TOPIC: 72 Merc 500 - Spark but No Fire

Hi...just emailed this to Dave but figured I would widen the net....pun intended.

Hi There. Even though I have a ton of experience restoring/fixing these inline 4 mercs, I am running out of ideas here on this 1972 Merc 50 with lightening energizer. It has great spark on a 4… it has good compression(actually I have only checked the top two holes now that I think of it). Somebody had been playing with the timing stops and trigger throttle advance arm so using a timing light I did the link n sync per specs. Battery is good and cranking well. Fuel is fine(same tank I use for other motors I am working on). Carbs removed and cleaned (weren’t very dirty actually). Floats/needles/seats are working as I can pump till hard.

Problem is I cannot get it to fire up and run. With premix in the the holes I have had it pop once or twice but will not run. I am stumped. With spark this good it should at least consistently rumble for a second or two burning off the premix in the cyls. This is why I thought it might be a timing issue and set it all up correctly(including of course getting the distributer pulley arrow lined up with TDC mark on flywheel). One thing I should have checked…especially with the distributor off was if the plug wires were actually connected to the rotor cap correctly? I should have also checked the reeds when I had the carbs off come to think of it.

Any thoughts?

Did somebody make the common mistake of taking the flexplate off the hub ?

I assume that you are referring to the flywheel..as in maybe not installed back on crankshaft correctly? If so it wasn't me but I can maybe pull the nut and be certain it is set down over the key and the key is intact?

And even if that was the case how would I have been able to adjust the timing with a light to the specs appropriate?

If the flexplate was removed from the hub than timing may be off.

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Have you confirmed timing marks with the position of the #1 piston at TDC ?

did you make sure that the top dead center mark on flywheel is correct that is where I would start then go from there as said some one could have taken the flywheel apart

Charles

I have confirmed that TDC and the mark on the flywheel are correct and that the ignition distributor sprocket arrow is lined up....and that with a timing light and cranking it is 4-5 degrees BTDC at the point of just contacting the throttle cam and is at 35 degrees at full throttle.

Therefore I believe I have ruled out a timing issue.

And a spark issue, so now we are down to fuel/air flow I think. Dave Bernard mentioned reeds which I am thinking might be it although avoiding like the plaque pulling the carbs again ( yea only a 45 min job I know but a real PIA with all the stuff you gotta remove to do so.)

I'm no pro... and don't mind me, because I'm just supposing here... but it makes me wonder if a broken reed valve would cause such a problem? Not even sure how to check for it without disassembly, but crankcase pressure being back-fed into the intake would throw the whole engine off (at the two cylinders served by the common carb)

Sounds to me like the new needles in the float chambers are stuck closed.
A common problem with new needle & seat components until the rubber 'titty' is thoroughly purged with carb cleaner or fuel mix & the brass 'seat' is also thoroughly purged as well.
The prime ball will become firm - as expected, but the needles refuse to release from their brass seats and consequently no fuel gets into the float chambers.
Squirt a bit of fuel mix - or carb cleaner - into the throats and the beast will 'pop off', but not continue to run as desired.
Doc F

Hi Thom, I am following this over on Johns as well. Tim it sounds to me that the flywheel key is sheared.

Hi Dave!
Definitely worth checking indeed!!!
Keep me posted guys as to the solution when discovered.
Doc

Dave/Thom,

I don't believe it can be that because timing is all set up in relation to verified TDC on 1st hole.

Tearing carbs off now to view reeds and will check needles but if they were sticking it would only occur at least the first time the bowl filled and the float closed the needle into the seat right? Which would cause ball to harden because fuel flow was stopped. If it was stuck and motor was otherwise running ok ....would it not at least run until the fuel was used up in the bowl(like it does when you pull the fuel line to clean out the carbs for storage)?
That is... if I pull the carbs and the bowls are full would that not indicate that the needles were not stuck?

Ok got them off....now what is the trick to inspect reeds? Do I need a dentist mirror or orthoscope ? And is there any way to try and clean the reeds without tearing apart the block? Spray something, compressed air????

Thanks Tim

Dr_Frankenmerc wrote:

Sounds to me like the new needles in the float chambers are stuck closed.
A common problem with new needle & seat components until the rubber 'titty' is thoroughly purged with carb cleaner or fuel mix & the brass 'seat' is also thoroughly purged as well.
The prime ball will become firm - as expected, but the needles refuse to release from their brass seats and consequently no fuel gets into the float chambers.
Squirt a bit of fuel mix - or carb cleaner - into the throats and the beast will 'pop off', but not continue to run as desired.
Doc F

Thom...I don't believe it is a sticking needle issue cause when I pull the bowl intakes off the floats are indeed floating nicely where they should be when pumped hard right? Therefore my theory is that if the bowls are full and jets are cleaned and set then getting fuel should not be an issue right?

Someone on another forum mentioned blocked exhaust preventing the block to breath... pulled lower and tuner is as clean as I have ever seen all the way up as far as I can see with flashlight.

THen I had another idea....replaced the ignition(lightening energizer) with another know good one....even though I have great spark...also wanted to ensure that the plug wires are connected to correct rotor cap ports. Still no run.

Only single pop on top 3 cyls igniting the premix I squited into plug holes.

I'm at witts end with this one but am not about to let them 4 banger beat me....it has never happened before after some tough issues.

Keep the ideas coming please/thanks.?

Tim

In no manner am I qualified to be diagnosing this,But from the sounds of it. The issue is clearly fuel related, could the set screws on the carbs have been overlooked when they where cleaned and reassembled ? If the set screw is bottomed out ( so to speak, because no one ever wants that screw bottomed out in a secure fit ) But if it is screwed in preventing enough fuel getting to the pistons .... Just a thought! And definitely something that I myself, have overlooked before.

Because of your timing light comments, I am probably full of prunes, but want to put it out there anyway.
Do you have the belt timing with the distributor pulley's arrow pointed at the flyhweel's TDC mark,...Or corrrectly aligned with the belt alignment mark on the flywheel, which is a different mark about (15) flywheel ring gear teeth counter-clockwise from the TDC mark??
Doc