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Discussion Starter · #1 ·
Hi All,
1987 Case 446 w/16hp Onan B43M. I'm dealing with a backfire issue since last fall. It gets pretty bad a few minutes after starting and then after lowering throttle and not mowing for 5 minutes or so and then it will run fine as long as need to mow. I had done some reading and thought maybe I just needed to decarbon (going on 600hrs). So I did that and it seemed to help at first. I could hear a very subdued backfire almost in the background the first couple times after. But it got worse and is just about as bad as it was now. I have put on new plugs, wires and air filter. I only burn rec fuel and have been adding a little seafoam lately too.
My next thought is to rebuild the carb (it is a Nikki) but I am looking for input from others. Any direction would be appreciated.

Thanks in advance!
 

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Discussion Starter · #3 ·
I was afraid somebody was going to say something about valves (not very familiar dealing with that), but made me recall one of them looked suspect while I was decarbing.
I took this picture with the head off, think I was mentally blocking it out since. Think this may be the issue and if so how do I resolve?
Gas Machine Circle Automotive wheel system Nickel
Thank you.
 

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I was afraid somebody was going to say something about valves (not very familiar dealing with that), but made me recall one of them looked suspect while I was decarbing.
I took this picture with the head off, think I was mentally blocking it out since. Think this may be the issue and if so how do I resolve?
View attachment 123433 Thank you.

Look at page 19 in the service manual which you can get here:

Directory Listing of . (Colt Case Ingersoll Manuals<hr><a href="http://ccigt.com">Back to the CCIGT forum</a>)

There's a file there with B43M in the title. The desired clearance (distance) between the tappet and valve is listed on page 4 of that manual.
 

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Discussion Starter · #5 ·
Yeah, good info. Thank you.
Pretty sure I have a bad valve seat on that exhaust valve. Yikes, pushing my limits here.
Suggestions on where get a valve seat from? Can I do just that one or should I do all 4 if I am doing one (I only noticed one seat with damage) ? Do I need new spring (s) or other parts while I am at it? Gonna need some new tools to do this too.
Thanks for advice and tips you may have.
 

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Hello OnekamaZac,

Wondering if you replaced the condenser? A bad condenser can cause the symptoms that you mention. To elaborate, the engine will start right up and run fairly well at idle or slightly above but as soon as you try to increase the throttle setting it will pop, spit, sputter and backfire as you mention. If you pull the cover off the breaker box the points will exhibit a good amount of blue spark and archng when running. If this is what you find I would replace the 312-0246 Onan condenser and file/clean/polish the breaker points then readjust or preferably set the gap using a timing light.

Hope this is an easy fix for you,

Bob
 

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Discussion Starter · #7 ·
I have not done the condenser. If that is the problem, will it straighten out after it gets warmed up and run normal after 5 minutes or will it continue to struggle? Mine will run fine after it seems to warm up and gets through a few minutes of backfiring.
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I would do a leak down test or very least a compression test if you think you have a bad valve and find out for sure. I would replace points and condenser with OEM ones for sure, condensers as stated do crazy things including often misfire and back fire.. Time the engine then. And adjust the valves if they check out. Unlikely but could be coil. Make sure you have the correct plugs too.
 

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Discussion Starter · #9 ·
Ok, I don't have a leak test kit yet, but I did a compression test with a cold engine and got about 85psi on each side, pretty happy with that. I tried to set it at TC and put compressed air in, sorta like a leak down test without being able to measure. If I was at TC, definitely do have air coming out the exhaust.
That said, I ordered the condenser and new plugs.
I'll see about getting a leak down test kit.
I found the TC mark on the flywheel. Do you just make sure that is at the top to be truly at TC?
Thanks
 

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Ok, I don't have a leak test kit yet, but I did a compression test with a cold engine and got about 85psi on each side, pretty happy with that. I tried to set it at TC and put compressed air in, sorta like a leak down test without being able to measure. If I was at TC, definitely do have air coming out the exhaust.
That said, I ordered the condenser and new plugs.
I'll see about getting a leak down test kit.
I found the TC mark on the flywheel. Do you just make sure that is at the top to be truly at TC?
Thanks
Yes timing mark at 12 O'clock, and with both valves closed, I believe it's called, the compression stroke.

Zero leakage out the oil filler tube, zero leakage out the carb throat, and zero leakage out the muffler, are the "ideal situations." I've put in 50 PSI into the cylinder, and that's pretty difficult to hold if the crank isn't jammed/locked in place, so make sure the crank is blocked from moving somehow.

Air out the oil fill tube, air getting past rings,
Air out Carb throat, intake valve leakage,
Air out the muffler, exhaust valve leakage.

I think, it was Gordy that mentioned timing and valve setup. You haven't answered that question from what I can see,
 

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Discussion Starter · #11 ·
Indeed. I will check /clean the points when I replace the condenser. Verify that the gap is correct. That will be the timing, right? I'll see what effect that has, and go from there. Maybe I will get lucky, but I think I will end up on the valves and that seat. Any suggestions on where to source the valve parts from?
 

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you could always try to apply some valve compound and lapp the valve to the seat before you go buying new parts. maybe a good lapp job could seal up to the seat. and yes get a new condenser same thing happened to me bought a new condenser and ran solid. member dealers on this site will have parts
 

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Discussion Starter · #13 · (Edited)
Well, got the condenser replaced and checked the points as well as new Champion plugs. Opened points just a touch to get to 0.21. Started up and mowed the lawn. No sign of backfire. Best it has run in over a year. Not claiming victory yet, but I'm encouraged. Will give it a few more runs first. I like the lapp job on the valve idea if backfire comes back. Thanks for all your tips guys!
 
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