Misfiring Sprint engine

[Beans]

During the Border Raiders in August the car developed an engine problem (Sprint engine on DCOE Webers and optical ignition module from Newtronic). Symptoms were lack of power especially at higher revs, running rather lumpy at idle. But running fairly well in higher gears at about 3000 rpm. So since then I have been checking various things;

•     The contacts inside the distributor cap were showing signs of corrosion as were some earth wires;
•     The spark plugs from cylinder 1, 2 and 4 looked OK, but number 3 was literally washed clean;
•     Removing spark plug caps one by one reveals that number 3 is not working on idle;
•     Measured the spark plug leads but the measurements were the same as a (new) spare set.
•     Ignition timing is spot on (in this case 11° BTDC, checked both static and dynamic);
•     Changed dizzy cap, spark plugs, spark plug leads and coil, no success;
•     Checked the compression to see if there was anything wrong with number 3. Should have done it on a warm engine but I just wanted to rule out any issues with cylinder number 3:
        o     # 1 - 8,0 kg/cm²     (114 psi)
        o     # 2 - 10,0 kg/cm²     (142 psi)
        o     # 3 - 9,7 kg/cm²     (138 psi)
        o     # 4 - 9,4 kg/cm²     (134 psi)
        Clearly nothing wrong with number three, but number one's too low. But I don't think that's causing the problem
•     Checked the carburettors but nothing visibly wrong there. Same applies for the in line filters and the electric fuel pump;
•     With the ignition switched on and the dizzy's chopper disc in the correct position I got a reading of approximately 6 Volts. So the ballast wire is OK;
•     Checked valve clearances, though not spot on, they didn't need immediate adjustment.
Al the above have had no effect whatsoever, so has anyone suggestions as to a solution for this problem?

[mikew]

Does the plug lead on 3 fire the timing strobe at tick over, if the strobe trigger lead is connected to it ? - I know it will indicate on the pulley at some distance from the no 1 firing mark, but this may be a clue

[Beans]

Haven't checked that, but certainly worth a try.

[32valves]

Check the distributor mechanical and vacuum advance then, assuming it's all moving freely, advance the ignition timing a couple of degrees and see what happens. In my experience the timing marks are purely arbitrary anyway.

You could also try swapping the coil for a known good one.

[paulsprint]

Check dizzy shaft wear,  rule out a faulty ing unit by refitting a set of points and trying it. Recheck the cap and leads.   I would then start checking for air leaks on the manifold and find some one who knows webers inside out

[trackerjack]

I must add that I have found that on sidedraughts the sprint likes a little more advance try 15 deg rather than 11 however this wont stop a misfire, I presume you have tried it in the dark as sometimes sparks can be seen where they should not be.

[nang]

Note that you don't seem to have changed the condenser. May be worth a shot.  :)
Tony.

[Beans]

Thanks for the replies so far.

No vacuum advance on this engine. It was set up on a rolling road and we found an advance of 11° worked best.
Will first try Mike's suggestion and see if there's a pulse through lead #3. As I still think the ignition module is on the way out.

Another thing to check is #3 three choke, as this was slightly loose the day before the event.
As only one bolt was loose I just tightened it up as I didn't like the idea of removing the carburettor completely in a hotel parking.

[yorkshire_spam]

Not familiar with the weber DCOE set up, but is it possible that it's flooding just no. 3?

[Beans]

nang wrote:

Note that you don't seem to have changed the condenser ...

Correct as there's no condenser, the car's fitted with an electronic ignition  ;)

6797 wrote:

Not familiar with the weber DCOE set up, but is it possible that it's flooding just no. 3?

I checked the float chambers but nothing visibly wrong there.

[logicaluk]

Hi theo
Did you check the plug to see if its firing. Holding the plug against the block {with insulated pliers}  I recon its more likely to be the carb now, from what you have said.
Dan

[junkuser]

Have you tried  trackerjack's suggestion of  having a look for errant sparks in  the dark?
Will see any surface tracking of HT.
A no-cost, simple test.

[Beans]

logicaluk wrote:

... Did you check the plug to see if its firing. Holding the plug against the block {with insulated pliers}  I recon its more likely to be the carb now, from what you have said ...

