Honda CT90 Engine Reassembly

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I have one 1971 CT90 where I was going to rebuild the clutch because the kickstarter just wasn't turning over the engine like it used too,  and I also had a Big Bore kit from DrATV that I wanted to install. My intent was to just do the top end and rebuild the clutch with new disks, but when I opened up the cases to rebuild the clutch pack I found a slick black slime in the bottom of the case.  I didn't like the idea that there could be black slime in the rest of the engine, so I decided to disassemble the entire engine to make sure I cleaned everything out.  Since I was tearing everything apart I thought I would document step by step how everything goes back together in this post in case others were interested in knowing what the inner workings of a CT90 engine look like. Links to Related Posts: My Honda CT90 Clutch and Headset Nut Tool Cut Away of a CT90 Engine - CT90 Engine Exposed! Repairing Damaged CT90 Spark Plug Threads Using a Time-Sert Thread Insert CT90 Clutch P

Adjusting and Jetting a Honda CT90 Carburetor

Recently I have had a couple of my CT90's start to give me some issues with how they were running and all the indications pointed at issues with the carb jetting and/or how it was adjusted.  All my CT90's were non running basket cases when I picked them up and I have very much enjoyed bringing them back to life.  Along the way, the process of getting these bikes back up and running involved digging through the pile of parts that I have to find something that might work, so each bike may not have the exact parts that the original model had, which may be part of my problem.

Links to Related Posts:
My Process to get a CT90 that won't Start to Start
The Basic Sequence and Process to Set or Adjust your CT90 Timing
Step by Step Instructions for the Reassembly of a basic Two Screw CT90 Carburetor
Low Cost Cleaner for your CT90 Carburetor Jets and Screws
CT90 Engine Assembly
Honda CT200 Engine Reassembly

To get to the bottom of my potential carb issues I decided to invest a little time and better understand what specific jets, needle valves, etc. were supposed to be in each carb and also educate myself on how each of those components effects the operation of the carb.  With a little bit more knowledge I can hopefully end up troubleshooting the right component to get my bikes running where I think they should be.

After going through all that effort, I decided to pull the information together in this post both for my own reference and also for the benefit of others in case it was useful in helping them in troubleshooting any problems they might be dealing with related to the fuel system on their CT90's. I also don't profess to be an expert by any means in this area and I'm sure that what I share below isn't perfect, so if you see an error or have additional insight to share please let me know and I'll be happy to update my post.  I also included a table near the end of the post with basic carb information on most CT90 models.


One of the first things I worked on was to better understand how each element/component within the carb influenced the performance of the carb in going from idle to wide open throttle.  In the end I learned that most of the carb components only impact performance through a limited portion of the throttle range, so understanding where you have a problem with respect to throttle position tells you a lot on where you need to focus when you go to make adjustments and or changes within the carb to fix your problem.

The chart below and also at this link and this link, as well as at other locations on the web, provide a graphical representation of the general effectiveness for each of the different elements of a Keihin slide valve carburetor verses throttle position.

Effectiveness of different elements of a CT90 Carburetor



To make this chart clearer for myself, I converted the graphical chart shown above into the Excel table shown below.   This is not meant to be an exacting reproduction of the data from the graphical chart, but a close approximation to help me better understand the effectiveness of each component verses throttle position.  I rearranged the data so that the components that had the most effect at idle and early in the throttle motion were towards the top the table and then as you work down the table the other elements that come in with increasing throttle position are listed.  I colored the cells reddish pink when that element was most effective (greater than 50%).  This was probably a little overboard, but I enjoy digging into things like this and this really has helped me better understand what goes on in my CT90's carb. You should be able to scroll the table to the right to see all of the information.


One of the things I didn't fully appreciate until now was how much influence the needle on the throttle valve/slide assembly has during a majority of the throttle travel.


The picture below is of some of the slides and needles I have in my spare parts bin and even though these all came from CT90 parts bikes of various years I had picked up, they all have different markings and are therefore different sizes and will have a different effect on a carbs performance.

Various CT90 carburetor slides and needles

Each needle has part markings around its diameter and just under the bottom groove that accepts the small clip that positions it within the throttle slide assemble.  These markings can be very hard to see without a magnifying glass.

Detail photo showing adjustment clip of CT90 carburetor needle

The markings on the needle correspond to a specific taper, length, and diameter for that specific needle.  I haven't been able to find information on the web to convert the markings I've found on the needles in my current CT90 carbs to know what the actual taper angle, lengths or diameters are, but when I do find that info, I plan on updating this post.

The following table are general guidelines I have found on the web on why and when you might select a needle with a specific diameter, taper or length to address a lean or rich condition with your CT90 carb. 

Chart showing the impact of features on CT90 slide needle


This link provides a good basic description of how each element of the slide needle impacts carb performance and it is what I used to build the table above.


Jetting
I found the following website at Shiny Red Tuning to be very helpful and a good generic description on the process to follow for jetting a Keihin carburetor. I would also recommend taking the time to look at the other write-up on this site related to spark plug heat ranges and jetting as it really is good info.

