That's good info on carbs. Share link do I can look at please.

I based on keihn due to jetting charts to make math easier to follow.

There are four TM38 carbs that I am aware of.
There is TM38-85, TM38-86, TM38-101, TM38-102.
The TM38-85 and TM38-86 have the same jetting.
The TM38-101 and TM38-102 I am not sure but the TM38-101 has a main jet that is 100 pts larger than the TM38-85.
Below are links to these carbs and a picture which shows the TM38-85 and the TM38-86.

http://www.nichecycle.com/ncs/mikuni/ca ... ikuni.html
http://www.nichecycle.com/ncs/mikuni/ca ... -43mm.html

I thought we had talked about this before.
Here is the post.

viewtopic.php?f=1&t=17376&hilit=TM38+carbs

Thank you posting info. I guess I did not view post since it was not kehin.
On the surface it appears to be more confusing than kehin. I'll dig into it more and wrap my head around it.
Did you understand the numbers I posted.
MM x 9.8=
= /1.6 this will be close for kehin then that number X ratio ( .90 etc)
Then ratio of adder's.
Then it can be plotted into graph for Comparison.

This can show a quick throttle ratio evulation.
Nice thing about kehin it is eaiser to plot
I'll stick with this for this discussion

The quick math shows your jetting to be around 155 to 160.at sea level and 86 f
What does the book say for stock ody?

Yeah I understand what the numbers you posted mean.
The thing that confused me was the part where you say: "Then ratio of adder's".
What is "adder's" ??
I assumed it was altitude.

Sorry for confusion
Adders are any change from stock.
An example : air box mod,pipe,carb intake change in the way of reeds,boot ,port timing or case CR.
This is where the .90.95 etc comes in.
I'm going to skip ahead a little so we all get an idea of where were heading.

So here we go:
Let's say we put on a larger carb mod only. Stock jetting will not work.
Why!
The crab is larger in diameter. 34mm verses 38 or 39 mm.
I provide a formula for this propee sizing later.
The air pump ( Engine) creates a given vacume that pull air from outside into the air box opening through the filter down the filter / carb tube into the carb.
At this point based on the given vacume strength it creates a velocity thru the carb. I'm skipping asome things as not to bla bla.
As the diameter increases the velocity changes. I generally refer to this as small straw big straw.

This velocity change at a given needle setting and main jet effecs the amount of fuel pulled which changes the air fuel ratio. Since the large carb does not pull as hard we need a richer settings. Larger main jet to pull fuel less restriction.
This also has an effect on fuel denesity. I'm going to skip the rest of the story through the intake cycle again bla bla for now. Bring back in later.
Based on the formulas given the math supports this. Real world testing does as well.
Higher velocity smaller main, lower velocity larger main.
Same for needle.

So far so good?

Now look at restriction to flow on the intake side before carb. Again let's not get ahead of me with carb single etc. Keep it sime for now.
This one pretty easy. Less resticarion more flow. More air needs more fuel.

Before we go deeper into adders let me address the fact that not adders(parts etc.) Requires increase in fuel metering. Look at boyseen reeds versus stock. Why does this require a metering change in a lean direction. Remember you changed nothing but this reeds.

This lighter (generic term)material will open sooner at a given vacume than before. So it pulls more (quanity) for a given time. Hench richens things up.
If you go to there Webb site they even give a jet chart for Honda's that reflect this. It's not much but has an effect.
Remember I said reeds only not block. That's another area all toghter no pun intended. He said area.

Now if you were to change velocity along with Reed lift/time. You have made a dramatic change.
Then throw in area/ lift/time/ velocity. All bets are off for any stock jetting chart.

Now don't forget about the denisity statement.

I took some info from the 85 post recall manual and made a j-peg for viewing. I made some notes and remarks worth review.
CO the Red Star is for you. You should add this to your list of general ody stuff. Remember this is post recall 78.5 and at 6 to 1 CCR if some one has the same info for pre recall manual, post me the pics and i will make a comparison.
FYI the 80 mm pre recall was at 6.7 to 1 if memory serves.
As always the Honda manual is loaded with a lot of info that is generally looked over. This will no doubt spark more interest I hope.

For fun some one look up just when did we switch over to unleaded? Ummmm

Any way enjoy and ghater some conclusion to what we have already talked about. More to come

I posted the pic of the carb for viewing for next set of comments. pilot guys may find this interesting.

Adnoh

Leaded fuel hummmmmm. Like I have said many times --- the fuel of today is not the same as the stuff we bought in the 70's.
Yes I did over look that in the manual. Missed it. Good one Adnoh.
The other thing I noticed with Avgas is it seems to give you a big safety margin. At least that's what I believe. Others can shoot me down over that belief but that's how I feel. I feel that way because of the Engine that lost the rod bearing. No sense getting into that one again but I did mention it earlier in this post.

Another item to take away from pic is the alitude statement ( black star).

Honda already new jetting was close with thin margian of error.

Yes, a higher octane rating will provide a safer thicker margin. What I mean by this in this case main jet sizing with needle/ air adjustments.

I mentioned the pre recall ccr as a side note which most are at (80 mm+). This takes the margin even closer to bad news. At 6.7 and uleaded fuel carb jetting and adders must be in check.

Honda took the easy way out. After all the mfg has a lot to loose.

I think the next step would to look at the jetting over lap and see what is best for the stock set up along with fuel choice. Then how to adjust for adders or at least what thete effects have.

