Snap Crackle Pop Go The Carbs

[North Coast Cab]

Ok, I've had my ups and downs on my 3.0 race motor. It's a suped up 3.0 with 46 PMO's. Anyway, it's running a little rich, which isn't a major problem, but it snaps, crackles and pops like crazy at idle and at higher RPM's on warm up. I've got an aggressice cam, Webcam 120/104, but I don't think that's the only issue. What can I do?

John

[EarlyPorsche]

Usually popping in the carburator means you are running lean. However this doesn't always mean that the setting on the carburator is bad. It could be a bad gasket under the intake or under the carburator. It could also be a hair line crack in the intake manifold. If you have used an exhaust gas analyzer and know the carburators are set correctly then you need to disassemble the setup and verify no leaks and reassemble with new gaskets.

[svandamme]

are you sure it's running rich???

crackles and pops , i usually heard in my intake when my mfi was running to lean. running rich usually was less noisy,
apart from the ocassional exhaust bang on decelleration, it was mostly poor performance that resulted from it... (2.4S)

[911quest]

Most carbs on race motors will pop on warm-up it's pretty normal does it run fine after it warms up?

[North Coast Cab]

I have an LM-1 mounted in the car and it's running a little rich. It has in the past, I have inly run the car four times, run well under load, but at times it pops much more than others. I'll check for an air leak somewhere.

Thanks, John

[svandamme]

lm1 in the muffler, or o2 bung in each side of the exhaust?
could be that one bank is just a little bit leaner then the other

does it pop in both banks ??
could also be just one or 2 cyllinders... but hat's near impossible to tell with lm1 , or you'de need 6 bungs , not practical

you could try spraying some carb cleaner in the individual velocity stacks one at a time to see if it goes away... but carefull with that...

[BK911]

I haven't worked with carbs in awhile, but mine used to pop if they weren't correlated properly.

[jpnovak]

Which cylinders are popping? All of them? Only one bank? individual within one bank?

If all of them, then your not jetted properly

If one bank consistantly the linkage is not sync'd.

If one cylinder check for improper air bleed or clogged jets.

If it runs well under load and acceleration that is due to the accelerator pumps overcoming a lean adjustment. My guess is that it is linkage related. I usually disconnect both sides and balance the carbs at idle. Then connect them both up and use the driver's side idle screw to drive the speed to about 2-2.5K. Then use the linkage to balance the two sides. Now back down the idle screw until the pass side just touches and slight slack occurs in the drop link. This method should give you a balanced idle and balanced off-idle.

[JP911]

When you say it pops and crackles on warmup, do you just mean during the first 30 seconds or so after you've started the car? I think that's pretty normal for carbs.

[len911]

On my 46IDA, 3.2 weber engine, the engine will snap-crackle& pop for the 1st 3 to 5 minutes after starting. After that, they run smooth as silk. You have to remember that there is a lot of unatomized fuel going into cold manifolds and turning into liquid. Once things warm up, atomized fuel gets to the right places and should smooth things out. I you are popping after a warmed engine, then things are not set up properly. Lou

[North Coast Cab]

Thanks all. I will check all of my linkage adjustment tomorrow and see if that helps.

[Green 912]

All good advice. One thing to think about if the other items all check out. If you have an aggressive cam and venturies (sp?) that are on the large end of the size range you might not have enough air flow at idle or low rpm to draw the right amount of fuel. A good hot set of cams can produce a low rpm reversion pulse that floats almost above the stacks. If it stops when the motor warms up you might want to live with it as to adjust for it might run the mix too rich for warm running at all but WOT. reducing the venturie (sp again) size is also an option if this is the case. you might or might not loose some top end depending on the setup and sizing but gain low end response.

[shbop]

What's our LM-1 telling you? Where's the O2 sensor(s)?

[North Coast Cab]

Lm1 is on he drivers side mid-pipe. I have moved it from side to side and seen the same results. The popping and cracking comes from both sides. Here's the carb configuration:
Carb Size 46
AV's 4.5
Venturis 38
Main Jets 150
Idle Jets 55
Idle Airs 130
Airs 180
Emulsion Tubes F11
FBVCV's 50

I will adjust the linkage and idle tonight along with making sure all of the manifold bolts are tight and there are no air leaks.

[tom1394racing]

I've had this problem on and off with my RSR spec 3.0. This engine is a twin plug with DC-60 cams and 46mm PMO's.

I had been running the car without a second fuel filter downstream of the chassis tube. Everything was fine until we ran it for two days straight at Watkins Glen on Race gas. The car was fine on the track, but when I got home and starting driving it on the street on part throttle...snap, crackle and pop.

Removing the idle jets and blowing the passages out with carb cleaner and low pressure air seemed to fix things. I'm guessing the race gas acted as enough of a solvent to dissolve some crud in the chassis tube and gum up the idle jets. You'd think I would have installed a second set of filters in the engine compartment..but i did not at that point.

Next track event at Lime Rock with race gas and the same problem on the street with part throttle afterwards. This time I installed a second set of filters at the inlet to both carbs.

Seemed to have solved the problem