Topic: Post Rebuild Carb Tuning

[Zeke]

EDIT:  Hey all, I know this thread is getting long but since I first posted I have worked on the bike about 20 more hours on it and still have the same problem.  If you can please look at the facts and check out the last post in the thread and give me some suggestions that I haven't already tried I will be truly grateful.  Thx.

It's the weekend again and I'm going to try again at tuning my CB650.

It runs, but so poorly that I can't ride it anywhere.  It is running on all cylinders.  #4 cylinder is popping a lot.

I rebuilt the engine top end and the carbs.  Rings, hone, new valve and guide, valve refacing/lap, the works.  Valves have been adjusted.When I took the carbs apart I recorded the position of the #1 pilot screw but forgot all the others.  In the end, I went with 1-5/8 turns out on all the carbs as the manual states before starting adjustment.

I'm ready to do the idle drop procedure to set the pilots -- but here's the problem.

It won't run off choke, and it won't run at idle speed of 1050 rpm.  It will idle reliably on choke at about 1500 rpm.

Last week I tried a tentative carb balance, to see if that would help.  I have a morgan carbtune.  Funny, I had to use the carbtune upside down -- I guess maybe the rings not being seated the carbs pull less vacuum???  Anyway, I haven't noticed a change I'm guessing because the pilot circuit hasn't been adjusted.

I haven't played with timing, as it's CDI ignition and I didn't touch any of that stuff -- before the valve guide went in November, it was running good.

Any words of wisdom here?  I haven't done this before, and I'm worried about fooling around so much to get it running that I lose my chance to properly wear in the rings.

If I had to do it again, I'd have waited to rebuild the carbs until the engine was worn in.

Thanks,

Zeke

[Zeke]

Here's an update:

I'm doing the idle drop procedure right now.

Basically, you turn out the screws out in 1/2 turn increments -- and while you do this the engine rpm will rise.  As long as it continues to rise more than 50 rpm, you continue the adjustment.  You do all of them equally.  Usually, my RPM jump came when I was adjusting #3.

My screws were getting pretty loose when the RPM stopped jumping -- the screws were nearly out.  It took me 6 steps to get here, which is 3 turns out from start.

Now, you have to turn in #1 screw until the idle drops 50 rpm -- once it does you've found the baseline.    Turn it out 7/8 turn from there and you're set.  Now, take that number of turns and match the other carbs to it.

OK, the problem I encountered here is that my idle never dropped.  I turned the screw all the way in and there was no change.  What to do now?

As I did the process I was able to ease off the choke but I wasn't able to take it off choke completely.  Can this affect the setting?  Why can't I take it off choke?  Motor was quite warm during the adjustment.

On a sidenote, I'm not sure that my dwell tach was working exactly right.  It doesn't seem to have 50 rpm resolution either.  The High range matched the bikes tach, but low-range was showing like 900 rpm when the bike showed 1500 plus.  Maybe I should get a better one?

Zeke

[Zeke]

Got a better dwell tach, one that does 50rpm resolution.

Hmm.  Dwell tach shows different speed than the bikes tach.  Dwell tach shows 700, bike shows 1500.

I can't get my bike to idle at 1050 -- it idles nice at 700, when I try to raise the idle by turning in the screw it jumps way past.

So I'm going to try and finish the pilot adjustment at 700 because I don't have a choice.

Must be you all are out riding today as the replies are a bit thin.  When you get back will someone please bestow some wrenching wisdom on me?

Zeke

[Chrisboden]

I can give you a little help, but not everything you like as I'm a bit of a bike Newb too.

Regarding your dwell tach;

Trust the tach on the bike first and foremost. The dwell tach is likely reading wrong, but I'll bet it's reading consistantly wrong. The reason for this is one of two things I'd imagine, and they both stem from that fact that your dwell tach works by counting the electrical impulses of a given spark plug (instead of actually measuring the RPMs of the Crankshaft directly).

The Tach assumes that your engine is a typical 4-cycle (Suck-Squeeze-Bang-Blow) engine. In which case your spark plug would fire once (at the top of the compression stroke, starting the power stroke).

