Honda Sky SGX50 Fuel problems By stoo (Read 1,353 times)

[stoo]

Hi all, just taken delivery of a '97 SGX50 in reasonable condition. BUT it's a non-runner :-(  Gets a good spark and has good compression. Only 4000 miles on the clock. My suspicions are taht it's something to do with the auto choke? I've got a load of old brit  bikes from the 50s n 60s where everything is manual and easily fixed. These sealed units befuddle me as they're so tough to diagnose. As I say the engine turns over fine, shows a good steady spark but will not draw fuel into the combustion chamber. Checked the reed valves - all seems fine. Replaced the vacuum petcock - no improvement. After turning over the plug is still dry.... One other thing is that the engine won't turn over on the start button. I have to run power direct from the battery to the starter relay. Can't find a wiring diagram anywhere. The alternative diagrams seem to miss a few components. I've tried getting the auto choke to activate by connecting power across the two wires - nada. The little needle does push in and sring out by finger pressure. I have poured fuel down the plug hole and it still won't start. Tried filling the float chamber and it won't fire. Poured fuel down the manifold and it won't fire. So....... does the auto choke cut out the fuel supply? Is there a fuel vacuum issue? Does the fact that the starter switch doesnt work have some bearing? Could someone perhaps give a basic description of how this fuel system works ? Any and all help very gratefully received :

Cheers,

Stoo 

[ellpee]

I know nothing about your bike, but had to deal with the "autochoke" on a couple newer-vintage Chinese scoots so can offer this: the "autochoke" on mine was the exact opposite. Rather than reducing air flow, which is what a choke does to enrich the mixture, it actually allows MORE fuel into the carb initially on startup. More fuel for the same air quantity coming in, presto, richer mixture. Then, after a few minutes the electric current heats it up, which pushes a wee needle valve into an opening to cut off that extra fuel. So if you're getting no fuel at all thru the carb into the manifold and ultimately the cylinder, I wouldn't consider the "autochoke" a likely culprit. Now that may not apply at all to your bike, it might well have an actual choke that does its thing by reducing air flow, but from what you wrote I suspect not.
As to the start button, clearly an electrical fault there somewhere, loose connection or bad ground or broken wire or maybe even the button switch itself is faulty. On every scooter I've had, the button wouldn't work unless at least one of the brake levers was depressed, and I've read that some scooters also have a similar cutoff on the kickstand so the button won't work with the stand down. Lots of places to check wiring and apply the ol' multimeter!

[ellpee]

I know nothing about your bike, but had to deal with the "autochoke" on a couple newer-vintage Chinese scoots so can offer this: the "autochoke" on mine was the exact opposite. Rather than reducing air flow, which is what a choke does to enrich the mixture, it actually allows MORE fuel into the carb initially on startup. More fuel for the same air quantity coming in, presto, richer mixture. Then, after a few minutes the electric current heats it up, which pushes a wee needle valve into an opening to cut off that extra fuel. So if you're getting no fuel at all thru the carb into the manifold and ultimately the cylinder, I wouldn't consider the "autochoke" a likely culprit. Now that may not apply at all to your bike, it might well have an actual choke that does its thing by reducing air flow, but from what you wrote I suspect not.

Curious, though, that even squirting fuel directly into the cylinder won't give you a couple pops, despite good spark. No idea how that could be, maybe someone else here will have some ideas.

As to the start button, clearly an electrical fault there somewhere, loose connection or bad ground or broken wire or maybe even the button switch itself is faulty. On every scooter I've had, the button wouldn't work unless at least one of the brake levers was depressed, and I've read that some scooters also have a similar cutoff on the kickstand so the button won't work with the stand down. Lots of places to check wiring and apply the ol' multimeter!

