Mech

The timing can't have been 180 degrees out.

A colder plug will oil up easier..

A spark plug missing only under low load is a new one to me.. for a carb engine anyway. EFI does that, but never found that with a carb engine. You should do the valve seals when doing the work. If they have sat for a few days and evaporated the fuel out of the carb, then the fuel pump sometimes takes a while to pump fuel through, especially if the engine has low vacuum.

Well you've come to the right place. Welcome along. You need to make ten posts to qualify to download a service manual, (which is what you need), so tell us about the bike's history as far as you know it, and perhaps a bit about you and your son's experience riding and/or repairing bikes. It'll help us a lot when we are giving advice if we know you or the son are familiar with bikes and engines etc.. or not.

Depends where the "pop' is coming from. If it's under load and out the exhaust it may may an electrical misfire. If it's out the carb then it's probably a lean mixture. But... the no idle and popping could be caused by valve clearance or valve timing. You really have to know one part of the system is absolutely right before you start looking at the next possibility. Valve clearance and timing is simple and doesn't take any discrimination or experience to judge. A new spark plug is simple and cheap to replace. The carb, which is from the sound of the description (symptoms and circumstances), the most likely, needs to be right for that bike. If the carb's been set up to be mounted flat it may not run well on a bike where it's mounted at an angle. You need to use some discretion when checking the jets and settings. If the float level's set different by the manufacturers because it's for another model bike, then the jets may all be suited to that float level. I always check the float level seems right for the way it's being mounted, then ride it taking careful note of all the symptoms and the circumstances they occur under trying to figure what part needs adjusting or changing. I only ever change one thing then go for another ride taking careful note of what effect that had to the fault, and whether it's made other ranges of throttle better or worse. The three jet systems in the carb, idle, mid-range and full throttle, all overlap, and there are special features that help with the transition from one system to the next. The usual way to set a carb up is, get it idling and running, test ride and set the main jet system, then recheck the idling system, then set the mid-range system. Sometimes we have to go around the cycle doing all three again if it was a really badly chosen or set up carb. It takes time, a lot of careful test riding, and a good understanding of what's going on.. That's why it's always best to stick with the original carb.

"when engaging throttle it runs smoother, but when disengaging or hovering throttle at a constant rpm, it bucks, no matter what gear its in.". Ok. Good observation. Sounds like a carb problem (or problems). The no idle after a run is probably going to be something that an adjustment of the idle will cure, and it may even help with the stuttering.. Another simple thing to try now would be to change the slide needle position in it's clip and flex and inspect the rubber diaphgram. That can be done in place. Then if that doesn't fix it I think I'd pull the carb off, clean it and check all the jets are clean and what size they are etc, set the float height carefully to a height appropriate for that bike. Carbs for some bikes sit at an angle and others sit flat, and the same carb can be used for both situations, but they have their float heights set different to suit the angle.

I'd imagine it can be done with the engine in place ? Most can. The service manual says what can be done with the engine in place.

You should be able to get all the gears if you jack it up and turn the rear wheels enough. When you are looking at the parts diagrams you'll find they generally have the gears, and the gear shift mechanisms on separate pages. The fault will be in one page or the other. It's unlikely to have problems with shifting as well as the actual drive. The shift mechanism is a ratchet mechanism to let the shift lever return to center between shifts, a start shaped washer that helps it move fully into gear once you've moved it over half way to the next gear, and another short lever and swash plate and ball bearings that disengage the clutch as you shift gears. The problem if it has one in the shift mechanism will be with the ratchet (lever, spring or pins the lever engages with), or spring loaded star washer part. The clutch operating part you can ignore in the diagrams. The pins the ratchet engages with may be shown in the gear section because they are on the end of the shift barrel which is normally in with the gears.

No. Just a side case.

Well rocking the bike as a way to get the gear's to change isn't the most reliable. It's a bit hit and miss which way the bike needs pushing to get the last shift fully done, and the next gear attempted, and sometimes you need to push hard enough to get the engine to turn over. To test a suspect shift fault I always jack the bike up and turn the wheels, or run the engine. If you rocked the bike enough and kept on trying it, and sometimes shifting down a bit, pushing, then trying the shift again, it should eventually shift through all the gears. If it really can't get each gear after a lot of turning the wheels, and sometimes turning them quite forcefully if needed to turn the gearbox input shaft, then it's perhaps got teeth missing off several gears, or, the shift ratchet mechanism isn't working properly. The ratchet mechanism is I think on the left side on that bike, under that cover.. If there is bad damage in the gears, the oil will have metal, either dust, flakes or even chunks. The dust and flakes might be obvious if you use a wire to dip a bit of oil out of the oil filler, or, take the oil filter out and inspect it. You could also test carefully to ascertain which gears it is and isn't getting, then figure out whether it's the shift mechanism or the gear teeth that have the fault. The fact that each cog us used for two different ratios will be how you tell if it's cog/tooth broken. If you check and tell me which gears you can get and which you can't get, I'll look at the exploded parts diagrams and figure if it's a gear or not. If you haven't got a service manual yet, Babbits parts have exploded diagrams of the gears and the shift mechanism.