The following link has a great flowchart on a process to follow when jetting that was intended for a two stoke, but the basic sequence outlined is representative of how you should approach jetting with most motorcycles.

Troubleshooting
As I stated earlier, most of my bikes have been basket cases that I have brought back to life, so I have had to deal with a lot of issues which forced me to spend time on the web coming up to speed on the CT90 and how it works.  There are a lot of knowledgable people who have shared a lot of great info on dealing with CT90 issues and I have tried to summarize what I have found that was useful in working through carb related issues in the comments below.

When troubleshooting you always need to fist make sure that the following have been addressed before making adjustments to carb settings of changing out the needle, main or slow jets.
1. Air filter is clean and unobstructed
2. Exhaust is not plugged or obstructed in any way
3. In-line fuel filters are not clogged and you have fresh gas in the tank
4. All internal passages within the carb are clean and have been blown out with compressed air
5. Engine has good compression, valves have been correctly adjusted, Timing is correct set
6. Spark advancer is known to be clean and that it operates freely
7. You have a fresh plug

In addition to the above items, within the carb you should do the following:
1. Make sure the Slide needle, main and slow jets are the correct size for your CT90 carb
2. Make sure that the float height has been correctly adjusted and that the float valve operates freely


With all of the above basics covered you can now focus on trying to work specific issues

Idle up through 1/4 Throttle Adjustment and Issues
You should always start with getting your idle circuit correct before moving on to address other issues at increased throttle positions.  As a side note, on older CT's like the CT200 where the air screw is on the intake manifold side of the carb and not the air filter side, the direction you turn the air screw to get a leaner of richer mixture is opposite to what is described below which is for later CT90 configurations where the air screw is on the air filter side of the carb.

Issue - If adjustment of the air screw has little impact on RPM's then the idle circuit may be clogged or the slow jet is the incorrect size
Issue - If your engine surges at low RPM's or bogs or cut's out when the throttle is opened quickly and has trouble idling back down then you are running lean and you should turn the air adjustment screw clockwise to richen the mixture and/or go to a bigger slow jet.
Issue - If your CT90 is hard starting, your plugs foul at low RPM's, or you get sputtering when you crack open the throttle then you are running to rich and need to turn air adjustment screw counter clockwise to lean out the mixture and/or go to a smaller slow jet.
Issue - Backfire through the carb.  This can be caused by an intake leak somewhere between the cylinder head and the carb. You can use the method of applying different materials like spray starter fluid to the joints in this area while the bike is running and if the RPM's increase then that specific joint should be addressed to eliminate the leak.
Issue - Your CT90 will only start and run if the choke is left on.  This is another indication that you have an intake leak somewhere between the cylinder head and the carb and you can use the method described in the previous issue to try and identify the location or just use some new gaskets and make sure the joints are tight.
Issue - Your CT90 won't idle correctly unless you leave the choke partially on.  This can be caused by the small o-ring on the emulsion tube being dried or cracked which can create an inconsistent air leak. The fix is to replace the o-ring and while your there also check and make sure all the tiny holes in the emulsion tube are free and clear.

1/4  through 3/4 Throttle Adjustment and Issues
The needle is the primary component affecting this range of the throttle.  Assuming the stock needle is adequate, raising the clip on the needle will make the mixture leaner and lowering the clip will make the mixture richer.  For some cases a different needle may be required (see the table above).

Issue - If the engine is zingy sounding in this range then it is running lean and there can be a risk of seizing or burning a hole in the piston if it is way to lean. Lower the clip on the needle to increase richness of mixture.
Issue - If the CT90 has lazy performance in the midrange and the exhaust sounds flat or sputters, then it is running rich and the clip on the needle needs to be raised to lean out the mixture.
Issue - Bogging on acceleration.
1. If there is no power at all then main jet could be clogged.
2. While the focus here is on the carb, if you do have this condition and have addressed all the potential sources in the carb it is worthwhile going back and rechecking your point gap and timing.


3/4 to Wide Open Throttle (WOT) Adjustments
One of the basic ways to dial in your main jet for wide open throttle is to perform what is called a throttle chop test. What the test entails is finding a spot where you can run your CT90 in high gear at speed and installing a fresh plug.  With the bike warmed up make a run through the gears and when you top out with respect to speed, kill the engine and coast to a stop. Immediately remove the plug and look at the base of the insulator, if its white/light grey its running lean and you need to go to a bigger main jet. If the insulator is brown or black then you are running rich and need to go to a leaner main jet. The ideal result is for the insulator to be a light brown after performing the test.

Issue - Lacks power from 3/4 to WOT - If power is fine all the way up to 3/4 throttle, but falls off from there to WOT then the main jet is partially clogged or to small.

Good General Guidance
An engine where the carb settings and jets are biased to being too lean is potentially a less reliable engine and more susceptible to seizing and or over heating. Once you have gone through and made all of your setting adjustments and jet changes to get your CT90 running just right, you should then make slight adjustments or jet changes to bias everything to be slightly richer.

Additional Conditions That May Impact Carb Adjustment
Altitude - If you're going to be operating at a higher altitude you may need to adjust your settings/jets to be slightly leaner then you would be at sea level.

