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SS18 Carb Conversion Marvel-Shebler DD to Nikki


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#1 Onanparts OFFLINE  

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Posted April 19, 2015 - 12:44 PM

My 1976 SS18 with the Onan BG series engine came with a Marvel-Shebler DD series carb. Although a good design it's a bit outdated and I decided to see how easy or hard it would be to convert it to the later style Nikki carbs used on the newer Onan's.

 

The conversion was basically a direct fit with a few minor twists. The original air cleaner will not work unless you do a lot of modifications. Good news is the later B series, B43, B48 etc air cleaners are a perfect fit with only one minor mod required as you will see shortly.

 

Main parts required: 

 

New style intake manifold # 541-0428, 146-0495 Nikki Carb, 149-2501 fuel pump or 149-1982.

 

Pics? You want pics! OK, here they come....

 

First up the original parts.

 

 

Attached Thumbnails

  • carb1.jpg
  • carb2.jpg
  • carb3.jpg
  • carb4.jpg

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#2 KennyP ONLINE  

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Posted April 19, 2015 - 12:51 PM

This sounds good. Count me in!



#3 Onanparts OFFLINE  

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Posted April 19, 2015 - 12:52 PM

Next up the test fit. No problems here. Fuel pump location can be either on the side of the carb as shown using the later style onan mounting plate that also retains the original choke cable setup. I'm looking for the plate part number and will post it when I find it. With the plate you use the 149-2501 fual pump. Without a plate a 149-1982 could be mounted on the flywheel shroud, Onan did this with many models. The only issue is routing a vacuum line to the pump from the block.

 

Throttle linkage was a direct fit connection.

Attached Thumbnails

  • carb5.jpg
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#4 larrybl ONLINE  

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Posted April 19, 2015 - 01:08 PM

I hope the one I bought will work, It looks like a direct fit, Only need to move the choke bracket over.

Attached Thumbnails

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#5 Onanparts OFFLINE  

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Posted April 19, 2015 - 01:14 PM

Pump outlet to carb inlet is a tight 180 degree turn. Just do what the Onan factory did. Get a small piece of spring that just fits inside the fuel line and make sure it's either in the middle at the u turn or use a longer piece that fills 75% of the length. If the spring is a tight fit you may have to push and or pull it. I made a small hook on the end of some welding rod to pull the spring into position.

 

Vacuum line from block to back of pump required no spring. 

 

Fuel line from tank was rerouted under the flywheel shroud and up the PTO side of block to make the pump connection. Pump # 149-2501-01 has the inlet and outlet on the same side but as you can see the breather tube is a tight fit and the inlet hose connection would push in on the breather. Opted for the no interference using the 149-2501 pump. The pump outlet to carb hose is a snug fit with no room to spare but it works fine.

Attached Thumbnails

  • carb9.jpg
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  • carb11.jpg
  • carb13.jpg
  • carb14.jpg

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#6 larrybl ONLINE  

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Posted April 19, 2015 - 01:18 PM

Neat trick with the spring! I shouldn't need to do that as my FP is mounted in the shroud tin, but I'll keep it in mind. Thanks!



#7 Onanparts OFFLINE  

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Posted April 19, 2015 - 01:38 PM

Neat trick with the spring! I shouldn't need to do that as my FP is mounted in the shroud tin, but I'll keep it in mind. Thanks!

The choke bracket is a direct fit to your newer carb larry. Looks like just a test fit on the manifold? Don't see the carb to intake gasket? One was in the gasket kit you got. Also don't forget to "remove" the limiter cap on the idle mixture adjustment screw. After you get it adjusted don't forget to put it back on.....those little plastic caps get lost......and if lost the world will never be the same. :)

 

Speaking of limiter caps...although it looks like you can't easily adjust the mixture on this conversion you can, it's the optical dillusion of the pic angle. I'll be certain to replace that limiter cap...if i can find the dang thing....Hmmmmm...now where is it? :)

 

Air cleaner installed. Only issue is the shroud to cleaner housing tube. Not exactly a perfect fit. I have a few ideas, might try a straight section of radiator hose and sleeve it up. Or drill three new mounting holes in the cleaner base to reclock it where it mounts to the carb. Breather hose has a little room to move so that might do the trick since it's not that far off.

Attached Thumbnails

  • carb15.jpg
  • carb16.jpg
  • carb17.jpg
  • carb18.jpg
  • carb19.jpg
  • carb20.jpg

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#8 Onanparts OFFLINE  

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Posted April 19, 2015 - 01:46 PM

You want to hear it run after the conversion? What a demanding bunch of Tractorholics you are!

 

And yes, the hood fits fine. I was checking for leaks etc. :) 

 


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#9 larrybl ONLINE  

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Posted April 19, 2015 - 02:29 PM

Wealth of tips! yes a test fit, I have the gaskets, and I would have tossed the limit cap as I thought it was shipping protection, Know better now! Thanks!



#10 trowel OFFLINE  

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Posted April 19, 2015 - 04:15 PM

 What a demanding bunch of Tractorholics you are!

 

 

We prefer passionate :thumbs:


Edited by trowel, April 19, 2015 - 04:16 PM.

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#11 KennyP ONLINE  

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Posted April 19, 2015 - 06:07 PM

Is the intake different for this carb?



#12 jhn9840 ONLINE  

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Posted April 19, 2015 - 06:26 PM

That is one sweet sounding machine. Looking forward to more.

jhn9840
John

#13 Onanparts OFFLINE  

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Posted April 19, 2015 - 07:52 PM

Is the intake different for this carb?

Yes it is. The early aluminum intakes had a smaller bore and carb mounting footprint. When Onan made the change to the later style Nikki carbs with a larger footprint they had a huge number of the early intakes still in stock, and they made up an adapter shown here that was used until the old style intake inventory was depleted. 

 

If you have an early style intake that is still good all you need is the adapter to be able to bolt up the newer style Nikki carb. Adapters turn up on epay from time to time. 

Attached Thumbnails

  • onan1542666.jpg
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  • onan5410428.jpg

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#14 toppop52 ONLINE  

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Posted April 19, 2015 - 08:39 PM

ASAP Onan parts in PA sells the brand new Onan dd carb and several others starting at $299, not the $500 you often see on eBay and other places. I almost did the conversion on my B 48 before I discovered them, two years ago they were only $239.

#15 sako222 OFFLINE  

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Posted May 01, 2015 - 08:39 PM

New (to me) SS-16 owner with throttle linkage question.

 

Hope this question is in line with the thread topic.  I acquired a neighbors 74 SS16 with almost a full suite of attachments for $250.00.  My wheel horse was lonely.

 

He reported problems with it only running at one low speed with little throttle response.. Hood & air cleaner off and digging into the DD carb & throttle linkage.  Trying to troubleshoot the tractor systematically.

 

When the throttle lever is run through the range, there is almost no movement of the control arm that actuates the throttle plate. It just hangs in the full open position. When moving the throttle lever to the slowest rpm setting, there is nothing that pulls the throttle plate back to the closed position.

 

  In Mr. Onan Parts excellent pictures above, I see a long, double-ended coil spring wrapped around the control arm that appears to perform the function of closing the throttle plate back to idle.  If my assumption is correct I am missing this spring.  Is this spring available or should I just find something similar and adapt it to fit?

 

Hope I haven't hijacked the thread with my new guy questions, but the photos showed me what I may have been missing.

 

Best Regards

Sako






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