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Offline Blitzburgh207

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Low back pressure or ?
« on: September 13, 2011, 06:58:58 AM »
My bike tends to be sort of bogged off the line and at 2.8-3k its like someone turns on the lightswitch and off she goes running and sounding beautifully.

Now my question is: Is this typically caused by jetting, timing, carb blockage, lean/rich condition, low backpressure? 

My plugs are tan at idle but could possibly on the lean side of it.

Setup: 78k, cycle-x 4-2-1 (wrapped), pods
Timing, valves & cam chain adjustment done w/in past 500mi as well as carbs cleaned and synced.
Jetting: slows drilled w/ .45mm bit (which makes them #45, right?), needle shimmed 2mm, 135 mains (140's later today  8))

Oh yah and the accelerator pump is working properly to all carbs.  I think thats it.
Thanks guys!
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Offline Blitzburgh207

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Re: Low back pressure or ?
« Reply #1 on: September 13, 2011, 07:00:22 AM »
Dang - forgot to add that it still happens but not as much when I run a little seafoam in the fuel.

Thanks again!
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1978 CB750 K8

Offline Blitzburgh207

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Re: Low back pressure or ?
« Reply #2 on: September 14, 2011, 06:11:41 AM »
bump.
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1978 CB750 K8

KingCustomCycles.com

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Re: Low back pressure or ?
« Reply #3 on: September 14, 2011, 06:23:32 AM »
I have a 750 that bogs if I run any larger than 40 idle jets.  I would put stock idle jets in and see if that helps.
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Offline dtmmil

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Re: Low back pressure or ?
« Reply #4 on: September 14, 2011, 07:06:27 AM »
Try adjusting the fuel mixture screws, give them 1/8th turn one way, try it and keep doing that till you get something.
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Offline fmctm1sw

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Re: Low back pressure or ?
« Reply #5 on: September 14, 2011, 12:07:28 PM »
Make sure your timing advancer is operating smoothly...
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This is a help or GTFO thread.

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Offline WarwickE36

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Re: Low back pressure or ?
« Reply #6 on: September 14, 2011, 12:41:44 PM »
RPM's and carbs aren't really related, per se.  Carbs tend to be related to throttle position.  I agree with fmctm1sw, Ignition timing and rpms are more closely related.  Especially at the rpm range you speak of.  I know my 550's full advance kicks in around 2,500 RPM.  When I bought my bike the PO had installed a dyna ignition and #$%*-ed it up.  There is a pin on the advance shaft you HAVE to remove when installing a dyna.  He did not remove it.  He then crammed the rotor over the shaft but it did not move freely.  He happened to cram it locked in the FULL advance position.  My bike acted just like yours. Rough idle, although it did idle at 1,000.  Hesitation off the line and boggy down low up to about 3k, the BWAAAAHHHH it roared. 

You should inspect the advancer for sure.  Make sure it isnt stuck open.  You should be able to manually open it and the springs should snap shut when you let go.  This is true for both points and dyna.  If it doesn open and close smoothly disassemble and find out why, it may just need a little anti seize on the shaft. 
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" Why does anyone get offended by what someone does to their own bike? I dont get it. "

You made me think about it after I cheered knowing someone else would like to know what these control freaks are up to.
 Every time any owner strips whatever precious Honda part the horrified purists parts go up in value. That's not the part that bothers them.
 What bothers them is they sat up late at night, their breast full of wonder and estrogen, unable to sleep, dreaming about their lovely darling and all her glory... and next thing you know someone else doesn't share their emotional deluge and their reaction is they must spread their mind museum as far as they possibly can, taking over as much of the real world as possible.

