Topic: Jetting fun

[killersoundz]

Ok so I drilled out an extra set of main jets I had yesterday with a 3/64 bit. That's equivalent to a 125 main Keihin jet from a size chart I've been looking at. Bike was still too lean on the top end. I just went ahead and drilled these jets out with a 1/16" bit which is equivalent to about a 160 according to the chart. Actually quite close up through the midrange and 3/4, but too rich at WOT and misfires.

I moved the needle clips back to the stock middle position. I did this after I started the bike up with the 125's with the needle up one and it was shooting out black smoke through the midrange. Too rich, no way that is ridable would cause my fuel consumption to go through the roof.

So knowing that 125's are still lean, and 160's are too rich, what number do you think I should shoot for? 140's?

This is a cb750k4 with 4 into 1 straight header and pods

[lucky]

This is a cb750k4 with 4 into 1 straight header and pods.



What is the number on the carb?

[wrenchmuch]

Drilling jets will leave a hole slightly larger than the bits diameter. If your going to do this by trial and error then I'd go for 135 first. I've had a similar setup in the past and that main jet size was good at wot. I know it costs money but the best way to dial in jetting is to test exhaust gases on a dyno.

[killersoundz]

Drilling jets will leave a hole slightly larger than the bits diameter. If your going to do this by trial and error then I'd go for 135 first. I've had a similar setup in the past and that main jet size was good at wot. I know it costs money but the best way to dial in jetting is to test exhaust gases on a dyno.

Ok thanks, I think I will try 135's, and maybe pick up 140's in case.

[lucky]

Drilling jets will leave a hole slightly larger than the bits diameter. If your going to do this by trial and error then I'd go for 135 first. I've had a similar setup in the past and that main jet size was good at wot. I know it costs money but the best way to dial in jetting is to test exhaust gases on a dyno.

Ok thanks, I think I will try 135's, and maybe pick up 140's in case.

You still did not tell us which carbs you have. 657's or 7A's.

[killersoundz]

Drilling jets will leave a hole slightly larger than the bits diameter. If your going to do this by trial and error then I'd go for 135 first. I've had a similar setup in the past and that main jet size was good at wot. I know it costs money but the best way to dial in jetting is to test exhaust gases on a dyno.

Ok thanks, I think I will try 135's, and maybe pick up 140's in case.

You still did not tell us which carbs you have. 657's or 7A's.

ooooo, where do I find this number?

[killersoundz]

Drilling jets will leave a hole slightly larger than the bits diameter. If your going to do this by trial and error then I'd go for 135 first. I've had a similar setup in the past and that main jet size was good at wot. I know it costs money but the best way to dial in jetting is to test exhaust gases on a dyno.

Ok thanks, I think I will try 135's, and maybe pick up 140's in case.

You still did not tell us which carbs you have. 657's or 7A's.

Found the number, says 657A


I also wanted to point out now that after riding with the 160 main jets (1/16 drill bit), at the low end range of say 5th gear (35mph), I can open the throttle all the way and it will run fine. Once the RPM's get up past 4k or so it will start to misfire from being too rich. Is this any hint that it's not too far off and I should go on the bigger jet side? Like a 140.

Also something weird started happening. The bike idles fine with this current setup but once it gets hot and I pull up to a redlight it will idle fine for 20 seconds then the RPM's will start to drop and wants to stall out unless I turn the idle up. I'm going to overlook this for now until I get to the final tune on the carbs and I'll worry about it if it still does it then. I wasn't able to get one of my intake boot clamps 100% tight so could be an intake leak.

[lucky]

Drilling jets will leave a hole slightly larger than the bits diameter. If your going to do this by trial and error then I'd go for 135 first. I've had a similar setup in the past and that main jet size was good at wot. I know it costs money but the best way to dial in jetting is to test exhaust gases on a dyno.

Ok thanks, I think I will try 135's, and maybe pick up 140's in case.

You still did not tell us which carbs you have. 657's or 7A's.

Found the number, says 657A


I also wanted to point out now that after riding with the 160 main jets (1/16 drill bit), at the low end range of say 5th gear (35mph), I can open the throttle all the way and it will run fine. Once the RPM's get up past 4k or so it will start to misfire from being too rich. Is this any hint that it's not too far off and I should go on the bigger jet side? Like a 140.

Also something weird started happening. The bike idles fine with this current setup but once it gets hot and I pull up to a redlight it will idle fine for 20 seconds then the RPM's will start to drop and wants to stall out unless I turn the idle up. I'm going to overlook this for now until I get to the final tune on the carbs and I'll worry about it if it still does it then. I wasn't able to get one of my intake boot clamps 100% tight so could be an intake leak.

Thanks for that info. Now I can help you.

[lucky]

Facts: My recommendations.

This is a cb750k4
4 into 1 straight header exhaust
Intake pods
Main jet #120.
Idle jet #42.
Slide needle clip position 5th down from the top.
One position down from stock because of the pods and open header.
Mixture screws Try 3/4 turn open.

Stock Settings for 657A.
110 main jets.
#40 idle jets.
Clip position 4 down from the top of the needle.
Mixture screws,1 turn open.

