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

[GTandcbr]

Okay so we have the essentials required to start the engine. Fuel,air,compression and a good spark. So my guess without standing next to the bike, is that the amount of fuel is a problem. As i said before the float level (which has been checked) is critical to this. The other thing that meters the fuel is the jets. The jet responsible for the cold start of the bike is the pilot jet together with the choke/enrichment circuit. I dont think anything mere can be achieved until the pilot jets are removed and preferably replaced. The internal pilot passageways need to be cleaned also.

[jabcb]

Not sure which model year GT380 you have. This doesn’t apply if you have an early model with the cable actuated choke.

For the later GT380s, make sure the choke lever stays engaged. The little metal part that holds the choke on can get weak with age.
It might not hold the coke in position for very long when the engine is vibrating.

Sometimes the problem is so obvious that we overlook it, like putting the carb slides in backwards.

[Ramjam]

but if the pilot jets were blocked surely the bike wouldn't start at all. I mean it runs well for three seconds and then DIES very suddenly. No misfiring or running rough. And we know gas is getting in cos of the wet plugs.
I think we are over looking something more fundamental

[GTandcbr]

Okay can you first tell us the year of the bike? Does it have a cable choke?. The pilots may not be completely blocked but just really dirty. Lets concentrate on the choke for a while. The symptoms you describe can be caused when the choke is not functioning. The purpose of the choke is provide a denser air/gas mixture until the engine parts become warm. Then the gas is atomised when it touches the hot metal parts and we then close the choke off as its no longer needed. If you look at your float bowl there is a brass tube sticking up. This is where fuel is drawn upwards from the base of the bowl. It may have small holes near the top to help atomise the gas. Check those tubes are not blocked.

[Ramjam]

the bike is a 76 model and the carbs have a single lever over the top of them pulling the carb links up together.

I haben't looked at those tubes (there are only two of them if i recall) but i hope you;re right that they could be blocked as this would be a simple issue.

But i think you're onto something with the choke theory

Is it possible that there is not enough gasoline sitting in the bowels, so that when the choke is operated, it only sucks a tiny bot of gas for a second?


And i thank you for it


MJ

[GTandcbr]

Yes there are only two chokes on your bike. If you look at the bottom of the bowls by the brass tube you will see a mall hole too. This is where the fuel is drawn in through a jet and sucked up the tube. Make sure this is very clear.on the top of the carb will be the choke pistons check they lift when you put it on. You also need to check the air passage from the choke piston into the carb is clear. I do think you are close to solving this problem. I know it seems there is only a little petrol in the bowls but you have checked the float height. Remember the gas is atomised before its burnt.

[Ramjam]

how can you check the choke pistons working?

[GTandcbr]

Hi
When the lever is put on, check that the two little pistons one on each of the two carbs are lifting to open the choke air passage. They have a habit of getting stuck in the passage. Take them out and clean with carb cleaner. You will see a rubber seal on the bottom of the piston. Quick spray into passageway blow out with compressed air.

[Ramjam]

thanks. Appreciate the advice.

[Ramjam]

Is there any way i could do this test/exam with the carbs still on the bike? It's almost 30 degrees C here

[GTandcbr]

Sorry to say you need to get the float bowls off to check those choke tubes and holes. You will need to get those little pistons out from the carb tops too. It will be worth it when you get the bike running...

[Ramjam]

so you have to take the carbs apart, then? I mean beyond just taking the bowels off?

[GTandcbr]

What i suggest is to do this in 2 stages. First and most important is get the bowls off and clean and check the choke feed tubes. Put the bowls back on. Make a visible check of the choke actuating shaft and plunger pistons. When you switch the choke on are the plunger pistons lifting up? Then try the bike. If it starts then great. If not you will have to remove the choke actuating bar and plunger pistons to clean them and the airway into the carb.

[Ramjam]

how hard is it to break teh top and bottom apart? The six nuts look pretty straight forward but is there anything you should be looking for which might ruin it? Will the two sections just lift away?

I have put carb cleaner into the tubes you mentioned but i dont see any sign of blockage. I blew threw them and although i had to blow hard, there was the sound of air passing through

From the outside, the two chock pistons are going up and down no problem \

My theory - and i can't explain this logically - is that the floats need to be sitting at a higher point before they close off the taps. I think the choke tubes are just touching the top of the surface of teh gasoline and there needs to be more there.

thanks

[GTandcbr]

Its been a few years since i took apart the top of your type carbs maybe someone else reading this can help you? I have the early 3 independent carbs(much easier to do) The choke draws fuel from the bottom of the bowl through that little hole. It is sucked up by air passing through the air passage in the carb. The plunger piston blocks that air passage until it is lifted by you on the lever. If that air passage through the carb is blocked it will not suck up fuel from the float bowl and therefore the mixture will be too lean to start the bike.