308 strange misfire
Discussion
HI all, I have a Euro Carbed Dry Sump 308 that has the single distributor ignition set up.
When I go out for a drive all seems well until I'm about 45 minutes in (its well warmed up by then). I then get a weird rough running as if its not running on all cylinders and sometimes a backfire that sounds like a wheezy turbo wastegate! Also if I stop at lights it is then reluctant to pull away and struggles under load to go over 2000 revs so I have to dump the clutch and get it revving again. The rev counter (tach) jumps about too, often going to 9000 revs suggesting its double reading.
I have changed coils to new ones then back to older ones that fit in the heatsink better, changed distributor cap, all the little distributor leads and rotor arm and also gone between silicone and copper HT leads.
Any thoughts as this is doing my head in!
When I go out for a drive all seems well until I'm about 45 minutes in (its well warmed up by then). I then get a weird rough running as if its not running on all cylinders and sometimes a backfire that sounds like a wheezy turbo wastegate! Also if I stop at lights it is then reluctant to pull away and struggles under load to go over 2000 revs so I have to dump the clutch and get it revving again. The rev counter (tach) jumps about too, often going to 9000 revs suggesting its double reading.
I have changed coils to new ones then back to older ones that fit in the heatsink better, changed distributor cap, all the little distributor leads and rotor arm and also gone between silicone and copper HT leads.
Any thoughts as this is doing my head in!
Take the carb filter off and have someone look down the barrel while you go through the rev range. If you see the fuel igniting in the carb then it could be an inlet valve problem. It could just be something simple like valve clearances - have you checked them?
If it’s not that, then it could be either the springs or the valve - but you’d be best taking it to a garage to check that.
Worst case would be a cam problem - worn lobe or maybe a valve seat.
You should be able to check and adjust the valve clearances yourself.
It’s possible it could be fuel mixture but that doesn’t really fit the problems you describe. I think if it was a mixture problem it would show up before the engine warms. To me, the misfire when warm sounds like a valve problem somewhere. Probably not expensive to fix if that’s the case.
I’m not an expert, although I have built custom engines, but I would start there.
If it’s not that, then it could be either the springs or the valve - but you’d be best taking it to a garage to check that.
Worst case would be a cam problem - worn lobe or maybe a valve seat.
You should be able to check and adjust the valve clearances yourself.
It’s possible it could be fuel mixture but that doesn’t really fit the problems you describe. I think if it was a mixture problem it would show up before the engine warms. To me, the misfire when warm sounds like a valve problem somewhere. Probably not expensive to fix if that’s the case.
I’m not an expert, although I have built custom engines, but I would start there.
AndrewE said:
The whole engine has been freshly rebuilt (by a renowned specialist) so I hope it is none of what you suggest but thanks I will ask them
Ah…new info…!So I would say even more likely it’s valve adjustment then. But diagnosing via the internet is going to be tough….
Strange that if it’s just been rebuilt that you then changed all of the ignition system ?
Anyway, FWIW, my bet is valve clearance. Backfires are generally due to the exhaust valve not being fully closed. And if it’s happening when the engine warms then the valve clearances will be changing.
andyr said:
Take the carb filter off and have someone look down the barrel while you go through the rev range. If you see the fuel igniting in the carb then it could be an inlet valve problem. It could just be something simple like valve clearances - have you checked them?
If it’s not that, then it could be either the springs or the valve - but you’d be best taking it to a garage to check that.
Worst case would be a cam problem - worn lobe or maybe a valve seat.
You should be able to check and adjust the valve clearances yourself.
It’s possible it could be fuel mixture but that doesn’t really fit the problems you describe. I think if it was a mixture problem it would show up before the engine warms. To me, the misfire when warm sounds like a valve problem somewhere. Probably not expensive to fix if that’s the case.
I’m not an expert, although I have built custom engines, but I would start there.
would any of that be happening at all times rather than when warm after 45 minutes of driving?If it’s not that, then it could be either the springs or the valve - but you’d be best taking it to a garage to check that.
Worst case would be a cam problem - worn lobe or maybe a valve seat.
You should be able to check and adjust the valve clearances yourself.
It’s possible it could be fuel mixture but that doesn’t really fit the problems you describe. I think if it was a mixture problem it would show up before the engine warms. To me, the misfire when warm sounds like a valve problem somewhere. Probably not expensive to fix if that’s the case.
I’m not an expert, although I have built custom engines, but I would start there.
How is the ignition firing triggered - is it points based or an electronic ignition module?
It could be a problem with detecting the position of the distributor rotor. If there's some form of electronic pickup coil this may be breaking down when hot/over time and causing the misfire. You can check this by putting a timing light on one of the ignition leads whilst the fault is present and see if the spark it present and/or the timing jumping around.
I have no experience of the 308, but had a very similar problem on one of my cars which ran fine until hot, at which point the pickup coil that determined when the electronic ignition should fire was putting out an intermittant signal.
It could be a problem with detecting the position of the distributor rotor. If there's some form of electronic pickup coil this may be breaking down when hot/over time and causing the misfire. You can check this by putting a timing light on one of the ignition leads whilst the fault is present and see if the spark it present and/or the timing jumping around.
I have no experience of the 308, but had a very similar problem on one of my cars which ran fine until hot, at which point the pickup coil that determined when the electronic ignition should fire was putting out an intermittant signal.
I had a very similar experience with my Mondial QV. Although that had a dual distributor 308 engine I had the fault traced down to one of the two bell housing ignition sensors that drove that particular ignition bank. One thing you could easily check is for the resistance of the suppression resistors at the end of the spark plug tops, assuming your 308 has them. They are quite unreliable and I have had a few failures. They are easy to remove, test with a multimeter and replace. I think they can break down with heat.
There is some good info on Ferrarichat about this: https://www.ferrarichat.com/forum/threads/308-qv-c...
From memory (I sold my car in 2007) the sensors have about 700 ohms resistance when good and should be very easy to see once you have the LH wheel and inner wheel arch off.
One other thing to check is the condition of all the small rubber hoses I'm fairly sure one of them does the ignition advance/retard although it could be different for your single distributor 308.
From memory (I sold my car in 2007) the sensors have about 700 ohms resistance when good and should be very easy to see once you have the LH wheel and inner wheel arch off.
One other thing to check is the condition of all the small rubber hoses I'm fairly sure one of them does the ignition advance/retard although it could be different for your single distributor 308.
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