Direct bikes DB50QT11, a 49cc with 139QMB engine, won’t rev
by: aaitken - Mar 18, 2021 5:54:48 GMT -6
Post by aaitken on Mar 18, 2021 5:54:48 GMT -6
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?
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?