all most there

[bchaffin]

Ok so after 2 months I finley got the bike on the road this weekend. I have been fighting some carb issues the whole time, so I thought. After going through the carbs and syncing them I took the bike out and it ran very well. So the next day I went riding with some buddys and about 40 miles into the trip the same problem came back. Fuel was running out of my air filters. Last time this happened I thought it was a stuck float. So I thought the same thing this time. I tapped the bowl and could not get the fuel to stop so I pulled the fuel lines off the petcock. The left petcock would not stop running fuel out. So unless I am over simplifing it I would guess it is the petcock. My question is do I just need to rebuild it or what would be causing this issue. Also on another note when looking in my gas tank I noticed it was rough inside like it was filled with gunk but it was hard. Could the tank have been lined with something or do I have problems?

[DGXSER]

Lets take the easy one first, yes, your tank sounds like it has a liner installed. It will appear kind of shiney with a light on it.

As to the petcocks and the carbs...you have two issues. The fuel system functions like this, the petcocks on the tank control when and if gas flows from them to the carb float bowls. That is it, they have no control over how much fuel goes into the float bowls, just if it is allowed to. Inside the carb float bowl is, yes indeedy, the floats. The floats press up against the float needle valve as the fuel level in the bowl rises (unless they get stuck on a gasket or the like) and eventually apply more pressure than the fuel is applying to the other side of the valve, stopping fuel from entering the float bowl.

So, on the standard tank, your petcock is supposed ot be vacuum operated, sounds like they need a rebuild, or the conversion to manual with an off position.

fuel flowing out the air box though, that is the float needle valves job not being done, and not the fault of your petcocks. I do not recall if you rebuilt the carbs or not, but new float needle valves and seats could be in order. If they are new, then add in-line filters to your fuel lines as gunk is getting to the float valves.

HTH

[bchaffin]

I have gone through the carbs completely. Also I just instaled in line filters. I guess I just don't fully understand how the petcocks work. With the tank taken off the bike why would one petcock stop fuel from comeing out and the other one not? Also I was wondering does one petcock send fuel to all carbs or just 2 of them?

[fredintoon]

I have gone through the carbs completely. Also I just instaled in line filters. I guess I just don't fully understand how the petcocks work. With the tank taken off the bike why would one petcock stop fuel from comeing out and the other one not? Also I was wondering does one petcock send fuel to all carbs or just 2 of them?

Hi Brent,
the stock Standard petcocks are vacuum operated and should automatically shut themselves off when the engine ain't sucking on them.
From what you say, one of yours has malfunctioned so it's not turning itself off like it should.
That, or one of them is switched to the "prime" setting which bypasses the vacuum shut-off to flow fuel at any time.
One petcock will flow enough gas to run all 4 cylinders.
You gotta Tee the two carb feed lines together to make that happen though.
You say you just installed in-line filters?
Then you should know how your fuel lines are hooked up?

[bchaffin]

I know how they are hooked up. I wasn't sure if each petcock fed all carbs cause when I broke down I just pluged one line and drove it home like that. I wasn't sure if I was just running on 2 carbs or just starving for fuel.

[bchaffin]

ok so I have been looking at post and looking at petcock rebuild kits. I know there is the spring issue when rebuilding these but any recommendations on what to get. Also I was going to replace the filters why the tank was empty and wondered if it was the same as the ones mikes xs offers.

[dbeardslee]

If this is happening when it's running, you may be barking up the wrong tree addressing the petcocks first. Reason I say this is that when it's running, and vacuum is supplied to the petcocks, they function like they do in the 'prime' position - gas flows. For some reason your fuel valves aren't shutting off the flow at the carb. Could be due to improper float height (should be 23mm on the 80/81 carbs), worn out fuel valve needles, leaking o-rings under the fuel valves, or a stuck float. Sometimes with repeated assembly and disassembly the float bowl gaskets can get flattened out, and if the get too flat they can catch the float and hold it in position. If that looks to be the problem, you can trim them with an exacta knife. Petcocks might need rebuilt too, but if you don't fix the problem at the carbs it's going to bite you again.

