Need help with tuning please

Well guys this evening after work I grounded the frame and took her out no change, So I just sat down and took a moment to try to think just what I have done so far. I really didn't do much in the way of taking notes as I went, So I think I was kinda wasting my time. So that's what I'm doing now. I looked at the tuning notes online and tried to understand where it was in relation to throttle opening and what jets were at work when it was going flat. So what I did was put a strip of tape on the throttle housing and a small piece on the grip and marked the fully closed position and the fully open position then marked half and so on down to 1/8. As it looks on the graph it is the needle jet and the pilot jet with the needle itself that are at work when it happens. Then took her out and found it was about 1/4 when it was starting to crap out. But at one point I tried to just go wide open and see what would happen ( hadn't done this before as this is a rebuilt motor) holy smokes it really took off, So now I know it's a carb issue right? Went home and realized I was just going up and down with the jets and not tuning the jets to work together. I was making changes in an guessing kinda way. Anyway I am getting close now as with just a couple of changes to the needle and pilot and she now will go to over 3,200 and just has a small rough spot in her above that. What's weird is the 850 has a P-0 needle jet and the new carb has a P-8 needle jet in it. I may still need to buy a different jet. But I ran out of time as it was getting dark and didn't want the old lady next store to come over with her pitch fork looking for me. What I think it needs is to lean out the 1/4 to 3/4 range as this seems to be helping, I really don't know why I kept wanting to go richer with the main as it was a waste of time as it is not even at work at that time. When I ordered the carb I told them it was for a Combat. But the cam is a Mega cycle 560 NR and is in between a stock cam and the Combat cam. So I think she was too rich to begin with in that range, I really don't know much about carb tuning but I guess I am starting to learn now. Once again I must thank you guys for all the time you have spent helping me figure this out. I just hope I'm onto something. :idea:

159 p2,6dh3 needle,is close.

Hang on a minute, didn't you say you changed carbs over and no change :?: :?: ,do a search on here for basic jet settings

That's right it didn't, It still had the flat spot in the 1/4 range. I think my main problem is the needle jet in the new carb is a P-8. And I was thinking I needed to go richer, So I was using main jets thinking this was the problem. Understand I know very little about carb tuning. And yes I did search this site and others looking for info. But now I know that changing the main jet will not help the 1/4 throttle opening and the needle setting in conjunction with the needle jet will help, Also the pilot is still is in effect a little too and the slide cut away. Why the 850's carb also was flat I don't know maybe the free flowing exhaust compared to the bean can exhaust had something to do with it. But what is strange is the 850 has a 2.5 slide and a P-0 needle jet and the new carb has a 3.0 slide and P-8 needle jet. Like I said I know very little about carbs, That's for sure.

Good going, one step at a time and it'll be right as rain.

Maybe you should think about taking off the Bub and running another set if you have one while you're sorting this out. It'll be easier on you and your neighbors; and the Bubs could have something to do with the problem you're having. BTW, I wouldn't use the Beancans. You can almost bet whatever carb setup that works on one won't work on the other. In my experience Beancans really have to have balanced pipes to function at all. Maybe you can borrow a set of peashooters?

Thanks Paul and Bob, The bean cans work great on the 75 and I am sure you are right about one not working well from one to the other. If in a few more tries it's not right I may just buy a set of peashooters and use those. I plan on looking into a P-4 needle jet for it as I am almost sure that's what she's asking for. Thanks again, Chuck.

If you are bothering to tune you need to do it with the exhaust you will use.

What needle are you using,mikuni needle jet sizes go up in very small steps compared to anals,2.5-3 slide either one should be close enough to run,a p8 needle jet is rich, and perhaps the carb was supplied jetted for something else,while stationary is there any black smoke at the rough patch.

