Fuel mixture help needed please

[mellorbo]

Hi all,
Firstly let me bring you all up to speed on my situation.
i have a 2011 sinnis apache 125cc ( yes i know its Chinese so please don't tell me to get a real bike that is not the issue at hand )
i believe its based on a suzuki gn or gz engine.

I have replaced the muffler/backbox for an aftermarket one but i have left the down pipe as stock. i have swapped out the stock air filter for a free flow foam one and put on a crank case oil breather filter and removed the egr system with a blanking plate.

I have been told to re-jet my carb to a bigger one if i have issues.
now the bike seems to be running fine with no dead spots or splutters on the acceleration but it does "burble" on deceleration (it did this as completely stock)
BUT i have a feeling that the bike is running lean as the engine gets too hot to touch after a short journey (approx 2 miles) which if i remember correctly it never got that hot before.

it has a mikuni carb (slider type) with the throttle cable coming out of the top of the cylinder which unscrews to show a spring and needle e.t.c
now i was wondering if i could just move the washer down a notch on the needle towards the point if that would make it run richer until i can get a bigger jet in a few week.

im just worried that if it is running too lean then i will cause damage to my engine, any help with this matter would be greatly appreciated and if you have any further questions i will be happy to answer if i can.

thankyou in advance
james

[Commuter_Tim]

In b4 "put it all back to stock" "they know better than you" etc. (you all know it was coming anyway)
____________________
The above post is most likely nonsensical.

I ride a Bandit 600... badly.

[mellorbo]

[Rogerborg]

Every little helps.

The colour of your spark plug tells you about the fuelling. Run it at a given throttle opening for as long as you can (if it explodes, you ran it too long) then hit the killswitch while holding the throttle steady until it's stopped. Plug out, check the colour.

https://img31.imageshack.us/img31/5442/sparkplugt.jpg

With that carb, you can do:

#1 3/4 to Full throttle (change main jet).
#2 Idle to 1/4 (adjust mixture screw).
#3 1/4 to 3/4 (needle height).

Idle mixture can be done by just tweaking until the revs increase at idle. Above that, you will want to look at the plug colour.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[mellorbo]

thankyou for that i shall give it a go tomorrow.
when i pull the plug out do i take it out as soon as i pull over or wait for it to cool down first?

[Rogerborg]

[stirlinggaz]

hi,
aye, definately a plug chop is in order & re-jetting of the carb will improve performance & make it less likely to go BANG!!!

you say you have replaced the "muffler/backbox", what with?
does it have a cat? (another restriction imo)
pretty much garauntee you will be running lean, but how much? luckily these bikes are set up rich from new.

what carb do you have? if its one of those mikuni rip offs from china, i'd be looking at replacing it, though how successful you will be with jetting is a bit of a lottery.

its a chinese 4 stroke, probably putting out around 9 or 10 bhp from new (to get it through the emissions test), but with a few mods, you might get close to 13 or 14bhp....revving the tits off it.

doesnt sound worth it does it.......but that would mean around 30/40% improvement.(max)

how well the bike will take this extra powahh....who knows but i'd give it a go.(but am a bit daft)

as an example i owned some chinese cbr125 replica, fitted with a weird all up or all down gearbox. engine was 80% cg rip off.(156FMI)

with a bigger main jet. a gutted exhaust, a cone filter & sprocket change i was getting around 65mph at wot (real speed, indicated speed was just shy of 80mph), but it rattled like f*ck, (no balancer shaft) & sounded.....a bit different & all plastics fittings were replaced with decent s/s stuff with fibre washers & any others replaced with s/s or threadlok. : this makes riding the bike a helluva lot more pleasurable.
far better than the original 55mph max, at the red-line.

the change of sprockets made it a bit more suitable for NSL roads,

cheers,
GAZ
ps : didnt there used to be a dedicated chinese bike owners forum....? im fairly sure there was.........................anyway the sinnis bikes seem to be one of the better chinese bikes we are getting.

pps : & more importantly, you do realise moving the clip on the needle only really affects the middle throttle openings?

whereas the main jet is for WOT (puting it simply).
you have to address the FULL throttle range.

