Carburetor Mystery, Assist Req. Hi forums, My 1975 DT125B is having a strange carburetor issue: the bike will idle and start, but runs roughly and will not rev beyond 4000rpm, no power. The air adjust and idle screws do not affect this condition when altered. The carb is an original Mikuni of the correct type.  I’ll save the time by listing what I’ve done to the bike (what isn’t the issue and has been thoroughly checked) what’s been checked in the carb and what the unknowns are. Any advice or questions are appreciated!  Non issues:-Brand new top end, excellent compression, bore to piston difference measured and new piston is confirmed the correct size, honed, new gaskets-Decarbonized exhaust, new packing and gasket-New crank seal and intake gaskets, leak down tested @ 10psi, no leaks anywhere. Reed valves are in good condition-Brand new plugs (gapped and B8ES) and coil, confirmed excellent spark-Timed within .002”, points gapped properly and clean-Carburetor is completely clean, all jets/holes etc. free of debris and unclogged, float set correctly (21mm) and pivot pin polished, slide polished and unobstructed, needle polished and unobstructed (set to second clip position as per manual), bowl gasket and drain bolt gasket sealed correctly, o rings all accounted for, air screw adjusted as per manual-Clean air filter and unobstructed

-Unobstructed fuel line of the correct size The unknowns/possible issues:-Unknown tolerances for carburetor throat/slide differences (possible leak)-Unoriginal gasket on drain bolt potentially setting an incorrect gap or spacing

-Leaking carb top gasket

I once had the same model with the same problem.

What my mechanic and I found was that the exhaust was totally clogged up, and our cure was to saw it in half, clean out the gunge and weld it back together! If yours is blocked and you are able/allowed to where you live, then put the exhaust on a very hot bonfire and burn out the blockage.

Mine was kick start only model (I'm in the UK). If yours is an electric start model, I wonder if the governor assembly might be seized, or maybe the timing has been set incorrectly (read the manual to check).

Oh, and I assume that the cable is correct and the throttle slide (valve) is lifting fully?

Welcome, yes i'd still suspect exhaust in the tail section deep down where the spark arrestor is or should be.
An easy experiment would be remove the complete tail muffler out of the main exhaust, then see is it'll rev out.
If not then look down in where that tail muffler pipe fits in as i've seen them grow carbon right where the pipe ends.
Garden hose seems about the right size so if you can push one down past that point it should be good.
Tail muffler i'd put in the log fire for the night.
Did that on my suzuki quad runner alloy head once to burn off all the grease & cow dung & all i got back was two valve guides & two valve seats haha. Lucky i didn't put the cylinder in as well.
Video to hear what it's doing would be great. I just link mine through youtube but i guess there's other ways.
Any pics must come via an external source as site won't take then direct. Cheers from NZ--in fact i has one the same as yours 30 years ago & was a little rocket.
I think Advancer was mentioned so be sure it was held open to set timing at 1.8mm btdc. because if set on closed it'd have guessing about close to 4mm advance once running.
Sorry maybe you mentioned all that i'll read tomorrow.

Exhaust (as others mentioned) or ignition.

Seriously doubt it's the carb.  When things act up starting at a certain RPM, it's almost always ignition (if maintenance like decarbonization of pipe has been done).

A carb issue would be "throttle slide position dependent", not RPM dependent.

First thing on the 12v e-start 125's is the battery and charging system.  A weak battery will often cause a misfire at higher RPM on the generator models if charging system is weak or broken.  Try charging the battery fully and monitoring the voltage.  If the miss stops (temporarily) with a fully charged battery that is at 12v + at 4,000 rpm then you found the problem.

Other potential issues are condenser and ignition coil.

Re ignition, lots of fake and crap repro parts out there, including NGK spark plugs, so buy genuine or a known and trusted repro manufacturer (eg Daiichi) from a trusted source.

Hi all, thank you for the responses:

I’ll reply to everything in this message to keep everything organized.
P.s. my apologies for the horrid formatting on the original post, the forum didn’t adapt my spacing.

