Cylinders different temperatures!

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Cylinders different temperatures!

Postby Cliff » 20 Jul 2013 19:59

Fired up my '77 Strada today for the first time in about three years today after having basically restored and refurbed all the cycle parts. This included a complete rewire, fitting NLM replacement transducer kit and replacing all jets and seals in the carbs. It appeared to be running fine. However, after running for maybe three minutes and switching off I noticed the front cylinder is considerably warmer than the rear! There are sparks to both cylinders and I'm stumped as to what may be causing it. Any ideas?

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Re: Cylinders different temperatures!

Postby George 350 » 21 Jul 2013 12:07

Hi,

First off, after just 3 minutes they are different? Is this just starting and running on the stand or riding it? I'll assume the former.
You have had the carbs apart for the new jets, but did you balance them afterwards? This will have a major effect on temp if you are just 'blipping' the throttle, as if they are unbalanced, the cylinder whose carb is opening first will be doing most of the work. Next up, are the idle mixtures correct? Bit difficult to have set them after just 3 minutes running, but again, these will not help the suituation. Dried up fuel deposits in the various drillings in the carb?
Final one, do you have the same ignition advance on both cylinders? Plug gaps the same?
Bottom line is, for one cylinder to be significantly hotter than the other that cylinder has to be:
1/ doing more work,
2/ running leaner,
3/ running retarded
or a combination of any of these.
Do a proper service - new belt (you say it has been standing for 3 years!) valve clearances, air filters, carb balance and mixtures and ignition timing and then see how it is.

Hope this helps you understand what is going on.

George F
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Re: Cylinders different temperatures!

Postby Cliff » 21 Jul 2013 16:31

Thanks for those pointers George. I've calmed down a bit - I think I got into a bit of a flap about it yesterday :oops:

I was running it purely on the stand and blipping the throttle. I've carried out a compression test today: Front cyl average was about 155psi rear was about 170. Perhaps the rings/bore are a little more worn at the front, any thoughts?
I've not balanced the carbs yet on a meter ( be great to know how to do this - I've got the set up with rubber manifolds and no balance pipe, just plugged holes) but merely tried to get the throttle cables with equal play and slides to the same height. The mixture screws have been roughly backed out the same from fully closed. Checked the plug gaps and one was nearly closed up completely although I could have sworn I had them at the same setting!

I changed the cam belt and had the ignition coil rewound as well as having the rotor re-magnetised. I had the carbs ultrasonically cleaned although there might well be some glitch still inside. One thing I did notice was the bike started easily with both chokes off ( I replaced these too - levers and plungers) but when I flicked the front one up it cut the engine instantly. Tried the same with the rear carb and the engine ran horribly lumpy. Also new air filters are fitted.

As for the ignition timing I've only manually set it ( original red pick ups) arrow aligned with the slot. If the ignition is retarded on the front cylinder does this point to pick up replacement? Could running the old original pick ups with the new transducer kit from NLM be causing some kind of ignition problem?

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Re: Cylinders different temperatures!

Postby julianharty » 30 Jan 2018 09:35

A quick note on an old topic.
I've had a similar experience where the rear cylinder was significantly cooler than the front after a ride. I used an infra-red camera (a Seek Thermal camera for iPhones) and found the front cylinder was about 50% hotter than the rear. In my case there were at least 2 major problems:
1. The rear carb was overflowing with fuel.
2. The rear exhaust valve's clearance was nil.

For the carb, I first changed the float valve and housing (under £10 on eBay from Eurocarb) and later, after trying to also compensate by adjusting the old float I fitted a new one bought from NLM . It's a 14g white plastic, unadjustable one.

For the rear valve clearance, the rocker arm was badly worn with significant pitting. I replaced the rocker arm block with one I'd bought online that had less wear. These two changes, together with manually balancing the carbs by adjusting the throttle cables has made a significant improvement in the equality in the temperatures. I've yet to measure them with the infra-red camera, this is a work-in-progress as I ride this bike (a 1978 3 1/2 Sport) to shake out various problems and foibles.
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Re: Cylinders different temperatures!

Postby robint » 11 Feb 2018 19:09

Having acquired a laser type temperature measuring gun I too have cylinders running at very different temperatures - 120C or more on the back, 75C on the front - and its on the stand in the garage so no wind effects! Temperatures are measured on head/barrel areas rather than the rocker covers which run quite cool in comparison as one might expect.

