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Post by buzzard82 on Jan 31, 2020 1:08:03 GMT
Rebuilt the carbs on my 775 as last time running seemed pto side was dead. Rebuilt carbs and same deal.fires up idles pretty good cold. Seems with heat you hear a skip. Put your hand over pto side carb and no change.do that to recoil side and kills it like it should. Im thinking coil/points maybe? With plugs out on the head cranking it over there is all kinds of spark. If it was seals it would be lean right not wet?
Weak points or coil?
P.s This sled has seen about 5 miles in 10 years.
Thanks
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olympic
Full Member
Fresh gas first pull got be a DOOO
Posts: 201
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Post by olympic on Jan 31, 2020 1:41:27 GMT
Do you think it is running on two cylinders when its cold.
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Post by buzzard82 on Jan 31, 2020 1:49:32 GMT
Do you think it is running on two cylinders when its cold. Yup it pops good and idles pretty smooth. After 5 mins can hear it idle different. Cover that pto carb and no difference. Pull plugs hot and the pto side is black. Head/cyl not cold on that side but seems not as hot. Intake gaskets looked all good too when it was apart. A bad coil geta worse with heat/load right ? Being a 70 the coils are mounted under the exhaust flanges.made me wonder if its bad. Could switch the coils around and problem should go to the other side if its the coil right? |
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olympic
Full Member
Fresh gas first pull got be a DOOO
Posts: 201
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Post by olympic on Jan 31, 2020 2:27:31 GMT
Yes switching coils will determine if its the coil but is the plug just black or is it wet. When do you know or not is the last time the crank seals been replace. I have an inline spark tester that I hook up to see if my spark is losing that would be a good test also.
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Post by buzzard82 on Jan 31, 2020 3:05:12 GMT
Yes switching coils will determine if its the coil but is the plug just black or is it wet. When do you know or not is the last time the crank seals been replace. I have an inline spark tester that I hook up to see if my spark is losing that would be a good test also. Black and wet. Engine was rebuilt back early 2000s. Not much run time. Bad seal can foul a plug? I figured it be lean with more air. But i guess covering the carb and not dying means it be sucking air somewhere or else no spark at all. Hard to believe my brothers 70 640 TNT has never been apart and runs like a top.only seals ever changed was intake. |
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olympic
Full Member
Fresh gas first pull got be a DOOO
Posts: 201
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Post by olympic on Jan 31, 2020 19:03:16 GMT
buzzard82 I'm not going to say the seal is bad or good so tackle one thing at a time here. So swap your coils to see if it follows to the other side if it does then you solve two birds at once the seal is good and the coil is bad. Like I said before if you hook up an inline spark tester that will tell you real quick if your losing spark from the ignition system that way you don't have to do any wrenching. Don't know where you're from but any auto parts store carries them or can get them.
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Post by 1970tnt on Feb 1, 2020 0:23:28 GMT
WHAT IS THE COMPRESSION ON THE PTO CYLINDER?
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Post by olyman on Feb 1, 2020 1:23:27 GMT
And if you ever suspect a seal spray it with carb cleaner when its running. If it changes the way it runs at all then there's a leak.
If the plug is wet it sounds like its probably spark related. If swapping the coils does nothing then look for loose wires. Could even be loose on the points.
I recently had a twin that was acting up quite similar to this. I was certain the points were clean, but went and redid them (file, clean, clean again) and no better. Tightened everything I could find, swapped coils etc, no better. The I had a really bright flashlight shining when it was running.. and I could see fine particles flying below the carb (it was dark out). Aha! Realized now that a seal between the carb and the isolation block was bad. Impossible to see without that light because nothing was visibly wet, until I looked really close. Teeny tiny spot, but that's all it took to piss it off.
Just ideas for ya
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Post by buzzard82 on Feb 1, 2020 15:35:26 GMT
Other day when first got it out lost all lights. Maybe something electrical.
Compression never checked. Enough to break an arm. New pistons hardly any run time.
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Post by olyman on Feb 4, 2020 22:24:55 GMT
Make sure both external coils have good grounding too. I agree with you that it really sounds electrical
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Post by buzzard82 on Feb 12, 2020 22:48:05 GMT
Make sure both external coils have good grounding too. I agree with you that it really sounds electrical Im back guys. Damn this 775 is starting to get to me lol. So had an awful time to get it going. Finally started sure sounded like 1cyl.before cold it seemed to run good. Not now. Had someone hold throttle as wont idle on its own. Soaked pto seal around base and intake gaskets with either. Zero change to how it ran. So i said must be fire. Put an inline tester. Shows spark pto side, also cranking it over with plugs on cyl head it fires although sometimes more orange on pto side. Now pto side head is cold plug too. Fuel running right out exhaust flange now on pto side. I swapped out the pto coil with a know working coil.Same thing. Had a new set of plugs,so put them in. No dice either. Im wondering if the points are sticky,or condensor issue? I will put a compression gauge on it for giggles. But it sure seems to have tons of compression like any other 775 ive ever cranked over. Sure seems like its a fire issue. Does anyone have the timing specs? Stay tuned |
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Post by olyman on Feb 12, 2020 23:59:07 GMT
Based on what you are saying, if the points are within spec AND clean then I blame the condensor. Point gap is 0.014 to 0.018 but i don't know the BTDC #. The 71 shop manual does the timing using the "edge gap method" which I have no experience with. I can take pictures of the book if you dont already have access to that. Starts on page 2-07-01 (202 / 375). This is the link to the same manual on "vintage snow" site www.vintagesnow.com/Ski-Doo_files/1970-1971_skidoo_SM.pdf |
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olympic
Full Member
Fresh gas first pull got be a DOOO
Posts: 201
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Post by olympic on Feb 13, 2020 0:25:29 GMT
At one time my 370 Olympic acted up on me would run beautiful cold as soon it warmed up would act up. Put an inline spark tester on and at cold nice orange light on the tester and when it warmed up wasn't as bright and when trying to give it throttle it would backfire and carry on. Hermit told me a good chance the condenser was bad so I changed and it seemed solve the problem but I have only ran it on the bench so maybe yes maybe no if I solved the problem. But look at the simply things first point gap and make sure points are good and clean double check all the wiring and connections.
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Post by buzzard82 on Feb 13, 2020 6:25:54 GMT
Thanks guys.Will dig into the points. Is there a way to test the condensor with a multi meter?
I noticed today also every second time cranking over a slight pop/backfire out that pto side threw the carb. This is all new it ran better than this before. Failing condenser/points.
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Post by olyman on Feb 14, 2020 13:17:56 GMT
I think there is yes. All a condensor is - is a capacitor.
Here's the simplest description I found:
Remove the soldered wire and get your meter set for resistance. Put the red wire on the button center, and black on the case of it for about 15-20 seconds. Don't interfere by touching both leads with your fingers (one is ok). This should charge the capacitor. Then swap the leads. This should discharge the cap, and the needle should jump to the right at contact, and then swing left as it discharges. This indicates that its working. No movement suggests that its shorted internally or open.
Still looking for the readings you'd get if you have a capacitor function on your meter...
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