Compression reading

Check all compression readings cold and warm. Disparity will give a better indication of wear or damage. Do not run the engine further or you will certainly cause damage.

You issue is on the ignition side and likely one set of burned or failed points. Have a knowledgeable shop examine your distributor and entire ignition system per the shop and overhaul original set up and tune up guide. CDs of this are available on the internet.

Best of luck,

Bill

 

compression readings are fine, not to worry, look elsewhere

clay

 

Cliff,

weak spark on some but not all plugs indicates a problem with the distributor cap, spark plug wires or plugs. A broken Dinoplex can have partial failures but these would then appear on all plugs not just specific ones. Would recommend the following tests:

1. Remove the distributor cap and check the inside carefully for dust, cracks or damages due to flashover. Also check the distributor rotor for wear and replace if necessary.

2. Check the condition of the points, with an S125BX one set of points could have corroded contacts or moved out of place leading to the symptoms you describe (the S125BX has two set of points, one for each bank), but then again your Dino should have had an S125C installed in '73 which has six cams and one set of points for normal operations (and one for the retarded setup).

3. Remove each spark wire and measure the resistance with a multimeter (ohm) from one to the other end, compare the values. They should not vary by more than 10-20% and the resistance of each wire is usually below 5000 Ohm for typical resistance wires. Also inspect the outside of each wire for damages/ground connects. If in doubt or if the spark wires are older than 2-3 years, replace them against a good set (don't forget the king lead).



Good luck,
Adrian

 

If I understand correctly, "weak" spark has not been established. What has been determined is that by removing the spark plug wires there is little impact on engine RPM or smoothness, with back firing on "the over run." Not sure what that means.

An easier approach to diagnosis would be to bring all of the "rear" bank idle mixture screws to the same setting (gently all the way in until they stop, and then out one full turn). Then start adjusting each screw, one at a time, to achieve maximum idle. If adjustment of the mixture screws fails to impact idle, then move on to electrical issues.

It is virtually impossible to attribute your symptoms to compression!

Timing might lead to similar symptoms, but difficult to check for the novice. Go with the carburetors first.

Bill - A bad set of points is a good thought, but not likely here. As the firing order alternates banks, one set of points will not limit itself to one bank. (I have to think that statement through a bit, but it seems logical as the firing order is linear and the lobes turn in a sequential (1,4,2,5,3,6)

Finally, an air leak from one of the small breathing tubes on the intake manifold might have split from age and heat, and could impact idle and lead to air leak and lean mixture (burbling and back fire on trailing throttle).

You are lucky if it is, indeed, the rear bank. This is easier to reach and easier to work with the idle mixture screws.

Jim S.

 

Jim,
I agree with Bill here regarding the points, with a three cam S125BX distributor one set of points triggers the 'even' cylinders, the other set of points the 'odd' cylinders, so a bad set of points can lead to the symptoms described by Cliff.

Best,
Adrian

 

Ok, with a 6 cam its unlikely that its a point trigger issue. 10K is a bit high, sounds like a carbon core type. Would recommend to replace those with inductance (spiral wound) wires on the next occasion, but unlikely that this is the reason for the symptoms you have described.

Looks like that there is a problem with the carbs (paging Dr. Selevan).

Have you had a look at the rear spark plugs, would be good to check if they are dark/soiled (mix too rich) or whitish (mix too lean on the rear bank).

 

Is the fiber cam follower in good condition?
Do the points fully open?
If pulling the lead off pot 3 will not make any difference to idle, then covering up the carb won't either 'cause it's not firing. My money is on ignition. Is there any occasional firing on that bank or is it always dead?

 

Having put my money on ignition, could this be a recurrence of an earlier carb or other problem?

 

Could Valve Timing on that bank be an issue?

 

Adrian - even cylinders and odd cylinders still won't explain front bank / rear bank. There are two odds on the rear and two evens on the front ????

Common things are common....sinking the carbs with a Unison won't fix mixture issues. Cliff - it sounds as though you know what you are doing...having pulled the carbs and distributor apart. Did you adjust the idle mixture screws. Of course, if the cylinders are not firing then if will be difficult to adjust for idle mixture. I assume that you have the idle mixture screws set correctly.

