Jetting and Temps

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Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
83
Santa Barbara, CA
Wow, I went riding it today and it is revving like crazy, I just can't get it to slow it down, when I release the throttle it continues to rev for a good minute before it will finally go down. It's vibrating like crazy and running louder too. Eeeeek, why did I do this? I'm thinking about putting on an old spare cylinder to try to get it back to where it was or maybe 2 head gaskets. It is running faster than it was but it is not running more efficient, it just won't slow down now. I haven't tried a bigger jet yet, maybe I will try that. Damn! I'm sorry I did this.
 

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
83
Santa Barbara, CA
Update:
Alright, I think I got this thing figured out. I spent a few hours today messing with my RT carb trying to get the engine to stop revving. I finally figured out that I can get the RPMs to come down into low idle if I set the carb at the lowest setting, so that just a small sliver a light can be seen shining through at zero throttle. Also, the idle screw must be about 80% screwed out. At this setting it will go down into low idle, although not very quickly, I mean, not as quickly as it was before I decked it but not unbearably slow. However, this might be expected, if I am getting more compression it figures that it might take longer to come down to low RPMs. It's running slightly hotter than it was before I decked it but not really that much more and definitely still cooler than the stock head was. Also, it is louder with more vibration, but again, I guess this is expected with more horse power. I found that I can help the RPMs come down by engaging the clutch at low throttle and letting the load bring it down but even if I don't do this it will come down by itself just slower than before. I am not leaking air anywhere around the head gasket or intake manifold.

I tried re-jetting to a higher jet. First I tried a .63, then .62, and then a .61 but the higher I had in, the more 4 stroking and the more I lose the extra top end that I got from decking it in the first place. Also, the larger jet did not help the RPMs come down any faster. I'm sure that the .60 is the right jet. It's screaming at wide open throttle! I hit 36mph going up a moderately steep hill today. I've never gone that fast going up any hill. I haven't tried it on level ground yet. But I think I have the carb tuned as best as I'm going to get it. Thanks again for helping me get more out of this thing. Am I still sorry I did it? No, I did get more speed in exchange for more vibration and noise and I expanded my knowledge. Will the engine last longer this way? I don't know.
 
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exokinetic

New Member
Mar 18, 2016
108
4
0
Lake Forest, CA
Okay!

That is a lot of good information to work with.

Generally when diagnosing jetting, I like to start from the top (WOT) and move down to idle.

It sounds like you got WOT figured out really well.

Anything bigger than the .60 and you loose power in the top end, I can verify for you (along with the verification of your experience) that is indeed what you would expect to experience as you go "too rich".

Now, you do not have anything smaller than a .60 main, and that I would say is just fine.

If you wanted to be EXTRA sure you had the jetting right, you would start by knowing that you loose power power going bigger than .60.

THEN you would want to at least see what happens as you go SMALLER than .60.

Does it gain even more power with a .58 for example?

Or does it immediately start to loose power again as you go smaller?

This would then tell you where "most power" is found, as relating to the main jet.

Now you have to make a decision:

A. Use the jet the shows the MOST power. This will give you the best power for a single run up to WOT. So like on a race track, where you do not spend very much time "staying" at WOT (depending on gearing obviously) you could use this jetting.


B. Use the jet that is one step RICHER (or more, depending on you conditions) than the jet that shows most power. This will sacrifice some power (not much at all) on your first WOT runs, BUT, if you are running at WOT for extended periods time (example: commuting) you will not loose NEARLY as much power as the engine heats up, and you will SAFELY be able to run at WOT for extended periods of time.


Now this is a decision you have to make for yourself.

And just for some guidance, rich enough to 4-stroke JUST SLIGHTLY when you first hit WOT is a pretty good setting.

As the engine heats up while you stay at WOT, the 4-stroking should go away.

IF after being at WOT for an extended period of time at a steady RPM, all of a sudden the RPM's start to slowly rise, this is a sign of the engine being so hot, that the mix is now going to the lean side of things, and is less safe.

If you continue for enough time after this, and all of a sudden RPM's fall, even though the ground hasn't changed its level (uphill, downhill) then you are now starting to experience detonation as a result of the lean condition, and this is robbing power, and this reducing RPM's.

If you continue long enough after THIS, you will develop a hole in the crown of the piston, although the engine will CONTINUE to run, you will have a sensation like it has lost ALL LOW END TORQUE, and will ONLY RUN at high RPM's.

This is bad.

Don't do this. :D



Now you said the RT carb only has one clip position....

Are you talking about this part:






And your "needle" only has one "needle slots" (also referred to as "clip positions")???

I don't think I have seen any that only have one slot... But I can't say I have seen 'em all.


BUT, not having an adjustable clip position on the needle REALLY limits your ability to tune the "mid-Range" and "off-idle" carburetor jetting.


At this point I would say you have the MAIN JET basically perfect.


NOW, about the engine REV's not wanting to fall.

I.E. you Rev the engine, and you let off the throttle, and the engine stays at HIGH RPM, and takes a full few seconds to fall (however long it takes to fall, you can hear really well with your ear when its lagging, and that is definitely what you are experiencing).


There can be two MAIN causes of this.

The first one is purely mechanical, and has nothing to do with jetting:

If the carburetor slide does not move smoothly up and down with the twist of the throttle (I.E. when you let off the throttle, the slide lags as it "pulls" the cable slack from the released throttle).

This lag in the ability of the slide to "drop" just as quickly as you let go of the throttle will be experienced as the engine lagging when the REV's fall.


If it is obviously NOT this, the slide is moving up and down butter smooth, then it likely jetting.


