Engine misses and loses power

[BigJim]

Help, anyone, Symptoms are loss of power and missing when held at a steady 50 to 60. When i slow it idles fine and resets itself and i turn round and come home, with less venom on the throttle, and bike runs fine.
Recent history:
Carb is clean and unchanged (was running well, if a little rich), fuel comes out tank fast and clean.
Spark is good when tested on kickover.
Have recently replaced head with new valves, guides and srm spring holder thingys.
Will be changing back to my old springs probably as nervous of these new stiff beggars.
Am looking at the new head as that is the only thing that has changed prior to running well recently.
Cheers
 
New ngk plugs fitted could they fail in this way?

[Greybeard]

I had something like this happen after I'd bungied a map in a plastic sleeve over the fuel tank. Short version; the vent hole was getting blocked, but only when the bike was going fast. Just check the vent is clear.

[muskrat]

G'day BJ.
You may have set the tappets a little tight. Back them off 2-3 thou and try again.
Cheers

[Triton Thrasher]

Check fuel flow.

If 400 cc runs out of the bottom of the carburettor in a minute, you don’t have a flow problem.

[BigJim]

Thanks chaps, brandy has kicked in.
Breather clear, blown through, big flow from both taps.
Tappets loose and noisy.
Am i missing a trick?

[bsa-bill]

New ngk plugs fitted could they fail in this way?

Modern plugs may work faultlessly in modern engines but can fail in our classic bikes if you haven't already done it change them back to the ones you had in when it worked well

[BigJim]

Will do,also loosen tappets to even noisier in the morning for a quick blast.

[muskrat]

G'day BJ.
As TT said flow from the carb not the taps. May have restriction in the hose or fittings.
Cheers

[BigJim]

Filter clean on carb. will change pipes.

[Scott and Jay]

I had very similar symptoms last year. I ran a long topic under "Does Needle Jet Behavior Change over time". I went through all the easier causes (and appreciated all the help I got through the forum), but in the end I had to rule out whole areas. I replaced the ignition system first (with a temporary coil system). That didn't fix it. I then replaced my new carburetor with my old one (which wasn't really that bad). Still no affect, so we had to conclude it was mechanical and I took the head off. My exhaust valves had got "sticky" after about 4k miles. I had gone the K-liner route, inlet and exhaust. My engineer rebated the exhaust ones a bit at the base (where the oil gets baked on to the valve stems) and, I think, increased the clearance (maybe to 3 thou "). He made that choice because the inlet liners were perfect. It wasn't causing the motor to seize, because if I slowed down (like you) - the valve springs overcame the inertia. Just in case this helps...

[BigJim]

Thank you Scott and Jay. Had been trying to find your post re valves. Am suspicious of these as newly fitted by someone else. Will change plugs and pipes first then if no joy the head is coming off (again!) and will be stripping and inspecting valves. It's really odd that performance at low revs is acceptable, will let me get to 50 then misfires etc. Never had this before.

[BigJim]

Ok, here's what happened. Changed to old plugs, new fuel pipe and opened out tappets. Exhaust to 20 thou and inlet to 14 thou. Thought i had cracked it but then the misfire and powerloss returned.
Will get back in the garage and will be taking the head off, removing valves and be chasing ideas on how to ease them out in their new guides?

[RichardL]

Jim,

Thanks to our excellent Search function, I see that you might have a concentric carb, and also found the following service sheet posted by JulianS. I did this mod after having power loss at around 65 MPH and have not had problem since. Granted, I've rarely gone that fast since that time.

https://www.a7a10.net/forum/index.php?topic=11619.msg88749#msg88749

Richard L.

[RichardL]

...and here is the tediously wordy post where I described doing it (uuuh, modifying the float bowl, that is).

https://www.a7a10.net/forum/index.php?topic=11619.msg89380#msg89380

Richard L.

[BigJim]

Hi Richard and thank you for response. My thoughts are valve based as the exact same set up (concentric carb/ignition etc) was working fine just prior to new head, guides, valves collets seats. The only issue i had with fast running previously was oil leaks through a crack and trashed threads on rocker box holding down bolts. In fact she ran well up to 70 mph. Maybe the new to carb inlet track is affecting it? The plugs look a normal colour?