One crazy thought- forgive the insinuation but not intended. Could you have the spark wires routed incorrectly? One must look inside the distributor cap to fully understand the proper routing, not simply from the outside top of cap.

If you did dismantle the distributor, then electrical looms large. Very easy to adjust points incorrectly vis-a-vis dwell and gap. But that should effect all cylinders.

Keep plugging along - you'll find the answer. Observe, observe, observe.

Jim S.

 

Cliff,

hmm, a defect with the Dinoplex should affect all cylinders, not only a single bank. Do you have an original Dinoplex AEC103A or a modified one? (Super4 with a Bosch coil, etc). Did you have the same issues with the rear bank also with fresh plugs?

Here's a simple health test, pull the king lead from the distributor and either connect a Sparktester or insert a screw/bolt in the king leads connector and set a distance of 0.8" to ground (engine block). When you now run the starter you should see healthy (clearly visible) sparks. If sparks are missing or don't manage to jump the gap, there is a good chance of a defect of the Dinoplex, a problem with the +12v/ground wiring to the Dinoplex (or an empty battery).

Jim, on the S125BX with three cams the two set of points are wired in parallel and closed one after another (thats what i meant with even/odd, sorry for the ambiguous wording), so the pattern would look like this "R1:1 R2:4 R1:2 R2:5 R1:3 R2:6".
If one set of points is not working correctly then just one bank would be affected. Cliff has a six cam S125C so this won't apply to him.

Best,
Adrian

 

Adrian - thanks for the lesson. Makes sense. Learn something new every hour. You'd think with all of these hours behind me I would be smarter than the sack of hammers that my kids (accurately) compare me to.

Jim S.

 

3 pots with compression at 170 to 175 which is even but possibly low, would 190 not be more likely? I believe that's what i saw on mine way back in the day.

One bank not firing, popping in the exhaust and seems to be ok above 3000.

Does it come on full song over 3k? are you getting full power or just smooths out?

If it was my alfa V6, i'd say timing belt slipped!

what else other than valve timing could could effect one bank only?

when you remove the lead from each plug do you get the same result or is there more effect with one over the other?

 

This is all a bit familiar, I had similar problems before my rebuild and I just lived with the mis fires and popping thinking this was the nature of the car. It was only when I stripped the engine I discovered the woodruff key in the timing chain sproket was actually fractured (from some previous disaster) and there was massive movement in the mechanism. In effect I had me own variable valve timing! With hindsight I suppose I could have checked by taking off the cam covers and checking where the timing marks end up when rotating the engine in either direction, but it's a lot of work.

 

It kinda ruins the experience.
Not sure what Cliff will find causes his misfires,
but It has to be frustrating!

 

Coil parameters sound ok. You might need to exchange the king wire though, if the corrosion of the wire/connector is too bad.

Did you do the 0.8" spark test with the king wire? The plug tester only tells you if the plug and fuel/air mix is ok but not if the Dinoplex/coil delivers a high enough secondary voltage.

 

I have read the entire thread but no one talked about this. Would this not imply carburation problems ? I would have swapped the plug leads, not just pulled the plug end of course but used a entirely new plug wire by switching from another long enough wire and also switched plugs, to see if the problem moved to the cylinder that HAD the no. 3 wire/plug on it, if not then because of the venturi issue mentioned above, I would have started looking at the carb.

It should not be valve timing with that good compression and since the problem is on no. 3 (not I said should not be).

The backfiring on overrun on my car was bad, I had bad advance weights in the distributor and once that was fixed I still had backfiring, it took a long time with the carbs to get them right.

 

Seems to apply to all three cylinders and seems to be "spark problem"

Seems to apply to the whole bank, seems to suggest bad carburation and spark,

So weak spark on #3, seems to indicate an ignition problem, but why are 1 and 2 not firing?
carburation over rich on that bank?

Cliff if you very gently ever so slowly increase the throttle, does it still clear up over 3000? and is it a gradual clearing up or a sudden coming on song?

 

When you pull 2&1 plugs what do you get?
In general it's best test one thing at a time.