lost compression
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Joined: Feb 2006
Posts: 3,035
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From: Houston Texas
OK guys..... If I have this correct, we have a hole in the front piston and some bent valve(s) in the rear head?
If that is the case it certainly explains the loss of compression in both cylinders at once.
That question has been bugging me since I first heard it.
Valves get bent because of high engine rpm. The valves start to 'float'. In other words the valves hang out there in the open while the pushrod is falling away. Here comes the piston.....whack! the valve gets hit and bends. Now no compression.
For the hole in the piston.......most likely it was a burned hole rather than a 'hit'. I say that because the valves would have been bent if this piston had collided.
Pics would be interesting to see as IronMick pointed out. And if it was a burned hole, I would still be of the opinion that taking the engine apart would be best. If there was some way of getting the junk out, maybe it would not be required. But, to stick money into an engine fix and miss removing a piece of piston down there may prove to be far more costly in the long run.
If I were removing a cylinder and had broken rings fall into the bottom end I would be taking the engine apart to clean it out.
Anyway, it sounds like the loss of compression on both cylinders may be answered now. Another purplexing question though is the engine starting up and running wide open. I just cannot get my mind around that yet. With the slide closed down, it just cannot happen.
The slide sits in the carb and a rather large spring pushes down on the top of it to hold it closed.
Attached to the slide is a tapered needle which seats in the main jet tube.
Slide opens and the needle is lifted up, gradually letting more fuel in to mix with the more air coming in as the slide opens. How an engine can run wide open with a closed slide just baffles me no end.
So, yeah, post some good clear shots of the piston if you can. Also, take the front head (that has no bent valves) and turn it chamber side up. Fill the head cavity with water and see if any has leaked past the valves' seats the following morning. If the water has leaked out, you have bent valves in that head as well.......... pg
If that is the case it certainly explains the loss of compression in both cylinders at once.
That question has been bugging me since I first heard it.
Valves get bent because of high engine rpm. The valves start to 'float'. In other words the valves hang out there in the open while the pushrod is falling away. Here comes the piston.....whack! the valve gets hit and bends. Now no compression.
For the hole in the piston.......most likely it was a burned hole rather than a 'hit'. I say that because the valves would have been bent if this piston had collided.
Pics would be interesting to see as IronMick pointed out. And if it was a burned hole, I would still be of the opinion that taking the engine apart would be best. If there was some way of getting the junk out, maybe it would not be required. But, to stick money into an engine fix and miss removing a piece of piston down there may prove to be far more costly in the long run.
If I were removing a cylinder and had broken rings fall into the bottom end I would be taking the engine apart to clean it out.
Anyway, it sounds like the loss of compression on both cylinders may be answered now. Another purplexing question though is the engine starting up and running wide open. I just cannot get my mind around that yet. With the slide closed down, it just cannot happen.
The slide sits in the carb and a rather large spring pushes down on the top of it to hold it closed.
Attached to the slide is a tapered needle which seats in the main jet tube.
Slide opens and the needle is lifted up, gradually letting more fuel in to mix with the more air coming in as the slide opens. How an engine can run wide open with a closed slide just baffles me no end.
So, yeah, post some good clear shots of the piston if you can. Also, take the front head (that has no bent valves) and turn it chamber side up. Fill the head cavity with water and see if any has leaked past the valves' seats the following morning. If the water has leaked out, you have bent valves in that head as well.......... pg
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Joined: Feb 2009
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From: Siler City,North Carolina
I went to a reputable shop today and told the guy my problem. He said that the reason for the high idle was because I probably had a leak in the intake somewhere and she was sucking air, causing her to run lean, which burnt a hole in her piston. This makes sense because I just put the new mikuni on her probably worked something loose. I will not rework the top end without going through the bottom end, inspecting and cleaning it. I can do the top end myself but to be honest I'm not comfortable tearing apart the cases. I can have my bottom end tore apart, inspected and ceaned for $200. As for the pics I don't have a digital camera but I can borrow one so it may take a couple of days to get one on here. Keep the input coming, it is very helpfull. Thanks guys,JTB
Last edited by jtbredneck351; Oct 3, 2009 at 02:46 PM.
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
Good move having a shop give you comment on it and that makes sense to me. When you said 'wide open', I thought you meant wide open.
An air leak will cause a very high idle, but in no way can it cause things to be wide open. A misunderstanding there on my part.
Where the air leak is going to happen is with the installation of the O rings in the inlet manifold. When you put these in they are very difficult to keep in place without them shifting (and failing to seal as a result) and producing an air leak.
Here is what you need to do concerning that......
Hold the manifold against one of the cylinderhead inlet flanges. Let's say the rear head. Now, look at the air gap between the manifold edge and the front head inlet flange. Is the air space parallel to both heads? If yes, that is good.
If no, then you need to do this:
Loosen all 4 nuts on one of the cylinder bases about a half turn each.
Take a heavy plastic mallet (hard rubber may work too) and bump the base of the cylinder in the correct direction to align the head with the manifold edge.
Check for parallel again and if it now lines up, tighten down the base bolts. And, Loosen the base bolts, NOT THE CYLINDER HEAD BOLTS!
