Compression issues?

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Train_Wreck

Train_Wreck

Member
Feb 9, 2015
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Sacramento, CA
So I assembled my motorbike today, had it running ok for an hour. got back on it a few hours later, started it up and it ran for a sec but I pulled the clutch lever and the engine died. Though nothing of it, then tried to start it again. it ran kinda weird, pulled the lever again and it died. I checked my fuel, made sure my choke was off, then tried again. This time it was really bogging and it didn't wanna go unless I pedaled. At this point I also lost most of the compression. I took it home, took off the cylinder head and noticed that the metal gasket is broken. Is this the reason why I do not have any compression and it will not run? I would assume so, there is a 1/8 inch tear in the metal head gasket and it was working ok earlier today. Thanks

:-||
 
maniac57

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
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memphis Tn
Sounds like you found the problem.
You might consider lapping the head to help prevent this issue in the future, but usually all it takes is keeping the head bolts tight. Most all new engines will need the head re-torqued after the first few rides until it settles in.
DON'T CRANK THEM DOWN, use a torque wrench unless you are very experienced as a stripped head stud or case is a HUGE pain in the behind to fix.
http://motorbicycling.com/showthread.php?t=44018
 
Train_Wreck

Train_Wreck

Member
Feb 9, 2015
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0
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Sacramento, CA
ok thanks. Sucks I blew a gasket on the first day of riding but it's better than something else severe. Yeah ill take your advice and lap the head while I'm waiting for a new gasket to come in the mail. I do not have a torque wrench tho, is it really necessary to get one?

thanks again dance1
 
fatdaddy

fatdaddy

New Member
May 4, 2011
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San Jose, Ca.
I dont use a turque wrench every time, BUT I've been doing this for 10 years or better, Just remember that the studs are threaded into a cheap Chinese made bottom end. Make it real snug but dont jump on it too hard if'n ya know what I mean. 10 to 12 ftlbs is recommended. Also make sure the studs are screwed in all the way. With the head off, lock two nuts together on them and use them to check the studs. you dont wanna jump too hard on that either. just untill you feel them snugging into the case. After you replace the head gasket ride it for a few miles and check the head nuts again. do this a few times. I just put my motor back together and got stuck without the 14 mm socket I needed to snug the head back down. But I ran into a guy that had a socket set in his trunk and now she runs great. I broke my own rule and took off without enough tools. Nobody ever said I was the sharpest tack in the box.
fatdaddy.usflg
 
2door

2door

Moderator
Staff member
Sep 15, 2008
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Littleton, Colorado
Something else we usually suggest is to get rid of the chrome acorn nuts and replace them with shouldered or flanged hex nuts. The shouldered nuts aren't an absolute necessity but they do make seating against the head nicer.

The acrorn nuts look good but they are notorious for sometimes bottoming out on the stud before they can apply the right pressure to the cylinder head. This happens when either the studs are too long or not seated into the case correctly. Take Fatdaddy's advice about getting the studs tight before installing the head. His method or double nutting is a good one.

120 to 140 inch pounds isn't a lot and is about as tight as you need to make the head nuts. Some builders go as far as 200 but in my opinion, that's pushing it.
Also do what was suggested by re-torquing the head after a couple of heat/cool cycles. You can check them peirodically but only 'check' them and don't try to get a turn on the wrench. Just make sure they're still at your original torque value.

And lap that head.

Good luck.

Tom
 
fatdaddy

fatdaddy

New Member
May 4, 2011
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San Jose, Ca.
I like the acorn nuts, They're PRETTY.LOL But I know about the studs hitting the top inside of the nuts before seating the head properly. If ya keep the acorns, just be aware of the problem. And Tom is right about lapping the head, It's easy enough to do and makes for a better head to cylinder match. And as a by product will increase compresion a little. The aluminum head gasket is soft enough to help with some small imperfections. But better safe than sorry.
fatdaddy.usflg
 
mapbike

mapbike

Well-Known Member
Mar 14, 2010
5,502
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Central Area of Texas
Just a note concerning the acorn nuts.

Some of the kits are coming with a set of taller shouldered acorn nuts now and those I assume were designed to help cure the bottoming out issues we seen a lot of with the shorty type acorn nuts.

the last four engines I have bought from thatsdax came with these taller shouldered acorn nuts, both the original 40mm stroke GenIV and the newest 38mm stroke version.

I have used these acorn nuts with zero issue, I put a lock washer under them along with a dap of blue loctite on the stud threads and they stay put.

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Train_Wreck

Train_Wreck

Member
Feb 9, 2015
47
0
6
Sacramento, CA
mapbike said:
Just a note concerning the acorn nuts.

Some of the kits are coming with a set of taller shouldered acorn nuts now and those I assume were designed to help cure the bottoming out issues we seen a lot of with the shorty type acorn nuts.

the last four engines I have bought from thatsdax came with these taller shouldered acorn nuts, both the original 40mm stroke GenIV and the newest 38mm stroke version.

I have used these acorn nuts with zero issue, I put a lock washer under them along with a dap of blue loctite on the stud threads and they stay put.

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Click to expand...
I have the longer acorn nuts on mine, I do have loctite but I am not using it, I am just checking the nuts after every ride to make sure they havent loosened. As i've been breaking the engine in, they aren't moving at all so that's good. Unfortunately I never got around to lapping the cylinder head cause I do not have the right sand paper or the other necessities but someone else said it should be fine as long as I check the nuts often. I only have about half a gallon left of the break-in fuel 'til I can let this baby go WOT! oh yeuuuh!

.wee.
 
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