Topic: 750: Shaving carb slides to richen pilot to needle transition (found on cycle X)

[DaveBarbier]

...if I get stuck riding in slow suburban traffic barely hitting 30mph it would seem quite ridiculous to maintain the revs buzzing at 6000 - 7500...

Different scenario. There is a difference between cruising at 2500rpms and getting stuck in slow suburban traffic.

[Garage_guy_chris]

I started the thread because i have never heard of this before and was curious what some of you real guru's had to say about the whole thing. Wasn't really meant to be a diagnosis and repair thread for my particular bike.

....couple things here...Garage Guy, I think your pinging at low throttle/high load is caused by an incorrect timing advance curve, not anything in the carbs.  Perhaps you have worn out advance springs or maybe try less initial advance or limit total advance to less than 30 degrees.  You need to fix it though.  Audible pinging means bad!

In my experience, a lean condition will cause the bike to spit, cough, sneeze (whatever you want to call it) through the intake side of the carbs.

Your symptom is pinging? Have you looked into the ignition system? Are you sure there are no vacuum leaks and cabs are synched?

Regarding the spark advance I was having idle problems due to premature advance so i removed the springs and shortened them by 0.050" and now the advance is starting around 1400 - 1500 rpm the base timing is set to the timing marks with a strobe timing light.

I do get the ocasional pop back through a random carb that with the pinging and white plugs is whats telling me its a lean condition

The carbs have been sync'd and checked for sync 4 times now and everything is fine there

If were considering the issue with my bike then I have to admit i have omitted alot of important information regarding my bike in particular.

Mods
-Crusin Image 65mm / 836cc
-Lightly ported head with stainless valves and F2 cam
-Stock opened up "K" airbox (openings cut an inch higher) with K&N drop in filter
-064A 1975 K5/F0 emissions carbs

Jetting:
Pilot:       40
Main:      130
Needle:   27201 / D17 in 4th from top clip pos
Airscrew: 3/4 to 1 turn out

The carbs may also have something to do with it as they are 064A casting 1975 K5/F0 emissions carbs. There is not alot of info about these but from what i can find there are some differences in carb circuits and crossover points for the jetting. Worst comes to worst i will rebuild a spare set of 657B casting carbs and start over with the jetting. 

I think I have the pilot and main jets sorted but I still have some screwing around to do in regards to the mid range jetting, In order of trials heres the plan of attack:

1. Try needles in the 5th clip pos
2. Try #42 pilots
3. Shave and try a second set of slides
4. Find alternate needles or modify existing ones?!?!
5. Rebuild a spare set of 657B carbs and abandon these 064A emissions carbs

[Don R]

Sounds like a plan.
  My K4 motor is similar, except mine's in a K0 with 4 cable carbs.  I liked the F2 cam, although I swapped it for a Kenny Harmon f grind road race cam I think it had better bottom end grunt.

[Steve_K]

I have heard of machining carb slides before,  but it was machining the cutaway angle on the slide.  On further thought,  maybe shortening the slide would change the mixture, too. (less cutaway)  Would be easier to machine that way too. 
Steve

[seanbarney41]

Garage Guy, I have not found 064A's to behave much different than any of the other roundtops...just tuned a couple different racks of 'em this summer, and they actually seemed to get into shape easier than the worn out 7a's I have.  Have you clear tubed your fuel levels?

[Garage_guy_chris]

Garage Guy, I have not found 064A's to behave much different than any of the other roundtops...just tuned a couple different racks of 'em this summer, and they actually seemed to get into shape easier than the worn out 7a's I have.  Have you clear tubed your fuel levels?

That is good to know It inspires some confidence in working with them rather than abandoning them. I didn't think to clear tube them. Doing so is probably a good idea as the needles and seats are aftermarket and could be causing a problem

New plan of attack:

1. Check fuel level with clear tube method
2. Try needles in the 5th clip pos
3. Try #42 pilots
4. Shave and try a second set of slides
5. Find alternate needles or modify existing ones?!?!
6. Rebuild a spare set of 657B carbs and abandon these 064A emissions carbs

[lucky]

Before you pay to machine slides everything else better be spot on or you will never detect ANY difference!

Get everything else right first.

Also .007 will make a difference But make sure your slides are sitting all the way down in the closed position with the throttle off.
Then make sure that when you start to turn the throttle they all start to move at the same time.
Put your fingers in the opening of the carb on the slide and feel when the slide starts to move. Check that they all start to move together.

[754]

Just use a rod to check that,make sure no burrs are on it..
 I found rather than pulling the rod in and out a bit, a twirling motion back and forth seems to be more accurate.

[PeWe]

I never had that kind of hesitation with my std 750 K6-76 carbs with pods. 836, ported head, moderate cam. No problems as many say about pods.
Pilot std (40)
needles 1 step up (4-:th notch from top)
mains 147.5 !!

I had hesitation as described here with my latest carbs, Mikuni TMR32. Too small pilot jets. I tested to retard ignition that solved  low throttle pinging but the power disappeared too.
Jetting for dyno was OK, low throttle and smooth street driving something else.
Carb boots are not leaking?

Ignition advance OK at idle as well as full advance kicking in not before 2500 rpms? Too early advance means sloppy adv springs. Tighten them a little will solve that. Some threads about it. I have one of them.

[ADW]

I don't know about the availability of those carb slides, but I hate the idea of irreversible changes to anything on these old bikes. There's only so many parts left in the world for them...