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Pressure Testing the Nikki Carburetor - Quick and Simple

One of the biggest complaints I hear about is often the Nikki Carburetor, and how much some techs hate them , how they seem to always leak and the leak just won't quit, ending up with the carburetor being simply replaced.

I, on the other hand, have rarely, if ever had any problems with them, and thinking about it, and discussing with peers, it seems to be the biggest factor is in technique and diagnostics.

So, this is one of the first 2-minute drill articles I wanted to do up - How to pressure test a Nikki Carburetor with that %$%$@ nylon fuel transfer tube.

Since the more common issues seem to come up with a particular design where the formed bowl gasket is seated in grooves in the transfer tube assembly (and a separate o-ring seal for the fuel inlet) I chose to start with that specific design.

To start with, you will need a good hand operated vacuum/pressure tester (or at least pressure) Such as This one from Mightyvac

The one I'll be using in the photos is a very expensive unit, but for those wanting the best of the best in tools, that's the Echo Dealer kit made by Power Probe, #LHRV90 shown here: PowerProbe LHRV90

To Begin with, the quick test is done on the carb bench - Start with baseline test- take the complete assembly and hook up pressure gauge to the fuel inlet - You'll do this test both before and after servicing. Ready to test - the initial hookup
Now you turn the carb upside down (float bowl up) to allow the weight of the float to close the float needle, and it should hold pressure - Just the weight of the float itself is sufficient to hold 10 PSI for 1 minute (Most vacuum operated fuel pumps won't produce much more than 8 PSI if that) - Here's the test - note it is holding pressure. (It is, after all, a brand new carb I'm using for this article)
Baseline Testing
If you have no leak there, then it should not be leaking fuel past the float needle. Assuming you do have pressure dropping (quickly or slowly, does not matter.), we'll pop the bowl off and separate the components: (Note the red line drawn between the points where fuel runs through carb to float needle)
The Nikki Carb - separated.
First, just to eliminate the carburetor body itself (Maybe it has cracks or other damage) , we'll pressure test just the body and fuel inlet (my finger pointing at it):
The Carburetor fuel inlet passage
What I do is seal off that inlet with a finger (a little soapy water helps to see if you're leaking!) and pressure test just that section (Can also pull a vacuum and see if it holds, but I rarely bother here.):
Inlet Pressure Test
If it holds pressure, you have eliminated the carburetor and fuel inlet as the leak source. If it loses pressure, squirt soapy water around and see where the bubbles form - if the carb body or fuel fitting leaks, only option is to replace the carburetor.

Next, I want to pressure test the transfer tube and float/needle As you can see from the photo below, there is a slight "nipple" on the transfer tube (that's where o-ring seal goes, by the way) which we'll use to pressure test the transfer tube
The transfer tube inlet nipple
It is pretty simple - just hook on a suitable hose between it and the pressure tester:
Hooking pressure tester to transfer tube
Now we can pressure and vacuum test the transfer tube and float needle/seat - again, soapy water can help locate any leaks (Cracks, leaking float needle, etc)
Pressure Testing Transfer Tube

If it holds pressure, just by the weight of the float, then you know that is all good, too, so the only other place it may be leaking would have to be the o-ring seal. If you have a pressure loss you can apply pressure to the float to see if you can force the needle to seat - if that works, then likely the seat (replace transfer tube) or needle (replace needle, obviously) would be leaking - otherwise any leaks are likely either around the hookup or a crack in the transfer tube. - Obviously if transfer tube is cracked, it needs to be replaced.
Finally, drop the float itself in a small container of gas, hold it under for a minute or two, with a suitable tool (make sure there's no ignition source around to light off the gas vapors!) and then remove it, shake it and listen/feel for any liquid sloshing - indicating that float is leaking - you'll need a new float if that is the case.
Now, for the MOST COMMON reason we find after *someone else* has tried to fix a leaky Nikki: Careless Assembly! - I got this photo using an otoscope - the little slice in that o-ring is not visible to the naked eye, unless you are really *looking* for it. (And even in this photo , it is hard to spot - but that's what was leaking.)
Nikki O-ring Damaged

So, to prevent that sort of problem - we have an assembly procedure for these carburetors, involving a little lubrication and some finesse.

First, make sure the carburetor and transfer tube are perfectly clean, no nicks, damage or corrosion in the above mentioned fuel inlet port of the carburetor body.
Then, assemble the o-ring and bowl gasket to the fuel transfer tube grooves as pictured here:
Ready for assembly
Next , a nice light film of 2-cycle mix oil smeared on the o-rings helps a lot...
Then, (and make sure the main jet is seated in the transfer tube, too, this method, that jet won't fall out of place either), holding the carburetor body in normal operating orientation (bowl down) , you can slip on the transfer tube upwards, keeping everything in place, and gently line up the fuel inlet ports and o-rings - it should slip together easily.
Assembling the Nikki
Once it is mostly all seated (don't force it!) you can invert the carburetor and check that the gasket and o-ring have seated flush and square to the carburetor body:
Gasket fully seated
Sometimes a slight adjustment and a little pressure while holding the transfer tube parallel to the carburetor body is needed, but it should go together very easily - if not, take it back apart and see what's stopping it. This is where the o-ring got sliced in the earlier photo - trying to force it in place.
From Start to Finish, the whole test procedure took less than 2 minutes.

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24.03.2014. 17:16 | by bgsengine


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