Carb. Jetting using an A/F gage

[XR250]

I need to adjust the jetting on a new build for a 125cc Honda Navi. Started as 110cc and added a cam, pipe and carb and bore. I just installed an A/F gage on it.
If for example, it says I am at 11:1 at WOT, and am running a 110 Main jet, should I try a 11/12.5 x 110 = 98 (say 100) main jet assuming I want 12.5:1 as my target mixture?
Also, is 12.5 a good number for WOT? What should I be shooting for a low and mid throttle?


Thanks

 

[BrianPetersen]

Jet nozzle diameters are, if anything, based on diameter, not flow area, and there area factors other than merely the flow area at work. On top of that, changing the fuel jet doesn't change the air-bleed jet, nor the orifice where the fuel circuit leads into the main throat (which is carrying emulsion).

For that air-cooled engine, low to mid 12 air/fuel at full load sounds about right. Idle can be as lean as it will take. Needle as lean as it will take. I doubt if it'll be happy lean of stoich, and even that's a stretch (it isn't a very sophisticated engine). I'd try about a 102 or 105 main jet and see what happens.

Having ridden a Navi, does part-throttle even matter? A big-bore (how big?) would probably just make it less underpowered ... When I rode one, all acceleration was at wide open throttle, and all straight-line riding was at top speed.

I put 70 000 km on a CBR125, mostly at wide-open throttle, before the big-end rod bearing let go. Still have it; needs an engine. Shrapnel from the shattered bearing did some collateral damage.

 

[XR250]

Thanks Brian.

"Having ridden a Navi, does part-throttle even matter? A big-bore (how big?) would probably just make it less underpowered ... When I rode one, all acceleration was at wide open throttle, and all straight-line riding was at top speed."

Ha. I have 2 Navi's. Even with the jetting off , the modified one (125cc) is night and day faster. Hits the rev limiter at 57 mph pretty quickly. So part throttle is a thing.
Honestly, the only reason I am doing this is to kick my buddie's ass on his Grom and to have a fun winter project. I may get a rev. adjustable ECU so I can break 60 mph.
I ride my SV650 when i want some reasonable speed.

 

[TugboatEng]

With an air cooler engine you may want to keep it a bit on the rich side, even if 12.5:1, if you want to run WOT for extended periods.

Do look into removing the cylinder base gasket to boost your compression ratio. This is an old school technique that costs nothing to do, especially if you can figure out how to remove the cylinder without pulling the head. This is something possible to do. When I had my 1987 CR250 I used to do pistons annually but never removed the head due to the cost of the head gasket.

 

[XR250]

With an air cooler engine you may want to keep it a bit on the rich side, even if 12.5:1, if you want to run WOT for extended periods.

Do look into removing the cylinder base gasket to boost your compression ratio. This is an old school technique that costs nothing to do, especially if you can figure out how to remove the cylinder without pulling the head. This is something possible to do. When I had my 1987 CR250 I used to do pistons annually but never removed the head due to the cost of the head gasket.

It has a high compression piston in it already. It is not possible to get to the base gasket w/o removing the head (this is a 4-stroke). Not sure I would want to remove that anyway. What do you replace it with FIPG? (RTV). I have been messing around with my jetting and have about 12.5:1 at WOT. Thanks.

 

[TugboatEng]

I'm a big fan of anaerobic gasket makers but the gray RTV works as well.

 

[Tmoose]

The Mikuni carburetor tuning manual from the 1970s got right to the point.

Selection of the main jet.
First ,do the following on a flat road.
(1) Select the largest main jet which affords the maximum revolutions per minute (the maximum speed).

 

[XR250]

The Mikuni carburetor tuning manual from the 1970s got right to the point.

Selection of the main jet.
First ,do the following on a flat road.
(1) Select the largest main jet which affords the maximum revolutions per minute (the maximum speed).

Nice. Unfortunately, I am hitting the rev limiter before hitting my top speed. Might invest in a ECU with an adjustable rev limit. Seems to be a common mod on these.

 

[gruntguru]

Or a sprocket change.

 

[XR250]

Or a sprocket change.

