600 single carb question

[Redmohawk]

Question for the orical, I'm doing up a yam 600 single XT600E motor in a srx600 the carbs (twin intake head) are sad but quite sortable interesting mix of one carb with a fuel bowl and mecanical operated slide and the other is an SU arrangment sharing the others fuel bowl. Knowing little about big singles like this and doing a ground up rebuild will be a challenge to say the least.

What I'm interested in knowing is whats the potential of upping the carb size on something like this, I assume the current setup is for great low to mid torque with an ok top end considering the motor power delivery of a big single etc, I have a hi comp piston to suit this motor and will be doing a full rebuild for reliabilty and the search for some more go. Highter comp for a start as its currently at a low 8.5 to 1 looking at more like 10.5 or higher if i can without compromising reliability to much and a warm cam. clean up the head/ports if needed and some larger carbs ?

The current carbs are 31mm inside dia at butterfly for SU carb with the area near the slide almost the same (only slightly ovaled) and 28.5 inside at aproximatly the same depth of the mecanical carb the slide area for the mecanical carb is the same dia at the top and bottom but aproximatly 3.5 mm narrower eather side center to center of carbe is 65mm.

I don't want to rev the crap outa this motor to make power, they only made about 35 hp around 5700 rpm stock I'd like to get this to say 45 and drop another 20 hp of NOS on top of it for some fun at times. Sujestions please.


http://www.asfphotos.com/upload/1285482968.jpg


http://www.asfphotos.com/upload/1285485015.jpg


http://www.asfphotos.com/upload/1285408363.jpg

[Large]

I've got a TTR600, that started out with a setup like that, but it's now running twin 39FCR's on long inlet runners....

When I got it, it also came with a pair of carbs off (I think) an FZR/ZZR 250. Aparrently this was a hot setup, but I've searched the interwebs and can't find anything about the mod- possibly because it was a Euro thingy.

You could try pming "speeddemon"- he hasn't posted for a while, but he might get your pm and he supplied the bike and bits.

[Large]

If you can fab up an inlet manifold, you could do the 41mm HSR/FCR single carb setup?

[Redmohawk]

Yeah sorting out a manifold wont be an issue mate have access to lathe mill and tig for alley welding, you still got the carbs off the FZR/ZZR 250 ? Did you ever ride it with them on ? I'm guessing the single carb would make things easyer to tune but it just feels like a step backwards to me, got the option of two ports Y not use em I say.

How's your bike go with the twin 39's? Big difference from the stock carbs? Has your's got the standard comp piston etc?

[Large]

I never ran it with the stock carbs cause we couldn't get them to fit up to the stock airbox.

It's got a high comp piston and porting work and (I think) a bigger cam.

Dynoed at 64hp on a generous dyno that was reckoned to be about 10% high

[Redmohawk]

Thanks for the info Large looks like the 39's are good stuff looking into finding some at the right price.

[speeddemon]

Hi, as large said the best power is with the twin FCR's or equiv. Best $per HP. That TTR engine had a 625cc kit, ported head and an older TTR inlet cam (was a bit more agro).

The other option is the twin banks off the 250's, a range of them fit. I had all the jet specs to get it ball park but dont know anymore. Think twin 133 mains..and carbs were Keihin. Those carbs give better overall fuelling, and hence better bottom and great mid range. Most of all the swap was just that you could fine tune them better than the yami horror carbs... Hope you like german because I think the info is on Euro sites as Large said, they luv the TT/XT over there.

Sorry I cant help anymore, been a while....

[Redmohawk]

Thanks muchly speed, Been chasing up info since large put me on the right track. Will be the twin 39's for sure, as far as german is concerned "Ich hasse aber kann"

[DiscoDan]

Hi Adam

Here's a bit of background http://www.thumperpage.com/articles/srx6faq.html

The 32mm GPX/ZZR 250 carb swap is a pretty easy job for 3-5 hp without any other mods.

For my 2 cents worth 39mm flatslides will be too big unless you're doing a fairly wild build and chasing revs for power. That's about the biggest the SoS racers will use on these motors and they're making 70+ hp at 6-7k. For the road I suspect you'll be bogging it down a lot unless you're carefull matching revs and not using too much throttle. 36mm flatslides is considered to be about right for a hot tune on a road bike.

They're a fairly lazy engine and respond well to a medium cam for road use. Pretty much the spec Large has got but with more forgiving carbs.

[Redmohawk]

Hmmmm more food for thought. Dont want to sacrafice the low down grunt for a peaky top end on this bugger , after all there grunt down low is the fun of it. A little NOS will give me a bit more shove when needed for angry moments, Might do a little poking around in the local bike wreackers and see what i can find.

[speeddemon]

Yes thats it the 32mm GPZ's... engine side fits perfectly into inlet manifolds, but airbox side is smaller and round (TTR is larger and oval) so you need to adapt..or ditch airbox..

I agree with disco that 39's could be a bit big unless u r on full throttle all the time, but bare in mind that equiv singles usually run as single 39 or 41 FCR for great all round dirt biking...and large has a heavily ported head and hi-comp so there is no way you will flow that much (singles act as a big pump and need compression to aid flow) stock. Then again just how much bottom end can a big bore cylinder loose...luv em...:D

[Redmohawk]

Might have found a set of 32mm off a gpx at the wreackers, as long as they fit on the engin side alls good was planing on pods on the other side anyhow.