Gas pouring out!

[RRHartzler]

81 CB750 Custom, just bought yesterday. Turn on Petcock and fuel just pours out the first carb (drivers side). Turn off the Petcock and it stops...I'm in serious trouble here boys, the wife paid for this bike on my recommendation and I have to get it running asap. I'm not the most mechanically inclined but am willing to learn, especially with this bike.

 

[amc49]

Trash in fuel to stick the needles open. Do they run ethanol in your local fuel? That adds to it immensely. Look in tank and see how rusty it is. There is a stick type filter sitting on top of the petcock, it may well be clogged but know that if rust then superfine ethanol rust can often get past almost any filter in the world, it is that fine. If the first time the fuel turned on the ethanol itself tends to stick the needles from it drying up earlier, the bikes now do NOT like to sit at all, problem can show as quick as a couple weeks.

You may well have a carb cleaning in your future..........ethanol is transparent in vehicles driven every day, let them sit though and the problems begin and carbs hate that stuff as they are vented to the atmosphere and that allows the ethanol to evap like 10X faster than straight gas ever did. Modern closed fuel systems and EFI in cars plows right through that but old school carbed bikes have utter fits. Why most cars now use plastic tanks.

Of course when switching us to that crap they never mentioned that at all did they? But then everyone thinks I see conspiracies everywhere (I do).

 

[RRHartzler]

Yes, the bike sat for a year in a barn. We also use ethanol here in Indiana/Michigan. The owner said he put coating in the gas tank but I'm not sure what he used or how well he did it. I will have to tear it down and pull the carbs...do I need a re-build kit or can I just clean it up and put it back in. Since I will be removing the Tank, is there something I should do with it as well?

 

[Dave]

A rack of 4 carbs is a very bad place to start "learning", especially in your "asap" mode.

They have to be synchronized afterwards which is done while it is running with tools you most likely do not have. I'd recommend a trip to the shop for a carb rebuild and tune up. The tune up should include an oil change, timing chain adjustment, drive chain adjustment, new spark plugs, and carb adjustment.

If you have the right size and length of Phillips screwdriver you can just take off that one carb bowl right on the bike. Stretch an old towel across the top of the engine so you don't lose the needle if it falls out. Carefully drop the bowl off taking care not to damage the gasket/seal as you do. It may stick to opposite surfaces in spots. With the bowel off you can slide the float pivot pin out and drop the float and needle out. Put a clean cup under it to catch the fuel and open the petcock for a few seconds to flush out the needle seat.

Look in the cup to see what type of contaminates your dealing with. Inspect the needle the see if it's damaged. If the needle is fine and there's junk in the cup, put it back together and cross your fingers. If the needle has the small clip on it you should be able to hold the needle upright by putting holding the float with your index finger on the clip. If it doesn't have the clip, you'll need to insert the needle then hold it in place with something like a paperclip bent straight with one hand while you move the float into place with the other. Then release the needle and use that hand to slide the pivot pin in. Not an easy task.

Here's a picture of what you'll see inside. (ignore the yellow text, I'm not sure exactly what carb that is but yours should look just like it)

 

[amc49]

Gotta wonder at someone that buys a bike that old and expecting it to start and run perfectly, no insult intended at all but it points away from the bike itself.

Be advised that these carbs often give fits to experts, they clog deep inside to have problems over and over even with person doing the work swearing they are 'perfectly clean'. You'll know when, bike will then begin to run fine.

Probably should run a compression test too to see how badly burned you got on the deal, often serious issues there and why then so many tear carbs down blaming them. It's an American male testosterone thing, 'it's not running right, it must be the carbs'. While possible the valves tend to wear to not close too and then low compression fools the rest of the guys.

I love them but 1st gen DOHC = money pit for many people. If you ain't got the love there will be problems in the relationship.

 

[RRHartzler]

Gotta wonder at someone that buys a bike that old and expecting it to start and run perfectly, no insult intended at all but it points away from the bike itself.

I love them but 1st gen DOHC = money pit for many people. If you ain't got the love there will be problems in the relationship.

I get it...I know it's old, had a nostalgic lust for the bike, learned on a CB450 when I was 13 from my Brother-In-Law. Always loved the 4-throated monster. But, I'm not new, I've been riding for quite a few 24 hours. I have done my own maintenance and although not a great mechanic, I love to learn.

I take to heart your comments, you're the expert, I get it. Just looking for some help to get me down the road.

I didn't pay a lot for this bike and I believe it is actually a basket-case. Needs new battery (knew that), new rear tire (knew that) and the leak from the number one carb (knew that as well). This is my wife's bike to learn on, I'm not planning on riding around the world...that's a different bike and a different story. This bike will then be handed down to my daughter for her to ride. It was nevere intended to be "restored".

amc49: Help a Brother out.

 

[amc49]

I have nothing against that whatsoever, it's just that reality is the great potential plan crusher. A small dose of that often sets the feet on correct path. Ask away.

Believing it to be a basket case says even more compression check. Senseless to do all that work with no payoff.

They don't have to be restored to run absolutely great, I have a CB550F that will easily run well over 100 mph and 9000 rpm and looks like junk. EVERYTHING works perfectly on it but it was left outside in the weather for six years before PO gave it to me.

 

[Dave]

This is my wife's bike to learn on

This is a VERY BAD bike to learn on. If that's the purpose you should get find an 86 VT500 shadow or the like.

A "learning" bike should be light and nimble. Easy to handle with good predictable brakes, a low seat height, and a low center of gravity.

The CB750 is the opposite of all this.

 

[amc49]

I never even thought about that and 100% correct, that's a 500 lb.+ bike fueled up and not many women can handle that much really well. My younger brother in teaching his wife to drive one then took her to a licensing place to test for license a year ago and they got ahold of an old CB500T which is same as the 750 but lighter (450 lbs?), she crashed it taking the test and got hurt. Just couldn't hold it up once it tipped out of right angle to the street she said. A little slow on speed going into corner and the bike fell to the inside.

Your life to do what you will but no way would I let any significant other I know and value get on a bike that I would describe as a 'basket case'. Women? We're supposed to protect them, not risk them. The bike has to run flawlessly.

Certainly no insult intended at all but I had to say that, Dave is absolutely right. Light and nimble. Basket case is the exact opposite of predictable, the brakes are not nearly the only part that has to be that, simply rolling into a turn to have bike stumble or sputter on rollout at all and there you have the accident.

Look at that HARD. I'll accept any insult back as a result of it, it is what it is.

It CAN be done, the 550 I just mentioned runs flawlessly and all day long. No miss, sputter or hesitation. Brakes perfectly. I can do an 8000 rpm burnout on it or drive around like a little old lady. That model had super small carb bores that held the engine back but they allow you to even take off from a dead stop in high gear if you know how to. Slow but doable.

 

[Dave]

I wasn't referring to one sex or the other only to a skill level.