Lean bog or four stroking

Some of you recently helped me put together a great setup for my Maxi, thanks again.

Now that I am completely done my break in, I want to dial in my vm20 carb. I will be using these as guides:

http://www.mikuni.com/pdf/vmmanual.pdf

http://www.3cyl.com/mraxl/manuals/carbtune/carbtune.htm#adj%20location

The only questions I have are:

1. What is the the easiest way to tell the difference between a lean bog and four stroking?

2. Is it possible to be running lean, while still maintaining low head temps?

I am almost possible I am running a little too rich right now, 160 Main jet, and stock idle jet, haven't touched needle jet setting yet. I have a "dead" spot at about 1/4 - 1/2 throttle where I think she is four stroking, but if I let off the throttle a hair, she will pick right back up.

Thank you

Re: Lean bog or four stroking

Dirty30 Dillon /

You aren't necessarily running too rich. I'm running a 170 on my ported kstar.

You need to do some needle fiddling to get that dead spot out.

Easy way to distinguish between a 4-stroke and lean bog, I've found, is exhaust sound. This is especially true with the vm20 on a piston port motor, which is loud as fuck.

4-stroking will be a sputtering, that is accompanied by an intermittent shudder in power. The sputtering is almost only audible in the exhaust note. Imagine the kind of feeling you get when you're running out of gas in your car, except not. A good way to see 4-stroking is to just turn on your choke and ride down the street.

Lean bog will be a consistent, drone, followed by altogether loss of power as long as you hold that throttle position.

Re: Lean bog or four stroking

What people refer to as four stroking is an overly rich condition that forces misfired strokes and is audible as well as felt. Its a breakup or preheat type of sound. You can also be rich enough to simply bog but generally it is accompanied by noticeable breakup and surging. Alean bog will just feel weak. For instance lets say that your main is lean but you have rthe needle at its richest setting. You may have great response and power at part throttle, but as you move beyond to full throttle you either notice no response to throttle (can also be rich) or a loss of power that feels flat. Usually people are either so lean that the bike does not runright, or are just lean enough that all feels well until they seize.

Re: Lean bog or four stroking

Brent Bublitz /

>1. What is the the easiest way to tell the difference between a lean bog and four stroking?

lean bog will be where the motor makes a WOOOOOOOHHHHH sound. 4 stroking will feel like a misfire. You can also lean over and cover part of the intake with your hand. if it gets a little better, you were lean, if it gets worse, you are rich.

>2. Is it possible to be running lean, while still maintaining low head temps?

Yes, to a point. There are a lot of ways to cause trouble for your motor. The safest thing is to start rich and tune back to a proper mix.

> I have a "dead" spot at about 1/4 - 1/2 throttle where I think she is four stroking, but if I let off the throttle a hair, she will pick right back up.

Does the dead spot get better or worse as the motor warms up? Warmer motors deal with rich conditions better than cold motors. I have two bikes that will 4 stroke slightly at WOT when cold, but they run great when warmed up. If the dead spot gets worse when you are warm, it might be that you have a lean spot in your setup.

Re: Lean bog or four stroking

Technically the engine is leaner when cold and an issue that gets better as it warms would indicate that it was overly lean, and the cold running exposed the problem better. A bike that runs like a bat out of hell but gets soggy when warm would be rich, or have low compression..

Re: Lean bog or four stroking

Thank you all for the quick responses, very useful information. Based on what you guys have said, I am definitely running rich. I am going to swap out my stock idle for the 17.5 I have, and I will try moving the needle to one clip leaner at a time. I do notice that as the bike warms up it runs better. Dillon, I am guessing that you may have a much higher RPM set up than me which justifies your 170 main jet. I am using 18x45 gearing with a proma circuit and no porting done to my treat reed kit besides case match. Thanks again guys.

Re: Lean bog or four stroking

Dirty30 Dillon /

Ahhhh, but yours is a reed kit. So that makes sense.

Mine's a PP kit.

Re: Lean bog or four stroking

Didn't she spit back out the carb into the air filter when 4-stroking? Always did on me. don-ohio

Re: Lean bog or four stroking

Overpriced Parts /

Chris Beers Wrote:

-------------------------------------------------------

> Thank you all for the quick responses, very useful

> information. Based on what you guys have said, I

> am definitely running rich. I am going to swap

> out my stock idle for the 17.5 I have, and I will

> try moving the needle to one clip leaner at a

> time. I do notice that as the bike warms up it

> runs better. Dillon, I am guessing that you may

> have a much higher RPM set up than me which

> justifies your 170 main jet. I am using 18x45

> gearing with a proma circuit and no porting done

> to my treat reed kit besides case match. Thanks

> again guys.

Don't kid yourself 18x45 is equal to 16×40 still a very high rpm possible .

Only 800 to a 1,000 rpm less at speed then stock 16x45 gear with a Proma pipe.

You will be lucky to get 8-8,500 rpm out if your Proma pipe so gear up to take advantage of the Proma's superior midrange power.

1/4 to 1/2 throttle is slide needing to be leaner but could be needle too.

Re: Lean bog or four stroking

What if, at almost full throttle, you have a "rev limiter" sound? Even if you are not near the limiter? Accelerating, at full throttle it pulls, but once the revs build, it sounds like it hits a rev limiter and gets jerky until you let off the throttle? Rich or lean?

Re: Lean bog or four stroking

That would be "four stroking" and rich. If it is consistent. If it is just surging and intermittent it may be lean. It depends on the sound eminating from the carb as this happens. Is this an underwater gurgling type of sound or a flat sound with no real characteristics? Running very rich will begin to sound like the bike is drowning in fuel.

Re: Lean bog or four stroking

A good way to tell the difference is to use the choke.

Re: Lean bog or four stroking

hold it where it bogs for a while down the road making weird noises people staring, then kill it, then reach and feel the manifold. is it wet? your rich, is your bike hot? its lean

also like when i was rich on the main. it would die, like with a sputter, like it spit, as it quit. like it shuddered.

if it was lean, it would just boaaww. and get weak then quit. rich sort of took its time, and even would backfire, when i held it, usually, how i found out, and i found out, it was rich. might not be so great if its the other way. i guess check for temps of the manifold. if you guess rich, hold it, as long, like 30 seconds or a minute. it might backfire. use the kill switch slam on your brakes, then touch your manifold. icy cold and wet your rich. just, run it like hell. but dont slam on your brakes if your going all fast like 30. just stop quickly, before heat from eveyrwhere else gets to there.

if you think its lean, hold it there, wont take it 10 to maybe 15 seconds to dry up anything it might of got there from a rich spot, then hit kill switch and check. if its dry and hot, your lean.

like rich like kept on wanting to run, or something. lean dies. ever heard them cars that backfire, and dont want to quit running? that makes me think of rich (edited)

Re: Lean bog or four stroking

I have never heard any mention of any of this method^ and I see disaster if you just wait around holding the throttle until "something happens"

FWIW My intake has ALWAYS been icy cold with condensation, do to the high vacuum in the intake and cooling fuel vapor. It's also hot as hell here..

Re: Lean bog or four stroking

Thank you all again for the help. Ken, 18x45 feels almost perfect for the terrain around my house (very hilly, and I literally live on top of a mountain). I only really use the moped for beer runs to the store, or quick trips to friends houses etc..Right now she pulls so hard from 20-40 MPH and tops out right around 50. With any taller gearing, I feel like I would lose quite a bit of low end, which I am finally happy with.

Anyways, I developed a vacuum leak over the weekend on the ignition side of the case :( Now I am starting to understand how mopeds ruin lives. Time to replace some fucking seals.

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