Strange one, appreciate views

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I've got (amongst others) a 750 Commando which was running very hot, appeared to have got to the bottom of that by setting the timing properly with a degree wheel. Now it won't run worth a damn, it runs like the choke is on all the time and smokes like a 2 stroke! I'm about to lift the head (tomorrow) but before I do I'll run the things I've done to cure this past you clever people and see if I'm missing anything!

Changed the carb (single Mikuni) with another Commando I have
Changed the complete ignition system including coils
Wired the ignition direct to the battery in case I was getting a voltage drop
Checked valve lift & Clearance (My Interstate has just run a bloody cam so I know what it looks like)
Compression reads over 150 PSI on each side and leakdown shows it still holding 110 PSI after 5 Minutes
Checked engine breather isn't blocked
Changed plugs coils & leads
Checked valve timing & put in new valve chain & tensioner as the old one was worn.

All to no avail!

Problem I have is I don't mind lifting the head as its only an hours worth of work but what the hell could be causing this?
I've reason to believe this motor is very low mileage standard 750
Very confused of Perth :evil:
 
Sounds like you've eliminated any possibility of cam/timing issues, carb issues, ring issues and spark issues.

What did the plugs look like? Were they fouled? If its smoking like a two-stroke, could you be dumping oil down the valve guides? This may not mean worn out guides. If the engine was running really hot, you may have baked the intake valve stem seals, causing too much oil to enter the chambers. I saw this on my wife's car (many, many years ago, when we were dating). She overheated the car, carbonizing the stem seals, resulting in poor performance and high oil consumption. I replaced the stem seals, and consumption disappeared.
 
The oil can lower the octane rating of the fuel to the degree that you are suffering from detonation. In this case what came first the bad timing or the oil burning. If the high heat was centered at the exhaust system you would think retarded timing,if it seemed to be centered at the jug and head than detonation. So what is too hot? the pipes, the head,the jug, the oil?
 
Hi Gino, was it smoking and faffing before you started or just hot ?

Did you put the degree disc on the cam timing ? It just occurs to me that if the wheels are marked incorrectly, a PO might have set the ignition timing incorrectly in an attempt to disguise the symptoms. Could it be that you're pumping unburned fuel out ?

If everything is to spec then it must be damage caused by the overheating I guess. You realise that if you've managed a hole in one, you've got to buy a round for the whole club-house, don't you ? :)
 
I guess I'd wonder about a couple of things. If it was over heating and then you set the timeing did it immediately start to run badly? If so I wonder if the timing is set correctly?
I remember when I put an Atlas crank in my Commando I had at least three goes with the degree wheel before I got it correct and everything in the right direction.
 
No Stone Unturned

Given the impressive list of things you have checked/adjusted/replaced the only item left, as has been mentioned above is your valve guides. A toasted seal and a worn guide will skew the mixture plenty, and with the compression/leak down test results you got I'd say that incorrect timing may have caused the extra heat necessary to fry your intake valve seals, adding oil (and additional uncorrected air) which compounded the problem (runs leaner and leaner), as oil burns hotter cokes up the combustion chambers increases the effective CR.

Gee at some point (small as it may have been) you must have had one really fast Norton :D

The perfect storm, things just get worse feeding off themselves.

If you see your head all coked up do look hard at the bore for score marks or major discoloration (heat effect). If any of these signs are present I suggest you pull the barrel and check pistions and ring end-gap.

Replace parts as necessary, call us in the morning...

RS
 
Did you ever put the timing back to where it was before you started. you could blank off the rocker feed to stop oilflow to valve guides.Oil coming down the guides will usually still burn a dry plug.Check down the plug holes with a small 12v led light for signs of a piston seziure,exsses oil in the bores will also raise your compression.As you only changed the timing and it runs badly with 2 different carbs Ithink I would go back to the start of your changes.
 
Thanks guys for all you ideas, looks like I really need to lift the head and investigate which knowing me will end up being a full engine rebuild!

I've got my 850 Mk1 in a million bits at the moment replacing the cam so don't need 2 Commando's spread over the garage at this time. I'm going to park the 750 till next winter (I'm lucky enough to have 3 others to play with in the summer)

I've noted (printed off) your ideas so will check all the good stuff when I get round to it, many thanks for the help guy's much apreciated Oh and 79X100 I'll see you at the BBA in Belgium hopefully and buy a round there anyway or remembering last year you may want to skip it! :lol: :lol:
 
Gino, I have run across about ten machines in the last year that have had bad rotors I do BSA work.... Is there a chance that you checked the mark on the rotor when you did the degree wheel? On real bad rotors you can feel the slop around the core but before this happens they beat up the key in the crank. Have you removed you rotor to check this? When you where strobe ing the machine did you use this mark on the rotor or the degree wheel to reset the timing? Is the heat you speak of at the pipes or the the head?
 
norbsa48503 said:
Gino, I have run across about ten machines in the last year that have had bad rotors I do BSA work.... Is there a chance that you checked the mark on the rotor when you did the degree wheel? On real bad rotors you can feel the slop around the core but before this happens they beat up the key in the crank. Have you removed you rotor to check this? When you where strobe ing the machine did you use this mark on the rotor or the degree wheel to reset the timing? Is the heat you speak of at the pipes or the the head?

Norbsa
I degreed the crank in with the rotor & Stator on (it is possible) then marked the stator to align with the mark on the rotor at 31 degrees. Also had the rotor off several times and its all a good fit. The head was getting very hot, before it started and ran fine but as soon as you stopped in traffic it heated up like hell and started to run very lumpy and smoke. It's probably only done 4 -5 trips of 60 miles or less since I finished it. (Reframed after the guy I bought it off hit a car) I'm going to pull it apart as I suspect the damage may be done now but like I said not till later, too many good Commando's in the garage giving me no grieff :lol:
 
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