Hi-Rev Bog Down

[Molotov256]

Hey again... thanks for everybody's help chasing down my last gremlin on the bike that met forcefully with a van! It's up and running again pretty strong.

Since that problem finally got worked out, I decided to create a new one on a different bike! Yea!

I ended up disassembling and parting out the first "Happy Time" motorbike I built because the bike itself was a P.O.S. and was getting too wobbly to enjoy riding without feeling the need for a full face helmet and flak jacket. The motor ran great though - better than the second bike I built.

I reinstalled the motor and all the other stock components on a similar bike with 1" smaller diamater tires. It's another 10 speed road bike. I also reinstalled a "boost bottle" expansion chamber which had been on the defunct bike.

The bike starts up and runs fine. My only gripe is on the new bike, it bogs down at higher RPM's when the throttle is open pretty wide. It didn't do that on the old bike. It doesn't sputter or feel like it's going to die, it just lags a bit until I back off the throttle and power it back up. I seem to remember it revving higher on the old bike without this problem.

My initial thoughts are:

• Smaller wheel is robbing me of top speed more than I anticipated
• Engine is flooding at high RPM
• Maybe I need to install a smaller rear sprocket for more top-end

I read one post after searching the message board about this that blamed engine break-in characteristics, especially if the engine is broken it at particularly high speeds, but I don't think that's the case with this motor because of the use it had on the previous bike.

Any thoughts or suggestions would be most appreciated!

 

[pdxrhett]

If you have a mixture screw, try tightening it up a bit, it might have backed out (I know mine used to, but I put loctite on it). Also, it does sound like it is getting too much gas, so maybe lean it out

 

[EnFlaMEd]

I had the same problem. I leaned out the fuel mixture by adjusting the needle clip up to its highest setting. I also found that when I didnt run the standard aix box and had the foam insert just screwed to the carb ( I initally had clearance issues ) that I also had this problem. With the mixture leaned out and the air box in place mine revs right out no worries now.

 

[Molotov256]

I've messed with the mixture screw quite a bit to try and smooth out the idle, but I'll give it some more attention as far as it affects the top end. If I back the screw out, it runs leaner, right?

You may be on to something with the needle clip, too. When I transfered the kit between bikes, I remember having to put the carb back together. I wasn't aware that the needle clip could sit in various positions, though, so I didn't give it any thought. How exactly do I adjust it? I searched the forum about the needle clip and found a thread about it adjusting it, but all the links it referenced are dead.

Thanks!

 

[pdxrhett]

The tighter the screw, the leaner I do believe.

If you have a screw, there is a possibility you don't have a needle (mine doesn't)

 

[Molotov256]

I found this explanation in another thread, so I stand corrected - the screw I have on the carb isn't a mixture screw, it's and idle screw:

Well, you fixed it and found the idle screw......

It's a very simple carb
there is only one screw which adjusts the idle and NO mixture screw
unlike more tuneable carbs

All the idle screw does is raise (or lower) the slide in the
carb throat by riding on a little ramp on the side of the slide

If you take the air cleaner off and look into the back of the
throat you will see how it works

The only other adjustment you can make on this carb is moving
the position of the clip on the needle which will make it richer or leaner

That is accessed by unscrewing the top of the carb where the throttle
cable enters
So, that being said, if I tighten the screw, it will raise the slide and make the idle higher, but won't have any effect on the overall fuel mixture.

I guess I've got to move the clip on the needle. I'm not sure what you mean by "highest setting", enflamed, but I'll probably figure it out once I get it taken apart and look at it again.

Off to the garage...

 

[Molotov256]

I found this in another thread, so I stand corrected. I haven't been adjusting the mixture screw, it's an idle screw:

Well, you fixed it and found the idle screw......

It's a very simple carb
there is only one screw which adjusts the idle and NO mixture screw
unlike more tuneable carbs

All the idle screw does is raise (or lower) the slide in the
carb throat by riding on a little ramp on the side of the slide

If you take the air cleaner off and look into the back of the
throat you will see how it works

The only other adjustment you can make on this carb is moving
the position of the clip on the needle which will make it richer or leaner

That is accessed by unscrewing the top of the carb where the throttle
cable enters

So, I guess I need to adjust the needle clip. I'm not sure exactly what enflamed means by "highest setting", but I'll hopefully figure it out once I pull it apart and look at it again.

Off to the garage...

 

[Molotov256]

Holy ****... a quick fix and a world of difference!!

I pulled the slide out of the carb throat and did like Enflamed said - I moved the E clip to the notch closest to the BOTTOM of the needle... so it's set on the notch closest to the long end of the needle, which I guess is the highest setting. Man - there's no stopping the engine now. I'm gonna go make the same adjustment on the other bike and see if it helps smooth out the ride on that one!

 

[Molotov256]

Wow, that's a pretty tight language filter. I didn't even cuss! lol...

 

[arceeguy]

If you move the clip lower on the jet needle, you raise it more from the needle jet. This will enrich the mixture from idle to mid throttle.

If you raise the clip, you lower the jet needle into the needle jet - restricting fuel alittle more - leaning the mixture from idle to mid throttle.

Anything past mid throttle is primarily controlled by the main jet. You can go to a MC shop and purchase a new main jet. (Dellorto jets will fit) I have found that in cold weather, you need to install (or drill out) the main jet to eliminate lean bog at high RPM, WOT conditions. Some have found that a smaller jet was needed because the mixture was too rich at WOT, limiting top speed. YMMV