1968 Commando: left cylinder does not start (2016)
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Sounds like an air leak to me, if you have some cold start aerosol, or suchlike, try spraying it around the exterior of the inlet tract, see if it helps it fire below 2500rpm.
Also you try an air gap spark test. A spark on a plug in the open air only requires a very low voltage (under compression it takes a lot more voltage) , so fix up an air gap tester, make a divider out of chop sticks or whatever (used to be able to get one by Gunsons, probably no longer available), so you can test the gap above which the spark stops. Unless you can get over 1/2" then you don't have enough voltage for a healthy spark in compression conditions. This however struggles to explain running above 2500 rpm.
Other thoughts, how old are the points? can you swap them and the condenser over from side to side and see what happens?
Also you try an air gap spark test. A spark on a plug in the open air only requires a very low voltage (under compression it takes a lot more voltage) , so fix up an air gap tester, make a divider out of chop sticks or whatever (used to be able to get one by Gunsons, probably no longer available), so you can test the gap above which the spark stops. Unless you can get over 1/2" then you don't have enough voltage for a healthy spark in compression conditions. This however struggles to explain running above 2500 rpm.
Other thoughts, how old are the points? can you swap them and the condenser over from side to side and see what happens?
Clanger said:
Hi and thank you.
To day i have changed the points with aBoyer E.I. (but a very old one).
The bike starts at first chick , but runs well only over 2500 rpm with the left carb with the air pilot asjiuster screw totally unscrewed.
I dont think is air leak but the carb.
Ciao
Piero
texasSlick
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Rohan said:
I would agree, but Piero claims to have changed carb with one from a presumably well running bike. What are the odds that the second carb would exhibit the same symptoms? That is the fact that I cannot resolve.
When I summarized all the swaps, change outs, and the results that followed, I am left with only blocked idle passages as the likely culprit, ..... except for that post " I changed the left carb. The left cylinder still did not start!"
The mystery continues ..... It will be informative to all when Piero reports the cause.
Slick
If you can give the throttle a bit of a tweak, and both cylinders chime in fully and strongly,
then it can't be anything but a plugged idle jet system on one carb ...
(Assuming all the ignition and spark plugs and throttle cables have been checked)
Have we seen that both spark plugs have been changed for NEW ones ???
First Commando I ever touched had 2 carbs like that.
It would start, with a good tickle on both carbs, but nothing under 2000 rpms.
Had to keep blipping the throttle to keep it alive.
A very thorough cleaning of the carbs brought back an idle on both cylinders.
After about 5 cleanings of the carbs, there are some very tough to reach places in those things...
then it can't be anything but a plugged idle jet system on one carb ...
(Assuming all the ignition and spark plugs and throttle cables have been checked)
Have we seen that both spark plugs have been changed for NEW ones ???
First Commando I ever touched had 2 carbs like that.
It would start, with a good tickle on both carbs, but nothing under 2000 rpms.
Had to keep blipping the throttle to keep it alive.
A very thorough cleaning of the carbs brought back an idle on both cylinders.
After about 5 cleanings of the carbs, there are some very tough to reach places in those things...
o0norton0o
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Classic plugged idle jet.... You can lead a horse to water Rohan... 8)
I've had this issue with the same symptons. I was sure I had cleared the jet, but I hadn't. I ended up pulling the carb and drilling it for the bushman's carb modification, so I could visually inspect the jet (which was plugged solid). I threaded the hole I drilled and put a set screw in it to reseal the passage after I cleaned the jet.
I've had this issue with the same symptons. I was sure I had cleared the jet, but I hadn't. I ended up pulling the carb and drilling it for the bushman's carb modification, so I could visually inspect the jet (which was plugged solid). I threaded the hole I drilled and put a set screw in it to reseal the passage after I cleaned the jet.
Hi.
Today i have opened the other side of the pilot adjuster screw of left carb.
So i have cleaned better the idle circuit.
The spray cleaner enters and exits trough all the holes.
Nothing to do.
The left cylinder doesnt start the same.
But, is the second time i can see, trought the left pipe/muffler exits a black liquid oil!
Valve?
Thanks.
Ciao.
Piero
Today i have opened the other side of the pilot adjuster screw of left carb.
So i have cleaned better the idle circuit.
The spray cleaner enters and exits trough all the holes.
Nothing to do.
The left cylinder doesnt start the same.
But, is the second time i can see, trought the left pipe/muffler exits a black liquid oil!
Valve?
Thanks.
Ciao.
Piero
texasSlick
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concours said:
Piero:
Have you tried concours advice as above? What is result?
You wrote that you CHANGED the LEFT carb and there was NO change in the problem. Are you sure? Your last post indicates a problem with the idle circuits, which many of us have taken to be the cause of your problem. But a blocked idle circuit problem should not have carried over to a second carb. I hope I am making myself understood, considering English is not your first language.
Keep us informed. We are here to help.
Slick
One more thing to check:
Early carbs had the pilot jet screwed into the bottom of the main body.
Suggest you remove the idle (pilot) air screw and shine a light inside to verify that your carb has the pressed in jet.
The early type do not have the jet in there.
I had this problem once on an old Amal carb, I needed to screw in the correct pilot jet.
Early carbs had the pilot jet screwed into the bottom of the main body.
Suggest you remove the idle (pilot) air screw and shine a light inside to verify that your carb has the pressed in jet.
The early type do not have the jet in there.
I had this problem once on an old Amal carb, I needed to screw in the correct pilot jet.
o0norton0o
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pierodn said:
... If you want to eliminate the carb as the problem, then maybe you could swap a carb which you know works from one of your other bikes to the problem bike in order to see if that cures the problem. If the left cylinder now works, then you know it's the previous carbs. If the left cylinder still doesn't work, then you know the carb isn't the problem...
I know it's a hassle to take a good running bike apart to test the bad running bike, but once you get to the point where you're not getting results, sometimes substituting known good parts helps to narrow down the search for the problem.
texasSlick
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o0norton0o said:
Your post is entirely. logical. In an earlier post, Piero stated he took the second carb from a running (we assume running well) bike. It is improbable that both carbs would behave the same. However, as this thread has developed, it begins to appear more and more that both suffer from a problem in the pilot (idle) circuits.
Bob Z. makes a good point Perhaps Piero's problem stems not from a lack of fuel, but from an excessively rich condition, which would not fire. When Piero screws out the air screw entirely, it leans out enough to fire as he reports.
I think a scrupulous examination of the idle circuits, with particularl attention to the pilot jet as Bob Z suggests, is the next step Piero should do.
Slick
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