Compression/Power loss Left Side

Based on all, I'm wondering about your compression tester - have you checked it against any other engine?

Reminds me of a rat hole I went down. Using a test oil pressure gauge from RGM showed only 30 psi. Took things apart looking for the problem. Found nothing. Finally, the light came on and I checked two more Commandos - all 30 psi!
 
marshg246 said:
Based on all, I'm wondering about your compression tester - have you checked it against any other engine?

Reminds me of a rat hole I went down. Using a test oil pressure gauge from RGM showed only 30 psi. Took things apart looking for the problem. Found nothing. Finally, the light came on and I checked two more Commandos - all 30 psi!
Click to expand...
Dont forget the numbers i last posted were on cold engine, so not as high as a recent previous hot test (first post on thread).
The absolute pressures less important than the difference left/right, correct?

When i had the modern air cooled bonne same guage gave around 175 psi both sides hot. Commando gave around 145-155 psi a year or so ago, hot. On my newly acquired Velocette MSS 500 single, gives 90 psi cold but some velo guys tells me its hard to get decent compression numbers with just the kickstarter bc it doesnt develop much rotational speed plus theres a fair bit of valve overlap.
 
Tornado said:
No bore scope presently.

Following rebore, bike ran well and continued to after i re-torqued head and set valve clearances....must have done 6k or 7k miles since then. Some point towards end of last seaon noted the power dips. Continues now. Became aware of bit of tappet noise on left side prior to noticing power dips. Did another clearance check/adjustment but never got tapping cured.

Plugs have been quite good, brown tan, not oily or sooty. Running needles on richest/ highest position to cure some exhaust popping.
Click to expand...

Much like ignition problems can turn out to be fuel problems and visa versa, could your miss be something like an intermittently bad spark plug? (I had one of those that tested good outside the bike, but under compression was cutting out)

What's your ignition system? electronic? individual coils? dual coil? Have you tried any of the left to right swapping of ignition parts to see if you can make the miss follow the swap??

In regards to the ticking and any thoughts about cam lobe wear, you can do a valve adjustment and observe where the adjusters fall in the rocker arm to see if the adjusters are locked at the same thread depth. That gives a decent idea whether the cam lobe heights are drastically different all of a sudden. (If you recall Semitone's recent thread, he later remarked that his adjusters were locked in vastly different positions which was a clue to his valve train being his issue)

Maybe pull the plugs and roll the bike over to inspect the valves going up and down. Maybe a sticky valve is causing the occasional miss. It would make sense. Ocassionally you get a miss, your compression is slightly less on one side and it's making a ticking sound. Maybe unburned fuel is depositing on the valve seat from a miss and now it' s showing up as a poorly sealed valve....

After you do all this observational diagnosis, you know what the next step is... a look inside.

Or you could buy a cheap bore scope and worm it up the oil drain plug hole to look at the cam lobes and followers, and then reposition it to see the bottom half of the bores. You can look at the top of the bores through the plug holes. From what I'm told they do a decent job for not all that much money. (but I don't own one so I'm just going on hearsay)
 
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Just a follow up on this old thread I started last year.
To summarize, bike was having power dip while at steady speeds, different gears, across a range of road speads and rpms. These dips began around 3k or 4k miles after rebore/headwork. Compression showing 5psi lower left side.

Nothing seemed out of spec, valve clearances, plugs looked good etc. Had been through the premier carbs multiple times, no improvements. Dips seemed to be getting worse over next 5k miles. This season they were annoying enough that bike was no longer nice to ride. More sleuthing and came across an Amal tuning guide on a year old thread on this forum. It mentioned the importance of replacing needles and needle jets every 5-10k miles and describe a very similar symptom of worn needle/jet as i was having. Checking my records, the preimers were fitted 4 seasons ago and some 9-10k miles done on them.

