Heat Imbalance Between Cylinders

[flatdeck]

Update on A7 Star Twin. Last night we spent some time taking the throttle slides out checking the needle positions, setting the pilot  and throttle stop screws, checking the float levels etc before attempting to start her. 2nd kick she fired and idled .... I nearly fell over but boy was I ever pleased. Anyway, forgot to shut off the choke so eventually she died but that was enough we ended the evening on a high note. Next week we have to see if the carbs are balanced and find out the cause of one cylinder getting alot hotter than the other (might have been weak mixture which we hopefully have sorted now).

[LJ.]

the cause of one cylinder getting alot hotter than the other

I was tempted to say this might be Induction bias, but I'm told or have read somewhere that it is only A10s that suffer from this... Is this correct? Any how both my A10s have suffered this at sometime in the past and I would put it down to the bike not having been run for a while. A good ride should cure the problem.

[flatdeck]

So induction bias can happen where there are twin carbs? I was under the impression that it was when a single carb was not delivering the same to both .... I'm going to have to read about it again because I have misunderstood LOL

[flatdeck]

Or is it more that one cylinder sucks in more than the other (slightly more capacity or whatever). If the mixture is right why would that make such a difference to the temp I wonder .... I mean we are seeing 120 on one and 60 on the other ...

[groily]

Dredging the far reaches of the memory cells, I recall you used to be able get an 'induction bias' gasket to mount a single carb at a very slight angle towards the weaker cylinder. Gasket was ever-so-slightly wedge-shaped and could go on either way, obviously. Probably a bit hit and miss, but I wonder if they're still available? Can't see why the prob couldn't afflict any single carb twin.
It's not IMHO the v small difference in capacity of suck and bang between cylinders that makes the difference in the mixture, unless something's well knackered in the valve/piston department. To have one cylinder running twice as hot as t'other, there is a major air leak, major fuel blockage, or mechanical mishap at the root of it.
Assuming on a twin carb bike that the jetting/needle/air screw setting are the same on both etc etc, and the throttle slides aren't very unequally worn out, the carbs are cleaned of all the varnish modern petrol can leave behind to reduce the jets or obstruct the flow, the filters on the float chambers are clean and the needles freely moving, and the valves are OK with the right clearances on both sides, the plugs are OK etc etc, I'd check the fit of the carbs to manifold gaskets v carefully to make sure nothing's partially covering the 'ole, I'd check that there wasn't a small air leak from a bowed flange on one or t'other (it's just so common, that), and I'd carefully clean out the inlet tracts.
I fitted my bias gasket as an extra precaution, along with raising the needle a notch etc etc after having a holed piston for no very obvious reason on the Winchester bypass in about 1973! According to the 'ColourTune' I had then and still have in the box, afterwards the mixture was the same on both sides . .  Groily

[a10 gf]

It's not IMHO the v small difference in capacity of suck and bang between cylinders that makes the difference in the mixture, unless something's well knackered in the valve/piston department. To have one cylinder running twice as hot as t'other, there is a major air leak, major fuel blockage, or mechanical mishap at the root of it.

Agree, think of potential induction bias as the least possible culprit. I'd say (without ever having felt the need to try it), when everything seems quite fine one may try a finetuning by slight carb tilting.

[flatdeck]

This is an interesting topic (since I know not alot in this department LOL). We have not checked the temps of various parts either side since we sorted the idling etc so it will be interesting. My workshop colleague has one of those laser temp gauges - just point and click - most useful - question is, which part of the bike would be the best indicator of temp in the cylinder?

[groily]

Very interesting Q given that one can't exactly climb in and feel it . . . exhaust gas temp (with separate pipes) might be measurable with one of those fancy analysers and give some sort of indication? Probe put on head in various places to see if there are major differences between sides? Or you could just check the combustion flame's health with the sort of ColourTune thingy I mentioned? State of plugs will give a reasonable clue - a plug will complain if run way out of its heat range after all. And how hot should things be in there anyway!? Hardest part would be to check through the rev range, under load variations . . . getting perilously close to the reasons why the darn car has to go to the garage whenever it hiccoughs, where there's a man who can plug it into something and make mysterious clucking noises involving pound/euro/dollar signs when he has the read-out in his hot little hand!
On an amusing note, for ages I couldn't understand why one bike I had kept baulking (as in it felt as if it was about to seize up) at sustained wide-throttle high-load up long hills - till I realised my personal profile is that of a lead brick and . . . . . . .I needed to open both fuel taps. Groily

[RichardL]

flatdeck,

No doubt, with the fun temp gauge at your disposal, you will be looking all over to find the best place to see a temperature differential. Nevertheless, my guess is that you will find it approximately halfway up the barrels, measured at the extreme outside (left and right). If you measure near the bottom of the barrels, the very efficient heat conduction of the aluminum crankcase will tend to even them out. Also, because the bottoms are coolest, there would tend to be a smaller difference to observe. At the top fin of the barrels, the likely hottest point, I am guessing that the accumulated heat in both cyclinders will be so high the apparent differences will be difficult to measure, even if they are a significant percentage (I am not so sure about this arguement). Also, if you have an alloy head, the same point regarding aluminum heat transfer applies. You wouldn't want to look near where the barrels come together on the centerline, for, I think, obvious reasons. So, I picked the middle ground. If I'm right, do I win something?

Richard

[flatdeck]

You would win .. er ... the satisfaction of knowing you are a clever clogs perhaps ... LOL. We have previously gathered temps from the following places: halfway up the barrels on the outside (as suggested), start of the exhausts, after the silencers and a multitude of points in between :-) Next time we run her I'll write down some temps and where they occur just for interests sake.

[a10 gf]

You all should win prices for all these great discussions and views.
Now I must buy a laser temp gauge!

[jfligg]

Hi ,

  Did you get your magneto back?  Did you get it timed OK?  Seems like your making progress.  I have had the same problem on my A7.  The right side is hotter than the left.  I am suspecting the mag, actually I was thinking it was the cam ring.  I have had no time to look at it lately.  But if your bike is doing it adter a new mag then it is not it.  Keep us posted,  or drop me a private line.  Jeff

[groily]

Can't rule out a mag completely, but unlikely to be the cam ring. If one cylinder is enough retarded to make a big temp difference, suspect contact breaker end bearing flopping around (perhaps due to crumbled insulator washer thingy between it and the mag body) or wear of the housing the ring rotates in if it's a manual, causing large points gap variations between cylinders. A couple of thou is not all that abnormal, however unwelcome. This was the (very good) man at Independent Ignition Supplies telling me this. Also check slip ring is clean, HT pick-ups ditto and not shorting to earth via tiny cracks, that the brushes move freely, and that the earth brush is there. IMHO mags get blamed for more than they deserve, they're among my very favourite things! Wish everything I owned had one. Groily

[groily]

Kitchen, Richard?. . . not my department! Unless I need an oven for proper oily purposes, of course, whereupon 'what's yours is mine and what's mine me own', just like the Rev. said all those years ago! Groily

[flatdeck]

Hi Jeff, Yes magneto is back and fitted and ignition timed. Everything from the ATD to the plug caps has been renewed/rewound/cleaned or whatever. Spark is huge, no probs there. We haven't had her running and up to temp since we set the carbs up properly earlier this week (started 2nd kick and idled OK). Hoping to spend some more time start of next week. We shall check the temps again then to see if we still have an issue. I suspect the issue was that the mixture was weak and that caused a higher temp. Dave