Running weak on one side

[muskrat]

G'day Fellas.
I haven't been happy with the Cafe lately. Seemed to be a little weak on the left pot. Hasn't had much running since new top end 80 over  . Bit hard to measure compression with a 23 engine sprocket and takes all my weight to kick it twice let alone 10 to do a proper test. Two identical carbs and identical spark on both. So something is wrong so I ripped the top end off. Tested the valves aren't leaking so the barrels were next. Surprise surprise, the oil ring on the right has a piece missing. Lucky it stayed in the groove, no damage to piston or bore. Still no reason why the left felt weak. The rings on both sides are identical gap 
Now to find a set of 72mm rings, probably end up wit Trihard T120 +40 if I can find a GOOD set. I won't use Hepolite or JP (most of them come from the same factory and distributed by Asia Pacific and put in their own packets). The hunt is on 
Cheers

[BagONails]

Good pick up Musky! I wonder if that happened on assembly? Did you use ring compressors? I must admit I'm not keen on them. When I put mine back together recently I popped the pistons and rings back into the barrels first, easing the rings in very gently with my finger nails and no pressure on the pistons to enter the bores. It was an easy job then to line the small ends up with the barrels supported and just push the wrist pins in with a bit of warming from a hot air gun on the pistons. Gotta make sure you put the base gasket on first though...

[chaterlea25]

Hi Musky,
Check the valve guides?
Maybe the left side is drawing air down the guide?

John

[muskrat]

G'day Ian.
I use poly pipe ring compressors that Ive used all my life. Only ever had a problem once with three piece oil rings. One "rail" got caught and ruined the bore. The rings were Hep's that I don't like, there is a chance it happened on assembly.

G'day John.
It could well be the guides. I'll pull the valves today and check. If I have to order new ones I'll convert the inlets to take seals.
Cheers 

[muskrat]

G'day John.
OK I pulled the valves. All 4 are good (as the day installed), inlet no perceivable rock at full lift and only the barest on the exhaust.
Where to now 
If one side would give trouble I'd expect the right side as this was the side that had the dropped valve and head repair.  https://www.a7a10.net/forum/index.php?topic=10630.msg79189#msg79189
Cheers

[muskrat]

G'day Fellas.
I inquired about a set of GPM +40 T120 rings at Steadfast Cycles in the US. The rings are $105AU but the freight to me is $115AU  . I could get the same size parcel (3"x3"x2") from the UK for 1/4 of that. Looks like the new Trumpet doesn't want to trade with the world, either in (tariffs) or out (post office).
Will keep looking.
Cheers

[Jules]

Hi Musky, I just had a similar experience with car vinyl roof material quote from the US, last year it was US$95/yard, they just requoted me US$125/yard plus postage of course!!

[Triton Thrasher]

It’s usually worth checking that both carb needles are clipped where you think they are.

At least I’ve found it worth checking!

[Triton Thrasher]

If it’s only “a little weak” on one cylinder, that doesn’t have to mean there’s a real diagnosable problem. 

One of the good things about twin carburettors is the facility to tune the mixture on individual cylinders.

[a101960]

Just to deviate a little bit: Mine is a single carb, and I have tried everything that I can think of to richen the weak mixture on the L/H/S cylinder without success. I even fitted a bias manifold spacer but it has made no difference.

[Joolstacho]

There used to be an adaptor you could get that was slightly wedge shaped, angling the carb, therefore angling the incoming charge towards the weak cylinder.
(Or did I dream that!)

[a101960]

Yes, that's what I was talking about. It made asolutely no differencs at all

[BagONails]

Be interesting to try running a velocity stack in front of the carb ilo. an air filter? (Not advocating running without a filter by the way purely as an experiment!)

A mate with a twin carb A65 couldn't start his at all (twin 32mm Amal Premiers) and it may not have the mixture completely right as yet but doing this made a huge difference to the starting and the slow speed pick up. Seems to smooth the airflow through the carb and help the fuel to mix better at lower air flow velocities.  What they were designed to do I guess!

Other than that you'd need to put the head, manifold, carby and filter housing etc on a flow bench to see what's happening.

[muskrat]

G'day Ian.
I don't run air filters on the Cafe, just short stacks with wire gauze. A flow bench is a good idea as I can't see any mechanical problems. The mob down in Windsor that did the head repair may have one. God knows how much that would cost (probably a weeks wage).
Cheers

[Yeti]

I found that adjusting the cam ring on the magneto so that both cylinders were running the same advance sorted (mostly) a weaker cylinder.