Our engines, however, don't do that.

They fire twice on each 4-stroke cycle of each cylinder (there's a second "Wasted Spark" on the Exhaust stroke).

Odd little beasties aren't they?

Because there's twice as many sparks, the dwell-tach thinks the engine is spinning twice as fast as it really is.


Now, all of the above could be completely wrong, I'm a Quantum Mechanic, not a Greasemonkey (I teach physics for a living), but it's a rather educated guess. If I am wrong someone please teach us both what's up as I'm very eager to learn

[Zeke]

Hey Chris:

You're almost exactly right.  I just replied to my other thread regarding SOHC and dwell tachs.

My meter is reading exactly half of the RPM it should be reading.  So you're right, at least it's consistent.  With my new dwell tach, it's actually a help because where each mark WAS 20rpm, it's now 10rpm -- pretty cool, eh?

Thanks,

Zeke

[Zeke]

OK, still going on.  I have read all the pertinent FAQs but most of these are written from a "previously running" perspective as opposed to a complete rebuild. 

Here's some simple questions:

Can I properly set the pilot screws with the choke on?  If not, why won't my bike run without the choke?  What would you try?

My idle isn't dropping when I turn in the #1 pilot screw.  Why not?  Is this because of the choke?

I've tried to test ride the bike -- when I take off from idle the exhaust pops wildly -- what causes this?

I readjusted my CAM chain today just in case that is a problem.  I'm also going to recheck valve clearance.  I tried to check timing today but the advance is a little rusty and so I couldn't see which mark was lining up.  I never touched it so I can't imagine it's too far out.  If anyone's experience says I should time it I will take the advance out and shine it up.  Other thing is I'm not sure you can properly time it if the idle speed isn't set correctly.

When I rebuilt the carbs I didn't change anything.  The jetting is the same as it was before, the pipes are the same.  The diaphragms and accelerator pump were all in good condition.  There wasn't any gum in there, and I blew everything out with compressed air.

C'mon, help a SOHC brother out!!

[SteveD CB500F]

Does your choke have a throttle lifter to increase the idle when cold.  If so then the idle won't drop (will it?).

On the older bikes (llike mine), there was no lifter and it won't idle when cold at all without holding the throttle open. My later 550 had this "modern" system and I could start it up on the choke and let it idle while I completed putting the helmet on.

[Zeke]

Does your choke have a throttle lifter to increase the idle when cold.  If so then the idle won't drop (will it?).

On the older bikes (llike mine), there was no lifter and it won't idle when cold at all without holding the throttle open. My later 550 had this "modern" system and I could start it up on the choke and let it idle while I completed putting the helmet on.

Hi Steve:

Yeah, it does have that -- but I've pushed in the choke enough for it to drop off the lifter, so I can adjust the idle with the screw.  Problem is, that it won't idle smoothly below like 1400 rpm, and not without choke (maybe half?).  Maybe because it's not broken in yet? 

Thanks,

Zeke

Update:

I rechecked/adjusted valves this morning -- only two were a tad loose, none were tight.  I decided to check the timing stuff out.  Advance was pretty rusty and I was convinced that was causing problems, so I removed it, disassembled, cleaned, lubed, put back in.  I did a static timing.  I tried dynamic but the timing mark doesn't show up in the window when I shoot it with the timing gun.

Runs same as before.  Popping out of the #4 pipe when I try to take it off choke.  I'm getting really frustrated.  I'm usually very good with engines, but I've never done a multi-carb bike before.  I didn't change anything with the carbs other than the seals.  They are clean, and were clean before I started.  I'll try the pilot adjustment again but since I can't run it without the choke on partially the mix doesn't seem like it'll be right anyway.

[Zeke]

Last Update of the weekend.

Really hoping that somebody reads this monday morning at work -- I'm running out of ideas.

I called a friend of mine back in MI -- he know engines very well and owns 4 of them, but none SOHC honda.