Will first try Mike Weaver's suggestion, using the strobe light's trigger lead to check #3 for a pulse, and check the alignment of #3 choke.
See if I can muster some inspiration to do that today  :X

[yorkshire_spam]

beans wrote:

Correct as there's no condenser, the car's fitted with an electronic ignition  ;)

I checked the float chambers but nothing visibly wrong there.

DCOE - float is shared by cylinders 3 and 4 isn't it? Wondering if the main or idle jet on #3 has worked loose or something so that JUST #3 is over-fueling?
(NB: it's possible that I'm very wrong, I know pretty much nothing about DCOE carbs)

[Raider]

junkuser wrote:

Have you tried  trackerjack's suggestion of  having a look for errant sparks in  the dark?
Will see any surface tracking of HT.
A no-cost, simple test.

That helped me with my 2000! Read that and went out last night under the car port and found the king lead arcing to the battery retaining clamp :)

Mind you, I also saw the plugs lighting up as they fired too - the ceramic bit was lighting up. Not sure whether that is normal though :-/

Good luck Theo - I know you will sort it out.

[Beans]

Checked a few more things today.

As Mike Weaver suggested, with the trigger lead connected to #3, the light  flickers rather randomly.
It is missing out about half the time, so it looks like the ignition module is on the way out.
So back to basics, putting the points back in the dizzy  ::)

Also did a quick check on #3 venturi but nothing visibly wrong there.

[junkuser]

"the ceramic bit was lighting up. Not sure whether that is normal though "
Not normal with normal plugs, but these days I would not be surprised to find plugs are available with LEDs on them Raider :)

That "lighting up" is surface tracking in muck on the surface of the ceramic.  Simply cleaning will usually overcome.
Wise to check the plug gaps as if they are too large the resistance is increased and so the HT may find a lower resistance route to follow. (either surface tracking or arcing.)

Tracking can also occur on the inner surface of the distributor cap so a good idea to clean that also.

[logicaluk]

Odd the ign module just missing no3 cyl. Have you triple checked the rotor, try pulling the plug lead and testing it with another known good plug earthed to the engine, im thinking if theres too much fuel in the chamber it might cause the plug to miss fire. This would show up on the strobe test as it has done. If it is the dizzy the plug will keep on miss firing.
Dan

[Beans]

Looks like I'll be removing the rear carburettor soon for further investigation.
I thought I had a spare points/condenser set for the Sprint but they are for the 8 valve's engine Delco dizzy  ::)

[Beans]

Short update, took of the carburettors for inspection, but nothing wrong there. Completely clean inside
But the flange holes on number three barrel were badly worn out due to a collapsed rubber spacer.
Sadly that wasn't the cause for the problem
So I borrowed a Sprint dizzy with points today for the final check. Hopefully will be able to put that in this Saturday.
Fingers crossed  :o

[Beans]

Wanted to swap the dizzy today but decided against it.
I first replaced the rotor only ... result. I knew it had to be something simple.

Though I can't figure out why the rotor only misfires on #3.
Remains the slightly lower compression on #1. Will see how that develops after next weeks "Nachtrit".

[Beans]

Did another compression check today to see if there was an improvement on cylinder number #1 after the 550 kilometres I drove the car last weekend.
But #1 was still approximately 20% lower compared to the other three. So to prevent major damage it is time to say goodbye and pull the engine ...



On the other hand the engine performed pretty well since it was installed well over 17 years and 110.000 hard kilometres ago  8)

[piman]

Hello Beans,

isn't it worth checking to see if the low compression is piston or valves? It may only need head work, assuming good oil pressure, save you some unnecessary work? That is one bonus with a pressure test as opposed to using a compression tester as you can listen to where the air is leaking.

Alec

[Beans]

The plan still is to use this engine in the FHC I am currently restoring.
And in that context I had scheduled to inspect this engine in the next year or two.
But I will start with pulling the head to see if there's something wrong with the valves.
If not ...

[logicaluk]

Have you tried the putting some oil in the bore and rechecking the compression?