Carby 250 dies when choke turned off
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Hi,
I’ve just got back into a project bike I’ve been away from for a few months and am having issues with idle and revving.
The carbs have been stripped, cleaned and reassembled. The bike startes and idles with the choke on and will remain running if left alone.
If I try rev the engine, the engine will “choke” itself and stall out (die) – choke is still on at this point. When I slowly back the choke off, the engine starts to struggle and will also die, usually when choke is about 80% off. The idle adjuster screw is wound in to give max idle.
Sometimes I can slowly roll the throttle on and the revs will climb, but not much. Sometimes when doing this, when I release the throttle, the revs skyrocket before stalling out.
I’ve taken the jets out and given them another blow out but still get the same thing. Reading through any carb related posts here, I should have a 95 and a 92.5 jet – I can see I have a 95 and a 9x.5 (I cant make out the second digit). The air mixture screws behind the black bungs were changed when I cleaned out the carbs a few months back. Reading through the carb posts last night, I now see I shouldn’t have touched them – looks like I’m in the abyss. I checked a spare carb that hasn’t been touched and found that both front and rear were wound 2 full rotations out from fully in. I’ve adjusted my air mixture screws to suit – it made no difference to my issue – same problem persisted. I’ve replaced all hose clamps to stainless but most of the hoses are original.
I’ll try put a video of it running.
Any ideas where I should look next?
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I’m leaning toward this as well… but I’m really not looking forward to trying to get the airbox/carb back on again – there was alot of swearing last time.
I’ve already stripped the carbs right down and cleaned all the fuel residue gunk from them. I was hoping pulling the main jets out and giving them a clean would solve it, but no such luck.
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It’s a big ask, but when the engine is hot. Turn it off. Then take out both spark plugs.
Can you do a compression test on both cylinders at full throttle for a few seconds and tell us what PSi each piston cylinder is making ?
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I don’t have compression test gauges but wouldn’t mind investing in some. Do you know what size/pitch type fits the plugs?
The engine won’t rev to full throttle – it dies if I try get to 1/4 throttle. I am curious, how would the comprssion test results aid in determining an issue?
Cheers,
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I don’t have compression test gauges but wouldn’t mind investing in some. Do you know what size/pitch type fits the plugs? The engine won’t rev to full throttle – it dies if I try get to 1/4 throttle. I am curious, how would the comprssion test results aid in determining an issue? Cheers,
Start it up and keep it running (by any means) until its hot for 10 minutes.
Take out both spark plugs, then fit one compression tester probe in to front cylinder, twist full throttle and press start for 5 seconds to get a reading.
Then wait for starter motor to cool down, repeat again on the rear engine.
Make sure battery is fully charged, as slower spinning cranks will give false positives. (unless starter motor itself
Amazon Prime, get one that says USPRO compression testor. Connect the “straight metal” pipe. ignore flexi tubes etc…
Now hold it down to the plug hole hard.
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Compression tester showed up yesterday. I got the below results on a hot engine with both plugs out when testing.
Front = 125 psi
Rear = 132.5 psi
Is that good or bad?
While I was waiting for the tester, I took out the jets for a 3rd time to triple check they were clear – they were. But I found that I missed the fuel inlet filter/gauze above the float stopper thingy. I took each of them out and one was semi-blocked with debris, the other was clear. Gave them a clean out, put the compressor blow gun on all holes and reassembled. Also made an auxiliary tank from a stainless steel water bottle so I can get the engine running without having to take the tank on and off everytime. This bypasses the ‘pump’ and goes straight to the carb inlet hose.
Few things I noticed:
- Good news is the bike will now rev to the limiter and everywhere inbetween as long as I keep the revs up, but it still dies at resting idle if I turn the choke off.
- It is also slow to decelerate when I release the throttle which I’m gussing is sticky slides/diaphragm? The throttle cable returns fine.
- As I bypassed the fuel pumps, I noticed the hoses from the inlet boots that create the vacuum for the fuel pump and the tank-tap pump had to be plugged well. They caused the motor to pop and cackle irregulary and the engine tune would improve when I blocked the hoses.
I’m not sure if it is running now due to the fuel inlet gauze being cleaned out in one of the carbs or it is because the fuel is gravity fed strainht into the carb hose. I need to hook the tank back up and see if the engine will still rev up. If it doesn’t, I think the pumps are the issue or the old vacuum hose’s have a leak in them (this is my pick).