We talked about the pilot circuit.  Basically, he told me there practically isn't a motor in the world that won't run at least OK with pilots approx 1.5 turns out (Of course, all other things being in good shape).  Even warm, I can't take my bike off choke or it dies.  He feels that pilot jets could be clogged.  So I agreed to drop the float bowls and check them, as well as to check compression.

As I already wrote, everything else has been checked already.  Timing, Valves, Spark Advance.  Compression checked out at about 140 psi -- it's a fresh rebuild, so that should be good enough.

Dropped float bowls, and removed pilot screws.  Poked a wire from a bread tie into the pilot jets, and used some spray carb clean to blow in there as well.  I did get some back-blow from the carb clean, but I assumed that's because the orifice is so small rather than a blockage.  Hell, anyway I did the best I could do sitting on the concrete for an hour and a half under my bike.  Put the pilot screws back in, turned them out to the manuals specifications.  Replaced float bowls.

Started right up on choke, as it has been all along.  Let her warm up for 10 minutes or so, hit the throttle a few times -- seemed a tad more responsive than before.  I started to drop the idle a bit, and tried to take off choke.  #4 starts popping again.  At this point, it was getting dark and after two days in the garage working on the same problem I decided to call it a day.

One last detail....#1 and #4 plugs are black, and #2 and #3 are brown.  If the pilots are adjusted the same, shouldn't the plugs be the same color?  I have looked at the spark and it seems good enough to me.

I really hope somebody can look at all the info provided here and give me a hint as to what is wrong.  I'll pull the carbs again if I have to but they seemed plenty clean when I put them on.

Thanks,

Zeke

[Gordon]

Sounds to me like you still have one or more clogged carbs. 

[TheRyanLilly]

 You are running Lean.(not enough fuel,  too much air.) This is why you get popping on cylinder four, also why you can only run with choke closed. Check your fuel supply. Petcock, Filters, fuel lines. If the supply is slowed this will happen, although the problem may be only one or 2 cylinders, number four is probably worse than the others if it is the only one popping. in that case i think there is a fuel blockage somewhere inside-needle valves, jets, float sticking/sinking(has a leak and is filling with gas?) I know its a pita, but pull the carbs, dissasemble and clean. Always clean your sparkplugs every time you make a change to the carb, if you fix the carb problem but have fouled plugs it will still run like crap and you may think that its still a fuel problem, but really its just the plugs. hope this will get you headed in the right direction.

One more thing, vacuum leaks, if there is a poor seal anywhere it can cause this. Spray carb cleaner or starter fluid around the intake and carb while running (cold engine, fires are bad) If you here an rpm change you have a vacuum leak. you may have tried all of this before; im just putting thi info out there mabe it can help some one else if not you.

[Harry]

Also, tighten your exhaust studs as slightly loose pipes can affect running a lot. If 1+4 are black, 2+3 tan you NEED to recheck your timing w. a strobe light. Typically, if you adjust 1+4 just right, then 2+3, when you get back to 1+4 their timing will have changed! Changing the one side affects the other side. Also, tightening the base plate affects the timing, very evident when using a strobe light. How are your condensors - is there any sparking at the points?

When you take choke off, it dies? Are you sure your idling adjustment screw was set on a warm engine - what happens if you screw the idle adjuster in until bike revs at say 3K (assuming you have the choke w. throttle lifter system) with choke on (warm engine) and you THEN take choke off?

[Zeke]

Morning Guys, thanks for the help.

Sounds like you all are in agreement with blockage.  I guess I'll be taking them out for the 4th time, and checking fuel delivery.  Intake boots are brand new, but I'll check them for leaks anyway.  Exhaust studs are tight.

I haven't tried turning in the idle screw to 3Krpm, but I went down the street at about 3K and tried pushing in the choke and it seemed to keep running though not smoothly and then when I slowed down it died right out.

My bike has CDI ignition -- no points -- only SOHC I guess that does.  OK, I did try to strobe time it, but ran into trouble.  I have it hooked to the #4 plug per Honda instructions.  The 1-4 timing mark doesn't show in the window though.  Why not?