Does your machine have the stock carbs on it? On an '80 you should only see two fuel lines coming off the carbs. On the earlier model carbs there's the fuel lines on the bottom, and a set of vent lines coming off the top. If the vent lines get restricted on the older carbs it will make them overflow. The way parts have been moved around on these old bikes, I just thought I would ask.

[bchaffin]

It sounds like I still have the original carbs. I guess I was just hoping it was the petcocks cause I have been through the carbs about 3 times in the last 2 months. I have replaced the bowl gasket and most of the other little parts on the carb. I just thought it was odd that even with the tank off the bike the petcock would not stop the fuel flow. Well I will still try to rebuild the petcocks but I guess I will go through the carbs again this weekend.

[DGXSER]

As has been mentioned before, look at the bowl gaskets. I have made a practice of trimming the sides of the gasket to match the sides of the bowl to prevent the float from hanging up on it.

I also had an issue with sticking brand new float valves, i would bench test them as Prometheus has outlined before, and even test with fuel in the upright and fully assembled position with no leaks, then load them in the bike and F'n things would leak everytime I got them installed!!! Turned out, I was using automotive style fuel line wiht the soft rubber inside lining. Well, that soft rubber was being abraided by the ribs on the fuel Ts and from using small hose clamps on the lines. And the small scrapings of rubber would get to the float needle valves and stick them open. I changed fuel lines and have never since had such issues.

[dbeardslee]

A few things on floats...

It sounds like I still have the original carbs.

Something to check - there's two tabs on the plastic floats - the big one that sets where the float closes, and a small one on the back that makes contact with the float post to keep float from dropping too far (bowls down orientation). It's not a problem with the brass floats, but if the plastic ones drop too far they can catch the float bowl. The small tab needs to be adjusted such that the float can fully open the valve but not allow for a lot of 'over travel'. You also want to make sure your float pins are straight, and that the flat sides of the floats themselves are as close to parallel with the sides of the bowls as possible.

When you set your float height, you need to use a measuring device that actually hangs over the float. I've heard of some members trying to use a ruler to do it, and you'll never get them right trying to eyeball across them. The depth gauge end of a caliper works well, or I like to use a carpenter's combination square. I set it 1mm proud of the mark (for yours that would be 24mm), and adjust the big tab until I can see about 1mm of light between the top of the float and the gauge. You don't want your measuring device to actually touch the floats - they're way too easy to push down. Once you've got the tab set correctly for one side of the float, check the other side and adjust as necessary by bending the cross-member that holds the float. Gotta set both sides - one side won't do.

And lastly, for whatever reason, I've noticed that some floats have more 'side to side slop' at the float pin than others. If you can push the floats from side to side on the pin, and they get very close to the sides of the bowl, you might want to put some teeny tiny washers on the pin to hold them centered on the bowl. Don't get them too tight though - you need a little slop.

Also, on the petcocks there have been a number of posts about modifying the lever so that you can turn it straight up - effectively giving you an 'off' on the standard petcocks. That'll shut off the fuel flow . Those vacuum operated petcocks can be a genuine PITA .

[bchaffin]

Thank you I will work on it this weekend. Adjusting the float hight was what I had the most problems with. I still want to rework my petcocks as well any advise on where to get a kit or what to watch for.

[dbeardslee]

If you were having difficulty adjusting the floats, that might be all that's wrong. They have to be set correctly, and even 1mm either way can make a difference in how it runs. Get 'em too low (upside down orientation) and they won't shut off the fuel before it overflows.

The main thing on the rebuild kits is to reuse the diaphragms from the existing petcocks (assuming they're serviceable), but replace the rest of the parts. The boss on the aftermarket diaphragms isn't the same length as the stockers, and sometimes they'll work at first but start leaking again later. I rebuilt mine several years ago and used the diaphragms in the kit. Sure enough, several months later they were leaking again. I finally got fed up with them and replaced them with manuals. If I had it to do over again I would have reused the old diaphragms and modified the levers for a positive 'off'.