Misfire cylinders #3 & #4

[NWElan]

Just finished putting 1967 Elan S3 twin cam back together after water pump replacement and new valves.

Engine idles roughly believe cylinders #3 #4 not firing regularly. Observations:
- engine shakes strongly around 1,200 rpm, will not idle at lower rpm.
- sooty #3 and #4 spark plugs
- engine slows down marginally when #3 or #4 spark plug wires disconnected
- IR thermometer shows #1 #2 exhaust manifold temperatures a lot lower than #3 #4. #1 low 200's, #2mid 200's, #3 low 500's, #4 low 500's
- engine rpm drops when spray starting fluid into #3 or #4 carb intake, eventually dies. no rpm change if spray into #1 or #2
- compression all four cylinders about 150 psi.
- varying idle adjustment screws doesn't have much effect.
- listening while running to intake trumpet #3 and #4 through rubber hose hear pop, pop, pop sound in addition to hissing. #1 and #2 only have hissing sound.

The sooty #3 and #4 plugs plus engine slow down when sprayed with starting fluid seems to indicate #3 and #4 cylinders are running too rich. I'd think any air leak around the rear carb would cause #3 and #4 to run lean and spraying starting fluid into them would cause rpm to rise. Don't understand why there is popping sound in rear carb trumpets and why exhaust manifolds much hotter.

Looking for any suggestions. Thanks.

John

[Concrete-crusher]

If you had the head off for the water pump check the valve timing again maybe it's off

Steve

[mbell]

What carbs are you running? Have you check or re-timed the ignition timing?

[2cams70]

Quite common to position the distributor rotor wrongly before refitting it to the block or to get the firing order wrong when refitting the spark plug leads.

[Billmack]

Likely 1 and 2 are not firing. Could be stuck float in carb so no gas gets in there or the number 1 and 2 plug wires are on the wrong plug. Manifold temp of 500 indicates those cylinders are running if a bit rich...try pulling cover off front carb and seeing if there is gas in there

[p.faurie]

Plug leads on wrong,tight valve clearances, spray your fluid over inlet - carb o rings ,shouldn’t alter revs check for air leak,servo pipe off,they are the things I would check first,hope it helps

[nmauduit]

not wanting to mud things out so I won't venture to a complete sequence (it would help to know where this is coming from, what was working and how when etc), but on impulse this is what I would try first :

+ good homogeneous compression on all 4 (150... ) : should not be a serious bottom end flaw, good odds for running (at least to a point)

+ 1+2 cold and not responsive : these may be the ones not running (depending how long engine was run ...)
as per previous comments, double checking there is fuel (in both carbs and also getting into the chambers, e.g. with a mirror and activating the accelerator pump) etc.

> since the engine is running somewhat (say on 2 cyl) the distributor is not completely out, so I would concentrate on something affecting only 2 cyl, possibly cyl 1 and 2, maybe 2 spark wires swapped or something the like, (then check also front carb and fuel)
> spark order 1 3 4 2 and spark quality : can be checked with all plugs out, on the starter motor and watching carefully one or two at a time to determine the sequence

I have the feeling you're not far from getting it going : keep trying ...

good luck !

[NWElan]

Thank you all for the many suggestions.

Confirmed spark plug wires correct. Re-checked timing and found too much advance. Correcting advance to around 10 deg BTDC brought #1 and #2 to life. Exhaust manifold temperatures now more even between front and rear pairs of cylinders.

#1 #2 not running cleanly and feels like needs lots of revs to run. #1 #2 spark plugs sooty while #3 #4 spark plugs clean light grey color. So #1 #2 spark may be weak or front carburetor not right.

[RichardHawkins]

John,

Glad to hear that the engine now runs.

I read a monthly magazine (practical performance car) here in the UK, one of the contributors who operates a rolling road service made a comment a few years ago, “spark plugs never recover”. I know this sounds a bit extreme, but this man has a good reputation, he went on to say that with modern fuel injected vehicles plugs do not foul, and he suspects that the type of ceramic used has changed so that if we people with carburettors have fouled plugs there is no point trying to clean them,

Try a new set of plugs, it might just solve your problem.

Hope this helps,

Richard Hawkins

[HCA]

Thank you all for the many suggestions.

