fuel starvation problem back again...where to start?

[Ramjam]

Hi dear friends
You haven't heard from me in Beirut for a while as about 15 months ago i moved flat. The gt380 rode well to the new place but after it sitting for a couple of weeks it developed a fuel starvation/flow problem. I noticed that it would start, run rough for about 30 seconds and then die. I also noticed that the lower chamber in number one pot often filled up with gasoline.

So, today i decided i wanted to get the only gt380 in Lebanon running again. But i knew number three float chamber was leaking so i replaced that and tested all three flat chambers for leaks. All good.

But i have found exactly the same problem. i fired up the bike - on either prime or reserve - and it runs for thirty seconds and dies. It often doesn't run on number one. If you pull the throttle, you hear that distinct sound of no gas getting in. Is there a step by step guide to testing the fuel tap? I fitted transparent tube as i wanted to see if the gas was moving but it seems as though it ssn't. After putting in five litres though, it flows out on the 'on' position when you pull the tub off. But when connected it doesn't seem to be moving. That would certainly explain why i have fuel starvation.

Does my problem of overflowing gas in number one pot explain the 'starvation'? I mean are all three fed independently or do they take their feed in sequence?

If anyone has a method of identifying where the problem is - fuel tap, carbs or other - i would be so grateful for your time in explaining.

thanks

MJ in Beirut

[GTandcbr]

Hi MJ.
Here is how to service your petrol tap https://youtu.be/qWNyvjaWfz8" onclick="window.open(this.href);return false;
The gas will only flow if the diaphragm is sucked out by the vacuum tube which is connected to the centre carb. Check both end of that tube are sealed.
You say the float chamber was leaking, was that from the overflow pipe or the gasket? Either way it would appear that your float needle valve is not functioning as gas is continuing to flow afterfilling the floatbowl and then the crankcase.
Are your floats in good condition ? If they are damaged in any way the float can sink or stick to the floatbowl and the gas will again continue to flow. You can test your floats for leaks by plunging them into hot water and looking for bubbles.
All three carbs are fed at the same time. Judging by your problems i would suggest it is not gas starvation you have but overfueling which eventually takes the spark away.
Finally what is the condition of your fuel tank inside? If it is rusty i guarantee you there will be rust in your tap, and float needle valves and pilot circuit in the carbs which will cause all sorts of problems. You can after cleaning everything either fit good in line filters or use a tank sealant.
Good luck

[Ramjam]

Hi MJ.
Here is how to service your petrol tap https://youtu.be/qWNyvjaWfz8" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;
The gas will only flow if the diaphragm is sucked out by the vacuum tube which is connected to the centre carb. Check both end of that tube are sealed.
You say the float chamber was leaking, was that from the overflow pipe or the gasket? Either way it would appear that your float needle valve is not functioning as gas is continuing to flow afterfilling the floatbowl and then the crankcase.
Are your floats in good condition ? If they are damaged in any way the float can sink or stick to the floatbowl and the gas will again continue to flow. You can test your floats for leaks by plunging them into hot water and looking for bubbles.
All three carbs are fed at the same time. Judging by your problems i would suggest it is not gas starvation you have but overfueling which eventually takes the spark away.
Finally what is the condition of your fuel tank inside? If it is rusty i guarantee you there will be rust in your tap, and float needle valves and pilot circuit in the carbs which will cause all sorts of problems. You can after cleaning everything either fit good in line filters or use a tank sealant.
Good luck

thanks for all that. All three floats are in good condition. Number one carb shows no signs of external leak but manages to overflow into the crankcase though - with nothing coming out the overflow tubes.
Is that vacuum tube a short transparent tube? Should it have a plug of some sort on the end?
No rust inside tank.

And if there is too much fuel getting in, then how come it starts and runs for 30 seconds?
thanks

MJ

[GTandcbr]

The vacuum pipe attaches to the flat diaphragm case on the back of the gas tap. The other end goes to the middle carb and is normaly black rubber. It uses the air from the airbox rushing into the back of the carb when the engine is started. This pulls the diaphragm back and the petrol flows.
Check the overflow pipe on your number one carb is not blocked as when the bowl overfills it should flow out. If not the pressure from the gas tank above will push gas through the carb into the crankcase.
The reason the bike starts is the plug is dry to start with but quickly soaked by all that gas sloshing around!

