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post #1 of 6 Old 08-29-2019, 11:29 AM Thread Starter
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Bolens 1203H engine trouble

I have the Bolens "Eliminator" 1203H with the Briggs 12.5 hp twin. I was mowing on a really hot day when I started losing power and a little popping. I turned off the PTO and it got a little better but it's got a problem. The guy I got it from said he replaced the head gasket and it ran fine but never had much power but it worked fine. Anyone know if it's possible to drop a bigger motor in it? It has the hydro "tiller transmission" that works fine. I can pick up an 18hp Briggs twin for 100 or less. Too big?

I have another Bolens - a 1450 with a Tecumseh, manual trans, and tube frame. That motor is tired, too. It's a horizontal shaft. Any suggestions? Thanks.
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post #2 of 6 Old 08-30-2019, 11:44 AM
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Re: Bolens 1203H engine trouble

Have you looked at the Predator V-twin from Harbor Freight? It would be higher than an opposed. As far as replacing a 12.5 with an 18, you should have no problems as far as power for the machine will only use what it needs, and that depends on how much the operator loads it. For example, you might slow down when in tall grass due to engine limitations, but with a more powerful engine, you could go faster without laboring the engine. It is still doing the same amount of 'work', cutting the stems, but at a faster pace. If you slow down to the previous rate of cutting, with the 12, you would be loading the equipment the same, no matter whether you had a 12, 18 or 24hp engine installed. Operator decides...
If the Briggs is a flathead, you may have a valve seat that got loose as I understand that can happen. I would try rotating the crankshaft manually, with the spark plugs removed. Using an available thumb, attempt to hold compression on both cylinders. If you get no compression on one or both, remove the cylinder heads or use a tiny camera to inspect for valve/seat problems. A sticky valve guide is also possible, which could make the valve less likely to seat.
In some cases, you can substitute a single in place of a twin. Some have used the small vertical Predator on riding mowers to replace NLA Tecumseh and others. The available hp seems to be enough. The smaller horizontals seem to work well also if you don't want to or can't fit a twin in place.
There is at least one youtube (backyard machinist?) who has installed a horizontal V-twin Predator on a zero turn, making a mount and modifying the PTO as needed. He was very pleased, FWIW.
tom

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post #3 of 6 Old 08-30-2019, 12:08 PM Thread Starter
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Re: Bolens 1203H engine trouble

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Originally Posted by tomw0 View Post
Have you looked at the Predator V-twin from Harbor Freight? It would be higher than an opposed. As far as replacing a 12.5 with an 18, you should have no problems as far as power for the machine will only use what it needs, and that depends on how much the operator loads it. For example, you might slow down when in tall grass due to engine limitations, but with a more powerful engine, you could go faster without laboring the engine. It is still doing the same amount of 'work', cutting the stems, but at a faster pace. If you slow down to the previous rate of cutting, with the 12, you would be loading the equipment the same, no matter whether you had a 12, 18 or 24hp engine installed. Operator decides...
If the Briggs is a flathead, you may have a valve seat that got loose as I understand that can happen. I would try rotating the crankshaft manually, with the spark plugs removed. Using an available thumb, attempt to hold compression on both cylinders. If you get no compression on one or both, remove the cylinder heads or use a tiny camera to inspect for valve/seat problems. A sticky valve guide is also possible, which could make the valve less likely to seat.
In some cases, you can substitute a single in place of a twin. Some have used the small vertical Predator on riding mowers to replace NLA Tecumseh and others. The available hp seems to be enough. The smaller horizontals seem to work well also if you don't want to or can't fit a twin in place.
There is at least one youtube (backyard machinist?) who has installed a horizontal V-twin Predator on a zero turn, making a mount and modifying the PTO as needed. He was very pleased, FWIW.
tom
Thanks for the info. I picked up one of the small predators to power a hydraulic winch. I was wondering if anyone tried to put one in place of the tecumseh. I have no experience with the opposed twins internally - just coils and starters, etc.
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post #4 of 6 Old 08-30-2019, 02:43 PM
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Re: Bolens 1203H engine trouble

I assume it is an opposed twin? I dont know that model, either way a 12.5 will swap for a 19.5 opposed twin... Same pretty much on the vanguard v-twins, a 12.5 will swap for a 16 etc.
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post #5 of 6 Old 08-30-2019, 11:15 PM Thread Starter
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Re: Bolens 1203H engine trouble

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Originally Posted by mrshaft696 View Post
I assume it is an opposed twin? I dont know that model, either way a 12.5 will swap for a 19.5 opposed twin... Same pretty much on the vanguard v-twins, a 12.5 will swap for a 16 etc.
yes, 12.5hp B&S opposed twin. Nice mower. The steering is a little funky and the forward/reverse takes a little getting used to but all in all a good machine - maybe a little under powered for my rural bush-hog type use.
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post #6 of 6 Old 08-30-2019, 11:23 PM
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Re: Bolens 1203H engine trouble

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Originally Posted by Rudy602 View Post
yes, 12.5hp B&S opposed twin. Nice mower. The steering is a little funky and the forward/reverse takes a little getting used to but all in all a good machine - maybe a little under powered for my rural bush-hog type use.

yeah the 12.5 opposed is pretty much the same block as the 16, 18, and 19 opposed. I like the 12.5 but it seems the one you have is running weak or something because they usually have pretty good power.
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