Author Topic: why does the CB550 have a "flat spot" in the torque curve?  (Read 5933 times)

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Offline jason41987

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why does the CB550 have a "flat spot" in the torque curve?
« on: September 16, 2012, 08:53:39 AM »
it seems, listening to 550 owners, and listening to some hondaman data, that the 550 does in fact have a flat spot in the torque curve... and he mentioned some ways to reduce this effect.. but what really causes it?.. what in the mechanics of the engine is reponsible for the curve flattening out, before picking back up again?

Offline Bailgang

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #1 on: September 16, 2012, 09:52:24 AM »
My guess and this is just a guess is that the stock 550 carbs don't do a very good job metering the air/fuel as it makes its transitions from idle to part throttle to WOT. I'm sure other factors may involved.
Scott


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Offline jason41987

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #2 on: September 16, 2012, 10:06:37 AM »
i know honda did something to this engine to smooth it out for a daily rider role.. but id like to know mechanically what they did so that i can perhaps reverse or change it to something a little higher performing, and even more fun to ride

Offline Stev-o

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #3 on: September 16, 2012, 10:54:37 AM »
I think you may be over thinking this.
I have a '76 550 with stock motor/carbs and do not notice a flat spot. The bike is a blast to ride!
Have you ridden your 550?
'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline Bailgang

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #4 on: September 16, 2012, 11:35:44 AM »
I think you may be over thinking this.
I have a '76 550 with stock motor/carbs and do not notice a flat spot. The bike is a blast to ride!
Have you ridden your 550?

I agree. Granted mine has a 650 cam but even with the stock cam I never could figure out what was meant by the flat spot. I'll admit it does come alive when I hit about 6 or 7k rpm much more so that any of the lower rpm's and you bet it's a blast to ride.
Scott


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77 cb750 F2
83 gl1100 Interstate

Offline jason41987

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #5 on: September 16, 2012, 06:45:28 PM »
i believe thats what i was referring to, how the power doesnt really take off until the higher RPMs, mid-range it just as alive... wondering how you can make these "come alive" at lower revs and continue throughout... maybe un-tame it.. do the 650 cams do this?

Online dave500

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #6 on: September 17, 2012, 01:58:15 AM »
5500 and up the power should come on strong,you can snap the throttle open at these rpms,if you snap it open at lower rpms itll flat spot right out,a smooth opening of the throttle all the way will show no flat spot.

Offline jason41987

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #7 on: September 17, 2012, 04:37:26 AM »
what is responsible for that flat spot when you snap it back though? im curious... do new cams fix it or is it something deeper than that?

Offline fmctm1sw

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #8 on: September 17, 2012, 06:47:25 AM »
what is responsible for that flat spot when you snap it back though? im curious... do new cams fix it or is it something deeper than that?

With slide carbs, snapping the throttle wide open at low RPMs isn't a good idea.  It's a carburetion issue.  Do a search for CV vs slide carbs.  Either Hondaman or TT explained it somewhere and I don't want to misquote.  An explanation might be on Randakks site too...
Quote from: 754
Dude is that a tire ? or an O-ring..??

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This is not a pod thread
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1973 CB350G
1975 CB550K
1983 GL650I
1973 CB750K3 (http://forums.sohc4.net/index.php?topic=92888.0)
1984 Kawasaki KLT-250 (AKA 3 wheeler of death)
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Offline strynboen

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #9 on: September 17, 2012, 07:06:28 AM »
my cbx 1050 have a flat spot..you have to break the wall..or just let it eat it a bit slover...vhen you are over the spot its eat rpm as nothing..to 10,500 rpm
but the 550 pull strong from bottom..2 valve heads give more pull in low..4 valve head gives hp...but at higher rpm
but hab ve to pull my caps,,to thek the acc pump this vinter
i kan not speak english/but trying!!
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http://forums.sohc4.net/index.php?topic=144758.0
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Online dave500

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #10 on: September 17, 2012, 12:32:58 PM »
put simply they dont have an accellerator pump,when you snap open the throttle it goes way lean and fuel stops flowing as the air speed drops over the jets,try it from idle in neutral,snap it full open and itll stall.

