to me its a cold seizure ...u should restrict the thermostat passage ...it will take alots of time to bring that coolant to the right spot .... check your side rad if they are not hot motor is not hot
yamalars
Member
I belive the cold sizure theori! T-stat or not. He is also lean but we dont know what jets his got in there yet!bufalobob said:
And if he has airbox mods?
So i think it's a little early to say if dons jetting spec is wrong.
Thoose 152,5 may sound small but remember that the spec includes raising the needles to.
And that's maybe the most importent thing to do,because most burndowns happens at part throttle.
Well i will leave my t-stat in, and if you guy's are getting 73 octane!! at the pump you better sue them or quit riding.
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yamalars said:
if its lean it will burn on the exhaust side ...............not only on intake side
yamalars
Member
I know that. I also think cold sizure did this.modsrx said:
But he is lean to and have to check his setup.
And if the engine runs cooler w/o t-stat it will of course need more fuel.
OZ_872
New member
bufalobob said:
Well said. Situation is probably a combination of lean jetting (piston wash) and inadequate warm up time (He said himself its hard to let it warm up when your buddies are taking off on you)
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mopar1rules
Active member
bufalobob said:
exactly....my '01 srx w/porting, cut box, and still stock bore, runs 155's and 153.8's. i wouldn't even start out w/mains any smaller than 157.5 or 160 on a 780 build, along w/like a 3.5 needle position and 45 pilots. also, putting needle clip on pos#4 and placing both shims on top, is the same as the stock 3.0 position, of clip in the middle and shims underneath. i don't even know why a jetting suggestion was already made to viperking, as we don't even know what all mods are done to his sled. once viperking lets us know what current jetting he has and engine mods, airbox mods, etc, we can then give a good baseline start point for his motor, after the rebuild.
mopar1rules said:
if u doesnt see the porting change u can't tell the jetting .......i wouldn't give a anwser about it ........
bufalobob
Member
below is an excerpt from an article written by jim on dynotech research's web blog:
"People who run high HP on the trail need to respect their engines, and expect to be running 87 octane on occasion. So that means tuning for 87 if you must run 100% pump gas. That means running enough fuel, with relaxed timing if necessary to get you around .70 lb/hphr for hot rod two-strokes. And, it means you must have a cooling system that will keep up with your HP/ riding style. How can a cooling system designed for 130 HP be expected to keep up with a 170 HP big bore if you ride like a madman? Coolant temp should be monitored, and when engine coolant gets to be much over 120 degrees F (like you might experience on hard packed roads or lakes) you must take it easy or risk deto since hot engines are most likely to detonate".
since a srx thermostat opens around 120 degrees you're already entering a danger zone. now add to the equation lean jetting which in itself creates more heat. there are ways to correct this situation. remove the thermostat and/or add more coolant capacity, retard the timing, jet up, reduce compression. why are his pistons scuffed so badly on the intake side? could be a number of reasons: belt dust or other junk sucked into the motor, out of round cylinder ( doesn't take much for this to happen on a 780 as they are bored wafer thin ), out of round piston or not enough piston skirt to cylinder wall clearance. if it was my motor i would start by having all cylinders checked for roundness & make sure the skirt to wall clearance is correct, remove the thermostat ( as valin said insert plug, w/ hole drilled, in top hose from tstat housing to coolant resivoir ) & jet up 2 sizes from what is in the carbs now. if you just take out the tstat & do not plug the hose you are sending hot coolant back into the motor. bob
"People who run high HP on the trail need to respect their engines, and expect to be running 87 octane on occasion. So that means tuning for 87 if you must run 100% pump gas. That means running enough fuel, with relaxed timing if necessary to get you around .70 lb/hphr for hot rod two-strokes. And, it means you must have a cooling system that will keep up with your HP/ riding style. How can a cooling system designed for 130 HP be expected to keep up with a 170 HP big bore if you ride like a madman? Coolant temp should be monitored, and when engine coolant gets to be much over 120 degrees F (like you might experience on hard packed roads or lakes) you must take it easy or risk deto since hot engines are most likely to detonate".
since a srx thermostat opens around 120 degrees you're already entering a danger zone. now add to the equation lean jetting which in itself creates more heat. there are ways to correct this situation. remove the thermostat and/or add more coolant capacity, retard the timing, jet up, reduce compression. why are his pistons scuffed so badly on the intake side? could be a number of reasons: belt dust or other junk sucked into the motor, out of round cylinder ( doesn't take much for this to happen on a 780 as they are bored wafer thin ), out of round piston or not enough piston skirt to cylinder wall clearance. if it was my motor i would start by having all cylinders checked for roundness & make sure the skirt to wall clearance is correct, remove the thermostat ( as valin said insert plug, w/ hole drilled, in top hose from tstat housing to coolant resivoir ) & jet up 2 sizes from what is in the carbs now. if you just take out the tstat & do not plug the hose you are sending hot coolant back into the motor. bob
valin
Active member
I do have to say that 152's do seem quite light for a 780. It takes fuel to make power, and 152's are not alot of fuel. Regardless of this, even though it appears lean, the motor didn't go down due to it being lean......it went down from inadequate warmup procedures.
no1chevyboy
New member
all i can say is with ideal conditions the restriction seems to work, but if it was a better idea, then i believe the factory would do it to there product, which they dont, i have seen first hand how a motor with a stuck open therm.runs compared to one with a working therm and its night /day
yamalars
Member
Messured the needle positions this morning and their is a diff.mopar1rules said:
Pos.4 with both on top is 0,3 mm. "richer" than stock pos.3.
