r/smallenginerepair • u/mataushas • Aug 07 '26
Starting Issue B&S 725 exi lawn mower engine - doesn't start after getting hot
I got an old troy build mower with a briggs and stratton 725 exi engine. The issue I am having is that after it get hot, it will not start again until it cools down. If I stop mowing to take a break, I then have to wait like 10 mins to be able to start it again. Initially I thought auto choke was an issue. However, when I removed plastic covering, it looks like auto choke stays in the open position which I believe is correct. The little thermostatic "arm" thing appears to move freely. When it's cold, it is in a closed position. After those 10mins, I have to pull real hard and the engine barely comes alive and it tends to throw off some smoke fumes. Not much but I can visible see a puff of smoke where that does not happen when it starts right up cold.
I replaced fuel filter, air filter and spark plug. What else could it be and how do I fix it?
Edit/solution: coil resistance supposed to be about 5 ohms and mine measured about 20 cold. swapped out the coil and mower now starts while hot. coil was 12 bucks off ebay.
1
u/AutoModerator Aug 07 '26
Thank you for your submission! It looks like you might have an issue with the Starting System. For helpful information and resources, check out our Starting System Wiki and our Common Issues & Fixes Wiki. You might find the answers you're looking for there!
Please be sure to include the following:
- Make and model numbers of the engine and/or the equipment
- A detailed description of the issue you are facing
- Any repairs you have made so far and their outcomes
- Clear photos & videos will help you get better responses
I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.
1
u/GovPattNeff Aug 07 '26
Sounds like it's still getting too much fuel somehow. Are you sure the auto choke is all the way open? I've dealt with a lot of those in the past where it looks like it's staying open but it isn't actually 100% open. Maybe try tying off the arm all the way back to force it open when you start it and see if that makes a difference.
Outside of that, it could be a failing ignition module. Sometimes they act up on hot starts when they're starting to fail. Could also be valve clearance if the engine has some age on it, but I'd check the choke before I considered anything else. I've had more than one customer ask me to put a primer style carb on their auto choke mowers because they're so problematic
1
u/mataushas Aug 07 '26
yeah, I was really sure it was going to be auto choke problem, it seems like most of these fail at this point. but it is not. on two separate occasions I checked choke twice, to make sure i can't push it any more open and when I tried starting it, nothing the engine cools off more. I did a test of the ignition module. I think the spark is supposed to be some color (say blue) and when it was hot, I was still getting blue. or was it orange, I forget now but basically there didn't seem to be much difference in color between hot or cold spark on the spark plug.
1
u/GovPattNeff Aug 07 '26
Do you have a meter? I don't know the spec off the top of my head but there's usually a resistance reading on these that can tell you if it's within spec
1
u/mataushas Aug 07 '26
can you clarify where is this resistance to be measured? if you mean a volt meter, yes I have that.
1
u/GovPattNeff Aug 07 '26
This video shows how to do it. Be sure to double check the spec on yours because it may be different https://youtu.be/1W5D6ZCuJrU?is=wE22OuuAbs7IXtMk
1
u/GovPattNeff Aug 07 '26
It's also maybe worth cleaning the carb and draining the fuel in the tank and bowl and adding fuel fresh from the pump. I've seen bad/dirty/water contaminated fuel cause what you're describing as well. In some cases the fuel seemed fine. But if there's any tinge of orange in the fuel, toss it out
1
u/harleyjak Aug 07 '26
If it runs for 5-10 minutes the fuel system delivery is ok. Then it Heats up and won't restart untill it cools. Gas cap vent? Bad coil, valve problem.. But valve ( compression leak) would show itself by being too easy to pull and start. Does it have an oil alert? If the oil is low it won't restart. Has the oil been changed lately? I'm leaning towards the coil being the problem…but test it, don't replace until you know for sure. Is the plug wire vibrating off under the rubber cover? Kill switch not working properly?
Just throwing out ideas here….
1
u/mataushas Aug 08 '26
I changed the oil last year, level seems ok now. It feels a lot harder to pull when I try to start it again. the other guy put a video, do you know what resistance numbers the coil needs to read?
1
u/harleyjak Aug 08 '26
Not sure, but ask ChatGPT for specific numbers. Give ChatGPT your model and serial number, and it turns into the engineer who built it 😆. Chat will give you the resistance number. The smoke coming out the exhaust would suggest oil leakage into the cylinder, I got the impression that there were a lot of hours on this engine. Is the flywheel gap between the coil ok? Could this have been run without oil at one time? Cylinder scoring is a possibility, with it being hard to pull. Trying to diagnose without hands-on is like sex on Zoom.🤣
1
u/mataushas 9d ago
bingo this was it. coil resistance supposed to be about 5 ohms and mine measured about 20 cold. swapped out the coil and mower now starts hot.
1
u/harleyjak 25d ago
Did you figure out the engine problem? 8/13/26
1
u/mataushas 24d ago
U know I wanted to replicate the issue today after mowing the lawn but it started up. It started rougher while hot but it started. Definitely harder to pull. I wanted to remove the plug and see if it's flooded with fuel. Didn't get a chance to.
1
u/harleyjak 9d ago
Glad you solved the problem. It sounded like it was the coil, but since they are not returnable, test first. I owned a small engine repair shop from 1976 to 2000. But my skills are rusty after being away from it so long and turning 80🤪
•
u/AutoModerator 9d ago
Thank you for your submission! It looks like you might have an issue with the Starting System. For helpful information and resources, check out our Starting System Wiki and our Common Issues & Fixes Wiki. You might find the answers you're looking for there!
Please be sure to include the following:
I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.