125cc issues downhill or at mild cruise

T

TNStreamerDreamer

Member
Hey Gang,

My K1 125cc motor swap has really good throttle response and run smooth from 0 to WOT on the accelerate, really don't get a terrible bog when I punch it or creep it up. The only problem, be it what I feel a serious one, is that downhill or cruising scenario in the mid range.

I've got great throttle response through acceleration and holding WOT.
HOWEVER, the engine vibrates so hard and really just sounds like it wants to rattle to death when I get to a cruising speed or go downhill and maintain that middle of the road throttle.

Does amount of flow through the carb correlate to engine RPM? I'm wondering if I could have some mild vacuum leak if the engine is creating low pressure but the throttle is keeping normal flow through the carb restrictive so it starts pulling air in from around some edges in this situation?

I'm a bit baffled. I feel a little bit of the same effect on my CT with a 140cc but it isn't as severe.

Any diagnosis? Thanks again for all the help.
 
cjpayne

cjpayne

Well-Known Member
Has it always done this since the motor swap? If so, sounds like the rotating assembly(crankshaft/flywheel/clutch) is out of balance.
 
T

TNStreamerDreamer

Member
cjpayne said:
Has it always done this since the motor swap? If so, sounds like the rotating assembly(crankshaft/flywheel/clutch) is out of balance.
Click to expand...
I really don't recall it being an issue before the swap.
I hadn't really considered this to be an issue though. I'm not too sure how to check this since it runs smooth on accel and seems to roll fine. I'll try and ensure the issue doesn't persist on engine braking. I could see that pointing towards the imbalance you mention here.
 
T

TNStreamerDreamer

Member
I tightened it all up. No luck. I feel I did get a little better result from richening the air fuel mix screw. I feel like I need maybe more mid-range fuel but I'm on the richest needle and I'm at my upper limit for jet size. Pretty wild...

When I pull the clutch, the vibration and noise stops. I'm going to try a different carb that is not tuned in as well and see if I at least get the same issue out of that. If I don't...I'll bite the bullet and get a genuine Mikuni.
 
6

69ST

Well-Known Member
Something about this isn't quite connecting all the dots, for me. With carburetion, it's usually either a steady-state misfire, often with strong low-rpm throttle response and easy cold-starting...when overrich; or, bogging/stumbling, loss of power, up to a complete dead spot, when lean. What you're describing sounds more like an engine issue.

Try changing the jet needle height, by repositioning the C-clip to a lower groove (makes the mixture richer) or higher groove (leas-out the mixture)...see if that makes a difference. If you have the right size main jet, you should be able to go too far in both directions. No difference...time to do some cursory testing of the rotating assemblies.

Can you replicate the vibration, with the trans in neutral? How about with the trans in-gear, wheel off the ground or chain removed (which is much safer for static testing). Really, you're making me think it's either chain whip, or a clutch problem.
 
T

TNStreamerDreamer

Member
racerx said:
Something about this isn't quite connecting all the dots, for me. With carburetion, it's usually either a steady-state misfire, often with strong low-rpm throttle response and easy cold-starting...when overrich; or, bogging/stumbling, loss of power, up to a complete dead spot, when lean. What you're describing sounds more like an engine issue.

Try changing the jet needle height, by repositioning the C-clip to a lower groove (makes the mixture richer) or higher groove (leas-out the mixture)...see if that makes a difference. If you have the right size main jet, you should be able to go too far in both directions. No difference...time to do some cursory testing of the rotating assemblies.

Can you replicate the vibration, with the trans in neutral? How about with the trans in-gear, wheel off the ground or chain removed (which is much safer for static testing). Really, you're making me think it's either chain whip, or a clutch problem.
Click to expand...

In neutral I was having trouble really distinguishing it. I didn't notice any variance from the issue when I tightened chain so I moved to running a different carb. Same issue. Cruising under light load around 4800-5400 rpm in all gears seems to cause the issue. In neutral it really sounded to be the same case but so much is going nuts when revving neutral I couldn't be totally sure.
 
6

69ST

Well-Known Member
I've tested a few bikes with the rear wheel off the ground...the swingarm supported with a paddock stand. Kinda difficult to see chain whip whilst riding.

You might temporarily reinstall the old, known, carburetor. If that makes no difference, then you may have a torque-sensitive harmonic vibration...I hope not. If it"s in the engine itself it could be a mutha to track down & fix.
 
T

TNStreamerDreamer

Member
racerx said:
I've tested a few bikes with the rear wheel off the ground...the swingarm supported with a paddock stand. Kinda difficult to see chain whip whilst riding.

You might temporarily reinstall the old, known, carburetor. If that makes no difference, then you may have a torque-sensitive harmonic vibration...I hope not. If it"s in the engine itself it could be a mutha to track down & fix.
Click to expand...
Not thrilled to hear that!
It seems to happen at the same room range with no load with two different carbs.
Here's the video of the link works:
 
6

69ST

Well-Known Member
Kind of a coin- flip as to where to begin the diagnosis. I think you'd feel better about checking the bike for something loose...could be the wheel sprocket (worn dampers, hub, sprocket bore), c/s sprocket, seized chain links, loose motor mount or swingarm pivot bolt.

