morecowbell
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Howdy Folks!
Recently adopted a set of neglected 1999 Yamaha Mountain Max 700s and am saving them from the graveyard. Both with about 5000mi on the clock, but they lived outside for a while.
I'm fresh off a complete teardown on the first. More or less everything consumable was replaced on it. When she's fully tuned up and trail broken I'll post before and afters.
Strictly from an engine perspective - during my first eval I noticed a fair bit of leakage out of the various gaskets, and the compression was a little low (~110PSI) so naturally i took the entire block apart, cleaned the crap out of it, and replaced main seals, gaskets, water pump seals, and the piston rings. Pistons themselves seemed just fine. That said it's my first triple rebuild.
After the rebuild, I primed the oil pump, fired her up, gave her the obligatory extra shot of oil in the fuel tank, gave it ~5 or 6 solid heat cycles and took it for the first ride. It ran pretty great, and I had some good fast straightways, although I didn't pin the crap out of it all day or anything. We probably rode ~12mi with multiple starts and stops.
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Issues / Details
This weekend, attempted second ride, ~18F (not that cold but also not warm), I broke the pull cord trying to get her started on the trailer (RIP, launched myself off the trailer into a ninja roll, but it was a cheap cord), took it home, new pull cord, small spritz of starter fluid, fired right up. I've adjusted the oil feed cable, choke cable, idle screw to spec. It starts, just not super easily. I discovered a few weird things tinkering more deeply with it today.
Here's the rub - it really doesn't like the choke switch. As in, it preferred to cold start with the switch in "warmed up" / disengaged position. AND - even cold - turning the choke on while the engine is running kills it. Giving 'er a little bit of throttle / rev while turning it over seems to be the most consistent clean start. Starting fluid will do it too but obviously not I interpret this as the pilot / idle having too much fuel.
Video is below. Idle is set a bit high in the video as I was in the middle of adjusting things but it ran fine around the spec'd ~1600rpm.
I've re-jetted the carbs to mostly match this chart here:
https://www.snowmobileforum.com/thr...running-poor.140433/#lg=thread-140433&slide=0
My altitude at home is ~3200ft but the riding will be largely between 4000-7500' and between 0F and 30F. Selected jets are 137.5 mains and 57.5 pilots. Fuel / pilot screw is turned out to spec at ~ 1 3/4 turns. Needle position 2. Obviously everything was deep cleaned in there.
Today I experimented with tightening the fuel / pilot screw, went as far as 1/2 turn out. This was by far the best result as the machine responded by starting with the choke on, albeit still running a little rough. Thinking I might need to re-jet for leaner pilots?
Brand new NGK BR9ES plugs. Confirmed they all have bright blue sparks. Machine seems to run good under a lot of throttle. At least, it's not rattly, doesn't sound like it's misfiring, and screams like a banshee. I can get a video of that later if I pull it off the trailer.
Does seem to sound like theres some kind of metallic rattling in the engine area that could maybe be a knocking piston (yay) or maybe just something rattling in the bay. Feels like it shouldn't be this aggressive.
I did another compression test tonight with the engine cold and my compression now appears actually lower with the new rings at about 105psi clutch side and center, and ~112psi on the magneto side. Will this pick up with more rides / as the rings break in or should I be pretty suspicious off the bat since it's already had a bunch of heat cycles? Cheap harbor freight gauge and I didn't hold the throttle wide open when I tested
Recently adopted a set of neglected 1999 Yamaha Mountain Max 700s and am saving them from the graveyard. Both with about 5000mi on the clock, but they lived outside for a while.
I'm fresh off a complete teardown on the first. More or less everything consumable was replaced on it. When she's fully tuned up and trail broken I'll post before and afters.
Strictly from an engine perspective - during my first eval I noticed a fair bit of leakage out of the various gaskets, and the compression was a little low (~110PSI) so naturally i took the entire block apart, cleaned the crap out of it, and replaced main seals, gaskets, water pump seals, and the piston rings. Pistons themselves seemed just fine. That said it's my first triple rebuild.
After the rebuild, I primed the oil pump, fired her up, gave her the obligatory extra shot of oil in the fuel tank, gave it ~5 or 6 solid heat cycles and took it for the first ride. It ran pretty great, and I had some good fast straightways, although I didn't pin the crap out of it all day or anything. We probably rode ~12mi with multiple starts and stops.
