lost compression
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Joined: Feb 2009
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From: Siler City,North Carolina
lost compression
1974 xlh.The Bike was running great. I just got my new mikuni vm 38 last week. Then today I was riding and and it started cutting out on me. I put in new plugs and checked my points. It was acting like it was'nt getting gas.I took my fuel line off and blew through it. The bike would start with the choke on and I finally got it to run with the choke off but I had to pump the gas. I got her going down the road still fluttering the gas to keep her running then she just stopped. No noise no poof no rattle or bang, just stopped. I pulled the plugs and put my finger over the plug hole and no compression on either cylinder. I don't have a compression tester but it wouldnt even think about blowing my finger off of the hole. It does have oil. The pistons are moving up and down so i don't think it's in the bottom end. How bout some input. THX, JTB
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Joined: Feb 2006
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From: Houston Texas
1974 xlh.The Bike was running great. I just got my new mikuni vm 38 last week. Then today I was riding and and it started cutting out on me. I put in new plugs and checked my points. It was acting like it was'nt getting gas.I took my fuel line off and blew through it. The bike would start with the choke on and I finally got it to run with the choke off but I had to pump the gas. I got her going down the road still fluttering the gas to keep her running then she just stopped. No noise no poof no rattle or bang, just stopped. I pulled the plugs and put my finger over the plug hole and no compression on either cylinder. I don't have a compression tester but it wouldnt even think about blowing my finger off of the hole. It does have oil. The pistons are moving up and down so i don't think it's in the bottom end. How bout some input. THX, JTB
A sudden loss of compression, especially on both cylinders, does not bode well!
Check the pushrod clearance first. If they are tight, set them correctly and then see if this brings back some compression.
If there is still no compression......... Then I would have to fear you have popped off the head of a valve or put a hole in a piston. The strange thing is for this to happen on both cylinders though. So that leaves me puzzled right now.
Anyway, after the pushrod check the next step will be to remove both heads and take a look inside............ pg
Road Master
Joined: Aug 2006
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From: 500 miles south from Artic Circle
As PG wrote, check pushrods first. Clocked carb may result extra lean mixture ->melting hole to piston but both of them is rare. If you have hole in piston there is usually excessive air/oil mixture coming from breather tube, when you try to start bike.
Cruiser
Joined: Jun 2008
Posts: 200
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From: Assonet MA
Advance Unit
If your not sure its the compression I suggest you check the advance unit. I had similar problems arise with my 74XLH, 38VM, and discovered a broken spring on the advance was the problem.
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From: Houston Texas
Actually, running with the choke on keeps the mixture rich on gas and is very unlikely to allow a hole to be put into the piston due to extreme lean mixture. This (choke on) cools the piston top.......... pg
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From: Siler City,North Carolina
When I started the thread i said that the bike cut off and would only start with the choke on at first. I failed to say that when the bike started it ran wide open. Wide open! After the first time i open and shut my throttle a few times, started it up again, choke on, and again wide open. After this I took the air breather off to see if my slide was stuck open in the carb. It wasn't. I started the bike again,choke on, and again wide open. Over all it ran several seconds absolutely wide open. I don't know why. Anyhow, I pulled the heads off of the old girl today and low and behold, a big ole hole in her front piston. I figure too many rpm's. It don't take a rocket scientist to figure that one out. The valves looked good, not broken, not bent. So what i need to know is how far do I need to tear into her? Did I maybe bend a rod. What else coulda went wrong? I figure off the bat I gotta get the broken piston pieces out of the bottom end. Any opinions appreciated. Thx, JTB
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Stellar HDF Member
Joined: Feb 2006
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From: Houston Texas
When I started the thread i said that the bike cut off and would only start with the choke on at first. I failed to say that when the bike started it ran wide open. Wide open! After the first time i open and shut my throttle a few times, started it up again, choke on, and again wide open. After this I took the air breather off to see if my slide was stuck open in the carb. It wasn't. I started the bike again,choke on, and again wide open. Over all it ran several seconds absolutely wide open. I don't know why. Anyhow, I pulled the heads off of the old girl today and low and behold, a big ole hole in her front piston. I figure too many rpm's. It don't take a rocket scientist to figure that one out. The valves looked good, not broken, not bent. So what i need to know is how far do I need to tear into her? Did I maybe bend a rod. What else coulda went wrong? I figure off the bat I gotta get the broken piston pieces out of the bottom end. Any opinions appreciated. Thx, JTB
Front piston has a hole? Not good!
Did you not say there was no compression on either cylinder, or am I confusing that with someone else?
In anycase, you will have to remove the pieces of broken piston out of the lower end. The only way to do that I am afraid is to remove the engine from the frame ans separate the cases.
Then you will have to determine if there is a bent rod. Did the piston hit the head? Any sign of it doing so on the head's surface?
I wish I had a better answer for you, but I am telling you straight out what my opinion is.
Myself, I would take the engine apart, fix what needs to be fixed and put it back together properly. This will be a good winter project is about the best news I can give you.
You are going to need a factory manual and perhaps a few special tools to get it apart. I would not separate the flywheels. Just remove the flywheel assembly with the rods attached.
Before you take it apart though..........
Look at where each piston high point is right now on each cylinder. If they appear to rise up the same amount, the rods are probably OK. Later you can look at them further.
Sorry to hear the news.......... pg
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Please post a pic of the piston with the hole. Use a tripod or whatever to steady the camera so the the pic is sharp and clear. PG, i am wondering if the hole might be a burn hole rather than a puncture hole, and if he could then avoid dismantling the bottom end? And if we [you!] could tell from such a pic?
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Joined: Feb 2009
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From: Siler City,North Carolina
Thx pg. I don't understand how it ran wide open with the slide down either but I swear it did. There is no sign of the piston hitting the head. And yes, no compression in either cylinder. After further inspection, one of the valves are slightly bent in the rear head. I do have a factory manual. Damn I hope that we have a cold cold winter.