Exhaust popping |
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Exhaust popping |
cal914 |
Jul 24 2018, 02:49 PM
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#1
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Member Group: Members Posts: 319 Joined: 12-April 12 From: United Kingdom Member No.: 14,366 Region Association: None |
I have a 6 cylinder engine in my 914 and every time I take my foot off the excelerator the exhaust pops and slightly backfires ,what causes this and how do I remedy it
Thanks Brian |
john77 |
Jul 24 2018, 02:50 PM
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#2
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Senior Member Group: Members Posts: 621 Joined: 21-February 14 From: Los Angeles Member No.: 17,027 Region Association: Southern California |
Does the 6 have carbs?
If yes, it could be running too rich, or it could be running too lean, the jets could be the wrong size, the mixture adjustment screws could be wound out too far, the throttle valves could be out of alignment, the throttle shaft springs could be shot, etc, etc... Is this a new issue or has it been going on for a while? If you're mechanically minded you can tune them and even rebuild them yourself, it just takes the internet, patience, a lot of adjustment, and a syncrometer. Whether they'll stay tuned is another story (IMG:style_emoticons/default/smile.gif) |
Rand |
Jul 24 2018, 03:05 PM
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#3
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Cross Member Group: Members Posts: 7,409 Joined: 8-February 05 From: OR Member No.: 3,573 Region Association: None |
Lean pops out the carbs. Rich pops out the exhaust.
I'm guessing you have an exhaust leak, which would exacerbate the rich and feed it with the air to ignite it. |
SirAndy |
Jul 24 2018, 03:26 PM
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#4
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Resident German Group: Admin Posts: 41,679 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California |
I'm guessing you have an exhaust leak, which would exacerbate the rich and feed it with the air to ignite it. (IMG:style_emoticons/default/agree.gif) |
Mark Henry |
Jul 24 2018, 05:52 PM
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#5
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that's what I do! Group: Members Posts: 20,065 Joined: 27-December 02 From: Port Hope, Ontario Member No.: 26 Region Association: Canada |
Another cause can be the carbs linkage out of sync one side opening more or a different rate than the other. This issue is worse on the /4 but is true with the /6 as well.
I found the stock weber/porsche drop link balls to be really sloppy. I made my own drop links using quality 1/4"-28tpi joints. I also use them on my /4 linkages. You can see them in this pic below: (IMG:http://www.914world.com/bbs2/uploads_offsite/img1.fastenal.com-26-1532476321.1.jpg) (IMG:http://www.914world.com/bbs2/uploads/post-26-1497554034_thumb.jpg) |
SO.O.C914er |
Jul 24 2018, 07:30 PM
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#6
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Senior Member Group: Members Posts: 1,265 Joined: 18-July 16 From: Mission Viejo Ca Member No.: 20,199 Region Association: Southern California |
Forrest’s 6 was doing that on our Temecula run last year and it was a new motor. He may be able to shed some light on this.....Paul (IMG:style_emoticons/default/popcorn[1].gif)
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IronHillRestorations |
Jul 25 2018, 10:09 AM
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#7
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I. I. R. C. Group: Members Posts: 6,731 Joined: 18-March 03 From: West TN Member No.: 439 Region Association: None |
There's a few things that can cause this, wrong cam timing, wrong valve lash, wrong ignition timing, incorrect fuel/air mixture, wrong exhaust system, exhaust leak, to name a few.
A simple and painless check is to have someone to follow you at night and see if they see any fireballs coming out the exhaust. If so then you are probably too rich, or have a timing issue. If no visible flamage then you could have an exhaust leak. Put a thick leather soled shoe over the exhaust pipe or a block of wood whilst idling, and you might be able to hear where it's leaking. There's another test some guys do using a shop vac to blow backwards through the exhaust system. I've personally not done this, and I"m not sure how effective it is for the cylinders with open or partially open valves. |
Retroracer |
Jul 25 2018, 02:18 PM
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#8
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Senior Member Group: Members Posts: 616 Joined: 7-July 13 From: Bend OR Member No.: 16,100 Region Association: Pacific Northwest |
Brian - One possible explanation is that you are running rich and unburnt fuel is accumulating in the exhaust. I'd suggest getting the engine thoroughly warmed up and pull the spark plugs to see condition - are they all coated in black film? Are some more than others?
If all the plugs are blackish, then I'd look at getting your carbs set up. Or, your fuel pressure / pump set up, to make sure your are not overcoming the float action in the bowls. If one or two plugs are blackened, then it may be various things - that specific throat is set up differently; or a stuck valve in the float chamber may be enriching 1 or 2 cylinders. If the plugs are a nice brown/gray colour, then I'd look at timing. Which I'm sure you have checked and verified - right? BTW, listing the carb type (weber / PMO / Zenith), fuel pump / regulator setup & engine type (T/E/S, etc) might help the group diagnosis..... - Tony |
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