Troubleshooting post-rebuild misfire

[JOHN COOKSON]

[QUOTE=Stevenph;99378]

(2) Pull the lead off #4 spark plug and put a screw driver in the boot and hold it near the spark plug while you turn the engine. If you can jump an arc to the plug then you will know that in fact #4 is firing. If no arc then put a different spark plug wire in place of the wire to #4 (such as the wire to #2) and try to throw an arc to the plug again. This will tell you once and for all if the spark is bad or if something before the plug is bad ie wiring , distributor cap, rotor.
You're getting close. More as I think on this.

I'm assuming you mean do this while the engine is off, all the other plugs removed and cranking?

Thanks for all of the help!

Yes, you don't want the engine to start when testing #4 plug so take the leads off the other three spark plugs. You can loosen the other three plugs to break the compression and be kinder to the starter and battery if you want to.
Another thing you can do if you can't throw an arc to #4 spark plug is to try to throw an arc to ground (the engine). This should give you more information which component is bad.

TRUE GRIT

[Stevenph]

i may have narrowed it down to a timing issue - i did all of the suggested tests and they all came out successful - all of the plugs are firing, checked compression again, checked the choke, and the entire exhaust system.

I did however find a slight oddity in the distributor setting - When i set the timing, following the manual, i used a timing light on the coil, and at TDC, i think i rotated the distributor to the point where the light turned on. Rechecking the manual it says to set it to when the points open, thus turning off the light.

Eureka! i thought...but... as you can hopefully see from the pictures, it seems like when i rotate the dizzy to when the point that the lights turns off, and the points open, the rotor is not lined up with the tab (which aligns with the #1 wire tower). It's about 15 degrees off.

I tried a few times to pull and adjust the rotor position, thinking i could slip it into the next gear tooth, but i think the range of motion for each gear tooth is bigger than the 15 degrees (or so) i'd need to move it to align with the tab when the points open at TDC.

Hopefully the pictures can give some clarity -

#1 - Rotating clockwise, this is the position just before the lights turns on.

#2 - rotating further clockwise, this is the position when the rotor is aligned with the tab, and is directly in the middle of the area of rotation when the light is on

#3 - Further clockwise, this is exactly when the light turns off, and where i think would be the ideal timing at TDC, however the rotor is not facing the tab, and it seems it would fire at the wrong time.


Unfortunately i had to leave before i could try start it up at the position of picture #3, but i also was a little nervous of starting it potentially that far off, and maybe causing a backfire or some other issue.

It seems like it could be a simple mistake i made - something i'm not thinking about correct, so i thought i'd check in with you all first...

[ndutton]

A simpler place to start

I'd put the light away for now and use the roll pin in the crankshaft as a visual indicator of #1 TDC. A vertical roll pin on the #1 compression stroke is precisely when the rotor should be pointing to #1 on the distributor cap. That should get the timing real close. You can fine tune it later with the light bulb, timing light or power timing.

The internal distributor/points/cam geometry will 'drop the coil' at the right time all by itself.

[Stevenph]

Indeed, that's been my method for finding #1 TDC and setting the initial timing. It was when i went back double and triple checked everything that i noticed that this all didn't seem to line up, at least according to what i understand of it.

I'm not too savvy on breaker point ingition systems - am i correct in thinking that at #1 TDC, the rotor should be precisely lined up with the #1 spot on the cap AND the points should begin to open (light turning off) at that exact moment?

It's quite possible i'm thinking far too hard about this and getting it entirely wrong... wouldn't be the first time

[ndutton]

Again, I'd stop worrying about when the points open. Don't get me wrong, it's important, the instant the coil field collapses and the stored charge is delivered but its timing is built into the distributor via the alignment of the cam and rotor keyway or in other words, the energy goes where the rotor is pointed. Set the point gap and/or dwell properly, rotate the distributor body so the rotor points to #1 on the distributor cap at #1 cylinder @TDC and call it good. Fine tune after it's running.

Accepting your analysis that #4 is not firing, improper timing would not affect only one cylinder. Get your timing set as described then follow Dave Neptune's advice earlier about rotor, distributor cap, plug wire and spark plug for the offending cylinder. Joe's advice about exhaust backpressure needs to be checked too.

[sastanley]

I am with Neil...if you get it running, you can adjust the timing later, and timing does not usually affect only one cylinder..it is pretty easy to loosen the dizzy clamp with a 1/2" wrench while running and it doesn't need to be real tight for this testing. Just watch out for the alt. belt.

I chased a bad plug for a while too. I now only run Champions in this motor, and I have even gone back to the Moyer recommended RJ8 in all cylinders after years of trying hotter plugs, different brands, one hotter in weak #4, etc., etc., - my engine runs best with RJ8's.

I'd swap out the #4 plug into another cylinder and see if the problem follows the spark plug..that is the simplest test at this point.

Incidentally, I remember a comment about choke..it SHOULD close the butterfly completely for starting a cold engine, and then there should be enough range of motion to open back to 90° when opened.

[tenders]

Have you checked the advance weights in the distributor? Might be frozen/missing?

[Stevenph]

Better luck today - I ran it after removing the exhaust components aft of the manifold, and all four cylinders came right to life! Looks like it could be the issue.

However, I looked through the system again, and just as before, found no blockages. The aft wet exhaust hose to the thru-hull is quite old, but doesn't seem to be collapsed. I want able to remove it, its access is terrible at the thru-hull, but I was able to get a decent look through it, as well as run a series of thick lines through it, all went through with ease.

I wonder if the system itself is of poor design? It's definitely an fyi job by the P.O., and while it did run for me when I bought the boat, it never ran great before I started this rebuild.

You can see in the picture - the short 1-5/8 hose connects to the manifold (which I'm pretty sure is not advisable since its only rated for wet exhaust), and then the pipe rises above the water line, where it's joined by the 3/4 wet exhaust. From there connects the exhaust hose that leads aft and out the transom.

Perhaps something in this design is giving me backpressure or perhaps allowing water to retreat back down my dry exhaust pipe?

[hanleyclifford]

Trying to understand that exhaust. Is the picture upside down? Which is the engine side?

[Stevenph]

Sorry - if you rotate the image 90° counterclockwise, it'll be right side up.

The short wide hose connects to the manifold dry exhaust, the thinner connects to the wet. The pipe end connects to the exhaust house to the transom.

[Stevenph]

When installed the top of the pipe tilts aft about five to ten degrees, in order to prevent water from flowing back to the pipe and down to the dry exhaust.

[hanleyclifford]

Then the water is not being injected with a down hill path. The design is flawed and allows water back into the engine; #4 would get it the worst.

[Stevenph]

What if i added an addition two elbows to the system, like in the (crude) drawing attached?

[hanleyclifford]

The whole idea of a riser type hot section is to create a down hill path for the water and present it with maximum difficulty of getting back into the engine. The higher the riser the better. All pipe ahead of the water injection point should be wrapped. Every boat requires a custom design. This is mine:

[Stevenph]

Very helpful - thanks. I should be able to design something that will work fairly easily.

Do you have a thought on the use of the hose as a connector between the manifold and the pipe? It's only rated to 250degree F and the one i pulled off was in terrible shape (the one in the photo is a new replacement). Do most have their pipes directly attached to the manifold like in yours?

The only flexible hose rated higher that i've found at my local supply is a silicone hose that is over 150$ for the minimum 3ft section... rather not go that route if not necessary.