jetting and air filter questions for a race bike

[petermbooth]

Hi all,
I haven't posted here since August so a quick refresher...

I'm building a T500 (1970--Titan) based race bike. I am really not doing much to it in terms of performance upgrades. I'm running it more or less stock.

In the interest of lean angle I have fitted a set of RD400 pipes to the stock header pipes. I cut off the top of the RD pipes at the spot where the diameter matched the diameter of the header pipes. Welded and fitted them.

I also have no airbox. (The bike was a donor.) I'm planning on running the biggest K&N filters I can fit into the space I have.

For emphasis the RD pipes and the pod filters are not meant to be performance upgrades or for stylish looks. They're simple solutions to the situation I have.


SO, with that backstory, what are people's collective thoughts on jetting? My carbs, I'm 99% sure are Mikuni 34's. (I read through a thread here from back in 2011 where someone posted a photo of the Mikuni 32's and my are definitely not those.)
I'm hoping for suggestions on jets, and needle heights based on the pod filters and RD pipes.

Thanks a bunch everyone!
Peter

[Jimroid]

Stolen from johnu's thread.

Post subject: Re: T500 Special Engine (Ported) Rebuild

PostPosted: Thu Jan 26, 2017 6:57 pm







Joined: Thu Sep 29, 2016 4:53 pm
Posts: 117
Country: United States
Bikes owned: T500

Engine is all back together now just working on carbs. Going to set them up as follows;
vm34's
Main Jet - 260
Pilot Jet - 40
Needle Jet - Q2
Needle - 6FJ6
Needle clip position - Start in the middle
Slide Cutaway - 2.5

As suggested by Zunspec.


Stolen from johnu's thread I,d probably start with main jets closer to #290 Jim

[petermbooth]

And you think his bike's set up is close enough to mine (stock with expansion chambers and pods)?

(Of course, you do, or you wouldn't have posted this, but I don't know you so, whatever, I'm double checking.)

[Zunspec4]

Hi Peter,

Getting std. carbs to work acceptably with pods & pipes seems to be a tricky process. Have you considered going for a set of Mikuni VM34 mm after market carbs and inlets. I am running these on one of my race engines with an almost stock engine.

Cheers Geoff

[petermbooth]

Geoff,
Are Mikuni VM34 different than my stock carbs (which I understand to be Mikuni 34's)? What does VM stand for or denote?
Thanks for taking the time to help me out.

[Zunspec4]

Hi Peter,

Here is photo of the "modern" VM series carbs as fitted to my engine including the manifolds to mount them to the cylinders.


These are brand new carbs available from most Mikuni distributors, in the UK I bought from these guys. You can see on their web site the available range of Mikuni carbs and manifolds.
http://www.allensperformance.co.uk/

Your bike looks to be progressing well and looks good. Sorting the fine tuning of the engine will be easier with after market carbs which ever type you use, not least because you have the availability of the necessary jets/slides etc. Setting up the engine is a vast subject with a bunch of variables, best debated over a cold beer (followed by dyno time).

Cheers Geoff

[Jimroid]

What jetting is in it now? Heck,I put an old set of T500 carbs on a 4stroke twin and they worked great for years. No kidding.

[tz375]

It all depends on what design your OEM carbs are. The early 34 were known as homo-pressure. Later 32mm carbs were the more normal design.

If yours are homo-pressure 34mm carbs, I would suggest NOT using the OEM "homo-pressure" carbs if possible.

If replacement is not an option, then the thing to keep in mind is that the way that those carbs adjust float bowl pressure changes the way that the jets react compared to normal VM carbs. The way that Suzuki described those carbs, they will run leaner at lower revs than a conventional carb with the same jets as a conventional carb.

Compared to say JohnU or Zunspec, you should probably start with richer (larger) pilot jets than they use. And those carbs had very large stock OEM main jets. They were #410 IIRC. That sounds huge, but was what they used. If yours have that sort of number main jets, start with a pair of say 440 or 450.

The way to tell if your carbs are indeed homo-pressure is to see where the float bowl breather exits. If it exits into the roof of the inlet, that's an HP type. If it vents to atmosphere it may have been modified or may be a later type.

All that said, your carbs look like later 32mm crabs with flat sides to the float bowl and not like the earlier 34s, so you may have dodged that bullet. What size are the throats or outlets - not the slide diameter.

[petermbooth]

Wow! That's a lot of helpful info. Tomorrow I will try to figure out what type of carbs I have. I will get some better photos and will also try to figure how to determine where the float bowl breather exits. (Step one will be to figure out what the hell a float bowl breather is, but I'll try to do that tonight. )

Thanks!

[petermbooth]

OK, went in to the shop and took more photos of my leftside carb than anyone will want to see.
Here's what I think. I think the carb IS vented to atmosphere. There's a brass nipple on the bottom of the float bowl which connects to a tall brass tube inside the float bowl. (See photos.)
So that means that either (a) it's been modified or (b) it's an older type. Correct?

Also, the slide that goes up when the throttle is opened is a barrel (round) and not a wall (flat). I'm assuming that is what is meant by a round slide as opposed to a flat slide. Is this correct?

Lastly for your viewing pleasure I have included a bunch of photos of the size (in mm's) both ID and OD both sides of the carb. I'm hoping that with all the photos someone will be able to say "Here's what kind of carb you have." with some confidence.

Oh and the main jet seems to say "97.5" (see photo)

Thanks so much for all the help everyone.

Peter

[petermbooth]

Photos were limited to 5. So here are the rest...

Again, thank you!

[Jimroid]

No picture of the body itself,that would help. But to me it looks like a late model good carb and the 97.5 mj suggests GT500 to me.

[petermbooth]

Here are the best two photos I have right now. I can get better ones tomorrow.
Do these two help?

[ConnerVT]

Sorry I'm late to the party.

You have what looks as late model 32mm carbs (1973-later). Not familiar with all of the Mikuni tops, they may/may not be T500/GT500 stock. The later carbs are vented externally, so they use the smaller Main Jets stock. You also have the late (1973-later) carb manafold/carb boots (as seen in your original post photos). With a 1970 frame, you would not be able to fit the stock airbox (without being creative).

The main influence on which Main Jet (150 vs 97.5) to use on a stock motor is based on early (homopressure) vs. late (external vented).

[petermbooth]

OK! Now we're getting somewhere!
So at least one vote for the opinion that I've got 32mm, later version carbs.(anyone with knowledge willing/able to second that diagnosis?).
If so, I'm back to my original question: given the carbs, stock motor and franken-pipes (RD400 with stock T500 headers), what suggestions do people have for jetting the bike for racing. The general sentiment seems to be "start rich and lean out from there". What would "starting rich" mean for main jet sizes?
Again, thanks so much!
(I'm always so surprised that anyone is taking time to read my posts and offer advice.)

BTW ConnorVT, are you in Vermont? Your profile says "north of Albany"
Peter (who is in VT)