Jump to content

Garden Tractors and Parts on eBay

- - - - -

Case 448 -- Can't get to run right... no matter what I try!

  • Please log in to reply
52 replies to this topic

#1 Case448 OFFLINE  

  • Member
  • Member No: 68323
  • 13 Thanks
  • 33 posts
  • Location: Michigan, USA

Posted November 20, 2015 - 08:32 PM

Re: Case 448 project tractor...


I just got this project tractor running, but no matter what I do, I CANNOT get it to run right. It's starting to drive me nuts!


First, I'll list whats been done, then I'll describe what its doing.




Engine: Onan B48M -- looked pretty good, so thought I would try to get it running rather than rebuild it right away.


Compression: 130 on drivers side, 105 on passengers side.


Valves:  Valve job done two weeks ago, valve lash set.


Carbeurator:  Newer style "DD" series, with diaphram type fuel pump on front side. Rebuilt last week, with complete "dd deluxe" kit -- including new fuel pump diaphram. Carb body not soaked.


Points/Condensor -- both replaced last week. Point gap lightly sanded, cleaned with brake cleaner, gap set at .021.


Fuel line and filter -- new filter (came with carb rebuild kit), line replaced all the way back to tank. (Fuel line was leaking / allowing air bubbles into the line.)


Spark Plugs -- new.





Completed everything listed above, ran great for the 15 minutes I drove it around the yard. Ran at all throttle positions, running great!


Went to do some work with it the next morning (had a case hydro vac/cart hooked up), thats when the trouble started...


Idled good, but overall got progressively worse after about one hour running time, started running terrible at higher RPM's, mild backfiring, would lose power, acted like it was running out of gas. Two times it just quit -- I guickly opened the hood, and the fuel line/filter was empty (no gas in the line, tank was full).

It's getting plenty of gas, or I should say, there is plenty of gas available in the tank and line.


Messed with the carb adjusting screws. Low speed screw works fine, adjusted it to best position. Main/high speed screw (brass screw on back right side) seems to work normally from around 0 to 1-1/2 turns out, but going from 1-1/2 turns out to 4+ turns out, doesn't seem to change anything.


Blew out main port 5 times with carb cleaner, port seems wide open / no obstructions, carb cleaner shoots heavy stream into throat of carb. The nozzle where cleaner shoots out into the throat of carb looks a little deformed, did not look examine as would need proper instrument to inspect.


Carb. main adjusting screw -- still no difference after 5 times with carb cleaner.


Motor is running worse by the hour --- was running fine 0-1/4 throttle, Now it starting hard, and it's loggy / terrible response at throttle lever. Higher RPM's are terrible. Acts real loggy across the entire RPM range. Increasing throttle shoots mist of gas straight verticle from within the carb throat. Mild backfiring at high RPM's, open throttle all the way, backfiring gets worse.


The more load I put on the engine (like engaging mower deck) causes more backfiring.


Let motor run at any continuous rpm -- low rpm not bad, anywhere from 1/4 to 3/4 rpm motor "hunts" / has a miss, seems like one cylinder might be cutting in and out, inrease to full throttle --- backfiring starts, so did not run past 3/4 throttle.


I can't figure out what might be causing this, but whatever it is, it's getting progressively worse. It seems like I've fixed the most common causes, but I might be missing something?


At this point its starting hard, and acts like it hardly wants to run.


Any help will be greatly appreciated.





  • boyscout862 said thank you

#2 shorty ONLINE  



  • Senior Member
  • -GTt Supporter-
  • Contributor
  • Member No: 6172
  • 5,500 Thanks
  • 5,267 posts
  • Location: Lancaster County Pa

Posted November 20, 2015 - 08:44 PM

Not knowing much about your tractor, it sounds almost like a partially clogged fuel line. Or a faulty pump.
  • boyscout862 said thank you

#3 petrj6 OFFLINE  

  • Senior Member
  • Contributor
  • Member No: 53717
  • 4,083 Thanks
  • 2,369 posts
  • Location: petersbrgh ny

Posted November 20, 2015 - 09:02 PM

   Yep I agree, if the fuel filter was empty than that's the place I would look into. replace all the fuel lines and filter and check for leaks and cracks then have a look at the fuel pump.  if the pump is good and it is getting plenty of fuel then when you turn the motor over with the fuel lines unhooked it will really spray fuel.


