carbed 308 Backfire?

I usually don't let the small "pop" from the exhuast get to me, unless I'm parking in a very public place with onlookers. Here's the solution:

Pull into wherever you're going to park, stop, leave the car in 1st with your foot on the clutch, pull the parking brake. Sloooowly start to take your foot off the clutch. As you feel it begin to catch, shut 'er down and smoothly release the clutch the rest of the way.

That should take care of it.

 

Keith,
Just curious, how excellent is your fuel mileage? I average 10 MPG around town, maybe 15 MPG highway..



Greg

1977 308 GTB

 

This is unburned fuel/air igniting in the exhaust. The fuel comes from the naturally rich mixture that these cars seem to like. The air could be from an exhaust leak or from the air pump.

Leaning out the idle mixture can help (car may not like it).

Reducing idle speed will help (car may not stay running).

Eliminating air pump system and blocking all air sources to exhaust can help (local government may not like it).

 

You can treat the symptoms two ways:

As mentioned, put the car in 1st, foot on brake and turn off the car, just before the engine stops release the clutch.

Or, just sit at idle 30 seconds, shut off car as usual. You may find the exhaust has cooled enough that it won't detonate.

But this is treating the symptoms. How about fixing the problem! First step is to check the spark plugs. Running lean = running hot. Are the spark plugs white? Just a tweak richer will eliminate the pop. My car damn near exploded on shutdown after I got a new SS exhaust and went too lean. I have never heard of the pop from being too rich but I'm just a shade tree mechanic. Since yours is a pop, I bet a tiny bit richer on the appropriate carb will not affect mileage but will eliminate the pop.

If that doesn't work, try changing the spark plugs to a hotter plug. It may give better burn and have less unburnt gas in the tailpipe.

Are the plugs gapped right? Try a wider gap than you're using.

How are your coils? Old and weak? Try changing to a new modern high output coil.

Just a few thoughts...

Ken

 

That's it as Ken says, let it idle awhile, cooling down the redhot exhaust...

REALLY avoid the 'cool guy revving the throttle a few times for the neighbors" syndrome........;D.

That'll charge the exhaust up, every time..if it gets bad you may need to put it on a gas analyzer and see what you got.....also lube the distibutors per the schedule, they can stick and cause heartache.....

 

Wow, thats a new "low" for mileage...is this normal? Might things need a tune up here or not? Who told me to expect 15-18city and 18-20 hwy??? C'mon......tell me the truth, now.....

Joe (on break)

 

No way 14 - 16MPG "Highway" per the books.......you haven't seen Greg328 drive, he complains about flat spots in the throttle during high speed cornering!

"You could try LIFTING OFF!"

That's not putting around town like ol' BigTex does...but I have a lengthy criminal record....

Regular is fine we have very low compression.....8:0 to 1 on a good day.....

Lubing the distribs is best left to the experts IMO, and I only mention it as being horizontal in our application is probably not optimal and as they are mechanical a "sticky" one can cause all sorts of devilment.....obviously that would throw the spark into the wrong point of the engine cycle.....

 

How fast is your idle? Setting mine at 900 stopped the little pop. Quick and easy fix.

 

Im not clear...so, what should you get hwy, maybe we should forget the books, as we know that is always ideal EPA, No? Also, lets asume normal car driving, not reving it high and all.

And if you say you're a criminal , how does that make you an expert (chuckle)?!

Joe (on break again, LOL)

 

It makes me an expert in "lifting off" of the throttle a little bit!

You really can drive these cars putting around but where's the fun it that? It all too easy to stay on it listening to the engine reach 7,800RPM...that's where the "Ferrari wail" lives...

At which point you have eight velocity stacks dumping for all they are worth..

That's when you'll drop to the lower numbers...I didn't calculate it but I felt I was getting pretty good range per tank with your car.....

 

Remember our books were published in 1976, so EPA methods were different then IIRC......

This was Ferrari's specification, not some real world info courtesy of our Federal Government, so consider it accordingly.......I'll look in the book and be sure I am quoting it correctly.

