‘75 GT550 carburetor questions

[GTandcbr]

Hi
Firstly when you pull the carbs of just make sure that the throttle body in the carb is very clean use plenty of spray carb cleaner. Also the throttle valve. What we are trying to achieve is smooth top to bottom movement because if any of those valves stick the bike won't idle down. Can I just check that the adjusters on the handlebars have been completely slackened off? +1 on DorT500 post there ìshould not be petrol behind the diaphragm. Problem is there is only one place I know to buy a replacement
https://hard-to-find-parts.de/epages/af ... ssicView=1" onclick="window.open(this.href);return false;

[dorT500]

These fuel taps are a PIA.....the ethanol in gasoline does not help the old rubber membrane any ......I've had a manual Pingel fuel petcock now for well over a year but it started 'weeping' fuel from the lever just before the warranty ran out but I procrastinated and missed a free repair/replacement. There is only a small fee from Pingel plus shipping to repair them but I have not done that yet.

I think I might have put one good OEM one together a few weeks ago from all these but I am not 100% sure yet. Some leaked from the lever, some from the body or outlets.




I do like the vacuum operated fuel taps and I might go the https://translate.google.com/translate? ... rev=search" onclick="window.open(this.href);return false;" route when I get through buying other wanted or needed parts.

[pgb123]

That's not actually a pic of the petcock on my bike, but would holes or pinholes in the diaphragm cause this specific issue? If so, I'll pull mine apart and check its condition. I just don't understand why it would cause fuel to spurt out of that little hole when the engine is running. Since the petcock doesn't leak when the engine is off and since the engine does run, i.e., the diaphragm is opening, it seems that everything is functioning properly. Is the hole in the picture and air inlet hole?

[pgb123]

What we are trying to achieve is smooth top to bottom movement because if any of those valves stick the bike won't idle down. Can I just check that the adjusters on the handlebars have been completely slackened off?

I haven't slackened off the handlebar adjusters! What's the throttle free play range?

[dorT500]

That's not actually a pic of the petcock on my bike, but would holes or pinholes in the diaphragm cause this specific issue? If so, I'll pull mine apart and check its condition. I just don't understand why it would cause fuel to spurt out of that little hole when the engine is running. Since the petcock doesn't leak when the engine is off and since the engine does run, i.e., the diaphragm is opening, it seems that everything is functioning properly. Is the hole in the picture and air inlet hole?

It may be a tiny rubber sealing washer damaged/shrunk behind that hole or on your fuel petcock there may be a tiny check valve.....just going by a vague memory (maybe Ian's oldjapanesebikes site or I could be completely wrong.

Don't have time to research it right now and maybe not until around noon tomorrow. In the mean time, maybe someone else can confirm about a tiny check valve in some fuel taps. Don't take it apart just yet.

[GTandcbr]

I don't have a set free play I just make sure that the throttles are down completely and don't lift for a small turn on the throttlegrip

[dorT500]

That's not actually a pic of the petcock on my bike...........

A little early, isn't it?



I guess this is what I had a vague recollection of......not your bike...... but on a much earlier T500. Not even sure if that is/was a check valve.

https://www.oldjapanesebikes.com/blog/2 ... fuel-cock/" onclick="window.open(this.href);return false;"

I don't remember seeing any type of check valve in any fuel petcocks I have seen....just a small rubber sealing washer between the recessed hole and the diaphragm.......like so.

One of my iffy T500 spares shown below. Like I mentioned earlier, that little sealing washer could be your culprit or any tiny breech in the sealing area marked. The fuel you are seeing leaking is probably coming out in very short/fast spurts as the engine is running, correct? As you probably know, that plunger and diaphragm is being pulled in and out very quickly by low vacuum and pressure in the crankchamber.
You can think of that hole you are asking about as an air intake and exhaust vent.

First thing I would check is to make sure all the screws are tight and tightened in small increments in a criss-cross manner to insure the plate is down evenly. I am thinking the only place you might find a fresh rubber washer exactly that size and thickness is walking into a local bearing and seal supply house and see if you can match one up if you think that washer might be to blame and not a tiny tear or hole in the diaphragm. It's that or buy a kit or partial kit if that German site mentioned earlier has what you need.








Time to start weighing your options I guess. Your fuel petcock will probably come apart without damaging the rubber if you want to inspect it but there is that chance that the diaphragm may tear at the edges. Just a heads up.

Good Luck on what ever you decide.

[pgb123]

Thanks so much for the info. You are correct, the gas is coming out in short fast spurts. There is, in fact, a small check valve on this petcock, but I don’t think that’s the issue here. One of the screw holes holding in the plate securing the diaphragm is stripped, so perhaps it’s just not sealing. In any event, I’ve given up on this. Instead of spending $70 or $80, plus shipping on that German diaphragm, I’m spending $110 on a new petcock. Autumn is slipping away too quickly!

Next issue: I notice that at idle, fuel is coming out of the overflow tubes at a pretty constant stream. Does this mean I need to lower the floats a bit?

[GTandcbr]

There are a few possibilities. Punctured float sinking to the bottom of the bowl. Floats sticking sometimes they catch on the side of the float bowl. Float needle valves either dirty or worn fail to shut off fuel. Or as you say float level too high.

[sportston]

There are a few possibilities. Punctured float sinking to the bottom of the bowl. Floats sticking sometimes they catch on the side of the float bowl. Float needle valves either dirty or worn fail to shut off fuel. Or as you say float level too high.

+1 on that.

[dorT500]

Yep and inside of fuel tank in pristine condition?

[pgb123]

The tank isn’t pristine. The petcock has filters on it and there is an inline fuel filter. I took apart and rebuilt the carbs. The floats are new. Since I’ve never done this before, it’s very possible I didn’t adjust the float height correctly. The needle valves looked ok, but I probably should have just replaced them to be safe. I’ll pull off the carbs and lower the floats a bit and see if that does the trick.

[GTandcbr]

I'm sorry but you will have to be 100% accurate on the float height you cant just 'lower them a bit' it simply wont function properly. It takes a bit of practice to measure the height properly. The height is measured from the top of the float to the carb surface NOT the gasket. The carb has to be tilted at an angle so that the tang on the float just touches the float needle valve.
This video is quite good https://youtu.be/2FENckAjNjA" onclick="window.open(this.href);return false;

[pgb123]

I appreciate the reply. The trouble I had setting the float height was that at the angle where the float just touched the needle valve, the needle valve seemed to unseat a bit. I tilted the carb ever so slightly more so that the valve was barely depressed and measured from that angle to the flange where the gasket would sit.

In looking at the video you posted, the guy tilted the carb with the float hinge on the bottom. I tilted it with the hinge up. I watched several videos before I did it and saw it done both ways, but perhaps I made the wrong choice. Any chance you can clear up which way I should be holding the carb?

[GTandcbr]

Okay yes sorry he does it a little different to me. I hold the carb in front of of me with my left hand the floats are hanging down. Then rotate the carb so that the tang just touches the float needle valve then measure. The needle valve top is sprung and is a shock absorber so vibration won't switch the valve on and off. Just make sure the tang is not depressing it before you measure.