GT550, temperature variances between cylinder heads, running

[bobotech]

I was having issues with my GT550 running well. When I put it back together, I just kept 2 of the 3 original plugs after I cleaned them and used a new BR8ES gapped to .028. I also regapped the old 2 plugs. Turns out that one of the cylinders wasn't firing (wasn't idling either). Swapped plugs with working cylinder and it fired. SO I bought 2 more new plugs and put them in today.

The bike is running MUCH better and idles nicely now however there are temp differences between cylinders that concern me. The idle isn't perfect though, just much better and it actually idles now with out resorting to tweaking the throttle and adjusting the idle stop screw til its max.

I first checked the exhaust pipes right out of heads with my infrared temp gun. When sitting on the bike, left cylinder is about 75 degrees at idle, middle is 85, and right is about 95. This is in Fahrenheit.

I checked the heads too right by the exhaust. Left is about 130 degrees, middle is about 170, and right is 220.

So at this point, points are original but properly gapped but I did not alter timing, new plugs, rebuilt carbs, all 3 carbs are feeding. I haven't synced the carbs or anything yet, I do not know how to do that. I turned the 3 idle mixture screws out 1 1/4 quarter turns from idle. When I rebuilt the carbs, I left the needles in the slot that they were in when I got the bike. I put the same needle/slides into the same carbs that they came out of during reassembly. I measured the smallest gap on the slides and adjusted the slide screws/stops on the top of the carbs so that all of the slide gaps (when looking into the carbs) were equal before mounting the carbs.

I am going to be replacing my points, I do know that I need to do that but I am not sure where to get points at this time.

[oldjapanesebikes]

You might want to first just try cleaning the points you have - if there is lots of material still there, then they don't really need to be replaced. The other point - and there are several schools of opinion on this topic - is to avoid using resister plugs as the spark plug cap does have (or at least did originally have) a 5K ohm resister in already. So just a B8ES, rather than using a BR8ES. Following that same line of thought, it is also worth checking that your caps are still good - the fact that your plugs do fire indicates the plugs should be OK, but are they all the same resistance of cap ? Also, sometimes just trimming back a 1/4 to 3/8 inch of the spark plug lead and re-installing the caps can often make a world of difference.

[bobotech]

Could I just cut the spark plug lead back a bit and put new boots on them that don't have the resistor and put the solid caps back on top of the plugs?

[tz375]

Yes. That would also work.

In my limited experience, variations in temperature are either timing differences, or jetting differences (partial blockages) but they may also be carb synch. If the cool cylinder is opening its carb slightly less than teh other two, that would explain everything.

I like to use the straw method to synch slide carbs. Remove the rubber boots on the air filter side and insert three short straws - one under each slide. I usually use three lengths cut out of a wire coat hanger but straws work just as well. They need to slid in under the slide and touch the needle/jet (engine off). Watch the straws and slowly crack the throttle open and see which one rises first and which one is last. They all need to open at the same time.

Then adjust the linkages so that they open together.

[H2RICK]

variations in temperature are either timing differences, or jetting differences (partial blockages) but they may also be carb synch.

And to that laundry list, you can add the following items that come immediately to mind:
Differences in compression between cylinders
Baffle air flow
Carbon buildup in combustion chamber(s)
Air leakage on intake side

I'm sure there are literally dozens more that could be listed.

[bobotech]

Well a better question is this, should I be concerned at this point about the temp differences or should I just ride it for a bit to let everything settle in place then test again?

I just don't want to kill my engine or something stupid after getting it running.

[tz375]

If it were mine, I would clean and synch the carbs and double check the points and timing and then clean out the baffles and check for air leaks and then I'd ride it.

Those heat variations are much wider than I would expect. One is rather low and the other is about as hot as our race bikes get after a good thrashing.

[barko1]

Is the thermometer good or from Harbor Freight?

[bobotech]

LOL it is a Harbor Freight thermometer. However the differences were consistent between cylinders.

Its not like I got widely varying temps when testing.