Another Engine Breather Question

Hi all,

I know the topic has been done to death already, and based on the collective wisdom of this forum (and others) I'm waiting for an XS 650 PCV to arrive from Holland for my '76 T140V.

The bike has the Harris (Gilardoni) Nikasil barrels fitted with low-ish compression pistons (Not sure what...), it has a Spitfire exhaust cam, 32mm Concentrics and the head flowed to suit. There are no air filters and it has standard downpipes and Norton reverse cones fitted. The cams were dialled in to spec and the ignition is an old Boyer, timed at (I think) 36 deg. BTDC FA

I swapped the standard flywheel for a lightened 650 Thruxton mumber.
It runs really well, and is the smoothest Bonnie I've ever had. Always starts first kick and accelerates well...to a point.

The problem I have is that I'd expect it to rev a bit more cleanly than it does; anything above 4,500-5,000 rpm feels like I'm wringing its neck for no good reason. I haven't done a plug chop, but the plug colour looks good.
I don't necessarily want to spend all my time at 6,000rpm, but I would like it to get there occasionally, and with a sense of more to come, if you know what I mean

Would this be symptomatic of poor engine breathing (which is out of the primary, i.e. per the design)?

I've just got hold of a belt primary drive, and I'm considering running the clutch dry. Would running the breather out of the crank timing plug hole be viable, or will it just chuck oil up the pipe?

As I said earlier, I have a PCV on the way, and I'm interested to hear what's the best location to mount it, and the best breather location on the engine.

I have an option on getting the timing cover modded for a breather, but for obvious reasons I don't what to travel this route if I don't have to.

I'd also like to know if there's anything fundamentally wrong with the setup - part of me wants to fit 11:1 pistons, but I rather like the absence of any pinking on modern sh*%e fuels, and the look of the alloy barrels, of course

Is 36 degrees the recommended spark timing? I thought it was 38 degrees.

If you persist with full throttle in 3rd and 4th gears, does it rev out to 6500+rpm? If so, then there probably isn't a fundamental problem.

Triton Thrasher said:

Is 36 degrees the recommended spark timing? I thought it was 38 degrees.

If you persist with full throttle in 3rd and 4th gears, does it rev out to 6500+rpm? If so, then there probably isn't a fundamental problem.

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I believe stock timing is 38 degrees, but I've also seen lower (32 degrees) for tuned motors. It's been a while since I strobed it, but it's either 36 or 38. I don't believe the difference would be enough to explain its reluctance to rev.

It doesn't like being revved beyond 5,500 in any gear! It's definitely not a 'topping out' problem and I'm only running a 19T gearbox sprocket.

You need to spend a dyno day with a couple of mates, fiddle the timing and carb jetting. You should get 7,000 RPM easily and cleanly, or you're not done fiddling.

grandpaul said:

You need to spend a dyno day with a couple of mates, fiddle the timing and carb jetting. You should get 7,000 RPM easily and cleanly, or you're not done fiddling.

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GP, I agree. However, I don't believe that the current issue is down to the 'last 20%' of fiddling: I'm not ready for that yet, primarily for this reason...

After the last (gentle) ride I parked the bike outside the house and it was oil drip-free. I came out a good quarter hour later to wheel it back into the garage and there was still no oil on the floor.
The next morning there was a nice big fat splodge of oil underneath the motor, and another big fat splodge of emulsified oil under the breather pipe. It hadn't run in the meantime.

The chaincase oil was 'clean' when I pulled the cover off

This is why I'm curious about the breathing - Once I'm satisfied I've done everything I can with the breathing, then I'll go plug-chopping - please rest assured I want this bike to perform!
I have done a fair bit of tweaking of the carbs, but I don't believe them to be an issue. Messing with the ignition may yield some results, but apart from going more retarded I'm not sure there's very far to go with it.

To summarise what's happening, at anything up to half-throttle it's perfect. Opening the throttle beyond this makes no appreciable difference - no surging when I roll it off, no blatting, no black smoke. I've never known a motor feel so held back since I rode a T140E with a duff RITA which wasn't advancing :|

Just a quick update....