Hopefully this combo I have suggested will work because the original Honda clip position was reported to be just a little rich in the first place.

[killersoundz]

Facts: My recommendations.

This is a cb750k4
4 into 1 straight header exhaust
Intake pods
Main jet #120.
Idle jet #42.
Slide needle clip position 5th down from the top.
One position down from stock because of the pods and open header.
Mixture screws Try 3/4 turn open.

Stock Settings for 657A.
110 main jets.
#40 idle jets.
Clip position 4 down from the top of the needle.
Mixture screws,1 turn open.

Hopefully this combo I have suggested will work because the original Honda clip position was reported to be just a little rich in the first place.

Respectfully man I have to say no way!! I had 115's in there with the clip 4th down from top and past 1/2 throttle it was so lean it would just bog out. I had to run the choke half on. When I drilled out the jets with a 3/64" bit (little smaller than a 125 jet) and I left the clip on 4th from the top, the mid range became super rich to the point it was shooting out black smoke when I revved it up BUT it was still too lean on the top end. So I moved the clip to 3 down from the top and have those 1/16" main jets in (160) and it is OK up until the very last 3/4 of throttle and then is too rich. All signs point me to clip 3 down from top and 140 main (maybe 135 to a 145 range).

Remember I installed the aftermarket rebuild kit idle jets and needles. I don't believe these needles are the same taper and whatnot as the stock ones are. Also these idle jets said they are 40's, but another thread here proved they were bigger then a genuine Keihin 42. My idle jetting is actually ok I don't need to mess with that much. Having  this large main jet in there actually overlaps the idle a little bit and made the idle a tad bit richer believe it or not.

[lucky]

Whoa!!!
You kind of threw a wild card in at the last minute.
non stock needles?  were they Dyno jet needles?

The needles control 1/4-3/4 throttle. So if it is running lean you would want to raise the needles to the 5th notch. Most of the entire throttle range is controlled by the slide needles.
The fact that you saw black smoke come out the exhaust is not a valid diagnostic  parameter. black soot can come out of the exhaust because the entire inside of the exhaust tube could have black soot all over it.

 A spark plug reading (not a chop) is what is needed after riding it around for a day or so.
Anyway up until now i thought we were talking stock needles.

[killersoundz]

Whoa!!!
You kind of threw a wild card in at the last minute.
non stock needles?  were they Dyno jet needles?

The needles control 1/4-3/4 throttle. So if it is running lean you would want to raise the needles to the 5th notch. Most of the entire throttle range is controlled by the slide needles.
The fact that you saw black smoke come out the exhaust is not a valid diagnostic  parameter. black soot can come out of the exhaust because the entire inside of the exhaust tube could have black soot all over it.

 A spark plug reading (not a chop) is what is needed after riding it around for a day or so.
Anyway up until now i thought we were talking stock needles.

Well they were from a "keyster" carb rebuild kit. I had bent one of my original needles thus rendering the set useless otherwise I would have kept them. Side by side these replacements and the originals looked a bit different.

I found a thread commenting about this:

I've been fighting a rich mid-range hesitation with my 450 since I installed Keyster carb kits. After comparing the old slide needles to the Keyster ones I found that the taper of the needle was quite different and the needle was quite a bit narrower at the tip. Since the kits don't come with the needle jets to match the needles I decided to re-install the old needles. That has solved the issue. The gaskets, jets and needle/seat assemblies seem to be OK but I would be very careful to compare the old and new parts in these kits from now on.

http://www.hondatwins.net/forum/viewtopic.php?f=18&t=4172

[wrenchmuch]

I think the idle problem might be a rich condition which is fouling the plugs at idle. Some fuel will be delivered by the main jet even at idle.
http://www.google.ca/images?q=tbn:ANd9GcRzDbnUKSsXfquCos8FGEOEhGMaJYYr5FlFHWKiJSgV9N0Fu2k1
Try and think of all the metering segments of the carb as having a limited effect on one another. People have had problems with aftermarket needles in stock configurations. Not so much of a problem for you since your jetting from scratch. The needles are only one half of that jet assembly. There is also the jet tube that sits above the emulsifier tube that holds the main jet. These two parts ( needle and tube )come as a set from Honda and Yamiya also sells them this way. I don't understand why Keyster and K&L only include the needle when it's the tube that wears the most. If you suspect the tube might be worn you should think about changing it now as this will save you grief while trying to figure out jetting.  Most rarely find a need for anything fatter than a #40 idle jet but results might vary. I still think #135 is where to start on the mains but armchair jetting can be inaccurate.

[killersoundz]

I think the idle problem might be a rich condition which is fouling the plugs at idle. Some fuel will be delivered by the main jet even at idle.
http://www.google.ca/images?q=tbn:ANd9GcRzDbnUKSsXfquCos8FGEOEhGMaJYYr5FlFHWKiJSgV9N0Fu2k1
Try and think of all the metering segments of the carb as having a limited effect on one another. People have had problems with aftermarket needles in stock configurations. Not so much of a problem for you since your jetting from scratch. The needles are only one half of that jet assembly. There is also the jet tube that sits above the emulsifier tube that holds the main jet. These two parts ( needle and tube )come as a set from Honda and Yamiya also sells them this way. I don't understand why Keyster and K&L only include the needle when it's the tube that wears the most. If you suspect the tube might be worn you should think about changing it now as this will save you grief while trying to figure out jetting.  Most rarely find a need for anything fatter than a #40 idle jet but results might vary. I still think #135 is where to start on the mains but armchair jetting can be inaccurate.