To eliminate the question of exhaust issues, I pulled off the muffler/spark arrestor and started the bike; no change.
Then I tried installing a known good and unclogged whole exhaust system (without muffler/spark arrestor) and no change either. The exhaust system I had installed prior was already thrown on the bbq!

The timing was checked post rough running condition, as a possible solution and was done properly as per the manual, to the T, with the advance armature held open. Prior, the timing wasn’t touched when it ran perfectly. I don’t believe it’s a timing issue. It ran roughly as described prior to checking the timing.
 
And yes, the slide works great (to full extent) and the cable is good as well. The timing assembly is not seized either.

Good point about the carb vs rpm issue identification, it’s not as if it suddenly stops working (and runs perfectly before) and won’t rev out @ 4000rpm, more or less it runs roughly up to 4k then will not go beyond. It also, bogs out after hitting 4k and decreases itself while the throttle position is unchanged. Another symptom is lots of smoke (as if built up).

To sow more confusion, when I tested an aftermarket carburetor I had as spare, it ran and stared perfectly, but actually over-revved right on start up and immediate went to 5000rpm, as it would with an air leak. 

Could someone provide a link to a decent and reputable aftermarket coil and plug? I buy most parts from Yambits, whom I presume to be trustworthy. Some stuff is genuine Yamaha others not. 

I forgot to address the battery: I have a good 12v battery which is fairly new, and was put on charge while trying the diagnostics as I know from experience the importance of a fully charged battery for the dynamo system. 

In conclusion, the aftermarket carburetor runs day and night differently from the Mikuni. Which is why I really believe it to be carburetor issues. Really can’t wrap my head around what though. 

Personally, I wouldn't trust any new repro ignition parts, either from Yambits or anywhere else - far better to use New Old Stock (NOS), or even used and tested, genuine or Daiichi.

I thought that one of Yambits' suppliers was a bit naughty in that they boxed their points in packaging with the same colour and design as Daiichi.

I buy my plugs from my local bike shop. 

We love videos. That'd have it diagnosed in no time--he said hopefully!!. Talking timing I wouldn't fully rely on timing marks being right if the timing tab is bendable but I don't suspect that's causing the issue at the moment. Should rev to red line easily in neutral. Could give it a quick rev with no exhaust if you have good neighbors & earmuffs. Check the crank seals left & right incase pushed out especially if lots of smoke with right side. Maybe I'm repeating myself but with timing if the crank key holding the armature is broken & armature slipped then timing could look right by the marks but be a long way from right so far as piston goes. 1.8mm on advance. I don't realy suspect that though. Sorry if i'm repeating myself, there similar post running with very similar problems.

Still sounds like it might be ignition to me.  I noticed you didn't mention actually MEASURING the voltage at the battery with it running?  Even newer batteries can go bad from vibration or whatever.  

While it may not be your battery, in my experience the best troubleshooting tool is an "open mind".  I wasted SO much time and money over the years because I'd dismiss simple things without really checking them properly...  like "it can't be the battery because it's almost new" (which actually happened to me years ago)...  Then I'd fire the "parts cannon" at the probelm and spend a lot of time and money...  only to eventually discover the problem was something I only half-ass checked because in my head I thought "it couldn't be that".

And now you've changed your description of how it's running.  A video would be very helpful.  (Upload to YouTube or somewhere and link here.)

Make sure your engine mount bolts are tight and/or temporarily run a jumper wire from engine case to a good ground on the frame.  (It you had the frame powdercoated or even painted, you may need to pull the engine and sand the paint down on a couple motor mount frame contact points to ensure good grounding)

Finally, if your dealer doesn't have a condenser, take your condenser to an auto parts store to and get an automotive one that has a long enough wire and similar bracket...  I'm in the US and like the early 70's Ford truck condensers...  and I use a name brand like Standard Ignition.  Condenser going bad will make it run rough and not rev.