Well, it is a belt CVT so not possible. I can go with a bigger rear wheel but I am then trading acceleration for top speed.
I just need about 700 more RPM's out of it so I think the CDI is the way to go.

 

[mfgenggear]

OP
What does the spark plug look like.
Lean rich, take small steps
Record the existing jet size.
Buy a set of jets. And go up or down.
How is the timing
Valve and timing to top dead center.
Compression level.
Hot or cold plugs.

 

[XR250]

OP
What does the spark plug look like.
Lean rich, take small steps
Record the existing jet size.
Buy a set of jets. And go up or down.
How is the timing
Valve and timing to top dead center.
Compression level.
Hot or cold plugs.

How does this apply to my original post?

 

[mfgenggear]

Op
This old school getting the jetting correct. If
My post is not helpful then please ignore.

 

[XR250]

Well, got my jetting finalized. 105 main worked with a pod style air filter. I then modified the stock airbox with some holes and a larger hose to fit the carb and I then needed 115 main. Much nicer to use the stock airbox as there is no intake noise like a pod filter. Finally raced my buddy with a modified Grom (pipe and ECU tune). Basically dead even up to 57 mph where I hit my rev limiter. Not bad considering how much loss I have thru the CVT. I still have the baffle in the exhaust so I still have someone headroom if I want to continue f'n with it. I am pretty tired of taking the carb off to jet though. So mission accomplished I suppose.
Honestly, I prefer to ride the stock Navi as it is so quiet.

 

[BrianPetersen]

Interesting. Usually any sort of airbox increases vacuum signal which means needing a smaller main jet, but so be it.

I hear you on the hassles with jetting. I'm kind of surprised that the Navi uses a carb. The cbr125 and the grom both have fuel injection.

 

[mfgenggear]

I have work on old school carburetors. Of all types. I had a rebuilt ford holley on my ol 1983 truck was just giving me fits. An AF gage might have been usefull. We use to use them on old cars for tuning.

 

[mfgenggear]

We use to use vacuum gage as well
An "AF gauge" refers to an "Air Fuel Ratio" gauge, which measures the ratio of air to fuel in an engine's combustion chamber, while a "vacuum gauge" measures pressure below atmospheric pressure, essentially indicating the vacuum level within a system; essentially, an AF gauge tells you about engine combustion efficiency while a vacuum gauge provides insight into engine health by showing how well the engine is drawing air during the intake stroke.

Key differences:

• Function:
  An AF gauge measures the air-to-fuel ratio, while a vacuum gauge measures negative pressure (below atmospheric pressure).
  
  
• Application:
  AF gauges are primarily used in automotive applications to monitor engine performance and tuning, while vacuum gauges are used to diagnose potential engine issues like leaks or worn components by observing the vacuum level.
  
  
• Reading Interpretation:
  A higher AF gauge reading usually indicates a lean mixture (more air than fuel), while a lower reading indicates a rich mixture (more fuel than air). A higher vacuum gauge reading generally indicates a healthy engine with good intake efficiency.

 

[XR250]

Interesting. Usually any sort of airbox increases vacuum signal which means needing a smaller main jet, but so be it.

I hear you on the hassles with jetting. I'm kind of surprised that the Navi uses a carb. The cbr125 and the grom both have fuel injection.

Yea, it would be nice to just pull out my laptop and tune it. I think they are carbed because:

1) they are built to a price point
2) they were designed for developing nations so maybe ease of repair and durability?

 

[XR250]

Interesting. Usually any sort of airbox increases vacuum signal which means needing a smaller main jet, but so be it.

It actually richened out my low and midrange a tad but boosted my WFO.
It may have to do with the long runner on the airbox maybe giving me some Helmholtz benefit?

 

[BrianPetersen]

If the airbox included a long intake runner and the pod filter did not, I'm not surrprised that it ran better with the airbox, because of that long runner. There might be something found in tailoring the length of that runner to the operating RPM of the engine.

On my race bike (yamaha R3) the stock intake runners are set up for mid-range torque to make it a better street bike, and are intentionally long, to deliberately cut down high RPM power, so that the engine's power output fit within licensing categories in some worldwide markets. They're also a different length for each cylinder. They're both shorter now ...