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Posted by: cal914 Jul 24 2018, 02:49 PM

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

Posted by: john77 Jul 24 2018, 02:50 PM

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

Posted by: Rand Jul 24 2018, 03:05 PM

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.

Posted by: SirAndy Jul 24 2018, 03:26 PM

QUOTE(Rand @ Jul 24 2018, 02:05 PM)

Posted by: Mark Henry Jul 24 2018, 05:52 PM

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:

Posted by: SO.O.C914er Jul 24 2018, 07:30 PM

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

Posted by: Perry Kiehl Jul 25 2018, 10:09 AM

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.

Posted by: Retroracer Jul 25 2018, 02:18 PM

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