I've started replacing the primary drive (got the belt drive with new clutch plates from Burton Bike Bits for just over £200 delivered, which seems pretty good value), and on removing the old setup, lurking at the back of the original crank sprocket were a couple of shims and a fairly large diameter washer which may be the culprit for effectively reducing the engine's breathing capability. Just waiting on a new alternator rotor to arrive so I can get it all back together.
Hopefully this may have been the problem...we'll see
Something else to note, the clutch shock absorber rubbers had started disintegrating fine style. I know from my day job that anything rubber has a finite life (based on cure date, not UV degeneration); does anyone have a rule-of-thumb regarding how long the rubbers should be good for?

B+Bogus said:

does anyone have a rule-of-thumb regarding how long the rubbers should be good for?

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No, not really, but I'd bet the replacements don't last as long as the originals!

L.A.B. said:

B+Bogus said:

does anyone have a rule-of-thumb regarding how long the rubbers should be good for?

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No, not really, but I'd bet the replacements don't last as long as the originals!

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Totally agree!

I'm guessing that alternative materials would be good here, perhaps silicon-rubber based?
Meanwhile, I'll put a slot in my calendar for regular re-builds!

If you are not running clean at high revs I would suspect main jets.
I run a timing cover breather pipe. I suspect it should be bigger. It throws out mist only.
I set my timing at about 38/39 degrees BTDC, fully advanced.
Stu.

bigstu said:

If you are not running clean at high revs I would suspect main jets.
I run a timing cover breather pipe. I suspect it should be bigger. It throws out mist only.
I set my timing at about 38/39 degrees BTDC, fully advanced.
Stu.

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I've thought about this, but even if the jets are a fair bit out (I think they're 220s on straight through reverse cone Commando silencers), I'd still expect it to rev in the lower gears...the last attempt was going down a steep hill in 3rd gear (on a 19T sprocket), and it was not at all happy - I think it got to just about 6k when I gave up wringing its neck!

At this point I concluded that there was something amiss. Hopefully the rest of the bits will arrive today so I can carry on with the investigations...I believe the big washer next to the main bearing may have had a *ahem*... bearing on the problem

Quick update,

The bike's now back together with a belt primary, and I've kept the standard breathing arangement with the exception of a PCV.
I can report that the bike's strong past 6k now, and feels pretty good
Haven't been far yet, so still shaking it down.
It's still got an oil leak but I've pinned it down; its coming from the gearchange shaft in the primary case, which was brand new less than a thousand miles ago :roll: I've already changed the O ring once, but to no effect - maybe the bush was out of spec whan it was fitted...now to find another...in spec this time :x

I hope Norton spares are better than this.... :wink:

Update to the update....

The gearshaft bush arrived today, so I set to work.... I checked the bore diameter on both the new and old bushes and they were both spot-on at .750" :!: :?:
I checked the O-ring size and the rebate in the shaft.....010" interference fit for the installed O-ring, so no problem there, either :?

I drove out the old bush from the clutch cover, and I was rather suprised at how difficult it wasn't.... I'd been trying to work out how to support the case while driving the bush out, and I gave it a tentative tap while holding it in my hands...2 taps later it was out, and covered in oil - problem solved!
It seems the new casing was bored oversize :roll:
I'm now waiting for the loctite to dry on the newly fitted bush, and I'm cautiously optimistic that I've nailed it at last (after 10 years!)

Now - hopefully - I can concentrate on the Commando!

Hi Andy; Have you sorted out your rev problem? I had a very similar situation with my 72 roadster. Motor is 75 880 w/
10.5/1, two 34 mm mikunis, megacycle cam. It ran great. I put new reverse cones on it and the header pipes wouldn't slide into them about 6" like on the old ones so I cut the header pipes off about 4". Then I didn't ride out of town for about 6 months. When I did get it out and run it hard it wouldn't run above 5000 RPM. OK up to that point and it would go a little higher if I eased it up, but if I tried to run it throttle open it was like I shut the fuel off at 5000 revs. It had been so long that I forgot about the header pipe mod. I jetted the mikunis down to the smallest jet and it helped just a bit. I spent almost a year fussing with everything. To make a long story longer I finally thought about the header pipe length and I got new headers and an origonal pair of mufflers from Phil at Fair Spares of America. BAM! 7000 and pulling on the next to biggest main jets. Life is good again.

best of luck, Gary