Ambassador problem. Solved.

[Bison]

Hello folks,
I visited a friend today to help him with his Ambassador, it won't run on the right, no 1, cylinder.
Now, it has a big fat spark, at the correct time, it has correct valve clearances at the correct time, it has fuel going to the carbs, float height correct and carbs cleaned, new points, capacitor, rotor arm, dist cap, plugs, leads, caps, coil. A compression test shows 170psi on the left cylinder, and almost 200psi on the right, more of which later. The cylinder is soaking wet, you can pour the petrol out of the plug when you remove it, air filter removed so plenty of air. Test the plug and it has a great spark, they all do,I've tried 6 of them, and the one from the left cylinder, swap components to the left cylinder and they all work. I fitted a  carb from another running Ambassador, exactly the same problem, even put all the replaced original parts back on, the same.
Here's my theory, the exhaust is blocked, and that's why it has 200psi compression on the non running cylinder. I know he had the exhaust off for cleaning, then a misfire developed and slowly got worse, until we got to this point. You can feel gas (Not petrol)escaping from the right hand silencer, but it has a balance pipe, so if the pipe was blocked towards the head you would still feel gas ( not petrol my american friends, gas) pumping out. The bike was running, so even though I checked all the timings of spark, valve openings etc, it was always unlikely those were going to be the problem.
Very happy to listen to other theories?.
Thanks,
Alan.

[Alfetta]

Your theory can quickly be validated...   pull the head pipes off, and light her up.

[pehayes]

I had a strange, similar symptom on a Tonti-framed bike.  Now with the Tonti you have two coils and two separate and independent ignition systems.  One for each cylinder.  But I had a visual fat spark and that cylinder would not run.  Turns out it was one borderline weak coil.  It takes much more energy to spark a plug in a chamber of compressed air than it does to spark the same plug out in uncompressed ambient air.  The coil wasn't up to the compressed air demand.  Hard to imagine with a single ignition system on a loop.  And you did say new coil.  But, you have very high compression in the failing cylinder.  Could that side demand more energy than the other?  Beyond the capacity of  your diminished coil?  Are you sitting on the fence with coil quality and working fine for one cylinder and not at all for the other?  You are only reporting good visual spark with the plug 'outside'.  We don't actually know if it is sparking inside the compressed chamber.

Find someone with a nice 'inductive' timing light.  Hook it onto the secondary wire of your failed cylinder.  Does the lamp flash when you crank?  It only flashes if the spark is passing inside.  If it doesn't flash, find another spark plug and hook up that same wire to the open air plug.  Does the lamp flash when you crank.

Easy and cheap to toss in another coil as an empirical test.

Patrick Hayes
Fremont CA

[MattP]

float not closing to much gass    pull float bole might have crap in seat

[Don G]

Does the camshaft have lobes? Eyeball the rockers to see if they are opening the valves equally, seen that before.  DonG

[John A]

Are the rocker covers off yet?

[Bison]

Hi Folks,
Thanks for all the replies.
What I did yesterday was to put in one of those spark plugs that have the hole so you can adjust the mixture, before it got so wet I could see it was sparking, but I'll take down my inductive strobe as suggested by another forum member tomorrow and double check.
I replaced the carb with one from another V7 which had been ridden the same morning, but I've told my friend to now take his carb and fit it to the other bike, that should rule the carb flooding out.
I sprayed carb cleaner into the venturi and it briefly fired, I think, but this to me sign that yes the spark plug is firing, but firing a very combustible gas briefly and not the very heaving fuel air mixture.
My friend it removing the exhaust today, I'll keep you all informed.
Alan

[Bison]

Oh yes, valve clearances and lift all spot on, and opening  when they should.
It had had the heads overhauled with new seats and valves by a very reputable engineer, and of course the compressions are high.

[Bison]

My friend had fitted the main jet into the pilot drilling and the pilot jet into the main jet drilling on the right hand carb, I suspect fitting the other carb didn't solve the problem because it was swimming in petrol. Another oil change and left overnight to drain is the first thing to do.
No wonder it was running wet!
All is well that ends well.
Alan.

[Tom H]

 

Tom