Mysterious loss of power!

The clue is "rebuilt", almost certainly the valve guides are too close finished ,in accordance with modern practise. It is just possible that with time and with many frustrating stops to cool down the problem will resolve itself,however the valve stems may scuff and the seats may burn. Get them reamed out.

Alloy push rods sound a plausible reason. Aluminium has as about twice the expansion coefficient as iron. Maybe 200C and 6" long - alloy could expand 12 thou more than iron.So if iron rods should have zero clearance it looks like aluminium ones might have to have 12 thou instead. Or you could have 12 thou gaps around the valves...

Hi Stuart,

I think Robert has worked this out, but check the valve material as you may have an austenitic exhaust valve. This material has a very high expansion rate and is identifiable by the fact it is non magnetic and may be stamped KE965, although you will need to take the valve out of the head to check. If it is you will need around 4 thou.of an inch stem clearance which feels like the valve is worn out even when it's new. The valve can also expand length ways which will close thetappet gap. Run the bike until it stops and then immediately take off the tappet cover and check the pushrods to see if they are tight or loose. Tight means it's expanded length ways, loose means it's tight in the guide. It is very unlikely the pushrods will expand to any degree as the tappet gap usually increases as the engine gets hotter. Alloy pushrods were used from around 1950 onwards so there are many bikes running without any problems.

if it does not make any tappet noise when it dies its most probably the push rods. if it does its more like sticking valve or valves. is the valve lifter cable routed ok and condition ok not pulling when moving handle bars ect. Baz.

I would be inclined to agree with David. I have a '54 Dominator, with iron head. A previous owner fitted alloy, instead of steel, pushrods which caused the same problem. I spent ages, playing with carbs, and mag timing, before Phil Hannam put me right. Increased tappet clearances solved the problem instantly.

Previously stuart_munroe wrote:

Thanks John - that is what I'll try first and then move to the head if not resolved. Any idea what gap you introduced and how you measure it.

stuart

Hi Stuart, Obviously the procedure for setting up clearances is different on a Dominator, but, I increased from 3 thou. inlet, and 5 exhaust, to 6 & 8 respectively. Normally clearances are set with the engine cold, but, a possible starting point might be to try 2 thou. with it hot.

If Stuart's engine has runenough milesto the point where it has required an overhaul, all with no apparent problem of suddencompression loss, duetohavingaluminium push rods fitted, I fail to see why they should suddenlybe a problem now after all these years.If by chance this were to be the problem, there would not be asudden loss of engine compression to the point the engine just stops, only a gradualreduction of power. The correct setup is what the book calls for, generally (although the engineyear isn't mentioned) isto be able to freely spin the rods with no percievable up or down movement, cold.

The valves should have .002"- .004" clearance in the cast iron guides.

Paul

I have a '51 engine with iron barrel and head and alloy pushrods. It's never caused a problem. The pushrod tubes on an ES2 are stuck out the side with an air flow over them, they wont get anywhere near as hot as the barrel, so won't expand as much. I belive on a Dominator they are tucked in behind the barrels, so may well get warmer.

I have however had a problem with valves siezing in guides and exactly the problem you describe. If you take the head off and the valves out, the stems will be marked and it will be obvious if that is your problem.

That being said, opening the tappet clearances up a little bit will be a lot quicker and easier to try than taking the head off, so try that first, but I would be suprised if that cured it.

Good luck

Andy

Glad it worked for you. With nearly all problems, it is best to try simple solutions first, before stripping an engine, to rectify faults that don't exist.

Glad you are fixed. We all have to start at the beginning, Basic maintenance on an old bike is a big job compared to todays bikes.

"Basic maintenance on an old bike is a big job compared to todays bikes." Really? Try changing the spark plugs on a Honda Deauville - just one of the reasons I sold it.

Just trying to be encouraging to a newby Gordon, If the basic maintenance was done he would not have needed to ask the question. I would not have any problem with Deauville plugs, as I would not have bought one in the first place!.

In the 50's "Basic" maintenance was just that, no electronic diagnostics, very few bikes with overhead camshaft, and many friendly bike dealers to offer advice and cheap spares. Still, many bikes received little attention, and, it is a tribute to their simple, and rugged, construction that so many have survived.