Backfiring

Does the car have an air pump?

 

Any chance you are slightly overfilled with oil? Not sure why that would cause such a problem, but it did for me many years ago (with stock exhaust). Corrected the level and no more backfiring. Just trying to help.

 

My L series does exactly the same thing (original ANSA exhaust). I assume the carbs are out of adjustment, though it runs/starts perfectly otherwise. If anyone has any further thoughts on this, I'd love to hear them!

 

Two things will lead to this symptom.

1) Timing off. Unlikely as the car is running well.

2) At least one carb is slightly lean. Easy to check, albeit with a bit of work. One approach is to inspect sparkplugs. The lean culprit will be gray or white, whereas the others will be dark/black. The other approach is to systematically turn the mixture screw on each carb 1/4 turn out (counterclockwise). If this solves the issue, then one-by-one turn them back in 1/4 turn until you find the one that starts popping.

I prefer to set the carbs to gently "burble", that is, slightly backfire on a trailing throttle. This tells me that they are set correctly. At idle, one can turn the mixture screw of a single carb in or out to achieve the optimum mixture by listening for engine RPM. When too rich or too lean the revs at idle will drop a little. This assumes, of course, that the linkage and airflow are equal. At optimum mixture the engine will pop on a trailing throttle.

You are likely hearing more of this now that you have changed the muffler.

Jim S.

 

Jim S. is right on. A third cause can be play in the throttle mechanism. I had some of this early on and found that providing a positive idle stop cured things. The standard procedure is to let one carb be the idle stop during tuneup. Any slop in the linkage to that carb will be transfered to the linkage as the linkage is pulled back by the big spring. The other 2 carbs will be spring loaded by their internal springs to follow the linkage. If all carbs were synched at 1500 RPM with a little throttle and then let back to the idle stop on one, the other two close more.
See my Dino Saga 050918. The main idle stop takes the force of the big spring and the individual carbs are equally spring loaded in the linkage. Once synched, allways synched. Only pops downhill with a trailing throttle. I set idle at 1,000-1,100.
John

 

Dan.

I put a stebro on my Dino about three years ago. Careful tuning and some re-jetting on the webers completely solved the problem.

I'll dig out the old order for the parts and get the details for you. Took the mechainc a couple hours of fussing, but we got rid of all the backfiring and bubbling on decel.

OF course, I changed the exhaust this spring, and now have the problem all over again as the new exhaust is not quite as free flowing as the old one.

AHHH the joys of old car ownership!!!

DM

 

John- Your next-to-last sentence implies that some popping is normal (!). Is it? Mine also mainly "pops" on "trailing throttle" (if I understand that term correctly)- how much of a "pop" is tolerable or normal? Maybe I have no problem if all Dinos do this.

 

i live on a steep hill, my 73 dino also backfires and pops when i go down. car runs and starts fine otherwise. has 65000 mi. dan z.

 

Robert,
Don't know if my Dino is typical but has always popped when using engine for braking with closed throttle. RPMs up above 3000. Like on a severe hill in mountain country. Even belches a flame now and then. Followers get a kick. I think it is because I have my idle set a little high. No harm and a little entertainment. No problem at all around town at lower revs.
John

 

Robert, your reading of John's next-to-last sentence is the same as my interpretation and my experience. I believe that the "correctly" tuned Dino will "burble" on a trailing throttle. The choice of muffler will dictate whether it is a burble or a pop. This is a characteristic of the Weber. My C4 does exactly the same, as does my Alpine Renault...all with Webers and all perfectly (my biased opinion) tuned. I have 28 cylinders fed by Webers, hence I have come to know them well. If your car does not burble, then, perhaps, it is running a bit too rich.

Jim S.

 

In the 25 years I have owned my Dino it has always popped through the exhaust. It runs a standard ANSA. Just accept its normal, and just one of those neat noises special to these neat wee cars. These things are what makes them so special eh??

 

YES!

 

One other thought on backfiring. If it is beyond what is considered reasonable, or cannot be corrected by proper adjustment of the idle mixture, then perhaps one of the idle jets is occluded with debris. Easy to blow out, but something to consider.

Jim S.

 

The new exhaust probably has lower back pressure and just calls for re-tuning of the carbs.
No big problem, no conspiracy - just the normal effects of changes in the system.

 

I was able to stop nearly all the "popping" on my 206 by spending a huge amount of time on the carbs, tiny barely noticeable adjustment and test drive, and adjust/drive and ... and ... took a week but I got it with almost no popping at all. Lasted about 2 weeks before it started again, so its all in the adjustments but it takes so little to cause it that I don't believe anymore that it can ever be completely eliminated. I have mine to a point where it barely pops and live with it.

It never backfired however.

 

Umm... having a well-tuned burble on trailing throttle is ideal. Jim S. was 100% correct. Additionaly, IMHO a burble and the occasional crest (actual term for when you have maximum burble and therefore what may seem as abnormal popping or 'backfire') are what really add to the beauty and sophistication of your engines... I wish my Colombo V12 would be so forthcoming...

Lucky you all.

Best,
M

 

It can also happen from the small amounts of unburned fuel (very normal) that are pulled into the hot exhaust system with the throttle off and the enginge in gear decellerating. That fuel will ignite in the pipes if there is air present. Even the tinest of leaks exhaust system leaks are sometimes enough to generate this effect. Coupled with a very rich idle mixture....

 

OK, here's what we put into my carbs that worked very well with the Stebro Sport Exhaust

I am gonna list part no's, and any other details I can remember

Carbs Unlimited
PArt # 76215.045 Pump Jets
Part # 64290.014 Pump Demand Valve 1.00mm (supercedes 79706.100)
Part # 72129.320 32 mm Venturi

www.carburetion.com

I'm gonna dig around in the spare parts. I think I bought extras, so I should have more details scribbled on the parts bags.

with the stebro exhaust and these jets, I got VERY little popping. Almost none, in fact. Just enough of a burble on decel to be satisfying.

New exhaust is MUCH burblier (hows that for an adjective) and will need re-jetting again, I'm sure, to solve the problem as adjustments didn't get it there. No backfiring, but way too much burble for me.

Dave

 

This subject seems to come up periodically and I always follow the threads with great interest as I've also wondered how much "burble/backfire" is "normal". I don't think a continuous loud "backfire" is normal with off-throttle; a mid-to-low-tone rumble with a some "burbling" is normal.

The reason I say that is from my experience replacing the Ansa with a new Stebro exhaust syatem. My car backfired quite loudly with the Ansa, eventhough I'd rebuilt the carbs and timed/tuned everything to the nth degree. I even machined new parts for the throttle linkage to eliminate any slop. During the exhaust replacement, I removed the last remnants of the airpump system and plugged all extraneous sources of air infiltration to the inlet and exhaust manifolds. Carbs are a lot easier to balance and adjust if the only air to them is through the venturis. The new Stebro sounded great - no loud backfires; just a nice rumble/burble with off-throttle.

After a couple years I noticed the exhaust rumble/burble was getting louder with more backfiring. I rechecked the carb settings several times with no improvement. Thinking it could be air leaks, I checked the exhaust/manifold connections. Bingo! All the manifold nuts were about 1 turn loose. After about 4 hrs and several bloodied knuckles I got the nuts tightened again. The exhaust note quited noticeably, back to the "normal" rumble/burble when Stebro was first installed.

That's my experience - hope it'll help some of you!

Bill