Topic: CB125S Engine HELP

[natez]

I had a perfectly good running cb125s which I decided should have a 145cc cylinder and piston on it because at the time I was bored and needed something to do. Got the engine back together and back in the bike and for the life of me I cannot get it to run right. Starts and idles perfectly and has great power until exactly half throttle when it starts to run like it is hitting a rev limiter. Absolutely no power, missing, but no smoke or anything. Does it in neutral and under power. Carb is clean as a whistle, originally had a 95 main jet in there but have tried everything from it to a 125 with no change. Valve clearances are dead on, points are set, I believe the timing is right. No airleaks on the carb or anything. Ignition advance is working. Can anyone help? Its driving me nuts :/

[grcamna2]

What exhaust you running ?

[natez]

No idea. Looks kinda like a small supertrap with the baffling removed. Ran fine before I did the cylinder with it.

[b52bombardier1]

Does giving it a dab of choke when it bogs down help at all? If yes, its still lean.   It is possible the throat size in your carb is simply too small for this much of an increase in displacement.

Rick

[natez]

No, choke does nothing, if anything it richens the mixture up a bit and it starts to smoke a bit, but doesn't change the bog/miss. It's not a "bog", more of a steady miss. Exactly like standing on the gas pedal in your car would produce once you hit the rev limiter. Does it exactly at half throttle to full. Does it on the bike moving, or if your sitting there in nuetral at a stop. I'm pretty sure others have gone from 125cc to 145cc, or even 200cc with the same carb? GRCamma2 can you chime in here I think you did it right?

At the point now that I think I'm going to have to just toss it. Don't know what else to do.

[grcamna2]

I used the stock carb. w/ a main jet of around #100 w/ a restrictive stock muffler.What carb. are you running ? and what color is your spark plug ?

[natez]

Mikuni 26mm (or is it 28, I always forget?) Whatever came with a cb125. With jets between 100-110 plug chops produce a plug that looks exactly like what you would want. Anything above that the plug comes out alittle darker, wetter. The miss/bogg does not change at all with any jet size. It's not fouling. There is no smoke (alittle smoke with a 125 jet). I think I have an ignition issue... perhaps the bigger jug needs a better spark and I have uncovered an ignition issue that was hidden by the smaller jug. My headlight has always been really dim and I never cared.

If I didn't set the timing quite right when I put the timing chain on it wouldn't run property at all? It Idles and runs through zero to just under half throttle better than it ever has. Such an F'ing tease I just want to ride this thing.

[Don R]

 I put a cb360 stator in a cl100 with a soid state reg/rec. I fried the coil and put one on with a ballast resistor in front of it. I used a gel battery standing on end and as long as I owned it it ran and charged great. It had an H4 in the headlight too.

 I got a wild hair and sold 4 bikes, it was one of them.

[natez]

Ran the bike with a timing light today and its dead on.

Any more help? My carb is clean as a whistle. Float high set correctly, etc.

grcamna2?

[grcamna2]

Ran the bike with a timing light today and its dead on.

Any more help? My carb is clean as a whistle. Float high set correctly, etc.

grcamna2?

That's a tough call to guess over the computer.. but the first place I would continue to investigate is electrical first.

[natez]

I feel like I would have seen the timing light start to miss as the engine started to though if something was amiss there? I even took my bulbs out and ran it with a 12V battery. Started and idles WAAAY better but still loses power past half throttle. Like I said I confirmed timing is dead on using a timing light and advances correctly. Perhaps my carb is too small but your using basically the same carb right grcamna?

