Running erratically

E

Eggsy

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Apr 28, 2025
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I just bought a 1973 750 commando that has been sitting for a while. It starts well and runs fine for a bit then starts missing and running on one cylinder then it goes back to normal.
I know the carbs are old. I’m wondering if I should get new carbs or ?
 
Eggsy said:
I just bought a 1973 750 commando that has been sitting for a while. It starts well and runs fine for a bit then starts missing and running on one cylinder then it goes back to normal.
I know the carbs are old. I’m wondering if I should get new carbs or ?
Click to expand...
Is this all at idle or what throttle opening
 
Eggsy said:
I just bought a 1973 750 commando that has been sitting for a while. It starts well and runs fine for a bit then starts missing and running on one cylinder then it goes back to normal.
I know the carbs are old. I’m wondering if I should get new carbs or ?
Click to expand...
Hi, welcome.
You've cast a pretty wide net there.
Please share with us what your skill set is... it will help us help you.
Cheers
 
The standard things you need to do after a bike has been standing a time.

1. Start at the battery and put a multimeter across every connection in the wiring system. Including the keyed main switch, the black plugin connectors under the tank and especially any kill switch if fitted. You will often find several connection with high resistance. Clean and apply dielectric grease. Check that no wires are jammed or broken at the steering head. Wires can be cut or broken with the plastic sheilding still in place and only fail if the bars are moved.

2. Check the spark plug wires and caps are in good shape and again check the resistance. Fit new genuine plugs.

3. Clean the carbs. Very carefully. Concentrate on cleaning out the pilot jet and idle passages. Check the various jets, needles and settings are standard for your model. Use new petrol.

Those three steps will solve 90 % of issues.

Note I'm assuming you have changed the oils filter etc. And given the whole bike a decent service and oily rag going over. The clutch may need a clean.
 
kommando said:
Plus are you using the choke, and do you know its works the opposite to a normal choke. Also are you tickling the carbs and if so by how much and how many times.
Click to expand...
Yes your right. First question should always be.

You know which way is on for the choke right?
 
johnm said:
Yes your right. First question should always be.
Click to expand...
Most of us on this board have had a Commando for decades and also it was not our first British bike so its easy to forget the problem the choke gives when you assume it works the same way every other choke does.

Choke on Amal's, when the choke cable is loose then the choke is on and the mixture is rich, when the lever is moved enough and the cable is tight the choke is off and the mixture normal.
 
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kommando said:
Most of us on this board have had a Commando for decades and also it was not our first bike so its easy to forget the problem the choke gives when you assume it works the same way every other choke does.

Choke on Amal's, when the choke cable is loose then the choke is on and the mixture is rich, when the lever is moved enough and the cable is tight the choke is off and the mixture normal.
Click to expand...

@Eggsy
Can you confirm the choke setting you are using?
 
Since it was stated only firing on one side, then suspect a failing ignition coil or its connections. If bike is still using points, there could be a problem with the contacts for tge faulty side.

A quick ceck is to swap the HT leads left for right and see if the problem moves to other side.
 
Make sure the throttle slides go up together , my cable splice had too much clearance on the outer, on one carb, and it got cocked and the result was one slide going up about 5 mm before the other , cut two bits of 2.4 welding wire and stick them up the back of the carb under the slides to check sync. And do everything everyone else says 😀
 
Thank you all for your input. My apologies for not replying quickly. The bike does not have a choke.
I have been busy changing the broken throttle cable, checking the valves, etc. etc.
cheers
 
If no choke is fitted then if it starts well and once its up to temp it also runs well, then its too be expected as it warms up it needs babying. There is no ECU to keep the mixture correct at all temps. So you tickle the carbs before starting to get a rich mixture by raising the fuel level in the float which you have right as it starts well. Then as the engine warms up the fuel level is also dropping and leaning out the mixture, if it leans out too quickly then the engine will falter, too slowly and the same will happen as the mixture will instead be too rich.

So next time you start the bike, tickle the normal amount and then as it starts to miss tickle for a second time. If it improves then the mix is lean, if it gets worse then the mix is too rich.

If the mix is lean then in future either tickle twice or fit a choke, too rich then apply some throttle for a min or so and live with it.
 
I was just reading the first post. It is unusual that the motor runs well for a bit then goes onto one cylinder. It sounds to me as though it might be spark rather than jetting. One of the plugs might of a different heat range to the other and might be fouling. If the motor coughs and spits when the choke is taken off, after the motor is warm - that is a sign of lean-ness. I would look at the carbon on the porcelain in the plugs. If one hasn't burned the carbon off properly it might have a different heat range. I would also take note if one of the plugs looks greasy - might be a rings problem. If you place your hand behind the silencers when the motor is running on one cylinder, you should be able to tell which cylinder has gone duff.
 
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What ya wanna DO , Is - Take the points cover off , and turn off the lights . In the Shed .

If it resembles a catherine wheel , its leaking .

A HOT WIRE , Battery to coils , to bypass gungy olde switches , will tell you if the gungey olde switches are gungey .

CARBS + inspect , if the SLIDE ratlles back & forward , ands scoured , Think about a Weber or other setup . Maybe .

Ya wanna getta flashlight & insect in the fuel tank . Drain , Clean Filters etc . Flow Test outta da TAPS . G per Hr .

It might not have Viton Tipped carb float NEEDLES .

Get to knowit before you stuff with it . Check the carbs arnt loose - a metre of 4 mm bore neoprene tube , in yer ear .

MOSTLY its fettling , Fine Adj. off mechanisms . If its a Boyer a naff battery they says ruins your day . Gotta Be FULL CHARGE .
 
If its a ' Mk V ' , 32 Mk II Amals , and a BSA A65 Advance Unit , will have it sit at 500 rpm idle , all day .
Better intake manifolds dont slowem down . But a 45 weber or dellorto wouldnt hurt . But a bloke threw
a H D FLX or somesuch carb on one , back a bit - So intakes V not S - For RAM induction .

Id think about a bigger oil tank , if Yr gunna Ride It . Throw away all the trash under the seat . K&N filters .
 
Dear Mr. Spenser: Apologies, but some of us Yanks do not speak Aussie. Please translate: What is a "catherine wheel"? Why would you want to get "insect" in the fuel tank? What is G per Hr? And I assume a naff battery is flat, no charge
 
Matt Spencer is probably a ten pound Pom, pretending to be an Aussie. Most of us don't talk like that. I had to laugh, my wife upset a British lady by calling her a Pommie. She said 'there are no convicts in my family' - class conscious ? With us 'Pommie' is a term of endearment. They do not know why we enjoy beating them at cricket, so much.
 
New spark plugs . Battery at full charge or new .
Matt has a good idea to temporarily hot wire to ignition to bypass almost all possible electrical issues . Detective work .
 


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