# Briggs 15.5 Compression Release



## JPBJustin (9 mo ago)

I've got an older Craftsman with the Briggs 15.5 HP engine. It's giving me the text book compression release issue, where you crank it and the thing hesitates. I took the whole thing apart and wouldn't you know it, the cam looked just fine and the little compression release pawl thingy appeared to work fine with no wear in sight. Put it all back together and it worked great for the day. Started and stopped several times. Now a week later it's right back to how it's always been. Starter cranks great with the plug out and I'm bypassing the battery with a jump right to the starter. Valves seem to do what they should, there is a slight hesitation and bump when the intake is closing. What am I missing!!??


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## SidecarFlip (Mar 25, 2021)

Intake valve setting too loose.


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## JPBJustin (9 mo ago)

SidecarFlip said:


> Intake valve setting too loose.


That's where I went first, and here I am. I'm at .004 on both. I think I'm going to go tighter and see what happens. Watching it with the valve cover off it seems to hesitate just as the intake valve is getting bumped in. Almost like it's not getting bumped enough. I can't see why that would be after looking at everything.


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## SidecarFlip (Mar 25, 2021)

it's very important that the starting battery is 'up to snuff' and can deliver sufficient start amps to spin the motor over. Setting the valves too tight can lead to burned valves or dropped valve seats if the motor has shrunk in hardened seats.


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## JPBJustin (9 mo ago)

SidecarFlip said:


> it's very important that the starting battery is 'up to snuff' and can deliver sufficient start amps to spin the motor over. Setting the valves too tight can lead to burned valves or dropped valve seats if the motor has shrunk in hardened seats.


I do not own a load tester yet. I've tried a small jump pack and hooking a second battery right to the starter while turning the key. The extra boost doesn't seem to help significantly. I'd give up on this stupid thing, but it ran so well last time I messed with it. Also during that time I was able to start and stop several times, and the battery seemed to handle it without any issues then.


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## JPBJustin (9 mo ago)

Update: Tried to go the other way with the valve clearance. Setting to .0025 made no difference. Setting to .008 made no difference. It would start with a truck battery, but still hesitated at that sweet spot. Put valves back at .004. Anyone have a good video of the valves working on a similar motor to compare? Best I can tell the thing looks like it's bumping the valve open as it should, but the way it's behaving still points to a typical compression release problem. Should I just start custom fabing up a spot for a truck battery under the seat?


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## SidecarFlip (Mar 25, 2021)

Sounds to me like it's new battery and clean all the connections time. I suspect you battery isn't providing sufficient starting amps to get the motor past that 'sweet spot'. Could be corroded connections or insufficient starting amps. You might want to replace the battery with a larger one (higher cold cranking amps). Batteries and connections are two of the most overlooked components on any engine, be it a tractor or lawn mower. People tend to run batteries well past their ability to provide sufficient starting amps. As a rule, I replace all the starting batteries in all my electric start equipment ay least every 3 years. I consider starting batteries to be a consumable item.

Far as valve clearance (intake not exhaust), the tighter the initial clearance is, the sooner the intake valve opens and relieves any compression pressure, allowing the engine to rotate past that 'sweet spot you refer to.


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## SidecarFlip (Mar 25, 2021)

JPBJustin said:


> I do not own a load tester yet


Harbor Freight has 2 carbon pile load testers and neither are expensive. Good investment because they will show how good or bad a starting battery is.


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## JPBJustin (9 mo ago)

I've been thinking about getting a load tester, and now I have yet another reason to buy one. The new battery seemed to do the trick. I thought the jump pack or adding the second small battery would have eliminated that, but who knows. Possibly the jump pack was not giving much output. I have no idea what the cranking amps were of the second battery I was using. I just had a good strong spark when putting the jumper to the starter, and assumed it was enough. I also learned that there are about 4 different cranking amp options with this same size battery ranging from 160 to 350. I bought the largest because it was what it had in it. Don't know if it's the correct one for the motor, or if having a "bigger" battery will be an issue in the long run. Also, all the oil that spilled on the exhaust from having the valve cover off made for one great smoke show!


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## SidecarFlip (Mar 25, 2021)

Did you clean all the connections from the battery terminals back or not? One oxidized connection will cause poor cranking. The tighter you set the intake clearance (not the exhaust), the sooner the valve will begin to open and relieve any compression. Corroded connections are the number one culprit of a no crank situation.


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## JPBJustin (9 mo ago)

SidecarFlip said:


> Did you clean all the connections from the battery terminals back or not? One oxidized connection will cause poor cranking. The tighter you set the intake clearance (not the exhaust), the sooner the valve will begin to open and relieve any compression. Corroded connections are the number one culprit of a no crank situation.


I had cleaned the starter connection when I took out the motor. At some point I must have done the battery, as they were both not only nice and clean, but greased as well. I'll admit that I didn't touch the connections on the solenoid yet. I have a much better understanding on the compression release now, especially after taking the thing all apart and putting eyes on the inner workings. In my case adjusting the valves in both directions had zero effect, so it was pushing me away from the compression release as the culprit. Without owning a load tester I had convinced myself that using the jump pack and adding the second battery was eliminating the battery as the culprit. At the same time adjusting the valves and taking the engine apart to inspect the cam was eliminating the compression release. I almost bought a starter as well, thinking that maybe it was worn and not able to turn as hard as it once did. I have to assume now that my little jump pack is not working correctly, as it should be able to start a lawn mower just fine. If I had been able to start this thing reliably with a jump pack I would have bought a battery months ago.


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## SidecarFlip (Mar 25, 2021)

EVERY connection needs to be clean and shiny and don't use regular grease on ANY connection, only Di-Electric grease or spray on corrosion inhibitor. The connection and the lugs that connect to them. Only takes ONE high resistance connection to spoil the party.


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## JPBJustin (9 mo ago)

SidecarFlip said:


> EVERY connection needs to be clean and shiny and don't use regular grease on ANY connection, only Di-Electric grease or spray on corrosion inhibitor. The connection and the lugs that connect to them. Only takes ONE high resistance connection to spoil the party.


I'm sure that it will eventually bug me enough that those two connections are still untouched. Right now the thing starts and runs great, and I'm forced to direct my attention to the rod I broke while trying to adjust the deck height. As far as di-electric grease, I usually use the grease on the connection, wipe off any excess and top it off with the spray. Maybe it's over kill, but I know I like seeing the terminal spray on batteries when I'm looking at equipment.


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