Compression Range for a LC fresh top end

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LC Cnd
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Compression Range for a LC fresh top end

#1 Post by LC Cnd » Thu Apr 06, 2017 1:58 am

Did a compression test and both sides which measured 120psi equally. There is 350kms on the jugs & pistons. Realise I still have a 150kms to complete the break in. Is the compression a tad high?

justind97
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Re: Compression Range for a LC fresh top end

#2 Post by justind97 » Thu Apr 06, 2017 9:27 am

Is this done when hot or cold?

And no, that's not high.

LC Cnd
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Re: Compression Range for a LC fresh top end

#3 Post by LC Cnd » Thu Apr 06, 2017 11:32 am

Done cold.

Angus McL
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Re: Compression Range for a LC fresh top end

#4 Post by Angus McL » Thu Apr 06, 2017 1:02 pm

Number sounds good to me.

A.

tacky1
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Re: Compression Range for a LC fresh top end

#5 Post by tacky1 » Thu Apr 06, 2017 6:42 pm

Thats about right,
1985 RZv500
1984 RZ500 Hybrid
1986 RG500 Walter Wolf
1986 RG500 Skoal Bandit
1984 RZ350
1984 RZ350 Hybrid
1981 RD350LC
1981 RD350LC Hybrid
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JonW
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Re: Compression Range for a LC fresh top end

#6 Post by JonW » Thu Apr 06, 2017 7:26 pm

I opened the thread and thought '120' as it opened, and lo n behold thats what you have... Im no expert but thats what I would expect.
80 XT500 Supermoto!
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid

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evan_calgary
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Re: Compression Range for a LC fresh top end

#7 Post by evan_calgary » Thu Apr 06, 2017 8:37 pm

Same thoughts as JonW when I opened but with no real science to back it up.

Bare
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Re: Compression Range for a LC fresh top end

#8 Post by Bare » Thu Apr 06, 2017 8:57 pm

Sub 100psi = Rebuild time
110 to 130 is Normal healthy engine range
150 = Too high!

AlexG
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Re: Compression Range for a LC fresh top end

#9 Post by AlexG » Thu Apr 06, 2017 9:04 pm

For me, rebuild time is when the thing refuses to start due to low compression, or when I have to stop between Detroit and Chicago to put more oil in a 4 stroke. :smt003

LC Cnd
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Re: Compression Range for a LC fresh top end

#10 Post by LC Cnd » Thu Apr 06, 2017 9:35 pm

There's nothing in the shop manual I could find regarding the psi - just ratios. The reason I was asking was when I momentarily hit the power band, last riding season, there is no 'fear factor' or lift from the front wheel like a typical LC - not even a hint of it. Thought perhaps I had seized a piston or two when on one break in ride the motor died and pulled over. The temp gauge show normal temp but I realize the LC gauge not accurate. Minutes later she cool and I drove her home deflated. Pull off the exhausts and looked at the piston skirts & no scoring. Pressed on the rings of both pistons and all mobile. Removed the head to see if the pistion crowns had a hole, prior to the compression test, and all good. The only other thing I was considering was the plugs are quite fouled black when ck'n the heads. Because its break in, I decided on premixing in the tank around 36:1. Yes, a bit rich but I'm using 94 octane with no methanol. Rather have over luberation than not at this stage. Thx all for the compression confirmation. There's hope. Leak down test next.

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JonW
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Re: Compression Range for a LC fresh top end

#11 Post by JonW » Thu Apr 06, 2017 10:52 pm

a leak down test is always worth doing, so yes do that if you have the kit.

I would be careful of adding more oil to the fuel, remember that while there is more oil you have less fuel in the same 'juice', this could make a lean situation worse etc.

Other factors... ie. Whats your gearing like?

