'74 246 GTS - backfire issue

Hi,

Read dino246blog - excellent source for restoration especially the carb balancing and tuning. I followed that and almost all issues are now gone but I’m still running engine in, so I expect some still.

Other culprits might be:
1. You have one or more of the air caps in the inlet manifold loose,
2. Your Idle jets are slightly blocked.
3. The idle is incorrectly adjusted
4. You have the wrong idle jests in one or more cylinders.
5. The carb’s are not balanced.
6. Wrong spark plugs
7. Fuel quality issues
8. You have bent inlet valves on at least 1 cylinder (worst case) - a PPI would have identified this anyway so I assume you don’t have this issue.

Don’t do anything till carbs have been fully tuned/balanced and it’s been on a gas analyser. Other members also recommended a rolling road to me to check the inlet jets were large enough for the quality of fuel I was using. You can tune the carb’s yourself, it’s easy.

What altitude are you driving at?

Good luck
Steve


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9. Timing?
10. Throttle Linkage joints worn, giving uneven idle?
11. Check colour of spark plugs (light brown OK, black = rich, oily black = not firing) My car hates NGK iridium plugs, others swear by them.?
12. Leakage at the ex. manifold gasket.
13. Air bypass screws incorrectly set.
14. Faulty accelerator pump circuit & or vacuum leak. (backfire during deceleration)

 

and timing otherwise you are wasting your time John

air bypass screws 1 turn out from closed, that way you have scope to adjust (+/-) and balance out the flow through each barrel.

 

Congrats again John. I’ve got a Stebro exhaust on my car which enhances the pops on deceleration. My friend Glen has the same on his. Both of our exhausts sound very “active”.

After reading here and talking with others, there seems to be debate on what’s “normal” on this subject.

My car idles well and not being able to hear and feel your car makes any assessment as to what is , normal, difficult from afar.

I do know that these old carb cars take a bit more revving from standstill to keep from bogging down when taking off. I rev mine to about 3k before letting out the clutch. I initially that the idle was set too low and causing this. It’s normal for a carb car.

I also get the occasional backfire particularly when shutting the car off when hot. It’s just unburnt fuel hitting the hot exhaust. No biggie.

 

Assuming the carbs are well adjusted and jetted and the ignition system works properly, a Dino will idle perfectly smooth and not stumble while accelerating from idle to redline in every gear. If your car seems to be running better one thing it could be are the spark plugs having cleaned themselves out from extended periods of being fouled up. I run NGK BPR7EIX plugs with great success. They foul less than the non Iridium ones and make for easy starts every time.

 

My first Dino had an issue with back firing cleaning the idle jets fixed it up, make sure you have the two filters in place the first one where it enters into the engine bay and the other one that sits on top of the carbies I have seen a few with no filters at all.

 

Neelfryer.
Some years ago, my Dino had an ongoing subtle backfire issue. Wasn't particularly bad (indeed sounded nice), but anomalous compared to other Dino's. The popping occurring under most driving conditions, hot and cold, however, mostly when warm and on throttle lift off. Carbs, fuel system had been stripped, cleaned and checked, but popping continued. When removing the engine (for other work), I noticed a small hole (~2mm diameter in the exhaust manifold. See white splatter mark downwards from the yoke below. This was a www.superperfomance.co.uk stainless manifold purchased around 2010. Exhaust is good, except for this small imperfection in the welding. Turned out to be complicated to get to for repair. I had to cut all the way through the yoke with fine hack saw, stainless weld the hole (which was more like a gap in the seam welding at the pipe junction), then weld up the cut, grind back and re-assemble. Solved the popping noises immediately. Been fine for the ~7 years since. So a subtle small hole in the exhaust system can be a cause. If you have an ongoing problem, may be worth a close look.
Kevin

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I think this is a very likely cause too.

 

Great to here John. It’s amazing what happens when these cars actually get driven. Italian tune up is real. I fought mine at stop lights as you describe. I believe changing the fuel filter cured my issue.

 

John,

I had mine about nine months before I learned the starting procedure I described to you. Prior to that, it was the extended cranking method. It seemed to go one forever sometimes and what was happening was simply taking time to get gas pumped up into the system. That’s why running the pumps for about 40 seconds prior to cranking will save your starter from extra wear. It’s a delicate touch that involves priming the pumps, feathering the gas, and cranking. Hope that helps.

Skipp

 

Change your fuel filter soon. Mine didn’t look too bad when I cracked it open (the simple in-line type), but changing it made my low idle at stop lights go away.

 

FYI. If you ever need to replace your alternator, the Superformance version doesn’t fit the US cars and they don’t seem to be aware of this. Get your original one rebuilt for cheap locally. And get the drip guard installed for it.

