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Seizure?

RD 400 SIEZEQ: I’ve just taken control of my uncle’s old 1977 Yamaha RD400, after pestering him for many years to let go of his old two-stroke.

I’ve got it running but the pipes were falling to pieces, so I’ve imported a pair of cool looking expansion chambers from Australia, which will make the bike sound better and go faster. I’ve kept the stock air box, but fitted a new air filter and cleaned out the carbs. The bike also had an electronic ignition fitted around 10 years ago and the timing was set correctly, so I’m told, so for now I’ll leave that part of the bike alone.

But my question is about the jetting and how to choose the correct main jet size? Do you have any idea what jet sizes to use and how to get the correct mixture; is the old ‘plug-chop’ method still applicable in today’s world of modern fuels, etc?

Eric Musson  Cape Town.

A: The only way to get the jetting correct is indeed to do a ‘plug-chop’. Hold the bike flat out in fourth or fifth gear and immediately kill the motor with the kill switch.

The plug should have a light brown tint to the white center piece, but if you see any sandy deposits or erosion of the centre electrode it can be dangerously weak and therefore cause terrible damage to your engine. If you blow a hole through the piston by running too lean it will spray molten aluminium all over the crank, and that will then become a major problem.

As a golden rule with old air-cooled two-strokes, if you’re not too sure what you’re doing please take it to a professional who does?

Bad box...

aprilia topboxQ: I’ve just fitted a top box (maker’s name withheld because my brother sells them) to my 2005 Aprilia Caponord. I’ve wanted to fit one for a while now because my backpack gets wet in the rain and I live just outside Durban where it tends to rain a lot. So it seemed like a good idea to fit a waterproof carrying device.

The fitment of the accompanying brackets in the kit was relatively easy, if not a little fiddly, but I came right in the end. So then it was time to go and out test the new ‘box on the open road.

As soon as I get to 140km/h the bike has now developed a constant weave, which is a bit unnerving, and I didn’t feel like going any faster to see if it disappeared at higher speeds.

So, is this trait common when one fits a top box and more importantly is there anything I can do, or try, to cure the problem?

Corbus.

A: If the top box you’ve bought isn’t primarily designed to fit this particular bike problems, like this, can occur. Also, there’s probably a label, or sticker of some kind, on the ‘box, advising on speeds after fitting it. So it might pay to find that.

Also, if a pillion sits on the back it sometimes stops this ‘weave’ because it smoothes out the airflow after it passes over your body, try that? Also, move your body position backwards and forwards whilst riding to see if that improves things. But as a rule you’re going to have to keep your speeds down, live with it, or take it off when not really needed.

 

Point hater!

1978 GS550Q: I see you’re getting a lot of questions relating to older Japanese motorbikes, so I thought I’d throw another one at you.

I have a Suzuki GS550 from the late seventies, just because it’s all I can afford. It still has points and condensers, which are always giving me loads of problems. I’m forever cleaning them and settings gaps, etc, which is irritating to say the least.

I’ve just priced a new set from Suzuki (two sets of points and two condensers) and they come to nearly R1,000, WTF! And within a few years they’ll need to be replaced again, and the entire mechanism seems to be worn out anyway.

So what other options do I have? I’m thinking of replacing all of it for an electronic system, and would you know who to get in touch with, and will it work OK with the stock ‘points’ coils?

Thanks man.

oldvsnew

Richard Shaw.

A: I would certainly throw away all that old stuff and replace with a modern conversion from Dynatek for example. You’ll get a much more accurate spark and better all-round performance, and it will work fine with your stock ignition coils.

Go to www.dynojet.co.za and order your parts from there, they are the official South African importer for the full range of quality electronic parts, like these, from the USA. Then your points problems will be over with your ol’ GS.

 

Take if off?

KawasakiZ1100GP81Q: Morning men. I’m about to rebuild an old Kawasaki GPZ1100 from 1982, the one before the Unitrack rear end arrived. I’ve taken off the head and barrels to check out the state of the piston, rings and valves. I’ll have the valve seats cut again for maximum compression and the piston and rings are within Kawasaki’s tolerances, so I’ll just hone the barrel.

But I’m thinking of raising the motor’s compression ratio by skimming the head and or barrel, which one would you recommend and more importantly, how much would you take off and what compression hike can I expect? Would this affect performance, and if so, by how much? According to the manual the standard compression ratio is 9.5:1.

Thanks for your time and now waiting for your reply.

Steve Cross

A: First off all do not attempt to skim the barrel; it will cause all sorts of problems regarding the piston height and therefore clearance issues. So you can certainly skim the head and it will indeed raise the compression ratio to a more modern figure.

We’ve done a calculation and recommend taking off 0.5mm from the cylinder head, make sure a proper engineer does this though, because if the head is machined incorrectly you might as well throw it into the bin, you have been warned!

This will raise the compression ration from 9.5:1 to close to 10.8:1, which is quite an increase. Set the ignition timing to stock and use 95-octane fuel and you’ll certainly feel a difference in performance, possibly a 10% increase, or even more.

Hope that clarifies the process for you.

 

 

 

 

 

 

 

 

 

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