GS drag bike updates

[Guest]

Moving an earlier thread here, since for now I'm just working on the bike, not actually racing it.


Thanks to some forum members (in particular Greg B!) finally got my hands on a GS-powered drag bike a few months ago.


I'm a long-time motorcycle road racer and MX racer (the story of my former GS1000-powered road racer is on the forum here), with a few endurance, flat-track, and cross-country races thrown in for good measure. I've never drag-raced, though, but am looking forward to it.


Here is a some photo of the starting point, the bike as picked up by me the fall of 2020.

 

[Guest]

another photo, showing the rear section

 

[Rob S.]

Very nice. Street legal?

 

[dorkburger]

...following.

 

[Guest]

The bike came with an extended swingarm in the spare parts bin, and hey ... I've never had a bike with an extended swingarm, so I figured I'd run it. (Terrible reason to make a decision about a racebike, by the way.) No clear labels, but it looks a lot like a Trac Dynamics piece, based on some interweb searches. (Side note: I might be going back to the stock swingarm; I've got a question in a separate thread on this forum about whether or not wheelie bars play nicely with extended swingarms.)


While I was working on the rear, I figured it'd be a good time to put a better rear wheel on. Tire size often drives lots of other things when building bikes, and the skinniest modern drag slick I could find was a 7"er. When I got the bike it had the stock wheel on it, which is 3.5" wide. It was running a Mickey Thompson 7" tire on the back. Don't get me wrong, the tire clearly worked for previous owners, but I also know that 5.5" is about as skinny a rim as you want to get with a 7" tire. (Per the tire manufacturer's specs too.)


A visit to Vicious Cycle did the trick! Owner Joe Pethoud set me up with a first-gen Honda CBR900 rear wheel. It's 5.5" wide, but uses the same axle diameter (20mm) as the GS, so I could use the same axle adjusters, etc. (Especially nice if I go back to the stock swingarm).


I installed the extended swingarm, then centered the rim down the bike's centerline. I made the measurement a half-dozen times (and asked some friends to spot-check my signtlines) to locate the wheel in the swingarm. I used the frame center rails and engine as visual references.


What looked good using those points as a centerline turned out to place the rim off to one side relative to the rear 'subframe'. That had (has) me scratching my head a bit, but I'm going to run what I have, and make adjustments (left-to-right) to the rear rim as necessary. Too often I've seen subframe get tweaked; using the center rails and engine as references made the most sense. We'll see. I found a big dimple in the LH upper 'subframe' rail since, which may explain why it looks catawampus. Any other GS1150 owners have this dimple in their frame? I don't know if it's damage or was part of the design or manufacturing process. I'll get a photo of it up shortly.

In the meanwhile, the rear wheel, in-place:

 

[Guest]

@Rob S. - the bike has a title but it'll be track only.

 

[Guest]

Stock carbs came with the bike, with a jet kit. I cleaned and kept them, but was looking for something a bit more ... and found a set of 37mm smoothbore Mikuni "Cone Carbs" on the interwebs. I was able to fit a Lectron four-into-one throttle cable to use. They have a hell of a throttle pull ... I'm considering putting in lighter springs (but also don't want them to hang up)!

Not too much info about these carbs; a few 'old-timers' around Portland know about them, and some forum members here. A friend of mine tracked a mention of them from Kevin Cameron's book "Classic Motorcycle Race Engines":

 

[Guest]

And the carbs themselves ... I'll post a photo of them installed soon.

 

[GS1150Pilot]

Wicked bike!

 

[Guest]

Here's a photo of the "crease" in my frame, in the upper rail around the seat area. Any other 1150 owners have this in their frame? The paint there is good / fair, but??

 

[Guest]

another "crease" photo

 

[Guest]

and a photo with the carbs installed

 

[Greg B]

That “dent” in the frame rail is normal.
It’s the same on my other 1150.
It allows the factory battery box to slip in place.

 

[Guest]

Thanks Greg - I love learning quirks about bikes like that!

 

[Guest]

The extended swingarm is substantially wider than the stock swingarm, to the point where the stock axle wouldn't work - it was a couple inches too short. A good friend of mine has a lathe, so I bought him a case of beer, got a 3' piece of machinable alloy steel from McMaster-Carr, and headed over. He cut it and threaded both ends; I put a nut on one side with permanent loctite, drilled a hole through the nut (about 3/16" diameter), covered a roll pin with permanent loctite, hammered it through one side of the nut, the axle, and the other side of the nut, then staked the pin in place with a cold chisel. I then drilled the other threaded end, for a cotter pin or R-clip (through a castle nut). Should do the trick for an axle.

 

[trippivot]

good work. just caught your thread and curious what's next?

 

[Guest]

Good question ... next step is to fire it up and confirm the cone carbs work OK, plus a test burnout. Maybe a bit overly cautious, but the carb choice (cones vs. stockers) drive some other decisions - I'll have details up on that soon.

I'm waiting to get the rear brake bled up before I ride it and/or spin the rear. I've spent a couple hours trying to get the rear line to pressurize; standard bleeding, reverse bleeding, etc. but no luck. I've got a new m/c on-order, a m/c rebuild kit on order, and am going to just open it up and clean it out this weekend.

I've got a question in a separate thread here about wheelie bars vs. extended swingarms. Assuming the clubs I'm running with don't have rules on the matter, any reason to run both or not run both? Based on reading and old photos, lots of this-era bike have lengthened frames and/or swingarms and wheelie bars, but?? The whole covid bit has been going on since I've had the bike so I haven't been able to head to the dragstrip and talk with folks.

I've built and/or set up a good # of road race and motorcross bikes before, but never a drag bike - this is new country, which is part of why it's fun! Any suggestions appreciated.

 

[Guest]

Progress today ... removed the rear master cylinder & disassembled it. The piston was stuck, I popped it out, reassembled, and the brake bled up in a couple minutes! I'd picked up a longer stainless brake line a couple weeks ago, it fit right up. A friend who works for an aluminum company took my rear brake torque arm and lengthened it a few inches, to fit the extended swingarm.

A couple odds and ends to button up before the first test ride ... like drilling a cotter pin hole in the torque arm bolt, etc. Photos soon.

 

[Guest]

OK! Did finished up installation of the rear brake torque arm, and did a once-over on all the fasteners. Thursday it's supposed to be dry (and relatively warm) so planning to leave work a bit early, do a final safety check on the bike, then fire it up!

Photo of the torque arm; lengthened a few inches to fit the longer swingarm. Stainless brake line from Oil Filter Services.

 

[Guest]

Close-up of the caliper carrier. It's ~1/2" aluminum plate. Can you tell I prioritize function over form?