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750 Ambassador Rebuild

rupeert

rupeert

Just got it firing!
Joined
Nov 15, 2022
Messages
5
Reaction score
12
Location
United Kingdom
Morning All,

I am currently restoring a 1970 Ambassador (US version) and I'm having trouble setting the ignition (I believe). I did a complete overhaul of the engine - most of it it did it myself, but got a machine shop to press small ends, re-machine surfaces of heads, fit new valve guides, grind valve seats and polish the crankshaft. Some time ago I found a Gilardoni pistons and barrels set for a V7 Sport (it was the only set I could find on internet at that time and I bought it) - I know the piston is 0.5 mm smaller in diameter compared to the Ambassador, however since it's a set I didn't think it was an issue. Another important modification is that I got the camshaft refurbished by Joy Engineering, however the only profile they could do was a B10. I've read of some big twins being ok with it (however I am very ignorant on this topic).

I left the valve timing as the original (aligned the paint marks on the gears and pin), I did the valve clearances as per the manual, did the ignition timing as per Guzziology (static timing (second) mark, compression stroke of left (starter motor side) cylinder) and set up the Dell'Orto VHB 29C carbs (I took them apart and cleaned them with an ultrasonic cleaner and reassembled with new seals).

I did a quick compression test and found it to be 125 PSI exact on both sides. Not sure it this is ok considering these are new barrels, pistons and rings?

I have primed the oil pump before firing the engine by undoing one of the banjo bolts that feeds the oil into the headers.

I can get the engine to fire up very irregularly and it dies after a couple of backfires.

I'd be grateful if you could help me understand how to take it from here please. Do I need to do something different considering I now have B10 lobe profile on the camshaft?

Thanks!!
 
Ignition timing may be off. That vintage bike has the timing marks on the generator pulley at the crankshaft. These marks can be off if the pulley is not correctly mounted. I suggest using a degree wheel mounted to the crankshaft to correct the possible timing issue.
 
I managed to fire it up yesterday evening and was purring like a kitten.
What I did:
-Turned the idle mixture half a turn out from what's suggested
-Increased the valve clearance to 0.25 mm intake and 0.3 mm exhaust, as the B10 camshaft has more aggressive ramp profiles.
-Advanced 3-4 degrees from what's suggested.

I don't know whether all this helped, perhaps it's only the advance. I will be turning the idle mixture back to manual recommendation and see if something changes, then I will proceed to changing the valve clearances back to what's recommended.
 
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