Keihin CR31...what a difference a jet makes....

T

taffyrully

VIP MEMBER
Joined
Jan 3, 2021
Messages
8
Country flag
Good evening,

.....with the CR31s(I know JS offers the CRs as 33, but a smaller carb seize also got its advantages....) I had installed in my 750 roadster the bike has been a pleasure to ride.....except from the cold starting.
Must have been plenty hundreds of kicks over the time.
When she was/is warm/engine hot no problem starting her:
no choke, almost runs at first kick.
Idling then perfect, plugs`colour(read after idling) ok.
When cold, she wouldn`t start without choke.....but with the choke lever raised for more than 1 mm she would drown....
So I went down the line of STARTER jets available from 50 to 35(the smallest size You can get)....didn`t make that big difference.
Endless searching and reading in the internet "keihin cr choke"/"keihin cr starting problems":
a lot of people seem to have problems with the choke/starting management of their bikes(mostly 4-cylinders...) tuned/upgraded with Keihin CRs.
Somebody wrote, that Keihins choke installation seemed to be quite sensible to the correct size of the PILOT jet....
So I ordered a set of pilot jets for my CRs at "jetsplus.co.uk"(reasonable prices, fast delivery, many thanks :))....one number bigger than mine and one number smaller.
My original size was #65.
After having installed the #62 I got from jetsplus things have changed dramatically:
bike starts cold with FIRST kick, the choke is "USABLE".....raising the choke lever raises rpm :)
.....things seem to be as they should be :)

So my actual setting is:
main 120
needle yy7
starter 35
pilot 62(instead of 65)
air screw 1 turn out

Perhaps I`ll try to go down to #60......anyway:
3 steps down the size of the PILOT jet makes the STARTER system work.
I would NEVER have thought things were "linked" like that....


Just want to share this experience...perhaps there is someone out there fiddling around with the same issue......













.
 
taffyrully said:
Good evening,

.....with the CR31s(I know JS offers the CRs as 33, but a smaller carb seize also got its advantages....) I had installed in my 750 roadster the bike has been a pleasure to ride.....except from the cold starting.
Must have been plenty hundreds of kicks over the time.
When she was/is warm/engine hot no problem starting her:
no choke, almost runs at first kick.
Idling then perfect, plugs`colour(read after idling) ok.
When cold, she wouldn`t start without choke.....but with the choke lever raised for more than 1 mm she would drown....
So I went down the line of STARTER jets available from 50 to 35(the smallest size You can get)....didn`t make that big difference.
Endless searching and reading in the internet "keihin cr choke"/"keihin cr starting problems":
a lot of people seem to have problems with the choke/starting management of their bikes(mostly 4-cylinders...) tuned/upgraded with Keihin CRs.
Somebody wrote, that Keihins choke installation seemed to be quite sensible to the correct size of the PILOT jet....
So I ordered a set of pilot jets for my CRs at "jetsplus.co.uk"(reasonable prices, fast delivery, many thanks :))....one number bigger than mine and one number smaller.
My original size was #65.
After having installed the #62 I got from jetsplus things have changed dramatically:
bike starts cold with FIRST kick, the choke is "USABLE".....raising the choke lever raises rpm :)
.....things seem to be as they should be :)

So my actual setting is:
main 120
needle yy7
starter 35
pilot 62(instead of 65)
air screw 1 turn out

Perhaps I`ll try to go down to #60......anyway:
3 steps down the size of the PILOT jet makes the STARTER system work.
I would NEVER have thought things were "linked" like that....


Just want to share this experience...perhaps there is someone out there fiddling around with the same issue......













.
Click to expand...
Thanks for sharing! Got CR31’s as well on my 750, runs great but can be hard tot start, was running with Fairbanks morse magneto, now changing to Lucas SR2 since that is single fire, so more spark per cylinder. But I will check pilot jet! Great carbs on all other aspects. Use 31’s because I could get a bank of 4 from some who couldn’t get his Honda to run good on them. Do you use an air filter?
 
