strange bogging at 1/2-wot

GoldenMotor.com

roughrider

New Member
Jan 21, 2012
10
0
0
victoria, B.C.
Hello, I'm new to this forum and built a MB a few months ago. ever since It's first ride, 1/2 to wot does not exist. as soon as I get past 1/2 the engine starts bogging down. It seems I can actually make the mixture leaner or richer with different throttle positions. slightly below 1/2 it is rich and starts to four stroke, slightly more throttle it stops four stroking and starts to bog down and act lean. I can still get up to a good speed, but I have to listen to my engine and get to that point slightly above four stroking. this makes it difficult to go up mild hills. i can only go up them at 1/2 throttle. any more and it looses power and boggs. I checked for air leaks and sealed off the intake manifold anyway to be sure. other than that she runs great. starts easy and idles nicely. I'm running 40:1.

thank you in advance

Colin
 

GearNut

Active Member
Aug 19, 2009
5,104
11
38
San Diego, Kaliforgnia
I would start by suggesting you do research on the carb tuning threads. The NT carb is very common and a popular choice for conversions and some even consider it an upgrade compared to recent factory "performance" carbs.

You might simply need to adjust the slide needle, or that along with a fuel jet re-size.
It also could be a fuel flow issue. Due to poor quality control at the factories, many of the stock fuel valves struggle to flow enough fuel at higher engine speeds causing a lean condition.
It could be a slow flowing fuel filter (you are using one, right?).
It could be an issue with the float setting.
It could be a poorly flowing muffler.
It could be a bad spark plug.
It could be a leaking head gasket.
It could be something I haven't listed here.
There is no one thing to look at for a solution.
It could be one of the above suggestions or a combination of them.

Check them out individually and write down any thing you do.
A written log of changes makes troubleshooting in the future easier if things go wrong.
Do not make multiple changes all at once because if one of them is wrong you only have multiplied the possible problems.
Example:
Which change actually helped?
Which change is now causing the new problem and masking the good result that other changes have done?
Do each change and test, then move on to the next change if the last one either helped or did nothing. If it did nothing at least you ruled it out....for now.
 

GearNut

Active Member
Aug 19, 2009
5,104
11
38
San Diego, Kaliforgnia
For now I do not recommend reaming it at all.
You have not done any of the other checks or adjustments to even know if that would be beneficial or bad. Assuming that one thing must absolutely done without checking all the other common culprits first can run you in circles or make more unnecessary work for yourself.
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Are you sure the throttle barrel is opening all the way?
Is the choke plate wide open?
What's your plug gap?
Are you still using the kit supplied spark plug?
How many miles (or gallons of fuel have you burned) ?

Let us know of any progress you've made.

Tom
 

roughrider

New Member
Jan 21, 2012
10
0
0
victoria, B.C.
alright, so I did some experimenting today and here's what I came up with...

-fuel filter had good flow
-the muffler is free flowing.
-I took the head off and checked the gasket. it was fine. while it was off I checked the ports for build up.
-the spark plug is looking fine. Tan/brick color. my gap is .050 I tried .020 a while ago but it ran rough.

I also played with the needle setting. the 4 stroking at low speed dissapeared with the leanest setting, but it still boggs at 3/4 throttle. I can't tell if it is a lean or rich bogg. performance at mid throttle seems to have improved, although I have very low power accelerating until I hit about 20K then it takes off.
 

roughrider

New Member
Jan 21, 2012
10
0
0
victoria, B.C.
Tom,

the throttle barrel is all the way open at wot
the choke is all the way open during operation
and I am using an aftermarket plug gapped at .050
I must have put around 200 km on it by now.

--Colin
 

leadfarmer

New Member
Sep 30, 2011
149
0
0
VA
Just a shot in the dark, but spark plug gapped at .050 sounds wide to me. Have you gapped at .030? YOur CDI may have trouble firing at higher RPM's
 

Nashville Kat

Well-Known Member
Apr 20, 2009
1,501
55
48
Jacksonville, Florida
Have you tried it without an air cleaner? I always drill a few extra holes in mine- if it runs the same without it, then something else is the problem.

I played around with the petcock when I built my first and closing it down had an effect- proving it was getting to much gas to air somehow

I also got better results back then adding a little more oil to the mixture- not sure why, but I tend to regulate mine now by smell and how much oil and smoke seem to be coming out- I've found that oil eventually tends to accumulate in the system a bit and have used less since break in.

Get a straight billet intake if your motors fairly level- its the greatest performance enhancer of anything else and less than $20- I like the regular without the "boost bottle" fitting. Anyway it gets the carb closer to the motor and that really helps the flow.
 

roughrider

New Member
Jan 21, 2012
10
0
0
victoria, B.C.
I think the problem was that the bike had 28 inch wheels. I put the motor on a different bike now, and it doesn't seem to bog. It has waaaaay more power for hills.

colin