Then install the inlet manifold and O rings. You can run your finger around the inside of the manifold, feeling the surface of each O ring. If it feels smooth and even all the way around, the inlet will not leak. Tighten down the clamp bands and check the O ring surfaces inside the manifold one last time.
Be sure to use the carb brace that attaches to the air cleaner back plate on one end of the brace and the pushrod/tappet base down on the engine. Without this brace, the weight of the carb can easily cause the O rings to leak air again.
pg
An air leak will cause a very high idle, but in no way can it cause things to be wide open. A misunderstanding there on my part.
Where the air leak is going to happen is with the installation of the O rings in the inlet manifold. When you put these in they are very difficult to keep in place without them shifting (and failing to seal as a result) and producing an air leak.
Here is what you need to do concerning that......
Hold the manifold against one of the cylinderhead inlet flanges. Let's say the rear head. Now, look at the air gap between the manifold edge and the front head inlet flange. Is the air space parallel to both heads? If yes, that is good.
If no, then you need to do this:
Loosen all 4 nuts on one of the cylinder bases about a half turn each.
Take a heavy plastic mallet (hard rubber may work too) and bump the base of the cylinder in the correct direction to align the head with the manifold edge.
Check for parallel again and if it now lines up, tighten down the base bolts. And, Loosen the base bolts, NOT THE CYLINDER HEAD BOLTS!
Then install the inlet manifold and O rings. You can run your finger around the inside of the manifold, feeling the surface of each O ring. If it feels smooth and even all the way around, the inlet will not leak. Tighten down the clamp bands and check the O ring surfaces inside the manifold one last time.
Be sure to use the carb brace that attaches to the air cleaner back plate on one end of the brace and the pushrod/tappet base down on the engine. Without this brace, the weight of the carb can easily cause the O rings to leak air again.
pg
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Joined: Feb 2009
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From: Siler City,North Carolina
pg, My carb did'nt come with a brace and I did'nt have one on there. Maybe that was the problem. I reckon I need to fab one up when I get the bike back together. Do you have a pic of yours?
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
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From: Houston Texas
A I did was take a 1/8 inch thick X 3/4 inch wide steel strap and bend it to shape.
Drill a hole in each end to bolt it on and you are set.
This brace is A MUST HAVE item on a Sportster. Without it I can guarantee that the carb will droop and you will have air leaks. It is not so much that the strap holds the carb up, but rather keeps it from drooping. If you want a better pic of the strap just let me know and I can take another shot and post it.
pg
I made mine essentially the same way - 1/8" X 3/4" flat steel stock, bent to shape, holes drilled. Attaches to front intake lifter block bolt, which i replaced with stud 24827-66A.
"bent to shape" was actually quite tedious. Measure exactly, bend, try it, oooo not quite right; repeat; repeat; repeat; ... Want it to be exactly right so that there is no stress on the lifter block thru the stud.
"bent to shape" was actually quite tedious. Measure exactly, bend, try it, oooo not quite right; repeat; repeat; repeat; ... Want it to be exactly right so that there is no stress on the lifter block thru the stud.
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
I made mine essentially the same way - 1/8" X 3/4" flat steel stock, bent to shape, holes drilled. Attaches to front intake lifter block bolt, which i replaced with stud 24827-66A.
"bent to shape" was actually quite tedious. Measure exactly, bend, try it, oooo not quite right; repeat; repeat; repeat; ... Want it to be exactly right so that there is no stress on the lifter block thru the stud.
"bent to shape" was actually quite tedious. Measure exactly, bend, try it, oooo not quite right; repeat; repeat; repeat; ... Want it to be exactly right so that there is no stress on the lifter block thru the stud.
What I have found useful is to take a piece of stiff wire, like coat hanger, and bend the wire to the shape and length you need.
Then transfer the bends of the wire to a piece of strap steel.
Not a cure all, but it does make things easier. And, throwing away a coat hanger is less painful than dumping several trashed pieces of strap.
pg
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Joined: Feb 2009
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From: Siler City,North Carolina
Thanks fella's. I'll have to brace my carb when I get the bike back together no doubt. Theres no telling when that will be. Funds are short around my house, me, the ole lady 3 boys, a dog, a pig, a cat, a turtle, and a snake. Food and power comes first.Thanks again guys. I appreciate the help. I'll get that pic asap.JTB
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From: Texas! Ya mean there's someplace else?
Where the air leak is going to happen is with the installation of the O rings in the inlet manifold. When you put these in they are very difficult to keep in place without them shifting (and failing to seal as a result) and producing an air leak.
Then install the inlet manifold and O rings. You can run your finger around the inside of the manifold, feeling the surface of each O ring. If it feels smooth and even all the way around, the inlet will not leak. Tighten down the clamp bands and check the O ring surfaces inside the manifold one last time.
Be sure to use the carb brace that attaches to the air cleaner back plate on one end of the brace and the pushrod/tappet base down on the engine. Without this brace, the weight of the carb can easily cause the O rings to leak air again.
pg
Last edited by pococj; Oct 3, 2009 at 09:24 PM.