I am a novice (but trying to learn) when it comes to carbed engines, and I could use some help in diagnosing some spark plug issues.

This is on a 1971 Chris Craft Lancer with a 200hp 307Q sterndrive.  My symptoms are as follows:

1. Hard starting when cold - I need to crank it 3-4 times, 5 seconds each time (even previous owner had this issue).  I've throttled with the engine off and looking into the carb and can't see any fuel jetting, only a small plume of fine smoke rising up when the valves open.  I think the fuel could be boiling off?  I'm planning to replace the impeller (regular maintenance) and rebuild the carb over the winter.  The carb has never been rebuilt on this boat (Carter AVS), so I'm sure it's far overdue for maintenance.

2. Loss of power - I was running near WOT two months ago and it suddenly/immediately dropped down to 2,500 RPMs.  I cannot get it above 2,500 since then.  The engine otherwise runs great at idle or low speeds.  .  I was waiting on the carb rebuild to see if this fixed the issue - could be related to secondaries not opening up or clogged jets?  The AVS uses vacuum-activated secondaries from what I can tell.

Since 1-2, I replaced spark plugs (1 cylinder was very carbon fouled, others were slightly fouled), distributor cap, fuel filter (it was filled with floating debris).  Last year I replaced the ignition coil as well, it was also converted to electronic ignition a few years ago.

3.  Only recently, I'm getting a little white smoke at start-up that goes away after a couple minutes.  I originally thought it was vaporization (after some heavy rains), but it's happened the last 3-4 cold starts now.

I ran a compression test and all 8 cylinders are between 135-140 psi.  However, when I took out the plugs to run the test (they've only been in two months - probably 6-8 trips), they looked like below.  Two plugs look brand new and the other 6 with varying levels of fouling and oil on the threads and what looked like oil stains on the metal shell.

I'm now thinking I have more serious issues than a need for a carb rebuild.  Longer term, I'm thinking of replacing the engine with a rebuilt 350, so now I'm wondering if this is the time to spring for that vs. putting more money into this engine since I'll gain the additional HP along with starting again at zero hours.

Any thoughts on the seriousness of the spark plug issue and what I'm dealing with?

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It is " normal" to have to crank the engine several times to get it started. The carb may help the fouling. It is probably fouling from ign wires or dist cap issues. This also would explain the difficulties getting the proper rpms. If it has the crab style cap, they are crap. With the higher voltage from the electronic ign, mine would fault internally on a regular basis. Replace the cap and rotor. Consider new plug wires. It will probably run like a champ. Also look to make sure the advance is free and working. The best thing I did for my former boat was to replace the distributor with a new Mallory unit with a standard cap.

Like bruce said, a few times to crank is normal. I added a momentary switch when I put my new electric pumps in and it allows me to prime the carb before starting and they fire right up as if I had fuel injected engines. I just need to use a stick to hold my chokes open because they are manual and my cables aren't hooked up.

My plugs looked like that after running my boat the first few times. I also had an electronic conversion done before I bought the boat. I replaced cap, rotor and wires (the wires that were on it when I purchased had so much resistance when I put the ohm meter on that they were far out of spec.)

I'm looking to switch the distributor like Bruce said to a standard chevy one with tall caps. The crab ones are notoriously bad and getting to be pretty expensive.

Matt, I have a question. What is the black residue all over the exterior housing on some of the plugs?

Also, how does your oil look? Any milkyness? Are you open or closed cooled? Compression ratings are great. No need for a new engine with those readings. My only concerns are with the cleanliness of those two plugs. Are they next to each other in the engine? The others are just plain rich. What temp does the motor run at?

Scott

Hi Scott,

I think the black residue could be oil that has been burned onto the sides of the plugs, it doesn't come off it all.  I'm thinking that some oil might be leaking from the valve covers.  The oil itself is fine, no milkiness at all.

The two clean plugs are right next to each other, the two middle cylinders on the right side (facing flywheel)

It's an open cooled system, and my temp gauge isn't working properly, that's another thing I need to fix.  The engine does appear to be running hot though.

Took it out for another spin today and the symptoms are getting worse unfortunately.  Idle was really low (less than 200 RPMs, normally at 600) and it wouldn't go above 1,000 RPMs even as I neared full throttle, and it started to knock or rattle a little as I gave it more gas so I took it back down quickly and returned to the dock. 

The bottom 2 plugs on your last picture really tell a story. Look at the nice clean one, especially the angled portion just above the threads. This surface mates with a corresponding angled surface in the cylinder head and makes the seal. This is what replaced a copper gaskets many years ago. This plug looks normal, almost like it was never in a running engine, or at least doesn't have many hours on it. Look at the blackened plug just above the pretty one. This plug was not tightened enough or the mating surface on the cylinder head is screwed up. The portion of the plug inside the engine is black, the threads are black, the angled seal surface is black, and the exterior body of the plug is black. Nor too hard to assume this plug was leaking like hell is it? The same is happening on quite a few of your plugs. I think you are going to have to at least pull the heads to see what the hell is wrong. BTW, when you put the plugs back in the engine, how tight did you make them? 

