popping through the carb or back fire, how can a lone driver tell what is what? Options

[VaccaRabite]

So, I got to go on a good 15 mile test drive today.

I am starting to second guess some of my assumtions on tuning. I don't know if the car is popping though the carbs or back firing out the tail pipe.

Driving around in 2nd and 3rd gears with RPMs between 3k and 3500, if I lift off I get little pops. If the RPMs go under 3K and I lift, I occasionally got a BANG that sounded to me like a back fire out the tail pipe.

I am not sure if the little pops are coming out the tail or through the carbs. I thought they were coming through the carbs, but I am second guessing myself now.

I did not damage my air filters, so I don't think that the bangs were out the carbs. But I don't know, for sure.

What I do know is that I have more tuning to do before the car is really streetable, and I am not sure what I am hearing, which is frusterating.

As far as drivability goes, the car was great. No hesitation at all up though 5K. I never really got to drive 4th or 5th, its all backroads with blind curves. Power curve seemed flat all the way up.

Only errors were caused by me. I must have forgotten to tighten the lock nuts on the pass side linkage, as I suddenly found myself at 4K idle. and one side an inch off the stops. Fixed the linkage and tightened down the screws, and that was fixed.

When I got home I realized that I had driven the last 5 miles back home with the parking brake on. (IMG:style_emoticons/default/slap.gif) I might have been a bit nervous when I made my road side repair.

Zach

[degreeoff]

Driving around in 2nd and 3rd gears with RPMs between 3k and 3500, if I lift off I get little pops. If the RPMs go under 3K and I lift, I occasionally got a BANG that sounded to me like a back fire out the tail pipe.

Yes thats rich on the idles..@ 3k idles will do that.

[r_towle]

Your intake charge is to slow.
Smaller venturis (aircooled.net) will help.
You probably have a leaky header still....I know you said it will always leak, but I would find a solution..

Your goal should be to tune the car for DD style driving. That occurs under 4k rpms 95% of the time.
This means you need a faster air charge for the idle circuit to get that mixture in check.
You will play around with three things...once you get the linkage fixed.

Venturis....with your somewhat stock heads and camshaft, you dont need outrageous amounts of air at WOT...so 44mm carbs are to big. You can fix them by getting smaller venturis and choke them down so the move less air, but at a faster velocity...that is key for the 0-4k rpm driving you will be doing.

e-tubes...
The f7 e-tube is a better choice for a smoother transition so you wont feel such a hug flat spot at 3k when you go over to the main circuit.

Idle jets.
You may want to try 50 idles....they are cheap.
Joe likes to ream his own, but for now, I would suggest you get a set of 50 idles, swap those in to see what you really need.
A 2.0 liter motor, 50 idles seem perfect in our part of the country...on our gas...seems like a nice setup.
Rich

[SGB]

Zach- don't second guess yourself too much. I think it may all be at the tail end though- I know I've got a new loose exhaust stud, and I found it when that side started to pop out the exhaust. Did I infer that you already know of a leak there? Thats at least a contributor. Also, are you 100% on the timing? I've had booming backfires before when tweaking several systems and that was b/c timing was wrong- - -

Also- I be thinking the stats from Rich above are spot on what I run.

[VaccaRabite]

Will I damage my engine if I drive the car like this until the proper idles and the new vents come in? Am I making noise and wasting gas, or burning the tops of my pistons off? Driveing this car felt REALLY good.
Zach

[degreeoff]

if you are rich you won't hurt it....might foul a plug or two.

[VaccaRabite]

I do need to go smaller on my idle jets a little I think, as you guys suggested.

I am going to return the 65 idlers that I ordered and buy some 50 idles.

I took the header pipes off the car last night, and gave them a very thick smear of Copper RTV (good to 750 degrees). I do not know how long that fix will last, but it sealed my exhaust leaks at the collector, and at no time during my 30 mile test drive today did I have a backfire.

Not that I have checked yet, but my fuel mileage is probably crappy, as I seemed to burn through gas pretty quick. BUT, I am pretty sure that my gas gauge sender is wonky too and not to be trusted. I put 4.5 gallons in the tank when I filled up today, and the gauge was reading 1/2 full.

Thanks guys. You have helped A LOT in getting my carbs working right. I bet they will be even better when I down the idle jets from 50 to 55.

Zach

[ME733]

(IMG:style_emoticons/default/popcorn[1].gif) Take a ...VERY CLOSE ...look at your spark plugs.(remove them only after the engine has cooled down considerably). the spark plug color, and condition, will tell you most of what you need to know... get from your local autoparts store (at least the pages) a spark plug condition ..chart. I have always used the Champion Spark plug book, which has comprehensive photos, and explanitions of various spark plug conditions. you will probably find it on line somewhere. The big "bang", upon deceleration...is usually caused from an over rich condition.(as already mentioned.)....(and)..BUT can be attributed to misfiring spark plugs also....fuel charge not being lit-off untill the next, firing cylinder, (flame) findes that charge in the exhaust pipe(s). checking the continuity of even brand new spark plugs is worthwile along with all the spark plug wires. (IMG:style_emoticons/default/popcorn[1].gif)

[Root_Werks]

Zach, I wanted to bring this thread back up top. My 914 is doing pretty much the same thing, popping, farting, occasional pop! out the tail pipe.

Running 40IDF's

I believe I am running the following combo:

32mm vents
50 or 55 idles
125's
F11 tubes? Not sure on that one?

I do have #4 and #1 not responding to adjustments so I know I need to take the tops off and do some cleaning.

