Author Topic: Not even a Fart  (Read 1655 times)

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Offline alexbrown64

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Not even a Fart
« on: May 12, 2019, 10:31:22 PM »
Hi guys,

Finally got my little 1979 YZ80F back together.  New piston and rings. Good compression. Big fat blue spark when kicking over. I am using the carb and reed block from my 1978 YZ80.  This carb and reed block starts the 78 right up. I have lots of fuel and am using a new petcock.  Plenty of fuel running into carb.  So i have fuel, air and fat spark.  But i kick and kick and don't even get a Fart. Dead as a door nail, not a puff of smoke, not a hint of ignition. 

Now i know i could have a leaking seal, timing slightly out etc.. but generally you get a whiff of a fart, smoke, life etc..

Ive pulled the plug a few times and still a nice fat blue spark. I have new head and base gaskets and everything is new and tight.  Bike has never run since i have had it.  Previous owner reckons it ran, but the old girl is a piece of shit and i doubt it ran in years.

I want to get her running to check gears, clutch etc.. before i tear her down for a complete rebuild.

I have used start ya bastard and she is still as dead as a dodo.  Any ideas on why there is no life what so ever?
Remember, she has compression, fat spark, fuel and air...

Cheers,


Offline matcho mick

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Re: Not even a Fart
« Reply #1 on: May 12, 2019, 10:59:32 PM »
rain delayed start WSBK,i'm in  ;)  pipes blocked? bloody wasps??,or timings way out??
work,the curse of the racing class!!
if a hammer dosn't fix it,you have a electrical problem!!

Offline pokey

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Re: Not even a Fart
« Reply #2 on: May 12, 2019, 11:15:27 PM »

Offline OverTheHill

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Re: Not even a Fart
« Reply #3 on: May 12, 2019, 11:17:17 PM »
Well, not making sense. Umm--people always say fat blue spark but cdi ignitions are usually concentrated & not fat/blue as such, bit ok assume it's still  normal as a cdi system usualy sparks at kick speed or you get nothing at all if voltage is too low to fire it. [not had a points much earlier mag fitted? of a yz80d]. Anyway--is it sucking to your hand over the manifold with carb off. If so then is the plug getting wet with carb 'on'. Mind you start fluid should get some life. Has the right side crank seal blown out of the case as that'll stop any suck. Well, so would the left seal if totaly shot out of the case but right side is harder to get at for a look, except you'd likely have gearbox/cluch oil in the crankcase. Right, so my first guess is the crank key has buggered off that locates the flywheel in the right place for timing. I'd still think start fluid would et a explosion of sorts even if no chance of running. I've seen at work [in 1962 lol] a Bantam with the base gasket NOT cutout for the transfers--did try to run. I've seen a pipe [weed eater parked for a year] & a mason bee filled the exhaust outlet solid. Couldn't suck so couldn't blow. or was that a girl i knew. Anyhoo, piston back to front with windows to the exhaust, but you'd still get a fart i'd thing. Rebuilt with a short conrod but has compression so forget that. Got me stuffed so check the flywheel key & if it sucks against ya hand on the inlet. Had a "79H new for the kids. Was a flyer too. Worked in the local Yamy shop. Great times, & that reminds me i had a 400F new that year. Bring back the "70's. Hang about--had a 465G in "80, so bring back the "80's. Hope you get it running without loosing too much hair.

Offline OverTheHill

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Re: Not even a Fart
« Reply #4 on: May 12, 2019, 11:21:04 PM »
sorry, typing so slow, matcho mick said it all in a few words lol.

Offline TT5 Matt

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Re: Not even a Fart
« Reply #5 on: May 12, 2019, 11:24:56 PM »
check to see your flywheel keys not sheared.most motors even if way down on comp go with start you bastard but only if the timing is near where it needs to be and check the pipe for wasp nest as had that beforeon a suzi 80cc scotter and they don't even think about firing as no air out equals no air in

Offline sleepy

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Re: Not even a Fart
« Reply #6 on: May 13, 2019, 09:17:48 AM »
Using a timing light on the plug lead while kicking over work out where the spark is happening. Had a YZ125G here with a replacement CDI box that was meant to be a direct replacement but it put the spark 180 deg out. Had a great looking spark though.

Offline ELC hunter

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Re: Not even a Fart
« Reply #7 on: May 13, 2019, 12:18:33 PM »
Is the piston facing the right way?

Offline alexbrown64

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Re: Not even a Fart
« Reply #8 on: May 13, 2019, 02:51:28 PM »
Thanks for all the help guys.  I took all the advice on board and re-checked everything and then also did some more digging.
I love a good murder mystery.........

I took the cylinder back off.  Ring end gap perfect, piston on correct way. Cylinder perfect.  I cleaned everything back up and torqued to spec.
Last night while crying in my wine and reading the wisdom on these pages, i remembered something.  I wasn't getting a smell of petrol and that sucking sound.

So, i also pulled out the old leak down tester and made some adaptors to check her out for leaks.



Pic above shows old crank seal on the shaft in the position i found it when i took the stator/rotor off.

I couldn't even get any pressure into her.  Air was just gushing out from behind the stator/rotor.  I took them off and low and behold.... the left crank seal wasn't even on.  It was just sitting loosely on the shaft and not even in the recess.  I slid it off the shaft and inspected it.  It looked OK, but i had a new genuine Yamaha one in my box.. so fitted it properly with lube etc and a length of 40mm pvc pipe.  Did the leak down test again and held 5 psi.

Whacked everything back together, sprayed a couple of squirts of "Start ya Bastard" in the carb and second kick she burst into life.
And i don't mean farts, smoke, bogging and rough running.  This old girl hasn't run in years, but she burst into life as crisp and clean as a brand new bike.  Revved to the moon, idled perfectly, sounded crisp and powerful.

I would of clicked her into gear and spun her up the street but that has to be another day.  I have no clutch cover or cable fitted, no brakes and no gear lever... That will be part two of getting her checked over before a full rebuild.

Most folk say you need 3 things to at least get a fart out of a 2 stroke.  Fuel, air and spark.  I am adding another one.  Vacuum.  If you have a big air leak, you won't get any vacuum to pull fuel through to the cylinder.

Cheers everyone,

Alex

Offline TT5 Matt

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Re: Not even a Fart
« Reply #9 on: May 13, 2019, 06:45:40 PM »
good you found the problem,as to reed valve yam engines running with the piston back the front yes they do but judging on the dt400 I brought that a farmer did his own rebuild on it suffered a lot of crankcase backfires due to wanting to suck in still burning exhaust gas when the piston was near t.d.c uncovering the windows in the piston skirt and would have pumped a lot of fresh charge out of the same piston windows on the way down