   Does the idle speed have to be exactly right for strobe timing process?  With the "extra spark" (1 and 4 spark at the same time) is there something else I need to do to use an inductive timing gun?  Can you explain why I need to time 1-4 and 2-3 separately -- my CDI pulsers are tied together on the same plate, so I think you can time everything at the same time.

Thnanks,

zeke

[Harry]

I haven't tried turning in the idle screw to 3Krpm, but I went down the street at about 3K and tried pushing in the choke and it seemed to keep running though not smoothly and then when I slowed down it died right out.

Aah! Something tells me that your idle adjustment screw is not set. Start with this first. If your bike aint idling properly you wont see the timing marks either.

[Zeke]

I haven't tried turning in the idle screw to 3Krpm, but I went down the street at about 3K and tried pushing in the choke and it seemed to keep running though not smoothly and then when I slowed down it died right out.

Aah! Something tells me that your idle adjustment screw is not set. Start with this first. If your bike aint idling properly you wont see the timing marks either.

Harry:

It's true my idle screws aren't properly set -- that's the problem.  Can you check out the early posts in the thread, I talk about this there.

I detailed the procedure (above) my honda shop manual states for adjusting the pilot screws -- but it doesn't say if it's OK to do it with the choke on.  My bike won't run without choke no matter how the screws are adjusted.

Do the older bikes use this same "idle-drop" procedure for setting pilot screws?  Maybe not. 

This crazy procedure was designed to allow dealers to set the idle screws, then glue tamper-proof caps to the screw so that the owner can't readjust them -- apparently some kind of anti-pollution thing?  So, anyway, I ground off the tabs so I can adjust them just like you would on any other SOHC.

The big question is:  all things being equal, should the bike run well enough with an initial pilot adjustment (meaning 1.5 turns out, adjusted at rebuild) that you should be able to take it off choke when it's warm?

[Harry]

Harry:

It's true my idle screws aren't properly set -- that's the problem.  Can you check out the early posts in the thread, I talk about this there.

I detailed the procedure (above) my honda shop manual states for adjusting the pilot screws -- but it doesn't say if it's OK to do it with the choke on.  My bike won't run without choke no matter how the screws are adjusted.

Do the older bikes use this same "idle-drop" procedure for setting pilot screws?  Maybe not. 

This crazy procedure was designed to allow dealers to set the idle screws, then glue tamper-proof caps to the screw so that the owner can't readjust them -- apparently some kind of anti-pollution thing?  So, anyway, I ground off the tabs so I can adjust them just like you would on any other SOHC.

The big question is:  all things being equal, should the bike run well enough with an initial pilot adjustment (meaning 1.5 turns out, adjusted at rebuild) that you should be able to take it off choke when it's warm?

Hey Zeke - you are talking about the idle air adjustment screws for setting mix, I am talking about the knurled knob for setting idle speed! Turn this clockwise until your bike will idle (albeit roughly) without choke. This may mean you will idle at 1500 rather than 1000rpm, this is where the fine tuning with the idle mix screws you talk about should take place!

[Zeke]

I have set the Idle screw.  But, even if I was to turn it to 2000 rpm it still won't run off choke.

I'm hoping that you guys are right about the jets being clogged somehow.  I'm taking them off 1 more time for a cleaning, and I'll try again.

If that doesn't do it, then I'll have to take her to a dealer because the carbs have to be the problem.  Everything else is as it should be, near as I can tell.

I'm going to use some wire, some spray cleaner, and compressed air -- anything else?


Zeke

[kghost]

Don't forget the carb body itself. Lots of little passages. Might try soaking them in one of the one gallon bucket of carb cleaner (berryman). Then blow air and carb cleaner (Can) through all the passages.

[Zeke]

Don't forget the carb body itself. Lots of little passages. Might try soaking them in one of the one gallon bucket of carb cleaner (berryman). Then blow air and carb cleaner (Can) through all the passages.

