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Mysterious husqvarna carb issue

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Hi guys, new to the forum but an old hand at carbs, yet I've got an issue with my missus' bike that makes no sense. It's an 04 TE250 with a mikuni TMR36.

The bike starts ok and pulls very well indeed under most circumstances. However it has what feels like a horrible lean surge at very slight throttle openings, it's something you don't notice when you're powering along but if you attempt to hold a steady speed at part throttle it jerks and bucks uncontrollably. It behaves identical to every bike with a clogged/poorly set pilot I've ever ridden. I've taken the carb apart numerous times, everything is surgically clean and set up correctly. The accelerator pump works fine and is set correctly, and indeed the issue disappears if you roll on the throttle even a tiny bit.

I know this sounds like a straightforward pilot jet/screw issue or an accelerator pump issue BUT the bike seems to be running rich, not lean. I'm able to screw the pilot screw all the way in and it not only continues to idle but actually runs better with a stronger idle and a slight diminishing of the surging issue. The float level is correct and the float o-ring/valve definitely aren't leaking (I've tested it on the bench with the bowl off and there's no leaking). I don't understand how it can continue to idle with the pilot screw shut because there shouldn't be anywhere else for it to get fuel from, yet it clearly is. I'm able to supply the carb with fuel on the bench and there doesn't appear to be anything unusual going on.

Any ideas?

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Surging off of idle is not pilot/fuel screw, they are for idle only. Not the AP, that olnly operates when the throttle is being opened. You issue is needle related, you need to determine WHERE on the needle you are when this is happening.

 

If your bike idles with the fuel screw closed, the pilot is too large. Fix this first.

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Thanks a lot! That makes sense. I'm realising now that the situation is a lot worse than I thought it was...

The bike has had an atehena barrel stuck on by someone who owned the bike long before us. Mechanically the engine seems to be in good-enough-to-run condition, clearances/compression/spark etc all check out and the bike itself is in reasonable condition and shows little sign of stupidity.

I don't know what displacement it is but it feels quite a bit quicker than every other 250F I've ridden. I'm assuming roughly 300 as it's still slightly slower and less grunty than my ancient KTM400. The jetting in the carb has been fooled around with for presumably that reason so the jet sizes don't match factory. All of this was done by some PO in the mysterious depths of the past. On the whole it seems to run relatively well everywhere other than at the very slightest throttle opening. But a delve into the jetting reveals some confusing things:

The pilot is a 30 and according to the FSM a 250 should have a 22.5, for reference a 450SM has a 30. So definitely need to go smaller there.

It also looks like I'm completely missing the plastic cap at the top of the AP shaft so there's about 2.5mm of travel before the pump cam can press on the shaft. I can't find a part number for that anywhere, it's immediately above #10 in this diagram. It isn't even part of a bigger parts group so no idea how I go about buying one.

Based on the FSM the needle is identical across various displacements, I don't know if I need to move it up a slot as it's currently at the factory position. Main is also chosen optimistically at 170 over the factory 150; a 450 has a 175.

 

What would be a good place online to buy genuine mikuni jets?

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Other than a competent dealer, JetsRUs or SUDUCO or possibly James Dean for that AP shaft. You might want to look at other parts listings for bikes that use that carb, you might find it listed there. All sorts of 'missing' parts can be found that way.

 

Big bore engines typically use SMALLER/LEANER mains and pilots due to the engine having a stronger vacuum effect and thereby drawing more fuel. A lot of people thing that the stronger sucking from a bigger barrel only sucks air. If it were dead even, you'd need no jetting changes. Most bikes, the larger the more, the smaller the jets. They are flowing more because the forces that draw the fuel are stronger.

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If it has pipe other than stock you should use oem jet sizes as a reference only. The 2.5mm gap for AP seems excessive although I'm not an expert. I have 0 gap on my crf with fcr carb and have inspected timing of squirt does not interfere with slide. I was amazed at how much squirt I get with slightest throttle movement.
I've gotten my mikuni jets off eBay, I believe seller was bobsjets or something like that. Marking grip and housing at full, half, quarter and eighth will help isolate circuit giving issues.

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