I have two 1997 Yamaha Timberwolf 4x4’s. They both have seen their better days. One is temporarily parked, the other I use for hunting. Recently, I decided to dig in and make one run more reliably. Here is that story.
Jill, that’s what my girls named this one, was running. The main problem is she would not idle reliably. She ran best in the winter, but the only way she ran was with the choke on – no matter how you adjusted the carb etc. The main concern I had was wearing out the starter and/or battery, because I constantly had to restart her.
So I decided for a carb rebuild. It was surprisingly easy, and made a difference. She did run better – not perfect, but better. I replaced all parts with factory equivalents, the main jet being an 85 and the clip on the needle in the 4th groove from the top. The jet for the choke had a stripped out head, so I had to machine it out and re-tap the threads - this was the most challenging problem I ran into on the carb rebuild. So I was careful not to over-tighten the brass jets during the install. After re-tuning, I noticed the idle mix screw had to be 3-1/2 turns out, which is way off from the factory spec of 1-1/2 turns. But again, no matter how I adjusted the carb, it still would not idle reliably (would occasionally stall, and the RPM’s at idle would vary.) Sometimes the engine would backfire through the carb. If I goosed the throttle, the engine would bog down, but if I gradually increased the throttle it was ok. It ran better, but still wasn’t good enough.
During the carb rebuild, I discovered the fuel shutoff valve on the bottom of the fuel tank did not work. It passed fuel even when in the off position. I don't think this was good for an older engine, and when I drained the oil I knew exactly why - the oil was thinned out with fuel. Ouch, not a good situation.
I had a feeling the coil might be going bad, because the spark looked a little weak to my eyes, and didn’t seem consistent. The coil measured 16K Ohm (should have been ~5K Ohm) on one side, and 2 Ohms on the other side (should have been ~0.2 Ohms-) so I ordered a new coil just to be sure.
To make matters worse, I accidentally poured contaminated fuel into Jill’s tank (fuel was 7 years old, had varnish and water in it.) Talk about an instant mess – an engine that was running marginally better, just tanked! I cleaned the main fuel tank twice, and the carb about 5 times. Lesson learned. But I still did not have a smooth idle.
Now decided to check the compression, and found a problem. After the first engine rotation, 30psi, then 60psi, 90psi, 100psi, ending up around 110psi - barely. Compression should be more like 125psi on this motor. Guess it’s time for an upper end rebuild.
This is where the problems started to compound. This is a 15 year old, well used, air cooled engine – so I ran into many screws which were broken (stripped heads, rusted bolts that broke in half upon removal), and required major surgery to fix – especially the two screws holding the exhaust pipe to the head. I had access to a machine shop, so this was not a big deal – just time and patience.
After cleaning up most of the parts, I discovered the cylinder was egg shaped and way out of tolerance, which explained the low compression. The bad rings and bore also allowed fuel to pass down into the oil, which further sped up wear inside the motor. Anyways, I decided to bore it out, and ended up +1mm (0.039”) over-bore. I did this on a manual knee mill, with a boring bar that had an indexable TiN coated carbide insert. It took a while, but that’s ok. After coarse and fine honing, I was exactly at 72mm. I then ordered a Wiseco 72mm top end rebuild kit, which had everything I needed (except the o-ring on the exhaust rocker shaft – local Yamaha dealer had it.)
Now came the valve job – which was another adventure. During the cleaning process, I noticed the Seafoam (to clean carbon out) would stay inside the exhaust cavity, but would quickly leak out of the intake cavity, so this was another source of the problems I had with this engine (intake valve was leaking.) Moving on, I had to make a special jig to pull the rocker arm shafts out - It was just a long M6 screw with several different spacers. It took a while, but I got them out. I had to purchase a special valve spring compressor (looks like a C-Clamp,) as well as the valve lapping compound and suction cup tool to grab the valves for the hand lapping process. The valves, guides and springs were in surprisingly good shape, so I decided to re-use everything. There was some wear on the cam lobes – in hindsight, I should have had the cam repaired (welded / ground) - maybe next year. The wear was undoubtedly from the thinned out engine oil. So after lapping the valves, installing new valve stem seals and reinstalling everything in the head (with lots of engine oil during assembly process,) the valves no longer leaked (I flipped the head upside down, and filled the combustion chamber with oil, and after 20 minutes or so, the oil level did not change – this was not the case before.) The other important step was to orient the intake rocker shaft properly, or you won’t be able to install the head bolts.
Installing the cylinder was fairly straight forward. Due to the limited space between the cylinder and crankcase, I had to use a hose clamp as a ring compressor to get the piston back into the cylinder – crude, but it worked. The biggest mistake I made here was accidentally dropping the dowel pin o-ring down into the crankcase – major bummer. I drained the engine oil, used compressed air, and a long hook – but I just could not find that dang o-ring. Oh well, how much damage can an o-ring due down inside this engine? Not sure, will have to see – because I am not cracking this engine open now! Lesson learned; place small o-ring on at the very last moment before the cylinder touches down on the crankcase.
After final assembly and re-tuning, the engine ran much smoother. Since I have a larger piston in this motor now, I decided to rejet the carb. I replaced the 85 main jet with a 110. Now the engine idles very smoothly, but I had noticeable hesitation in the mid range (low end and high were nice, just no power in the mid range -) so I moved the clip on the needle from the 4th to the 3rd slot from the top. This pushes the needle farther down, which leans out the mix a little. When I restarted the engine, the idle was way high – and this puzzled me, but as it turns out, I had to reset the throttle linkage since the clip location changed – duh! Anyways, readjusted the throttle cable and stop screw. I played around with the idle mixture screw, and it seems to run good right at 1-1/2 turns. She’s running really nicely now, like an engine should. Upon cold starts, I turn the choke on and she starts right up. When the engine starts to heat up, the RPM's start to increase, and this is when I know to turn the choke off. After the engine is heated up, the idle is smooth and reliable. When I restart the hot engine, I don't need the choke. The throttle response is as it should be, when I goose it, bam - instant RPM's and power - no more bogging down. Now I just need to finish breaking her in.
We took Jill out today to the lake for a test, and she did fine. No major issues. Can't wait till deer season!
Now all I have to do if fix a stubborn oil leak near the shifter - I have a feeling that's going to be another major job for another day.
