dynojet question, still running lean

[jordraatt]

i have never oiled my K&N's...most people oil them to much.
i don't live near a beach or down a gravel lane so i have never had any engine problems from the K&N's not being oiled.
the information i offer is from 20+ years of working on these GS turds.
do with it as you wish.

i'm not an expert, so i'm thankful for all tips

[jordraatt]

What about pilot jet?

i've never heard anything of changing the pilot jets, only the main jets..

[EvilEnfield]

I'm no expert, but when I did mine I think I went up 3 or 4 sizes on the main jet, 1 on the pilot and 1 or 2 on the air and moved the needle down a notch. I think the pilot is part of the low to mid range system, there is a very helpful link on BassCliffs site.

***EDIT*** Lots of helpful links on BassCliffs site.

[MisterCinders]

I'm no expert, but when I did mine I think I went up 3 or 4 sizes on the main jet, 1 on the pilot and 1 or 2 on the air and moved the needle down a notch. I think the pilot is part of the low to mid range system, there is a very helpful link on BassCliffs site.

***EDIT*** Lots of helpful links on BassCliffs site.

The jet circuits affect different throttle positions. That's why you see everyone ask about throttle instead of RPMs for carb issues. RPMs mean nothing.

The jets and throttle relationships are:

Pilot = idle to 1/4 throttle
Needle = 1/4 to 3/4 throttle
Main = 3/4 to WoT

If you change to pods and pipe, you have more air at all circuits than originally contemplated by the designer. So you need to adjust the jetting to accommodate the extra air at every stage. The air increase has a bigger impact in the needle and main circuits, but the pilot circuit needs some love too.

The pilot circuit has the most margin, since they can be adjusted without new parts. Most people can tune the pilot for pods/pipe with changes to the screw setting or settings. If you are still lean/rich at the edge of the screw settings (e.g., idle screws set at the rich end, but plugs show lean), then you can change the pilot jet for a larger size.

[jordraatt]

The jet circuits affect different throttle positions. That's why you see everyone ask about throttle instead of RPMs for carb issues. RPMs mean nothing.

The jets and throttle relationships are:

Pilot = idle to 1/4 throttle
Needle = 1/4 to 3/4 throttle
Main = 3/4 to WoT

If you change to pods and pipe, you have more air at all circuits than originally contemplated by the designer. So you need to adjust the jetting to accommodate the extra air at every stage. The air increase has a bigger impact in the needle and main circuits, but the pilot circuit needs some love too.

The pilot circuit has the most margin, since they can be adjusted without new parts. Most people can tune the pilot for pods/pipe with changes to the screw setting or settings. If you are still lean/rich at the edge of the screw settings (e.g., idle screws set at the rich end, but plugs show lean), then you can change the pilot jet for a larger size.

okey, now i see.. but there's one thing. the dynajet stage 3 kit does NOT include any pilot jet, and in the instructions it says turn the mixture screw 2,5 turns out.. i guess this is the screw i'm supposed to experiment with??

[MisterCinders]

Yes, IIRC your bike has CV carbs which use a single screw to adjust the pilot mixture. Dyno is telling you to set that screw at 2.5 turns out, which I suspect is a bit richer than the stock setting.

If you end up too many turns out and are still lean, then you may have to increase the size of the pilot jet itself.

[NeyborDayfid]

On the VM carbs, IIRC, you have 2 mixture screws.

On the CV carbs, IIRC, you only have the one mixture screw right at
the front on top of the carb right before the intake boot. (to the engine)
Doesn't taking this out make it leaner, while turning it in makes it richer?
I thought opening this up allowed more air. I could surely be wrong though.

[jordraatt]

Yes, IIRC your bike has CV carbs which use a single screw to adjust the pilot mixture. Dyno is telling you to set that screw at 2.5 turns out, which I suspect is a bit richer than the stock setting.