Sorry Adnoh but I adjusted your quote to suit my argument .
Like you mentioned above Honda already new jetting was close, had a piston size recall, and took the easy way out by posting an altitude statement.
When they started blowing up engines they recalled it. Tried a new head with angled fins, tried bigger side scoops, tried smaller piston ---- then they gave up.
I bet the machines ran fine depending on outside air temp and altitude.
Can't believe they didn't catch on that the fuel was detonating under certain conditions.
Even factories make mistakes, but that's how we learn.
The reason I say factories make mistakes is because of my racing experience with rotaries.
Back in the 1970's when I was a teen and first started racing cars, Mazda came out with the 12A rotary. Everyone started racing these as soon as they came out and so did the factory. Many blew up. Same goes for the factory race cars. As soon as you went to a peripheral port design it was a death sentence. Reason: there was an Engine flaw. A friend of mine, an American that lived in Washington state, and built these engines figured it out. In fact some Mazda factory people came out to watch him race at SIR, I think it was there anyways, it was a long time ago memory fades a bit (Seattle International Raceway). Doug's engines never blew. The factory boys had a chat with him and went to his backyard shop. He took an Engine apart for them and showed them what he does. At the end of this tale is that the factory sent him a letter and told him to go to any dealer he wanted and take any Mazda he wanted "FREE" of charge. He did. Moral of this true story: EVEN THE FACTORY F'S UP.
Edit: The rotary Engine is actually a two stroke.

No its not.... LOL With three lobes, its a three stroke!

Rand

Since we have discovered the octane rating form the manual. Let's look at basic math for mixing octane numbers.
Let's say we do a 50/50 mix. 50% 87and 50% 110.
.50 x 87= 43.5
.50 x 120= 55
43.5+55= 98.5
Pretty close to book standard
Now let's look at 93 and 110
.50 x 93=46.5
46.5+55=101.
See no magic involved.
So now how do mix other than 50/50.
More easy math. I teach this one all day and sometimes every day.
Being a electrical it's key in most of our math.
Simple fractions.
Let's say 3 gal of 110 and 2 gal of 93 for 5 gal total
The fractions look like this 3/5 and 2/5
Three divided by five= .60 or 60%
Two divided by five = .40 or 40%
110 x .60 = 66
93 x .40 = 37.2
66 + 37.2 = 103.2

As always, some one check my math.

One another note. The black star I put on the picture.
It is pretty clear one size jet or two can be real bad.
Unless you compensate in another manner. An example would be math above.
So is this a crutch or a tool.
Adnoh

I gota disagree with part of this.
You said: "It is pretty clear one size jet or two can be real bad."
Well I say NO because the octane will stay the same. The problem would be a lean out and high combustion temps. Like I have said here, my Engine ran incredibly lean with that cracked intake but it didn't damage the piston. It was so lean the rod bearing didn't get enough lube. I run 50/1 oil mix. Now add a lean condition -- that's almost no oil now, and it just said F it and gave up. It would have been real cool if I knew what the actual air/fuel ratio was with that cracked intake. I bet my rent money it was a hell of a lot more than two or three jet sizes. My argument is that high octane gives you a real big anti detonation and lean out protection curve.

The Americans got the patriot missle, the Jews got the iron dome and I got AVGAS.



Edit: You also need to keep in mind your oil mix ratio as this will effect your jetting. A 145 jet with 30/1 mix will not be the same as a 145 jet with 50/1 mix. The fuel rate will not be the same. And here we are talking about one jet size. Take a look at your main jet -- see that little hole ??. Now remember one fiftieth of that little hole is oil the rest is fuel .

I want to bring this one back to life for a brief moment because I found a better plug reading video on youboob. I bring this thread back to life because sometimes it's hard to find old threads that you are looking for and many here do plug chops. I don't. If it blows up I will rebuild. Haven't had that problem for a while now. AVGAS.
Here is the vid, start at the 3 min mark: https://www.youtube.com/watch?v=Zx44uRFKRD8

Hmm. I admit I didn’t click that, but honestly I just get her running fat just shy for blubbering at each 1/4 throttle. Last time out I didn’t change main jet much, but I thinned our needles 4 times and every time the machine ran better.

Honestly the only way to really and truely know your mixtures corrrct would be to use a wideband sensor and tune for specific readings at each 1/4 throttle.

I’ve personally had more than one crank bearing fail likely due to slightly lean lack of oil over time slowly starving the bearings; while the top end was great up until the bottom end failure. Nowadays I refuse to let that happen and push more fuel as a priority.

So if it doesn’t rain tomorrow I’m supposed to take the buggy out and try and do some plug chops, in my research I see this and I’m wondering now if I should even bother since I am running av gas?

If your fuel pump is putting out 4.5 in 10 and you are running the factory/manufacturer suggested jetting for that carb you're good to go. The only thing you will be playing with is the pilot jetting. You do that in the driveway with a small induction tach and a screwdriver.
Provided you don't have a port race Engine --- then all bets are off.

By the way, my brother and I went out for a ride last week and both our machines had great bottom end and then suddenly fell on their faces at mid and full throttle. We were riding at just above sea level to 4200 ft. When I got home I did a flow test on the pumps. Both put out 2.5 oz in 10 sec. That's half of what it should be and neither machine blew up. AVGAS
CO

EDIT: By the way this exact same thing happened to the death machine on one of the first rides of this year. This is the machine with the homemade head and welded piston. It didn't blow up either. AVGAS
CO

Well I I don’t know if you would call it a race Engine but it’s definitely not stock running that CR 500 piston.

Do you plug chops.
There more than color to consider.
Gas, fuel or a combination of each. The plug reading is important.
It can help with the effect of of fuel choice on jetting.
Plugs cheap compared to Engine rebuilding