GS450 (81?) vs. GS425 engine differences?

[pozzi]

[long version - scroll down for cliff notes and actual question if you'd like]

well, i finally got a day to go up and wrench on the GS this past week.

its one of the first times i have had some time to really sit down and look at everything I have and make a true project list of what I need to get it up and running.

the bike is a '81 (i believe) GS450T. the ringlands on one piston on the original 450 engine cracked so the owner pulled the head and then sold the bike to the guy i bought it from. he in turn got a replacement engine for it, never installed it and I picked it up from him (one of those too-many-project guys.8-[...sorta like me:-D).
so i got from him a complete GS450T w/ no head on it (but he did give me the original head from the blown engine) and the rest still bolted into the frame and a spare engine.

anyway, i was looking at the sprocket cover panel on the bike and noticed it was different than the spare engine cover. so i started to look at things in more detail and noticed the engines (at least the bottom cases) were slightly different. then i realized the spare had a kick starter...something the engine on the frame lacks.
i then started looking more closely and noticed the spare is stamped: GS425:shock:.

the GS425 has all the wiring, carbs and is complete the bike has all the wiring and is complete.

so i'm not worried that it is a 425cc. a little bummed maybe but it's my first bike, i'm not too worried about having Hayabusa power to start.

so, my plans were to take the spare engine to my wife's uncle who owns a bike shop and have him do a leak down and comp test to make sure its in okay shape to install but NOW i want to make sure i'm not going to run into any problems using the GS425 engine in the 450 frame.

he knows tons about bikes, but he mainly deals w/ either old Nortons, BSAs, Ducatis or newer dirt bikes (his main business in the race season) so he's a great mechanic but not as familiar as some of you when it comes to minute differences in the GS bikes.

i did some searching and found some info but i would like more.

so...this is where you guys come in:\\/
[cliffnotes: bought blown GS450T w/ spare engine, spare turns out to be an older GS425 engine]

• what are the main differences i should be concerned with if i install this GS425 engine into my GS450T frame/bike?
• will there be any issues w/ wiring/electronics?
• i assume the 425 will bolt to the frame of the 450, correct?
• will the stock 450 exhaust work on the 425?
• I know the 450 is more similar to the newer 500 twin and the earlier 425 is a slightly reworked 400, but besides the obvious difference in displacement and HP, what are any other differences between the 450 and earlier 425?
• its been off the books for a long time, so will DMV in California give me grief when i go to register this bike for the first time with a different engine?

i am kinda stoked i get a kick start (it matches the cafe racer look i'm going for) and still glad the 425 is a 6-speed but everything else i'm not too worried about at this point. i just don't want to run into any unforeseen issues.

TIA!

~poz

[Steve]

Not sure about the frame and mounting differences, but because your 425 has a kick starter, I would guess that it is 1979 or older (not sure what years they made them). Most, if not all, of the GS line went with electronic ignition in 1980, so your 425 would have points feeding the coils, while your current 450 has pick-up coils feeding an ignitor, which then feeds the coils. Would be nice if the pick-ups from the 450 would bolt into the 425, but I have no idea. 8-[


.

[Nerobro]

the 450 pickups and ignition should bolt up to the 425 engine with no problem. The motors are the same. As far as i know you could even bolt in a GS500 motor or a GS300 motor. :-)

[pozzi]

good to know. so it sounds like i can just swap out the ignition off the 450 to the 425 and that should be the only modification?

that would be awesome!
i'll look at the factory manual and compare the 425 points to the 450 pick-ups.

[Nerobro]

you need more than just the "pickups" to switch from points to electronic ignition you need to grab the ignitor, coils, and pickups from the more modern bike.

Points coils are higher resistance than electronic ignition coils. They're 3.3 ohm I think And the Electronic ignition coils are 2.2? The transisitors used to switch the coils on and off have a certian voltage drop, so they alter the resistance of the coils to have the same current through them.

[pozzi]

well the frame already has the late model 450 coils mounted on it and its already wired up for it, so i just need the engine side stuff from the 450 moved over to the 425 (igniter & pickups).
hopefully it will be a bolt-on affair.