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! |
TheCabinetmaker |
May 17 2018, 06:25 PM
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#2
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I drive my car everyday Group: Members Posts: 8,299 Joined: 8-May 03 From: Tulsa, Ok. Member No.: 666 |
sounds like your mech might be on the right track. Those are terrible numbers.
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worn |
May 17 2018, 06:52 PM
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#3
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can't remember Group: Members Posts: 3,129 Joined: 3-June 11 From: Madison, WI Member No.: 13,152 Region Association: Upper MidWest |
Valve job completed meaning valve adjustment or replacement?
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VaccaRabite |
May 17 2018, 07:07 PM
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#4
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En Garde! Group: Admin Posts: 13,407 Joined: 15-December 03 From: Dallastown, PA Member No.: 1,435 Region Association: MidAtlantic Region |
So... When was the last time it had the valves adjusted?
If recently, then I agree its probably time for a top end rebuild. Zach |
mobymutt |
May 17 2018, 07:49 PM
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#5
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Senior Member Group: Members Posts: 548 Joined: 16-December 13 From: Kingston, Ontario, Canada Member No.: 16,770 Region Association: Canada |
I also have terrible compression numbers, and I also had a lot of backfiring. I got rid of most of the backfiring through some combination of new rotor and cap, adjusting valve clearances, adjusting timing and syncing the carbs.
Now I have terrible compression, but no backfiring. I call that a win! |
Don911 |
May 18 2018, 07:52 AM
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#6
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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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#7
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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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#8
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can't remember Group: Members Posts: 3,129 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. |
Don911 |
May 18 2018, 08:17 AM
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#9
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Newbie Group: Members Posts: 17 Joined: 30-March 18 From: Charleston Member No.: 22,007 Region Association: South East States |
A leak down has been completed as well.
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Mblizzard |
May 18 2018, 08:26 AM
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#10
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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 |
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Don911 |
May 18 2018, 09:00 AM
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#11
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Newbie Group: Members Posts: 17 Joined: 30-March 18 From: Charleston Member No.: 22,007 Region Association: South East States |
It is a 1.8 with dual Solex carbs. I'll upgrade to big webers at the same time.
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Mblizzard |
May 18 2018, 11:46 AM
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#12
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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 |
Are you going to build it?
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TheCabinetmaker |
May 18 2018, 12:15 PM
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#13
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I drive my car everyday Group: Members Posts: 8,299 Joined: 8-May 03 From: Tulsa, Ok. Member No.: 666 |
You did not answer the questions about valves. What do you mean by valve job? Were the heads removed and taken to a machine shop? Or were they just adjusted? Your symptoms do sound like seriously misadjusted valves. Did your mech do them, or did you do them yourself?
Btw, you know we like pcs, right? |
VaccaRabite |
May 18 2018, 12:25 PM
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#14
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En Garde! Group: Admin Posts: 13,407 Joined: 15-December 03 From: Dallastown, PA Member No.: 1,435 Region Association: MidAtlantic Region |
Don't put big carbs on a 1.8 unless the engine warrants it. Bigger does not mean better in this regard.
Zach |
Don911 |
May 21 2018, 08:34 AM
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#15
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Newbie Group: Members Posts: 17 Joined: 30-March 18 From: Charleston Member No.: 22,007 Region Association: South East States |
It was a valve adjustment done by a mechanic.
Point taken on the carbs. Thanks! |
Don911 |
Dec 11 2018, 10:55 PM
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#16
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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 thought I'd follow up. I started this thread in May and as of yesterday I finally got the 914 back with a little 2.055 liter hot rod. Took a LONG time, but I'm enjoying it.
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porschetub |
Dec 11 2018, 11:16 PM
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#17
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Advanced Member Group: Members Posts: 4,695 Joined: 25-July 15 From: New Zealand Member No.: 18,995 Region Association: None |
Great ,but we are funny on here lots of pics and replies to what you did to get the motor sorted are always welcome viewing and reading, a lot of folks are receiving help on sorting engine issues and that's what its about (IMG:style_emoticons/default/biggrin.gif) .
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VaccaRabite |
Dec 12 2018, 06:21 AM
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#18
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En Garde! Group: Admin Posts: 13,407 Joined: 15-December 03 From: Dallastown, PA Member No.: 1,435 Region Association: MidAtlantic Region |
Yeah, post up your combo and build pics. I'm assuming you built a 2056 and not a 2055 (96mm pistons and a 2L crank?)
What cam? Carbs or FI? I have a 2056 in my car and I love it. Enough power to keep up with most mini-vans. Lol! Zach |
TheCabinetmaker |
Dec 12 2018, 08:23 AM
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#19
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I drive my car everyday Group: Members Posts: 8,299 Joined: 8-May 03 From: Tulsa, Ok. Member No.: 666 |
No pics. Didn't happen! (IMG:style_emoticons/default/alfred.gif) (IMG:style_emoticons/default/thisthreadisworthlesswithoutpics.gif)
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dr914@autoatlanta.com |
Dec 12 2018, 08:51 AM
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#20
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914 Guru Group: Members Posts: 7,772 Joined: 3-January 07 From: atlanta georgia Member No.: 7,418 Region Association: None |
you say "valve job" did you mean vale ADJUSTMENT? If adjustment and you know that the adjustment is correct, and still low compression then certainly a rebuild. Carbs are very hard on a 914 engine.
So without decent compression trying to get your engine to run correctly, is probably futile, although with carb rebuild, adjustment, no intake leaks, dwell and timing and a good ignition system, it will run better than it is now, 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! |
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