Carb guru's....please step in!!

[Mr. Schweer]

Okay, I have two specific questions. What specific part(s) of the carb deal with low RPM fuel delivery?
What specific part(s) deal with high RPM?
I put a 4-1 and pods on and the bottom end is very lethargic until 3000 then it pulls nicely. If I'm on it hard, it starts chugging, popping and slows down at about 5000! If I roll on slowly, it's not as bad.
I put in 145 main jets but I don't think that's enough and I'm not quite sure what to do for the bottom end. I had some extentions between the carbs and the pods and that just about elimitated the low end issue, but would rather not use those.
Any suggestions?

[GARTHXS]

what bike?

if you are talking about the one with the kerker, you shouldnt have to richen the mains , set them stock and imcrease the size of you pilot jets

[Mr. Schweer]

My '79 SF. Not sure of the brand. The header collects under the bike and the pipe joins under the bike and comes out and back. The center stand is retained and the pipe ends almost at the rear portion of the rear brake disc. Another note, I'd prefer to run the bike without the baffle which really kills the bottom end. I have the baffle in their now. What would you suggest for the pilot jets?

[CatatonicBug]

What specific part(s) of the carb deal with low RPM fuel delivery?
What specific part(s) deal with high RPM?

Here's a picture from Dynojet that pretty much answers those questions:

[Mr. Schweer]

Okay bug, so from that I'm getting that the mixture screw sets the idle mixture. The pilot AIR jet deals with low RPM. The needle deals with the transition. And the main is WOT.
Correct?
So, and please correct me if I'm wrong, I would need to reduce the pilot air jet size to compensate for the pods and pipe on the bottom end and increase the main jet for WOT.
The needle is okay with regards to transition??? since it runs fine between low rpm and wot??

[natemoen]

Where is the pilot jet on the diagram?

pilot air jet is not changeable. its pressed in and drilled at the factory.

[CatatonicBug]

pilot air jet is not changeable. its pressed in and drilled at the factory.

Not correct at all...

The Air jet is on the air filter side (intake) of the carb, and unscrews like any of the other jets. There are 3 jets in the Dynojet kit, and that is one of them. The pilot (fuel) jet does not get changed with a Dynojet kit, perhaps due to the lack of a need to change it when re-jetting.

I won't pretend to be a jetting expert, but I have to say that Dynojet has put a LOT of engineering into dialing in the jetting for all types of carbs, and I tend to trust them on this.

[natemoen]

Yup my bad its the main air jet that is pressed in at the factory.

[GARTHXS]

So you are talking about the 79sf are the carbs early model or 80 + models. If early model you could run a 42.5 pilot or 45 for mains 145 should work if late model carbs 42 and 120. That should get you running. Tune as needed. Check plug color also check all other systems first air fire and gas and valve adjustment

[Rasputin]

http://oldmanhonda.com/MC/Rcarbs.html
http://www.factorypro.com/tech/carbt...m_engines.html
Just a couple of sites that helped me understand carbs a bit more. Hopefully of some use to you.

[WMarshy]

Stock '79SF carbs are the earlier style carbs unless the PO changed them. I would suggest bumping the idle jet to 45, stock should be 42.5...

[dbeardslee]

First of all, the mains will affect operation through the entire RPM range - particularly on the 78/79 carbs. The pilots draw their fuel through the mains via the orifice that connects the main jet tunnel to the pilot jet tunnel, so bigger mains will get you more gas at low rpms. Gotta get the mains right first.

If you've got pods now, but you don't plan on keeping them, don't waste your time trying to tune it with the pods - as soon as you swap the airbox back in it's going to change everything. Set it up the way you're going to ride it, jet it appropriately, and then tune it properly (for me that involves a colortune and sync gauges). If your airbox isn't drilled, your mains should be 142.5 (plus or minus, depending on how the bike runs at high rpms), and your pilots should be 45. With the pods your looking at 147.5 and 45. With the 145's you're right in the middle for both setups, but don't kid yourself into thinking that will make it right for both - it won't. One jet size off is noticeable.

You could drill the airbox and put it back on, and that should put you pretty close on the mains. IMHO it's a good idea to drill the box - otherwise you're drawing all your air through the snorkel (and relatively hot air, at that), which is a pretty major restriction and negates a lot of what you can get from the 4/1.

As far as the pilots are concerned, I'm thinking it's a good idea to go to the 45's. You can usually tell by colortune if you need to bump the pilots. When you start getting in the 4-5 turn out range on your mixture screws to get bunson blue (after you've got the proper mains installed), it's time for bigger pilots.

Float height is critical too. I've found setting them dead nuts on 25.7mm to be most bestest on mine.

Here's a link to an excellent resource for carb tuning - CV Carb Tuning

[Mr. Schweer]

WOO-HOO.....
gotta love this site! Thanks guys for the input.
I'm keeping the pods on this bike, so that is how I'm going to be tuning it.
The carbs are original to the bike and the build date for the bike is 1/79, so I'd figure that is realativly early.
So should I just get a dynojet kit??

[dbeardslee]

Personally I'm not a big dyno jet fan. Jet it properly for your mods, and tune them properly and they'll be fine. Just make sure you use genuine Mikuni jets - the numbers seem to mean different things from manufacturer to manufacturer. You can get main or pilot jets from Mikes XS for $6 a pair. I doubt your going to find a dyno jet kit for that kind of money.

[natemoen]

IIRC dino key does not have a kit for the early style cabs, only the later carbs.