Topic: Beginner Question about Carbs

[clarettom]

First of all, thanks in advance for the help. I have an 81 650 that I got in over my head restoring. I have gotten almost everything fixed up but I'm still having problems starting the bike. I'm almost certain it has to do with fuel delivery to the carbs. I don't know alot about carbs but I have been reading dozens of posts on this site to learn.

My Question - Should I be able to blow air into the gas line leading in to the carbs. I understand how the floats work and since the bowls are empty i feel like I should be able to blow air into it. I know its not clogged up because I can blow through it fine when the bowls/floats/etc are taken apart. Am I doing something stupid? Thanks again.

[Zig]

do you want to know "can you" or "should you"??
am just guessing here but enough air pressure should lift needles and allow air into float bowls.
standing by to be corrected....

[ChortlingSandcaster]

DISCLAIMER: I don't own your model. What's below is general knowledge only.

Some carbs require suction from the engine turning over to operate a diaphragm-switched
fuel flow out of the petcock. 

Here's a test to *really* illuminate your seemingly implicit concern that fuel from the gas tank is not getting into the carb:  Loosen the drain plug at the bottom of the carb bowl.

Because yours is probably a diaphragm-type petcock due to the model year
-- check your manual to locate the source of engine suction that is typically a nipple on one of the intake manifolds or from a nipple one of the carbs.

On *some* (keyword 'some', this is not specific to your model) diaphragm petcock'd bikes,
the intake manifold has a nipple that sucks when the motor is cranked over.  On other bikes,
a nipple on a carburetor supplies the suction.

The suction is then routed via a rubber hose from the suction nipple (wherever it's located, carb or
intake manifold) -- the rubber hose is routed from the nipple to the diaphragm on the fuel petcock.   When the engine turns over, the intake sucks and that suction pulls open the fuel diaphragm in the gas petcock and then fuel will flow to the carbs.

Did you leave off that suction hose that operates the diaphragm in the fuel petcock during re-assembly after your restoration?  I have done it and it causes exactly your symptom -- no
gas getting to the carbs.

This diaphragm design allows you to be able to switch the petcock to the 'On' position and leave it
'On' all the time.   When the engine suction goes away, that rubber diaphragm inside the petcock closes and shuts off fuel to the carbs.

On my preferred type of bike, 'OLD,' you actually had to switch the fuel petcock to the 'OFF' position or the gas kept flowing out of the gas tank and into the carbs then the carbs leak the excess gas.

So check your manual and find out if you have a diagphragm petcock and check to see if that suction line is hooked up.

[scottly]

If you are blowing into the fuel line where it disconnects from the tank, and you do have the vacuum actuated petcock Chortling referred to, then no, you can't blow air through it. There should be a "PRI" (prime) setting on the petcock to allow fuel to flow without the engine running.   

[scottly]

If you are blowing into the fuel line where it disconnects from the tank,

Duh, the petcock is usually mounted directly to the tank, so if you are blowing into the hose where it disconnects from the petcock, and the bowls are empty, yes, air should pass. If not, the hose may be plugged, or kinked.

[clarettom]

Wow thanks guys for the advice. I'm still uncertain, so bear with me. It's my understanding that the bike originally had two petcocks. One off the tank and one on the carbs. The previous owner must have bypassed the petcock on the carbs before i got it. I just checked the one on the tank and it flows fine. I dont think its a blockage because when i take out the float valve, put it all back together, I can blow through the carbs easily. I still don't completely understand this whole carb system. There seems to be an infinite number of small holes and passageways inside these things. I really appreciate the detailed explanation, ChortlingSandcaster. I dont think the fuel petcock has the suction hose that you mentioned but could it still be possible that the whole system is under a vaccum and only allows gas to flow (in this case, allow me to blow into the carbs) when the engine is turning over? I just put the carbs back on the bike and started it up...still no gas coming out of the drain in the bowl. Any thoughts? And I cant tell you guys how much i appreciate you helping me.

[cameron]

When you say "started it up".. do you mean "started it up", or "failed to start it up"?

And lets go through systematically... although I bet you already have.

From tank- does gas pass the petcock?
After fuel lines... does gas spill out there if you pull them from the carbs?
Do you have any inline filters or anything?
Does any gas get into any of the carbs ever?
Has it ever started?

[eurban]

The "vacuum operated fuel safety device" on the 650s (I believe it is only on the later 650s with CV carbs <big round caps on top, your 81 probably has CVs>) is an inline device rather than being incorporated into the petcock.  It is bolted to the top of the carb rack and has the fuel hose from the traditional petcock going to it, as well as the fuel hose that then proceeds to the carb.  There is also a vacuum hose attached to it.  The safety device only allows fuel to flow when vacuum is applied.  These guys do fail and even when working properly, they seem to significantly increase the amount of time required to start a bike that has been sitting for a while. There is no prime setting on the petcock. You can easily remove the safety device and connect a fuel hose directly from the petcock to the carb rack.  Plug the vacuum hose and now make sure to shut off the fuel whenever you park the bike.

[clarettom]

Yeah I guess I should say "failed to startup". It tries to start but doesn't get the gas it needs, therefore it just keeps trying to kick over. But yes, the bike was "running" as of last year. And I put that in quotations because it had electrical issues but nothing that would be causing this problem.

Ok systematically...The gas leaves the tank when its not connected to the carbs and the petcock is open. It does not leave the tank when it is connected. It has a filter in the tank but this couldn't be the problem. Not a drop of gas makes it into any of the carb bowls.

Alright, youre right about it having the CVs. And I looked in the online manuals and youre correct about it supposed to having the vacuum operated fuel safety device. That must have been bypassed before i got the bike. Its long gone. Is that the only vacuum system in the carbs that requires a vacuum to pull gas into the carbs?

The floats seem to work fine. The rods are all straight and the floats move easily. Do the tiny float valves that allow the gas into the carbs go bad? They seem to be fairly new and they look like they work fine when the bowls are off and I blow into the gas line. Any thoughts?

[cameron]

So.. digging through your post, I noticed you said when you  " take out the float valve, put it all back together, I can blow through the carbs easily."

Are they upside down?

If you pull off a bowl and manually work the valve... does fuel come out?

[Ved]

Sounds like to me that your lower jets (idle jets) are clogged. The hole is so small that I had to use a high E string for a guitar to poke through it. (.010) When I cleaned the carbs on my CB750A, 3 out of the 4 idle jets were clogged solid. I had to use a can of starter fluid to get it started before I cleaned them out. Now she fires up with no problem with half choke.

Also, note that those idle jets are pressed in, NOT threaded.(the upper jets are threaded) So ya gotta grab em with some needle nose and twist them back and fourth to work them out.




Good luck and happy riding!