I do understand I need to tune the carb to the exhaust, When I was talking about a different one I knew I could not just change back and all would be fine. The needle is a 6DH3 and the needle jet is P-8 which is 8 steps above the carb on the 850. When I ordered the carb I told them it was for a Combat, This was my first mistake. The cam is not the same as a stock Combat cam, And the exhaust is not the same and that makes this Mistake #2. Then assuming to know something about jetting and thinking it should need more fuel was mistake #3. I had it in my mind that changing the main should help (WRONG) #4 Should I go on? It has been a learning thing I guess and If I have confused some of you I am sorry but think how I felt knowing next to nothing. But at this point I feel much better as I am starting to feel like I am slowly getting it. It seems close now as the bike will rev higher and it does not just start to die, I just don't know if lowering the needle all the way will be enough with the P-8. Thanks again.

hi chuck, i agree with cookie regarding tuning to the exaust you,re using but as you changed everything else i cant see it doing any harm to change the exaust and see what happens,

It's quite frustrating trying to get it right. I'm in the middle of a similar task without much time to complete it. Luckily I have a few runners to ride while I play with this one.
Stuff I have leared painully would include.....
Too much cam, the hottest is not always the best.
Too large carbs, I'm not really on a race track, sometimes a good mid range is best.
The loudest muffler may not be best.
Make jet changes very carefully, always after adjusting float level, and small increments.
Never make more than one change if possible and keep good records.
Start with a working setup somebody else you trust has worked out if possible. The best Chevy engine I ever built was from a recipe from Smokey Yunik.
Make sure you have enough breather for the air you are going to move internally.
Make sure you have enough oil and keep it cool.
If you double the engine hp something else will break.
These are only my personal mistakes.

chris plant said:

hi chuck, i agree with cookie regarding tuning to the exaust you,re using but as you changed everything else i cant see it doing any harm to change the exaust and see what happens,

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Two into one headers are very tricky business, tuning wise. For any given length you are going to get benefits at one rpm range and penalties in another. I looks like Chuck has finally reved through to its sweet spot. There's no way to simply jet for it. It's a personal question if it's worth the hassle. I've read that a muffler tempers the up and downside a lot. Besides though, Peashooters really do sound so sweet. This is all based on the assumption that this is what's going on, which could be wrong, BTW.

Hi, first time on the forum. I expect your Boyer ignition is mismatched to the coils you are using. I have tried the Dyna (the green one) coil with the Boyer and the bike will run to only 4000. The green coil is three ohm try a five ohm coil and see if that makes a difference. Also using that NR 560 cam make sure there is 13 thou clearance. Let us know how you make out.

norsa1 said:

Hi, first time on the forum. I expect your Boyer ignition is mismatched to the coils you are using. I have tried the Dyna (the green one) coil with the Boyer and the bike will run to only 4000. The green coil is three ohm try a five ohm coil and see if that makes a difference. Also using that NR 560 cam make sure there is 13 thou clearance. Let us know how you make out.

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I found the Boyer system gives a nice fat spark with a pair of 6V PVL coils - they're cheap, they work and they fit the original brackets.
I don't want to say anything bad about Dyna green cois as I have a pair for sale...only used once. Any takers :wink:

According to Fred Eaton's site (Old Britts) the 3 ohm Dyna coil is fine with points, whereas the 5 ohm coil should be used with the Boyer. I ordered a Dyna coil from him years ago specifying a Boyer ignition and it's worked fine.

Thanks for all the help guys, Bike is running much better now and after lowering the needle all the way made a big difference. It is still a bit rich so I have ordered a smaller needle jet and will continue when that arrives. I really need to have it right and also have some leeway so when I head to Colorado I can lean it out a bit more for the high elevation. With a few more miles on it I am now able to open it up a bit more and it really has some great pull when it gets on the main jet, Just need to sort the small rich mid range. Have a great day, Chuck.

Forgive a newbie for chipping in :roll: .Dont overlook the obvious.have you simply run the fuel taps disconnected into a container to check flow.things like this get fixed by elimination,it can only only really be sparks or fuel somehow. plugs running a nice colour?

Yea that was checked too, I now have a p-5 needle jet in it and the plug looks good and the needle is in the center groove. I hope I can get away with just lowering the needle if I have to, Put about a hundred miles on it this weekend and all seems good. It was just hard to understand how it needed to go so much leaner, My mistake was thinking otherwise. Thanks guys for putting up with all the questions and complaints, But she sure does run good now. Ride safe, Chuck.