Confirmed spark plug wires correct. Re-checked timing and found too much advance. Correcting advance to around 10 deg BTDC brought #1 and #2 to life. Exhaust manifold temperatures now more even between front and rear pairs of cylinders.

#1 #2 not running cleanly and feels like needs lots of revs to run. #1 #2 spark plugs sooty while #3 #4 spark plugs clean light grey color. So #1 #2 spark may be weak or front carburetor not right.

A bit confusing here - originally you had sooty 3 and 4 plugs, and by advancing ign, you now have 1 and 2 sooty with 3 and 4 light grey.

Sooty plugs are carburettor related or blocked injector or engine idling too long. Sorry but advancing ign will not alter carbon build up significantly. You need to concentrate on setting the carburettors - first check mixture as this is the single most influence on carbon deposits.

This so much illustrates the difficulties in armchair diagnosis and remedy.

[2cams70]

Are the carburettors Weber’s? Did you touch or adjust these at all when you rebuilt the engine or did you just remove and replace? Where are the cold start levers set? You may have set them with the cold start enrichment on. Just leave them permanently set to off. They are unnecessary for starting the engine even when it’s cold

[NWElan]

I have Weber 40 DCOE 18's and didn't make any adjustments for the rebuild except idle speed screw.

I initially had sooty #3 & #4 and clean #1 & #2 plugs. I believe the reason was #1 & #2 weren't firing at all thus clean looking plugs. After retarding timing to about 10 deg BTDC, I had clean #3 & #4 with sooty #1 & #2 plugs. Cold start levers are off.

Have since tried adjusting synchroniser screw while listening to trumpets through plastic hose and fiddle with idle mixture adjustment screws without much improvement. Engine won't idle below about 1,500 rpm on the tach. Can't see much effect playing with the mixture adjustment screws. Continue trying different settings.

[2cams70]

Recheck that you timed the cams properly and didn’t get exhaust and inlet sprockets mixed up

[HCA]

I initially had sooty #3 & #4 and clean #1 & #2 plugs. I believe the reason was #1 & #2 weren't firing at all thus clean looking plugs. After retarding timing to about 10 deg BTDC, I had clean #3 & #4 with sooty #1 & #2 plugs. Cold start levers are off.

I hate to labour the point, but sorry - altering the ignition timing will not transfer problems of two cylinders to another two. That is, with everything correctly set. The only conceivable way that this might happen is if the firing order is incorrect.

The fact is that a sooty spark plug is a sign of unburnt fuel - either sucking too much from a carburettor, or blocked injector or not burning the charge efficiently (as poor ignition timing will give you). So think about it logically, if 3 and 4 were bad and after a twist of the distributer they are good, but making another two bad, what is the reason for this...?

I would forget any ideas that you have or think. Instead, go back and check every single setting

By the way, if a plug is not firing at all, then it will be wet.

[nmauduit]

a close look at how the distributor cap is seating on the distributor may confirm that the cap is indeed seating squarely (so that the finger passes at equal distance of each 4 spark wires contacts) - again, all 4 spark plugs out to observe spark quality and consistency would confirm that this part is in check (the change of color between 1-2 and 3-4 would tend to indicate the problem is elsewhere - just like it would also clear a carburettor problem without having to swap carbs, at least for now).

How long do you run the engine, can it idle 1500 smoothly enough til the engine temps rises to normal operating temp? fine tuning the idle circuit comes at the end of the re-tuning process, to get started just put all idle screws equally from the stop according to your carbs (x or y number of turns whether it's a coarse or fine thread). The idle tuning is to be done engine at operating temp. Also, to resynch the carbs, the use of a synchrometer is quite handy (a few tens for the rugged snail type plastic version), I find it more reliable than my ears (which are needing a tune themselves).

I suppose you did set the spark timing with a strobe light... if you are reasonably sure that spark timing is as before or close, one point may be that you're off by one tooth on the chain (on one or both camshafts) then attempting to run at a way suboptimal cam timing :as suggested above , if all else is checked, verifying this would be in order. You can start to get a glimpse of that just by checking if TDC on #1 aligns with the marks you're using for strobe timing.

good luck, keep at it...