Mercruiser 4.3 Vortec Backfire and no start

[Scott Danforth]

with spark plugs out, dizzy out, crank engine with your finger over the spark plug hole for #1.

stop cranking when the air pushes your finger out of the hole

on the exhaust stroke, you will get a little pfft
on the compression stroke you will get PPPPPPPFFFFFFFFTTTTTTTT

now rotate the motor CW or CCW as needed the smidge to get the balancer and timing marks to align

now grab the dizzy off the bench, rotate the shaft as needed to align with #1 tower, and turn it about 30 degrees...... and here is why. the gears are a helix, as you install the distributor, the rotor will turn about 30 degrees.

the distributor is now aligned roughly with #1 tower, however still has not engaged the oil pump tang yet. the dizzy will also be sitting about 1/4" up from the top of the manifold boss.

crank the motor until the dizzy drops into the oil pump shaft.

now, bring the motor back around to #1 TDC on the compression stroke. this is to verify that you are still pointing to #1 spark plug tower on the cap.

install the dizzy hold-down, however to not fully tighten, you must be able to grab the dizzy to turn slightly to set timing

now install the cap, wires, plugs

crank the motor and set timing. (motor doesnt even need fuel as you should be able to set timing while cranking)

 

[MichaelBC]

I will show you a picture.

When the piston is up the mark on the balancer should match the mark on the housing at 0°. Sorry that I do not have the picture at 0° on hand, but I think you can imagine.





In this position both valves of the first cylinder need to be up.




I can turn the piston rods with my fingers when there is no load on it and the valves are up and closed.





If they are not closed it looks like this for example. You can clearly see that one valve is down.




Then I checked where contact number one is on my distributor cap....




... and marked that position on my distributor housing.



So when the distributor is mounted, the valves are closed and the mark of the balancer is in the right position then the finger of the distributor should match the #1 cylinder mark on the housing. Screw on the cap and the engine will start.

 

[Superjetjim]

BLUE TEXT - No, TDC lines on the harmonic balancer (front of the engine), not the flywheel (back of the engine).

Yes that’s what I meant - front of motor, sorry!

RED TEXT - No, you want both valves to be closed (up, not down). You should be able to wiggle the rockers and verify they are not pushing down on either intake or exhaust valve when you are at TDC.

Understood

 

[Lou C]

Normally on a Chevrolet small block your #1 will be in the front, on the V6 it will be in the front right behind the carb & flame arrestor....
You want the line on the harmonic balancer to be lined up with the Zero or TDC mark on the timing chain cover and both valves on #1 cyl (front cyl on port or left side as you face the engine front) to be closed (all the way up, slight looseness may be felt in the rocker arms). Firing order of the V6 is 165432, note the distributor turns clockwise.

 

[Superjetjim]

Right chaps, firstly thanks for all the help / pics.

So removed rocker cover / plugs and turned over until on compression stroke / valves closed and aligned notch on case as seen.

Dissy marked case marked and rotor marked for cylinder 1 and spun about 180 degrees and reinserted. Screwdriver shows where the rotor is pointing under the dissy cap now. How does this look (motor still at TDC)?

 

[Lou C]

Looks about right. Just make sure you have the correct plugs wires in the cap in the correct position/sequence. Refer to the diagram I posted.
Years ago my brother changed the dist cap on his wife's Subaru GL 4x4. Car wouldn't start. I asked him did you put the wires in the cap in the correct order? He asked, oh there's an order lol! Had a good laugh on that one.


6 5
4 3
2 1


front

 

[Superjetjim]

Normally on a Chevrolet small block your #1 will be in the front, on the V6 it will be in the front right behind the carb & flame arrestor....
You want the line on the harmonic balancer to be lined up with the Zero or TDC mark on the timing chain cover and both valves on #1 cyl (front cyl on port or left side as you face the engine front) to be closed (all the way up, slight looseness may be felt in the rocker arms). Firing order of the V6 is 165432, note the distributor turns clockwise.View attachment 356496View attachment 356499

Thanks this really helped. Timing set and leads removed, traced and labelled to help in future - will try and start it tomorrow. If its a runner and sounds OK I assume we're into timing lights to do the dynamic timing? - or does that need to be done before I try and start it?

 

[Superjetjim]

Normally on a Chevrolet small block your #1 will be in the front, on the V6 it will be in the front right behind the carb & flame arrestor....
You want the line on the harmonic balancer to be lined up with the Zero or TDC mark on the timing chain cover and both valves on #1 cyl (front cyl on port or left side as you face the engine front) to be closed (all the way up, slight looseness may be felt in the rocker arms). Firing order of the V6 is 165432, note the distributor turns clockwise.View attachment 356496View attachment 356499

Quick check (and this might be a red herring) - but on this video the guy has his No1 dissy post at the back (where mine was) rather than at the front as per the above diagram - does that matter?

