yamaha jog 50 with 70cc kit won't start...ideas?

hi all,

so I'm trying to get my spare/donor ped running as a hack bike but so far no joy. If anyone has any ideas please shout..

here's what I know:

- it has good spark (new plug, gapped correctly, can see strong spark when plug is taken out and earthed to the cylinder casing/head)
- it seems to be getting fuel (plug is wet when pulled after cranking, petcock seems to work when activated manually and I see some movement when it's hooked up to the vacuum pipe just before the inlet manifold so there looks to be some vacuum to activate the petcock)
- it has compression (builds to ~150psi after a few cranks)
- carb seems to be clean (took it apart and cleaned, blew thru main & secondary jet, was generally clean and not gunked up, tho i've not touched carbs in years so could well have missed something)
- I get a flicker of lights on the speedo/dash when I crank it manually even without battery so figure the charging system is possibly ok
- it cranks on the elec starter and apart from sounding a bit "scratchy" it turns over ok, kinda sounds like it wants to start but then doesn't. When cranking it basically sounds like if you crank a working scoot with the ignition switched off.
- tried a different coil from my working neos, no difference
- the big red 4-way plug near the ignition barrel had some corrosion on terminals..unplugged and replugged it a few times to try and clear that. May still be an issue though.
- It looks to have a cast iron cylinder so figure possibly a malossi 70cc kit. Whether it's jetted right is anyone's guess, tho i figure it would at least start even if it didn't run right on current jetting.
- I've tried the adjustable screw (idle speed?) on the side of the carb in a number of positions - right the way in, right the way out, somewhere in the middle, no diff.

When i bought it i was told it would run *eventually* after a lot of cranking, but that is just the sayso of some dodgy geezer so am not holding much faith in it.

Any ideas? I feel like I'm missing something obvious..

thanks in advance

 

having found a similar thread on here I did some more checking today, particularly of the wires around the ignition. unfortunately to no avail...

- checked the brown wire had continuity to and from the key/switch (and to the cdi)
- checked the black and white wire had continuity from the bullet connector to the cdi
- wires to the 4-way connector on the ignition seemed to be getting power and were making contact across the switch, cleaned up the contacts anyway
- brown wire on the cdi getting power
- tried the cdi from the neos
- both pairs of wires on the stator plug had continuity and seemed to generate voltage when cranked
- changed the rectifier for a new one from neos..may have been a different type because then it blew several fuses. changed back
- all the stray wires at the front end are accounted for (headlight, position lights, indicators)

i noticed the fuel lcd comes on sometimes but not other times, haven't worked out what influences that yet.

the "scratchy" sound when starting on the electric starter seems to be to do with the electric starter. no scratchyness when cranking on the kick starter.

it's still getting plenty of spark when cranked (earthed to the cylinder so we know the engine earth connection is ok), about 2-4 sparks per kick of the starter - and doesn't get spark when cranked with ignition off which is leading me back to thinking it might still be a mechanical/fuel/air issue

if it's getting sparks, that should be the only electrical part of the process surely - right? so I guess it must be a fuel/air issue or the timing of one/all of those components is off.

I'm trying to get the inlet manifold off just because it's one of the few things I've not removed yet (possibly the reed valve is in there?), but the two smaller allen bolts are rounded/stuck so that's going to have to be a project for another day. yay !

 

Shout louder? Swearing always helps...

 

If its sparking then electrics sound good as long as the spark is strong enough, I can't recall the colour coding but probably a blue spark is good and yellow is weak? Wet plug suggests fuel so that leaves a firing problem, either too much fuel or not enough. I would firstly check the choke is working properly to negate that as being the issue, then I would remove the air filter and spray 'sure-Start' or similar damp start product into the inlet to the engine. This is more combustible than the fuel. Crank the engine a number of times and see if the spray vapours get it fired up.

 

Might be over-fuelling.
What carb is on it, standard 12mm or 17.5 dellorto?
What size is the main jet?

 

Sounds like jetting or needle setting needs changing.

 

Im telling you, swear at that mother plucker....

 

thanks for the replies guys, some good points in there ...so in sequence:

scuba - swearing: way ahead of you lol. I'm sure my neighbours learned some new swear words

sparks: defintely blue. Didn't realise the colour made a difference but it's a nice crisp light blue.


> Wet plug suggests fuel so that leaves a firing problem, either too much fuel or not enough.

right..that makes sense and is sorta where I ended up conclusion-wise..


> I would firstly check the choke is working properly to negate that as being the issue

I've seen a test of how to check the choke in the service manual - will re-find that and test. I figured it was a solenoid of some sort so checked the choke had continuity but wasn't sure what it was supposed to do beyond that. will revisit that. good idea.


> then I would remove the air filter and spray 'sure-Start' or similar damp start product into the inlet to the engine. This is more combustible than the fuel.

