Carb slides, 1 hole or 3 ..... an explanation.

[John Orchard]

Well I've been working on the bikes carburation, nothing new for a Firestorm owner that likes to tinker but, after wearing out 3 sets of VTR carbs over the years, I have learnt something that I must share with you Guy's.

I know there are many varied carb set-up combinations that seem to work for many, mine is even more away from others due to the 2 into 1 and opened-up airbox. There has been plenty of discussion over the VTR years of 3, 2 or 1 slide lift holes, also the use of Dynojet slide springs.

When I got this, my second Firestorm, I fired into it, changed the pipe, hacked into the airbox, set the carburation how I thought it should be, fantastic, loved it! Then with the talk of 1 slide lift hole being the go, and even HRC venturing down that path, I figured I better give it a go (both carbs are running short stacks), which I did, the bike didn't go as good as it did before, me thinking I need to play with the carb settings again, nope the bike had lost power and had a slight stumble at high rpm during fast gear changes that I could not jet-out, I chased my tail for a while, thought maybe I'd have to live with it?

Next change I thought I'd try; Dynojet slide springs (short), my high rpm fast gear shifts had an even bigger hesitation! So I learned, the issue with slide holes, is not how fast the slide lifts but how slow, it DROPS. Snapping the throttle back on after a quick shift, the bike had a stumble because the slide was still high and the sudden opening of the butterfly gave the engine a gulp of air (lean stumble), I'd tried fixing it with pilot & needle settings ....... WRONG! Maybe the single slide lift hole might work for a stock airbox, the slight restriction might help pull some fuel out when the throttle is snapped open? I guess this why the Dynojet kit states that you need three slide holes when the short springs are used.

So I go back to a previous setting, the slide with the single hole I had drilled the hole from 2.5mm to 2.75 and araldited the other hole, so I drill out the Alaldite, so now I am at 2 holes, 1 x 2.75 and 1 x 2.5mm. Went back to my original slide springs, stockers with 50mm cut off and stretched back out to the 300mm length. Awesome! No stumble, full power.

If you are finding a stumble at high rpm with a sudden off then back on throttle, my guess is the slide can't keep up, if you are using 1 slide hole and/or short springs.

John Orchard

[8541Hawk]

Like most, when talking about the single lift hole one important bit seems to be overlooked by most. The single lift hole is ONLY to be used with the short stack. Running both carbs (with stock stacks) with a single lift hole will not work (and was never recommended.) Also if you run something like the Dr Honda (or other equivalent stacks) you will want to run 2 lift holes in that application.

[fabiostar]

Good reading folks

[sirch345]

I also enjoyed reading that, thanks for taking the time to post

Chris.

[John Orchard]

Like most, when talking about the single lift hole one important bit seems to be overlooked by most. The single lift hole is ONLY to be used with the short stack. Running both carbs (with stock stacks) with a single lift hole will not work (and was never recommended.) Also if you run something like the Dr Honda (or other equivalent stacks) you will want to run 2 lift holes in that application.

My post says that I am running two short stacks. I'm interested in your reasoning on why the OEM short stack would require one slide lift hole yet an aftermarket stack requires two?!

[8541Hawk]

Like most, when talking about the single lift hole one important bit seems to be overlooked by most. The single lift hole is ONLY to be used with the short stack. Running both carbs (with stock stacks) with a single lift hole will not work (and was never recommended.) Also if you run something like the Dr Honda (or other equivalent stacks) you will want to run 2 lift holes in that application.

My post says that I am running two short stacks. I'm interested in your reasoning on why the OEM short stack would require one slide lift hole yet an aftermarket stack requires two?!

The single lift hole for the short stack is what the instructions that came with the HRC jet kit stated was recommended and why they supplied single lift hole slides in the kit. For the aftermarket stacks, even though they are shorter than the stock long stack, they are tuned like the long stacks. Which means 2 lift holes. I have tried both 1 & 2 lift hole slides with the aftermarket stacks and in both road tests and on the dyno shows cleaner curves and better power delivery with 2 lift holes. I also run much smaller jetting than many folks with no issues (no it is not running lean and makes over 120 RWHP) so YMMY

[John Orchard]

Hmmm, thanks for your reply. What does YMMY mean?

What makes you say the aftermarket stacks are "tuned like the long stacks", do you say that just because of the slide hole requirement?

Just so you know, I was working for the Australian arm of HRC when the Firestorm was released (I had frame No 1), and when the HRC kits were produced, my first Firestorm was modified & tuned on their dyno. I competed in the Phillip Island World Superbikes on the Firestorm but crashed in qualifying with a broken hand & foot, which put me out.

I am running OEM short stack but with an added curvature added to entry of the stacks, maybe that change required the two slide lift/drop holes?

I too run much different jetting to most here, I guess with it warmer here in Oz and I run a 2 into 1 exhaust with 2" core straight thru muffler, an opened-up airbox with Pipercross filter and radical custom needles. My brief settings below....

Mains: #195 F & R
Needles: OEM with 3mm shims, the bottom half (aprox) sanded down to close to Dynojet specs.
Emulsion tubes: stock F & R.
Pilot jet: #42 F & R.
Float height: Stock rear, up 1.5mm on the front.
Mixture screws: 1 3/4 front, 2 1/4 rear.
Stacks: As mentioned.

I also have various airbox mods that I won't go into here now.