Assist my knowledge: side draft Carbs

This isn't quite a learn me but I do need some refreshing. It is time to dial in my carb for the new frankenwankel. The issue I am having is off idle bog/lean condition. This is an issue on any rotary with a twin barrel, you are fighting physics on initial crack of the throttle. The current carb is too small and I making due with what I got, it is a Mikuni 44 PHH. I am working on a smoother transition to main circuit.

Cliff notes:

1. I have stepped up to a larger 210 (the largest avail) fuel/main jet from 205 and that helped.

2. Low throttle application is fine, (sub 15% throttle) Maybe it can be a little richer (next route, go up a size or two)

3. Accelerator pump is adjustable and is adjusted to highest fuel setting

4. Pump nozzle?

5. float system appears to be working fine.

But does duration translate to volume? With the larger bore am I getting more fuel at a shorter duration or is it just same amount of fuel at a shorter duration?

What I haven't tried yet it is stepping down to a smaller air jet which would also richen the fuel mixture but right now I lack a wideband and non snowy roads to be able to fine tune WOT.

good question, to which I will give a semi-related but probably not helpful answer to. (I am interested in seeing the solution).

In CV style sidedraft carbs (like on pre '75 MGBs) you would need to check the oil viscosity and level inside the carbs. Opening the throttle draws airflow which lifts the inner slides, but the slides will lift much too quickly on their own. The oil (of correct viscosity) dampens the rate at which the slides lift, and therefore prevents the carb getting 'ahead' of the engine.

Dunno anything about your Mikunis, or rotaries.

EDIT: oops, also, (again on my MGB) fixing my off-idle bog was not really the carb at all. It was ignition timing. (More specifically, vacuum advance). Make sure your vacuum advance is hooked up properly and running correctly before messing too much with the carbs (I know, I know, everybody says to check ignition when you think you have a carb problem, blah blah blah - sorry about that.)

Just remember, coming off idle to throttle (especially a big, quick, change) really changes the engine's vacuum situation. If your vacuum advance is not correct, you could be losing 10-20 degrees of advance at tip in (and THAT would explain a bog).

In reply to rcutclif:

hahah yeah understandable but it is hooked up. I can feel it is fuel related because the transition from pilot to main circuit drivability improved with the richer fuel jet without changing the ignition. I may try and advance/retard it a bit and see. I really can't advance it much further than I had it because under moderate load the other day I did hear some knocking so I dialed it back.

If you pump the throttle a bit it fills the manifold with fuel then you can crack the throttle to off idle easy and its more seamless.

Honestly I do need a bigger carb but I'm gonna make this do. (for now) Going to a bigger carb is mucho dollars and then I need to spend money re-jetting and new air filters/horns. almost worth it to EFI retrofit at that point, from a cost standpoint.

First off, you mention that the carb is too small. That is not the cause of your bog. That would only come into play in the top of the rpm band, and would actually play to your advantage in low speed situations by increasing air velocity in the ventury.

I see that you have a Mikuni, so since my experience is with WEBER DCOES, this might not all apply. The DCOE tune has to be built from the idle up. You need to have the throttle adjust screws out pretty much all the way and get the carb to run on the idle circuit before you do anything else. If it wont run smooth on the idle circuit with the idle adjust screws only half a turn in, then you need to find the cause.

Warning, do not screw the idle mixture screws in with excessive force, it is easy to damage the seat and damage the screw tip.

I agree with NOHOME, assuming it's not a timing problem, go richer on the idle jets. That thing looks a lot like a loose copy of a weber, as was a popular Japanese design philosophy back then, and most webers lean heavily on the idle circuits at part throttle.

In reply to NOHOME:

Yeah I do get great mid range because of the tiny carb but yeah this isn't the root of my issue, it makes me want mooaarrr carbbbbb though.

anywhoos. With the Mikuni they want you to screw in the idle mixture all the way (gently) then back off 1 and 1/8ths turn. Right now it runs smooth on the idle circuit, but it is a bit high for improved drivability, about 1000-1200 rpm. I just turned the idle speed screw up a tad.

Right now I have the idle screws turned out much farther than 1 and 1/8th turn and it doesn't seem to affect the idle.

EDIT: I normally have to go through wolfcreek racing for mikuni parts, never really enjoyed dealing with them on the phone they always seemed angry when you were trying to give the money but I got a few email answers back and it looks like main jets go up to 240! so I'm gonna get a 215 and 220 main and then up my pilot a size or two. I have a variety of air jets as well so we shall see.

Does that carb have an accelerator pump?

DaveEstey wrote: Does that carb have an accelerator pump?

Yes, and it is adjustable.

That's where I would be looking first. Is the bog instant or does the engine start to accelerate and then fall flat on its face?

According to the manual the pilot circuit is active from closed to 20% open on the throttle.

Page 19 from the manual

Trouble shooting points to pilot jet as possible culprit: Page 22

and here's the page with links to the table of contents: Mikuni manual

DaveEstey wrote: That's where I would be looking first. Is the bog instant or does the engine start to accelerate and then fall flat on its face?

Its instant, once I can get some fuel in there it tends to pick up steam and rev fine.

its always had this issue but with the new engine and manifolds its even more present.

Also, thanks for the links! my mikuni manual is pretty beat up and it looks like its a copy of a copy of a copy.

I want to look into bleed pipes but mine is the indepedant type, I truly have not figured out how it comes out of the jet block. Maybe learn me bleed pipes? aka emulsion tubes?

Well I did some digging last night and found out a few things:

1. I have the largest pump nozzle they make for that carb
2. my jetting was closer to a stock 12a than a heavily modified 13b
3. new jets are ordered.
4. they DO make main jets up to 240 (now-a-days)

They did sell 2 different jet blocks: I current has the OA labeled one and the other one that 8x1.2 bleed hole has a well diameter of .4 larger. Not sure what this does or changes, can anyone shed light on this? I assume bigger means more fuel? Maybe I want that. Not sure but they are 25 bucks each...

I will be obtaining a wideband shortly and do some trial and error by flooring it around the highway and monitoring AFRs.