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Poor running on newly rebuilt model 50

Submitted by jak_lemon1 on Sun, 27/06/2021
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Hi, I don’t know if anyone can help and I realise that diagnosing problems over the Internet is difficult. I have recently got my Model 50 going after a long rebuild. It has had new main bearings, valves and guides. I cleaned and replace the jets and needle in the carb. It had been converted tom12v +vie earth before I got it. I have replaced the petrol with new. 

The bike starts from cold easily and will run on the stand. If it is on the flat (difficult where I live) it seems to run okay, but going up hill or if you open the throttle it tends to cough and falter and will stall. I have checked the needle is on the second groove from top which I believe is the correct position. Changed the plug cap and checked to see if I can see any loose electrical connections. Before I start doing anything else is there anything obvious I should check.

Thanks

Submitted by david_cooper on Sun, 27/06/2021

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Even if you cleaned the carb, it might be worth looking at the main jet. Is the air slide down? If the easy things are clear  I'd check the timing.  Given more than one figure in the books (and some do vary), I'd choose the more advanced. You can always retard it a bit if need be.  All a bit obvious...I'm sorry if its familiar stuff.

Submitted by neil_wyatt on Sun, 27/06/2021

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Jak, you don't say what year your bike is. Assuming you are using points, given the symptoms you suggest I would first check to see if the points gap is too big.

I have two model 50's, great machines. one on standard gearing = 18 teeth on the engine sprocket and 19 on the gearbox and no problems going up hills etc.

The other machine I put a 20 tooth gearbox sprocket on and it struggles a bit up the hills. 

This is where I would start.

Submitted by michael_sullivan on Thu, 01/07/2021

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Jak:

If it has new valves there is a slight chance that now that the engine ran a little, the valves might have recessed into the valve seat pockets to the point that the clearances have closed up and are causing a leak of compression.......

Let us know what you have found out

Mike.

.

 

Submitted by andrew_westmore on Thu, 01/07/2021

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Jak, I had a similar problem with a different bike. It turned out that the float bowl wasn't filling at a rate to support fast running or up hills. Once the fuel level dropped the motor would lack power, pull over and the bowl would be replenished ; until the next hill or throttle opening.

Check the fuel tank vent, any fuel filters and fuel lines.

 

Submitted by michael_sullivan on Thu, 01/07/2021

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Are there any identifying marks on the old juts and needles?

Also what are the marks on the new jets and needle?

Mike

Submitted by lloyd_turner on Thu, 08/07/2021

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Those fine screens on the petrol taps can look good but sometimes are plugged up enough to restrict the flow into the carb.  Disconnect the fuel line at the carb and direct it into a container.  With both taps off there should be no fuel flow of course.  Open one of them and make sure the flow is steady if it is close it and open the other.  Same thing make sure it's a steady flow.  It doesn't need to gush out but it should be sufficient to flow steady.  You may need to remove them and spray with carb or brake cleaner.  My M50 is a 55 with push/pull taps.  My bike sat for over 20 years and was not stored properly.  The Carb was ok,  but those fine screens needed cleaning.

 

 


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