Right cylinder running hotter than left.

Technical Big Block Tonti and Loop Frame models
1

I have a 75,000 mile 1989 Lemans 1000 with r/h cylinder running hotter than left. 160 vs 95 Celsius at the head after a 60mph run. (According to the temp gun which I bought because one header was blueing more.) I’ve no idea what the temp is meant to be but they can’t both be correct. If anything 95 sounds low to me.

Valve clearances good, carbs stripped & cleaned, new intake rubbers. Ignition timing & advance checked with strobe & seems ok apart from the flywheel advance mark for the right cyl dancing one tooth each side of the static reference point at 5000rpm rather than aligning nicely as the left does.

Carbs balance well at idle but the vacuum gauges diverge significantly as the revs rise… and yet it feels no lumpier than usual. The bike runs, pulls & sounds fine with no pinging or popping and the plugs are both looking good.

I’m wondering whether to take it somewhere for a leakdown/compression test before or after a proper carb refurb.

My preferred option is to schedule a top end inspection & rebuild for the winter and buy raffle tickets for something with fuel injection and OBD whilst I have my fingers crossed anyway… but I’m open to other ideas.

2

Check for air leak?

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3

Thanks Brian. Nothing obvious and I have pod filters so i can’t realistically spray easy start or similar to find a leak, but I replaced the mounting rubbers just in case.

4

Intriguing. At what throttle opening do the vacuum gauge readings begin to diverge?

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5

I was thinking of an exhaust air leak as well.

I use to balance my throttles by just making sure they both opened at the same time, using my fingers on the slides.

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6

I think maybe you’ve kind of answered your own question there. I’m not saying that is definately the problem but its a good starting point.

If you are still running Points and Condensors, check if your have EFFE or Magneti Marelli brand. With EFFE you can never get both Cylinders timed correctly

With reference to the ‘vacuum Gauges diverge significantly’ Check the ‘High Top’ action on the Carbs if you still have them in place.

I would consider removing the K&N’s !

good luck, Nik

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Hard to be precise but certainly less than 1/4 which is about when the main starts having more influence than the idle.

8

Nothing obvious with the exhaust either. TBH the last 30 years I’ve done the same when balancing the carbs (and set the idle & mixture by ear) I only bought the gadgets because I was getting nowhere with the temp imbalance.

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Thanks NIk, I’m using a Newtronic that’s been v reliable & maintenance free so far. I’m not sure what to make of the “dancing” mark but whatever the cause I its potentially a 3 - 4 degree variation each side of what it’s meant to be. Probably worth investigating.

Not sure I understand the “High Top” reference but the pods went on about 50,000 miles ago, mostly because changing the stock filter was so awkward.

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Hmm. I agree that 95ºC sounds implausibly low, but let’s assume that side is running rich. The difference starts at less than 1/4 throttle, so in the transition phase. I’m assuming you’ve checked all jets, needles, atomisers, and throttle slides to ensure they’re both exactly the same on both carbs. Leaving the part numbers aside, have you tried swapping over 1) the throttle slides and/or 2) the needles, from one side to the other, to see if wear in either of these components, often hard to detect by eye, could be implicated?

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Not yet but I’ve had the bike for 30 years so wear or a partial blockage seems more likely than mismatched settings or parts. Hadn’t thought about swapping the slides though. Thanks for that.

I’m wondering whether it’s worth doing a leakdown/compression test first to avoid attempting to fix engine wear with carb settings. It’s time of year when I’'d rather be riding than tinkering or waiting for parts / repairs.

12

Another thought – and forgive me if it sounds like I’m teaching you how to suck eggs!

Have you tried reducing the free play on the cable adjuster on top of the carb that is running behind? In case the slide is still sitting on the throttle stop? Very basic stuff, I know, but this, and swapping the slides/needles, is 10-15 minutes’ easy work, and might identify or obviate an easily overlooked and remedied issue, quite a way short of arranging a leakdown/compression test, which I agree is the next obvious step.

I’ll shut up now!

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I would suggest rebalancing the carbs. - it sounds like they are opening different amounts (diverging as the throttle is opened) The vacuum can be even at tickover and completely out of kilter as the throttle is opened as these are adjusted by different things.

Use the following method

  • Warm the bike up and stop the engine
  • Set the idle mixture screws to the manuals setting (normally 1.5 to 2 turns out)
  • Wind out the tickover screws until they are not supporting the slides at all.
  • Ensure there is some slack in both throttle cables (5mm ish)
  • Fit the vacuum guages and start the engine, it wont tick over at this point and will need to be kept running with the throttle.
  • Adjust the throttle cables at the adjusters such that both vacuum guages react together and when held at around 3000 rpm both are showing the same vacuum.
  • Adjust the tickover screws so the bike ticks over at the correct revs with even vacuum
  • Adjust the idle mixture screws slowly in or out for maximum revs, you will hear the engine pick up when its correct.
  • Readjust the tickover screws so the bike ticks over at the correct revs with even vacuum
  • Ensure there is still a little slack in the throttle cables

Russell

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14

Remember that as you adjust a slide upwards that cylinder doesn’t need to suck so hard, so its vacuum will fall.

Counter-intuitive to me but still true.

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15

Thanks Russel. It was I think exactly this.

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Thanks again. It was indeed the throttle cable adjustment. Right was a fraction of a mm ahead Enough to make a the difference it seems.

17

Thanks everyone It all helped. The problem was the throttle cables. I replaced the originals with Venhills last year. The lock nuts on the adjusters where the twin cables screw into the splitter barrel interfere with each other and I think I may have loosened one as I tightened the other when I set them up initially. It was an almost imperceptible difference, but after lengthening the cable on “hot” side about 3 turns they seem to have equalised.

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Sometimes it’s the simplest things :+1:

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Delighted to hear it. Even on this august forum it’s worth repeating the old adage: “First, eliminate the obvious.”

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Assuming the thread is 5 or 6mm the pitch is between 0.8 and 1.0mm. Three turns of the nut with adjust the cable length by between 2.4 and 3 mm. That is lot of movement of the slide. Glad you found the problem and fixed it. Cheers Phil

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