harder to start....

[t20sl]

I have been reading this topic with interest.  Now I am ready to chime in.  Let's try the basics.  Why did you replace your original Mikuni that ran good?  Was it the same size as the new one?  If so what was the old jetting?  Have you put the old one back on and tried it?  
ted

[skiracer]

Tom, I had an older 34mm and thought I would but a new one; tighter clearances, etc.  Yes it was the same size.  The only change was a new top end.  I had originally installed the same jetting new carb.  It wasn't running right, so I started changing the jetting to get the bike running better.  Okay new needle jet is ordered.  Question:  what is the advantage of a leaner needle jet and a larger pilot jet as opposed to a richer needle jet and a leaner pilot jet??

And yes, I understand that the clip needs to go back to the middle.

quote:Originally posted by t20sl

I have been reading this topic with interest.  Now I am ready to chime in.  Let's try the basics.  Why did you replace your original Mikuni that ran good?  Was it the same size as the new one?  If so what was the old jetting?  Have you put the old one back on and tried it?  
ted

1976 MC 5 Original Owner
1982 Suzuki PE 175
1976 Penton 175 XC
1985 20' Hi Point trailer

[t20sl]

What size was the pilot in the old carb?  Does the bike start with the old carb fine?
Ted

[brian kirby]

quote:Originally posted by skiracer

Tom, I had an older 34mm and thought I would but a new one; tighter clearances, etc.  Yes it was the same size.  The only change was a new top end.  I had originally installed the same jetting new carb.  It wasn't running right, so I started changing the jetting to get the bike running better.  Okay new needle jet is ordered.  Question:  what is the advantage of a leaner needle jet and a larger pilot jet as opposed to a richer needle jet and a leaner pilot jet??

And yes, I understand that the clip needs to go back to the middle.

It is not really an advantage, its understanding what part of the carburetor controls fuel in what range of throttle opening, which I did not understand it for a VERY long time and made a lot of mistakes jetting because I didnt understand and correctly identify which piece of brass I needed to change and changed the wrong one. What you are doing by changing the needle jet instead of the pilot is changing the part that actually controls the rich point you are having. Many times main jets or pilot jets are changed when the problem is the needle jet, I did it myself for many years as I said. Going leaner on the pilot is used to cure what is really a rich needle jet, but on the Mikuni the pilot jet ONLY controls fuel when the slide is completely closed, the instant the slide cracks open even the tiniest amount the needle jet and slide cutaway are what is controlling the fuel mixture. The pilot will continue to flow fuel after the throttle is open so it does effect jetting after the slide opens but the effect is extremely tiny and you cant really effectively use it to tune above the closed slide position. The only functional purpose for a pilot jet in a Mikuni is starting (closed throttle) and idling. You are having a rich condition right after you open the slide, but you are also having hard starting, which tells me the pilot is too lean trying to compensate for a rich off idle/mid range which is the needle jet.

Also, and some of this might be obvious to the more experienced folks but it was not obvious to me for a LONG time, the tuning parts of a carburetor function purely on the slide position, IE 1/4 or 3/4 throttle open, the carb does not know nor does it matter what RPM the engine is running, the carb only sees the vacuum signal coming from the engine. Many times in the past I would get slide opening and "engine low end or top end" mixed up. When you are test riding, ignore (or mostly ignore) the engine RPM and focus your attention on what the jetting feels and sounds like relative to the amount of throttle you have applied.

Brian

[skiracer]

The pilot was a 22.5.  Yes it does start, however, as the reasoning for all the changes is that it has become harder to start since I re-did my top end............

quote:Originally posted by t20sl

What size was the pilot in the old carb?  Does the bike start with the old carb fine?
Ted

1976 MC 5 Original Owner
1982 Suzuki PE 175
1976 Penton 175 XC
1985 20' Hi Point trailer

[skiracer]

Well said Brian!  I learned more today than I have in a long time...  Thank you!  So, I went down two sizes on the needle jet.  I am thinking that I will start with a 30 pilot....  Until Tuesday......

quote:Originally posted by brian kirby

quote:Originally posted by skiracer

Tom, I had an older 34mm and thought I would but a new one; tighter clearances, etc.  Yes it was the same size.  The only change was a new top end.  I had originally installed the same jetting new carb.  It wasn't running right, so I started changing the jetting to get the bike running better.  Okay new needle jet is ordered.  Question:  what is the advantage of a leaner needle jet and a larger pilot jet as opposed to a richer needle jet and a leaner pilot jet??

