Backfire, Simple question from a simple guy... Options

[Coy]

Okay… This might sound stupid…

I just love 914s, but I’m not really that good at working on them. Sure I’ll do brakes and oil and tune-ups, door latches and stuff like that, but when it comes down to actual engine work I just don’t have the proper space or tools, and to be perfectly honest; when the garage is colder than 60 degrees I don’t even have the inclination to do it myself. I used to over haul a Harley every winter and to tell you the truth, I’m just old now; I don’t want to work in the cold. Ad to that the fact that I am totally self-taught as a mechanic and you’ll see that I’m not great at diagnostics, and that I can quickly get in over my head.

So… I have this intermittent exhaust backfire that comes and goes and I don’t know why. It frequently backfires when it’s cold starting before it gets warm, but that stops after just a minute or less.

The car was running great Sunday afternoon (no backfire) so I took it out and had a good time on a twisty little road not far from my house and I have to say… it was running like a champ (I’m tell’n you, I love this car).

So Monday I drive it downtown and it starts doing the back fire thing in earnest. Not only that, but the exhaust seems especially loud. It doesn’t backfire when the accelerator pump is squirting, but it does when it’s at a constant speed. You guys are all real smart; does an exhaust leak at the head have the potential to cause a backfire when the car is running at about 3000 rpm? It's a 2.0 with dual Webbers and hydraulic lifters.

I have other cars, so I’ll just wait till the floor is warmer before I crawl underneath it, but in the mean time does this make sense that it could just be an exhaust leak, or does that kind of crap happen when the carbs are out of sync too? I don’t know squat about dual carbs.

Also... Does anyone in the Portland/Salem area know of a shop with anyone who is epecially good with 914s that can diagnose?

Thanks!

[dr914@autoatlanta.com]

Okay… This might sound stupid…

I just love 914s, but I’m not really that good at working on them. Sure I’ll do brakes and oil and tune-ups, door latches and stuff like that, but when it comes down to actual engine work I just don’t have the proper space or tools, and to be perfectly honest; when the garage is colder than 60 degrees I don’t even have the inclination to do it myself. I used to over haul a Harley every winter and to tell you the truth, I’m just old now; I don’t want to work in the cold. Ad to that the fact that I am totally self-taught as a mechanic and you’ll see that I’m not great at diagnostics, and that I can quickly get in over my head.

So… I have this intermittent exhaust backfire that comes and goes and I don’t know why. It frequently backfires when it’s cold starting before it gets warm, but that stops after just a minute or less.

The car was running great Sunday afternoon (no backfire) so I took it out and had a good time on a twisty little road not far from my house and I have to say… it was running like a champ (I’m tell’n you, I love this car).

So Monday I drive it downtown and it starts doing the back fire thing in earnest. Not only that, but the exhaust seems especially loud. It doesn’t backfire when the accelerator pump is squirting, but it does when it’s at a constant speed. You guys are all real smart; does an exhaust leak at the head have the potential to cause a backfire when the car is running at about 3000 rpm? It's a 2.0 with dual Webbers and hydraulic lifters.

I have other cars, so I’ll just wait till the floor is warmer before I crawl underneath it, but in the mean time does this make sense that it could just be an exhaust leak, or does that kind of crap happen when the carbs are out of sync too? I don’t know squat about dual carbs.

Also... Does anyone in the Portland/Salem area know of a shop with anyone who is epecially good with 914s that can diagnose?

Thanks!

yes yes yes Exhaust leak will cause backfire especially with carbs, but usually only when letting off the gas. I would suggest that a backfire at a steady speed if not ignition related (bad condenser/points) would be dirt in the carbs.

[Joe Bob]

Backfire on decel is an exhaust leak....on accel or steady speed, crappy gas, crap IN the gas, fuel filter, ignition issues, bad ground someplece (good luck WITH that), etc....

[degreeoff]

Could it be an air leak @ the manifold (carb side)??

[Coy]

I checked the intake manifolds by spraying starting fluid around the base, it seems good. I have electronic ignition so the points/condenser are out. When electronic ignition fails it just quits running doesn't it?

Fuel sounds like a likely culprit, since Oregon gas quality isn't regulated. It could be that it starves at high RPMs, but it idles okay. I'll have to crawl underneath to change the filter, I haven't done it yet this year. Where does the accelerator pump get it's fuel?

Otherwise a bad ground is always possible. If the ground is loose at the terminal, will it cause the engine to die, or will it just not start? I did have a loose ground wire before and when I fixed that it seemed to fix the intermitten backfire thing (For a while).

See I knew you guys were smarter than me. Thanks!

