Engine sucks but will not blow

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Gordon

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Some time ago I built an Upshur Horizontal Twin and was never able to get it to run. I just could not get compression. I blamed it on rings or poor bore and finally just set it aside for a while. Later I built a similar engine of my own design but larger with 1" bore c 1 1/4 stroke. I again have a non running engine. I have made new rings and honed the cylinders but still no compression. The thing which has be baffled is that the engine has great suction on the intake stroke but no compression. With the head removed when I put my hand over the bore it has great suction and will even hold the vacuum for a long time but almost no compression. It obviously is not valve leakage or head gasket because this is with the head removed and just my hand sealing the bore. Also there are two cylinders and both act the same way. It would seem that if a piston works in one direction it should work in both directions. What am I missing?
 

Gordon

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I have used cast iron rings, both ones I made and purchased rings, and I have even tried viton O rings. Nothing changes.
 
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I understand that you have tested with the head removed and have good vacuum on the down stroke but no or little compression on the up stroke.
Does the engine have a piston liner in the block?
I had a similar problem with a liner not sealing around the outside and letting the compression escape. The liner was slightly lower than the block deck.
Worth a look.
Ray
 

Gordon

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I understand that you have tested with the head removed and have good vacuum on the down stroke but no or little compression on the up stroke.
Does the engine have a piston liner in the block?
I had a similar problem with a liner not sealing around the outside and letting the compression escape. The liner was slightly lower than the block deck.
Worth a look.
Ray
No liner. I also had that problem on one of the engines that I had built.
 
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I'm having something of a problem seeing how this can happen - Idon't understand the engine configuration so it's hard to tell what to look for specifically, but if you have sealed off the top of the cylinder, on compression the gas must go somewhere. here are some thoughts

  • I don't know how many rings are on your piston, but if the gas is getting past them on compression but not on intake, then maybe invert the rings
  • try filling the bore with a liquid, and then try to compress it - don't force too hard, bring it up to 50 to 100 pounds - if you use alcohol or some liquid that has very low viscocity, it will go somewhere. and when you see where it goes, that is probably (high certainty) where the air is going.
  • the standard way to test for leaking rings on a car is to put a few squirts of oil in the cylinder and run a compression test - if the compression goes up, the rings are leaking.
  • can you replace the head with a piece of metal with a 1/8" pipe theread in it, and see what the actual pressures are? if you put a hose adapter into the threaded hole and pressurize the cylinder you will be able to hear the air leaking out. I've done that before with great success. If you can't tell where the sound is coming from, that's the one good thing you can do with cigarettes, make some smoke and blow that into the cylinder, and you will see where it's coming from.
  • if you can't tell by pressurizing the cylinder, create a vacuum on the crank case side and use your cigarette smoke to see where the leak is
  • if you remove the crank and everything and just put hte piston into the cylinder by itself, and you do any of the above leak tests, does air pass by the piston? if yes, invert the piston so the top points to the crank case and try again. if that stops the leak, you at least know what to puzzle over
 

arnie

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In my days as a M?C mechanic the top inside edge of the rings had a bevel that let the compression leak around them a little and push the rings outward to seal in the bore.
 

minh-thanh

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With the head removed when I put my hand over the bore it has great suction and will even hold the vacuum for a long time but almost no compression. It obviously is not valve leakage or head gasket because this is with the head removed and just my hand sealing the bore. Also there are two cylinders and both act the same way. It would seem that if a piston works in one direction it should work in both directions. What am I missing?
Maybe ....
A.jpg
 

johwen

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Some time ago I built an Upshur Horizontal Twin and was never able to get it to run. I just could not get compression. I blamed it on rings or poor bore and finally just set it aside for a while. Later I built a similar engine of my own design but larger with 1" bore c 1 1/4 stroke. I again have a non running engine. I have made new rings and honed the cylinders but still no compression. The thing which has be baffled is that the engine has great suction on the intake stroke but no compression. With the head removed when I put my hand over the bore it has great suction and will even hold the vacuum for a long time but almost no compression. It obviously is not valve leakage or head gasket because this is with the head removed and just my hand sealing the bore. Also there are two cylinders and both act the same way. It would seem that if a piston works in one direction it should work in both directions. What am I missing?
Have you checked your Valve timing or direction of rotation sounds silly but worth checking the timing for the direction of rotation. If rings and valves are sealing it has to be be something as silly as this. Johwen
 

lee webster

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As stupid as this sounds..... Did you cut the groove for the piston ring with a flat tip cutter or one with a bit of a taper, like some parting tools? If so then perhaps the ring is being held tightly in one direction, but a little looser int tother. I know, stupid. I was cutting the grass when I came up with this one!
 

Gordon

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Rings are purchased from Dave Reed so I am pretty sure that the problem is not in the rings themselves. Possibly in my ring groove but it would seem unlikely that all four grooves would be wrong. I am presently in the process of reassembling the engine and readjusting everything. I am having a problem wrapping my head around the proper valve timing and ignition timing as they relate to the opposite cylinder. Some really strange things going on here. I was getting proper intake and exhaust on the LH cylinder but the RH cylinder was having intake and compression in the wrong ports. I swapped the heads around an now they are both working properly. Evil spirits at work I think. The more I work on this thing the more confused that I get.
 

stevehuckss396

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Have you checked your Valve timing or direction of rotation sounds silly but worth checking the timing for the direction of rotation. If rings and valves are sealing it has to be be something as silly as this. Johwen

This was my first thought. Maybe the valve timing is off or something is machined incorrectly and is allowing escape on the compression stroke?
 

lee webster

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Ahhh, evil spirits. Of course. Well, thats got that sorted then.
The solution to this problem is going to be very interesting.
I thought minh thanh has posted a very credible suggestion. Have you checked the bores for runout/taper?
 

rklopp

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Does the engine have separate cylinder liners and cooling jackets? If so, how’s the fit? I could see your hand sealing on the liner during your suction test and trying to seal on the outer edge of the jacket during compression, and the air bypasses down the joint between the liner and jacket. I am skeptical of my own theory and think there’s a ring groove problem. Pics?
 

Gordon

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No separate liner. I am currently in the process of reworking and remaking parts. Piston. Valve cage. Valves. If I keep working on one thing at a time eventually I will find the problem or problems.
 

bluejets

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It obviously is not valve leakage or head gasket because this is with the head removed and just my hand sealing the bore.
That sounds like a valve problem to me......as most are in fact.
 

rklopp

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How can it be the valves if the suction is good and compression is poor with the head off?
 

bluejets

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Many ringed engines will have near zero compression (initially) when wound over by hand.
See how it responds with the same arrangement only with a drop of oil in the bore and spun with a drill at a hundred revs or so.
 
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