Direct bikes DB50QT11, a 49cc with 139QMB engine, won’t rev By aaitken (Read 1,262 times)

[aaitken]

The problem.

My daughters Direct bikes DB50QT11, a 49cc with 139QMB engine, won’t rev up and has sooty plug.


The moped will start, with difficulty, often using starter fluid. It will then idle until warm, and may occasionally cut out, so needs a throttle tweak to keep alive. But it won’t rev to power. It partially revs up, but not really enough to move the back wheel when on the stand, and can’t propel itself forward even if you stand by it and rev it to the max.


The spark plug looks very sooty when pulled. I can wire brush this off, and it will be sooty again after a few minutes of running at idle.


I am getting I think white (not blue) smoke from the exhaust. It doesn’t smell oily.



Things I have inspected:

The plug is sooty on any inspection. It is an NGK and I have checked the gap, it is about 0.63mm.


Valve clearances. Inlet and exhaust are currently gapped at 0.10mm which I think is about 0.004”.


Compression: I was able to borrow a gauge and when warm I was getting up to 125psi on the starter motor. Only took a few seconds to peak there. I no longer have access to this tool, so won’t easily be able to repeat the measurement.


Sucking on the inlet vacuum hose will cause fuel to flow from the tank.


Choke unit (enricher) will extend out and retract when I connect it direct to a car battery.


Exhaust is sooty inside.


Drivetrain all looks to be free moving, I wondered for a while if the drivetrain was stuck, but apparently not.


The inlet elbow is not cracked. The spacer underneath isn’t great, it looks to have been overtightened in the past. It doesn’t have any cracks that go all the way through.
I've not looked at this before, so any cracking predates our ownership of the moped.

The vacuum lines to inlet and from airbox are dry, have no fuel in them. 




Things I have tried:

Closing fuel/air ratio screw fully (clockwise): still idles, still won’t rev up.


Opening up the current carb and blowing compressed air through all jets and tubes.


Swapping in a spare plug, probably the original one, not new. Makes no difference. Gap on this one is the same, 0.63mm.


Disconnecting vacuum lines, makes it worse, and unable to maintain idle.


Swapping in a brand new Keihin CVK PD18J carb and autochoke and makes no difference, difficult to start and won’t rev up.


Disconnecting air filter at the carb inlet improves throttle response very slightly.


Trickle charging battery to ensure best charge.


Disconnecting exhaust at the three bolts. Makes it louder, doesn’t help with revving up.


Spraying starter fluid around inlets and hoses, doesn’t affect running.


Reducing valve clearances down to 0.05mm, probably made it worse, in that it would rev up even less.



Other information.

This machine has history. When I first checked the valve clearances, because it was surging and cutting out, the clearances were hard zero.


I had a badly routed vacuum hose to the air inlet a while back which resulted in it cracking originally, so I shortened it and rerouted it. That then came off the connector at a later date so I replaced it with a 3mm internal diameter vacuum hose from the parts shop. This is a different internal diameter, the original was I think 4mm. I don’t think this, or the length of this hose is important. Could I be wrong about either of these?


As I have done a complete carb/choke swap I am ruling out jets, diaphragms, needle heights, floats and so on.


My daughter doesn’t live with me and so I got reports of problems with the machine by text message, so I don’t have all the details. After the last service I did, when I did valve clearances and fixed the vacuum tube, the machine ran Ok for a few days, then as she says, died, while she was out. She spent some time trying to start it. She got it going again, enough to get home, but it has behaved as now ever since, around a month.


I don't know where to go with this, hence joining the forum.  The plug and the improved running without air filter attached makes me think its running rich, but I haven't changed the jets.  Swapping in the new Keihin carb doesn't resolve the running issue, so if it's getting too much fuel still could that be a petcock issue?  It operates as expected, ie fuel does not flow until vacuum is applied to the air inlet hose.

[aaitken]

[aaitken]

Well 6 months have passed and guess what? I think it's fixed.

One of my early tests was to swap out the entire carburettor for a new Keihin piece, and it wasn't a successful swap. Which led me to believe there was no problem with the old carb.

However in the meantime, my advice from my learned step-dad, who has sadly passed away since this saga began, was that the valves probably weren't closing properly and I should remove the head and lap/grind the valves.

So I did that, which required purchase of a valve spring compressor for small valves. They were grim. As was the head.





I ground them a little, they didn't need a lot to get a shiny face to both the valve and the head.

Generally cleaned everything, the piston face was disgusting.



Inspected the piston rings, looked OK.

Replaced the gasket at the bottom of the cylinder block and the head gasket. Replaced the cracked spacer under the air inlet elbow and replaced the o-ring in there as well. Replaced the speedo cable.

Put it all back together and miraculously it started, and did not need any easy-start fluid to get going.

But the saga didn't end there. During the winter months, a slow puncture in the back tyre had let all the air out and at some point the side of the tyre had come off the rim. I got it reinflated but was then able to find the puncture. The tyre was close to the limit, so I could either put in a tube for £20 or replace it for £30, and as I intend to sell, chose to replace it.

Got all that refitted and then found, that even though it was starting, it STILL wasn't producing much power and wouldn't propel itself even without a rider

So...
Check the variator and the clutch. Had to borrow an impact gun to get those open. The variator ramp plate was pretty much stuck to the drive shaft so I don't think that was helping. But it wasn't the problem. I lightly sanded out the inner diameter of that plate to allow it some more movement on the shaft, and reassembled. Clutch plates looked good. Belt looked like it was in tolerance.

So everything was pointing back to the air/fuel ratio again, maybe there was an issue there. Taking off the airbox and using my fingers to restrict air into the intake gave better running, which I think means insufficient fuel getting in.

So I opened up the carb again, the old one was still fitted at this point, and i was going to move the clip on the float chamber needle down a notch and retest. But disaster, on refitting the clip to the needle, it pinged off, never to be seen again.

I had no choice then, but to refit the new Keihin carb, which previously didn't help. I did that, took extra care to make sure both connection to the airbox and the air inlet were good, used some silicon spray to assist with getting a good seal.

Fired it up, and it was all fine. Revved as normal, back wheel spinning on the stand. Moved under it's own power without a rider. Hurrah!

So it was just a matter then of reassembling everything and taking it on a test ride. I did that this morning. All good, I'm a big guy, 6' 4" and I weigh a lot, and it was able to get up to 30mph+ with me on, eventually. Did a mile or so, and came home.

Switched off and back on again for a warm start test and .... nothing. No power on the key. Nothing working on the dash.
The main fuse had given up.

So I've just replaced that, and put a spare in the holder also.

I think that the valves were so coked up they weren't ever sealing, which meant too much air getting in leading to more coking. Switching in the new carb when all that was still a problem was inconclusive, the carb alone couldn't fix it.

But there was clearly something up with the old carb. Unless I find the clip, or a source of clips very cheap I'll never know.

And now it's taken so long the MOT is about to expire, need to get that done to sell.

Anyone want to buy a moped?