AR125 running WAY too rich

[tshort2007]

In need of some serious help, before i take a lit rag to the petrol tank. Got an AR125 (1989) 2 stroke with mikuni VM26 carb. The bike is totally stock, apart from mikomi can. Thats how it was when i bought it.

The bike had a flat spot aroung 5-8k then almighty power.

I have uprated the main jet from 100 to a 105 jet, to lessen the flatspot, but now the bike struggles to idle, the idle screw does almost nothing, and the bike wont go above 3000 RPM in ANY gear. When i pull the plug, its dripping wet with fuel. I know the main jet has no effect until 75% throttle, but am still getting problems.

Further to all this, i have no idle air or fuel screws, so i cant adjust the mixture.

Any help would be seriously appreciated

Oh, and i tried puttin the 100 jet back in, and still problems

[Blau Zedong]

Where have your idle and pilot air screws gone?
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18:54:48 Rob Fzs: jews don't give away stuff for free

18:54:59 Rob Fzs: unless its their clothes/ hair/ golden teeth

[tshort2007]

[Timmeh]

If you haven't got the big silver screw STOP what you are doing and get a replacement carb from eBay, before you destroy your rings, piston and bore with massive over-fuelling.
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GSXR400 x2 | '94 RVF400 | '93 TZR125 4DL (again)
20:22:30 BLUEX5: i would love to be forced to undergo a**l plugging with different sizes
20:48:18 Temeluchus: comp you hunk of smouldering homos3x you

[tshort2007]

Yes, i do have the big silver screw. Its the screw marked 17 thats missing on mine. When i say missing, its never had one. the recess where it goes was blanked off when the carb was made. Mr cousins AR125 is exactly the same.

PS thanks for the diagram!

Was thinkin, if i cant control my idle air or fuel, these must of been set at the factory? correct? if so, and i'm runnin rich, is it possible my idle air jet is blocked?

[Timmeh]

No probs about the picture

Definately take the carb apart and check it for correct operation.

Something could be blocked, a float stuck or at the wrong height.
____________________
GSXR400 x2 | '94 RVF400 | '93 TZR125 4DL (again)
20:22:30 BLUEX5: i would love to be forced to undergo a**l plugging with different sizes
20:48:18 Temeluchus: comp you hunk of smouldering homos3x you

[tshort2007]

Ok, i sprayed carb cleaner in the air jet. Started bike, and had a very boggy tickover, similar to a problem i had last week, which just went away! after a few minutes, it idled fine. Killed the engine, and stripped the carb. Blew through the airjet with carb cleaner, but couldnt see the spray coming out anywhere on the carb, apart from back in my face! (Note to self, wear glasses, carb cleaner burns eyes!)
Took main jet out, and plugged hole with my thumb, put nozzle inside needle jet and blew. Cleaner fluid came out of every hole, apart from the air jet. A few seconds later, "POP" and out the airjet it came!

Rebuilt bike, fired up engine, and proceeded to give it a 5 mile thrash, using varying throttle positions (i allowed a 5 minuite tickover warmup first @1250 RPM). Acceleration was poor, with lots of gear changes required.
Got back, and pulled the plug. Looks like

https://www.spark-plugs.co.uk/pages/technical/diagnosis.htm

The second image down.
Now, as i cant seem to adjust my fuel/air mix, could this be to a poor spark? or shall i just buy a new carb? Funding is limited, so i can only replace 1 componant!

Thanks in advance!

[Kickstart]

Hi

Idle jet will make no real difference when you are riding it, so if after a thrash the plug is showing signs of it being rich then don't worry about the idle jet (for now).

From an old book, it looks like a 100 main jet is standard for a restricted AR125 with a 95 used in the full power versions (but with a larger idle jet). Does seem a bit odd (and I don't believe the book or my reading of it entirely) but might explain the issue you have.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[tshort2007]

lol, that book must be old! I belive my bike is a NZ import, for many reasons, mainly the colour was only officially released over there. As for the fact i got 75 out of it this afternoon, with it running rich also indicates that it hasnt been restricted to 12 BHP like other uk models, but is actually 22 BHP. The original jet was a 100 in the carb, and is now running a 105.
Now, IIRC, the position of the throttle slide indicates which jet is being used, not the RPM, so in my interpritation, if acceleration is poor, open the throttle more (or less) to provide fuel using a different jet. this should resolve "bogging" indicating a blocked or wrong size jet. I get poor acceleration with all throttle positions, which is why i think electrical. BTW, new plug (7 and 8) used with new performance HT lead.

EDIT:- forgot to mention, what i said about the jets proves right, because if you pull the main jet out compleatly, you bog out big time at 75% throttle or above, but below 75% throttle is unaffected

[Kickstart]

Hi

Full power ones were sold in the UK as well (plenty sold in Northern Ireland as they didn't get the mainland 12hp / 125 learner law). A UK restricted one would show 75mph indicated fairly happily (they were just very expensive to derestrict properly).

On a slide carb you are right that throttle position determines which jet it in play (and this is why they are less idiot proof and why CV carbs became so common on large bikes). Trouble is that a wide open throttle at low revs probably means a low air speed through the car and so a low pressure drop, hence failing to pull in fuel from the float bowl.

The main jet is the ultimate restriction on fuel flow, but if the needle position is such that it can't flow as much as the main jet will allow then removing the main jet won't make a difference. Think the needle jet should be on the 2nd position.

What do you mean by "performance HT lead"? Fairly sure it should just be a length of copper cored HT lead

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[tshort2007]

[Kickstart]