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Nissan 5HP 2-stroke NS5B strange running issue

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keaton85

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Older Nissan NS5B that ran all last season and now it will run fine at low RPM and then with higher throttle it will just die and not restart with fuel in the venture. It will not restart and run unless I strip the carb down and put it back on dry. Run again fine and then at high RPM stall and never restart normally.

Things that have been done:
- Lower unit pulled and impeller done
- New head gasket
- Good compression
- New plug
- Replaced the carb, same issues! plenty of fuel.

I'm at a loss, it's like the float is set at the wrong level and then it just floods, but it won't come out of it.
 
pvanv said:
Verify float height.
Click to expand...
The very first thing I did, original carb checked twice and perfectly parallel with carb body when flipped. The new Chinese carb is fixed higher without the damper needle like the original, same parallel setting. Bought the new carb since the idle needle on the old one had been warn out from incorrect use.

I just pulled the flywheel to check for corrosion on the coil, all looks perfect except rust on the Hall effect sensor tip. I don’t think this would cause this issue.

it does seem like over fueling, although if the spark was weak at higher RPMs that might cause the buildup of unburnt fuel.

I’ve done a ton of 2-strokes in my day, in and out of the marine industry and this one is very perplexing.
 
I have a very similar issue.

M5b will run great for about 5 mins, but after that it dies and won’t restart, or if it does restart it wont rev.

I’ve replaced the carburettor too, made no difference.

So am wondering if an ignition component is failing when hot ?
 
SDTohatsuM5B said:
I have a very similar issue.

M5b will run great for about 5 mins, but after that it dies and won’t restart, or if it does restart it wont rev.

I’ve replaced the carburettor too, made no difference.

So am wondering if an ignition component is failing when hot ?
Click to expand...
Ive been using an electric outboard for a few weeks and had the Nissan sitting in my shed, pulled the flywheel to check the pickup and coil which looked fine yet the pickup had a bit of rust on the sensor tip which i cleaned. Now, put the engine back on and its been running great, although I actually think the original issue was water in the fuel. under low speed the high speed jet at the bottom of the bowl wasn't being used, just the idle jet, yet once i throttled up it would inject a big shot of water. Just a theory because now it runs perfectly. so odd!
 
Thanks for the update.
My issue isnt water or ethanol related as it’s the same with fresh non-ethanol fuel.
I’ll check all the resistances at the weekend.
PS, as a hobby i rebuild old motorbikes so am very familiar with carbs and electrics, just not 2-strokes!
One other thing to check is the spark plug, i had terrible issues with an old Honda eventually traced to new but dodgy ‘ngk’ spark pkugs that must have been fakes even though bought from reputable supplier.
 
SDTohatsuM5B said:
Thanks for the update.
My issue isnt water or ethanol related as it’s the same with fresh non-ethanol fuel.
I’ll check all the resistances at the weekend.
Click to expand...
ethanol can actually help a set percentage of water since it will keep the water in suspension and allow for it to pass through the engine. The issue is only once you reach total saturation you get the fall out of water and it settles to the bottom. Ironic that more ethanol would actually help mitigate this issue.
 
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