Low Compression, running rough, backfire, Technical help please |
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Low Compression, running rough, backfire, Technical help please |
Don911 |
May 17 2018, 01:59 PM
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#1
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Newbie Group: Members Posts: 17 Joined: 30-March 18 From: Charleston Member No.: 22,007 Region Association: South East States |
So I have searched the forum (and internet) before posting...sorry if this a tired old topic.
Trying to narrow down the cause for some backfiring through the carbs and rough running. Compression: #1-125, #2-90, #3-70, #4-85 Some oil around the air filter housing. #3 plug fouled so all were replaced. Carbs synched Valve job completed. Still: runs rough in lower RPM ranges especially when starting out and now I'm getting an occasional backfire through the carbs under acceleration. Mech says it caused by low compression and needs at least a top end rebuild. A lot of what I'm reading points toward...well a lot of things, but nobody says low compression causes the chugging from start and the backfire. Agree? Disagree? Not enough info? Thoughts please...thanks! |
Don911 |
May 18 2018, 07:52 AM
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#2
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Newbie Group: Members Posts: 17 Joined: 30-March 18 From: Charleston Member No.: 22,007 Region Association: South East States |
Thanks guys! I have accepted the inevitable (IMG:style_emoticons/default/smile.gif)
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Mblizzard |
May 18 2018, 08:06 AM
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#3
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Advanced Member Group: Members Posts: 3,033 Joined: 28-January 13 From: Knoxville Tn Member No.: 15,438 Region Association: South East States |
Old trick and not sure just how reliable it is as far as firm diagnostics but repeating the compression check on #3 after adding some oil can give you an indication of if the loss of compression is due to rings or valves. if you see an increase it suggests rings. But with the backfire, I would typically think valves would be the more likely cause. There are a lot smarter people than me here but with 3 of 4 very low I would have to go back to rings. Keep us posted. |
worn |
May 18 2018, 08:08 AM
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#4
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can't remember Group: Members Posts: 3,154 Joined: 3-June 11 From: Madison, WI Member No.: 13,152 Region Association: Upper MidWest |
Old trick and not sure just how reliable it is as far as firm diagnostics but repeating the compression check on #3 after adding some oil can give you an indication of if the loss of compression is due to rings or valves. if you see an increase it suggests rings. But with the backfire, I would typically think valves would be the more likely cause. There are a lot smarter people than me here but with 3 of 4 very low I would have to go back to rings. Keep us posted. How easy is it to do a leak down on the type 4 in place? Not that it probably matters in this case, because the inevitable seems...inevitable. |
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