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Diaphragm needles. (SOLVED)

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Yan101
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Diaphragm needles. (SOLVED)

Post by Yan101 » Wed Aug 03, 2022 4:11 pm

Hi. Just replaced the diaphragms in the carb on my 1992 xv535. I noticed the screw top that holds the needle in wasn't as deep as the previous one and therefore the needle isn't quite as long. Fired the bike up and it ran fine and revved perfectly. Then slowly died. It now won't start. Could this shorter needle from the bottom of the diaphragm assembly be the cause?

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faffi
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Re: Diaphragm needles.

Post by faffi » Wed Aug 03, 2022 4:27 pm

If the needle is basically sitting a little higher, the mixture will be richer. I would let the bike sit for at least an hour and try and start it again without choke to see if it will run then. Also, is there any chance that the needle(s) have fallen out and into the emulsion tube? That could prevent the engine from running because it would lean out the mixture.
The journey of a thousand miles begins with a motorcycle.

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faffi
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Re: Diaphragm needles.

Post by faffi » Wed Aug 03, 2022 4:34 pm

Just one more thing; you are sure you are getting fuel to the carbs?
The journey of a thousand miles begins with a motorcycle.

Yan101
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Re: Diaphragm needles.

Post by Yan101 » Wed Aug 03, 2022 5:09 pm

Yea the bike was running before hand. Just hard to rev. I checked the diaphragms and they were in a bad way. The replacements had a different top screw which wasn't as deep so wouldn't allow the needle to come down as far. I've just come back out and put the screws on from the old units which has lowered the needle to the same as they were before but still not starting

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faffi
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Re: Diaphragm needles.

Post by faffi » Wed Aug 03, 2022 5:28 pm

Well, we are down to basic trouble-shooting then. It seems unlikely that compression went away on both cylinders at once, so we are looking for spark and fuel, then.

Spark is easy to check. The best would be a strobe timing light, but you can remove the spark plug caps, one at the time, and the accompanying spark plug and crank the engine while looking for a good, strong blue spark. Both should be equally strong.

If you lack spark, you must find out why.

Fuel is next. You either have too little, or too much. I believe you can reach the float drain screws fairly easily? If so, open them while trying to catch as much of the fuel coming out as you can. You want to drain a float bowl of fuel from each. If little or no fuel comes out, you have some kind of obstruction or a bad pump. If you have too much fuel coming out, chances are your needle float valves are not closing, drowning the engine in fuel. You should be able to detect the latter by repeatedly turning the ignition on and off - if the fuel pump never stops, or takes several seconds to stop every time, chances are it does not get enough back pressure to stop, meaning float valves are likely not closing fully.
The journey of a thousand miles begins with a motorcycle.

Yan101
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Re: Diaphragm needles.

Post by Yan101 » Wed Aug 03, 2022 5:58 pm

Well the bike initially did run when the diaphragms were re installed then it slowly popped to a death. The bike is a project my dad and me got running on Saturday. The pump is fine. The bowls are filling. It has spark, just pulled a plug and it was wet. Is it possible I've flooded the bike with the needles being so much shorter on the new diaphragm bodies? I think my next move is to re use the old needles in the new diaphragms (I bought full diaphragms and not just rubbers) and see if its those. They don't look as tapered for a start and the new ones only have one e clip setting.

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faffi
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Re: Diaphragm needles.

Post by faffi » Wed Aug 03, 2022 6:27 pm

If the spark plugs are wet, you have/had a too rich condition. In my experience, NGK spark plugs in particular tend to die when subjected to drowning. They may work outside the engine, but not properly under compression, making starts difficult at best. A new set of spark plugs may be all you need.

What you need to determine is if the too rich condition stemmed from the the wrong needle position, or overflowing carbs.

If the new needles are identical to the old ones, there is no reason to put the old ones back in. But if they are thinner, shorter, or have a different taper, fitting the original ones make sense.
The journey of a thousand miles begins with a motorcycle.

Yan101
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Re: Diaphragm needles.

Post by Yan101 » Wed Aug 03, 2022 6:31 pm

I just re fitted the originals. They seem more tapered and to be honest the new ones looked "cheap" the diaphragms were covered in fuel and there was a strong smell of petrol as I removed them. I'm hoping it's a case of me flooding the bike. The length of the needles was 3.5cm from the bottom of the diaphragm body to the tip of the needle. I've searched the haynes and have installed the needle in the same order as in the book. I'll give it a try again in the morning

Yan101
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Re: Diaphragm needles.

Post by Yan101 » Thu Aug 04, 2022 12:23 pm

Just an update. Left the bike overnight and all today. Still wouldn't start. So I removed the plugs and the wiring to the fuel pump, ran the starter a bit to dry up any un burned fuel. Reinstalled and she fired up. My only thought is that I flooded it somehow after installing the new diaphragm needles that were too short. I'm not overly confident with carbs and I'm still learning bit surly that had to be the issue as that's the only change I'd made.

It does have the original needles on the diaphragms back in though. As long as it runs I'm not bothered. Reset the mixture screws to factory settings too as one was about 3.5 turns out.

Next job is oil change, drive shaft oil and I need to paint it as its been sat since 2016

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faffi
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Re: Diaphragm needles. (SOLVED)

Post by faffi » Thu Aug 04, 2022 4:54 pm

Good you got that solved!
The journey of a thousand miles begins with a motorcycle.

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