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Your thoughts on a random misfire.

Posted: Fri Aug 16, 2019 3:27 am
by mark rudder
Hi all,

My Australian based GTE 6B is playing games. On cold startup it fires instantly, but often only runs on 5 cylinders.
Most times the other one comes in after 30 seconds or so & then all's good for the day.
But (there's always a but) on the odd occasion it doesn't come in. I have pulled the plugs & one random cylinder is
showing & smelling petrol wet. The problem shows on any cylinder on either bank. It has also had carbon fouled random
plugs cleaning them fixes them. My starting (cold ) procedure is half choke, 4-6 pumps turn key &, no accelerator
it runs, sometimes on 6 cyls!
Facts:
Standard Cologne, with good even compression, standard exhaust.
Weber DGAS, serviced, manual choke.
Lifters adjusted. Uses little oil. No oil on any plug.
Correct heat range plugs under 1000k's old (NGK BPR7ES).
When removed after an expressway run they indicate a little on the hot side still.
Good leads.
Factory Weber correct air cleaner.
9 degrees timing (on light)
Happy hot idle at 800 rpm.
Manual O/D box.
Uses no water, doesn't over heat.
Can occur one day or one week since last used.
Car is garaged.

It's REALLY annoying me. I even have a 7th plug to speed the changeover process.
If someone has an idea it's most welcome, I have pondered that I'm partly flooding it.

Your thoughts on a random misfire.

Posted: Fri Aug 16, 2019 6:26 am
by rebel alliance
Manual choke , why the six pumps ? Likely your flooding the engine before it even starts plus your washing the oil off the bores..i just give mine full choke turn the key till it starts then leave it on half choke till warm never touch the throttle

Your thoughts on a random misfire.

Posted: Fri Aug 16, 2019 9:48 am
by TrevorG
Agree with reb, but the other first check......are you putting the plugs back in the same bore they came out? If not, could it be that simple? A plug breaking down....not in the normal way, but insulator giving up. Had it happen on the race bikes....difficult to start, but once hot it reseals and comes alive....bit like two stroke fouling.

Your thoughts on a random misfire.

Posted: Sat Aug 17, 2019 8:37 pm
by DARK STAR
DGAS is auto choke, it needs ONE pump to pull the throttle linkage off the choke step (if you see what I mean, sorry, I can't think in english any more).
Did the factory really fit Webers to the 2,8, I thought they all had Pierburgs originally?
As Trevor says, verify if it is that same plug which is wet even if isn't the same cylinder.

Your thoughts on a random misfire.

Posted: Sat Aug 17, 2019 9:41 pm
by rebel alliance
Can be Auto or Manual choke...this one is manual,no not factory fit on a 2.8 that was some other cheap carb

Your thoughts on a random misfire.

Posted: Sat Aug 17, 2019 9:43 pm
by AJL Electronics
The factory method of starting is two pumps. It sets the autochoke and puts some neat fuel into the manifold. Six pumps is going to put far more than required, but it shouldn't cause a cold engine to flood.

I always fit BPR6ES plugs, so I be.ieve you have the wrong ones. Can't recall which way the heat designations run, but it could be that you are under heating the plugs which is why they are fouling.

Your thoughts on a random misfire.

Posted: Sat Aug 17, 2019 10:02 pm
by DARK STAR
DGMS then :mrgreen:
Not sure Pierburgs were cheap, the service kits cost a fortune.
And are necessary.
They are certainly not the same quality as a Weber.

Your thoughts on a random misfire.

Posted: Sat Aug 17, 2019 10:07 pm
by rebel alliance
He has a manual choke !...therefore not really any pumping required is there ?

Your thoughts on a random misfire.

Posted: Sun Aug 18, 2019 11:52 am
by mark rudder
Hi all again.
TrevorG, wish it was that simple, the problem doesn't follow a plug movement.
AJL, the car had those plugs in it when purchased, it was running-on & they were heat stressed.
Counter intuitive to me but NGK's go colder as numbers go up, 7's recommended in the paperwork.
The Weber was retro fitted by a previous owner. Tried the starting methods suggested, they make
sense, started straight away, twice, still with a misfire that cleared within 30 seconds.
Thanks for the advice so far, anything else welcomed.

Your thoughts on a random misfire.

Posted: Sun Aug 18, 2019 6:18 pm
by AJL Electronics
rebel alliance wrote:
Sat Aug 17, 2019 10:07 pm
He has a manual choke !...therefore not really any pumping required is there ?
So how does neat fuel get into the inlet manifold before cranking otherwise?

Your thoughts on a random misfire.

Posted: Sun Aug 18, 2019 7:25 pm
by rebel alliance
I don't understand why you think it needs neat fuel ? ..I just been out and started one been sat for two weeks manual choke mechanical pump.Choke full on turn key it starts, choke back to half job done..if i could post a vid i would 😀....SU carbs have no accelerator pump didnt stop my mini starting ok for 6 years

Your thoughts on a random misfire.

Posted: Sun Aug 18, 2019 8:14 pm
by AJL Electronics
rebel alliance wrote:
Sun Aug 18, 2019 7:25 pm
I don't understand why you think it needs neat fuel ? ..I just been out and started one been sat for two weeks manual choke mechanical pump.Choke full on turn key it starts, choke back to half job done..if i could post a vid i would 😀....SU carbs have no accelerator pump didnt stop my mini starting ok for 6 years
It isn't what I think, it is the factory recommended starting procedure. Many cars will start from cold without any choke, but the factory designed a method that suits every car. The choke has nothing to do with fuel delivery, it merely strangles the air supply. A couple of squirts of neat fuel helps the engine to catch.

Your thoughts on a random misfire.

Posted: Sun Aug 18, 2019 9:04 pm
by rebel alliance
Yes they do recommend that for an auto choke ...my point is a manual choked essex will start every time with out pumping the throttle i don't actually care if someone wants to do a dance on there throttle pedal every morning 😀

Your thoughts on a random misfire.

Posted: Mon Aug 19, 2019 4:45 pm
by Andrewsait
I had exactly the same symptoms on one of my cars (not a Scimitar), it turned out to be a weak ignition coil which I tracked down by measuring the resistances on the primary and secondary windings. One othe test is do you get a misfire under hard acceleration, this was another one of the symptoms. The coil was OK when in normal running but when asked to produce a bigger spark, e.g. hard acceleration or when the plugs were slightly fouled, then it just wasn't up to it.

If I remember correctly the primary winding (measured between the + and - low tension terminals) should be between 1.5 to 2.0 ohms) and the secondary winding (measured between the HT terminal and the + terminal) should be between 8 to 18 kilo-ohms). On my car the LT side was OK but no reading at all on the HT side.

Fitted a new coil and everything has been fine since.

Kind regards,

Andy

Your thoughts on a random misfire.

Posted: Wed Aug 21, 2019 9:37 pm
by Roger Pennington
Still catching-up after Curborough - it's worth checking what kind of "manual" choke you actually have. Many so-called manual carbs are effectively converted auto chokes i.e. the act of pulling on the choke control in the cabin simulates the carb being cold, but it's still necessary to pump the throttle, just like an autochoke, to actually close the choke plates. It's worth having a look down the carb throats while someone pulls the choke lever, and seeing if the choke plates move.