…nice to hear you‘re another lucky one with the 31s. ….
also I was fiddling around with the ignition….new 6v in series coils, changing ignition timing, plugs with/without resistance….caps with/without resistance…..
surely good to have all this done/checked…in the end the reason seems to be the (minimsl…) smaller pilot jet.
surely worth a look!

my bike is running with velocity stacks(the longer ones)

take care :)
 
taffyrully said:
Good evening,

.....with the CR31s(I know JS offers the CRs as 33, but a smaller carb seize also got its advantages....) I had installed in my 750 roadster the bike has been a pleasure to ride.....except from the cold starting.
Must have been plenty hundreds of kicks over the time.
When she was/is warm/engine hot no problem starting her:
no choke, almost runs at first kick.
Idling then perfect, plugs`colour(read after idling) ok.
When cold, she wouldn`t start without choke.....but with the choke lever raised for more than 1 mm she would drown....
So I went down the line of STARTER jets available from 50 to 35(the smallest size You can get)....didn`t make that big difference.
Endless searching and reading in the internet "keihin cr choke"/"keihin cr starting problems":
a lot of people seem to have problems with the choke/starting management of their bikes(mostly 4-cylinders...) tuned/upgraded with Keihin CRs.
Somebody wrote, that Keihins choke installation seemed to be quite sensible to the correct size of the PILOT jet....
So I ordered a set of pilot jets for my CRs at "jetsplus.co.uk"(reasonable prices, fast delivery, many thanks :))....one number bigger than mine and one number smaller.
My original size was #65.
After having installed the #62 I got from jetsplus things have changed dramatically:
bike starts cold with FIRST kick, the choke is "USABLE".....raising the choke lever raises rpm :)
.....things seem to be as they should be :)

So my actual setting is:
main 120
needle yy7
starter 35
pilot 62(instead of 65)
air screw 1 turn out

Perhaps I`ll try to go down to #60......anyway:
3 steps down the size of the PILOT jet makes the STARTER system work.
I would NEVER have thought things were "linked" like that....


Just want to share this experience...perhaps there is someone out there fiddling around with the same issue......













.
Click to expand...
And what is the needle jet?
 
62 slow jets worked best for Taffyrully. I would like to hear some feedback if others have success with Taffyrullys suggestions. But I've been using 68s because that usually gives you an idle screw adjustment of 1 turn. Going to a smaller slow jet may require less than 1 turn of the idle mix screw on some motors and then the idle mix screw may become too sensitive.

I found that the #35 starter jets idled much too high with the choke lever pulled so I have been going to 40s lately.

In my testing the bike starts easily with 68 slow jets. But you can only choke it cold for one or two kicks and then you have to unchoke it and it will start. Leave it choked and it will just flood. Keep kicking with it choked and it will never start. Once its started or warm it never needs choking.

I'll go back and test Taffyrullys jetting and make comparisons.

Tuning video:
 
If your motor is warmed-up and the choke is off, if you lower the needles one notch, it should misfire when you ride the bike. The slightest bit too rich anywhere will cause the motor to be slow, but too lean can cause damage. If the motor misses when you ride the bike - stop, and raise the needles one notch. When you become accustomed to tuning, you know immediately when you have carburation too lean. It should not miss at all, when the choke is off. If you cannot make the motor miss by lowering the needles, you need the smaller needle jet. But do not continue to ride the bike, if the motor is misfiring. The needle shape affects how fast you can open the throttle, - for more speed it is better if you are forced to open the throttle slower. For speed, a big gasp of rich mixture is not as good as a smaller gasp of leaner mixture. The gearing of the motorcycle can affect the needle shape which is needed. With higher gearing, the crank inertia causes you to tend to use more throttle when you are accelerating. A quick taper needle causes the mixture to richen quicker. Slower taper needles can be better.
For racing my Seeley 850, I use very high overall gearing with close ratios, and very slow taper needles, with methanol fuel - it is really good. The gaps between the gears are small and it pulls like a train. With bigger gaps between the gears, you use more throttle - I feed the throttle on.
 
