Topic: 1979 CB650 airbox

[Scotiabiker79]

So I’m a newbie to restoring the old bikes.  Well at least a newbie in terms of going into depth of getting motor running.  I recently rebuilt the carbs and put the bike together without the airbox.  Mistake.

I found a video on YouTube last night and a couple threads talking about how the motors don’t run properly without breather box.  The bike was running like crap

So to the newbies.  Put the airbox on.  Don’t make mistake I did.   After a lot of struggling I got the airbox and new air filter on

Runs awesome now. I’ll post video under project forum

[Scotiabiker79]

Now carb 2 is leaking out of overflow tube after bike shuts off.    I’ve read all the posts of this problem and fixes.  After fighting to get the airbox on I don’t want to take it all apart again.   I’m at my wits end I think. 

[Alan F.]

Just the #2 carb?

This could be as simple as a stuck float valve. It could be stuck or there could be a tiny piece of debris caught there helping it leak.

*Knock on the float bowl with the plastic handle of a screwdriver a few times, that's the time honored trick to free a stuck float valve. Like 4 knocks then check if it's still leaking, repeat if necessary. The plastic handle won't damage your carbs.

Always turn the fuel petcock to OFF when the engine isn't running.

How clean is the inside of your tank?
Many people will replace the float bowl screws with Allen head screws, also knows as Socket Head Cap Screws or SHCS. These allow for much easier float bowl removal with the airbox and carbs still in place.

If it's not a stuck float, your float height may be too high. Search the forums for the Clear Tube Method of checking fuel level in your float bowls. I'm not sure about a 650 but this can probably be done with the airbox in place.

[Scotiabiker79]

Thanks for the tips Alan. 

I’ll try the tapping method this morning.  As far as the tank goes.  I did the vinegar bath and eliminated rust. Then rinsed with water.   Then filled with metal rescue soaked and rinsed with gas.  I filled with gas and installed tank.  I put an inline fuel filter before starting and a new petcock. 
When I look in tank I see a little rust forming on top portion of tank.  Another struggle. 

[Alan F.]

Yes fighting tank rust can be a terrible struggle sometimes. It helps if you can do it in a really hot room, or outside on the hottest day. Rinsing with large quantities of boiling water is good to help with evaporation, airflow from a shop vac at the petcock bung will help too. Nothing works it's best at low temps, flipping the tank over and shaking by hand for a few minutes each hour is a pain but it helps. I remember someone even put their tank on the back runners of a rocking chair so it would slosh just a bit when the wind blew overnight.
Someone else filled their tank with used golf balls to take up the majority of space then added MetalRescue or Evaporust or whatever, that seemed like a good idea.

[Scotiabiker79]

Does anyone know any place to get an aftermarket new gas tank?

[fizzlebottom]

No such thing for CB650s unless you want to fab your own. You can attempt to repair yours, sit on eBay (or other CMSNL), or work with what you've got.

[Alan F.]

I suppose there might be one out there, new in the box, but it'd cost more than most CB650s are worth.

There are some steel and aluminum tanks being made in India up on ebay, but none are specific fit for your bike.

Best bet would be searching around for a different tank that someone fitted to their 650, and find one of those if you like the look.

There are a few other ways to adapt a tank from a different bike, finding one with similar capacity and outside dimensions is a good start. But the "tunnel" on the bottom of the tank should fit your frame like the stocker, and it's also important that the petcock position won't interfere with carb linkages or make it tough to run fuel lines.

Some eBay sellers hold up a tape measure in their photos. Also if you happen to have a good bike junkyard nearby, maybe you could test- fit tanks to a 650 like yours before you buy?

[Alan F.]

If your tank doesn't have rust holes, you might be better off cleaning out the rust and maybe using a tank liner kit.

After looking around it appears there are frame differences among the 79-82 CB650. 80&81 tanks are interchangeable but 79 and 82 there are differences in mounts and tank length. Tanks from other years may not fit right with regard to the seat.

There are a ton of Chinese 9 and 10 liter tanks on ebay, I'm sure you could mount one of those up with a little fablication, but you may need a liner kit with those tanks as well, and again 9 liters compared to your stock 18?

[Alan F.]

If you're still interested in swapping tanks, I'd look first for a tank from a carbureted bike, fuel injection tanks have differences and will cost more. So start around 2008 and work your way back. Find a tank you like from a bike that was made for many consecutive years, so there will be plenty of them out there, maybe even with factory paint you can live with. Tank capacity and fit to your bike will be important, if you're building a bobber or cafe you might not be keeping the stock seat so you'd have more freedom to choose.

Are you planning on keeping the bike stock? What kind of riding will you be doing? Long day rides, or just basic transportation? Maybe a smaller tank will suit your needs, there are many things to consider depending on your goals for this bike. Some people will just bolt on a Harley Sportster tank and happily ride 100 miles between fill-ups.

[Scotiabiker79]

Hey Alan
Thanks for the replies.  Lots of good info.  I want to keep it stock. I already have the seat base cleaned up and ready to put the new seat cover on.   I was hell bent on getting the tank panted professionally with decals etc but I’m gonna wait til I get the bike done.  I’m gonna just do a flat black for now   My tank has started to form a little rust inside since cleaning it.  I have an in-line filter which hopefully will catch any foreign particles.  I’m still going to be on the hunt for another better tank if I can find one.  I’m just going to use the bike for occasional rides.

Have you done a tank liner kit before?  I was talking to a guy a few months back and he was recommended not doing a liner liquid because it could peel off and clog things up from changes in gasoline formulas. 

[Alan F.]

For occasional rides with a stock 79 tank (which seems to be the rare one) I'd definitely consider lining it or it'll eventually rust through with pinhole leaks. I'm not experienced enough with liners to advise which to choose, but avoid Kreem brand like the plague. Ask or search here for Caswell, Redkote and POR15 as those seem to be the ones most go with. And just like good paint work, preparation is the key to a job well done that will last. Follow all directions to the letter no matter which one you choose.

[Scotiabiker79]

Thanks for the info.  I’ll research the heck out of how to do it. 

[sloburn]

I had the exact same thing on carb two after install. Broke down and just took it back off. Flipped it over and inspected the bowl on carb two. It had hair in it…. Cleaned them all. Worked great. Literally can be super simple don’t get overworked like I did.

[newday777]

I had the exact same thing on carb two after install. Broke down and just took it back off. Flipped it over and inspected the bowl on carb two. It had hair in it…. Cleaned them all. Worked great. Literally can be super simple don’t get overworked like I did.

"It had hair in it...."

As in human hair???


BTW welcome to the forum
Introduce yourself and your bike in the new members section.