[gopack]

Backfire on decel is an exhaust leak....on accel or steady speed, crappy gas, crap IN the gas, fuel filter, ignition issues, bad ground someplece (good luck WITH that), etc....

I know less than nothing, but when my teener was driving around, I had the back fire ( more of a pop.pop.pop thing) n decelleration, and I always thought it was a intake leak on a carbed car. if it is an exhaust leak, where would it be? anywhere in teh exhaust, or is it specific to a leak right at the head?

[So.Cal.914]

Backfire on decel is an exhaust leak....on accel or steady speed, crappy gas, crap IN the gas, fuel filter, ignition issues, bad ground someplece (good luck WITH that), etc....

I know less than nothing, but when my teener was driving around, I had the back fire ( more of a pop.pop.pop thing) n decelleration, and I always thought it was a intake leak on a carbed car. if it is an exhaust leak, where would it be? anywhere in teh exhaust, or is it specific to a leak right at the head?

Usually at the head but not always, replace the copper seals and see.

[Joe Bob]

Wet you hand and pass it under the car when running....it helps in finding the draft. It's usually in a joint or a manifold but can be a pin hole, crack or seperation at a weld.

[r_towle]

I keep finding them at the Heat exchanger to muffler connection...and they are really not that significant, but require a yearly tightening of the three bolts on each side to attach the muffler...

[TheCabinetmaker]

I have electronic ignition so the points/condenser are out. When electronic ignition fails it just quits running doesn't it?

No! I had a car in my shop last year that I chased a problem for two months. Car would just die till it cooled off. Finally reinstalled points and problem dissapeared.

[SGB]

I have 2.0 w/ 40 webers and hydro cam too. My car did this too, after years of faithfull service, I developed an intermittant POP at cruising. My first thought was crap in the carb circuits ( epecially the idle volume screw and air bypass screw passages). I would blast out the carbs and it would be better, then return. I pulled my gas tank and found it was full of rust & crud, so boiled it and coated inside with por-15 gas tank liner. I also did a full rebuild of the carbs and found one float level was WAY off. The problem seems to be gone. I really think it was the float level.

[SGB]

Oh- for working on cold concrete, take a corregated cardboard box ( or several) and flatten it out to be insulation. Works great, it is a nice crawling surface, and it is disposable!

[gopack]

Oh- for working on cold concrete, take a corregated cardboard box ( or several) and flatten it out to be insulation. Works great, it is a nice crawling surface, and it is disposable!

You get "COLD" floors there? chilly I would say!

[Joe Ricard]

Well on decell you can also get a back fire if you are too rich on the main jet.

My buddy told me I was throwing foot long fire balls out the pipe when I let off from a WOT run. I leaned it out and it went away.

CHECK YOUR PLUGS FOR RICH / LEAN CONDITION 1ST

This is of course providing everything else is correct Hydraulic valves? If you are absolutely sure then OK but tight valves can make popping sounds

[Coy]

The plugs actually look pretty good. Also the rotor and cap are realtively new. It's not so much a detonation as just a "Pop....Pop Pop.... Pop....Pop Pop Pop...." while I'm driving down the road. Then it might sound kind of loud, but not much power like an air compressor. If I stick my foot in it it clears while the accelerator pumps are squirting, but then it goes flat again when it levels out and starts popping again. Very weird.

As for the cold floor; well, I'm sure it's kind of relative. Cold to me in western Oregon is probably just chilly to you in early February Minnesota. I don't mean to complain... I'm just admitting I'm kind of a fair weather garage guy. I have a motorcycle waiting for 65 degree weather too.

[PinetreePorsche]

Although not initially stated, it's now clear you've got twin carbs. My similar set-up, done by a previous owner, used a round, not hex, bar across linking the two. The clamp on one was too big around, so that even when the bolt was tight, that one was not really gripping the rod very well, letting them get a little out of sinc. That apparently led to backfires, medium ones. Then once it got way out (later diagnosis) and it made a mighty explosion (think of, in Winnie-the-Pooh, when Piglet falls on the balloon, and thinks maybe the whole forest has exploded). I drove home a nervous wreck, thought I'd done some real damage. Found the two throttles, thus accelerator pumps as well, coming up out of sinc, ground the clamp so it came tight, matched them, and never a backfire again.

[Coy]

Actually, I did state in my initial post "It's a 2.0 with dual Webbers and hydraulic lifters."

I filled it up with premium and a bottle of fuel additive, and went out and drove down the highway. It seems to be better. Must have been bad gas. Oh, and the exhaust leak is at the muffler (Thanks MikeZ great idea with the wet hand and all).

Thank you everyone for all of your input. I learned a lot from asking a simple question. You guys are smart.