SZR compression

[Tamaco]

Hi Everyone,

I'm new to the forum, but have been reading up on topics as a guest for about 6months. I don't own a MZ, I have a SZR 660 (hopefully I won't be exiled for that ! haha).

I've been trying to get the SZR back to its former glory, a lot of the servicing etc has been done, new battery, new air filter, braided lines, I even rewired the entire thing.

Now my attention is drawn to the engine, the engine runs fine, it pulls in final gear, and doesn't appear to be using a lot of oil, but when I checked the compression it was reporting about 90 PSI which seems very low. The valve clearances had been done prior to the compression test, so would this indicate piston rings?

Cheers.

[Srinath]

It has a pimple decompressor that "retracts" if it runs a bit higher than about 4-500 rpm. Spring controlled, I guess its about 400 on an old bike (like mine).
Cool.
Srinath.

[Tamaco]

Sorry Srinath, don't entirely understand. Are you saying if run at a higher rpm than 500, the decompression device dumps any compression (through the exhaust port by sounds of it)?

If so 2 more questions:

1. Does the starter motor run at higher than 500 RPM required to activate the device?

2. And if so, can I not accurately measure the cylinder compression?

Cheers

[edfmaniac]

The auto decomp system is always engaged until the engine fires and the rpms rise high enough for the centrifugal weights on the cam drive gear to disengage it. You can't check compression until you disable it. If you decide to mod the engine, some cams don't need the decomp system. With my Megacycle X4 cam profile the starter doesn't even think twice about turning over the engine without the decomp system intact. Actually, to be honest, it has a little bit of a hard time when it's cold out. But that's because the new super light battery chemistry doesn't like to be cold, and I'm running a smaller battery than the manufacturer recommends. That problem should be resolved with the proper battery.

[Tamaco]

Ok cheers for that info, I never knew. Almost feels misguided that the service manual even tells you to test the compression.

So here's the current state of the bike, it starts on the button every single time, unless its been standing without starting for a week, in which case it need about 3 seconds of cranking before it turns over. No noticeable blue smoke, Haven't had a chance to ride it far enough to check oil consumption. It pulls in every gear.

Should I not be concerned at that obviously erroneous compression test, and ditch the idea of sorting the rings?

Thanks

[DAVID THOMPSON]

see how it runs before doing too much time and money on it ..

i have the 660 traveller and its only problem is it LOVES lots of gasoline
the term here in the usa is GAS HOG

dave

[Tamaco]

Haha Gas hog, I like it.

I was probably misleading when I said I'd not done enough miles on a trip to check oil consumption - when I take it out on a weekend, I'll regularly do 150 miles in a single sitting, and I've commuted for a couple of weeks to work on it, just haven't had to for thought to check oil level before and after.

As far as fuel consumption, I'd say its very economical. It'll easily do 50 MPG, as opposed to my street triple that gives me 35 MPG, now that is a gas hog haha.

It's more of a toy than a tool so I don't mind spending time and some money on it - I do intend on getting the ignitech sparker for it and sorting out the carbs for a exhaust I've yet to bolt on.

Hopefully I'll give it a good run this weekend if I manage to make this tool for removing the front forks cartridges...

[edfmaniac]

50 mpg sounds about right and I wouldn't worry about the rings/compression if you aren't smoking or using lots of oil. If you don't already know about the delay adjustment between the primary and secondary carbs, it's on top of the carbs in between them and can add quite a bit of fun to your riding if you crank it down a bit. You'll lose some mileage, but the bike will be a little more punchy. I also recommend installing the "Off the Road" carb kit for these carbs. http://www.off-the-road.de/en/Other-bik ... stiff.html

[Srinath]

Most kits involve drilling the main on the right carb to 51 or 50 numbered drill, putting a 42.5 VM mikuni jet in the left carb and 140-145 mains and washers to lift the needle.
$2 worth of crap.
Cool.
Srinath.

[edfmaniac]

That's nice. But I think the reputation of this mod speaks for itself. The diaphragm on the CV carb is a known issue once it is exposed to any fuel with alcohol in it. The kit that I linked to has a spacer that goes in between the rubber and top plate so that the slide can actually open all of the way and it comes with a special venturi that you silicon into the lift hole once it's drilled out with the provided bit.

There are drill bits with this kit for the CV main jet but I can't remember what size they are. I ended up running 150 or 155. Needle position is pretty damn important once you start modifying carbs. The needles on my Honda v twin had to be shimmed or the bike wouldn't run right. No way around it other than to have custom needles machined.

[Srinath]

That kit may be different from the 6sigma one ... That kit is a jet drill kit ...
Cool.
Srinath.

[Tamaco]

Thanks for the replies guys. Got the forks on and managed to get it out on Sunday for a 200 mile blast. Again seems to pull well, much more punchy between 5k and 7k rpm, but I guess that's the big carb opening and something that can be tweaked to come in earlier using that screw edf mentions.

There is a fair amount of white smoke when the choke is on, but becomes clear when choke is off. Oil was topped up on Saturday so I'll commute all week on it and check the oil on Friday night.

I did want to jet the carbs, and if that kit has a good rep for these engines I'm all for it. Are they preset sizes? In the past I've only ever swapped the jets on feedback from the spark plug and what it rides like.

[edfmaniac]

It's NOT a jet kit. It's a kit designed to fix the shortcomings in design of the CV carb. It comes with drill bits to enlarge the main jet on the CV kit, but like I said above, you'll probably have to jet it yourself. Mine ended up with a leaner jet than the bits in the kit will give you, and that's with a modified engine pumping out 60hp to the wheel.

[Tamaco]

Right, the CV diaphragm issue.

That "off the road" website is pretty good, never knew about it. Any other places you guys generally buy jets / 660 engine bits from? (guess this is more a question towards the Europeans as customs would hammer me)

* That note about your honda v-twin not running until you had new needles machined - I'll admit, I don't think I'm even close to being knowledgeable enough in this field to go down that route, I get the theory, tighter taper = more fuel, but how much tighter, and how do you know etc ... Am I being ignorant in thinking I can more or less get away with switching main jets on the carbs and shim the needles (obviously with some trial and error) to get the engine running right with a slip on can and normal airbox with K&n filter? (hopefully eventually have the ignitech sparker).

[edfmaniac]

I think you misunderstood me or I wasn't clear enough. I was saying that if you don't use shims, then the alternative would be to have needles machined. The needles that are in my v-twin are in fact the needles that came with the Dynojet kit made for it, but they still require some modification in the form of shims and/or cutting the end of the needle off. Carb set up isn't really that hard. Ignore the fact that I ever mentioned custom machined needles and just do some reading to figure out how to diagnose needle and jet issues.