Forum

[Bandit12]

Hi guys, I've a head scratching issue with my 1959 big head bullet. Starts easily and goes up to 60+mph but after 5 miles or so starts to run out of power,speed drops, engine starts to vibrate and increase in noise, it feels like its going to seize. If I push it then the engine will stop. After 5 minutes rest by the side of the road it starts fine, I ride off and then the cycle starts again. If i dont push it and slow down to 40 then the problem does not occur.
I've double checked ignition timing, valve timing, replaced auto advance springs in distributor and fitted new premier concentric. There is plenty of compression. Oil appears to be circulating but wonder if the oil pump is worn out and if so how to check it?
Any advice is appreciated.

Bandit

[Jamesy]

Fuel starvation?At higher speed petrol not being fed quick enough to carb.Tank clean?Tap clean?carb neading cleaned?

[Aethelric]

Petrol tank breather blocked?

[vince]

Hi, does the engine backfire as it comes to a stop?

[Bandit12]

thanks for the responses guys. tank has recently been cleaned, sealed and re chromed, new fuel tap, filter and petrol cap from hosts. Bike does not backfire when it comes to a stop. Spark plug is chocolate brown with a bit sooty round the edge. Currently running a 260 mainjet but will try a 250. I do wonder if this is an oil issue. Any way of checking the efficiency of the pump?

[Adrian]

It sounds like you might have had/are getting a **partial** seizure, with the heat causing high spots to form on the piston skirt. Thus the engine kicks over fine and runs when cold, but once everything starts getting towards working temperature and expanding, the high spots are effectively giving you a slightly oversize piston which now has insufficient clearance against the cylinder wall. I had this happen with a recently-rebored BSA C15 years ago. The guy who did the bore had this wonderful new machine that left the perfect-but-rough running in surface that needed less piston skirt clearance, or so he said.



Once someone (else) explained what the most like source of the problem and its fix, I was able to salvage the piston. After stripping the top-end the high spots were apparent as shiny patches on the piston skirt. I was able to relieve these by careful draw-filing so that the piston had adequate clearance, and the engine ran fine after that, though I was lucky in that the cylinder bore was still OK. You will have to see what condition your bore and piston are in, if they're too badly mauled it will be new piston and rebore time. Maybe post some pictures of what's been going on in there?



Our hosts technical notes have some advice about oil pump checks, I believe.



A.

[PO51UHD]

I had a similar problem on ‘Connie’, my ‘59 Constellation. Turned out to be the cheapo fuel tap I’d installed when rebuilding. MrH sells a range of taps, some with bigger flow rates, and I fitted one and solved my problem.

Stephen

[Adrian]

That would be a much easier fix, of course.



Meanwhile, if it IS something as simple as fuel starvation don't forget to check the little breather holes in the fuel tank cap as well as the tap. I have had Indian Bullet tank caps that have brought a bike to a stand still, if the air can't get in fast enough to replace the petrol that leaves the tank, a partial vacuum builds up, and after a few miles the remaining petrol can't get out until more air finds its way into the tank. If you're still using the original tank cap it's had a few years to create a partial or complete blockage of the vent.



A.

[Adrian]

Sorry, missed Aethelric's post about the filler cap. Please ignore my last one.



A.

[Bandit12]

Thanks for all the advice guys. I have had bikes suffer with fuel starvation and vapour lock before it feels more like a seizure with increasing top end noise and vibration till it comes to a stop. Adrians explanation of hot spots sound likely so I will take the head and barrel off and inspect the bore and piston. Another idea a mate suggested was the spark plug might be too hot. I'm running the NGK B8ES supplied by our hosts. Never fully understood about hot and cold plugs. Worth trying a different one?

Bandit