#4 propane1 ONLINE  

  • Senior Member
  • Member No: 54307
  • 8,725 Thanks
  • 4,132 posts
  • Location: Charlottetown, Prince Edward Island, Canada

Posted November 20, 2015 - 09:13 PM

I would also check the gas cap to make sure the vent in the cap is working. Simple test is to loosen the cap to let air in and see if that changes any thing. And I would recheck your points setting, and to see if points are loose and if the setting changed. Also check timing. I don't know how to do that on that engine of if it can be done. Just some ideas, Noel.
  • JDBrian and boyscout862 have said thanks

#5 Rock farmer OFFLINE  

Rock farmer
  • Senior Member
  • -GTt Supporter-
  • Member No: 10759
  • 1,198 Thanks
  • 1,234 posts
  • Location: York Maine

Posted November 20, 2015 - 09:15 PM

First thought is, your valves are set too tight.
I'd try adding some more clearance to them.
You set them cold? Maybe when worm their not closing tight.


#6 Case448 OFFLINE  

  • Member
  • Member No: 68323
  • 13 Thanks
  • 33 posts
  • Location: Michigan, USA

Posted November 20, 2015 - 09:49 PM

Earlier this week, I had excessive air bubbles in the fuel line, which (I assume) were getting sucked into the carb, and so there must have been a leak somewhere which supplied the constant stream of air bubbles.


I replaced the fuel line all the way from the tank to the carb. I ran the tractor using the new hose submerged in a 5-gal gas can, no air bubbles. Then tested with the tank, no air bubbles. So installed new line and filter.


Now, no air bubbles.


But the tractor still runs terribly.


Could be fuel pump issues, but the diaphram was just replaced with carb rebuild this past week.


Valve lash sounds like a possibility, but then why the "hunting" at any RPM (ie. variation in motor RPM, with governer trying to compensate (moving back and forth about 1/4" to 1/2"), and the sound of (possibly) running on one cylinder, then two, then one, then two, etc. Overall sound is not smooth, but choppy sounding, like its hitting on only one.


I watched the spark using spark testing (not sure if correct name) hooked between plug and wire, spark looked normal on both sides, I did see a slight variation in spark every time it that cylinder would 'miss' --- not sure the cause, or if this means anything.


Also, is it normal to see sparks between the points (ie between the point gap) when running?

#7 Eric OFFLINE  

  • Senior Member
  • Member No: 41778
  • 1,323 Thanks
  • 813 posts
  • Location: Wisconsin

Posted November 20, 2015 - 10:03 PM

I have recently learned that I am not as perfect as I thought I was, just because I have done a rebuild does not mean I don't have to do it again. Sometimes even us godly mechanics get it wrong and need to go back and redo some of what we blessed with our awesome hands. I would check that fuel pump again and possibly the carb also as you could still have an obstruction. Just my two cents, good luck.
  • boyscout862 and karel have said thanks

#8 gopher OFFLINE  

  • Senior Member
  • Member No: 2755
  • 764 Thanks
  • 758 posts

Posted November 20, 2015 - 10:15 PM

If getting a lot of arcing in points bad condenser. You said lower pressure on pas side  pull plug wire see if rids backfire  if so that's the valve you need to look at.

Edited by gopher, November 20, 2015 - 10:19 PM.

  • Alc said thank you

#9 boyscout862 OFFLINE  

  • Senior Member
  • Contributor
  • Member No: 8923
  • 12,864 Thanks
  • 9,176 posts
  • Location: N.E. Connecticut

Posted November 20, 2015 - 11:35 PM

I too suspect that you have a fuel problem. You have done the normal remdies. I have to suspect that you have a fuel filter in the valve at the bottom of the fuel tank and that it is partially blocked. Use teflon tape when you replace it. The other possibility is that some crud that was in a fitting has moved into the fuel pump or carb.

Whenever you have problems with an engine, use the troubleshooting guide in its manual. Good Luck, Rick

#10 crittersf1 OFFLINE  


    GT Lunatic

  • Senior Member
  • Member No: 26142
  • 4,207 Thanks
  • 5,805 posts
  • Location: Newport, N.C.