Got new needles/jets on order but found some used jets in my stock from my previous non premier carbs, which should be roughly half mileage...though they are wassel branded. Popped them on the bike today and BINGO! Power dips hugely diminshed...just hint remain and likely to go once replacing both needles and new jets.

Bike is once again nice to ride 😀
 
Tornado said:
Just a follow up on this old thread I started last year.
To summarize, bike was having power dip while at steady speeds, different gears, across a range of road speads and rpms. These dips began around 3k or 4k miles after rebore/headwork. Compression showing 5psi lower left side.

Nothing seemed out of spec, valve clearances, plugs looked good etc. Had been through the premier carbs multiple times, no improvements. Dips seemed to be getting worse over next 5k miles. This season they were annoying enough that bike was no longer nice to ride. More sleuthing and came across an Amal tuning guide on a year old thread on this forum. It mentioned the importance of replacing needles and needle jets every 5-10k miles and describe a very similar symptom of worn needle/jet as i was having. Checking my records, the preimers were fitted 4 seasons ago and some 9-10k miles done on them.

Got new needles/jets on order but found some used jets in my stock from my previous non premier carbs, which should be roughly half mileage...though they are wassel branded. Popped them on the bike today and BINGO! Power dips hugely diminshed...just hint remain and likely to go once replacing both needles and new jets.

Bike is once again nice to ride 😀
Click to expand...
Great you followed up. Reportbacks are as important as initial symptoms/diagnosis posts for being helpful to the forum
 
> It mentioned the importance of replacing needles and needle jets every 5-10k miles and describe a very similar symptom of worn needle/jet as i was having.

Is there a mentioning of wear at needles and jets in the workshop manual? The need for replacement after 10k miles sounds excessive. I wonder if a rubber-mounted carb would perform better?

Apart from carb wear, increased wear in the left cylinder appears to be a frequent occurence. Anyone having a reasonable theory why the left cylinder and the associated piston rings wears more rapidly than the right cylinder and the rings therein?

- Knut
 
mdt-son said:
> It mentioned the importance of replacing needles and needle jets every 5-10k miles and describe a very similar symptom of worn needle/jet as i was having.

Is there a mentioning of wear at needles and jets in the workshop manual? The need for replacement after 10k miles sounds excessive. I wonder if a rubber-mounted carb would perform better?

Apart from carb wear, increased wear in the left cylinder appears to be a frequent occurence. Anyone having a reasonable theory why the left cylinder and the associated piston rings wears more rapidly than the right cylinder and the rings therein?

- Knut
Click to expand...
I would think a lot depends on how the bike is spending most of its running time...at city street speeds or opened up on the highway. My bike does a fairly high mileage per year (typically5-6k per annum until very recently...thanks to a new romance with my Velocette!) and a lot is urban riding. At city street speeds, carb needles spend a lot more time nearly fully lowered down in the needle jet, just rattling away and wear rate likely higher b/c of that.
 
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Here is the older thread I mentioned that put me on the the right track....inlcudes a link to the tuning article mentioned... https://www.accessnorton.com/NortonCommando/carb-questions-amal.37704/page-2

Curiously, I'm replying on that very thread a year ago,and I'm even relating how my bike had a worn jet with lots of pops and bangs and misfiring when I was running on a single Amal carb back when I first got the bike.... not making the connection to my own issue at the time!?!? In my defense, those original issues with the single setup were much more severe than what's been happening over past 3 seasons...
 
Tornado said:
Did the oil down plug holes test today, all values on cold engine:

Dry: Left 120 psi Right 125 psl

Wet: Left 130 psi Right 135

So oil made 10 psi more both sides.

Significant? Needs to be hot to be valid?
Click to expand...
Are you doing the compression test with a warm engine and the carbs wide open and about 8 kicks. All my bikes have 170 PSI.
 
Worn needles and needle jets should not cause a miss. A miss means either too lean or electrical. If it is jetting, you might have spark plugs which are the wrong heat range - too cold does not burn the carbon off the porcelain inside the plug.
 


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