Topic: Vacuum leak on carb body? '78 550K/PD carbs

[TwoTired]

How does it respond to this test:
Idle RPM, top gear engaged/rolling, snap the throttle to one half of total travel.  (mark your twist grip.)

Does it accelerate smoothly if slowly, or does it just wheeze and slow down with no power while holding at 1/2 throttle?

[Scott S]

 Let me make sure I'm following you right....

 Is this more than one test? Or are you saying have it rolling in 5th gear but only at idle speed and then snap the throttle to 1/2 travel?

 And how exactly should I get there? Accelerate to 5th gear, coast down in gear until RPM's drop to idle/near idle, then crack it to half throttle quickly?

[TwoTired]

Let me make sure I'm following you right....

 Is this more than one test?

No

Or are you saying have it rolling in 5th gear but only at idle speed and then snap the throttle to 1/2 travel?

Yup

And how exactly should I get there? Accelerate to 5th gear, coast down in gear until RPM's drop to idle/near idle, then crack it to half throttle quickly?

If I were watching in person I'd say you should push and run next to it and hop on.  But, that would just be to amuse me.    And a video might be fun!

But really, it doesn't matter how you get to the rolling-in-5th-at-idle-with-the-throttle-in-a-steady-state-to-maintain-that-speed for a few seconds.  It's what the engine does to the sudden throttle twist (only 1/2 of available travel) where the behavior needs to be monitored/reported.

[Scott S]

 OK, marked the throttle with tape and a pin. Close and WOT. I'm assuming that 1/2 throttle is....half way.





 I really think that my tach isn't correct, because it sounds best at 1800 rpm at idle. I have another CB750 tach in my stash somewhere. I'll dig it out and see if it's better/different.

 Anyway, the idle is better...not hanging...but there's still some sneezing and snapping just off idle and up to 1/4 throttle.
  It pulls OK to there, but is popping a little.

 Around 1/4 throttle, it starts running bad. From there all the way to WOT, there's a surging, the engine sounds like it's not running on all four, the smoothness is gone, etc., etc.

 I took it up to 5th gear and let it coast back down to just under 2K (remember, I think my tach is screwy). Snapped the throttle to 1/2 way and it WILL accelerate, but slowly, sluggishly. There was a pop like a back fire the first time I did it. Second time....accelerates sluggishly, sounds like crap, surges and jerks, etc.

 This bike ran great after I first built it, so I know what the exhaust CAN sound like. Anything over 1/4 throttle and it just doesn't sound or feel right. I'd swear I'm dropping a cylinder, but the head pipes are all getting hot.

[TwoTired]

OK, the idle mix is likely rich enough.  Maybe even too rich. 

But, the surging and jerking is a different problem.  Plugs sooty?

Having no tach to report true RPM, handicaps remote diagnosis.  There is no normal reason to have an 1800 RPM idle and that kind of invalidates the idle mixture test I described.  It doesn't sound too lean at that speed, anyway.

Has the cam been degreed in?

Perhaps it is time to start with a basic evaluation to eliminate possibilities, and put some verified, know good items in the known column?

Check all spark plugs to verify equal deposit patterns.  Ensure the insulators inside aren't coated with soot.

Verify compression equal all cylinders.

Verify all slides are able to reach the carb throat floor with the idle knob backed out.

Verify vacuum sync.

Ensure floats aren't upside down.   (The flat bit points to earth.)  Float levels equal/ correct.

Divide and conquer, isolate and verify, bit by bit, methodical.
The test I requested is usually done toward the end of the jetting carbs process.  You probably aren't ready for that just yet.

Are you doing all this without a Dyno or a test track? ...to obtain reportable definable data?  Or, are you just changing things and driving for effect?

[Scott S]

 To answer a few of your questions:

 No dyno or test track. Just riding a bike that I've put many miles on and now has problems. Testing is done on the rural streets where I live.

 Floats were set when I built the bike. No reason they should have changed....right?