[stoo]

Thanks ellpee, I've stripped the carb and cleaned it to within an inch of it's life. Still no joy. I've put a new starter relay in - no joy. Tested the autochoke on the bench, which is working fine. I've got a new cdi unit and regulator on the way. Diagnostics are difficult as I can't get a wiring diagram for it anywhere and the diagrams offered as alternatives seem to vary quite a bit and only add to the confusion. Weather permitting, I'll get out and have a look at the brake/stand cut-out switch possibilities. This was my next port of call. Having old '50s brit bikes, we rarely have keys, never mind any kind of safety features - apart from brakes(which more often than not don't brake!) 4 of my bikes don't even have brake lamps.
Anyway will poke the multimeter around, look for shorts etc. The annoying thing is that the guy I bought it from mananged to get the starter switch going by jiggling the battery terminals! After transporting it - nada!
Keep the faith :
Stewart

[stoo]

Progress! But not yet Success...
Faulty safety cutout on brake lever repaired so now, Starter button working. Headlamp and tail light working when engine cranks over. But still not the slightest whiff of a bang in the combustion chamber. I've installed a new vacuum petcock already. So the two things that I think it could be are 1. dodgy CDI unit? Got one coming from a piaggio that is supposed to improve performance anyway or 2: some vacuum-related issue that isn't pulling fuel into the carb -> cylinder. Will investigate further. Hopefully getting closer. Should run for another fifty years with the amount of new parts that I'm buying !! $$$$$

[ellpee]

Congrats on getting that starter button working. Headlights and taillights likely run off the stator when the engine is turning, so that part makes sense. Your writeups seem to say no fuel at all is getting from the carb into the cylinder. Is that the case? If it's now cranking, and the spark is good, I don't see that there's an electrical problem, although if the CDI is sending that spark at the wrong point in the cycle that could be it I guess. Replacing the CDI with a known good one should eliminate that possibility. However, if no gas is flowing that says fuel lines/carb to me. Seems logical something should be happening in the cylinder if any gas at all is getting there. Not to belabor the obvious, but there is gas flowing INTO the carb, ja? Puzzling, that part. Keep posting, I'm curious how this will turn out.

[stoo]

It's this vacuum issue that's puzzling me know. I'm wondering whether the 2T ol pump which is connected to the carb by a tube could be at fault or whether there is an issue in the reed valve block. Although when I checked it the other day, it all looked fine... I've blown out all the fuel lines so that doesn't seem to be a problem..... Put new O-rings in the inlet manifold faces. As i said new vacuum petcock. Again compression seems good but I can't measure it by meter as there isn't enough space to use it. Like you say, ellpee, it's not likely to be electrical now but I'm absolutely stumped as to why it won't suck in any fuel. Ah well, tomorrow is another day....

[ellpee]

Okay, so though you haven't specifically said, I presume the fuel line to the carb IS providing gas, ja? So there's gas sitting in the float chamber ready to be sucked into the manifold and engine, but that's not happening? Sounds like your scoot is quite a bit different from any I've messed with, know nothing about reed valves and no idea what a 2T ol pump is, much less why it's connected to the carb.

[stoo]

Petrol in float chamber. Tried it today with an external auxiliary fuel tank and no joy. Carb just ovrflowed. Still no bang! The 2 stroke oil and the petrol aren't premixed but the oil seems to be added direct to the crankcase by means of a pump which appears to be activated by a wormwheel off the crank and a vacuum pipe from the inlet manifold..... All the jets are clear. The manifold is tight. Now tending toward the reed valve assembly. will investigate further.

[ellpee]

Hmmmm. Well, as I understand it, cranking the engine creates a vacuum back through the manifold to the carb, and that vacuum is supposed to suck petrol from the float chamber through the needle valve into the carb throat to mix with intake air and flow into the manifold, on through the intake valve and ultimately into the cylinder. Pretty straightforward. So if you're saying no gas is making that trip, that the cylinder is dry after cranking, 'tis indeed a puzzlement. What in that simple sequence could be the problem, given you've checked the needle valve and it's clear? One unpleasant thing that comes to mind is that the intake valve isn't opening at all, but maybe on your scoot that's what the "reed valve" is all about? Darn if I know; will join you in thinking about it some more.

[stoo]

Ordered a new reed valve. Cant do any harm, especially as teh bike's been sitting for a while. Hopefully that'll sort it. Updates to follow. :