Triumphrat

markbvt · 07-29-2008, 04:52 PM

Hey all. Please excuse the fact that this question is actually not about my Triumph, but about an elderly Kawasaki, but the principles are the same and there are a lot of people on this forum who are very knowledgeable. And I tried asking this question on the Kawi forum, but got no responses.

I picked up an '83 Kawasaki KZ550F1 Spectre for my dad so he can see if he wants to get back into riding after a 40-year absence. The previous owner took the bike to a mechanic for general maintenance, and the mechanic took it upon himself to remove the stock airbox and fit K&N pod filters onto the carbs -- presumably the stock air filters are no longer available, but as I came to find out, he didn't rejet the carbs (pipes, by the way, are totally stock). The PO, being a novice rider, didn't really notice the performance difference.

I took the bike out for a test-ride on Saturday after doing some unrelated work on it (I hadn't ridden it before). It ran okay and was easy enough to take out on a 20-mile loop, but it quickly became apparent that the jetting was off. The top-end is totally gutless. The midrange is okay, but I could feel some slight surging.

Clearly, it needs richer main jets -- but not massively richer because the bike does run okay, just not as well as it should. The stock ones are 120s. I was thinking of picking up a set of 125s and 130s... or would it be better to go straight to a 130 and then maybe try a 135 or 140?

Any of you carb jetting experts have any suggestions? Again, only the air filters changed -- exhaust is stock (anyone done an airbox delete on a Bonnie without changing their exhaust? If so, what jetting did you end up with?).

Thanks in advance!

--mark

sweatmachine · 07-29-2008, 08:50 PM

sounds good, go up a few steps at a time. Hell, drill the baffles in the 'zhaust while you're at it.

B02S4 · 07-29-2008, 11:09 PM

From the description it sounds lean, however did you pull the plugs for a look? Presuming they are white & dry, I would try the 130 mains first before making any other adjustments.

gm16sw · 07-29-2008, 11:20 PM

Well, it is about 25 years old. I would also check the floats. They might need an upgrade. Obviously just try one thing at a time and see how the bike responds. It took me about 8 or 9 trys to get my mixture figured out after the mods I have done to my bike...

Sheepdog · 07-30-2008, 12:04 AM

First, make sure that the bike has not developed any intake leaks (such as loose or shrunken intake boots or cracked balance tube caps). These must be completely sealed up before any rejetting is undertaken. Once you are sure that there are no leaks (and assuming the bike didn't come from some really different altitude), start by just changing the mains by two steps...like the 125s. Then evaluate starting, overrun, high rpm running, and throttle response. One of the problems with bikes the vintage of your Sceptre is the lean idle mixture tuning that was commonly used to pass EPA tests. Drilling out the mixture caps and enrichening the mixture will often transform a bike of this period. Also, shimming the needles is a nice trick for picking up some response in the midrange...my 1982 Suzuki 550 Kantana responded beautifully to shimming the needles and nothing else.

If you have difficulty with the tuning, be aware that airboxes manage more than clean air. The volume of the airbox and even the shape are used to manage combustion pulses and stock jetting was conceived with this dynamic in mind. Don't be surprised if all three circuits (low end, midrange, & wide-open-throttle) of your machine need retuning in the absence of an airbox. Good luck with your Kawi...

amanger · 07-30-2008, 04:00 AM

I d nearly put money on the diaphrams being holed at that age. The slides wont lift properly at full revs.

markbvt · 07-30-2008, 02:08 PM

Thanks all for your replies. I ordered up a set of 125 and 130 mains and needle shims, and I'll check the carb diaphragms.

I should have mentioned that the bike actually idles fine -- previous owner was having some idle issues after the pods were installed, and took it back to the shop where he'd had the work done. The shop owner adjusted the pilot screws and synced the carbs, so all's well at the low end.

Also, I suspect strongly that these carbs have never been apart. And overall the bike is in very good shape for its age. So I'm hopeful that the carb diaphragms and floats are fine.

--mark

sweatmachine · 07-31-2008, 01:44 AM

might as well order some float bowl gaskets before you take the carbs apart, unless you know that they've been replaced recently. Those old float bowl gaskets don't take kindly to being taken apart.

Sal Paradise · 07-31-2008, 10:26 AM

+1 on checking the diaphragms. I am going thru this with an older bike right now. Take the carbs off and do this easy test of the diapragm seal by pushing the slides up, one then the other- you should hear air coming out of the vacum port. With the slide up seal up the port with your thumb or duct tape - the diaphragm should stay up by vacum. If one drops down then you know its a leaker and is probably flopping around.

markbvt · 08-14-2008, 11:47 AM

Update for those who are interested:

125 main jets made a big difference. The bike pulls nicely now at WOT. Midrange is better too, but still not what it could be; I'll try shimming, but I'm going to need to grind down the edges of my shims first as the needle tops sit in little cups inside the slides, and standard shims are too wide.

Diaphragms look brand-new, by the way. I'm pretty sure these carbs have never been apart before.

I'm going to continue tweaking, but the bike is very rideable already. In fact, my dad was up this past weekend, and after an hour of parking-lot practice, I took him out for a 65-mile loop on Saturday and a 100-mile one Sunday. He had a great time.

Thanks everyone for your advice!

--mark