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Discussion Starter · #1 ·
Hey guys, just a question noodling for ideas....

Have a '94 Lawn Chief with a 15Hp Briggs 28Q777 0128. When she runs, she runs great.

After winter storage this year, I went through the Nikki carb and replaced the needle and float. She seemed to be doing fine. However now it seems like shes choking down or perhaps vapor locking after 20 -30 minutes run time. Like shes running out of fuel but she isn't.

After she stops/dies and cools off a bit, she'll fire right back up and run about the same time length and start stumbling again.

Fishing for your thoughts on this one .. ;) Appreciate it.

Chap
Kentucky
 

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Typical symptoms of an ignition module going bad. Once they heat up, the broken wires expand. Once it cools, it'll work fine.
 

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Like the guys say, it sure sounds like a fried coil. Try running without the gas cap first, though; that'd be the cheapest fix.
 

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Discussion Starter · #6 ·
Thanks guys.

The ignition seems more likely as it has a tupperware cap with a hole in it for a gas cap. Can't find the right size cap that will thread onto the tank.

I'll be scoping it out again this weekend. :)

Chap
 

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Discussion Starter · #10 ·
Well, we put the ignition module/coil in. There is a fairly repetitious engine surge. The surge is mostly under a load, light or heavy, and slightly at idle. Reckon we'll start looking at the carb or carb gaskets or something.. ;)

Chap
 

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Why did you install a new coil? Did you check for spark when it stumbles...? Unless that surge of rpm is constant from the start up...carb is probably Not the problem..,
Does it still take 20-30 min for choking/surging to act up...?
 

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Discussion Starter · #12 · (Edited)
Hey Sus,

Appreciate your thoughts!

Why did you install a new coil?
Well, it made sense to me at them time. Plus the 28Q is a '94 model and it was the original. I figured it can' hurt. :)

Did you check for spark when it stumbles...?
No, I sure didn't. Dang... didn't even think of it. I'll sure be doing that today!

Unless that surge of rpm is constant from the start up...carb is probably Not the problem..,
Cold engine ran smooth. Took about 10 minutes of running then the surge started up and gradually became worse.

Does it still take 20-30 min for choking/surging to act up...?
Will follow up on that today.

Chap
 

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Discussion Starter · #14 ·
Ok...

On cold engine, I took the Nikki carb off and clean it good and blew it out. There was a tiny bit of gunk in the gas when I removed the float bowl. Needle didn't look bad. No holes in plastic float. Reassembled and reinstalled the carb.

On a choked start, engine idle as normal. After a minute or so in choke I moved it to normal run. And that's when the surge starts. If I move back to choke the surge goes away. Every move to run normal (fast .. or slow) throttle caused surge. After about 20 minutes running in this manner the surge began to show up in choke. Ultimately the engine quit running while surging and in choke mode.

Letting it cool down for a couple of hours ... and then we'll see if it'll do it again.

Chap...
 

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Thnnx for the detailed description of start and run...It definately looks to be a "lean running" eng...causing the surge...Usually a partially clogged main jet or tube in the carb...perhaps some of that gunk is still in there..Or, leak at gasket [intake pipe]..Myself, I have had bad luck with the Nikki carbs..but only for leak thru at needle seat, and flooding..So, I'm no help here..Perhaps others can clarify how to better clean the Nikki..You may also want to spray carb cleaner arround the intake pipe when running, see if surging stops..if so, you found the problem..Good Luck....Hope you can avoid the pricey buy of a new carb..
 

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I don't have much experience with Nikki carbs; I've worked more on Walbros and Zamas. Anyway, it sure sounds like you have a vacuum leak or a partially clogged jet. A tiny piece of debris that you can barely see will suffice to clog your jet. I would go over it real carefully with a magnifying glass, and I would also look for a vacuum leak, like suspicionofignorance suggests.
 

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To test for vacuum leaks, take a can of carb cleaner and spray CAREFULLY around the intake, without spraying directly into the intake. Spray where the intake manifold meets the block and where the carb meets the intake. If the engine revs up whenever you do this, you have a vacuum leak.
 

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Discussion Starter · #18 ·
Hey guys...

Thanks a ton for all your input. I sincerely appreciate it. I wasn't able to get back on the 28Q this evening. Had other crises happening. :)

In pulling the Nikki carb I didn't even think to remove the jet. I should have, no doubt.

In the meantime on Monday I'm going to check with my local B&S parts house and pickup or order a full carb overhaul kit and jet kit.

May take a day or two to get back and report.... but now I'm on a mission... ;)

Again, thanks! You guys are aces!

Chap

PS. Here's a graphic of the Nikki I'm working on....

http://www.chapmanjones.com/Equipment/nikkicarb.png
 

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If that fixes it, look on the bright side, you probably have a good spare coil now.
 
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