If you end up too many turns out and are still lean, then you may have to increase the size of the pilot jet itself.

allright, thank you very much.. i'm going to experiment with this screw.. but what if i have to go up in jet pilot size?? where do i order??

[BigD_83]

The stock setting on the air/fuel mixture screw was 1.5 for the '83 650. That info is somewhere in a chart here. There is no listing for an '81

\\http://members.dslextreme.com/users/...at_height.html

[EvilEnfield]

allright, thank you very much.. i'm going to experiment with this screw.. but what if i have to go up in jet pilot size?? where do i order??

Drill it out yourself with a pin vice and micro bit, just twist it gently and it will nibble away at the soft brass.

[BigD_83]

allright, thank you very much.. i'm going to experiment with this screw.. but what if i have to go up in jet pilot size?? where do i order??

You can order pilots from Jets R Us if you need to:

http://jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_BS_series.htm

EDIT_ I'm pretty sure Z1 could help you out as well, even if they do not have the jets listed for your specific model

[Steve]

... yes the carbs were fully stripped and cleaned, many times. ...

That tells me that they still might not be clean. Done properly, it only takes one time.

... the valves were not adjusted, as it ran just perfectly before installing the pods. ...

All the more reason to suspect the valves. They can run "perfectly", right up until the point where the valves get burnt.

..., if that doesn't help then get yourself some of these;

http://www.ebay.com/itm/20-PC-MICRO-METRIC-3-1-6-mm-SIZE-DRILLS-PRECISION-DRILLING-JEWELRY-MODELS-TOOLS-/180893263963?pt=LH_DefaultDomain_0&hash=item2a1e14285b#ht_500wt_1180

http://www.ebay.com/itm/4PC-Hollow-Wire-Pin-Drill-Vice-Airfix-Hornby-Modeling-Craft-/400299391842?pt=UK_ToysGames_ModelKits_ModelKits_JN&hash=item5d33b3fb62#ht_5191wt_1163

Then you can drill out the jets which you know are too small to see if it helps.

Please don't drill out any jets. Starting with the fact that you have NO idea what the starting diameter of the jets is now (you have a number, but not a size), you also don't have any idea how much bigger to make them. Then, if you do ever get the bike running and sell it, the new owner is going to say, "wow, how did he ever get it to run with these little (numbered) jets?" New jets are only a couple bucks a piece.

to drill the holes bigger or change the main jets will probably work if the problem is in the upper rpm area.. but both of us are struggling with lower/mid rpm area.. that means changing needle position and/or adjusting float height

The float height should be set to the STOCK height, as it affects ALL circuits in the carbs. Give them all a proper baseline from which to work.

Believe it or not, the main jets can be a problem at 2000 rpm.
Yep. The circuit that is being used is more dependant on throttle position than engine speed. Open the throttle at 2000, the mains are called to duty.

The jet circuits affect different throttle positions. That's why you see everyone ask about throttle instead of RPMs for carb issues. RPMs mean nothing.

The jets and throttle relationships are:

Pilot = idle to 1/4 throttle
Needle = 1/4 to 3/4 throttle
Main = 3/4 to WoT

Also note that there is some overlap on these settings. For example, the pilot will still be adding a bit of fuel through most of the range, bit it is such a small amount compared to what the main will be adding when at full throttle. The main is actually providing the fuel that is regulated by the needle circuit, differences in its metering ability only show up when the needle is fully open.

On the VM carbs, IIRC, you have 2 mixture screws.

On the CV carbs, IIRC, you only have the one mixture screw right at
the front on top of the carb right before the intake boot. (to the engine)
Doesn't taking this out make it leaner, while turning it in makes it richer?
I thought opening this up allowed more air. I could surely be wrong though.

Yep, it's another one of those times.

