Gt550a won’t fire

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Johninreading
Posts: 18
Joined: Sun Feb 17, 2019 9:49 pm

Gt550a won’t fire

Post by Johninreading »

My 1976 GT550 does not fire at all. It had poor starting but traced this to a short in the ignition circuit causing a voltage drop on the electronic ignition. Also one choke fork not connected properly. Fixed both problems and it started and ran for about 5 mins, then stopped and won’t fire at all now. Checked, I have good spark, new fuel, choke working and good compression. Can’t fathom out what could be wrong. Can anyone help?
lairdofsmoke
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Re: Gt550a won’t fire

Post by lairdofsmoke »

Have you tried turning petcock to prime for a couple of minutes.
Can get back to you later if no luck. Or send me a pm
On fragrant cloud
barney01
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Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Gt550a won’t fire

Post by barney01 »

Hi. Firstly welcome to the forum. There are plenty of 550 owners on here so you have come to the right place. Obviously there are many reasons why your bike wont start but it can be solved but first we need to eliminate a few common causes. Have you just bought the bike and do you know the history of when it last ran?
What sort of electronic ignition is fitted? Is the petrol tank clean inside as any rust will end up in the carbs unless you have a fuel filter fitted.
The petrol tap has a diaphragm fitted which can stick closed preventing fuel flow. Suggest turning to prime on the tap to bypass this.
After trying to start it pull the plugs are they wet or dry.
If the carbs have not been cleaned they will be full of all sorts of crap blocking jets and pilot circuits.
Is the timing sorted so the motor fires at the right time?
NO SMOKE .......NO POKE
Johninreading
Posts: 18
Joined: Sun Feb 17, 2019 9:49 pm

Re: Gt550a won’t fire

Post by Johninreading »

I have had the bike for 5 years. I had the engine stripped and rebored (oil pump failure on centre cylinder) and the crank rebuilt with new seals (as it had been standing for many years). It ran fine after that, but was hard to start. I diagnosed the fault to a choke problem (one fork not engaged). After that it started pretty good (but only if the battery was 100% charged or kick started). It has a Newtronics Electronic Ignition system, but a wiring fault (4 ohm short) was pulling down the battery voltage, which stopped the electronics working, I fixed that and now it gives a a healthy spark. After that it started and ran pretty good. I then fitted an electronic regulator/rectifier for good measure. When I started the bike to test if the battery was charging, it ran for about 4 minutes and then died. It appeared to have run out of fuel. I removed the carbs and sure enough, the float chambers were empty> There was about 2 gallons of fuel in the tank, the prime worked and so did the vacuum switch. The pipes between the carbs however were perished, so I replace the, and the feed pipe with clear cotton reinforced PVC tubing.

Now I see fuel running into the carbs and out of the float drain plus when open, so I know fuel is getting onto the carbs. I have had the tank resprayed so I know it is clean and I replaced the fuel with new high octane petrol yesterday (just in case it had lost its light ends). So I have spark, compression and fuel, and rapid cranking, but still not a single pop from the engine! The plugs are not damp after cranking, so I think its fuel, but can't work out why?
barney01
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Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Gt550a won’t fire

Post by barney01 »

One thing to try to locate the problem. Remove the airbox and try the starter and spray easy start or something similar into carb venturi it should at least try to fire then.
Have the carbs been cleaned?
When operating the starter whilst on choke make sure the throttle is fully closed otherwise the starter circuit won't be any use at all.
I too have Newtronics system fitted they are very reliable.
How clean is the air filter. Clogged it will not allow air into the cylinder for combustion.
NO SMOKE .......NO POKE
Johninreading
Posts: 18
Joined: Sun Feb 17, 2019 9:49 pm

Re: Gt550a won’t fire

Post by Johninreading »

Thanks for the tip about the throttle when on choke.
I cleaned the air filters with air filter solvent and re-oiled them with air filter oil. I tried with them out, but didn't think of the easy start test.
I'll try that
Johninreading
Posts: 18
Joined: Sun Feb 17, 2019 9:49 pm

Re: Gt550a won’t fire

Post by Johninreading »

Good news, problem sorted, the problem was the dopey organic think on the end of the starter button. Thanks for the tip about keeping the throttle closed when on choke. it fired first time! Thanks Barney01
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slosher
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Re: Gt550a won’t fire

Post by slosher »

Good thread that guys a lot of food for thought there ;)
And Welcome John :)
GT750L
GT550L
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Loctite Trained Engineer :lol:(since having the Bonnie)
barney01
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Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Gt550a won’t fire

Post by barney01 »

To those who are interested in the theory from the photo hole F is the choke air tube. With the main throttle valve at the bottom air is forced down choke tube. With main throttle valve lifted air pressure into F is lower or even minimal. In the choke air tube or starter circuit is the choke plunger. With the plunger lifted (on) the air passes over a tube which suck's petrol from the bottom of the float bowl and this is then drawn directly into the cylinder. The tube in the float bowl that feeds the starter circuit has little holes near to the top which causes the fuel to froth.
A two stroke engine needs to be heavily enriched with fuel to fire.
But once the engine is running the choke needs to be closed as soon as is possible.
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NO SMOKE .......NO POKE
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