---
Issues / Details
This weekend, attempted second ride, ~18F (not that cold but also not warm), I broke the pull cord trying to get her started on the trailer (RIP, launched myself off the trailer into a ninja roll, but it was a cheap cord), took it home, new pull cord, small spritz of starter fluid, fired right up. I've adjusted the oil feed cable, choke cable, idle screw to spec. It starts, just not super easily. I discovered a few weird things tinkering more deeply with it today.
Here's the rub - it really doesn't like the choke switch. As in, it preferred to cold start with the switch in "warmed up" / disengaged position. AND - even cold - turning the choke on while the engine is running kills it. Giving 'er a little bit of throttle / rev while turning it over seems to be the most consistent clean start. Starting fluid will do it too but obviously not I interpret this as the pilot / idle having too much fuel.
Video is below. Idle is set a bit high in the video as I was in the middle of adjusting things but it ran fine around the spec'd ~1600rpm.
I've re-jetted the carbs to mostly match this chart here:
https://www.snowmobileforum.com/thr...running-poor.140433/#lg=thread-140433&slide=0
My altitude at home is ~3200ft but the riding will be largely between 4000-7500' and between 0F and 30F. Selected jets are 137.5 mains and 57.5 pilots. Fuel / pilot screw is turned out to spec at ~ 1 3/4 turns. Needle position 2. Obviously everything was deep cleaned in there.
Today I experimented with tightening the fuel / pilot screw, went as far as 1/2 turn out. This was by far the best result as the machine responded by starting with the choke on, albeit still running a little rough. Thinking I might need to re-jet for leaner pilots?
Brand new NGK BR9ES plugs. Confirmed they all have bright blue sparks. Machine seems to run good under a lot of throttle. At least, it's not rattly, doesn't sound like it's misfiring, and screams like a banshee. I can get a video of that later if I pull it off the trailer.
Does seem to sound like theres some kind of metallic rattling in the engine area that could maybe be a knocking piston (yay) or maybe just something rattling in the bay. Feels like it shouldn't be this aggressive.
I did another compression test tonight with the engine cold and my compression now appears actually lower with the new rings at about 105psi clutch side and center, and ~112psi on the magneto side. Will this pick up with more rides / as the rings break in or should I be pretty suspicious off the bat since it's already had a bunch of heat cycles? Cheap harbor freight gauge and I didn't hold the throttle wide open when I tested
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I would guess initially your motor was fine at 110 with stock head gasket at. Compression Gauges very also when you check it you put a few drop oil in cylinders and hold it WOT and pull. Also sounds to me like your inlet needles or floods are sticking flooding the motor.
Also plugs foul really easily if the choke is not working correctly.
morecowbell
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Make sure that choke is off completely when it’s flipped down.
morecowbell
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Yeah, it is, I adjusted it to the spec'd ~1/16" or so of free play in the lever, you can visually see the plungers seated all the way in when it's down.
Just gave her a true cold start after resting overnight with the fuel screws at 1/2 turn out. This time it was happier with the choke wide open but still needed a squirt of starter fluid to turn over. Letting it rest while I pull the carby rack.... gonna reset to the spec ~1 3/4 turns out, deep clean the carbs and check everything incl the inlet needles.
On another note - I pulled the primary clutch and the god awful rattling seemed to reduce dramatically so that is a good sign.
EDIT: Oh hey also @MrSled thx you da man, man
Just gave her a true cold start after resting overnight with the fuel screws at 1/2 turn out. This time it was happier with the choke wide open but still needed a squirt of starter fluid to turn over. Letting it rest while I pull the carby rack.... gonna reset to the spec ~1 3/4 turns out, deep clean the carbs and check everything incl the inlet needles.
On another note - I pulled the primary clutch and the god awful rattling seemed to reduce dramatically so that is a good sign.
EDIT: Oh hey also @MrSled thx you da man, man
Your welcome. Sound like you are on the right track.
JM.02c
Compression:
-the Red Head triples don't usually crank over 120psi on a good gage
-the compression tests need to be completed wit throttle wide open, all spark plugs removed, and five good pulls of the starter chord
-compression gages vary a lot in readings and its the comparison between the cylinders that tell the tale normally
Jetting:
-regardless of the jetting chart I think 57.5 is too big on the pilots for your current altitude
-I would install 52.5 or 55.0 pilots and 1 1/2 open on the fuel screws even for the altitude
-I always installed remote fuel screw adjusters on my Yamaha triples for this kind of application and quick adjustments but unfortunately they have not been available for a long time.
If I remember correctly one turn of the fuel screw was equivalent to 2.5 on the pilot jet size..
