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Bruce P

Hotter plug???

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Bruce P
When I rebuilt the engine on my 3410 a few years ago I had to have the cylinder bored ten over. Should I be running a hotter spark plug now??? It's been hard starting since the rebuild but mostly in the winter or cold months, not bad in the summer. It's a 10 Horse B&S. Thanks in advance Bruce

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Kent
Bruce, "Conventional wisdom" would say that you'd use a colder plug having bored an engine, which raises the compression ratio -- and the fresh rings would have raised the compression from before the rebuild (regardless). My guess is that your harder starting, in winter especially, is because the engine turns over slower due to higher compression (and thicker oil in winter) and the magneto is producing a weaker spark.... Converting to a coil-based ignition system, like show in this DIY article would address that issue, since it will produce a hot spark regardless of how fast the engine turns: http://www.simpletractors.com/do_it/magneto_to_coil.htm

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BLT
Bruce the standard rule of thumb on spark plugs is for extreme idling time use a higher heat(hot) range plug as it slows down the fouling process and vise/versa for constant heavy pulling. Mowers wether riding or walk behind for the most part do not require a heat range change. Another avenue to look at is how well your battery is and have your starter checked. Contrary to what Kent said your compression ratio does not change because the stroke of the crank is still the same and the piston pin to firing dome dimension is the same, the compression will not increase. But the bore being a slightly larger hole will give you an increase in HP but to what I can't say, my gut guess being maybe 1/4 to 1/2 .

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HubbardRA
BLT, I beg to differ, but Kent is correct. The formula for compression ratio is: Cylinder displacement volume plus head volume, divided by head volume. When the bore is increased, the cylinder displacement volume is increased, since this volume is bore times stroke. .010 is not much, but it will make for a small increase in compression.

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BLT
Rod you and Kent are right. I was always told, in my industry, diesel engines, that the compression ratio didn't change when going to O/S pistons that the ratio didn't change. And this could be maybe by closing up on the firing dome slightly or maybe for the engine MFG it was a non issue. For us it is now a mute point as we have been using ovesize liners and std pistons.

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Kent
I beg to differ on the compression ratio change -- if you have increased the bore without changing and the reshaping the size of the combustion chamber in the head, you WILL increase the compression ratio. I've done the calculations and measured the change in PSI many times on VWs, where you could bolt in new oversize jugs and pistons... Increasing the bore on a VW from stock 85mm to 87mm would jump the compression ratio from stock 7.5 to 1 to around 8.0 to 1. Going to 88 mm (the largest bolt-in, though the best ones in this size required a bore job) would go to about 8.2 to 1. If you bored the case and step cut the head to correspond, without reshaping the combustion chamber and went up to 92mm pistons, you'd go over 8.5 to 1, I've seen 9.0 with a slight refacing of the head... and you would have to run premium gas and really start worrying about heat... Never measured, nor calculated the change on these one-lungers....

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BLT
I erred, but to help Bruce out it might be more than a compression ratio change. Another thing to verify is the spark itself. And the best device I found for that is the B&S spark tester. If the spark can arc with this device, the spark plug should fire real well when in the head. Also not mentioned if oil is changed to 5W-20 for cold weather use.

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Kent
Certainly could be Bob... I mentioned the thicker oil... IMO these starter-generators combined with magnetos are kinda "marginal" starting systems at best in cold weather. I also know mine are very sensitive to the choke and throttle setting, since they turn over so slow...

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BLT
Maynard you are correct. But for fit that kind of battery in some of those spaces would require some hacking and or modifications. And after what Dutch started last week I not so sure I want to take that road again (grin)

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thedaddycat
Unless the 3400s are a lot different than the 3300s, you can use a 51-4 from Wal-Mart. It's rated at 610 CA and 500 CCA, footprint is 5" X 9", about 8" tall. It will slide right in, but is a tight fit. This was about the bigest battery I could find to fit the space available on the 3310. I posted pics on this related post about batteries. http://www.simpletractors.com/club2/topic.asp?TOPIC_ID=11781

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