 

[Lou C]

Ok what this procedure does is it gets the timing close enough so it will start; doing it this way you are more or less setting the timing at zero degrees. The actual spec will vary by the tune but typically it is 6–>8 degrees before top dead center. Set at idle speed and if you are using an advance timing light with the dial make sure it is set at zero. Timing light gets hooked up to the battery and #1 spark plug.

 

[Scott06]

Quick check (and this might be a red herring) - but on this video the guy has his No1 dissy post at the back (where mine was) rather than at the front as per the above diagram - does that matter?

Number on cylinder on tdc of compression stroke & rotor pointed at number one cylinder is all you need to worry about, get it running then set timing with light, idle speed and mixture.

 

[Lou C]

As long as the rotor is at the #1 plug wire position and the harmonic balancer is at the zero mark with cyl #1 at TDC of the compression stroke it should not matter what position the distributor is facing. I’m not sure if Merc set them up that way due to the module connections sticking out or not. But in the old school Chevrolet engines # 1 was always in front.

 

[Superjetjim]

Got a second attempt to start the motor today - no backfires, but sadly I think the motor is toast. I’ll check the compression again tonight, suspect the oil that was in the pots and providing good compression has now gone and its low again. I went through a couple of good batteries cranking it / letting the starter cool down between attempts. If it was going to go I think it would have.

I have the same smoke / vapour coming out the exhaust as the rocker cover breather as it tries to start (cold motor so not oil vapour) . I’ve tried all sorts - pouring fuel into the carb as it turns over, spraying Easy Start in, manipulating the throttle, advancing / retarding the timing a bit but nothing more than this.

Possible a cracked block somewhere?

 

[MichaelBC]

Did you check timing with a timing light ?

 

[Scott Danforth]

step 1, check compression

 

[Lou C]

Agreed you might have a few valves hanging open which can result on low or zero compression in one or more cyls. Ideally do a leak down test as well because if your compression is low that will pinpoint the source. But first have you checked for spark at all the spark plugs?

 

[Superjetjim]

Agreed you might have a few valves hanging open which can result on low or zero compression in one or more cyls. Ideally do a leak down test as well because if your compression is low that will pinpoint the source. But first have you checked for spark at all the spark plugs?

So I have bright blue spark in all plugs.
Compression readings from a moment ago:
1) 155
3) 157
5) 135

2) 125
4) 175
6) 90

Even with no6 on 90 it would run and ‘carry’ that piston right?

If I wanted to check the timing with a timing light is that all you need, the light? Do you need to put the ignition in some special mode?

 

[Superjetjim]

Did you check timing with a timing light ?

No I don’t have one, plan was to test it at 0 degrees and if it ran go from there.

 

[nola mike]

I'm not convinced your timing is correct still (might be the fault of the direction of this thread...). Looks like you're getting an occasional fire. Those compression numbers should start the engine, though it would be worth seeing what's going on in #6. You've obviously screwed with the timing significantly. I'm not sure what's keeping you from spending $20 on something you'll need anyway to get the thing running.

 

[Lou C]

Yep keep in mind that cold comp test results are not near as accurate as those on a warmed up engine.
Are you SURE that the order of the plug wires is correct on the cap??? Double check that and that your distributor was installed right (cyl # 1 at TDC of compression stroke and dist rotor aimed # 1 terminal in cap).
Because even with one cyl at 90 psi & your timing set at zero or TDC is should start.

When I did my top end overhaul 5 years ago due to past overheat I had to change the cyl heads and installed new exhaust at the same time. First time ever I had taken a "real" engine apart. I half expected it not to start but it fired up so fast I was startled.
Started with the bottom pic and wound up with the vid.
View attachment top end rebuild 4.3 test run.MOV
Started with this:

 

[Scott06]

So I have bright blue spark in all plugs.
Compression readings from a moment ago:
1) 155
3) 157
5) 135

2) 125
4) 175
6) 90

Even with no6 on 90 it would run and ‘carry’ that piston right?

If I wanted to check the timing with a timing light is that all you need, the light? Do you need to put the ignition in some special mode?

Yes you need to put it in base mode to set the timing. As others have said the compression is good enough that the engine should run. It does seem to cough and with no backfire clearly timing should be closer. Agree with NOLA timing or firing order must still be off…
can you post a video of cranking it while turning the distributor? If yiu dump a little gas out down the carb first it should couh and run momentarily unless maybe there is a vacuum leak making it very lean.
did you have the heads intake or carb off it?