 I've already thought about grabbing 4 new plugs. Will do that ASAP.
 
 Carbs were synched when I got the bike on the road and I checked them on the bench (just eyeballing) and nothing seemed out of whack. Non-scientific, I know...

 Cam was set using the method in the Honda manual. Lined up the notch on the head. Again, put many miles on it.

 All slides fall to the floor. The return spring is very strong.

 I don't think it's actually 1800 rpm. Going by ear (again...non-scientific, but I have owned several SOHC4's), a "normal" idle reads 1800 on the tach. A 1,000-1,000 rpm idle and I can hear the clutch, primary, cam chain, etc. all knocking around. It sounds "right" when it shows 1800 rpm and doesn't sound like it's too high.

 Gonna go look for that other tach now.....

[Scott S]

  Found the other tach and I get the same readings. So.....here's what I think....

 I think I/we over-reacted to the "performance mods". I think that I should not have raised the needle one clip and that a 105 main (versus 90 stock) was too much.
 
 And I also think I went the wrong way the other day when the carbs were off and I went UP to a 110, thinking that the sluggish revs in the upper RPM range was from being lean when, in fact, it was probably a little rich.

 These carbs have pressed in pilot jets. I can't go up on them, so that may explain why I ended up at 2.5 turns out on the idle mixture screws.

 Using the tape method posted above, I start feeling the "poor running" at 1/4 throttle and up, about when you start lifting the needles and getting into the upper circuits on the carbs.

 I tried the other tach and I get the same readings. I think the clunking, etc. at anything below 1800 RPM is a bad carb synch.

 My plans:
 -New plugs
 -Adjust valves (they have about 1500 miles on this adjustment)
 -Adjust cam chain
 -Re-check timing
 - Pull carbs and drop needles back to stock setting.
 -Go DOWN on the mains to 100 vs. 90 stock (since I do have the 650 cam, mild P&P, etc.)
 -Set IMS screws back to 1.5 turns out

 Start over and go from there.

[bollingball]

  Found the other tach and I get the same readings. So.....here's what I think....

 I think I/we over-reacted to the "performance mods". I think that I should not have raised the needle one clip and that a 105 main (versus 90 stock) was too much.
 
 And I also think I went the wrong way the other day when the carbs were off and I went UP to a 110, thinking that the sluggish revs in the upper RPM range was from being lean when, in fact, it was probably a little rich.

 These carbs have pressed in pilot jets. I can't go up on them, so that may explain why I ended up at 2.5 turns out on the idle mixture screws.

 Using the tape method posted above, I start feeling the "poor running" at 1/4 throttle and up, about when you start lifting the needles and getting into the upper circuits on the carbs.

 I tried the other tach and I get the same readings. I think the clunking, etc. at anything below 1800 RPM is a bad carb synch.

 My plans:
 -New plugs
 -Adjust valves (they have about 1500 miles on this adjustment)
 -Adjust cam chain
 -Re-check timing
 - Pull carbs and drop needles back to stock setting.
 -Go DOWN on the mains to 100 vs. 90 stock (since I do have the 650 cam, mild P&P, etc.)
 -Set IMS screws back to 1.5 turns out

 Start over and go from there.

Scott I don't know what to tell you about the issue you are having. But I wonder why you said this (in the red). Not that it is your problem but why can't you go up on the slow jets? Just asking? Not that it is what you need to do. But can you not make them larger or get a different size?
Ken

[Scott S]

 To remove the pressed in jets, you have to pretty much destroy them. Drill them, thread in a screw, etc. Once removed, you can press in a new one, but selection is very limited. Maybe one size other than stock.

[Scott S]

 Also, I may leave the needles where they are and drop the mains, since most of this part throttle mess wasn't around until I upped the mains.
 Thoughts, TT?

[TwoTired]

The larger displacement, cam, etc. ought to leave the cylinders sucking on the carb harder, which would deepen the vacuum and make all the existing jets pass more fuel along with gulping more air.