The sprag/oneway clutch in the centrifugal clutch isn't working. You'll have no engine braking once the engine revs get low enough for the centrifugal clutch to disengage. When you were testing it before you must have have not let the bike get slow enough down hill for it to do it, but if you go down a steep slope in gear and leave the throttle off till the bike's almost come to a stop, it'll accelerate and start rolling away again uncontrolled once the centrifugal clutch disengages. If we know the bikes got the fault and take precautions when going slow and don't rely entirely on the engine braking it's not too dangerous, but people that haven't known their bike had the fault, have been hurt or given a scare when their bike having almost stopped, then started rolling and rolled off a bend or into a gate..

Hi. Welcome along. You'd better explain in more detail how the shift's working (or not). I take it you're used to bike shifters ? Know about using the throttle to get them to change nicely etc.. ? Rocking the bike to get it to go in or out of a gear sometimes, (especially if the engine's not running) ?

The idea with the test riding is to figure if it's a load (mid revs /steep hill between 1/4 and full throttle openings), or certain revs (any throttle or load), or the jet it's using at the time it starts missing ( any load or revs but at a certain throttle range. If it's always the same throttle setting then It's going to be a blocked jet. If it's load that does it then probably spark-plug, lead or coil. And if it's always at the same revs no matter the load or throttle, then it's some electrical component failing. Going up then down a slope means you can have certain revs or throttle settings with or without load on the engine. If you suspect a failing electrical component then it's a good idea to turn the headlights on. The voltage regulators(or wiring) can make them miss bad/buck as you seem to be describing. Good idea to try the headlight anyway probably to se if it makes a difference.

You could check the air gap on the trigger coil..A big air gap means the magnet has to get closer to the pulse coil as it's rotating, before it makes enough voltage to fire the cdi. A small air gap advances the ignition timing. At slow(cranking) revs the magnet has to get really close to the pulse coil as it's rotating before it fires the cdi, and that's supposed to make it very retarded at cranking speeds. A big air gap might reduce the knocking when the starter's operating. Later model bikes have a signal from the starter into the cdi unit to retard the timing at starter operation, but older cdi didn't have that and relied on the air gap and pulse coil position to set and advance the ignition timing. That bike will be one of the old system I'd think.It'll probably respond to the air gap being changed. Later designs don't respond much by altering the air gap.

Ha. Good one Gw. Patience always wins..

I've been fixing LTs since they were first made, and I've owned three. I live on very steep land and the low low is brilliant. An air leak, either at the inlet manifold, or on the exhaust at the head, or even a blown head gasket, can make that squeaking noise you mention.. So can a seizing piston or head. You undo a certain head nut to check the oils getting to the cam. The spluttering could be water in the fuel, or an electrical problem. I'd try it at different speeds and loads (going up then down long slopes), and throttle openings, and slowly figure whether it was directly connected to revs, load, or throttle position..

Is it possible to clamp or weld a pipe bend on aiming downwards ?

They also have a steel brake pipe between top and bottom cylinders I think, and they might have rusted. It looks like it's coming from inside the drum though, so that would be seals. If the bike's been sitting the smoke might get better after a few runs so I'd ignore that for a while.. Oil's cheap.. haha. If it is smoking a lot then Gw's pretty much right about the circumstances... If it blows a cloud of smoke at startup, especially cold start, it'll be valves, smoke under load, rings. It could be a combination of both, but it's best to do rings and valve grind and seals at the same time anyway.

A blown seal in a brake cylinder probably.

The Japs were quick to realise that people world wide wouldn't buy their vehicles if they couldn't get them serviced easily, anywhere, any time, and so they all made their manuals readily available. The Americans prefer to keep their information to them selves unless you want to pay for it, or go to a dealer. It's the same with their software, they don't want us to use it. I's hard to get any information from J.D., unless you want to pay. The Japs are happy for us to understand and repair our cars, the Americans would happily sue you for sharing "their" information !

Then you could try one step higher to see if it's too much... after the first test ride.

Well check the manifold for air leaks.

I could look up the original carb's jet sizes, but an aftermarket carb might have a different emulsion tube, and/or needle, and might need different main jet, and/or main air jet.. That's the trouble with after-market, if they aren't the same design, then the jetting is just guess work. You should try changing the slide needle setting..

If it pulls strongly up to the 3/4 mark where it starts to falter, then runs out of go abruptly, then it's likely to be main jet more than the slide needle, but if it's weak from 1/4 to 3/4 then runs right out of steam, it's almost certainly the slide needle. A bigger main jet may cure it's shortness of breath at 3/4 if it's the needle, but it may not run well at full throttle and it may stumble accelerating. I could make an educated guess by riding it for a bit up and down hills and along some flat.. But... The slide needle position is the easy thing to change. That will do something and give some indications about what to do next. It's easier than changing main jets. Oh.. The jet threads need to match the emulsion tube threads, which needs to match the carb body threads, so you have to be careful if changing parts from one make to another, and, not all jets are measured the same way, or numbered with the same system.. You need to check them by eye or gauge to be sure they do seem the same measure.

Yeah I think you're right. I suggested those numbered parts were main jets earlier. DTLB.. Those are called main jets, and service manuals and parts places will all call them that.. If you read on the internet about tuning your carb, they will mention main jets. That's them.. So you know.