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Hey all, i just joined this site be side I am in need of some desperate help here. I'm working on this '81 cb750 and when I got it, it was only running on one cylinder. So I checked the spark, turns out the pickup for #2&3 is not great, the wires coming out of it are a bit loose, and it wasn't adjusted well. I adjusted it to 6 thou like the other one, then it started to work. I also pulled the bottoms off the carbs and cleaned all the junk out of them and cleaned up the jets. the float levels were all over the place, and I cant figure out how you adjust them. I just put them back together and tried to start it. It started up and it was running on all 4 cylinders this time, albeit not well. It wasn't idling and and would hesitate before it would rev up. I decided to check the valves, and they were all alright, not prefect, but within a couple thou. I did not adjust them. I did a cylinder leakage test as well, and all 4 cylinders were between 7% and 17%. The rings aren't the greatest. I then tried to start it up again, but it wasn't firing. So I pulled the carbs all apart but left them on the bank, and soaked everything in solvent for a day then cleaned them all up. I just rebuilt them now, and everything looks alright but the slides are worn pretty good. I tried to start it up again, but like before, it was backfiring like a bitch out both the carbs and pipes, like really loud bangs. I'm lost, I don't know what else to do, I'm 16, I have the clymer manual for it, but any help would be greatly appreciated!!
Thanks so much, Sean
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Floats on the 81 aren't adjustable unless you use washers. For the most part this isn't advised.
The manual says there is a tab to bend, but it's wrong.
Download the fsm from the link on the the front Page of those sites. Much better than the Clymer and free.
When is this the backfiring? At idle? On acceleration? Don't. You know what jets you have installed? What is your setup? Pods? Stock exhaust? Straight pipes? What jets are currently installed?
Pics will help.
Are you sure your carbs are clean? Search for dohc carb cleaning procedure for a good thread containing resources on how to do this so properly. Dependent on the solvent you used you may have damaged some rubber when you soaked it in solvent.
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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In general, suck, squeeze, bang, blow is what will get an engine to fire. Not sure if you're getting the right gas which is why I asked the carb questions. What are your compression numbers? You mentioned percent losses but not psi. Are you getting more than 100psi on a simple compression test?
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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The bike won't even start now man, it'll be cranking with the choke on or off, doesn't matter, but seems to go bang more when you open up the throttle. As far as jetting goes, I don't know, whatever was in it when it was brought in, but I believe stock. Pilot is turned 2 1/2 out, main? has the number 28 on it. They're all clear, I cleaned the carbs very thoroughly, but I didn't soak any rubber as to not destroy it. The exhaust isn't stock, it's 4-2 with side pipes but they're pretty quiet, so it wouldn't have leaned it out much if any.
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Are you sure that isn't a 68? Stock jets were 68 And105. 28 would be tiny. Lots of rubber in the bank of these cv carbs. You might not notice it if you didn't know if was there. Sent from my BlackBerry device From: 2stroker_4_life [via Honda CB750'S]
Sent: Friday, December 28, 2012 11:58 PM
To: seestheday
Subject: Re: HELP!!! 1981 cb750k won't fire
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The bike won't even start now man, it'll be cranking with the choke on or off, doesn't matter, but seems to go bang more when you open up the throttle. As far as jetting goes, I don't know, whatever was in it when it was brought in, but I believe stock. Pilot is turned 2 1/2 out, main? has the number 28 on it. They're all clear, I cleaned the carbs very thoroughly, but I didn't soak any rubber as to not destroy it. The exhaust isn't stock, it's 4-2 with side pipes but they're pretty quiet, so it wouldn't have leaned it out much if any.
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Also if I had to guess I'd say your slow jet was plugged. They're pressed in and hard to clean. Check the pdf linked to in the dohc carb cleaning thread to learn how to clean and test to see if it's clean.
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Alright, thanks, ill play around tomorrow and see how it goes down, and let you know. I'm sure ill need more carb help! :p
-sean On 2012-12-29, at 12:25 AM, "seestheday [via Honda CB750'S]" < [hidden email]> wrote:
Also if I had to guess I'd say your slow jet was plugged. They're pressed in and hard to clean. Check the pdf linked to in the dohc carb cleaning thread to learn how things. Clean and test to see if it's clean.
1981 CB750K
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Hey guys thanks a lot for your help, I got it running, the pickups were messed up so it was firing at the wrong time. While it does 'run' on all 4 cylinders, it still won't idle after playing with the idle adjustment, it still hesitates to rev although it idling would probably cure that to some degree, and I hear a slight knocking sound from #3 or 4...any ideas thanks alot,
-sean On 2012-12-29, at 12:25 AM, "seestheday [via Honda CB750'S]" < [hidden email]> wrote:
Also if I had to guess I'd say your slow jet was plugged. They're pressed in and hard to clean. Check the pdf linked to in the dohc carb cleaning thread to learn how things. Clean and test to see if it's clean.
1981 CB750K
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Your pickups are off... do you mean your timing isn't right? Do you have a timing light to check it?
Are you sure all of your jets are clean?
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Yeah I do, and I checked the timing, it's good. I fixed the idle today, the screw was waayy out and all messed up but it seems to be running ok now. I just need to fine tune the carbs, however you do that. Sync them and adjust the pilots and whatnot, I'm not sure. Cleaning the carbs made a huge difference though, I think that was mostly what was wrong with it.
-sean On 2012-12-30, at 9:01 PM, "seestheday [via Honda CB750'S]" < [hidden email]> wrote:
Your pickups are off... do you mean your timing isn't right? Do you have a timing light to check it?
