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  1. #1

    Default Honda cb750 1981 dohc - tuning issues - help please!

    Hi guys...

    LATEST UPDATE :

    The focus is getting this bike running for one - then running right.
    I wondered if you guys could take a look at a few vids to see if there's anything you can pick out to help point me in the right direction, which I'll try explain below:

    Vids Link is dropbox : https://www.dropbox.com/sh/udhujtob1...SDXRWZSqa?dl=0
    They're listed in order and with a description!

    The Situ...

    I was having issues getting the bike to really run at all before, so hit the internet and decided to up my Secondary Main Jet to 80 (from 72 - now running 80/118) and shimmed my Jet Needles with x2 Keihin Shim Washers. FYI - F/A Mixture Screws are 3 turns out.

    Gave the bike a turn over today and SHIT, the thing fired up almost right away with Choke and sat idling pretty nicely.
    From there, I gave it a few revs to check that side and first time round, which also sounded on the money.
    This seemed to be all good for a minute or so, but right at the end of the vid you'll hear that when I stop throttling for revs the bike just drops off on idle and it cut itself out. [Video 1 & Video 2]

    So I figured no biggie - maybe just needed to warm up right and really, the bike has barely been running for more than a few minutes in nearly a year...

    Tried starting it up again (Choke on) and giving it a rev...
    This is where I'm stuck -
    It seemed to hold on to the revs pretty highly for a prolonged period after revving (definitely not the throttle sticking), and then dropped back down. When it drops down it sounds fairly patchy and then goes weak - and this happens over and over for a few rounds of revving up and letting it drop back to idle. [Video 3 & Video 4]

    It eventually cut itself out again when it dropped down, so gave it another few tries and some seemed almost perfect and some with this really weird revving (and sometimes abnormally high as if it was almost WOT). This wasn't always just from throttle, sometimes on start it would just whip high and for fear it wouldn't stop itself I'd cut Choke or just turn it off and try again. [Video 5]

    FYI - I also gave it a try altering the Choke level a few times whilst running to see if that helped after a few minutes - but it wasn't really distinguishable any particular way to make a positive difference.

    For next steps :
    I have a Carb Sync tester tool which I need to run and Im aware playing around with Fuel mix or adding / reducing on those Shims could alter my current results - but as before, would be great if anyone can offer up any immediate guidance based on the current sounds...

    Hope you guys can help - it's nearly bike riding weather, even in Scotland!

    Thanks a mill.
    BP

  2. #2
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    I say your carbs are dirty along with the tank and lines.....

    Are you running an air box are pods?????

    I didn't watch your video..... I didn't see a need to watch it.......
    Last edited by Tattooo; 03-28-2019 at 3:41 PM.

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    Quote Originally Posted by Dragstews View Post
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    Yea I will get a pic of mine tomorrow.....

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    Those vacuum guages are terrible! Try these. Be amazed.
    Click image for larger version. 

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    Please do yourself a favor and forget about that contraption. Seriously!

    No leaking fluids ,no vacuum fluids getting sucked into the intake just because the idle went too high for a second. No messes.
    Mount them on a board,
    Click image for larger version. 

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    Last edited by Luky; 04-04-2019 at 1:54 PM.

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    I prefer the liquid manometers because of their rapid response and easy linear comparison of vacuum, but I don't drop or spill them or let the engine suck them dry. The one we use at the shop is a mercury manometer from the 1970s similar to Dragstews pic. It's been on countless motorcycles. Bourdon tube gauges work OK though.

    the bike has barely been running for more than a few minutes in nearly a year...
    Did you drain the carbs when you parked it??? I expect not. This is typical garage sitter fun. I'd pull the low speed jets (they are famous for clogging with varnish) and clean them. If no joy after that I'd strip and dip the carbs (leaving them connected on the rack so as not to fuck up the sync).

    When bikes sit carbs turn to shit. Tattooo is wise.

    Inspect any vacuum hoses and the carb boots for cracking. Old carb boots commonly leak.

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    Quote Originally Posted by Luky View Post
    Those vacuum guages are terrible! Try these. Be amazed.
    Click image for larger version. 

Name:	s-l640.jpg 
Views:	0 
Size:	38.1 KB 
ID:	92640
    Please do yourself a favor and forget about that contraption. Seriously!

    No leaking fluids ,no vacuum fluids getting sucked into the intake just because the idle went too high for a second. No messes.
    Mount them on a board,
    Click image for larger version. 

Name:	Super Magna Carb Balance setup..jpg 
Views:	0 
Size:	92.0 KB 
ID:	92642
    I disagree what you have there is junk...... Let me go take a pic of my setup.....

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    Ok luky here is a pic of my tuners...... I used them on all of my 1100F bikes and many other Honda's....... I had a set of those gauges like you have and sold them because they bounce around to much when you rise the RPMs up....

    By the way mine has steel balls in it so no mess at all...............
    Attached Thumbnails Attached Thumbnails MVC-043S.JPG   MVC-042S.JPG  

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    Oh yea Luky here is a pic of one of my 1100s......
    Attached Thumbnails Attached Thumbnails 11RedHonda.JPG  

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    Quote Originally Posted by Tattooo View Post
    Oh yea Luky here is a pic of one of my 1100s......
    Very nice Tatoo! That 1100 is now back in the modern Honda line up!
    Passengers butt is not up high in the air ruining the low center of gravity either.
    Last edited by Luky; 04-05-2019 at 7:49 AM.

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    On those carbs that have been sitting... I always replace the float needles too because
    the rubber tips get deformed and can leak
    .
    Then a quick check of the vacuum diaphragm on top of the carb.
    Just remove the cover and don't remove the diaphragm but just look for cracks in the rubber diaphram.
    If any of them have a split or crack it will never idle correctly!

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    Quote Originally Posted by Tattooo View Post
    Ok luky here is a pic of my tuners...... I used them on all of my 1100F bikes and many other Honda's....... I had a set of those gauges like you have and sold them because they bounce around to much when you rise the RPMs up....

    By the way mine has steel balls in it so no mess at all...............
    Steel balls eh? LOL

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    Quote Originally Posted by Luky View Post
    Very nice Tatoo! That 1100 is now back in the modern Honda line up!
    Passengers butt is not up high in the air ruining the low center of gravity either.
    At one time I had 4 of them, 2 blue and 2 red....... They were fast as hell for there time..... I sold them all as every time I rode one of them I would get over a 100mph..... I figured I needed to sell them.... LOL

    Here is the blue one that I rode the most.....
    Attached Thumbnails Attached Thumbnails TattoosHonda.jpg  

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    Quote Originally Posted by Luky View Post
    Steel balls eh? LOL
    LOL Yes sir...... It's the best out there it's made in England I think it was......

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    Quote Originally Posted by Tattooo View Post
    At one time I had 4 of them, 2 blue and 2 red....... They were fast as hell for there time..... I sold them all as every time I rode one of them I would get over a 100mph..... I figured I needed to sell them.... LOL

    Here is the blue one that I rode the most.....
    I really like the blue color!
    That engine looks good too.

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    Quote Originally Posted by Luky View Post
    I really like the blue color!
    That engine looks good too.
    Yea they only were made in Red and Blue......

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