Pos.3,5 is 0,2 mm. richer than pos.4 with both on top.
Wanted to know becouse i'm going to put my 780 back in soon.
So i will try the pos.4 and 152,5 mains in my setup with stock box and se what happens.
But with box mods its a different story,then you may need the 3,5 pos.
and bigger mains.
mopar1rules
Active member
yamalars said:
that's interesting....i'm going to measure that up myself when i go out in the shop today. nice finding yamalars. in my viper manual it looks like .5mm increments....weird.
WATCHMAKER
Member
yamalars is correct.........The manuals only show the .5 increments using both shims below or one above and one below the clip. Both shims above is an option that the manual does not show. Clip in 4th and both shims above is between 3 and 3.5.
no1chevyboy
New member
i always thought you didnt put the clip resting on the slid because of wear? ive seen someone do this and the clip wore its thickness down it to the slide? just two cents
mopar1rules
Active member
went out to the shop today and measured up the needles, w/clip in groove 3 and groove 4, w/shims underneath and w/them on top....i got a difference of .12mm between clip in groove 3 w/shims underneath and clip in groove 4 and washers on top. there is a difference i guess.
mopar1rules said:
Was that .12mm difference resulting in a richer needle position or leaner for 3 w/washers under neath compared to 4 w/washers on top
harness racer
New member
mrviper700 said:very simple:
COLD seizure
Removing the thermostat is the most foolish thing you can do in a trail sled, these 780's use a wiseco piston which expands much quicker then the cylinder does, by making the engine take even longer to warm up, youd need to let that engine sit and idle for every bit of a half hour to put heat into the entire system to expand the cylinders at the same rate the piston expands,this is not going to happen in the real world, nobody is going to wait 30 mins or more to ride a sled everytime you run it!
put the thermostat back in!!, check the jetting as well because the wash is on the lean side and if you rattle these pistons in the bore, next youll break off the rear cylinder skirts as well.
carb settings:
jetting: 42.5-45 pilots
needles on the 4th groove with both shims on top of the clip
152.5 mains in all 3
93 octane fuel, not 91, not 87, if you cant run premium fuel and all you can get is 91 then you should at all times have 2 bottles of octane booster in trunk and add 1/2 bottle at every full fill up.
seeing how this engine was built for someone else and run for a couple years,then you bought it second hand,you wouldnt have had the warnings I tell all people about 780 engines when they want one built.(no fault of yours)
They run reliable and strong, but you have to let them warm up thoroughly and you need to run them rich enough to keep from detonating the pistons. There is no way around this using a wiseco piston. Theres lots and lots of my 780's out there and they run reliably and strong, but these warm up and fuel rules have to be followed or you will have problems,and its no differnt for any other maker of a 780 kit using a wiseco piston! 3 of the people here in this very thread have my 780 kits and they have all had the talk before the engines were built to make sure they understood the basic rules to having a 780.
For the future anyone buying a sled with a engine built by someone else should always contact the builder for basic carb specs and general information about what they have purchased, it saves alot of headaches in the future.
just wondering if this statement is for 780's or wiseco piston engines only but why did yamaha engineeres not put any thermostat in the most reliable 2 stroke ever built " the red head " ?
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mopar1rules
Active member
sideshowBob said:
richer
lots of misconceptions....you guys are comparing apples to bannanas, banshees, doo's, redheads and all other low output yamaha sleds for that matter, dont use a thermostat because it has a differnt designed cooling system, its routed differnt, hose size differnt and the heat exchangers are all completely differnt by design. This stuff is engineered for the paticular engine by design,volume and routing,this is what controls "thermal conductivity". This is also the reason yamaha uses a thermostat in the high output srx700 sleds, they use it for consistant performance,repeatability the trail, the same exact reason those little aluminum pipe shields are also used to keep the pipe at optimum operating temp so the engines rpm range is available at all times, cool off the pipe and your low on peak rpm. Thermostats provide this optimum temp range for consistant performance,again.... this is a trailsled not a race sled, looking to extract 2 more hp from a cooler watertemp and the associated risk involved with doing so. I would be willing to bet a dollar to your doughnut youd be lucky to have 1 out of 10 srx owners that have a watertemp guage installed on thier sled,because its not necassary, yamaha engineers already did the work for them, so they can go out and have fun not worry if the watertemps are right,the thermostat does this for them automaticlly! My last comment on cooling system: I assure you the yamaha cooling system design on the srx is quite capable of handling more then 141hp,(lol..) as designed its quite capable of cooling up to 170hp without any mods what so ever. Above that hp level, yes, I agree, it needs help and can be helped by adding "more coolant volume" to the system, not increasing water speed and losing thermal conductivity because the water moves too fast and doesnt have time enough to transfer heat efficicently, but added volume makes for more liquid to absorb and move the heat thru the system at the correct gpm effectively bleeding it off. ( prime example of this is adding a srx exchanger on a viper),So your trail ports and basic 780 trail engines run just fine with the added hp and no cooling system mods required,again... this thread keeps straying from the purpose here,ITS A TRAIL SLED!!