And yes, I understand that the clip needs to go back to the middle.

It is not really an advantage, its understanding what part of the carburetor controls fuel in what range of throttle opening, which I did not understand it for a VERY long time and made a lot of mistakes jetting because I didnt understand and correctly identify which piece of brass I needed to change and changed the wrong one. What you are doing by changing the needle jet instead of the pilot is changing the part that actually controls the rich point you are having. Many times main jets or pilot jets are changed when the problem is the needle jet, I did it myself for many years as I said. Going leaner on the pilot is used to cure what is really a rich needle jet, but on the Mikuni the pilot jet ONLY controls fuel when the slide is completely closed, the instant the slide cracks open even the tiniest amount the needle jet and slide cutaway are what is controlling the fuel mixture. The pilot will continue to flow fuel after the throttle is open so it does effect jetting after the slide opens but the effect is extremely tiny and you cant really effectively use it to tune above the closed slide position. The only functional purpose for a pilot jet in a Mikuni is starting (closed throttle) and idling. You are having a rich condition right after you open the slide, but you are also having hard starting, which tells me the pilot is too lean trying to compensate for a rich off idle/mid range which is the needle jet.

Also, and some of this might be obvious to the more experienced folks but it was not obvious to me for a LONG time, the tuning parts of a carburetor function purely on the slide position, IE 1/4 or 3/4 throttle open, the carb does not know nor does it matter what RPM the engine is running, the carb only sees the vacuum signal coming from the engine. Many times in the past I would get slide opening and "engine low end or top end" mixed up. When you are test riding, ignore (or mostly ignore) the engine RPM and focus your attention on what the jetting feels and sounds like relative to the amount of throttle you have applied.

Brian

1976 MC 5 Original Owner
1982 Suzuki PE 175
1976 Penton 175 XC
1985 20' Hi Point trailer

[brian kirby]

No problem, I am always happy to help especially since carburetor jetting can be counter intuitive and confusing. I didnt figure this stuff out by myself, everything I know about motorcycles and engines, or virtually everything, I learned from people much wiser than me. I listen to guys like Speedy, Ron Carbaugh, Gary Ellis, JP Morgen, Peter Villacaro, Bob Bean, Larry Perkins, and too many others to name. I try to remember what I learn from them and pass it on.

Brian

[t20sl]

Same question.  If you put the old carb back on does it help the starting issue?
Ted

[Larry Perkins]

What Ted is asking could solve a big piece of the puzzle.  Probably worth the effort if doable.

Larry P

[brian kirby]

I agree, changing back to the old carb will eliminate some thing, for example if the old carb exhibits the same symptoms is something else causing the hard starting, like an air leak.

Brian

[skiracer]

No, putting the old carb back on does not change the starting issue.  I received a new needle jet over the weekend and will test it this evening..... stayed tuned!

1976 MC 5 Original Owner
1982 Suzuki PE 175
1976 Penton 175 XC
1985 20' Hi Point trailer

[Larry Perkins]

If it started fine with the old carb before and does not now it is not a jetting problem in my opinion.

Larry P

[brian kirby]

I agree. Sounds like an air leak to me.

Brian

[skiracer]

Where?

quote:Originally posted by brian kirby

I agree. Sounds like an air leak to me.

Brian

1976 MC 5 Original Owner
1982 Suzuki PE 175
1976 Penton 175 XC
1985 20' Hi Point trailer

[skiracer]

Here is the update:
Went from a Q-0 to a P-6 needle jet
Changed the pilot jet from 20 to 32.5 Needle clip is in the middle.
Bike ran rich on the bottom; zero to 1/4 throttle, very rich.
Went to a 27.5 pilot running much better.
Moved the clip to the top, bike is running very well.  

Mention of an air leak....  where? Would that not make the bike run lean, and idle very high?

1976 MC 5 Original Owner
1982 Suzuki PE 175
1976 Penton 175 XC
1985 20' Hi Point trailer