On the CV-type carbs (the Mikuni BS-series carbs in our GS bikes), that screw is the "idle mixture adjustment screw". The mixture is pre-set by the pilot air jet and the pilot fuel jet, then run through some passages inside the carb to the top of the throat. There are three small holes, one is always open, right at the throttle plate, a second that is opened just as soon as the throttle plate moves, and a third one that is a bit downstream. The one that is always open can not supply as much mixture as the engine needs at idle. The two at the throttle plate can not supply what the engine needs at low throttle openings. That is why the third hole is adjustable with the "mixture screw". Turning the screw OUT will add mixture, richening up what gets to the cylinder. A good starting point to start your tuning is with the screws three turns out, then slowly tweak them from there, listening for highest engine idle speed (best running).

.

[BassCliff]

Hi,

On the CV-type carbs (the Mikuni BS-series carbs in our GS bikes), that screw is the "idle mixture adjustment screw". The mixture is pre-set by the pilot air jet and the pilot fuel jet, then run through some passages inside the carb to the top of the throat. There are three small holes, one is always open, right at the throttle plate, a second that is opened just as soon as the throttle plate moves, and a third one that is a bit downstream. The one that is always open can not supply as much mixture as the engine needs at idle. The two at the throttle plate can not supply what the engine needs at low throttle openings. That is why the third hole is adjustable with the "mixture screw". Turning the screw OUT will add mixture, richening up what gets to the cylinder. A good starting point to start your tuning is with the screws three turns out, then slowly tweak them from there, listening for highest engine idle speed (best running).

Mr. Steve, I've seen you explain this hundreds of times. It gets better and more eloquent with each telling. I just wanted to say "thanks".

All the best to you and yours,

Cliff

[NeyborDayfid]

Thanks Mr. Steve.
I now see how that works. An eloquent telling indeed.
Regards.

[BigD_83]

That tells me that they still might not be clean. Done properly, it only takes one time.



All the more reason to suspect the valves. They can run "perfectly", right up until the point where the valves get burnt.




Please don't drill out any jets. Starting with the fact that you have NO idea what the starting diameter of the jets is now (you have a number, but not a size), you also don't have any idea how much bigger to make them. Then, if you do ever get the bike running and sell it, the new owner is going to say, "wow, how did he ever get it to run with these little (numbered) jets?" New jets are only a couple bucks a piece.




The float height should be set to the STOCK height, as it affects ALL circuits in the carbs. Give them all a proper baseline from which to work.

Believe it or not, the main jets can be a problem at 2000 rpm.
Yep. The circuit that is being used is more dependant on throttle position than engine speed. Open the throttle at 2000, the mains are called to duty.



Also note that there is some overlap on these settings. For example, the pilot will still be adding a bit of fuel through most of the range, bit it is such a small amount compared to what the main will be adding when at full throttle. The main is actually providing the fuel that is regulated by the needle circuit, differences in its metering ability only show up when the needle is fully open.




Yep, it's another one of those times.

On the CV-type carbs (the Mikuni BS-series carbs in our GS bikes), that screw is the "idle mixture adjustment screw". The mixture is pre-set by the pilot air jet and the pilot fuel jet, then run through some passages inside the carb to the top of the throat. There are three small holes, one is always open, right at the throttle plate, a second that is opened just as soon as the throttle plate moves, and a third one that is a bit downstream. The one that is always open can not supply as much mixture as the engine needs at idle. The two at the throttle plate can not supply what the engine needs at low throttle openings. That is why the third hole is adjustable with the "mixture screw". Turning the screw OUT will add mixture, richening up what gets to the cylinder. A good starting point to start your tuning is with the screws three turns out, then slowly tweak them from there, listening for highest engine idle speed (best running).

.

Steve,

Do I understand your explanation correctly:

The mixture screw isn't changing the RATIO of air/fuel going into the cylinder, but just adding more air/fuel of the same ratio to the carb throat so there is a greater mass of gasoline in the cylinder? i.e. more gasoline mass in the same volume in the cylinder under compression = richer?

I will delete this comment if it unnecessarily confuses the issue.