Nikki carburetors overview and information [Archive] - Case Colt Ingersoll Lawn and Garden Tractor Forum

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Bob MacGregor
07-23-2020, 05:10 PM
Hi Troops,
Nikki carbs in various venturi diameters are found on later production B43M, B48M and all 16, 18 and 20 HP Performer series Onan engines. Later production P series Onan engines have a Nikki carb which has the addition of an accelerator pump.
All Briggs & Stratton Vanguard twin cylinder horizontal shaft engines from 12-18 HP have Nikki carbs that are similar to the ones on Onans but with one major difference which is the addition of solenoid valve. This valve is electrical and with key switch off, the spring in the solenoid pushes the plunger, which has a Viton tip, making contact with the high speed fuel metering jet and stopping fuel flow from the carb bowl into the jet.
Key on, the plunger in the solenoid retracts allowing fuel to flow thru the jet from the carb bowl. The high speed jet in Vanguard carbs differs from the jet in Onan carbs as it has a counter bored recess in it that allows the Viton tip of the solenoid plunger to make contact and close off fuel flow.
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Bob MacGregor
07-23-2020, 06:50 PM
More Nikki carb info:
Generally, the choke plates are the same size, most throttle plates are the same size or close, the difference is the diameter of the venturi. Examples, the 14 Vanguard carb is marked as a 130 across the air inlet, this is actually a restrictor as the 14 Vanguard is a has a small displacement. 116049
This carb is from a B43M Onan and although only 2 more HP than the Vanguard, it is a much larger displacement engine, so the air inlet is larger in diameter and marked as a 200. As you can see, this carb needs to be cleaned which I have done. The contamination in this carb is a granular substance that is probably from gasoline with ethanol and not treated for long term storage. This substance did not dissolve in the gasoline and was partially blocking the high speed jet. The fuel tank nor fuel filter on this tractor had any of this contamination showing in it. This tractor is a 1986 226 that had been here for some TLC which it received and has gone back to the owner.

Bob MacGregor
07-23-2020, 07:41 PM
Nikki carbs on Onan B43M and B48M engines can be easily removed for cleaning. Close the choke while removing the air filter and housing. The oil breather can be slipped off the connector on the engine, but the intake manifold must be removed with the carb still on it, so the correct carb kit and two intake manifold gaskets are needed to reinstall after cleaning. Don't count on the carb top gasket or the intake gaskets being reusable. Special screwdrivers are needed to remove the jets for cleaning and reinstallation. The choke cable needs to be disconnected and the link from the governor to the throttle lever needs to be disconnected, don't loose the connectors and note how the link is installed before removing it. Disconnect the fuel line, remove the intake bolts and remove the exhaust heat shields and lift off the carb/intake manifold.

Bob MacGregor
07-23-2020, 08:26 PM
Intake manifold and mounting bolt hole conditions to be aware of on the following engines!:shock:
B43M,B48M, P216, P218, P220 and similar Linamar engines!
The intake manifold bolts of which there are two different lengths, were installed by Onan with no thought of having additional items added. These bolts are borderline too short to begin with although the threaded holes are deep enough to accommodate a longer bolt providing the threads have been extended, confusing? I will explain!!!!
When these engines are installed in Case/Ingersoll garden tractors, the exhaust heat shields, ground cable connectors and other items were added and installed during assembly, but the same bolts were reinstalled. This caused even less intake bolt to thread engagement and when torqued to 6-10 FTLBs can strip the threads in the engine which is aluminum. As I service Onan powered Case/Ingersoll tractors I find this condition often.
How does one fix this problem, well I'm glad that you asked!!!
Follow me carefully on this!
During manufacture, Onan drilled these holes with the correct tap drill for 5/16-18 thread and then tapped the holes to 5/16-18 thread, however they didn't tap the threads all the way to the bottom of the holes. With the bolt holes clean and using a 5/16-18 bottoming tap, the threads can be extended deeper and be able to accommodate longer bolts. Clean and dry out the bolt holes out after tapping them deeper. Should any of the holes have stripped threads, then a thread insert such as a Helicoil can be installed. Bear in mind that the stripped bolt hole will need to be drilled and then tapped with the correct tap drill and taped to receive the insert which needs to be installed to the correct depth and the tang broke off and removed from the hole.

Bob MacGregor
07-23-2020, 08:40 PM
Pic of a 5/16-18 bottoming tap.
The bolt configuration is shown in these pics. The four stock bolts are 2" long, the replacements are 2 1/4" and 2 1/2" long.


Bob MacGregor
07-23-2020, 08:55 PM
The other consideration with Onan intake manifolds is leakage. The manifold is not one casting but several pieces put together with fuel resistant sealant and rivets. Sometimes leaks can develop and cause mysterious engine running problems that are difficult to diagnose. To rule out or confirm a leaking intake manifold, with the engine running, carefully spray brake clean around the manifold as it is highly flammable and observe any difference in the running of the engine. Repairing intake manifolds is a nasty job, I will get into this in a later post.

Bob MacGregor
07-23-2020, 09:18 PM
Early production P series Onans, spec A-F have a Nikki carb without the accelerator pump. Later production P series Onans and Linamar XL of spec letter G-H have the Nikki carb with the accelerator pump. The flexible material of the check valve in the pump will deteriorate and allow fuel to leak into the carb venturi as the engine is running or shut down. This fuel causes several poor engine running problems in addition to fuel dilution of the engine lube oil. I have run into this problem with almost every spec G-H Onan and Linamar that I have serviced. The reason for adding the accelerator pump to the carb was to be able to comply with ever increasingly stringent emissions requirements from the EPA. As Onan/Linamar/Cummins wasn't able to clean up the emissions on these engines, production was stopped. Ingersoll and other manufacturers who had been using Onan/Linamar engines had to go to other small engine manufacturers such as Briggs & Stratton, Kohler and Kawasaki.
Ok, I'll be back and I have a 1987 448 coming in soon which will need a lot of TLC and some Mac Mods done to it, hopefully I will be able to take more system specific pics particularly of the EZ Adjust clutch repair and adjustment and post them in Mac's Corner!!:geek: