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Vastly different manifold temps ..
Posted: Mon Mar 02, 2020 9:26 pm
by Neil G
Hi guys, first post here, joined at the classic bike show, Stafford last October and have slowly been tinkering with my ‘75 S3.
I’ll post a few pics later but in short it has K2 Tec pipes and I believe standard carb settings - carbs rebuilt with aftermarket kits.
It has Boyer electronic ignition and starts first kick. It sounds like it’s runs / idles ok, my distant memory recalls as I had a B5 back in the day. After approx 1 minute of idling I checked the manifold temps with an infrared thermometer and running left to right I had 103 degrees, 75 degrees and 54 degrees ..... Cant be right, right ?
Lots of other S3 questions but can I start with this one ?
Cheers
Re: Vastly different manifold temps ..
Posted: Mon Mar 02, 2020 10:10 pm
by Doctorpayday
Howdy
Wolfy has done a bit of research on cylinder temps.
With some interesting data.
Aftermarket carb kits have a bad reputation with some guys, based on jet sizes not being correct and giving inconsistent running. Your sample temperatures could just be reflecting this.
I’m sure that some better explanations will be forthcoming.
Doc
Re: Vastly different manifold temps ..
Posted: Mon Mar 02, 2020 10:17 pm
by Mitsui Spy
Hi, most likely culprit is simply the carbs not synchronised.
Re: Vastly different manifold temps ..
Posted: Mon Mar 02, 2020 10:19 pm
by Triple Parts
Have you actually ridden/thrashed it? That will probably tell you more. Then maybe check the temps after a good run?
Re: Vastly different manifold temps ..
Posted: Mon Mar 02, 2020 10:31 pm
by Neil G
Thank you guys for your replies .... I set up the slides using the ‘finger method’ and pretty sure they are damn close ... I have not checked float heights as yet ( need to check up on how exactly to do that ) carbs have been ultra sonically cleaned and rebuilt with ‘those kits’ maybe I need to put original internals back in ? .... the choke cables all have slack, not checked timing - is that adjustable being electronic - again something I need to look up.
I have not ridden it really other than a couple of miles last September prior to getting stuck into it over winter, maybe it could do with a run to see what’s what !
Re: Vastly different manifold temps ..
Posted: Mon Mar 02, 2020 10:57 pm
by Wolfie
Fingers are rubbish for setting carbs I'm afraid although many recommend it.. but as above it does simply sound like the carbs aren't pulling evenly at tickover. Turn the coldest one up a bit... simple as that.
Oh yeah, also as above get rid of "those kits" jets, they have a terrible reputation, just use the valves and gaskets.
Setting float height is simple, it's on MrAxl.. but am fookt if I can even find MrAxl at the moment.. Setting Fuel height is a harder proposition.
An Airflow meter is the great for setting the tickover balance, and then for slide sync, 3 'somethings' of around 10mm (I have 3 small sections from a broken fibreglass tent pole) inserted in the mouths and trapped under the slides (erm... not running or you might lose them).. and the cables then adjusted so that they all dip at the same time as the throttle is opened.
This should set all the slide heights to match, at a level somewhere around a good tootling around town speed.
Hmm.. .I didn't know I'd researched manifold temps though, maybe I forgot.. It's quite likely these days..
Re: Vastly different manifold temps ..
Posted: Tue Mar 03, 2020 8:22 am
by SAM
It goes with out saying. These bikes need everything spot on for best performance. Points/ignition set 2.6mm btdc, float height all same and then carbs set up by synchronisation. Mixture screws last adjustment.
I have a heat meter and find not easy to get too close. I believe cylinders probably run at different temperatures due to position of course.
Re: Vastly different manifold temps ..
Posted: Tue Mar 03, 2020 10:06 am
by Wolfie
Ah I see what you mean now Doc 
...
Yep the Cylinder temperatures do vary, day by day in fact, I can go out somedays and one will be hotter than another... the day after, it can be a different one.. it also varies depending on what speed (Throttle opening) you are crusing along at. The WIndward Cylinder tends to be coolest, but even that's not always.
Centre is NOT necessarily the hottest one..
But wildly different temps at tickover just has to be as simple as the hottest cylinder pulling the other two along.
Re: Vastly different manifold temps ..
Posted: Tue Mar 03, 2020 1:05 pm
by fieldsheer
Re: Vastly different manifold temps ..
Posted: Fri Mar 06, 2020 12:52 pm
by Neil G
Thank you all for your valued advise ... I'll try a run out on it ( when its a little warmer ! ) then I'll set the float heights & sinc the slides and see if it makes any difference. I may well change the carb internals back to original kit rather than aftermarket stuff ... Are the mikuni original jets & needles readily available ?
Cheers
Re: Vastly different manifold temps ..
Posted: Fri Mar 06, 2020 3:19 pm
by RacerRB159
Jets are readily available from Steve at Motocarb, needles and needle jets may well be an issue though. H1 jets and needles come up on eBay periodically. The other option is to buy an Ivan’s jet kit, they are very good
Good luck
Rich