Compression:
-the Red Head triples don't usually crank over 120psi on a good gage
-the compression tests need to be completed wit throttle wide open, all spark plugs removed, and five good pulls of the starter chord
-compression gages vary a lot in readings and its the comparison between the cylinders that tell the tale normally
Jetting:
-regardless of the jetting chart I think 57.5 is too big on the pilots for your current altitude
-I would install 52.5 or 55.0 pilots and 1 1/2 open on the fuel screws even for the altitude
-I always installed remote fuel screw adjusters on my Yamaha triples for this kind of application and quick adjustments but unfortunately they have not been available for a long time.
If I remember correctly one turn of the fuel screw was equivalent to 2.5 on the pilot jet size..
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morecowbell
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Well - good and bad news - i yanked the carby off and deep cleaned it all. Inlet needles in good shape and not sticking. Carb's pristine now. Set fuel screws to 1 3/4 turns.
Got it back on the machine, ran another compression test as y'all described, got ~107PSI on all three cylinders. Consistent, at least.
It turned over second pull with choke full on. ~30F ambient. Then the symptoms reappeared after the sled warmed up - became harder to turn over, responded better to a bit of throttle when pulling on the starter cord. The rattling / knocking also returned, and the engine was shaking like a mofo. Suspecting piston slap / bad wrist pin bearing or potentially a bad crank bearing? The crank spun alright free hand with the plugs out, smooth enough.
Personally I think it sounds like the center piston
Regardless she's not very happy. haha
Got it back on the machine, ran another compression test as y'all described, got ~107PSI on all three cylinders. Consistent, at least.
It turned over second pull with choke full on. ~30F ambient. Then the symptoms reappeared after the sled warmed up - became harder to turn over, responded better to a bit of throttle when pulling on the starter cord. The rattling / knocking also returned, and the engine was shaking like a mofo. Suspecting piston slap / bad wrist pin bearing or potentially a bad crank bearing? The crank spun alright free hand with the plugs out, smooth enough.
Personally I think it sounds like the center piston
Regardless she's not very happy. haha
Pilots too rich
Did you inspect orings on needle and seat?
Sounds like its not firing on a cylinder or 2
Did you inspect orings on needle and seat?
Sounds like its not firing on a cylinder or 2
morecowbell
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@MURDER YAMAHA Yes - O rings on the seat are new. Inlet needles were clean and the rubber cones clean, firm, looked good.
I agree that it does not sound like all three are firing happily. What's my next move ... pull the head / cylinders and inspect the wrist bearings? Wiggle around the connecting rods to look for slop? Nothing felt "off" when reassembling.
Seems it needs to go back in the garage and likely not run it much more than that. Maybe pray it does not explode between the trailer and the garage
EDIT: Pilots #55 on the way. Only part of the problem though, I think.
I agree that it does not sound like all three are firing happily. What's my next move ... pull the head / cylinders and inspect the wrist bearings? Wiggle around the connecting rods to look for slop? Nothing felt "off" when reassembling.
Seems it needs to go back in the garage and likely not run it much more than that. Maybe pray it does not explode between the trailer and the garage
EDIT: Pilots #55 on the way. Only part of the problem though, I think.
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Agree, way too big on the pilots.
morecowbell
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FWIW the original pilot jetting was even richer at #60... 
One possible cause of the new engine issue is the wrist pin circlip not being completely seated and popping out resulting in a damaged piston and probably more....lets hope I am wrong.
The wrist pins circlips on the Yamaha 2 stroke pistons can be difficult to install at times.
That engine definitely has to come apart.
The wrist pins circlips on the Yamaha 2 stroke pistons can be difficult to install at times.
That engine definitely has to come apart.
Did you put the ring in upside down?
morecowbell
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don't believe I was that careless but we'll see. They snapped into place firmly.
Same here. They said "top" on them lol
I'll pull it apart again when i get some free time and see what we find. Realistically it was not that bad of a job. Hope it's not a crank bearing.
I stand corrected on the pilots. At your home elevation of 3200 feet they should be 50.
Yamaha specs say 5000-6700 should be 57.5
Yamaha specs say 5000-6700 should be 57.5
morecowbell
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I think 55 is probably a good middle ground. I'm not really running it at home other than ... well, now. 95% of the rest of its life will be starting it at ~4000' and pushing up to 6000-7500' for ski access.
May want to check clearance with stator and pickup coil. have seen where pickup coil wasnt tightened all the way, came loose, and hit magnets.
Could accont for the noise and running condition.
Could accont for the noise and running condition.
morecowbell
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I'll get it off the trailer today and pull the engine up at some point this week. Stay tuned.