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Compression tester showed up yesterday. I got the below results on a hot engine with both plugs out when testing.
Front = 125 psi
Rear = 132.5 psi
Is that good or bad?
While I was waiting for the tester, I took out the jets for a 3rd time to triple check they were clear – they were. But I found that I missed the fuel inlet filter/gauze above the float stopper thingy. I took each of them out and one was semi-blocked with debris, the other was clear. Gave them a clean out, put the compressor blow gun on all holes and reassembled. Also made an auxiliary tank from a stainless steel water bottle so I can get the engine running without having to take the tank on and off everytime. This bypasses the ‘pump’ and goes straight to the carb inlet hose.
Few things I noticed:
- Good news is the bike will now rev to the limiter and everywhere inbetween as long as I keep the revs up, but it still dies at resting idle if I turn the choke off.
- It is also slow to decelerate when I release the throttle which I’m gussing is sticky slides/diaphragm? Odd as I cleaned all the sticky fuel residue off of them when I stripped the carbs. The throttle cable returns fine.
- As I bypassed the fuel pumps, I noticed the hoses from the inlet boots that create the vacuum for the fuel pump and the tank-tap pump had to be plugged well. They caused the motor to pop and cackle irregulary and the engine tune would improve when I blocked the hoses.
I’m not sure if it is running now due to the fuel inlet gauze being cleaned out in one of the carbs or it is because the fuel is gravity fed strainht into the carb hose. I need to hook the tank back up and see if the engine will still rev up. If it doesn’t, I think the pumps are the issue or the old vacuum hose’s have a leak in them (this is my pick).
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The float seat was a little restricted yes, so it’s a good job you have cleaned out the mesh cone that’s under it. Now the bike can correctly meter its fuel.
Feeding fuel straight to the carb is okay with a clean auxilary pump, just make sure there is an inline fuel filter as you will never know , micro debris may be present in the fuel than our eyes can see.
Was the airbox still connected and fully secure ? And also if you spray under the airbox rubbers and carb engine rubber pipes, with carb cleaner, do the revs change ?
It may explain the slow drop of revs as it’s either
– Hunting for fuel to idle down (clogged pilots / mixture screws too lean / airbox suspicious)
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– intake manifolds on the engine have a micro leak somewhere
or
– Vacuum hoses have a leak (double check they are blocked hard if not using fuel pump and fuel tank)You can clean the fuel pump yes , but open it. Don’t use air compressor, just a gentle washout with the carb cleaner helps.
No need to spray carb cleaner inside the “silver” hole of the pump , as that’s just for air vacuum.
If the fuel pump is faulty , it will be slow to deliver fuel or if bike is switched off , fuel will leak in to the silver vacuum hole , then leaks in to intake manifold pumps (which is not good for engine health as fuel washes down piston walls and oil is like water and smells very very strong of fuel, this is a sign to immediately flush engine and piston rings may need changing
Which means your compression should be around 150-175 psi hot healthy. It will run at 130 psi but it isn’t ideal, i believe that’s a limit , which means rebuild time soon.
I assume clearances are spot on ? (can affect compression values too)
Or if possible, expect to change head gaskets while replacing piston rings or pistons with rings , this would be an opportunity to really inspect cylinder walls and valves are sitting nicely in the valve head.
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Have’nt touched the bike for a couple weeks as I needed to put the battery back into the race bike for a race that came up – I only have 1 battery between the 2 bikes.
In repsonse to your comments Marcel;
- The auxilary tank has a new inline filter on the feed hose.
- The air box and filter were fitted. I didn’t have a carb cleaner spray can at the time – will have to try this to see if there is a leak in this area.
- I only put my fingers over both inlet vacuum hoses at the same time when running – I’ll try plug them and clamp them tight.
- I had opened both fuel pumps at the time, and they were both clean and seemed to be functioning fine. Rubber seals all looked good.
- I haven’t opened the oil cap to smell for petrol but am familialr with this problem. I’ve had it on a dirt bike. – I’ll have a sniff to make sure it hasn’t occured.
- Compression – I just realised 2 mistakes I made, 1) I didn’t hold the throttle open when testing. 2) I hadn’t checked the valve clearances – I’ll have to check the clearances and re-do the test.
Hopefully get a chance to try this weekend.
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