Posted November 21, 2015 - 08:05 AM

Sounds like a fuel delivery problem to me.

#11 glgrumpy ONLINE  


    Getting Out!

  • Senior Member
  • -GTt Supporter-
  • Member No: 8360
  • 8,618 Thanks
  • 7,926 posts
  • Location: Huntington, IN 46750 North East in State

Posted November 21, 2015 - 11:28 AM

First, .o21 seems like big point gap, but don't know these engines. Usually .o20 for new, but used better at .018 on most K engines and such I have worked on. Second, the pumps on carbs on these are not know to be good items. Many gut those and use diff pump, electric or the pulse kind. Briggs type is cheap and easy to run lines. Some Onans have a two-piece tinny intake manifold that is glued and pinned together and they develop leaks along there sides at joint. Spray something along there and see if any changes in running from spray being pulled in there. That would be a "Vacume leak then.  As noted, IF after running is worse, might be valves expanding, I think that is place to try again also.

#12 cmn135 OFFLINE  

  • Member
  • Member No: 56682
  • 36 Thanks
  • 55 posts
  • Location: S.E. Connecticut

Posted November 22, 2015 - 03:58 PM

Just for giggles...grab the top of your throttle shaft where it attaches to the carb and give it a shake. It should have very little play. If it does have excessive play it introduces dirty air into the combustion process and also results in erratic idling especially at the mid ranges where the excessive play shows up the most. I don't think this is your only problem as I think the majority of your issue is fuel related but it could be a contributing factor. I had this issue and fixed it with a bushing kit.
  • boyscout862 said thank you

#13 Case448 OFFLINE  

  • Member
  • Member No: 68323
  • 13 Thanks
  • 33 posts
  • Location: Michigan, USA

Posted November 24, 2015 - 04:54 PM

Apparently I have two problems: 

Problem #1 (likely solved):   Was an air leak between the top and base of the carb, so that it would be drawing air through this leak, and not have sufficient vacuum to draw gas through the high speed jet, located at that same level. Air is easier to move than gas, so it was drawing air, rather than sufficient gas. Which now makes sense as to why the high speed screw would not change anything when turning out past 1-1/2 turns, as there wasn't a sufficienct supply of gas to change anything.


Motor now running better, no longer backfiring through the exhaust, did not apply a load to the engine, but seems to longer be leaning out.


Motor now runs better, but I still have problem #2...


Problem #2:  I still have a miss -- one of the cylinders is only firing intermittently, it comes and goes about every 2-5 seconds. Mild backfiring through the carb at higher RPM's. Seems to want to run a lot better, much healthier sound, good pulse at the exhaust -- but the pulse I feel is mostly just one cylinder, with the other coming and going.


Idles good -- miss seems to come in at about 1/4 throttle (I could be mistaken - maybe its just not audible), stays constant up to full throttle, but back off full throttle because backfires through carb.


Has two new spark plugs, new points.


Has had valve job two weeks ago, valve lash set.


I now believe, because of todays repairs, that its getting enough gas.


Old plug wires.


Any ideas on this one?  . . .

Edited by Case448, November 24, 2015 - 04:56 PM.

#14 olcowhand ONLINE  


    Red Tractor Nut & Diesel Addict

  • Staff Admin
  • Staff
  • -GTt Supporter-
  • Sponsor
  • Contributor
  • Member No: 20
  • 41,255 Thanks
  • 31,447 posts
  • Location: South Central Kentucky

Posted November 24, 2015 - 06:53 PM

Old wires can definitely cause issues like this.
  • propane1 said thank you

#15 cmn135 OFFLINE  

  • Member
  • Member No: 56682
  • 36 Thanks
  • 55 posts
  • Location: S.E. Connecticut

Posted November 25, 2015 - 09:14 AM

Could be wires as indicated. I'm only familiar with the single cylinder kohler models. Does the twin onan have two coils? If so it could be the coil or condenser (if yours has one) on the failing side. Coils and condensers sometimes only begin acting up once they warm up or are put in higher throttle as they near full failure.

Also...can you elaborate on the air gap? Was the needle not seated properly or did you mean the carb was not mounted tight enough against the motor and was drawing air in that way?

Edited by cmn135, November 25, 2015 - 09:16 AM.