WOT 1st, plug chop takes care of main jet,
idle-1/4 throttle is the pilot jet & the rest is the clip on the needle. (WAAAAAY over simplified)

[mellorbo]

the engine in the sinnis is sound its built in a suzuki factory its the pulse adrenalin and superbyke rmr which are the knock offs. its not got a cat its a straight through down pipe to a performance can with a k&n filters.
the spark plug was changed last month to an irridium one. i had it spare at the time and its the correct plug for the bike so i put it in. it has been gapped correctly too and i did the valve clearances 2 weeks ago too
i believe its a genuine mikuni carb as all part numbers and markings match up to other mikuni carbs i have had so im 99% sure
this is the exhaust i got https://www.chinesemotorcyclepartsonline.co.uk/partno_EXHKIT001.php

EDIT:
yes i understand that the needle clip was not the be all end all i just thought id give it a try as set in the middle it seemed to be ok ish. thankyou all for the help so far im going to go get a jet tomorrow but im going to pay the £30 and get it dyno tuned to find the current air fuel mix its on and get it re jetted properly. i shall let you all know how i get on.

p.s i have come into some money today not a great deal but enough to do what i need. i started this thread with the thought that it was going to be a while when i could afford it. but as i use the bike daily i diddnt want to blow it up

[mellorbo]

hey guys i couldn't get in to the dyno shop today so i went to my local back street garage and got a jet. i went bigger from a 97.5 to a 112.5
as expected the bike pulls harder now quite noticeably throughout the rev range.

i have noticed however sometimes when i let off of full throttle the bike lunges forward in a split second power burst. is that anything to be concerned about?

[Nobby the Bastard]

[mellorbo]

[talkToTheHat]

The lunging on throttle off is dangerous and will catch you out on a slippery roundabout or similar.

The GN/GZ/GS125 engine design is pretty tough, I don't have personal experience of clones. Both the two I had would burble a little on a closed throttle and were fine temperature wise.

Hot's relative, but mine would both be too hot to touch after a spirited ride and when I stopped would make the plink plink plink sound of cooling metal for a few minutes. Plugs were fine, the K2 drank oil, the K6 didnt. Too hot smells different.
____________________
Bandit. does. everything.

[spriddler]

[Rogerborg]

[mellorbo]

[mellorbo]

[spriddler]

[Rogerborg]

ADMIRALTY LAW, I AM A FREEMAN ON THE LAND, NOT A SHIP!
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[Robby]

Having skimmed down the thread, let me see if I have this correct:

1. You did some mods. The bike was running fine, but you thought it was too hot.
2. You moved the needle down. Moving the needle down means the engine will run lean on 1/4 to 3/4 throttle. You need to be more accurate in your explanation here. Did you move the needle to a lower position (move the clip up the needle) or vice-versa?
3. You fitted a bigger main jet. This makes the mixture richer at full throttle, but has little effect at other times.
4. It's running poorly.

You have an air-cooled engine. They get hot. The only good indication you have of the heat in the engine is the spark plug. You don't need to go a plug chop - a chop is a technique for assessing fuelling under one particular set of conditions. You are looking to assess how the engine is working during normal riding.

Put the carb back to stock. Fit a new plug. Go for a 20 minute ride. Let the engine cool down. Remove the plug and look at it. If it's tan brown, your engine is fine.

Carb tuning is not that simple. You have four main things to adjust - idle, mixture, needle, main jet. An adjustment to one of them will have some effect on the others, varying from negligible to significant. To make adjustments accurately, you need to have the engine under load and be able to measure the air/fuel mix from the exhaust.

Luckily, you have a ace up your sleeve. The mixture doesn't need to be perfect, in fact it can be pretty awful. To foul a plug, it needs to be very rich. To melt something, it needs to be very lean. The stock settings allow for a fair bit of leeway, so you don't need to mess about with it after doing some minor mods.

[mellorbo]

[mellorbo]

hey guys i went out for a ride today for approx an hour. some slow parts cruising in 5th 4k revs 40mph.
some just before redlining it in 5th gear on dual carriage ways 8500rpm 70-75mph
i would say 70% of the ride was full throttle changing gear from 8k revs (limiter 9k)

i got home waited half hour took plug out and took pics, will post link below.

this is the same plug i was running lean on too.

plug number (NGK)
https://imgur.com/m4a1izs,TpQMgh8#0

plug tip
https://imgur.com/m4a1izs,TpQMgh8#1

the ground electrode looks like a tan brown but the rim around the electrode looks black. it is an iridium plug if that makes a difference.

[Robby]

The colour looks right. 70% throttle means the fuelling is being controlled by the main jet and the needle.

Surging when you snap the throttle shut is a concern. My best guess is it's running too rich on wide open throttle. Throttle snaps shut, a bit of fuel is still floating around, surge.

The main jet you have fitted is 17% bigger. I don't feel like doing maths, but as a ballpark that means it's flowing about 20% more fuel. The modifications you have made are unlikely to mean that you are flowing 20% more air. I would try an intermediate size of main jet.

[mellorbo]

[wr6133]

When I did free flowing pipes and cone filters on a Chinese bike it needed a 10% upjet. So if you started on a 97 I'd be trying with 106, 107,108... plug chops on each of those I'd bet one is right.

[Pete.]