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Author Topic: What's an air filter worth in jet size?  (Read 827 times)

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Offline jawntybull

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What's an air filter worth in jet size?
« on: August 14, 2011, 05:16:40 PM »
It's taken me a while to get my 750 K7 running well - stock pipes, carbs from the F model not the K (PD41B), standard but earlier model K airbox. Its now running nicely and a plug chop at 80kmh looks good (except cyl 1 running rich, which I need to look into - see photo). The trouble is, in order to fix an over rich condition I've done a few things including taking the air filter out of the box. I'm running 110 mains at the moment.

I want to put the airfilter back and have to buy mains to suit - so my question is, what is an air filter worth in terms of main jet sizes? ie how much change in jet size do I need to compensate for the pressure drop across the air filter? Eg if it's worth one jet size, then I would drop the jet size down to 105.

I'd like opinions on this before buying the jets. Of course I'll test myself but at $40 for an OEM set I don't want to buy too many main jets!
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Bandit 1250S, DR650, CB750K

Offline dtmmil

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Re: What's an air filter worth in jet size?
« Reply #1 on: August 14, 2011, 08:37:47 PM »
Not sure about your airbox trouble, but to me, you have 3 rich, and one just a little leaner, but still rich.
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Offline jawntybull

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Re: What's an air filter worth in jet size?
« Reply #2 on: August 15, 2011, 04:58:16 PM »
Really? You should have seen them before - all black soot, with no. 1 so caked up you could chip the soot off with your fingernail.

Looks like I need 105s in any case, so I've ordered them. I'll then re-set the mixture screws and do a carb sync, then see where I'm at.

BTW had a strange thing going on with the mixture screws; I went to set them by screwing them in until the engine stumbled, but it didn't stumble at all. I had no. 4 screwed in all the way and the engine was still running at 1000rpm. Anyone else had this? Right now I just have them set one turn out and then idle is good.
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Offline Duke McDukiedook

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Re: What's an air filter worth in jet size?
« Reply #3 on: August 15, 2011, 05:57:28 PM »
Keep the filter in, sounds like you may have some unsynchronized carbs, you will have to synch them. I would recommend cleaning the carbs before you do that though.
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Offline jawntybull

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Re: What's an air filter worth in jet size?
« Reply #4 on: August 19, 2011, 01:26:08 AM »
Spot on Dukiedook - I changed the jets down to 105 today, put the airfilter back in, set the mixture screws to 1.5 turns out, then synced the carbs with my shiny new Morgan Carbtune. Man does it make the job easy, even fun! I found the sync went out when you unscrewed the top of a carb - must be some leakage down through the needle chamber. The rich no. 4 cyl was showing significantly higher vacuum. Took me a few iterations to sync them all as one seems to affect the other, but they're now in sync.

Its raining and dark now so no test ride tonight - will give her a spin tomorrow. Fingers crossed!
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Bandit 1250S, DR650, CB750K
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