Front cylinder does have a clear misfire when cool/cold but then seems to settle down and the bike then actually idles quite well at around 1600rpm (rev counter reading). Clearly need to look at swapping a few items from front to rear pots in a logical way and also look at oil flow and compression over next few days (may be weeks as its cold in the garage!). Did a full service on the carbs (which cured fuel leaks) so they are at the moment no1 suspect, but bike was not running that well beforehand with poor idling and a top speed (actual not Vaugelia) under 80mph. Will report back in due course on what seems to cause the temperature difference (and probably the misfire).
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Re: Cylinders different temperatures!

Postby 72degrees » 11 Feb 2018 19:36

Certainly worth trying an ignition swap to see if the cold misfire is transferred. I've never tried to get my compression tester on the rear cylinder of a Morini. Access is a bit awkward for the hose. I've not checked a front either to be fair, but if you could get comparison readings (with the other plug out and throttle wide open) that might be interesting. One of the advantages of the Morini design is that you don't have to have different 'handed' carbs, so a swap of those might be illuminating.
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Re: Cylinders different temperatures!

Postby 72degrees » 11 Feb 2018 19:42

Cliff wrote: As for the ignition timing I've only manually set it ( original red pick ups) arrow aligned with the slot. If the ignition is retarded on the front cylinder does this point to pick up replacement? Could running the old original pick ups with the new transducer kit from NLM be causing some kind of ignition problem?
Cliff


Not spotted this query before. In my experience the NLM module and coils (either the first small scooter type or the later advised larger ones) work very well with an OEM type 2 red pick up if it's in good working order.

Is it the ignition retarded when checked with a strobe - more than you can compensate by adjusting the pickup 'stator'?
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Re: Cylinders different temperatures!

Postby robint » 26 Feb 2018 21:11

Despite the slightly chilly (!) nature of the garage I have been playing a little. No evidence of electrical issues, timing certainly as it always has been and plugs looked similar (rather rich). Tappets were fine on both inlets but front cylinder exhaust 2 thou too big and rear cylinder exhaust almost no clearance - and the front was the one running badly. Adjusted and no obvious difference to misfire or temperatures. In the meantime got some 44 idle jets, new throttle cables and carb top adjusters. All fitted and idling better but still a bit of a misfire and temperature mismatch. Swapped carbs over and temp difference moves with carbs......... But idling is much better and temp difference now much smaller (less than 10C on a particular point on the inlet side of the heads at about 120C each) and misfire has gone.

Realise there are rather too many variables to be certain of anything (except that the ignition system seems fine) but it does seem as if I can now try to balance the carbs and improve the response and smoothness. Clearly the carburation can make a huge difference to the performance as one might expect, but it seems that even small, non obvious differences could be significant. Will add any more as I find it!
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Re: Cylinders different temperatures!

Postby 72degrees » 27 Feb 2018 10:36

Hmm, if the carbs are identically jetted, I can only wonder if the float levels are slightly different. Have the carb bodies been well cleaned (preferably ultrasonically)?
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Re: Cylinders different temperatures!

Postby harrymuffin » 27 Feb 2018 12:14

Once you have balanced the air flow with vacuum gauges - Morgantune - or with a flow meter - Cryptontune - then you can balance the fuel mixture with the adjustable idle mixture screw which is probably why you have uneven temperatures. I have an engine that three carburettors feed two cylinders each and after setting the air flow, then set the idle mixture with the infrared thermometer aimed at the exhaust pipes next to the head. I can balance the exhaust temperature of the six pots by about 10 deg with paired cylinders shewing different individual temperatures up to 10 deg. With the thermometer impinging on the exhaust pipe at tick over I can directly see the exhaust temperature vary as I open or close the mixture control. You will never get them the same and work on the principle of near enough. Remember your carbs are probably old, well worn and were never 'precision' instruments in the first place especially with square slides. See if you can have a look at an Amal GP carburettor which were precision instruments and compare to those fitted to our engines.
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Re: Cylinders different temperatures!

Postby 72degrees » 27 Feb 2018 12:28

What harrymuffin said. I assumed the OP had already done a proper balancing job and idle mixture screw setting, but if not it's long overdue.
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