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Author Topic: 78 750 hondamatic power issues  (Read 480 times)

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Online Evilneversleeps

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78 750 hondamatic power issues
« on: October 11, 2020, 03:39:26 pm »
Hi all! I’ve been restoring a 78 750 hondamatic since August. Super fun bike and a great learning experience. I’m noticing a power shortage and need help figuring out why. The bike starts fine and idles fine. When I put it in gear with the break on - especially when cold-  it dies. I can give it a little throttle and put it in gear and it’ll drive but very slowly at first. I can’t find any leaks via carb cleaner  test but maybe a pinhole in my Petcock to carb line is #$%*ing things up?

I also have the timing plate pinned all the way counter clockwise. I need to readjust the points and time it properly, but would this account for low power when throttle is opened? Or when bike is in gear?

Idle regulator system works properly btw.

Could this be a symptom of a bad fuel pump ( the one in carb 2?) or just mal adjusted timing?
« Last Edit: October 11, 2020, 05:20:49 pm by Evilneversleeps »

Online Evilneversleeps

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Re: 78 750 hondamatic power issues
« Reply #1 on: October 16, 2020, 08:24:06 am »
So adjusted timing and there was an instant improvement.

Online Evilneversleeps

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Re: 78 750 hondamatic power issues
« Reply #2 on: October 23, 2020, 12:47:01 pm »
And am Now replacing airbox where there was a leak

Offline HondaMan

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Re: 78 750 hondamatic power issues
« Reply #3 on: October 23, 2020, 05:33:48 pm »
Good job!
:)
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Offline Randy Nevrotski

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Re: 78 750 hondamatic power issues
« Reply #4 on: November 02, 2020, 06:36:29 pm »
you have to lift off the gas tank a little bit get a small screwdriver . now find the screw on right hand side top of carbs not idle knob between #3 & #4 carbs turn that screw a half turn at a time to the right . that's to kick up idle when in gear . if to high it will take off on you . put on center stand with brake on . its in shop Manuel

Online Evilneversleeps

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Re: 78 750 hondamatic power issues
« Reply #5 on: November 07, 2020, 04:41:44 pm »
Good job!
:)


Thanks Hondaman!

So now it’s runs but runs lean as hell. I can’t find any vacuum leaks with carb cleaner. I ordered a new air filter with the new box. It’s a hi-flo filter. Would this cause it to run lean? Jets are all clear, airbox seals, boots from box to carbs and carbs to head are all nice and tight valve solenoid functions properly.

I do have some valve lash. I opened the Tappet clearance to .003 and .004.

Would timing affect the way the bike runs in this way? Valve taper clearance?

Somehow getting some 70 degree days in Chicago in November so I’m out riding every day. Would love to nail this down before winter really hits

Online Evilneversleeps

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Re: 78 750 hondamatic power issues
« Reply #6 on: November 08, 2020, 11:12:50 am »
So replaced points and reset static timing. Re gapped valve tappers all to .003. Runs great but still needs a little choke when taking off from a stop in low speed.

Online Evilneversleeps

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Re: 78 750 hondamatic power issues
« Reply #7 on: November 08, 2020, 05:23:55 pm »
I ran that bike all day up and down lakeshore drive. Rode like a dream. I didn’t go more than 60 mph but it definitely had a good bit left in her. Was able to run without choke in stop and go traffic about an half hour into riding.

Offline HondaMan

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Re: 78 750 hondamatic power issues
« Reply #8 on: November 22, 2020, 02:12:29 pm »
Good job!
:)


Thanks Hondaman!

So now it’s runs but runs lean as hell. I can’t find any vacuum leaks with carb cleaner. I ordered a new air filter with the new box. It’s a hi-flo filter. Would this cause it to run lean? Jets are all clear, airbox seals, boots from box to carbs and carbs to head are all nice and tight valve solenoid functions properly.

I do have some valve lash. I opened the Tappet clearance to .003 and .004.

Would timing affect the way the bike runs in this way? Valve taper clearance?

Somehow getting some 70 degree days in Chicago in November so I’m out riding every day. Would love to nail this down before winter really hits

Those particular carbs were quite lean because of new emissions regulations that were forced on Honda in the late 1970s. The simplest way to improve it is to just go up a #5 number on the current mainjet sizes. Be SURE to use Keihin jets (JetsRus sell them) or, if you get off-brand ones, get them a #10 size larger instead. Look for a tiny "K" etched on the present jets: if it is not there, then you have Keyster jets, installed by someone 'rebuilding' the carbs. Don't go too crazy with even bigger jets or it will start fouling plugs. After you put in the richer jets, run on low-octane gas, at least at first, and check the sparkplug color after 100 miles with that gas. If they are white, you are OK to use premium gas, if they have color to them, then just stay with the regular grade. Also: turn the air screws on the idle adjustment out about 1/8 turn form where they are if the cold-bloodedness is a problem in cold weather: turn them back for summer riding. This will help the idle and low end be stronger when the weather is cold. In hot weather, it will likely cause fouled sparkplugs after a lot of in-town idling, though, so be sure to reset them back then.
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Online Evilneversleeps

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Re: 78 750 hondamatic power issues
« Reply #9 on: November 25, 2020, 09:16:12 pm »
Oh. Nice one Hondaman. I’ll make that a winter project. Putting her away til spring this weekend. Love the insight.

 

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