motor won't pull

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lawman

lawman

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May 29, 2013
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I have a 73 850 that is running very poorly and I must admit that I am not very mechanically minded.

It won't accelerate very well and coughs and splutters. When the throttle is rolled off it backfires very badly.

When I took it for a run to see how it was going a guy in a car pulled up beside me and said that the bike was running very rich and all he could smell was petrol.

If this is correct how do I set the carbs to sort it out?

Thanks

Frank
 
The air:fuel mixture is regulated by the pilot circuit at idle. This is adjusted with a screw on the side of the carb body. The mixture is regulated mostly by the needle and jet assembly at higher RPM. This is a function of both the size of the jet and corresponding needle and how the needle is adjusted (by a clip in grooves) in relation to the jet body. Raising the clip (lowering the needle) will cause the taper to drop deeper into the jet and decrease the amount of fuel (lean out the mix).

At wide open throttle it is all about the size of the jet. But my guess is you are not getting there. It is also possible that if the float is adjusted very high or the float valve is sticking open that you might be getting fuel into the venturi. It would be worth checking those items while you have the carbs off.

Go to the tech section of the forum and scroll down the list. There are links there for the Amal Carb Company and various tunning guides:
technical-information-how-post-photos-t2357.html

Russ
 
TROUBLESHOOT, don't adjust. If it was running well, don't start changing jets, screw settings. Check for things like a sunken float, failed needle/seat, etc.

"not very mechanically minded" :oops: owning a vintage BritBike... you WILL be! :lol:

Is there a mechanic that you use that knows these old bikes?
 
Yes to troubleshooting as you are way past fine adjustment techniques. Change out sparkplugs. Fit viton tipped float needles if old. Check needles and needle jets for ovalitys or replace as sets new. Check floats are floating,no fuel inside. New stay-up type an option. What's the mileage on Amals and age of components ?
 
Thanks for the info and the Amals are all but new.

I will learn how these bikes operate as that was part of the reason for buying it.
 
Hi
Didn't you just put in the choke, are you sure it is in the correct position. Also are the cables fitted all the way into the slides, mine was fitted incorrectly at manufacture.
 
You don't say what carbs you have, but if Amals could be your main jets have come loose. The symptoms you describe fit this. Remove the fuel bowls, have a look.
 
lawman said:
I have a 73 850 that is running very poorly and I must admit that I am not very mechanically minded.

It won't accelerate very well and coughs and splutters. When the throttle is rolled off it backfires very badly.

When I took it for a run to see how it was going a guy in a car pulled up beside me and said that the bike was running very rich and all he could smell was petrol.

If this is correct how do I set the carbs to sort it out?

Thanks

Frank
Click to expand...

Since you refer to "carbs" it would be very helpful if you could ascertain which may the one with the problem.
Any chance you could ride it with someone following to see if one pipe putting out dark smoke.
Maybe a sniff test while standing.
Maybe a coil breaking down if problem on one cylinder.
If so try swapping coils to see if problem follows.
Try new plug on bad side since may be a failing plug.

If problem on both cylinders it may be an ignition problem causing intermittant firing.
Standard points or electronic ignition? If EI check all wires and connections.
I'd try a timing light to see if sparking is regular.
Possible problem in ignition switch or kill switch.
 
Chokes are best lost , asit doesnt snow in Sydney that often .

You should have a mnimum 3/8 spark ( 10 mm ) from the H T .
best to put a strobe on ea. cylinder and see thats correct .

Do you know if its electronic or points ??

A few specs on the Bike / & where in Syd.
Could glare at it to dislodge gremlins .
 
lawman said:
I have a 73 850 that is running very poorly
Click to expand...

Lets start at the beginning.

Bike was hard to start. So you fitted starter and choke and Trispark. Trispark is good, choke maybe not so. A primitive motor, easy to troubleshoot. Pull the spark plugs and let us know if they are black and wet. Pics really help.

Sounds like choke is the place to start.

What area are you ?

Cheers

Mark
 
You sure your not running around with the choke on.
Choke lever on handle bars should be pulling cable tight to have choke off.
Your better off just leaving it that way. Even when cold. Tickling is better for cold starting.
Whatever you do though remember the golden rule for fault finding. Change only one thing at a time. :wink:
 
The bike is going back to have carbs looked at by the guys that installed the chokes. The choke is definitely in the off position. It may be best if they are removed. (the chokes not the carbs)
 
Are you sure the choke is out? If the bike ran well previously you may have the choke all the way in and not realize it. Did this once many years ago leaving a flat track down in Delaware. It was dark and I couldn't see that I had the choke on. At that time I always used the choke to start, not smart but I was young. Ran terribly and I was pulling the plugs and setting the points and timing in a parking lot before I realized how stupid I was. Cable out for choke off.
 
The choke is definitely off. The bike is in with the Mechanics who tell me it is an ignition issue.

Frank
 
In a previous post he said Tri-Spark, Frank which part of the Emerald City do you hale from?
Chili
 
We hope your mechanic has heard the quote that
99% of carb problems are electrics, and
99% of electrical problems are carbs.

Chugging black smoke and raw fuel is a strange sounding ignition problem, it must be said.
Not running on one cylinder may be more an issue, but that doesn't usually involve black smoke.
Especially after newly fitted choke slides are involved....
 
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