing correctly.

There also has been some issue with aftermarket carb needles properly seating with stock needle jets. The pitch can be slightly different, leading to an almost unsolvable rich condition. Also stick with the stock marts, if you have the, They very rarely actually wear out.

Patrick

[Dirtdevil]

Yes, I did indeed do a "clear tube test" and I can assure you that the levels are fine. Also, I am using all Honda OEM parts, so no funny business with needles and seats. I'm fairly certain that #1 isn't starving. 2, 3, and 4 are  just VERY rich.
Ive jetted my fair share of bike and I know what a lean/rich plug looks like, and I can tell you that #1 is coco-brown, right where it should be.
Maybe the picture doesn't show it well, but it is.

My money is on the jets. No clue what it could be other than that.

[ekpent]

 That #1 plug looks like it was not firing/burning at all. Did you touch the exhaust head pipes with a wet rag and see if all four were sizzling hot and equal. Did you have the cylinders bored or honed and new rings or pistons installed ? Does it smoke either blue or black when you start it up or run it down the road. How many miles have you put on it so far and is the engine broke in yet ? Useing a stock new paper filter ?  Are you using just regular low octane gas or premium which may blacken plugs. Did I ask enough questions ? 
  I doubt a leaky head gasket would be reason for someone to drill out the jets but would be nice to have a known good one to compare to eliminate that possibility.  Good luck on it.

[Patrick]

How are you running he motor to get these plug colors? Is this idling in your garage? Riding around the block at low rpms? Racing at oval at close to redline?

There are only so many circuits in the carb to deliver fuel. One of yours is not working. Without knowing which you are using, it is hard to tell where the problem is. You have checked all the cleaning and installation issues I can think if. Others probably think deeper than me.

If this is you first time through these carbs, you might be using someone else's mismatched parts.

Patrick

[Dirtdevil]

Yes, all pipes are hot. Running premium fuel. New OEM rings on a fresh home, the engine only had 13k on it prior to my overhauling it. It does not smoke even a little bit. Stock paper filter with stock air box and stock EVERYTHING. The entire bike is original.
I have seen mechanics try and solve head leaks on air cooled bikes by fattening up the mixture. This is what lead to my thoughts on that.

I am warming it up in front of fans and then riding it normally (how one would on a casual Sunday ride to the super market. Not too far into the revs. Mostly under 4K. Mostly at, or under 25% throttle. Engine has about 1.5 hours on it.

If you could see the #1 plug from a different angle, you would see that the electrode is coco brown. Or, at least the initial plug was. That one was from the second set of plugs that I put in because the first set fowled on 2, 3, and 4.

The jetting is the only thing I can come up with. I'm scratching my head.

HOWEVER! What I will do tomorrow is take the jetting from #1 and swap it with #4 and see what happens!

[Dirtdevil]

Swapped jets with no change in behavior. so i put in a new set of plugs and took it out on a good long ride up out local mountain road, Mt baldy. I ran it pretty good up into the RPMs. It felt good once the throttle was open and the RPMs were up above 4k. Took it out on the freeway an ran it down the road opening it up from time to time. I let it cool and checked the plugs today, they are all uniform! They all look the same. Not sure how that worked, but it did. it still seems just a little fat across all four cylinders, but its better than it was! I'm not sure if this is due to me running it at a much more open throttle position, or is something seated...
I did do a compression test and  found that #1= 130, #2=115, #3= 120, #4=130. So, I'm pretty sure that they are all close enough, and the rings aren't completely seated yet, i'm sure.
It does still hesitate on the lower end though. Almost as though it has a slight dead spot right off of the lower end.

[ekpent]

  Hopefully getting it out,running and breaking it in a little more has helped your cause from it fresh rebuild. Post up some pics of it sometime,sounds like a nice bike !

[tlbranth]

Running the bike can loosen up stuck rings and can also clear out a partially blocked jet. Glad it's running better. Your compression might improve after a while too.

[HondaMan]

If you have a strobe timing light...check the timing of the points on the 1-4 side ("F" mark at idle speed) and the 2-3 side, using the light. If the 2-3 side is 'jittering' badly, the shaft holding the spark advancer is bent a bit too much. This can cause the 2-3-4 plugs to be darker than the #1 plug, or vice-versa (#4 white, others dark). Also: today's fuels burn MUCH slower than the fuels when these bikes were made, plus the K4-5-6 were made to run on Regular grade gas (because of the fuel shortage we had in the USA at the time). Add to this the fact that today's regular gas burns at about the same speed as 1970s premium gas, and the plugs get too wet.

If the timing is too far advanced, it will cause dark sparkplugs, too. This is because it makes the cylinder(s) spit back up the intake tract, slowing the airflow into the engine. As the air flows slower, these carbs mix richer: this is the nature of them. The troubles erupt when the multiple carbs are running on the same crankshaft, which causes the next cylinder in the firing order to 'stumble' slightly, slowing it down (#1 in this case) relative to its carb slide opening.

BTW: if the timing is jittering, you must remove the points plate and advancer and straighten the shaft. After that, never turn the engine CCW direction using the big nut at the points (especially if the plugs are in the engine): this is what bends the shaft under the advancer. It needs to be within 0.002" to work exactly right: less than 0.005" will work OK in most cases.

[Dirtdevil]

Yikes! I never would have considered that. The timing on 1-4 and 2-3 is solid and steady at idle. However, when i rev it to 2,500 to check the advance, it is jittery. I figured that the timing light was receiving interference due to the close proximity of the other wires. If the shaft were bet, would it cause the timing to appear jittery at idle, or is it possible that it could appear as normal at idle and jittery at advance?

[Don R]

 I had a 76F that had drilled jets, fubared floats and many other PO caused maladys. If you get new jets get genuine kiehens.

[PeWe]

Agree about floats and correct fuel level. I had problem with that on my CB750  K6 build 2013, got the hint here to check floats which were floading. That time Mikuni Smoothbore carbs

Same thing at my last CB750 K2 build that has got my stock K6 carbs. I understood direct that floats were wrong when it ran too rich when idling.
Upon that as Hondaman wrote, to advanced ignition. I have learned the hard way.... But know now after a lot of adjustments and test runs, finally solved it. Points can be adjusted to work really well.

I wrote about this in another thread earlier today.
http://forums.sohc4.net/index.php/topic,176642.msg2053918.html#msg2053918

CB750 will run with too advanced ignition if not taking care about it. As the Base plate can be moved sideways making one point to get smaller gap, the other wider.
If moving plate to the right... 1:4 point gap will then be wider (later ignition on 1:4 ) while 2:3 will get smaller gap (advancing 2:3)
If checking ignition now with a lamp you will probably advance the base plate since you see 1:4 as retarded. If you do not check 2:3 that now is even more advanced you can damage the engine.
I blew pistons 2-3 ( 836 with good compression) back in the 80's that I finally understand why...