[grcamna2]

Yes,same,stock carb.
 I had a similar problem when i tried advancing my valve timing 2degrees;I bought an adj. camshaft sprocket from Megacycle and adv. 2 degrees and that's exactly what it did,but only after 1/3 throttle.I think you should remove the points housing and double-check the cam timing marks.The other problem that can cause this is having a 'wiped-out' cam chain which needs to Not be worn or stretched-out;did you already say you replaced that along w/ checking the alignment of the lower cam sprocket on the crank? The small cam sprocket(on the crank) needs to have the tooth exactly in the center/inline with the middle of that woodruff key-way behind the alternator rotor.When they press that little sprocket onto the end of the crankshaft and do their best to keep that tooth in the center of the woodruff key-way it can be sort of 'hit or miss'.I have had to 'slot' my upper camshaft sprocket just a bit on one of my engines to keep the valve timing directly inline to compensate for the lower crank camshaft/timing sprocket which was a bit out.
The index/alignment 'notch' in the head needs to have the small '0' in the large camshaft sprocket exactly in the center of that notch for correct valve timing;in that position(TDC)the 2) camshaft sprocket bolts/bolt-pattern will be both inline pointing straight up at that notch in the head.

[tbpmusic]

Check your compression - maybe you broke a ring or something while assembling.........

[natez]

Now I feel like we are headed in the right direction.

Got a solid 155psi compression

Grcamna2, couple of notes on your great post. Perhaps we can get to the solution. I really appreciate the help, its so frustrating not being able to get knowledgable help locally.

I put a brand new timing chain on. If you remember a few months back i ordered the wrong part number and got one one link too big. Ordered the right one and man it was really tight. I barely, i mean just friggin barely got the sprocket on the cam. I was going to post about it but then figured what the hell, it will prob stretch out.

I did not check the alignment on the lower sprocket... book didnt mention anything about this, also i didnt take it off the crank or touch it when i took the engine apart so i didnt even think about it.

What do you mean by "sloting" your upper camshaft?

I dont remember exactly why i did this, but when I put the sprocket in the chain and lined it up, the "O" mark was on the bottom, meaning the sprocket was 180 degree out, but everything including the sprocket bolts was lined up with their respective index marks (i think i left it because its such a pain in the ass to fiddle with the sprocket and chain in there)... its a perfect circle, and the index mark on the rotor lined up with the T and the cam was where it was supposed to be (believe the cam lobes were pointed down as to not push against the lifters)  so I said what the hell and bolted it up.


Thoughts on this info? Based on what you said it does sort of seem like the timing chain/sprocket/cam is off just a bit doesnt it? (Is there a name for that timing vs the siming set by the points plate/advancing mechinism so we can talk about each without getting the two confused?)

[grcamna2]

The '0' mark on the upper camshaft sprocket is a reference mark and I think it's very difficult to attempt to correctly set the valve timing w/o using it in the correct position;your valve timing might be one tooth off.. I would remove the 2) bolts and get that '0' lined up at that notch in the head while the 'T' mark(top dead center on the compression stroke)is set on the alternator stator reference mark.Does your cam chain tensioner loosen and tighten correctly? if it was me i would want to check all these things. Do you have a good service repair manual? I like the OEM Honda book myself.

[grcamna2]

The '0' mark on the upper camshaft sprocket is a reference mark and I think it's very difficult to attempt to correctly set the valve timing w/o using it in the correct position;your valve timing might be one tooth off.. I would remove the 2) bolts and get that '0' lined up at that notch in the head while the 'T' mark(top dead center on the compression stroke)is set on the alternator stator reference mark.Does your cam chain tensioner loosen and tighten correctly? if it was me i would want to check all these things. Do you have a good service repair manual? I like the OEM Honda book myself.

Slotting the camshaft upper sprocket bolt holes is for minor adjustments in camshaft timing once you already have the camshaft correctly set on both the lower and upper cam timing marks by the book,you could proceed to do this to 'fine-tune' your camshaft timing a degree or two according to instructions on aftermarket performance camshafts..

[natez]

Well, I flipped the sprocket around, and checked the cam chain tensioner operation. Fired it up and no change at all. I have a Clymler manual. Not the best but everything is checked/followed in the book (especially now that the sprocket is positioned properly). Thing idles and revs so beautifully until just at or just below half throttle, then just starts to fall flat.

Should I try positioning the sprocket one tooth either side of the "0"/Index mark and try that or just trash it at this point?