Hesitates and jerks until you get off the idle circuit or deep enough into the throttle it smooths out.

Running an 050 dizzy.

I get "tweep" up the carbs
"pop" out the tail pipe

Timing is good, compression seemed really good even cold:

1 - 120
2 - 140
3 - 120
4 - 140

I adjusted the valves after that test, found all were set to 6, so I moved the exhuast's out to 8. Haven't re-tested compression, but figure it's not the issue. Warm, I'm sure I'd pull in higher numbers closer together now.

It's probably just the carbs, no plugs look fouled.

Ideas? (IMG:style_emoticons/default/idea.gif) How did you finally get yours resolved?

[VaccaRabite]

Clean out your idle jets. I bet you have something clogging the #4 and #1 jets. You can pull the idles out w/o taking the tops off the carbs.

For my problem, I did two things.

1) I sealed my exhaust better.
2) I built a set of 40IDF carbs to replace the set of 44IDF carbs.

Zach

[Cevan]

I am having a similar problem, but only under certain conditions. If the engine is cold, it starts up fine and I have no popping out the carbs or exhaust under de-acceleration.

If I restart an already warmed up engine, I get this popping for about the next 5 minutes of driving and then it goes away. I'm not sure if it's carbs or exhaust. It isn't very loud and it is not a loud bang.

I'm not sure if this is related but starting the engine after it's been warmed up takes a bit of cranking with the throttle at about 1/2 to WOT. This is in contrast to starting the engine when it's cold where after a quick blip of the throttle it fires up instantly.

I'm sizing up my idle jets based on a stumble below 3000 rpms. I guess this just takes some experimentation.

[Root_Werks]

Thanks guys.

Yeah, I'm betting it's just clogged idles on the #1 and #4.

Tweeping is lean up through the carbs.

Popping is rich through the exhaust.

Carbs were put on in 2006 and the 914 has mostly sat since that point I believe. I gotta pull off the fuel filter and see what's inside the tank. Could be some crud in there getting by clogging up the carbs?

I'll do some cleaning today and report back.

[charliew]

When you try to read plugs, you have to cut the motor And push the clutch in at the rpm you want to read the plug at. Driving home and then looking at plugs doesn't tell you how the mixture is at wot or part throttle. When a plug is partially fouled it will cause rough running. Over rich in the exhaust can be smelled usually at the exhaust tip if it is at idle. Too rich at idle will dilute the oil real quick.

[rhodyguy]

dan, seeing as how your car has sat for a bit, inspect the tiny o-rings that seal the idle jet holder assem and the idle-air-mixture screws. they need to be in excellent condition.

for dealing with jets and whatnot, the cans of compressed air are great for cleaning circuits and jets. for me, a compressor and air nozzle is a bit of overkill for this task. remove the idle jet and the main stack for one venturi, stick the plastic straw in the idle jet port, cover the top of the main stack area with a paper towel and give the air a few blasts. beware...the fuel will come flying out the top.

a cyl that is unresponsive to closing the idle air screw would be the place to start. if you close a needle screw there should be a pronounced change in the idle after a few bit. remember to wait a for up to 30 seconds even after EVERY adj, allowing the adj to take effect.

[Dr Evil]

Zach, the stuff I used on my corvair headers seals very well. It is like a spackle kind of compound that comes in a tube. You can get it from the NAPA down the street from you in a tube. It seals all exhaust leaks at joints and such.

[Root_Werks]

That's a good tip Kevin, thanks. I'll grab a can of air from the local store on my way home. See if I can clean these out before I head down to Garret's tonight.

[Gint]

I do need to go smaller on my idle jets a little I think, as you guys suggested.

I am going to return the 65 idlers that I ordered and buy some 50 idles.

65 idles are too big for a stock 2.0 or even a mild 2056 if you're running 44IDF's. I'd get a set of 50's and a set of 55's and try them out.

And little carb backfires won't hurt your filters or air cleaners at all, so unless you're getting big ass backfires from the intakes, you can stop second guessing that.

Edit: This is last year's thread. I should have red the whole thing before I replied.

[Gint]

Carbs were put on in 2006 and the 914 has mostly sat since that point I believe. I gotta pull off the fuel filter and see what's inside the tank. Could be some crud in there getting by clogging up the carbs?

IDF's don't like to work well after sitting around for a year or two or more. Brant taught me that one and ove rthe years I've learned that he was absolutely correct.

You definitely need to clean out the idles. I'd get a couple of kits and rebuild 'em.

[Root_Werks]

Thanks for the tips Gint.

I've cleaned out idles many times before on many carb'd 914's and 914-6's. I took Kevin's tip and used a little bit of canned air which worked great. Engine was purring like a kitten again, smooth idle, very snappy response. Cool, did a little adjusting and off we went. About 1/2 a mile later, popping, tweeping, popping which put it back to where it was before I blew out the idles.

Gotta be crud in the tank, filter clogged or even torn through and letting more crud by.

Going to empty the tank, pull it, clean out everything, carbs as well and get a good, clean base-line to start from.

The car sat more than it was driven for the last 4 years that I can tell. I'm sure there is gunk in the tank.

I brought this thread back becuase I try to search for threads to answer questions I have and then keep updating existing ones. This one is a good, search friendly thread that popped up right away with good information. Hopefully, it has more good information now. (IMG:style_emoticons/default/biggrin.gif)

[rhodyguy]

what's odd is that you're fouling in the circuits on the left rear and right front venturis/idle circuits. with the jets cleaned and a good idle, measure the flow on each venturi. use one idle adj screw to pull the idle up just enough to smooth out the pulsing.