 

On New Year's Day this year I went for a drive with a group and we had a stop in this remote place with no cell reception. A girl came to me and asked for a ride back for help because her Dad's Porsche would not start. Before giving up I went and had a look. The car had spark and fuel but turned over totally dead. When I asked her boyfriend to crank the car I saw he was pushing on the gas 4 or 5 times before each try. At that stage I told him to let me work the throttle while he cranked and I would tell him when to stop. I put the throttle on 100% and the engine cranked over a few times and then roared to life. I had to be quick on the throttle but had it idling perfect in only a few seconds.

I quite often see people struggle to start carburetted cars by constantly using the same 'sure fire' sequence someone told them about when in reality different situations call for different approaches. Understanding what is happening during cranking and pumping the gas goes a long way to choosing the correct approach for starting the car. I learned these lessons the hard way in years of racing direct drive 100cc karts. These karts required a push start and were hyper sensitive to carb tuning. Running at some 18,000rpm these engines were being tuned constantly while driving and ran on the ragged edge of seizing all the time. It was very much a trial by fire with lots of blown engines as part of the education.

Let's start with the basics:

1. If your car has been sitting a while you must run the fuel pumps to fill the carb bodies properly. Note that you cannot over-fill the carbs. When they reach the correct level the extra fuel will merely flow out an overflow and back into the tank. Running the fuel pumps for 10 seconds or so will achieve this.

2. Most people recommend at this stage to pump the gas one or two times, hold the car at half throttle, and then crank. In general this is good advice but it is important to understand WHY this is good advice and how it can be bad advice in different conditions.

This is where understanding how the accelerator pumps work and affect things is critical. Normally the carbs operate by atomizing a mixture of fuel (metered by the jets) and air (metered by the throttle position) that is then fed to the engine, combusted , and then creating power, rpms, etc. When you are driving along and put your foot hard on the accelerator, the throttle plates will open and rush the engine with AIR at a rate much faster than what the jets can immediately keep up with. In the absence of an accelerator pump what happens is that the fuel takes very obvious amount of time to catch up to the higher air flow so there is a lag between putting your foot on the accelerator and the engine speeding up. Some old cars like my 1964 FIAT 500 do not have an accelerator pump and you are VERY aware of its absence.

So what is an accelerator pump? Quite simply it is a direct jet of liquid fuel that is pumped into the engine when you press the accelerator to speed up the combustion process. Think of a camp fire that you throw some gasoline on. You will get a big and immediate flare of fire. If you look down the throats of the carburettors and press on the accelerator you will see these jets squirt raw fuel right in. This squirt makes up for the lack of gasoline flowing through the jets and accelerates the engine as the carb jets catch up and do their job.

Here is a quick vid I found showing the accelerator pumps in action:



Getting back to our Dino engine here is why the common advice of 2 pumps and 1/2 throttle make sense for a cold engine:

- Because the whole system is dry and the engine will be turned relatively slowly by the starter motor, the two squirts of gas are a good extra jump start for the ignition. Think about again a camp fire and using some gas to get the fire started.

- The 1/2 throttle works because it gives a little more air to balance out this extra fuel and help combustion. In addition, the extra airflow during cranking helps the idle jets to start to deliver atomized fuel.

3. (back to numbering the basics) If you cranked and the engine did nothing then one more round of 2 pumps should be ok as the engine may still be too dry. Once the engine has started or kicked over here is where most people get it wrong. A short start or light fire is the sign of an engine that is just ready to fire and only cranking or some part throttle is necessary. Pumping the gas again will put too much raw fuel in and will wet the spark plugs causing them to foul and not be able to create a spark. It is here were people enter a viscous cycle of pumping and cranking only making the problem worse. Smelling fuel is the tell tale of a flooded engine and requires a different approach to fire. Flooded engines can crank for days and not fire so you need to get the plugs dry.

4. Waiting for the plugs to dry is one way but this can take a lot of time. Another highly effective method is to put your foot flat on the accelerator, wait 10 seconds or so (for most of the accelerator pump squirt to evaporate) and then crank the engine. The accelerator pump only gives one squirt per application so if you keep your foot hard and flat on the pedal, after the first squirt all you are pumping is lots of air that does a great job of drying out the plugs fast. Once the plugs dry enough the residual fuel on them will ignite and then the engine will start kick and give signs of firing. Depending on the situation you can keep your foot on the throttle till the engine really starts (but be ready to take it off fast so you don't race the engine when it catches) or a light feather on the throttle as the engine kicks will be necessary to get it to fire. Here is where feel and experience comes in but these techniques are not car specific and can be applied to any carburetted car.

Hopefully this post will help broaden your understanding of what is happening during all that cranking and pumping of the accelerator. Knowing what does what goes a long way to consistently and reliably being able to start one of these cars.

 

Rob, thanks for taking the time for that excellent tutorial. I learned a lot and I’m sure John will to.

Skipp