Thanks that's an interesting theory about the weird idle.

[wrenchmuch]

Could be a charging issue as well .

[lucky]

Could be a charging issue as well .

Of course that would have been resolved before any carb tuning.

[lucky]

It is unfortunate that you bent one of the original needles.

The needle does not actually touch the jet. It just goes through the main jet.
When the throttle is closed the slide needle should not actually touch the mainjet.

On the older CB 72's and 77's the needles did touch, and the needles got worn and caused a lot of problems. Then the needles were made free floating side to side, so they can center themselves.

When you are comparing needles you would need to use accurate calipers.
Below is a diagram showing how the main jet is positioned in the main jet.

[killersoundz]

Could be a charging issue as well .

How would a charging issue cause the idle to drag and not just stall though?

[wrenchmuch]

At idle charging output drops. As you sit at the light battery voltage drops with it weakening spark intensity. A faulty charging system can be due to things like a bad rectifier, regulator, wiring coming from the alternator or even a battery that's on it's way out. The rectifier can have one or more failed diodes. This will have the effect of losing the power contribution of one or more of the 3 alternator coils supplying charging current. A malfunctioning or incorrectly adjusted reg unit can cause lower charge voltages. It's important to make sure you have proper spark output before making carb changes as bad spark can look a lot like a rich condition. A engine with points can run with lower electrical system voltages. Just doesn't run as well.

[killersoundz]

I figured out my sagging idle seemed to be due to poor fuel delivery caused by poor line routing. Charging is ok.


Well I got the 140's and 130 main jets in the mail. Tried the 140's out first. Still way too rich 

Put the 130's in. Better but WOT still too rich    I got a couple WOT pulls that ran clean throughout without any misfiring up to 8k, but most of the pulls started rich misfiring on the very top end.  I missed a gear and must have hit 10-11k. Know the valve springs must be alright nothing bad happened haha

I can see how the 120's and needles up would work well Lucky. However with these needles that are in there the midrange still runs great with the 130's. There is a slight hesitation in 5th gear cruising at 35-40mph but it could have been a crossdraft pod issue as well. Seems very responsive through the midrange. Is it on the lean side through there? Maybe, I don't know. I might get it on a dyno tune.

Should have got 125's I think those hit the sweet spot.

[killersoundz]

I'm going to put new spark plugs in the bike tomorrow, they were fouled out from the get go and I cleaned them once. It also does need new spark plug caps I tested the resistance on them all and 2 don't appear to be in spec. And going to check my points again make sure everything looks ok tune up wise.

[lucky]

I figured out my sagging idle seemed to be due to poor fuel delivery caused by poor line routing. Charging is ok.


Well I got the 140's and 130 main jets in the mail. Tried the 140's out first. Still way too rich 

Put the 130's in. Better but WOT still too rich    I got a couple WOT pulls that ran clean throughout without any misfiring up to 8k, but most of the pulls started rich misfiring on the very top end.  I missed a gear and must have hit 10-11k. Know the valve springs must be alright nothing bad happened haha

I can see how the 120's and needles up would work well Lucky. However with these needles that are in there the midrange still runs great with the 130's. There is a slight hesitation in 5th gear cruising at 35-40mph but it could have been a crossdraft pod issue as well. Seems very responsive through the midrange. Is it on the lean side through there? Maybe, I don't know. I might get it on a dyno tune.

Should have got 125's I think those hit the sweet spot.

You may be right about the #125's.
The main jet size controls mostly WOT.

You do not ride at WOT most of the time.
Do not focus so much on WOT issues.
Focus on every day driving.

[killersoundz]

I figured out my sagging idle seemed to be due to poor fuel delivery caused by poor line routing. Charging is ok.


Well I got the 140's and 130 main jets in the mail. Tried the 140's out first. Still way too rich 

Put the 130's in. Better but WOT still too rich    I got a couple WOT pulls that ran clean throughout without any misfiring up to 8k, but most of the pulls started rich misfiring on the very top end.  I missed a gear and must have hit 10-11k. Know the valve springs must be alright nothing bad happened haha

I can see how the 120's and needles up would work well Lucky. However with these needles that are in there the midrange still runs great with the 130's. There is a slight hesitation in 5th gear cruising at 35-40mph but it could have been a crossdraft pod issue as well. Seems very responsive through the midrange. Is it on the lean side through there? Maybe, I don't know. I might get it on a dyno tune.

Should have got 125's I think those hit the sweet spot.

You may be right about the #125's.
The main jet size controls mostly WOT.

You do not ride at WOT most of the time.
Do not focus so much on WOT issues.
Focus on every day driving.

I am focusing on WOT now because I'm planning on going to a vintage drag racing day this weekend.