And... Did you sit back a bit or lean over the bars? Im not the weight I was when I was 18, my bikes need to be much more powerful to lift my girth from the ground now to give me the same hit they did when i was young and dumb... just saying ;)
80 XT500 Supermoto!
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid

Like Watches? www.PloProf.com & www.DeskDivers.com

LC Cnd
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Re: Compression Range for a LC fresh top end

#12 Post by LC Cnd » Fri Apr 07, 2017 2:01 am

Check, built a leak down kit. Gearing is stock. Leaning on the tank - not in the habit of that just yet thought I don't mind putting my feet on the back pegs like when I was 18. Probably not now as I like the seating position as is - actually one of the chief reasons I was attracted to this bike. The only reason Jon I went with the oil ratio was a local banshee owner acquaintance advised not to trust the oil pump and go with the oil ratio he suggested. Theortically, the juice issue you pointed out may have been the reason for the temp engine conk out/scare. I leaning toward that thinking. The other option is hook up the oil lines, bleed them out & utilize 'em as engineered. Using Micron pipes/jetting, at this point, is probably not wise considering what I hope to accomplish - but man I love the noise they make; distinctly guttural. Going with the stock pilot/mains, with stock pipes, is more applicable for break in. Yep, completing the leak down test is logical for sure.

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JonW
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Re: Compression Range for a LC fresh top end

#13 Post by JonW » Fri Apr 07, 2017 3:58 am

Hmm, its always hearsay that gets you into trouble... and wives tales... or just telling the wife... hmm, LOL. To be honest the 'oil pumps are bad' myth is bizarre and must have come from the early 70s somewhere. Plenty of people have run the OEM pump as it came from the factory with no issues, for 30+ years... Nowadays we have Arrow and his pumps are tested and proven to be right, not all the factory supplied pumps are correct so we have a better than new source now. Ive had lots of pumps dome by Arrow, he does superb work and is a nice guy to boot. Use OEM lines and clips and youre golden, and theyre all available brand new still from yam. too easy.

if you lean forwards when the power hits and youre more than 50kgs then I reckon on a standard bike you will see very little wheelie action (cue everyone saying theyve power wheelied in 3rd stood on the clocks and they weigh 200kgs... bring it on haha). I reckon if you sit straight up (no need to sit on the grab rail or put feet on the passenger pegs lol) and get used to bring the power up fast you will do it, if not then gradually lean back a tad until you feel the front come up, if that fails then slide back a little perhaps for those times you really want to show off :) All this should be easy to accomplish in first for sure. Of course you can clutch it up for sure, but power wheelies are less easy Id say unless youre used to doing them, plenty are it seems from the pics on the forums ;) FWIW I can clutch or power wheelie a big KTM motard from a standstill and pretty much any 2 stroke enduro bike (u have to if you ride off road), but on the road I rarely would even try nowadays... maybe im just gettin old lol
80 XT500 Supermoto!
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid

Like Watches? www.PloProf.com & www.DeskDivers.com

Bare
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Re: Compression Range for a LC fresh top end

#14 Post by Bare » Sun Apr 09, 2017 11:28 pm

Black Plugs is an Odd condition.
Speaking more of being Wayyy too rich than too much oil.. Or possibly incomplete combustion.
Oil pump puts out a lot of oil when needed. No idea what your claimed oil ratio means.
I routinely run 22~25 :1. Dependent on how careful I am when adding the 'bit extra' of oil to the fuel tank.. and yess the pump is fine and running.
My plugs are milk chocolate brown. Never been black.
Only see black plugs on my Shindaiwas' .. when they refuse to start due to their airfilters being clogged or chokes stuck closed.

As above; Wheelies are more to do with wheelbase and how far back one sits (ie: trying to do them :-0
Only genuine Wheelie machines were the earlier RD 350 Air coolers.. as these had a much shorter wheelbase.
Son's Yama BWS scooter wheelies on demand.. despite single digit HP.. as it has a V short wheel base ... one pretty well sits on top of the rear wheel.

Your power loss suggests Somethings amiss.

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