UL help.
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Intermediate
Joined: Dec 2022
Posts: 38
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From: Lakeside, ca
UL help.
I am banging my head against the wall at this point. Lets start, I have a '41 UL I built from a set of cases. The bike has fired and ran, but not perfect. Initially the bike would fire up and after a few seconds of running it would start missing on the front cylinder. I did a compression test to find that the front had low compression compared to the rear (~30 psi front, ~ 60 rear). I put new rings in the front and had equal compression but this didnt change the issue at hand much. Next I checked all my cam timing and that all checked out fine. The only thing that never was gone through during this build was the magneto that I bought used, I had a friend check it out and he said it was good. But just to be sure I sent it out to Joe Hunt to get checked out. They said the magnets and coil were bad and I thought this was the light at the end of the tunnel. Now the magneto is back in, and all I am getting is back fires and flames out of the intake. I have timed the mag off of what the service manual says (front cylinder at 11/32'" BTC and adjust mag to where points just start to break). I have advanced and retarded and get no difference, I have timed it off of the actual advance mark on the wheels with the line being more forward of the timing hole, no difference. I believe my next steps at this point is to double check my valve adjustments and do another compression test. I am running out of ideas and may have to swallow my pride and take it to a shop to have another set of eyes on it. This is not the first engine ive built but it is the first BT flathead. I appreciate any ideas you all may have. Thank you
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Joined: Apr 2010
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From: Long Island, New York
Certainly sounds like a timing issue.
Check the valve adjustment and do a leak test of the manifold. After that if she has fuel she should run. What is your current compression? 60 seems low, pretty sure my little 45, when stock, was around 90 psi. Good luck.
Check the valve adjustment and do a leak test of the manifold. After that if she has fuel she should run. What is your current compression? 60 seems low, pretty sure my little 45, when stock, was around 90 psi. Good luck.
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Joined: Mar 2010
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From: Honah Lee
Not sure what the ignition has to do with compression. You said 60 rear and 30 front. I assume properly done, warm engine and throttle wide open.
Do a leak down to determine cylinder or valves. Listen for the leak. Your going to air leak big time in exhaust or intake carb mouth or crank case.
What kind of clearance do you have piston to cylinder? Did you have a good cross hatching for rings? What is the ring gap about an inch down in cylinder made square with piston?
Do a leak down to determine cylinder or valves. Listen for the leak. Your going to air leak big time in exhaust or intake carb mouth or crank case.
What kind of clearance do you have piston to cylinder? Did you have a good cross hatching for rings? What is the ring gap about an inch down in cylinder made square with piston?
Last edited by Jackie Paper; Mar 26, 2025 at 12:54 PM.
Thread Starter
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Intermediate
Joined: Dec 2022
Posts: 38
Likes: 14
From: Lakeside, ca
Not sure what the ignition has to do with compression. You said 60 rear and 30 front. I assume properly done, warm engine and throttle wide open.
Do a leak down to determine cylinder or valves. Listen for the leak. Your going to air leak big time in exhaust or intake carb mouth or crank case.
What kind of clearance do you have piston to cylinder? Did you have a good cross hatching for rings? What is the ring gap about an inch down in cylinder made square with piston?
Do a leak down to determine cylinder or valves. Listen for the leak. Your going to air leak big time in exhaust or intake carb mouth or crank case.
What kind of clearance do you have piston to cylinder? Did you have a good cross hatching for rings? What is the ring gap about an inch down in cylinder made square with piston?
Cross hatching is good, they were sleeved at LA Sleeve and honed to pistons. Piston ring gaps are within spec per manual. The low compression in the front was due to me messing up a set because I had two feeler gauges stuck together, and I left those rings sitting on the bench right next to a new good set and then accidently put the large gapped rings in. Totally my fault.
Last edited by T_levan84; Mar 26, 2025 at 01:25 PM.
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From: Honah Lee
Ill clarify, The compression test was done while the engine was cold due to the fact that I could not get it to run long enough to get hot. I had 30 psi in the front and re-ringed it and then had 60 psi on both while cold.
Cross hatching is good, they were sleeved at LA Sleeve and honed to pistons. Piston ring gaps are within spec per manual. The low compression in the front was due to me messing up a set because I had two feeler gauges stuck together, and I left those rings sitting on the bench right next to a new good set and then accidently put the large gapped rings in. Totally my fault.
Cross hatching is good, they were sleeved at LA Sleeve and honed to pistons. Piston ring gaps are within spec per manual. The low compression in the front was due to me messing up a set because I had two feeler gauges stuck together, and I left those rings sitting on the bench right next to a new good set and then accidently put the large gapped rings in. Totally my fault.
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Intermediate
Joined: Dec 2022
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From: Lakeside, ca
like I said my gauges were stuck together, if I remember correctly it was my .017 and .018 so .035. I should have thrown them away when I pulled them but I seem to enjoy clutter on my work benches. And also there was a bunch of raw gas dumping in there because the magneto wasn’t firing properly.
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From: la la land jerzey
Today you must use the peeek seals for the intake to seal - you must the brass are king kong rings only
once you set the location of the flywheels at what you think is the timing mark for setting the mag - oh yea the mag not one chance if you send a mag out to be checked they will tell you its ok - not one chance ——- 5 or 6 hundred they will tell you it works now
the rear intake is just opening when the front is at timing location - put a 3/8 pice of hose in the plug hole and blow in the 2 cylinders the one that is resistance and not easy to blow through is the one that is firing the plug - it should be the front but i think your timing the rear
once you set the location of the flywheels at what you think is the timing mark for setting the mag - oh yea the mag not one chance if you send a mag out to be checked they will tell you its ok - not one chance ——- 5 or 6 hundred they will tell you it works now
the rear intake is just opening when the front is at timing location - put a 3/8 pice of hose in the plug hole and blow in the 2 cylinders the one that is resistance and not easy to blow through is the one that is firing the plug - it should be the front but i think your timing the rear
Thread Starter
|
Intermediate
Joined: Dec 2022
Posts: 38
Likes: 14
From: Lakeside, ca
Today you must use the peeek seals for the intake to seal - you must the brass are king kong rings only
once you set the location of the flywheels at what you think is the timing mark for setting the mag - oh yea the mag not one chance if you send a mag out to be checked they will tell you its ok - not one chance ——- 5 or 6 hundred they will tell you it works now
the rear intake is just opening when the front is at timing location - put a 3/8 pice of hose in the plug hole and blow in the 2 cylinders the one that is resistance and not easy to blow through is the one that is firing the plug - it should be the front but i think your timing the rear
once you set the location of the flywheels at what you think is the timing mark for setting the mag - oh yea the mag not one chance if you send a mag out to be checked they will tell you its ok - not one chance ——- 5 or 6 hundred they will tell you it works now
the rear intake is just opening when the front is at timing location - put a 3/8 pice of hose in the plug hole and blow in the 2 cylinders the one that is resistance and not easy to blow through is the one that is firing the plug - it should be the front but i think your timing the rear