Pilot Jet question

[shagertymma]

Hi all, this isn't actually my first post but couldn't remember my login info!

Anyway, I have a 1979 GS425. I put POD filters on, and slip on mufflers.

I ordered a jet kit from signma6,,, with specs that I had drag pipes and POD filters.
The pilot jet with the kit is a size 30. anyway, it worked out quite well.. the bike idles nicely, I synced the carbs. It has plenty of power throughout; however, when I'd cruise and release the throttle the exhaust would pop. Not backfire but more popping. As I started riding more and taking the bike in traffic I noticed after sitting at lights it would kind of bog off the start and lately I've even stalled a couple times, Bike always started back up reasonably easy.

Last trip out I noticed fuel (maybe oil? or a mix?) coming from the hose that would connect to the airbox (since I have POD filters its no longer connected).

Also the plugs are fairly wet. The carbs are clean, new jets, I dont believe my bowls are stuck or anything, and both plugs were about the same looking.

I'm leaning towards the idle jet being too large since it was speced for drag pipes and I added slip on mufflers?
Troubleshooting: bike starts without the choke, and if i flip the choke on it runs like garbage.

just wanted opinions before I order new jets. and 1 size smaller or 2?


thanks in advance!

 

[shagertymma]

a picture of the bike

 

[Steve]

Regardless of your intake and exhaust modifications, you very seldom have to change the pilot jet, and if you do, it is usually just going up ONE size. I don't have specs for the 425, but the 400 had a 22.5 pilot jet. Assuming the 425 might have had a 25.0, you have gone up two sizes. And, ... that is assuming that Sigma6 uses the stock Mikuni jet sizing.

If your plugs are wet, you are either running VERY rich and not burning the fuel or you have them in backwards and are riding in the rain. Next time it starts to run a bit less than 'perfectly', pull the plugs to see what they look like. Check the color, especially. If they are really dark, you are running RICH.

There are also other things to consider besides jet size. What is your pilot screw setting? Normally, we would suggest starting at three turns out, then adjusting the screw IN for best idle and throttle response, but with your larger jets, you will likely need to go farther in than that.

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[shagertymma]

Thanks for the response Steve. I actually put the stock pilot jet back in, and it started up and idled nice and smooth. (adjusting the screw I've been starting all the way in and backing it out, I may use that 3 turn thing, so thanks for that idea as well).

I thought that since I leaned out the pilot, I should richen the needle and moved it down one. (its now at the richest spot). figuring they would balance out. It idled smooth and when I went for a ride it was AWFUL mid throttle.

I figured it was because of the pilot jet, but maybe it was actually the needle being too rich in the first place? and since I made it more rich thats why it was worse. tomorrow I will lean it out 2 and see if that helps.

Seem logical?
thanks again!

 

[Steve]

Rather than guessing, do some tests. They are called "plug chops".

Basically, you would start with some fresh spark plugs. Mark your throttle. I used some white adhesive tape. Place an index mark at idle and another one at FULL. Now mark 1/2, 1/4 and 1/8 by simply dividing the sections in half.

Idle around town for a few minutes with the throttle as steady as possible, but without turning the throttle over 1/8. When you find a safe place, pull the clutch lever, hit the KILL switch and coast to a safe place to stop and pull your spark plugs for inspection. Either take some notes or take pictures of the plugs. Now go find a hill. See if you can keep the bike at about 1/4 throttle for a minute or so. Higher gears and lower engine speeds will help, as it is throttle position, not engine speed that will determine which carb circuit is in use. Again, 'chop' the throttle, pull the clutch, hit the kill switch, coast to a safe place to check the plugs. Repeat for half throttle, but it won't take as long, maybe only 20-30 seconds. Finally, the fun one, a blast at full throttle. Probably only need second and/or third gear, as you will be going plenty fast at the top of third gear. One blast of acceleration should do, without shifing, if possible.

Reading the plugs at those various throttle positions will tell you which carb circuit needs the adjustment, but keep in mind that all of this assumes that your carbs are CLEAN and the fuel height is properly adjusted. Also, the valves need to be properly adjusted and the spark plugs need to be the correct heat range. ANYTHING that is non-standard can cover up a problem, so you will never really know.

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[Guest]

Pilot jet shouldn't effect things much at 1/2 throttle, probably up to 1/4 or so. Needle clip is the main thing working at 1/2 throttle. You will have to tune 3 circuits; pilot(jet size as well as air/fuel mixture screw), needle position, and main jet size. You will tune them separately and in turn. Check out this chart for an idea of which circuits kick in when. Look at slow jet, needle position and main jet.



Go here: http://members.dslextreme.com/users/bikecliff/images/highest_rpm_plug_chop.html --there is link for CV carbs on top.
Do plug chops at different throttle setting to SEE which circuits are rich or lean instead of guessing. It will probably save you a lot of work taking the carbs off and on. You will get it right sooner with less frustration. Take notes with all or your carb screw and needle settings for reference too.

 

[shagertymma]

Good points... I thought of plug chops but I guess its now a must. I like the idea of using tape to mark the positions... when i said 1/2 throttle earlier i could have been mistaken since it was a blind guess. Do I need to use new plugs each time or is that not necessary?

 

[Steve]

New plugs are only necessary for the first time, then continue to use them for the other tests.

That is just to make sure that they have at least half a chance of showing up clean, if the bike is actually running right. If they are really coated now, they might not get clean enough to show you what's really happening.

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[2BRacing]

New plugs are only necessary for the first time, then continue to use them for the other tests.

That is just to make sure that they have at least half a chance of showing up clean, if the bike is actually running right. If they are really coated now, they might not get clean enough to show you what's really happening.

.

Hi Steve

Please send me a PM - need to communicate with you. My e-mails do not seem to get through to you.

Thanks!

 

[shagertymma]

Took me forever to update, sorry... between working nonstop and rain days I didnt get a chance to work on the bike until today. I did plug checks and noticed one looked brand new... i had recently synced the carbs but must have messed them up somewhere along the line and one cylinder was closed. Bench synced them, leaned out the needle, kept the stock pilot jet... and the bike ran better than ever! Dont have time today to plug chop at all the throttle positions but i am pretty confident itll have good results. Just wanted to thank you Steve... without your help i would have ordered $20 pilot jets for no reason!