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Offline Blitzburgh207

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Re: Low back pressure or ?
« Reply #7 on: September 14, 2011, 06:09:22 PM »
oh very cool - i didnt even know that was back there haha. Well i have about 10 hours of hw to do tonight so that check will have to wait til tomorrow ): in the meantime ill also source some slows - but the problem seemed to lessen when i worked my way out from 1.5 to about 3 turns out on the idle/mix screws...?  :-\ does seem like an awful lot of fuel at zero throttle tho.
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1978 CB750 K8

Offline WarwickE36

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Re: Low back pressure or ?
« Reply #8 on: September 14, 2011, 06:13:00 PM »
Im pretty sure the only homework at Orono is upsidedown with a keg tap line in your mouth.  I should know I went there. 
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" Why does anyone get offended by what someone does to their own bike? I dont get it. "

You made me think about it after I cheered knowing someone else would like to know what these control freaks are up to.
 Every time any owner strips whatever precious Honda part the horrified purists parts go up in value. That's not the part that bothers them.
 What bothers them is they sat up late at night, their breast full of wonder and estrogen, unable to sleep, dreaming about their lovely darling and all her glory... and next thing you know someone else doesn't share their emotional deluge and their reaction is they must spread their mind museum as far as they possibly can, taking over as much of the real world as possible.

Drink fast, drive slow, but ride it like you stole it

1974 Cb550 with style
2004 SV650s

Offline BobbyR

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Re: Low back pressure or ?
« Reply #9 on: September 14, 2011, 06:43:04 PM »
Quote from: Blitzburgh207 on September 14, 2011, 06:09:22 PM
oh very cool - i didnt even know that was back there haha. Well i have about 10 hours of hw to do tonight so that check will have to wait til tomorrow ): in the meantime ill also source some slows - but the problem seemed to lessen when i worked my way out from 1.5 to about 3 turns out on the idle/mix screws...?  :-\ does seem like an awful lot of fuel at zero throttle tho.
On the K8 you have a accel pump and they clog easy. I cleaned mine and set the tab that works the pump so she comes in early. It made a big difference. Also, these motors don;t really have a lot below 4K.
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Offline seanbarney41

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Re: Low back pressure or ?
« Reply #10 on: September 14, 2011, 07:18:49 PM »
Quote from: Blitzburgh207 on September 14, 2011, 06:09:22 PM
oh very cool - i didnt even know that was back there haha. Well i have about 10 hours of hw to do tonight so that check will have to wait til tomorrow ): in the meantime ill also source some slows - but the problem seemed to lessen when i worked my way out from 1.5 to about 3 turns out on the idle/mix screws...?  :-\ does seem like an awful lot of fuel at zero throttle tho.
make sure your fuel level in your float bowls is a lot closer to the gasket line than you probably think it should be...4mm from the gasket on older carbs, not exactly sure on yours...correct fuel level in your bowls will make carb tune up a lot more logical...
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Offline Blitzburgh207

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Re: Low back pressure or ?
« Reply #11 on: September 15, 2011, 06:57:41 PM »
Quote from: WarwickE36 on September 14, 2011, 06:13:00 PM
Im pretty sure the only homework at Orono is upsidedown with a keg tap line in your mouth.  I should know I went there. 
Oh I never do homework without beer  ;D

Quote from: BobbyR on September 14, 2011, 06:43:04 PM
On the K8 you have a accel pump and they clog easy. I cleaned mine and set the tab that works the pump so she comes in early. It made a big difference. Also, these motors don;t really have a lot below 4K.
That sounds like a good trick!  I'm not looking to gain any power below 3k persay... I just want to clean it up a little - eliminate the sputtering/gargling as much as possible.

Quote from: seanbarney41 on September 14, 2011, 07:18:49 PM
make sure your fuel level in your float bowls is a lot closer to the gasket line than you probably think it should be...4mm from the gasket on older carbs, not exactly sure on yours...correct fuel level in your bowls will make carb tune up a lot more logical...
I set the floats a little while back, but neglected to use the clear tube method.  I know the "measured" level is different on the PD carbs than the earlier models (don't remember the difference off the top of my head) but does anyone know if the level using the clear tube method should still be just below the gasket?  I noticed the #3 bowl had a slow leak in it the other day so maybe that one is too high...

Lots of checks for me again! Looks like my girlfriend is going to be jealous again this weekend!  ;D

Anyways, back to my, uh, "homework"   ::) Thanks guys!
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1978 CB750 K8
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