[grcamna2]

TDC on the stator mark and then the '0' is at the top index mark on the head center-top w/ the cam chain adjusted? if so that eliminates that as a cause.I wouldn't trash it but just keep trying to diagnose the problem.
Can you post any pics  of the sparkplug color,eng. in general,etc. ?

[natez]

welp... was messing with the needle and put the collar on the very top position (leaner) and ran it and it ran alittle better... put it all the way at the bottom and it ran worse, hmm... Put it back at the top and then stuck my 95 main jet, which I was running with the smaller cylinder and holy cow it ran right... not perfect, but damn close considering. Went back to a rich needle with this jet, ran way worse again, put it back to the top leanest setting. It almost seems like I need to drop to a 90 main jet (bike idles very lean and can't adjust it properly with the pilot screw with the needle setting all the way on the top, in the middle it tunes and idles fine so I want to drop another main jet size so I can put the needle clip back to the middle setting). So... I guess that was it the whole time. The plug did not look rich at all, and there was no smoke to speak of but regardless, I put a bigger cylinder and piston on the thing wouldn't you think it would need MORE fuel than before and not less??

[tbpmusic]

While it's running down the road, slip it a little choke - if it runs better, you're lean.
The interaction between jet and slide needle can be counter-intuitive sometimes.
Don't just assume that rich=smaller jet to fix. Sometimes larger jet plus leaner needle will fix a rich condition.
Conversely, sometimes a smaller jet plus richer needle solves a lean condition.
Intuitively, you'd think more displacement might require a larger jet, but maybe not.........

[natez]

Put a 90 main jet in, runs as good as it has since I did the big bore. I can pin the throttle and it pulls hard to about 80-85% RPM (engine RPM, not throttle position this time. Before it would do it at any RPM dependent on throttle position past 1/2) and then I start to get alittle of that burble again. They don't make a main jet smaller than a 90. I have a pretty free flowing exhaust, if I put something alittle more restrictive on it, will that richen or lean the air mixture at WOT?

[grcamna2]

Put a 90 main jet in, runs as good as it has since I did the big bore. I can pin the throttle and it pulls hard to about 80-85% RPM (engine RPM, not throttle position this time. Before it would do it at any RPM dependent on throttle position past 1/2) and then I start to get alittle of that burble again. They don't make a main jet smaller than a 90. I have a pretty free flowing exhaust, if I put something alittle more restrictive on it, will that richen or lean the air mixture at WOT?

That sounds like you set the fuel mixture a bit leaner changing that MJ;a more restrictive exh. usually makes the mixture richer.

[natez]

I don't understand what you mean. Yes I meant to lean it out by going from a 95 to a 90.... Seems like now I need to go down one more (they don't make one smaller than a 90) or I am indeed being intake air restricted and it is richening up a bunch once it hits a certain number of RPM's, as I mentioned it pulls hard at full throttle until whatever RPM it starts to burble at, where as before it would bog at any RPM anytime the throttle was past half. Just weird to me that you had to go up to a 100 MJ and I had to go down and I have a freeflowing exhaust grcamna2.

[grcamna2]

I don't understand what you mean. Yes I meant to lean it out by going from a 95 to a 90.... Seems like now I need to go down one more (they don't make one smaller than a 90) or I am indeed being intake air restricted and it is richening up a bunch once it hits a certain number of RPM's, as I mentioned it pulls hard at full throttle until whatever RPM it starts to burble at, where as before it would bog at any RPM anytime the throttle was past half. Just weird to me that you had to go up to a 100 MJ and I had to go down and I have a freeflowing exhaust grcamna2.

Nate,
My 1981' CB125S came with an #88 stock but I thought that was too lean w/ Ethanol gasoline so I then installed a #90.I have it set up now where i buy jets from jetsrus.com in like #85,87.5,90,92.5,95,97.5,etc. in 1/4 size increments.That plus you can either adjust the clip on the needle or shim it a little w/ one .020in. washer for better low-midrange response.I think getting the correct size main jet for your application is a first priority.