Although the pilot jets are pressed in, they are what is called a swagged fit, or interference fit.  They can and most certainly should be removed for cleaning and inspection.  Grab them with soft jawed pliers and yank them straight out.  I've done this to each and every PD carb that has crossed my bench for attention.  After cleaning as any screw type jet would be, they simply tap right back into the same swagged hole until they seat on the shoulder feature machined into the jet brass.

However, there is no reason, at this time, to replace them with another size, as it has yet to be proven whether the pilot screw adjustment range is currently insufficient.  That will be determined near the end of carb tailoring/rejetting, with that throttle response test I outlined.

With the changes you made to the engine's volumetric efficiency, there isn't a single factory book value spec that can be relied upon to be correct for your set up.  You have effectively re-engineered a portion of the machine, and since the carbs aren't smart enough to automatically adapt to the changes, you will have to find the correct adjustment and metering values for each of the three fuel supply devices inside the carburetor.  You may even have to change the float level and or emulsion tube holes or the slide needle taper, to make all throttle positions achieve ideal fuel ratios.
I don't expect you will get it perfect, perhaps close.  But, I'm pretty certain that unless you like lottery odds, you will not get it correct without placing the machine into controlled test conditions, getting some feed back from either a dyno fuel map printout, or loading the engine on a test track and reading the spark plug deposits for combustion deposit residue, to determine a direction to change the fuel metering closer to ideal.

Perhaps I've misunderstood the problem.  Are you saying you've been happily riding it about, happy as a clam with all operation parameters and something suddenly changed?

Has it had issues for a while now and they've gradually deteriorated to become unacceptable?

Have you checked the spark plug deposits to see if it has slowly built up enough carbon to begin misfiring and develop poor idle traits?
How have you determined which spark plug heat range is proper for the new configuration?

I guess I may not be able to help you further, as I cannot ascertain bounded data with which to formulate a corrective approach.

I do wish you luck in random changes and casual driving about for testing.  But, I have limited confidence that will result in a well mannered machine, without a great deal of luck.  I hope I'm wrong about that.

Best regards,

[Scott S]

 Well, I'm really sorry that my "casual driving about" will have to suffice, as I don't have access to a dyno or test facility. I doubt many people do.

 The bike ran pretty well before. Certainly no popping or sneezing at idle and low RPM. I always felt like it didn't pull as hard in the upper RPM's as it should, hence my bump to a bigger main, which I know think was the wrong way.

 After posting that about the pressed in pilot, I found a thread about the 750 PD carbs and they mentioned being able to pull out the jets. I was unaware of that. I will attempt that today when the carbs are off.

 While you are absolutely correct in that I made changes and I have to deal with it, do you have any thoughts on the needle position? If you were in my predicament, would you put it back in the stock position?

[Scott S]

 Changed the mains to 95's.
 Set the IMS screws at 1.5 turns out.
 Tried to pull the pilots but was unsuccessful. Sprayed carb cleaner and air through them.
 Pulled and checked emulsion tubes, just in case.

 Adjusted cam chain.
 Adjusted valves. The intakes were a little loose, exhausts were fine.
 Set timing.

 Started bike and was able to get idle better right off the bat.
 Vacuum synched carbs. #4 was off a good bit.
 Idle got even better.
 Backed out on IMS screws.
 Idle got better.
 Ended up at 3 turns out on IMS screws. Stock is 1.5.
 Checked synch and timing again.

 Idle is much better and the clunk and knocking is gone. That was probably #4 being out of synch. However, it seems to like 1300-1400 rpm best, if my tach is to be believed. That's not TOO much above the ~1,100 factory idle.

 Performance is better, but it's still not as clean and crisp in the mid range, above 1/4 throttle, as it should be. I really wish I had lowered the clips while I had the carbs off.

 I'm gonna take a break from it. I'll pull the carbs again and lower the needles back to the stock clip position and maybe leave the 95 main.
 It's sort of odd that I'm going to end up so close to stock settings on these things.