Sounds like the right frequency for a chain with seized/seizing links. Pull the chain and check for an arthritic section
...an easy test.

The next step is pulling the clutch cover to inspect the clutch & primary drive.

I'll stop the guesses here, for now...before it gets out of hand.
 
T

TNStreamerDreamer

Member
OLD CT said:
Are you running the stock countershaft sprocket that comes with most china engines? BTW, they are not the correct 420 sprocket. OR you have a very worn out chain and sprockets.
Click to expand...

My stock 125cc came with a 428 sprocket and I swapped it for 420 sprocket(s).

I am hopeful that it could be the chain. I have about 30 hours on the chain but it is a real cheapy. Also, I cleaned then lubed it with gear oil as I do for my big bike o-ring chain. I'm not so sure that was the right move. (I was going to use chain wax on the non o-ring chains from now on unless you have other suggestions?)
I will report back after swapping the chain out for a DID 420 I have fresh in the box. Really hoping to not have to crack this one open.

Thanks!
 
6

69ST

Well-Known Member
You'd be lucky to get 1000 miles from a cheap chain if the sprockets are new and all 3 items are meticulously maintained. Otherwise, service life will be shortened. A worn chain will eat sprockets...and vice versa.

Heard through real speakers, it sounds like a chain + sprocket issue. Before you take the chain off the bike, try lifting it off the rear sprocket...midway along the seated length. If you see ~1.5mm of daylight between the sprocket & the bottom of the chain, the chain is worn out. How's the wheel sprocket look? Do the teeth have a wear pattern that sits anyplace other than the base of the teeth? Are they curved/hooked/angled in the direction of wheel rotation? Those are all indications of a worn out sprocket. With the chain off the bike, place the two open ends together, then move the chain so that each link has to move on its pin. If there's a seized link, you should see it. The noise that I'm now hearing is consistent with a dying chain...and the right frequency for the speed you were traveling.
 
T

TNStreamerDreamer

Member
racerx said:
You'd be lucky to get 1000 miles from a cheap chain if the sprockets are new and all 3 items are meticulously maintained. Otherwise, service life will be shortened. A worn chain will eat sprockets...and vice versa.

Heard through real speakers, it sounds like a chain + sprocket issue. Before you take the chain off the bike, try lifting it off the rear sprocket...midway along the seated length. If you see ~1.5mm of daylight between the sprocket & the bottom of the chain, the chain is worn out. How's the wheel sprocket look? Do the teeth have a wear pattern that sits anyplace other than the base of the teeth? Are they curved/hooked/angled in the direction of wheel rotation? Those are all indications of a worn out sprocket. With the chain off the bike, place the two open ends together, then move the chain so that each link has to move on its pin. If there's a seized link, you should see it. The noise that I'm now hearing is consistent with a dying chain...and the right frequency for the speed you were traveling.
Click to expand...
You nailed it... That chain was long beyond gone (and also a #41). I installed a quality DID 420 and it cleared up! I feel so silly.

Also, now my (slightly wider) chain is rubbing the rear swing arm when I'm cruising (oddly not at low speeds or on the stand). I didn't have the OEM spacers when putting the rear axle together so I'll have to revisit that setup.

Now on to why my 140cc won't break 60 mph... I'll make a new thread for that one. Thanks again guys and @racerx .
 
6

69ST

Well-Known Member
You shouldn't feel silly, unless you repeat the same mistake. Many people wonder how the "experts" came by their expertise. The answer is: "like this...by trying new things and learning from them, i.e. what works and what doesn't". Repeating that basic process with enough different elements of building/tuning, filing away the info (mentally) can grow into a large database, given enough time & projects.

FYI, once total chain "stretch" exceeds ~0.500" (beyond the initial stretch you'll get in the first 50 miles, or so) it's time for a new chain. The only true "stretch" occurs in the first few miles; after that, it's wear. With an 88L chain (typical with the sprocket combos used with 110cc & larger displacement), that works out close to 0.006" per link...enough to change link-to-link spacing, and begin eroding sprocket teeth.

Where, exactly, are you getting chain contact? What size sprockets, what swingarm & rear hub are you running?
 
T

TNStreamerDreamer

Member
Running the original hub and rear swing arm. I suspect this would not be an issue with a large sprocket but the 31t puts the chain near the weld for the rear shock mount.
 
6

69ST

Well-Known Member
IDK, something seems "off" about this. I ran 17/31 on my daily rider and there was no clearance problem with the stock swingarm. Only chain contact was closer to the pivot and because of that, I went to an aluminum swingarm, fitted with Delrin plastic rub strips. Kinda wondering if you have the rear wheel spacers swapped L-for-R. Been a long time since I checked but, from memory, there's ~1mm difference. And when the chain is just clicking the swingarm, a "free" millimeter of added clearance can make a huge difference.
 
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