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Author Topic: My 750 won't rev properly. <Kinda solved>  (Read 2197 times)

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Offline SOHCiro

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My 750 won't rev properly. <Kinda solved>
« on: May 14, 2018, 10:57:45 PM »
It's a 76 750f.
Starts right up and idles fine, so I assume it's not a coil issue.
In 1st, it'll rev up to about 6500rpm, begins to hesitate and lose all the power, backfires a bit.
put it in 2nd rev to 6000rpm, 3rd 5500rpm and so on...
On the highway it'll pull to 5000rpm on 5th at around 60 mph when it stops accelerating.

My initial idea was that it wasn't getting enough fuel due to clogged jet, so I pulled out the spark plug #1 and bingo it was white.
Gave the jets good cleaning, no change. Adjusted float height to 26mm on all four carbs, still no change.

Valve gaps, points gaps, and timing were all adjusted accordingly to the manual and it ran perfectly before this problem appeared, and haven't changed the setting since then.
What do you think the problem is?
« Last Edit: May 15, 2018, 03:12:54 PM by SOHCiro »
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76' 750F SuperSport

Offline BobR

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Re: My 750 won't rev properly.
« Reply #1 on: May 15, 2018, 01:47:15 AM »
   Presuming all the work you mentioned is correct perhaps a clogged strainer in the petcock? Bob
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Offline robvangulik

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Re: My 750 won't rev properly.
« Reply #2 on: May 15, 2018, 02:19:24 AM »
What size jets are in there?
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Offline ekpent

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Re: My 750 won't rev properly.
« Reply #3 on: May 15, 2018, 04:30:32 AM »
 Could be several things. Bad condensers if your running stock ignition can give those symptoms as well as possible carb or fuel flow problems.
« Last Edit: May 15, 2018, 04:43:25 AM by ekpent »
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Offline SOHCiro

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Re: My 750 won't rev properly.
« Reply #4 on: May 15, 2018, 12:17:18 PM »
Petcock was replaced by a brand new one a couple of weeks ago,
jets are 40/120 with pods and aftermarket reverse cone muffler with no packing in it,
condensors were replaced a month ago...
Could it be a tiny intake leak? Or possible a weak spark plug/cap connection?
I've redone the entire electrical harness a while back but can't tell if its electrical or carb related issue.
Thanks a bunch
« Last Edit: May 15, 2018, 12:28:47 PM by SOHCiro »
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76' 750F SuperSport

Offline SOHCiro

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Re: My 750 won't rev properly.
« Reply #5 on: May 15, 2018, 12:20:04 PM »
She's the one ;)
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76' 750F SuperSport

Offline 2wheels

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Re: My 750 won't rev properly.
« Reply #6 on: May 15, 2018, 12:48:16 PM »
Possibly a compression test is in order.
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Offline SOHCiro

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Re: My 750 won't rev properly.
« Reply #7 on: May 15, 2018, 01:04:48 PM »
Well I haven't tested compression yet, so it's on my list when I have the chance!
Can low compression cause a problem like this to suddenly occur on an otherwise good running bike?
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76' 750F SuperSport

Offline SOHCiro

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Re: My 750 won't rev properly.
« Reply #8 on: May 15, 2018, 03:11:40 PM »
I decided to check for clogged passages again and found this crack on the #1 carb main jet housing ;D

I believe the problem came from the carb sucking up extra air from the crack and leaning the mixture, hence the hesitation to accelerate beyond the main jet range and the white plug.

No way in hell I'm buying a set of carbs for this one hairline crack!
I found this neat handgun shell trick (link below), but I'm probably gonna try JB weld on the crack tomorrow.
http://forums.sohc4.net/index.php?topic=100144.0

let me know if you guys know better sealant!
« Last Edit: May 15, 2018, 03:19:17 PM by SOHCiro »
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76' 750F SuperSport

Offline kmb69

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #9 on: May 15, 2018, 03:31:44 PM »
Quote from: SOHCiro on May 15, 2018, 03:11:40 PM
I decided to check for clogged passages again and found this crack on the #1 carb main jet housing ;D

I believe the problem came from the carb sucking up extra air from the crack and leaning the mixture, hence the hesitation to accelerate beyond the main jet range and the white plug.

No way in hell I'm buying a set of carbs for this one hairline crack!
I found this neat handgun shell trick (link below), but I'm probably gonna try JB weld on the crack tomorrow.
http://forums.sohc4.net/index.php?topic=100144.0

let me know if you guys know better sealant!


Jim French, a forum member here, can micro tig or micro laser weld that carb. Send him a PM.
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Offline SOHCiro

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #10 on: May 15, 2018, 03:42:10 PM »
Thanks!!!
Do you know his user name?
The search function is still not working, have to look up on the memberlist.
Nothing comes up with "Jim French"  :-\
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76' 750F SuperSport

Offline kmb69

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #11 on: May 15, 2018, 03:50:03 PM »
He's Jim F.
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Offline SOHCiro

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #12 on: May 15, 2018, 03:51:39 PM »
Sweet, just got in touch with him. Sounds like a super nice person!
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76' 750F SuperSport

Offline my name is nobody

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #13 on: May 15, 2018, 10:25:15 PM »
That jet holder ,which holds the main jet and serves as the emulsion tube, screws into the carb body..
When installed in the carb., it should still have several threads showing below the carb body when it's
tight. It looks like somebody has possibly installed that one and overtightened it, forcing it in
to the carb body farther than it should be, cracking the housing. If you remove the jet holder now, you
will more than likely find the soft brass jet holder/emulsifier tube has been mushroomed and crushed
on the top end also.
Are all 4 like that?
« Last Edit: May 15, 2018, 10:35:21 PM by my name is nobody »
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Offline evanphi

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #14 on: May 16, 2018, 07:04:37 AM »
Do you have an inline fuel filter? They have a nasty habit of clogging up.
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Offline SOHCiro

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #15 on: May 16, 2018, 03:45:59 PM »
I've just sent the carb to Jim, will see if that solves the problem and I'll throw an update.
Thanks guys!
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76' 750F SuperSport

Offline American Locomotive

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #16 on: May 16, 2018, 06:00:25 PM »
While that was certainly an issue, I don't think one bum carb would make the whole engine not rev out. I've seen plenty of people ride 4-cylidner bikes right up to redline with a cylinder or two not firing at all.
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Offline SOHCiro

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Re: My 750 won't rev properly. <Kinda solved>
« Reply #17 on: May 16, 2018, 09:38:55 PM »
That's true, I'll go over coil connections etc once I get the carbs back 8)
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76' 750F SuperSport
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