I've only tried with wd40 which is what I've used in the past on other vehicles (rightly or wrongly)..wasn't sure about its combustibility tho so will get some actual starting spray.


> Might be over-fuelling.
> What carb is on it, standard 12mm or 17.5 dellorto?
> What size is the main jet?

Pics of carb and main jet.. looks to be "110"..though currently I don't really know what that means. Looks to be a dellorto phva carb and the choke diameter looked to be more 12mm than 17mm and it does say "12" on the side of the carb.




> Sounds like jetting or needle setting needs changing.

ok well I'm hoping the pics will give a clue as to how right or wrong the current jet is. I was wondering about just wanging a new carb on there as by the time I've messed around with buying jets and sprays and whatnot i could probably get a new replacement carb of optimal choke size etc (if this one isn't optimal). I'm thinking i can build this motor up and then swap it onto my neos once I've got my CBT and can ride over 50cc

compression pic just for ref. That's after a few cranks. It started around 8 on the dial and then built up to 11 after a few turns and then stayed there.



engine pic just for ref - looks to be a cast iron cylinder, right?


Thank you for the feedback!

 

Yeah thats a 12mm carb for sure , the 12mm stands for the inner slide bore ( when you look inside from either the intake - outlet sides ).

im going to hedge my bet on that 110 jet being slightly to big for the 70 jug , just a little to much fuel and the sparks just getting damped out and not triggering a ignition .
104 or below jet i recon you will see a fire up.

could replace it with a 17.5mm just so you got plenty of meat and a fairly healthy bowl size, tend to find most 70cc kit scoots running a 17.4mm or a 21mm carb thats jetted low, also find the choke jets are lightly bigger to ( my 17.5mm has a 60 choke jet and im running low high 70's to low 80's on jetting (without a 70 kit installed). gonna feel a difference in throttle to its just more snappy and responsive with a larger carb bore.

 

thanks Tamiya - that makes sense. Sounds like someone tried to just drown it in fuel for the bigger cylinder and over did it. That would be in line with the general half-assery evident in the rest of the bike.

I might see if i can source a 104 or below and get this carb running and then look into getting a 17mm carb jetted accordingly after that. bigger bore makes more sense re power..like running twin 40s on a golf

Are there particular carb makes you'd recommend or avoid? there seem to be lots on ebay all promising the earth, so I would welcome input based on actual experience.

Also it may be a dumb question but where's the best place to get said jets for this carb? I'd default to ebay but am open to other options if they're better...




Very helpful, thanks

 

pedparts
donotlink pedstop
euroscooterparts

those will have jet kits and sell half decent carbs to . dellorto are great but they cost a good fair bit, stage 6 ( but dont go with naraku )

 

Fleabay is good for jets, just choose a reputable name and opt for a small selection rather than one size jet to enable you to size up or down if necessary.
Testing the choke is easy, remove from bike lay on a piece of card and mark the position of the needle, attach to 12v source and as it warms up the needle extends. Takes about 5 minutes and you will clearly see the end of the needle moved from its original position. The idea is simple, intial cold engine requires a little more fuel then as it warms up the needle extends a reduces the flow.

 

110 is WAAAAY too big for that carb and kit.
I would start at 100 and go down.
Don't buy single jets, buy the packs of 10, 72 upto 100 pack should be about right.
Adrenaline is where i buy mine

 

awesome info, thanks very much guys

 

Does anyone happen to know which size/diameter jets I should be getting? They seem to come in 6, 5, and 4mm sizes (at a guess I'd say 6mm) but whether that refers to the thread size or the diameter of the jet "head" I don't know. thanks in advance

 

Thread size in mm much like a nut and jet hole size is your 100, 104 etc.

 

righto - I guess I'll have to pull the carb apart again to know for sure then. cheers

 

apparently the PHVA 12 has 6mm jets (according to adrenal1n-pedstop) so have ordered a set of 60-80 jets (closest match I could find in stock), as well as a couple of individual ones (85, 100) so hopefully that should cover all bases. I guess we'll know how it turns out in a couple of days

It should also let me re-jet my neos for the sports pipe which is a nice bonus \o/

 

hmm still no joy on starting the jog so far. Tried various jets today from 90 right the way down to 64 or so which is the smallest I have. I *think* it did sound a little more like it was trying to start but still didn't actually start.

I've yet to test the auto-choke, but while changing carb jets I made a note of the smaller jet number (34). I dunno what bearing that has on things but thought I'd mention it just in case it prompted any ideas.

i'll report back once I've tested the auto choke.

 

small 34 jet is your pilot jet, the pilot jet is for idle to 1/4 throttle

did you yank the plug see if its getting wet at all, be a good sign that fuel is getting in , also did one check the sparks bright and blue

Oh and that red box up front, decamp the connector grab some sandpaper and rub those contacts to shiney again dont go mad just get that surface crap off, if you can try get inside the connector to rub them contacts a bit to so your getting a great contact area and not intermitent