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B&S 18.5 HP Engine only starts when fully choked and then barely runs


jam1020's Avatar
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09-08-08, 11:07 PM   #1  
B&S 18.5 HP Engine only starts when fully choked and then barely runs

I am pretty handy, but no where near a small engine person. So here it goes.
I have an older (early to mid-90's) Powrkraft (MTD) riding mower with a Briggs & Stratton 18.5 HP TwinII Industrial/Commercial engine (Model: 42A707 Type: 1238 01 Code: 9604095B). I have been having a progressive issue that has finally forced me to post.

The engine has been running this season, but typically I have to choke it from 50%-90% to get full power. I was going to have it looked at this fall, but...

It has not been run in about 3 weeks due to our drought and non-growing grass. Thanks to the rain we finally received, I needed to pull it out again and when I did, it would not start.

I started by checking oil and gas and the basics and everything looks good. I removed the air filter cover and tried starting it full choke, no choke, half choke, full throttle, idle, etc. The only thing that seemed to work was a full throttle while using a shop towel to completely choke the carb (which I figured out by total mistake). Once it was running (having to leave it fully choked with the rag on top), I could adjust the throttle normally; however, when it reached normal operating temp, the RPMs at idle/full throttle did decrease a little overall. Here is the initial symptom/issue: If I let any air into the carb it would begin to die. (I assume that since it runs fully choked (manually sealed with a rag) that it is getting some sort of fuel flow and even some air, but I don't know what that means.)

I know there are ways to adjust the carb and I walked through those to no avail (probably shouldn't have before I asked for advice, but hey...).
I am not sure what to do or where to go next. I would probably put it in the shop, but I am cheap and the two shops in town are backed up for 2-3 weeks anyway. So I would like to get this fixed in the next few days.

I have replaced the inline fuel filter and the fuel line seems to be getting plenty of fuel to the carb. I also replaced both spark plugs, and I will be replacing the air filter although it looks pretty good. I am afraid this might require me getting into the carb, but I don't really know what I would be looking for.

Any help or suggestions would be appreciated. Thank you, in advance.

 
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09-08-08, 11:52 PM   #2  
You're right, you need to go into the carb and clean it out. There's no need to be afraid though, it's a very simple carb. Remove the top, using spray carb cleaner, spray out all passages, remove the drain plug and the main jet behind it. Clean the main jet and the area where it fits. Reassemble, and if you got it cleaned right, it will run.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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09-09-08, 12:07 AM   #3  
Wow, thanks for the fast response (and I thought I was a night owl). I will give the carb cleaning a shot and let you know. I might take me a day to get to it. Do I need to take the entire carb off the engine?

 
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09-09-08, 06:09 AM   #4  
Posted By: jam1020 Do I need to take the entire carb off the engine?
Not initially, unless the carburetor is so bad that you need to soak it in a bath type cleaner. Most of the time you can do a pretty good job by removing the top and using a spray type cleaner, compressed air helps if you have it available, otherwise just do as cheese suggested and you should be alright. I would say that about 90% of the time I clean them without removing the whole assembly.

Best of Luck...

 
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09-11-08, 06:43 PM   #5  
OK...so I had to charge my battery up from all that initial "playing around" I did before posting here. Once I did that and cleaned the carb as you guys suggested it started up pretty quick. Now I have a few more small issues. In all my fooling early on, I may have tweaked the idle screw and that other screw that I think you all call the jet. There is also a screw/spring assembly on the throttle cable at the base of the front of the engine that I might have adjusted. In my defense these probably all needed adjusting anyway, but I am sure I did not help much.

While it was running full throttle/full choke to warm up, it sounded like the RPMs where a little high. It could just be that I had not heard the engine run that strong in awhile either (hats off to carb cleaner). So I tried to pull the throttle back, but the engine did not drop RPMs. I figured that was part of that spring/screw assemble on the throttle cable and after tightening it up a bit that seemed to give me some control. Once I got it up to operating temp with the throttle down, I goofed with the idle screw and I think I got that pretty much figured out; however, I could not lower the idle as much as I think it ought to be based on the sound of the engine before it would try to sputter a bit and try to die. That leads me to the next issue.

I could not seem to lower the throttle all the way without it dying or coming close (really sputtering) even a backfire. Another part of that is that when I moved the throttle back up to full, the engine would surge a few times before smoothing out. Same thing happened when I lowered the throttle back down. Except when the throttle was lowered, it never seemed to fully smooth out. So I am down to the last screw, the jet. Does that actually control fuel flow into the engine from the carb, by the way?

I have not yet done a search of the forum, but if you know of another post that answers how to set the idle, fuel mixture and throttle cable on this engine (see first post for engine specs) feel free to post it. Otherwise, I am going to need you guys to give me a quick primer on tuning this thing. Thanks again for your help.

 
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09-12-08, 12:48 AM   #6  
The jet is behind the drain plug inside the carb. It sounds like the carb is still not clean, and you need to remove the jet.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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09-12-08, 11:57 AM   #7  
OK, guess I missed the jet...so I will look for the jet and clean it. But what is the spring screw on the front bottom of the carb for? and what, if anything, does it adjust? (It tapers like a pencil at the end to a pretty fine point.)
Do you know any reference where I could find a schematic of this carb? I always like looking at pictures.
Thanks.

 
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09-12-08, 06:44 PM   #8  
Posted By: jam1020 OK, guess I missed the jet...so I will look for the jet and clean it. But what is the spring screw on the front bottom of the carb for? and what, if anything, does it adjust? (It tapers like a pencil at the end to a pretty fine point.)
Do you know any reference where I could find a schematic of this carb? I always like looking at pictures.
Thanks.
That is the low speed or idle circuit adjustment screw.

You can look at an illustrated parts list in pdf format at the Briggs website.

http://www.briggsandstratton.com/mai...0Parts%20Lists

 
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