[Ramjam]

So it's not possible to start it then with no air filter?

[GTandcbr]

It will start with no air filter but its not recommended as too much air will enter the mixture and the bike will run lean ending in a seizure.

[Ramjam]

It will start with no air filter but its not recommended as too much air will enter the mixture and the bike will run lean ending in a seizure.

I thought you might be interested to know that i checked the floats again. It seems that my adjustments for height were wildly out. Mine only measured about 20mm when in fact it should be 25mm so i'm guessing it was not closing off any of the fuel float needles, hence the flooding. But i still don't know why the crankase chamber filled up in number one. And i also don't understand why none of the gas came out of the overflow tubes.

Any ideas?


MJ in Beirut

[GTandcbr]

If your float height was 5 mm out that means the level in the bowls would be probably covering the bottom of the main jets which would mean as soon as the engine started it would suck pure gas into the carbs and it would therefore spit it straight into the cylinders. The float height MUST be set at 25.8 mm or the bike will run rough. The method of setting it is very important too. Hold the carb upside down. Tilt it forward until the tang between the 2 floats is just touching the float needle. Measure fron the carb not the gasket. Look here https://youtu.be/fuHOrBYOeqo" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;. I have no idea why the gas doesn't go out the overflow but this happened to me and i found out it was a sticking diaphragm in the tap

[Ramjam]

I have put the carbs and filter back on. But have same problem. Engine starts immediately but only runs now for 3 seconds. Looks like all three are firing. This to me - now i have corrected the float heights - seems more than ever like fuel starvation. I don't wish to be ungrateful to those who have advised me, but it might have been possible that i didn't give all of the info

Is it possible that now with less amount of fuel in the bowels that it could be the diaphram system which opens the petcock? If this is not working, would that not explain the 'starvation'?

thanks


MJ

[GTandcbr]

Hi easy way to find out if its the diaphragm run the engine with the tap in the 'prime' position. This bypasses the diaphragm. If the bike wont run i would get the tap off and service it.

[Ramjam]

But wht about just making a temp fuel tank out of a plastic bottle and feeding it directly? Would that be a way to detect whether the problem is with the petcock or not?

[GTandcbr]

Yes that would be good. But goes without saying be very careful..... Just another thought try running the bike with the petrol cap open as there is a breather hole in it which can sometimes become blocked. I would try everthing first before making a temporary fuel tank.

[Ramjam]

Quick update. Makes no difference with a make-do gas feed. The problem is inside the carbs and im beginning to wonder if after leaving the bike standing for 1.5 years that the fuel line are partially blocked. When i took the bowels out there was a fair amount of what looked like a wax. Perhaps it got into the lines.

the bike starts now but is not at all running on no 1 pot. And it can't rev up at all without dying. It's like its being drip fed gasoiline.

Is it possible that i didn;t set the flat chambers exactly to 25.8mm? Could that have such a dramatic effect? And do you push the floats down when you measure or let them sit touching the needles?

it will tick over. But just on no 2 and no 3

Any ideas?


MJ

[GTandcbr]

So now we are getting somewhere. We can eliminate to tap as the source of the problem. The float height is CRITICAL to the running of the bike and needs to be set at 25.8 EXACTLY. To set the height follow this proceedure. Assemble the float on the carb with the float needle valve in place. Grasp the carb in your left hand and hold it in front of you carb top to the left and floats to the right. Set the vernier caliper to 25.8mm. With your right hand Put the protruding slide to the edge of the carb top(not the lip) Now tilt the carb to the right so the floats dangle forward. Very carefully watch the tang between the two floats to see it just touch the top of the spring centre (without pressing it down) Now looking at the bottom of your vernier gauge the float top should just be touching the bottom of the gauge. If not carefully bend the tang untill it does.
If your carbs had anything in the gas bowls it will also be in the internal passages patricularly the pilot jet and circuit. If this is blocked and my guess it is the bike will struggle to run. The carbs will need to be cleaned and the gas tank flushed out too.

[Ramjam]

thanks mate. I'm going to yank 'em off tomorrow and open them up again. My guess is that it is BOTH the float levels and the jets and fuels lines. I'll update you here when i have some info

thanks for your time. Much appreciated

MJ