 Oh...and we managed to knock the tank off the stool and put a nice dent in it, too. DOH!
 

[TwoTired]

Well, I'm really sorry that my "casual driving about" will have to suffice, as I don't have access to a dyno or test facility. I doubt many people do.

Some tools are essential for the task to be performed.
Eliminate all screwdrivers from your tool box and see how many corrective procedures can be performed successfully.  Though common tools, they are essential.  The Dyno and test track are specialized tools essential for a straightforward re jet process.  Because they are not common, makes them no less essential.

You are right, not everyone or even "many" do have a dyno or a test track.  But, these same people are unable properly address carburetor jetting issues either.

While you are absolutely correct in that I made changes and I have to deal with it, do you have any thoughts on the needle position? If you were in my predicament, would you put it back in the stock position?

The only clue I might have would be based on spark plug deposits.  Which I have not seen or heard described in sufficient detail to identify present mixture conditions.  Sorry.  Without knowing where one is, it is pretty hard to make a definitive decision on how to get to where one wants to be.

The "roll on" test I described above, is just one part of the test process when not having a dyno.  I neglected to mention that the spark plugs should be fresh for the test because I thought that was assumed, or a natural condition following prior idealization of the main jet and slide needles.

With your current corrective direction, any guess is as good as another.  Below is a matrix that you can look forward to in defining your remaining variables.  You won't have to try them all if you get lucky with your first selections.  Put it on your dart board, and change whatever the dart hits, blindfold optional.  The individual items are all somewhat interactive, as one item can have an effect on others.  But, you may or may not have a lifetime project, unless you are the lucky sort.

Good luck!

[Scott S]

 The roll on test was better, but mid range still suffers.

 

[Scott S]

 Dropped the needles one clip today, back to stock position. I even did it on the bike! WAY easier than I thought it would be. The rear slide screws on #2 and 3 were a little tight, but if I had a longer screwdriver, it would have been a snap! I bet I did the whole job in less than half an hour!

 Synched the carbs and took her for a test ride. Best she's ever performed. I still get a little funky...something...right around 1/4 throttle somtimes. And, sometimes, it feels like it's maybe dropping a cylinder. I think I'll change plugs and make sure the caps are connected securely.

 Overall, it's the best she's ever been. Pulls to redline. Idles better. Plugs look perfect. So far, so good. Since I couldn't remove the pilot jets (I tried....they're tight and I didn't want to break anything. I did spray carb cleaner and air through them and could hear air moving through the orifices), I'm going to run some SeaFoam through a tank of gas and see if that helps.

[brewsky]

(I tried....they're tight and I didn't want to break anything. I did spray carb cleaner and air through them and could hear air moving through the orifices), I'm going to run some SeaFoam through a tank of gas and see if that helps.

You'll never know for sure unless you pull them. It's only scary the first time.

I wrap a thin piece of leather 3/4 around the jet, use needle nose vice grips, and pull straight out, twisting slightly, only if necessary.

Don't try it with the carbs mounted, and remove the float and main jet holder first.

[Scott S]

 I tried it on the bench with a pair of flat grip pliers...whatever you call those things...the kind where the jaws open and close squarely...and I wrapped a rubber "lid remover" gripper from the kitchen around the jet for grip.
 Tried pulling straight out, tried a slight twist. Couldn't get them to budge.

 I'm really thinking that the flatness or stumble or whatever...I'm not sure what to call it..is ignition related. It's intermittent. If I had a plugged jet or passage, it would be in the same spot all the time. The bike runs great and sounds great at times, then it feels/sounds like maybe it's not firing on all four. I wish there was a way to see what's happening with the spark WHILE I'm riding it.

[Lostboy Steve]

I had a leaking carb body that caused a terrible misfire on cylinder 3. I never could find the exact leak but it was the body. I swapped with another body and never had the problem again.