Topic: How can 750 roundtops run this rich..? ?

[754]

I am stumped on this one.. Had lots of 750,s..never had this.
Roundtops..been off and cleaned. A few times..

 Needle clip moved 2 notches leaner..  First or 2nd notch from fat end.

 Stock 105 jets not been able confirm, but a. 0463 drill bit will not enter..(approx 1.2 mm).

 Stock numbered pilot,#42 or #40.. The size listed in manuals is what it is.

 Idle screws 7/8 turn out.

 Finally today I pulled the breadbox filter off,  ran slightly better and cleaner.
 But...
 It still has visible black smoke coming out of the 4-2 pipes.
 If someone told me you could make one run that rich..with those parts, a month ago,  I would not have believed them.
 How is this even possible???. And what is left to do to make it leaner... I hit the wall here.. Can't think of anything else.
 Any ideas..?

[goldarrow]

Sometimes the carbs are cursed...try different sets of carbs?

[BPellerine]

they begin with the letter w,haha bill

[DH]

. My k5 did that. New needles and needle jets cured it. IIRC, Honda sold them in sets. They wore thinner.
Needle is constantly moving thru jet with each movement of throttle. Sometimes jet wears out.
Only way I found out was to try them. Just my experience...


Gotta add that they were replaced as last resort, everything else was vverified correct.

[Don R]

 Float levels? My F1 had it all, drilled jets and screwey float valves bent up tabs etc. I was shocked how much high float levels richened the fuel mix.

[seanbarney41]

. My k5 did that. New needles and needle jets cured it. IIRC, Honda sold them in sets. They wore thinner.
Needle is constantly moving thru jet with each movement of throttle. Sometimes jet wears out.
Only way I found out was to try them. Just my experience...


Gotta add that they were replaced as last resort, everything else was vverified correct.

+1...high mileage carbs need new needles and needle jets...might be easier to just find low mileage carbs



in my experience, it only really affects the transition from idle to part throttle

[Henning]

The idle screw - makes a huge difference to the mixture, in my experience. Ignore the manual and turn it out to say 1 1/4 or more and see if it helps.

On mine ('71) the manual says 1 turn out and I'm at 1 3/8 and need to go further out. Otherwise stock exhaust, intake etc., needle middle position, new Keihin 115 mains and 40 idle jets.

[754]

Okay reading hondamans book, he says turning in makes leaner..
 If this. Don't have this part correct, then I would be going opposite.

[BPellerine]

needles and jets def worth a try.bill

[754]

Made some progress,  pulled the breadbox filter, it helped.
Changed the car fuel filter this morning, it appears to be letting fines thru.
I have been thinking that now my tank is mostly clean, no new debris getting to filter. But filter was still letting thru some powdery grit that could still be causing float jets to leak.
 Will know in a few miles.
 I have a rough set of cards,  if I can find them,  I swap the brass or at least float needles, or I have at least one set of 72 carbs that were stored clean..

[BPellerine]

was it still leaking fuel,or just running rich?I hated my pd carbs spent lots on them and still were not right,they were very dirty when I got them.bill

[754]

Hard to see but puffing black smoke at idle..

[Spanner 1]

+1 on float levels wrong ( fuel level too high ). BTW, on screw top carbs, the air screw is 'in' for richer idle air mix., the opposite of what you posted above 

[scottly]

Slides installed backwards? (Never done it myself, but I know it has been done; just don't remember the result.)

[evanphi]

Roundtop carbs, idle screw in for more rich, out for lean.

[ofreen]

Frank, are those needle jet sets stock or are they aftermarket (Keysters). I tried Keysters years ago and they were hopelessly rich no matter where the clip was set.

[754]

Not sure now, I think original. Bike has 33K miles on it. Looking for my other cars just going to swap them.. Are you back now Greg?

[754]

Just put a set of 657A on the bike.. It had a 657B rack on it.
 4 is touring gas thru the float, took it off cleaned it up, forgot to check height.. Still leaked. So off again, will measure float. Checked, it has a Keihin 2.0 seat.

[rotortiller]

A buddy of mine had a similar issue and not saying this is your issue. Turned out someone drilled the emulsion holes in the jet holders.

[ofreen]

Not sure now, I think original. Bike has 33K miles on it. Looking for my other cars just going to swap them.. Are you back now Greg?

I got back last night.  I looked at my records and see I replaced the first needle jet sets at 41,000 miles.  I remember fuel mileage started noticeably dropping at around 35,000 miles. By the time 41K rolled around plugs were fouling and mileage was at about 30 MPG.  The choke wasn't needed no matter how cold the ambient temperature was and the exhaust was eye watering.  Then I got hip to what was going on and replaced the needle jet sets and everything went back to normal.  So if yours are original I'd expect to see some wear at 33K but not enough to cause what you are seeing.

[754]

Greg will send you a pm..
 I put a new rack on,bit of floods ng, and float massaging, whole new bike now.
Ran over some rough road testing it, started runners ng rough in the midrange quite bad but cleared up mostly.. Will dropped mains and holder a clean up
 Got about 40 mpg. Some open road but not hiway speeds.
 Double the mileage it was getting and pretty nice idle..
 Whoo hook..

[BPellerine]

California dreamin!!hahabill