jetting comments:
I see others can state "they know" the jetting that can be used for a engine they didnt build, without knowing any of the specs,nor did they purchase, or even tune,thats pretty tough to do. Also adding to this they compare thier own sleds with gutted airboxs,removed thermostats, and other mods that result in needing a richer mixture. My specs are for a "trail sled" with center shelf still in airbox..added holes in top "ok", thermostat remains , fuel used is 93 octane premium. Some of you may need to do a little more tuning time with your needles as it where 85% of all burndowns occur,simply using oversized richer mains is a band aid fix for lack of fine tuning thru out the entire fuel range,which results in more power.
The very guy who started this thread didnt burn the sled down,he "cold seized" it from inadequate warmup time. I said it looked a bit lean but it has no signs of burndown,it could and likely is the result of a colder watertemp(leans down a engine) if mods have been done with removing the stat and no bypass is used. There is likely a 100 of these 780's here on this site alone using these very specs, and yes, by golly the needle s are richer with my specs....imagine that.. lol!!
I am glad some you guys learn something during your parade of attacks , maybe some actual time spent tuning and understanding how things work and why would yield better results for you on your own sled. It basicly boils down to make mods on your machine as you see fit to do I would encourage you to read and read prior post on these subjects to see what other peoples results were with those mods. You can either put your trust into the yamaha engineers and the aftermarket product or you can try and redesign your sled by doing mods taken on the word of someone you only know who post on the worldwide web!
In the end, if those mods as some have found out dont work,its always your dollar to repair the sled.
viperking, wiseco is currently making a new batch of pistons, will be ready on/after feb 12th. I know I didnt build the engine for you but I never close the door on any of my products regardless of how many times it changes hands. pm me for details,thanx
jetting comments:
I see others can state "they know" the jetting that can be used for a engine they didnt build, without knowing any of the specs,nor did they purchase, or even tune,thats pretty tough to do. Also adding to this they compare thier own sleds with gutted airboxs,removed thermostats, and other mods that result in needing a richer mixture. My specs are for a "trail sled" with center shelf still in airbox..added holes in top "ok", thermostat remains , fuel used is 93 octane premium. Some of you may need to do a little more tuning time with your needles as it where 85% of all burndowns occur,simply using oversized richer mains is a band aid fix for lack of fine tuning thru out the entire fuel range,which results in more power.
The very guy who started this thread didnt burn the sled down,he "cold seized" it from inadequate warmup time. I said it looked a bit lean but it has no signs of burndown,it could and likely is the result of a colder watertemp(leans down a engine) if mods have been done with removing the stat and no bypass is used. There is likely a 100 of these 780's here on this site alone using these very specs, and yes, by golly the needle s are richer with my specs....imagine that.. lol!!
I am glad some you guys learn something during your parade of attacks , maybe some actual time spent tuning and understanding how things work and why would yield better results for you on your own sled. It basicly boils down to make mods on your machine as you see fit to do I would encourage you to read and read prior post on these subjects to see what other peoples results were with those mods. You can either put your trust into the yamaha engineers and the aftermarket product or you can try and redesign your sled by doing mods taken on the word of someone you only know who post on the worldwide web!
In the end, if those mods as some have found out dont work,its always your dollar to repair the sled.
viperking, wiseco is currently making a new batch of pistons, will be ready on/after feb 12th. I know I didnt build the engine for you but I never close the door on any of my products regardless of how many times it changes hands. pm me for details,thanx
harness racer
New member
there may be alot of misconceptions here aswell as some confusion , although cooling systems maybe different by design and routed differently to control thermal conductivity , @ what point does hp become an issue "iddle , start up" if the engine is going to be cold seized then it wont be @ 141hp .
Therefore the controled design of coolant flow is really what's @ question !!! and if installing an inline plug to re-route the coolant is adaquatetly as efficient as a red head system then it may not be the "silliest " thing ever ......
Therefore the controled design of coolant flow is really what's @ question !!! and if installing an inline plug to re-route the coolant is adaquatetly as efficient as a red head system then it may not be the "silliest " thing ever ......