Took the carbs back off again last night.  I will disassemble and clean one more time, but I'm not gonna dip them again.  They ran excellent before the valve guide broke -- the only reason I cleaned them at all was to get the oil from cam cover leak off the carbs and give them fresh o-rings.  So they've been cleaned like hell and bead blasted.  When I blasted them all the holes were plugged and I cleaned them again like crazy, blew out all the orifices with carb clean and compressed air.

Entertain me on this thought -- how important is it to have the EXACT float height measurement? 

When I was rebuilding them, I felt that the picture in my manuals was pretty vague about how to adjust them.  So I asked here and Bob W. showed me a picture of adjusting a 750 float which is a bit different.  My floats are more rectangular shaped than round.  The 750 measurement was something in the 20mm range, measured from the carb body to the BOTTOM of the round float.  My manual calls for 14.5mm, and I wasn't sure if I should measure 14.5mm to the TOP of the float OR the BOTTOM.  At this point, without looking again I can't be sure what I did.

I adjusted the first one, because it wasn't 14.5mm.  After doing so, it looked much different than the others, so I decided to leave them alone.  Lo and behold, when I put the carbs back in my bike the #4 bowl was empty.  All the rest seemed fine.  So I removed the bowl from #4 and carefully removed the float.  I worked it out in my mind and bent the tang so that when hanging, some gas would come out when I turned it on.  I couldn't easily measure it on the bike and it was kinda dark, and of course I was over-eager to start the bike.  Put the bowl back on, turned on the gas, and she started right up!  YeeeHaaa, right?

OK, so what happens if your floats aren't right?  I know the two extremes -- adjust too low and you get no gas.  Adjust too high and it comes pouring out the carb.

But what about in between?  Is this critical?  I see that the main jet is a good deal deeper than the pilot.  The pilot needs to be submerged, right?  I guess what I'm wondering is whether or not the popping of #4 could be attributed to the gas level of #4 bowl being too low?

Thakns,

Zeke

[Robert]

Zeke, are you shure about the 14.5mm float height? All my available literature on the Cb650 (C) says 12.5mm.
Float height affects carburation pretty much (lean/rich). If it is only a tad off, it is possible to correct with pilot screws but not when it's completely off.

[Zeke]

Zeke, are you shure about the 14.5mm float height? All my available literature on the Cb650 (C) says 12.5mm.
Float height affects carburation pretty much (lean/rich). If it is only a tad off, it is possible to correct with pilot screws but not when it's completely off.

Robert,

Thanks for your reply.

It probably is 12.5, I was going from memory (and not a good memory).

Well, if you are correct then we might have found the source of my problems!

Now the real question is, is the 12.5mm measurement to the TOP of float (edge closest to carb body) OR to the BOTTOM of float (edge closest to bottom of float bowl.

If I can get an answer here we might have a winner!!

Thanks,

Zeke

[Bob Wessner]

See this link, it might help. Bare in mind, your carbs and floats will look different, and he explains why the angle he is holding the carb body in is incorrect. It should be tilted slightly so the float tab is 'just' barely touching the tip of the float valve to measure. Hope this helps.

http://www.salocal.com/sohc/tech/carb/asmpg_mgs/adjsttang.htm

[Zeke]

Hi BOB!

This is the picture you posted earlier to my inquiry.

Thing is, it's true that my floats are alot different than these.  So I don't know if the 12.5 mm (being a much smaller number than 20+ ) is to the top or bottom.

I'll try to make an educated guess as to which side it is,  OR, if any of you have had your 650 apart maybe you can tell me.

Thanks Again,

Zeke

[Bob Wessner]

Hmm, bad memory, forgot the earlier post. Unless the 650 is different in some way, the picture was really to show where the measurement is taken, from the carb body edge where the bowl attaches, to the bottom of the float, that is the bottom as the float would be sitting on the bike.

[Zeke]

Bob:

OK, thanks.  I'll try 12.5 mm from BOTTOM of float to carb body and see if that looks crazy.  Man, the contrast of the pic in my Honda manual is so bad you really can't tell -- the honda tool is also black!

I hope to god this works before I set her on FIRE!!

Zeke