This condition, of the REV's not wanting to fall (when it IS IN FACT jetting) is ALWAYS due to a LEAN condition.


SO, not being able to move the clip position on the needle to one richer position (moving the clip to a "lower" position) is kind of a bummer.

BUT, there is still another option for adjusting "idle" and "off idle" mixture, and that is a tuning measurement referred to as "slide cutaway".

Here is an example:




This shows a STOCK NT Carb slide, and one that I modified (through judicious use of a hand file) to have MORE "slide cutaway".


Unfortunately for YOU, MORE SLIDE CUTAWAY makes "idle" and "off-idle" LEANER.

...exactly what you don't want.

And the STOCK Slide has VERY LOW "slide cutaway" to begin with, and is USUALLY TOO RICH.


SO, this leads me to think that the "clip position" on the "needle" in your carburetor is in a position that is "TOO LEAN" (meaning the clip is TOO HIGH on the needle, and its position needs to be LOWERED).

If after further examination of your carburetor, you find that your "needle" does in fact have multiple positions for placing the "clip", I would recommend "lowering" the clip position.

Because of how the needle is assembled into the slide, as you "lower" the clip position, you "raise" the needle, so that it is "higher". This means it will allow MORE FUEL past its "tip" witch goes down into the "fuel chamber". This is why "lowering" the "clip position", and thus "raising" the "needle" has the effect of making the mixture richer.

The Needle Position in the carburetor has the MOST effect from 1/8 throttle opening, to 3/4 throttle opening. It also has a slight effect lower and higher than this.

This "slide "cutaway" in this carburetor has the MOST effect from 0 Throttle opening to 1/4 throttle opening. It also has a slight effect higher than this.

The MAIN JET in this carburetor has the MOST effect from 1/2 throttle opening to WOT. It also has a slight effect lower than this.



Let me know if you need anymore guidance after you digest all of that ;)
 

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
83
Santa Barbara, CA
Update:

Okay, I was having trouble tuning my bike, had extra vibration and couldn't get it to stop revving and go into low idle. I played with the carb and idle screw and just couldn't figure it out. I decided that maybe a new intake might help so I ordered a 15mm Billet, "Big-Mouth" Intake Manifold and I also got some port matched gaskets. It shows up in the mail and I go to install it and I remove the intake and there is no gasket on there!! It must have fallen off when I went to reinstall the cylinder. I had taken the intake off to try to shorten it a little bit on the bench grinder so that the air filter would fit better. No wonder it wouldn't tune, it was leaking with no gasket!! You see, this is the kind of stupid stuff I do, how lame can I be? With the new intake and gasket its back to purring like a kitten again!! How cool is that? Decking it had nothing to do with my tuning problem. The vibration has been reduced and the thing goes right into low idle and I'm still getting better high end too. The new intake should make it run better anyway so I'm still happy I got it. Perfect!! Thanks again man, you can punch me in the face when you see me again for this, I deserve it.

brnot
 
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allen standley

Well-Known Member
Oct 22, 2011
1,126
238
63
Bangor, Maine
I would love a Fred Head but it would spoil my "It's all stock and even de-tuned, officer, this thing wouldn't pull a sick ho off a toilet seat!" story. We aren't allowed to go fast in Nohio.
I think the colder plug and riding at partial throttle now and then should work for ya.
sick ho off a toilet seat! Ha Ha that is a funny bit Slogger - HeH
 

allen standley

Well-Known Member
Oct 22, 2011
1,126
238
63
Bangor, Maine
, you can punch me in the face when you see me again for this, I deserve it.

Nobody will do that. You beating yourself up too much. We all have a twitch now and then. Glad you got it figured.
 

allen standley

Well-Known Member
Oct 22, 2011
1,126
238
63
Bangor, Maine
Exo, I had to read this 3 times. Talk about pulling it all together. Makes perfect sense and not only can I relate, I'm also inspired to practice a finer tune. I gain from your effort. sincerely thanks Much!,
Allen
 

exokinetic

New Member
Mar 18, 2016
108
4
0
Lake Forest, CA
Update:It shows up in the mail and I go to install it and I remove the intake and there is no gasket on there!! It must have fallen off when I went to reinstall the cylinder. I had taken the intake off to try to shorten it a little bit on the bench grinder so that the air filter would fit better. No wonder it wouldn't tune, it was leaking with no gasket!!
Don't beat yourself up about it man!

You probably wouldn't believe how often I do something exactly like this.

It's all part of the process, even expert mechanics make stupid mistakes sometimes.

I am stoked you got it all sorted!
 

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
83
Santa Barbara, CA
Don't beat yourself up about it man!

You probably wouldn't believe how often I do something exactly like this.

It's all part of the process, even expert mechanics make stupid mistakes sometimes.

I am stoked you got it all sorted!
I know, it was just such a dumb thing... Anyhow, I took my bike out today for about a 10 mile ride. Oh man it is running good, my high end is flying now and its very responsive when it purrs into low idle. I can't believe how fast I can get this thing to rev now, I can really feel the extra RPMs. I wonder how many RPMs it is actually measuring at wide open now? It's funny how such a tiny change in squish gap can make such a difference. Also, I'm sure the new intake made a difference too. I guess I will never know exactly how much of a difference the squish gap made unless I put the stock intake back on it (with a gasket!) and test. When I make changes, I try to do things one thing at a time so I know what does what but I have a feeling that changing the squish gap made more of a difference than the new intake but really it doesn't even matter. I gotta learn a lesson that I should never make assumptions. I assumed that decking the cylinder caused the tuning problem instead of looking at the standard things that cause a tuning problem.
.wee.
 
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