Author Topic: 75' CB750 Carb Issues  (Read 1318 times)

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kerber

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75' CB750 Carb Issues
« on: May 05, 2010, 12:25:41 PM »
Hey everyone,

This past fall, as a college graduation gift to myself, I picked up a 1975 cb750 bobber. It had some issues, but I figured it was nothing I couldn't figure out, being technically adept. I may have overestimated by skills.

Thus far I have worked on the brakes, some other misc work, and cleaned the carbs. The carbs were completely filled with rust and sludge. The previous owner had left it sitting for a couple years without gas in the tank, then to sell it, filled up the tank and fired it up with no fuel filter.... One thing I notice in cleaning was that the idle jets don't seem to have a hole going through them from one end to another. I am not sure if this is just how they are or if they are so clogged with junk that even after cleaning, I couldn't see through them. After trying to push a pin through to no avail, I figured that is how they were supposed to be.

After putting everything back together, it fired up, but is having some major firing issues. With the choke on, it starts up and a couple of the cylinders seem to fire all right, but the other two flood and pop if I rev high enough. As soon as I turn off the choke, the engine dies. What I am hoping to find is some advice from people who are more experienced in the world of these carbs, not to mention some general guidance.

Also, I do not have an air filter in the filter box yet, as I am not sure what kind will fit. Would this be enough to completely mess up my mixture? I will post some pics tonight and maybe, if I am lucky enough, someone will be able to advise me on what filters will fit.

Thanks in advance for anyone that can help me getting this running.

EDIT: Cleaned it up. Clarification.
« Last Edit: May 05, 2010, 02:03:43 PM by kerber »



CB750Winger

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Re: 75' CB750 Carb Issues
« Reply #1 on: May 05, 2010, 08:09:12 PM »
   Was the gas tank stored empty too? If so, you may have rust in it and have issues until it's cleaned or even seal coated. Your carbs might also need another cleaning and the jets need to be clear. You will more than likely have float issues if a needle and seat gets rust or crud stuck in them. They will leak out the overflow. Make sure you get everything clean first.



kerber

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Re: 75' CB750 Carb Issues
« Reply #2 on: May 05, 2010, 10:49:00 PM »
The tank was stored empty, but I cleaned it out and put a fuel filter inline. As far as the carbs go, they are as clean as they are going to get. I spent over an hour on each one. As far as the floats go, there isn't an issue. The float needle and the floats are in good condition and the carbs aren't overflowing.



kerber

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Re: 75' CB750 Carb Issues
« Reply #3 on: May 05, 2010, 10:57:00 PM »
Here are the pics. I have seen these filter boxes on a few other modded bikes, but I am not sure what air filter it takes. Anyone know if the stock filter will fit?



Offline scondon

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Re: 75' CB750 Carb Issues
« Reply #4 on: May 05, 2010, 11:02:24 PM »
Remove the jets and soak them overnight in carb cleaner. A guitar string is a good tool to use to try and poke stubborn gunk out of the hole. If you can't clear the jets you have, replace them with new ones.

Did you remove the jets, slides/needles, floats, needles, air screws and soak the carbs in dip to clean them? Sorry to ask this, it's just that when someone says "I cleaned the carbs" this is what is assumed, not just spraying carb cleaner over everything to remove surface gunk.
Give me..a frame to build a bike on, and my imagination will build upon that frame



kerber

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Re: 75' CB750 Carb Issues
« Reply #5 on: May 05, 2010, 11:13:05 PM »
I actually did not soak them! I used can carb cleaner and a toothbrush to scrub them clean since they weren't very dirty. Chalk it up to inexperience, but this is why I came here. Tomorrow night I will pick up some dip cleaner. Any idea where I could get some new idle jets for it? I am sure I could find some eventually, but worth checking if anyone has a preferred source. Any thoughts on the air filter affecting it?



kerber

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Re: 75' CB750 Carb Issues
« Reply #6 on: June 02, 2010, 10:01:00 AM »
Well, I have progress!

I got some carb cleaner soaking solution and let all the carb parts sit in it for a day. I then found some insanely small wire and ran it through the idle jets. After putting everything back together it ran like a dream!

All I had left to do was change the oil. What's that? The previous owner did a spin on oil filter conversion? Oh, the filter is bigger than any bike shop in town has ever seen? AWESOME.

Now, the search is on for a chromed spin on that has a 3/4" mount, is 3.6" diameter on the outside of the gasket, and just about 3" long. I don't suppose anyone seen one this size?



Offline Eydugstr

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Re: 75' CB750 Carb Issues
« Reply #7 on: June 02, 2010, 01:07:58 PM »
Kerber - give this guy a call, he offers a spin on oil filter kit pictured with a chrome filter.

Nice looking bobber!



kerber

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Re: 75' CB750 Carb Issues
« Reply #8 on: June 02, 2010, 01:09:31 PM »
Which guy?



Offline Eydugstr

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Re: 75' CB750 Carb Issues
« Reply #9 on: June 03, 2010, 04:00:15 PM »
Sorry, Kerber - http://www.cyclexchange.net/

His website seems to be going through some construction, but he lists a chrome filter spin-on kit, so he'd probably be able to hook you up with a filter.



kerber

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Re: 75' CB750 Carb Issues
« Reply #10 on: June 09, 2010, 01:37:45 PM »
Well, I went ahead and called the guys at www.cyclexchange.net. The guy I talked to went through the same oil filter search 15 years ago and he didn't have any luck. I have decided on just removing the oil cooler and going back to the stock oil filter setup. At least then I will be able to find filters. I went ahead and ordered an oil filter cover from ebay, so I will post some more pics after I get it all put back together.



kerber

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Re: 75' CB750 Carb Issues
« Reply #11 on: August 06, 2010, 02:19:45 PM »
It took a few weeks, but I have the new filter housing on and have it running! Now I just have to deal with trying to find a new throttle cable... I will put up a new post about that...