18 hp Opposed twin wont turn over

Hello everyone,
this engine is absolutely driving me insane. I have a briggs 18 hp opposed twin model 42a707-2238-e1. The engine will not turn over with the starter. I have tried another starter, did a valve job, all new gaskets (sump, valve cover, head gaskets) cleaned all the carbon out of cylinder heads and valves and it still will not turn over!! Before i did all this work, i pulled up my other tractor, put a belt on the mandrels of both engines which spun the twin over and it fired right up! it runs i just have no idea how to fix this. There is no decompression on the camshaft and the valve stems have been ground down please help!

Thanks,

To me it sounds like you aren't getting enough power to the starter. Have you tried connecting jumper cables to the starter and seeing if it will start? If not I would try that

theswampmachine wrote:

To me it sounds like you aren't getting enough power to the starter. Have you tried connecting jumper cables to the starter and seeing if it will start? If not I would try that

Do this. If it turns over fine my other means, by hand for example, it isnt seized. Your starter either isnt getting enough juice like swammachine said, or your starter is going bad.

Get a pair of jumper cables. Turn the key to the "RUN" position. Hook the black cable to the block of the engine and to the negative terminal on your tractor battery or a car battery. Then hook the red cable to the car battery and then touch it to the post on the starter. If it doesnt spin over good, your starter is bad. If it does, your battery may be dead in your tractor or you have bad connections.

Hello, thanks for the help.

The motor isnt mounted right now and i have been jumping directly to the starter. I have tried another starter too and its still doing the same thing and I don't think both starters could be bad. I tried 2 batteries but I will try a car battery tommorow. It still wont turn over all the way with one plug out and with both plugs out it turns over but not as fast as opposed twins I see on youtube even with their plugs in.

Thanks

maybe make sure their is no paint or corrosion were your wires hook up

you might want to check the solinoid and see if it will start by bypassing it

Saw your PM, and thought I would respond here.

Opposed twins are a notorious for being slow cranking over. If you have too much clearance between the valve and lifter, that will slow it down cranking.

Bigger wires, I would say 4 gauge. The factory small wires will not allow enough amperage to start the engine sometimes. Clean, tight connections, and good connectors on the wires. maybe take the starter apart, and clean the brushes and contacts, then grease the bushings.

Does it spin easy with no plugs in it? If so, you have too much compression, or something is weak in your starting system. Only way I could see too much compression is excessive valve clearance, or cam is mistimed.

A larger, fresh battery is required, old worn out ones won't cut it. Even if everything is perfect, they still can crank slow.

Thanks for the response stretch.

Right now the motor isnt mounted, i have a very heavy duty pair of jumper cables ive been jumping straight to the starter. This is the second starter ive tried, but i may grease it and clean it anyways. Ive tried to fairly new lawn tractor batteries, a 300 amp and a 230 amp, i will try a car battery to see if it makes a difference. There wasnt enough valve clearance, so i ground them down and lapped the valves too, I also cleaned out all the carbon build up. The engine wont even spin over with one plug out, it spins over with both plugs out, but not as fast as opposed twins i see on youtube spin even with their plugs in. It doesnt really crank at all, i wish cranking slow was the problem. It spins a tiny bit until the compression stroke, and then it stops. With one plug out (doesnt matter which) it spins free and then stops at the compression stroke of whichever cylinder has the plug in it. The engine has sat for a while if that helps, Im wondering if I should put cheap oil in it, belt start it with the mandrel to my other tractor, let it run for 10-20 minutes then shut it off and see if it turns freely then.

Thanks for your help, Im only a teen and ive already put countless hours and almost 100 bucks into this thing and I need all the advice I can to get it going.

Stretch has it right, another thing to consider, quite often clamping a booster cable on a threaded post does not give a good connection.

4 gauge wire is overkill, but not by much. I used 8 gauge DLO (diesel locomotive) cable for the 6 foot run on my Ford. 8 gauge can handle 70-80 amps depending on the temperature outside. and as the cable heats up the amperage drops as the resistance increases.

4 gauge wire is good for over 100 amps, I'd be stunned if these things pull more than 50 amps when starting.

My 750 4 cylinder only draws 30 amps when cranking. It will crank for a minute or so before the circuit breaker pops. It usually starts after about 4 seconds.

I don't have an opposed twin here to measure rotating torque on a good engine.

Some one with a good deflecting beam style torque wrench could tell you how much torque is required to spin a known good engine.

Is there any oil in the cylinders by any chance. even a small amount of oil in a cylinder would cause hydraulic lock as it nears TDC

rt_smallengines wrote:

I have tried another starter too and its still doing the same thing and I don't think both starters could be bad.

Actually it's very possible they could both be bad if they're used; Opposed twins are known to eat starter bushings (most commonly the bushing in the top plate). I've seen Many top bushings that are egg-shaped. The starter will bench-test just fine, but when on the engine that egg shape allows the shaft to move & bind up, causing slow or no cranking.

Thanks guys,
The torque wrench suggestion is a really good idea, if anyone in the forum has one and has a twin please let me know. I didnt add oil yet, I just wanted to see if it would crank normally before all that so it is not hydro locked. I will take the starter apart, clean it, inspect the bushings and grease them.

Thanks again,

I'm assuming it has oil in the crankcase?

When you had the heads off, did you notice any gouges in the cylinder walls?

As mentioned, twins are ornery motors - theres twice the mass to turn over , not to mention fuel is being pumped to the motor instead of gravity feed .

Small 200- 300CCA batterys just dont have the oomph to continuously turn the big motor over - actually my parts GT6000 that had a 20HP onan twin says right on the battery cable " 600CCA minimum battery required" .

dangeroustoys56 wrote:

As mentioned, twins are ornery motors - theres twice the mass to turn over , not to mention fuel is being pumped to the motor instead of gravity feed .

Small 200- 300CCA batterys just dont have the oomph to continuously turn the big motor over - actually my parts GT6000 that had a 20HP onan twin says right on the battery cable " 600CCA minimum battery required" .

Thats enough to start a small car.

RT you could try wiring 2 or 3 batteries up in parallel. You would still have 12 volts, but you could maximize the amps.

Yeah i used a 700 CCA battery from my truck till it finally died - tractors turned over super easy. Been using a 600 CCA battery for a while as well.

The onan GT needed the extra juice, for the electric deck clutch and electric deck lift.

dangeroustoys56 wrote:

Yeah i used a 700 CCA battery from my truck till it finally died - tractors turned over super easy. Been using a 600 CCA battery for a while as well.

The onan GT needed the extra juice, for the electric deck clutch and electric deck lift.

My Ford LGT100 has a battery box built for a group 24 ( automotive sized) battery.

Its running a '77 Kohler cast iron 10 horse. with just the electric clutch. Thats what was nin it when I got it. nSerious tracotrs need serious batteries.

Thanks for the suggestions, I was thinking of trying to batteries in parallel but I have to get another set of cables. I'll try the parallel and the car battery and see if it will stop being a b*tch lol.