 

I need to figure out why that happens. Feels like it's just gravity, but I wonder if my carbs aren't adjusted right. The power is smooth and steady in a straight line....

Yep, I get 10/15 MPG--sucks...

Greg

 

OK....confirmed that mileage is 10-15mpg, and not the environmentally friendly range of 15-19 mpg which would go a long way to placate the "green" people based in Stockholm. That does it. Im looking for a high mileage ferrari....probably real expensive though......

 

I run 93 in my '78 308. When I shut it off a second or two later there is a *very* slight wumphf. It's kind of funny, doesn't happen every time and is nowhere near a backfire. It's almost like a <sigh> from the engine as it gives up the ghost at the end of another great run through the wind.

As for lubing the distributors... oh boy. I just did a s h i t e load of work on my car. I really never intended to attack the distributors but in doing the valve adjustment, belts, and cam seals it became inevitable. Turns out last summer's miles were ignorant bliss .

First, all the points were basically damn near closed. No wonder I couldn't rev over 4K rpm. Plus, I still had the R2 setup in there. After putting in new points and condensors I used a degree wheel to help set the points gap. When timing it once set at 5000rpm I found the marks would not line up at idle. The Rube Goldberg centrifugal advance probably needs cleaning, re-lubing, and adjustment. But, the car now actually runs fairly well (actually many times better than when I bought it), I can run it up to 5 to 6rpm easy, and it idles right good. The distributor cams need to be lubed as part of a regular service. The other lube job is the centrifugal advance mechanism.

Seamus

P.S. Do some more searching here. I think the backfires are claimed to be due to idle being set high as a cheap way to fix a rough running car that was running rough in the first place due to another problem (too lean, air/fuel mixture wrong, carbs not synced, etc.).

 

I have a 78 GTS. When the engine is warm and the fast idle cam has let go to bring the RPM down to about 1000, and you shut her off, you will get the "whumf" that was mentioned above. When she's cold and you turn her off, she will backfire like gunshot and people leap for the bushes. It is about a 2-3 second delay. Mileage somewhere around 12-14 and runs very smooth.

-Rjay

 

Yup! That what my car used to do. That can be delt with by following the afore mentioned fixes. Don't get resigned that you have to live with it!

Ken

 

I agree here as my 79 308 gives that "whumf" a second or two after shut off. I asked a Ferrari mechanic and he said that was just excess gas and typical. Reducing the idle a little will help...idle at 900-1000 rpm's. Average mileage with my foot out of it...in and out of town around 15.

 

Neither of my 77's have ever backfired, or did it on shutdown. But neither car has the air injection, and all exhaust plumbing is sound with NO leaks. Leaks always cause backfires because air is actually sucked in as the exhaust pulse turns into a super high vacuum. The first 308 had a blown piston and big fat P6 cams with rich carbs and big reversion clouds hanging over the airhorns, so if that beast wouldnt backfire, our leaned out stockers shouldnt even think of doing it. Just like 2NA said, its airleaks in the exhaust.

 

Oh yeah. mileage with the stocker has been "averaging" between 10-12, and 18-22 MPG.

 

Yes, I have the air injection with the pump removed so these little injectors are probably open somehow. Also, the exhaust is original and while I cannot see corrosion or damage, there may be a hole somewhere. I have elected to keep the car as original as possible. The emissions gear is important given that it is the seller's responsibility to pass in Colorado and I live in a very green area called Boulder. The bang is at least, manageable. The only time it is a problem is when I start her up and need gas right away. I have had to do a few loops before getting to the gas station. The "whumf" is so minor and not really noticable except to the trained ear. The bang, however, really freaks people out and is a bit embarrassing given ...it IS a Ferrari.

-Rjay

 

I only run premium fuel with the highest octane I can find. I doubt the car will run on anything less.

I've gotten a pop from the exhaust ocasionally before when the carbs were way out of adjustment. They're pretty close to being right on so I don't get a pop anymore though. A high idle will give the idle a nice pop too. I need to lower my idle but having to pull the bonnet just to do that is a pain. I'm guessing that the individual aircleaners are in my future. Then I can finally get things right no matter the altitude.