Are you sure all of your jets are clean?
1981 CB750K
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You can bench Sync the carbs to get them in the right neighborhood, bit you'll need a carb Sync tool to do it right.
You can check for vacuum leaks easily at this point too. Sent from my BlackBerry device From: 2stroker_4_life [via Honda CB750'S]
Sent: Sunday, December 30, 2012 10:19 PM
To: seestheday
Subject: Re: HELP!!! 1981 cb750k won't fire
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Yeah I do, and I checked the timing, it's good. I fixed the idle today, the screw was waayy out and all messed up but it seems to be running ok now. I just need to fine tune the carbs, however you do that. Sync them and adjust the pilots and whatnot, I'm not sure. Cleaning the carbs made a huge difference though, I think that was mostly what was wrong with it.
-sean On 2012-12-30, at 9:01 PM, "seestheday [via Honda CB750'S]" < [hidden email]> wrote:
Your pickups are off... do you mean your timing isn't right? Do you have a timing light to check it?
Are you sure all of your jets are clean?
1981 CB750K
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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how do you do that?
Date: Mon, 31 Dec 2012 13:59:51 -0800 From: [hidden email]To: [hidden email]Subject: Re: HELP!!! 1981 cb750k won't fire
You can bench Sync the carbs to get them in the right neighborhood, bit you'll need a carb Sync tool to do it right.
You can check for vacuum leaks easily at this point too. Sent from my BlackBerry device From: 2stroker_4_life [via Honda CB750'S]
Sent: Sunday, December 30, 2012 10:19 PM
To: seestheday
Subject: Re: HELP!!! 1981 cb750k won't fire
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Yeah I do, and I checked the timing, it's good. I fixed the idle today, the screw was waayy out and all messed up but it seems to be running ok now. I just need to fine tune the carbs, however you do that. Sync them and adjust the pilots and whatnot, I'm not sure. Cleaning the carbs made a huge difference though, I think that was mostly what was wrong with it.
-sean On 2012-12-30, at 9:01 PM, "seestheday [via Honda CB750'S]" < [hidden email]> wrote:
Your pickups are off... do you mean your timing isn't right? Do you have a timing light to check it?
Are you sure all of your jets are clean?
1981 CB750K
1981 CB750K
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Read the dohc carb cleaning thread. Sorry, way too much detail is already in the docs in that thread. Sent from my BlackBerry device From: 2stroker_4_life [via Honda CB750'S]
Sent: Monday, December 31, 2012 10:45 PM
To: seestheday
Subject: RE: HELP!!! 1981 cb750k won't fire
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how do you do that?
Date: Mon, 31 Dec 2012 13:59:51 -0800 From: [hidden email]To: [hidden email]Subject: Re: HELP!!! 1981 cb750k won't fire
You can bench Sync the carbs to get them in the right neighborhood, bit you'll need a carb Sync tool to do it right.
You can check for vacuum leaks easily at this point too. Sent from my BlackBerry device From: 2stroker_4_life [via Honda CB750'S]
Sent: Sunday, December 30, 2012 10:19 PM
To: seestheday
Subject: Re: HELP!!! 1981 cb750k won't fire
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Yeah I do, and I checked the timing, it's good. I fixed the idle today, the screw was waayy out and all messed up but it seems to be running ok now. I just need to fine tune the carbs, however you do that. Sync them and adjust the pilots and whatnot, I'm not sure. Cleaning the carbs made a huge difference though, I think that was mostly what was wrong with it.
-sean On 2012-12-30, at 9:01 PM, "seestheday [via Honda CB750'S]" < [hidden email]> wrote:
Your pickups are off... do you mean your timing isn't right? Do you have a timing light to check it?
Are you sure all of your jets are clean?
1981 CB750K
1981 CB750K
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Ok thanks a lot kind sir!
On 2013-01-01, at 7:01 PM, "seestheday [via Honda CB750'S]" < [hidden email]> wrote:
Read the dohc carb cleaning thread. Sorry, way too much detail is already in the docs in that thread. Sent from my BlackBerry device From: 2stroker_4_life [via Honda CB750'S]
Sent: Monday, December 31, 2012 10:45 PM
To: seestheday
Subject: RE: HELP!!! 1981 cb750k won't fire
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how do you do that?
Date: Mon, 31 Dec 2012 13:59:51 -0800 From: [hidden email]To: [hidden email]Subject: Re: HELP!!! 1981 cb750k won't fire
You can bench Sync the carbs to get them in the right neighborhood, bit you'll need a carb Sync tool to do it right.
You can check for vacuum leaks easily at this point too. Sent from my BlackBerry device From: 2stroker_4_life [via Honda CB750'S]
Sent: Sunday, December 30, 2012 10:19 PM
To: seestheday
Subject: Re: HELP!!! 1981 cb750k won't fire
|
Yeah I do, and I checked the timing, it's good. I fixed the idle today, the screw was waayy out and all messed up but it seems to be running ok now. I just need to fine tune the carbs, however you do that. Sync them and adjust the pilots and whatnot, I'm not sure. Cleaning the carbs made a huge difference though, I think that was mostly what was wrong with it.
-sean On 2012-12-30, at 9:01 PM, "seestheday [via Honda CB750'S]" < [hidden email]> wrote:
Your pickups are off... do you mean your timing isn't right? Do you have a timing light to check it?
Are you sure all of your jets are clean?
1981 CB750K
1981 CB750K
1981 CB750K
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Locked