Stubby Air Filter - If your not running the stock air filter on your CT90, but a stubby aftermarket configuration that attaches directly to your carb, you may need to adjust your settings/jets so that they are slightly richer then the stock settings and jets.  The stock air filter is more restrictive then the typical aftermarket stubby.

Using Fuel that has 10% or Greater Ethanol Included - Having to use fuel with ethanol kind of sucks, but if you are in that situation like I am, you may find that biasing your settings/jets to the richer side may make the situation more acceptable.


Basic CT90 Carburetor Information



Model
CT200
CT90 K0
CT90 K0
CT90 K1
CT90 K1
CT90 K2 & K3
CT90 K4
CT90 K5 & K6
CT90 '76 & '77
CT90 '78
CT90 '79

Year
1964 thru 1965
1966 thru 1968
1967 thru 1968
Early 1969
Late 1969
1970 thru 1971
1972
1974-75
1976 thru 1977
1978
1979

Bowl Configuration
4 Screw
4 Screw
2 Screw
4 Screw w/O-ring
2 Screw
2 Screw
2 Screw
2 Screw
2 Screw
Round Bowl
Round Bowl

OEM Carb#
16100-033-014
16100-053-305/315
16100-053-044
16010-077-305/004
16100-077-014
16100-102-034
16100-102-702
16100-102-702
16100-102-702/734
16100-102-734/774
16100-102-734/774

Keihin Carb Model
DPW-14H15A
T90D
?
T90KA,  up to CT90-234851
K29A
K29B
556A
556A
556A
B27A
B27A

Slow Jet
#38
#40
#40
#35
#35
#35
#35
#35
#35
#38
#38

Main Jet
#75
#72
#72
#75
#75
#65
#62
#62
#62
#65
#65

Slide Needle
16151-033-004, size #18507 16151-053-004 size #18301
16012-053-024, 16151-053-014, D32
16012-053-004 size #16332
16012-077-014 or D136
16012-077-014 or D92
16012-102-004 size #16333 or D91
16012-102-004 size #16333 or D91
16012-102-004 size #16333 or D91
16012-102-004 size #16333 or D91
16012-102-730 or D416
16012-102-730 or D416

Float Valve Needle and Seat
#1
#1.2
#1.2
#1.5
1.8
#1.5
#1.5
#1.5
#1.5
#1.7
#1.7

Float Height
23 mm
23 mm
20 mm
23 mm
20 mm
20 mm
20 mm
20 mm
20 mm
10.7 mm
10.7 mm

Initial Air Screw Setting from Honda Shop Manual


1 3/8 turns
1 1/8 turns


1 Turn
1 Turn
1 Turn
1 1/4 turn
1 1/4 turn

Keyster Kit #
KH-0676N
KH-0202N
KH-0078N
KH-0203N
KH-0079
KH-0080/-0081
KH-0081
KH-0081
KH-0214N
KH-1160N
KH-1160N
Complete Standard Carb Kit
SCI

Beatrice Link



WinCycles









4 into 1









Jerry's Cycles




Just Carb Packing
Beatrice




4 into 1









Additional Information
The link here to Shiny Red Tuning is very helpful and has a very in-depth discussion on jetting Keihin carburetors.

This is a very good link of how to read your spark plug to get a feel in if you are running too rich or lean an if your timing is too advanced or not.

Here is a very good article from Cycle World on how to adjust a carburetor

Here is an additional link that has a clear process to follow for adjusting your CT90 carb through the complete throttle range.

This link is a good resource for troubleshooting various carb performance issues.

If you're looking to get a better understanding of how in general a carb works (not specific to a CT90) and also troubleshoot basic problems you should click through the slide presentation at the following link as it is a very well done presentation.

Here is a link to another site that has a good discussion about carb tuning for newer Keihin cabs, but is still very much applicable to the older versions that came on your CT90.

This link is another good site with basic information on how a carb works along with troubleshooting info and additional info on cleaning and getting an old carb up and running.

This is a link to Dan's Online Motorcycle Repair Course which has a lot of great info in general, but if you scroll down the list to the section on carb's he provides some great information in a number of key areas. A good place to start on carb tuning is at this link on Dan's site and this link.

Here is another link to a site that has a good general description on jetting and also with doing a throttle chop test.

One of the easiest things to do to help your CT90 to run better is to use gas that does not contain ethanol. This link will provide a list of gas stations by state that sell ethanol free gas.

Other Related Links
Honda Keihin Adjustment & Balance
Keihin Carburetor Tuning
Oldmanhonda Carburetor Discussion

Links to Related Posts:
My Process to get a CT90 that won't Start to Start
The Basic Sequence and Process to Set or Adjust your CT90 Timing
Step by Step Instructions for the Reassembly of a basic Two Screw CT90 Carburetor
Low Cost Cleaner for your CT90 Carburetor Jets and Screws
CT90 Engine Assembly
Honda CT200 Engine Reassembly

Helpful Links (Shop Manuals, Wire Diagram, Model Information, etc.)

Link to page with listing of CT90 parts available on Amazon


Comments

  1. Another excellent witeup that will help many people.

    Rick

    ReplyDelete

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