Offline oldfart

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #11 on: September 17, 2012, 12:38:14 PM »
When today's bikes have up to 13mm lift per inch of valve diameter, you can't expect a machine having only 5mm valve lift per inch of valve diameter to be very robust power-wise. You can rev a 500/500 til the tach wraps its needle all the way around the instrument the valve action os so mild. 
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Offline Bailgang

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #12 on: September 17, 2012, 02:46:46 PM »
i believe thats what i was referring to, how the power doesnt really take off until the higher RPMs, mid-range it just as alive... wondering how you can make these "come alive" at lower revs and continue throughout... maybe un-tame it.. do the 650 cams do this?

First off I have to agree with Steve-o because I too think you're over thinking this. It's good to see you're actually reading up on the subject but you're beating yourself up on it. I highly recommend that you get the bike up and running in stock configuration first THEN start playing with it if you're not satisfied. There are plenty of stock 550 owners here who are happy with their bikes just the way they are and you may too.

I can tell you this from my own experience when I first put my 550 together, I too did what you're thinking of doing and started doing mods before I even had the engine running and then couldn't figure out why it wouldn't run worth a s..t and it took me a year to gather up the money and parts to get it back to stock config and it was then that the bike started to run good and from that point on I did 1 mod at a time but even then aside from the 650 cam, dyna ignition, carb jetting and exhaust the bike is pretty much stock.

There are 2 main reasons I installed the 650 cam, 1 is because I can't help messing with stuff and 2 is because my wife rides with me a lot and I wanted a bit more power to deal with our combined weight but even then I didn't mess with the 650 cam until after the bike was running good with the stock 550 cam.
Scott


71 cb350 twin
77 cb750 F2
83 gl1100 Interstate

Online dave500

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #13 on: September 18, 2012, 12:46:55 AM »
these carbs also dont have an off idle or power circuit,they also dont have a vacuum ignition advance system.

Offline jason41987

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #14 on: September 18, 2012, 04:57:44 AM »
hmm.. doesnt the CB450 use CV carburetors? is that why they tend to not have the slow response?
« Last Edit: September 18, 2012, 05:05:45 AM by jason41987 »

Offline jason41987

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #15 on: September 18, 2012, 05:46:14 AM »
just to verify, im just trying to learn more about the mechanics of, and the nature of these bikes, and the systems required for them to operate.. myself, im more likely to aim for better fuel economy rather than better throttle response

Offline singedebile

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #16 on: September 18, 2012, 09:14:04 AM »
I feel like there is a noticeable flat spot (not talking about snapping the throttle), its between 4k & 5k and the motor always sort of pauses slightly for a moment... of course the motor has all its power above 6k, but around town I find myself hitting this flat spot all the time. Its not that bad, or annoying but it is noticeable.
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Offline jason41987

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #17 on: September 18, 2012, 09:36:40 AM »
im curious... is it the 550? or do other bikes with the same carbs have a similar effect?, such as the 500F

Offline fmctm1sw

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Quote from: 754
Dude is that a tire ? or an O-ring..??

Quote from: inkscars
This is not a pod thread
This is not a #$%* on my vacuum gauges thread
This is a help or GTFO thread.

1973 CB350F
1973 CB350G
1975 CB550K
1983 GL650I
1973 CB750K3 (http://forums.sohc4.net/index.php?topic=92888.0)
1984 Kawasaki KLT-250 (AKA 3 wheeler of death)
1994 Honda TRX300
1999 Honda TRX250

Offline mono

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #19 on: September 18, 2012, 01:45:57 PM »
I feel like there is a noticeable flat spot (not talking about snapping the throttle), its between 4k & 5k and the motor always sort of pauses slightly for a moment... of course the motor has all its power above 6k, but around town I find myself hitting this flat spot all the time. Its not that bad, or annoying but it is noticeable.

mine too.  not bad, but i do feel it till 5K, then i can nearly yank myself out of the seat if i crank it.  :)

Online dave500

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Re: why does the CB550 have a "flat spot" in the torque curve?
« Reply #20 on: September 18, 2012, 02:01:13 PM »
when you hit your flat spot try and hold those rpms,if the engine seems to falter or blubber badly you need to do some slow speed jetting,if its minor and clears away with any throttle increase its ok.