[mverdill]

Hi,
I recently acquired a mid 80s Case 448 with the Onan 18HP engine. This is my first tractor that has the Nikki Carb (146-0414). I'm having problems with the engine surging and I believe the trouble is the carb. I've swapped the intake manifold and that didn't help. I took a DD series carb off another tractor, installed in the 448 and the engine ran just fine. So I decided to tear down the Nikki carb and soaked in carb cleaner, blew out the air passages, etc. That didn't help either.

My question, am I better off biting the bullet and buying a new carb or is there someone out that that overhauls them? There is some play in the throttle shaft, so I'm guessing that it's sucking air and causing the engine to surge.

Thanks for any suggestions.

 

[Bob MacGregor]

The Nikki carb was an option on B48 Onans. It is a fairly simple carb but to clean it correctly both the high speed and low speed jets need to be removed. The high speed jet is under a a plug and it needs a particularly shaped flat tip screwdriver to unscrew it to apply cleaner into it and the passage, the jet itself is brass and has a metered ID passage. The brass nozzle in the carb cover is another possibility where crud gets stuck and plugs passages. The float setting usually needs to be radjusted also.
You can find a parts breakdown in the Onan manuals on this site in both the B48 and P series manuals. The Onan P series manual will show several versions of the Nikki carb as later P series engines used a Nikki carb that has an accelerator pump installed. Your carb probably doesn't have an accelerator pump.
Bob MacGregor in CT :mrgreen: :mowlawn:

 

[boomers_influence]

mverdill
i have found the nikki carbs to be virtually trouble free.
i hope you did not SOAK the float.
thank you boomer

 

[Bob MacGregor]

With 1,000 hours on each of two P218s that I had, a repowered 448 and a 4018 I did have to do a carb cleaning on each several times. I use internal engine cleaner (marine engine decarb)for carb cleaning as it digests the crud and cleans the brass. This cleaner is not the solvent type and I wash the carb parts in water and air blow dry. Been using this product since the late 60s. It is in an aerosol can and I never let it touch any gaskets and I don't use it on plastic floats.
As soon as the RPM of the P218s start surging then I do a carb cleaning. Much easier to remove the intake manifold and then remove the carb. The P218 on the 448 was the later type with an accelerator pump and at every surge in RPM it squirts fuel into the engine, some of this fuel ends up in the engine oil diluting it, not good.
Mad Mackie in Taxonnecticut :mrgreen: :mowlawn:

 

[f350ctd]

Make sure that when you blew the passages out you didn't blow the plug out that Bob refers to. I did that and it took me awhile to figure it out. It is near the bottom of the carb opposite the flywheel end. I also had a high rpm surge after cleaning my carb and adjusting the only mixture screw on the Nikki (idle air mixture?) a little richer cleared it up.
f350ctd

 

[mverdill]

This particular carb is definitely not "trouble free". I completely disassembled it including both the high and low speed jets and soaked in carb cleaner (no, I didn't soak the float) for several days. I ran a small wire through all the passages and blew them out with compressed air, so I know there is nothing plugged. Again, when I put a carb from my 446 on it, ran fine and idled smooth.

My question is: can someone overhaul this or should I just buy a new one? Boomer - do you have any good used ones for sale??

 

[mverdill]

Here is a link to a NOS Onan carb on eBay. Would this work OK?

http://cgi.ebay.com/CARBURETOR-ONAN...783?pt=LH_DefaultDomain_0&hash=item20b81e6b5f

 

[Hydriv]

I'm not trying to be argumentative here but let's look at this logically and see if we can agree on the following.


At some point, the carb you have was installed on an engine, the engine went into a tractor and the tractor was sold to a customer. If the carb was defective, then logic would dictate that it would have been replaced and hopefully....scrapped by the dealer or by Case or by Onan ....depending on how the chain of warranty worked.


I don't doubt your word when you tell us that you soaked it for days and then "rodded out" the passages. What I am saying is this. There is something about this carb that you have missed. I don't know what it is. Perhaps you missed some dirt in a passage. Perhaps you have a damaged needle or jet. Purchasing a used carb, may or may not put you ahead of the game but I certainly understand your frustration with this problem. It could be that someone else messed with this carb prior to you owning it and they did something foolish. Have you closely examined the needles? Are they smoothly tapered or is there a ridge indicating that a heavy-handed person with a screwdriver bottomed it out? Is the needle broken off?

Have you compared what you have to an exploded diagram of the carb? If this carb has always been a problem since owning the tractor, perhaps the PO put a needle from a different carb into it. As you say, the engine runs fine with a different carb and that proves the problem is the carb you are trying to repair. All too often, the real problem is very simple but we just don't see it.

Of course.... at some point we are forced to assess the time factor involved vs the cost factor of replacement. I'm just tossing out some ideas that may make you want to take one more look at the problem.
:thumbsup: :thumbsup:

 

[Hydriv]

Here is a link to a NOS Onan carb on eBay. Would this work OK?

http://cgi.ebay.com/CARBURETOR-ONAN...783?pt=LH_DefaultDomain_0&hash=item20b81e6b5f

The Seller is willing to answer that question if you send him the number off the side of your carb. Seems to me, that would be the best way to determine if his carb is right for you. :trink:

 

[mikebramel]

Would you want to buy an adjustable one

 

[troll]

mverdill- Don't give up! When I first bought my 448 (B48M) it ran rough. Despite dismantling and cleaning the carb no less than 3 times, I couldn't get the damn thing to run right. Turns out the throttle body shaft had an 'intermittent slip', according to the Cummins mechanic.
I must admit the 448 has been a treat to use since the carb issue has been solved.
Troll