I figured out that the exterior of these particular plugs are smaller than the normal plugs and without the copper gasket and was causing the leaking issues on the outside of the plugs.

However in replacing the plugs, I verified that only 3 of the 8 cylinders are firing.  Cylinders 3, 5, 6, 7, and 8 are not firing (I took the plug wires off while running, no change).  The firing order is 1-8-7-2-4-3-6-5, the non-firing cylinders are not in order.

I swapped out distributor caps and tried swapping plug wires between working/non-working cylinders and same issues.  Based on the progression of symptoms and the state of the original plugs in my photo, I'm guessing that cylinders 3 and 5 originally stopped firing (they are the middle, next to each other) and that is what caused the original power loss and then the other 3 stopped firing more recently.

I know the carb needs a rebuild, but not sure if I should try any other troubleshooting before taking the carb out?  The fuel filter was recently replaced and just checked as well.

Thanks for the advice so far.

Matt,

The correct plugs for a Q motor should be the tapered seat plugs.

Considering that you removed tapered seat plugs from your engine and they were leaking suggests issues.

Chevrolet did switch to straight seat plug with a seal washer when they went to aluminum heads. There are also some performance heads that can use either.

You need to check the plug holes and see if they are tapered. It would also be prudent to check the head casting numbers and the block casting number to verify that you actually have a 307.

All of the SBC parts interchange with some exceptions which means the previous owner might have swapped in something else and just installed the Q parts.

Proceed cautiously as you don't want to mess up the plug holes.

I would also suggest that you go to the files section and download the Q manual. Why I say this is that having 4 out of 8 cylinders not firing is out of the norm for what is normally found. This suggests the firing order of the engine does not match with engine rotation and or cylinder layout.

Hi Ronald, I can look into the plug holes when I get back over to the boat, but I still have the original 307 manual that the previous owner kept and it called for Champion UJ6 spark plugs, which are discontinued, but pretty sure they are flat seated based on internet image search. 

I'm getting the firing order directly from the manual as well.  I double-checked that the wires were hooked up to the distributor cap correctly, just in case these got crossed up along the way.

Update - just figured out that the Pertronix ignition magnetic ring on the distributor is missing a magnet, and others might be polarized incorrectly.  Will order a new ring and see if that helps to solve the problem.  Stay tuned...

Just to close the loop on this problem.  I replaced the ignition magnetic ring, fired up the engine and then confirmed that all 8 cylinders were firing using a timing light.  I took the boat out for a spin and I'm back to healthy 4,000 rpms and running great.  Between the the new distributor cap, new fuel filter, new spark plugs (I'm using the Autolite 303), and ignition coil..oh, and using non-oxy gas with a lead additive, it's running as well as it's ever had since I have owned it.  And just in time to take it out for winterizing!

Thanks for posting a follow up.

Hi Ronald - I know it's an old thread, but just found it due to some investigation. I was confused yesterday, when trying to replace the spark plugs of my 307Qs: I have an old manual which documents UJ-6 as the spark plug for the 307Q. Also this document specifies UJ-6 for all 3 engines in common. Believing in written stuff I concluded that the wrong type of plugs were installed (NGK BR6FS) and ordered a set of NGK B6S (the UJ-6 spec). But those can't be installed, since the shaft of tapered type is a tiny bit smaller than with the gasket type. I will thoroughly inspect the seats next visit, but since the gasket type didn't even fit, the conclusions are:

#1 - You are right, the 307Q definitely needs tapered seat type spark plugs.

#2 - Some documentation is wrong.

, but just found your post

Ronald Zick said:

Matt,

The correct plugs for a Q motor should be the tapered seat plugs.

Considering that you removed tapered seat plugs from your engine and they were leaking suggests issues.

Chevrolet did switch to straight seat plug with a seal washer when they went to aluminum heads. There are also some performance heads that can use either.

You need to check the plug holes and see if they are tapered. It would also be prudent to check the head casting numbers and the block casting number to verify that you actually have a 307.

All of the SBC parts interchange with some exceptions which means the previous owner might have swapped in something else and just installed the Q parts.

Proceed cautiously as you don't want to mess up the plug holes.

I would also suggest that you go to the files section and download the Q manual. Why I say this is that having 4 out of 8 cylinders not firing is out of the norm for what is normally found. This suggests the firing order of the engine does not match with engine rotation and or cylinder layout.

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