1974 Commando No starting or running

I need some help guys. My father and I run a small motorcycle shop in northwest Ohio and our mechanic is a pretty darn good British bike tuner. We have a '74 850 Commando in the shop right now that I can't wrap my head around. Our mechanic Steve is off on medical leave right now (Cancer) and has been trying to coach me over the phone with this thing. The bike came in with carb issue's and needing muffler grommets replaced.

The ticklers were sticking on and it ended up being an issue with the new floats that were in the carbs. I removed his new floats from his new 932's and stuck in a old set from my '70 and his issue with the ticklers sticking went away. When I installed his floats into my bike, my bike had the issue of sticking ticklers/floats. So one new set of floats later, his carbs were not sticking anymore. Bike started and ran fine, idle was great, everything was working awesome. 2 days later, it wouldn't start, nada, nothing, new plugs and I got a little fire, but no start or running.

With Steve off and my limited knowledge, I think I am in over my head right now. The bike will fire up for a second, but only on starting fluid and won't try to do any rpm less then 3Kish. The pilot circuits were clear, mixture screws are a turn and a half out, the bike has choke slides installed and choke is off. I have substituted a known good 12v battery for his marginal at best one. The bike has unmarked coils, which across the positive and negative terminals have 4ohms, which leads me to think 12v coils, but he had a ballast resistor in line before the coils, dropping the voltage from 11.8ish volts to 7 volts coming out to the coils. He has ran this set-up for a while, but I am not positive if this is where my problems lies.

Help!!

Info on bike:
'74 850
Points and condensers are new
Battery is good
Carbs have been cleaned/gone through a good number of times not sure on jet sizes
Power is going into and out of right hand kill switch, contacts were dirty and have been cleaned up
Timing is set at 28 degree's

72westie said:

The bike has unmarked coils, which across the positive and negative terminals have 4ohms, which leads me to think 12v coils, but he had a ballast resistor in line before the coils, dropping the voltage from 11.8ish volts to 7 volts coming out to the coils. He has ran this set-up for a while, but I am not positive if this is where my problems lies.

Click to expand...

The ballast resistor only needs to be used when two 6V coils are wired in parallel (original points 1971-on, as earlier Commandos had 2x 12V coils and no ballast resistor) .
The original 17M6 6V Lucas coil primary resistance would be approximately 1.7-1.9 Ohms and 12V Lucas (17M12) coils would have a primary resistance of 3.3-3.8 Ohms.

I have a couple known (marked) 6v coils here. Both test right around 2.0 ohms. Would having 12v coils wired in with a ballast resistor run though? The bike did start and run pretty good, then one morning, no go.

It would seem odd that it ran fine and then quit unless something failed. The points haven't shifted? The good ole kill switch has been checked? I would be tempted to bypass the handlebar switch. A length of wire with aligator clips from the fuse at the battery to the ballast resistor would be the most direct route. Might not be it, but eliminate the easy stuff first.

Russ

Sorry, I see at the bottom of your post that you have checked for continuity on this circuit. But the advice is probably still sound. Eliminate the easy stuff. You might have a number of small faults that add up.

One point had closed up, but I have adjusted it and the timing was reset. There was a bad exhaust valve adjusting screw, it had mushroomed out and that valve had a ton of clearance, I swapped it out with a good one and re-set the valves.

I had been in the kill switch and cleaned up a bunch of dried brake fluid out of it. I have power going in and out of the kill switch, roughly 11.5 volts in and out and down to resistor and 7ish volts out of resistor. I can go back and get all the volts at the different points, but those are pretty close I think.

Sorry, I see at the bottom of your first post that you have checked for continuity on this circuit. But the advice is probably still sound. Eliminate the easy stuff. You might have a number of small faults that add up.

And how did you adjust the float height?

I agree. I will go through and take apart each connection, clean and use some di-electric grease and follow my power flow again.

swooshdave said:

And how did you adjust the float height?

Click to expand...

I didn't. How do I do that and at what height should it be?

72westie said:

swooshdave said:

And how did you adjust the float height?

Click to expand...

I didn't. How do I do that and at what height should it be?

Click to expand...

As far as adjusting the height, the best way to do it is order some stay up floats and adjust the tang.

http://www.amalcarb.co.uk/TechnicalDetail.aspx?id=14

If you use the old style floats you have to heat the bowl and hammer the seat up or down with a drift. Not too bad, but it's a messy way to set float height and you risk bunging up the needle seat.

What the float height should be is a bit of a debate. Amal puts on their website 0.17" - 0.25" below the bowl level. Other sources say 0.08" below bowl level, but as far as I can tell that has been based on a technical bulletin specifically discussing Amals fitted on Triumphs. With the canted angle of the Amals on a Norton (or at least my Norton) setting it 0.08" below the bowl level causes the floats to never seat the needle and consequently gasoline spills out the tickler nonstop...

Best way to check float height is to check it with the bowl resting on a flat surface and dribble in gasoline until the needle seats. If you don't want to mess with gasoline though due to safety or ventilation issues in your shop, isopropyl alcohol (rubbing alcohol) is very close in specific gravity and a good surrogate.

Others with more experience than I have will tell you to ignore the factory recommended levels and adjust the float level until the mixture screw is 1 1/2 turns out. That seems a bit odd to me though as there are other variables into that issue (throttle valve cutaway, small air leaks, etc.).

In my limited experience on my bike, setting the float level 0.25" below the bowl level gave good tickler action and a decent starting point for setting up the rest of the carb...

Some good sources of info for Amal tuning in general:

Bushman's Guide (note uses the 0.08" float level height)
http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

Rebuild notes from Burlen
http://www.amalcarb.co.uk/TechnicalDetail.aspx?id=11

AMR notes for setting idle
http://www.amr-of-tucson.com/idle_adj.html

Cheers,

- HJ

Check those connections to earth (ground) as well!