kdx 175 small throttle open running poor

[kdx Geoff]

Hi

Trying to get this bike running right. Standard Mikuni carb

The problem is at small throttle openings it 4 strokes very strongly, if lm going up a small incline, it stutters along between firing every stroke and miss firing or 4 stroking quite roughly when the engine is clearly under some light to moderate load at a small throttle opening, there is plenty of blue smoke from the exhaust too.

Once l get around half throttle or more, away we go and it pulls strongly for a 175 with almost 100kg on board. When its happening, putting the choke on makes no real difference so makes me think its not lean.

l have replaced the 28 year old Bosch plug and it now idles better (no surprises there), removed the air filter, cleaned the carb within an inch of its life and paid particular attention to the air jet which is wound out 2 1/2 turns.
lm using castor 927 at 30/1, but l tried different fuel/oil for the same result.

It starts easily and idles well.

As it pulls well at full throttle lm not thinking its electrical.

So its sounds like the pilot jet is too big and its rich, yes ?

lts been a good many years since lve ridden this bike and it had never run like this in the past and other than cleaning the tank, carb, new fuel line and fresh fuel, nothing has been changed from how it was back then.

Am l too strong on the oil ? or is all the smoke from the intermittent firing ?

ls the fuel different these days that l need to look at the jetting ?

Should l use prem unleaded ?

Geoff

[PEZBerq]

Hi Geoff

Blubbering at low throttle opening and air screw 2 1/2 turns out sounds like pilot jet way too big. I had similar problem with one of my Husky's and had to drop down several sizes of pilot to get it to run clean down low. Higher throttle openings and the pilot doesnt contribute much to the fuel supply. Good luck.

[dalesween]

Yes ,the first quarter throttle being blubbery does sound like the pilot jet,i would be tinkering there first.

Dale.

[OverTheHill]

without thinking 'too' long & hard at this stage, one thing that's often over looked is a worn needle jet & needle itself [not to be confused with the float valve--often people call 'that' the needle jet], so needle jet or metering hole the needle slides in at top of emulsion tube gets worn oval & not easy to see unless quite bad. Needle also can get visible wear at small openings. Dropping the needle [raising the clip] might help but not going to cure a bad case. One more thing, some carbs have the metering bit as a separate item [like Honda xr200 Keihin carbs] & has to be carefully pushed out after emulsion tube is wound out. all above is of course providing the trouble 'is' carburation & not electrical but if smoking heaps [but not taking in gearbox oil] it sounds like a good starting point. Smoke can also be greatly attributed in older bikes to oil built up in the muffler & just smoldering away under heat so neads the big burn out [right through chamber & all] or if pos just re-pack the muffler. Sorry about the essay--gettin carried away, not gonna proof read it now. Cheers.

[80-85 husky]

Café Husky had a similar thred with a 430. lots of suggestions including the jet needle etc. guy's last post was that he had cleaned the fully cleaned carb and there was junk in the pilot jet. but... the kickstarter broke and he couldn't test it

How is the float level?, needle and seat? not dribbling a little extra into the carb and engine? that clears on xtra throttle

[kdx Geoff]

Thanks,

l replaced the fitted number 45 pilot jet with a standard 40 for an improvement but its not how it used to ride, l have a 37.5 to try next and l feel lm heading in the right direction.

l think the smoke was less too.

The bike has only done 1700 km so lm not thinking its excessive wear in the carb but l will keep that in mind.lf the 37.5 pilot doesnt make much more difference l might drop the needle, but the throttle is just open when this is happening and its definitely before quarter throttle.

lm sure lve never ridden this bike on unleaded fuel before and thats the only thing thats changed with the bike after many years at the back of the shed.

l did flush and clean the tank, clean the carb, replace the fuel line and set the float level and using fresh 91 unleaded and new oil.

Will sit and idle for extendend time and no fuel comes out of the overflow so l didnt think l had a problem there.

Geoff

[OverTheHill]

think i forgot to say--providing it's not electrical-- but for you to decide that.

[Davey Crocket]

Use 98 BP fuel.

[PEZBerq]

I went from a 45 and ended up at 25 before it ran well. Steps of 5 would be my recommendation to get in the ballpark.

[GMC]

I think I dropped my KDX back to a 20 or 25 pilot, it didn't run that poorly with the larger pilot though.
I have been Ossa'sised for Harrow as my KDX is in pieces so if you want to borrow my carby for the weekend then let me know by midday as that's when I hope to leave. (Thought I read somewhere you were going to Harrow?)

[kdx Geoff]

Thanks for the offer Geoff, unfortunately lm not able to attend Harrow but based on the last couple of posts, l might skip 37.5 and go to 30.

Geoff

[OverTheHill]

[my theory] the more you need to go down in the pilot [below what was std & good for years say] means it's getting its fuel from somewhere else--in other words up past a worn slide needle & needle jet. Good trick i did once was to wind the idle up so it would keep running then shine a toch down the spout & [in my case] i could see the fuel being sucked past the needle [so compensating the idle mixture]. Then to prove a point i found a needle that totally blocked off the needle jet--just dropped it in with no clip so it stayed there [still run the slide of course] then started on choke then choke off to see if it would idle clean--that's using the normal pilot jet size. Never really thought about it but an easy way might be to totally block the main jet for the test & don't worry about the needle idea as the needle jet gets its fuel from the main jet anyway, so if blocked off you'll only be running on the pilot. Someone will tell me if i'm a bit off track but--all good. Of course with older two strokes, restricted exhaust [carbon'd up not factory restricted] can greatly effect performance at the top end not slow running [sort of]. Talking of carbon build up i'll put a pic up one-day of the headers of my LS2 100 twin, that were blocked to about a 10mm hole & would barely drag itself down the road. Mind you the bike was 40 years old & ridden around town slowly all its life. Sorry, turning into bed time stories again. Cheers.

[80-85 husky]

Did you sort the fuel tank on the ossa Geoff???

[kdx Geoff]

Hi OverTheHill,

l take on board your thoughts about the extra fuel coming from somewhere else.

Here is a pick of the needle jet and the jet needle.



Slightly off centre but looks round.

The jet needle shows little sign of any wear either.

I got thinkng about the choke too, as it idles well, needs the choke to start and dosnt carry on untill the throttle is actually open, lm not thinking theres anything wrong there.

Gets back to to the pilot jet being too big.

Geoff

[OverTheHill]

Hi, hope you get it sorted [i give up now] but had one more thought, had a bike once that had got a bit of 'crumbled' old sponge air filter sucked into the airbleed to the  needle jet which caused it to give running rich symptoms--[think 'not' emulsifying the fuel in the needle jet], mind you it was a super small air jet in a small motor. Cheers. ps, thanks for the good clear photo.