Last edited:
jseng1 said:
62 slow jets worked best for Taffyrully. I would like to hear some feedback if others have success with Taffyrullys suggestions. But I've been using 68s because that usually gives you an idle screw adjustment of 1 turn. Going to a smaller slow jet may require less than 1 turn of the idle mix screw on some motors and then the idle mix screw may become too sensitive.

I found that the #35 starter jets idled much too high with the choke lever pulled so I have been going to 40s lately.

In my testing the bike starts easily with 68 slow jets. But you can only choke it cold for one or two kicks and then you have to unchoke it and it will start. Leave it choked and it will just flood. Keep kicking with it choked and it will never start. Once its started or warm it never needs choking.

I'll go back and test Taffyrullys jetting and make comparisons.

Tuning video:
Click to expand...

thanks for Your thoughts….
Surely not an abysmal difference…..
but my CRs are 31….instead of the 33 You‘re using?
Maybe therefore a 33 runs/starts well with the #65….and my 31 „prefers“ the #62?

„Don‘t let me be misunderstood“:
I‘m not claiming universal wisdom with my findings….just wanted to share my experience, as looking up something like „Keihin CR/choke system“ in internet seems to be not an unknown issue….
hopefully might spare time and headsche for someone fiddling with the same problem
 
taffyrully said:
ps:
my ignition is pazon surefire.
isn‘t the brightest spark….but seems to be more than sufficient „as is“
Click to expand...
The Pazon replaces the points/condensers. The actual spark is generated by the coils/plugs. The Pazon only controls the timing of the spark. A weak spark is not caused by the Pazon.
 
Sometimes the carbon on the spark plugs can change and start to conduct electricity. If you have trouble with the ignition, sometimes fitting new plugs can fix it. My bike always starts first kick, but one day, it did not matter what I did - I could not get a murmer out of it, until I fitted new plugs.
 
taffyrully said:
thanks for Your thoughts….
Surely not an abysmal difference…..
but my CRs are 31….instead of the 33 You‘re using?
Maybe therefore a 33 runs/starts well with the #65….and my 31 „prefers“ the #62?

„Don‘t let me be misunderstood“:
I‘m not claiming universal wisdom with my findings….just wanted to share my experience, as looking up something like „Keihin CR/choke system“ in internet seems to be not an unknown issue….
hopefully might spare time and headsche for someone fiddling with the same problem
Click to expand...
I'm all for discovering improvements. Sometimes its just a starting technique. I think flooding is an issue that needs to be avoided. I'd like a 2nd opinion made by another CRS owner on smaller #62 slow jets.

There are also two options for the tiny starter jets attatched to the carb bowl. #35 is leaner - it works well but will rev too high at start up. #40 won't run away with the revs but its richer and will flood if you leave it choked for more than one or two kicks. And you never want to choke it once its run.

And there is the option of starting with it half choked. Everyone needs to find the starting technique that works best for them. For me its choke it and hope it starts 1st or 2nd kick (and it usually does). If not then unchoke it and it usually starts with a couple more kicks. Then it seem to start 1st kick the rest of the day with no choke.

Keihin CR31...what a difference a jet makes....
 
Last edited:
I suggest it might be better to get the jetting correct from top to bottom when the motor is hot, and only use choke or enrichers for cold starts. For performance, as lean as possible right through all throttle openings, without doing damage - is usually better. You should be able to tell when the jetting is too lean - if you ever get a miss, stop and fix it. Raise the needles one notch. If you tune your bike on a very cold day, that should be as cold as the air ever gets, and possibly make the mixture too lean.
I usually lean the mixture off until I get the miss, then move very slightly richer. Main jets should always be a bit too rich. I do not seek more power by leaning-off main jets, unless I really need more top end power. If the carbs are jetted lean right up to 3/4 throttle, the rest does not matter much. Some people open the throttle too quickly - lean with slower throttle opening is usually much better.
 
Last edited:
Back
Top