Mid RPM Stumble Predator 212

MKW

Active Member
#1
So, I just recently finished a stage 1 Predator for my Simplex. Problem is that it stumbles when I try to cruise half-throttle, or so. It idles great and runs full throttle great. It's just the mid-range that ain't acting right. It never dies fully, just acts like it's skipping.
I set it up almost exactly like another one that I have that runs great all the way through the RPM range.

Standard (non-Hemi) Predator
governor intact
Air filter kit
140 emulsion tube
36 main jet
disabled low oil sensor
Autolite Racing plug
"Box Stock" shorty header with mini RVL silencer

I have switched back and forth through different combinations of stock emulsion tube and 140 tube and jets of 36 and 37. None of this seems to matter except it seems to do a little better with the stock emulsion tube. I'm thinking maybe it's the "box stock" header that might be the issue because the other engine has a typical 1" diameter header and this "box stock" header is only 3/4" outside diameter.

Where would you start when trying to fix this thing?
 

Triley41395

Well-Known Member
#2
So, I just recently finished a stage 1 Predator for my Simplex. Problem is that it stumbles when I try to cruise half-throttle, or so. It idles great and runs full throttle great. It's just the mid-range that ain't acting right. It never dies fully, just acts like it's skipping.
I set it up almost exactly like another one that I have that runs great all the way through the RPM range.

Standard (non-Hemi) Predator
governor intact
Air filter kit
140 emulsion tube
36 main jet
disabled low oil sensor
Autolite Racing plug
"Box Stock" shorty header with mini RVL silencer

I have switched back and forth through different combinations of stock emulsion tube and 140 tube and jets of 36 and 37. None of this seems to matter except it seems to do a little better with the stock emulsion tube. I'm thinking maybe it's the "box stock" header that might be the issue because the other engine has a typical 1" diameter header and this "box stock" header is only 3/4" outside diameter.

Where would you start when trying to fix this thing?
I don't know much about engines but seems to me that if you think the header might be the problem just switch them and see if there is any difference. That's where I would start.
 

SAS289

Well-Known Member
#3
A 3/4" header is still better than the stock muffler. You should be able to run it without issues with your jetting and e-tubes. I ran my 196 with a header made out of 3/4" conduit. It has a 36 jet and 140 e-tube. It's pretty much a stock and governed engine like yours and runs well.

Are you sure it's not just the governor reacting to changes in the load at a fixed throttle position?
 

MKW

Active Member
#4
Are you sure it's not just the governor reacting to changes in the load at a fixed throttle position?
No, I'm not sure it's not the governor somehow. Hard to explain, but it's not just a sputter and go...it's more of a constant skipping at normal cruising speed. When it's happening, I can go full throttle and it smooths out nicely.
 

SAS289

Well-Known Member
#5
If nothing else you try takes care of it you could verify whether it's the governor or not by temporarily doing the zip tie thing and putting the bike in the same riding scenario. If it behaves the same with the zip tie it's not the governor.
 

MKW

Active Member
#6
If nothing else you try takes care of it you could verify whether it's the governor or not by temporarily doing the zip tie thing and putting the bike in the same riding scenario. If it behaves the same with the zip tie it's not the governor.
That's a good idea. I'll just be careful not to over-rev it.
 
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