Topic: Carb tuning: bike running rich at idle?

[jahmic]

bike:  78 CB750 F

Question (abridged version):  Can improper float height cause my bike to run rich at idle?

The details:

Well, I'm at the point where I've pulled the carbs off the bike multiple times for cleaning, have done all the other required tune-ups (points, timing, tappets, etc), and have definitely solved my vacuum leak problem.  I've been reading and re-reading through these forums for the past couple weeks, and THINK that I've figured out what my problem may be.  Just wanting to get some opinions on whether or not I'm approaching this correctly before I end up going in circles trying to get these carbs dialed in.

The problem actually started this past weekend, when I pulled my carbs, re-cleaned them, and fixed my vacuum leak.  Prior to fixing the leak, the bike was running a little lean at idle and would hesitate a bit when rolling onto the throttle.  I should mention that my leak consisted of lightly cracked intake boots, and a completely shot airbox with multiple gaping holes...it didn't seal at all.  The other issue I addressed this past weekend was that the posts for the accelerator pump had gotten clogged again.  They were BAD initially, and I wasn't surprised that I needed to clean them again.  The symptom that they were clogged was basically just hesitation as I rolled on the throttle...but the engine definitely kept running.  In fact, the bike actually didn't run bad at all, aside from the lean condition at idle.  But then, I fixed the vacuum leaks...

Now, the new issue, after fixing the vacuum leaks, is that the bike is suddenly running noticeably rich at idle.  Disclaimer:  I was impatient this past weekend after fixing the leaks and took a short ride of about 10 miles just because...and didn't do another vacuum synch after fixing the leaks.  I'm going to go home after work and vacuum synch the carbs tonight (no internet at home, so posting prematurely from work).  That being said, the bike IS idling smoother than it was before, but seems to "load up" after a while and start stumbling.  Whereas before I could fiddle with the idle adjustment screw to keep things in order, it now takes quite a few turns in to make any noticeable difference in improving the degrading idle.  At idle, with the engine warmed up, if I roll on the throttle at all, the RPMs dip down.  If I immediately back off the throttle, it grumbles back to life...if I keep the throttle open, it dies.  This seems to be the opposite of what it was doing before (hesitating and surging), which is why I think it's running rich at idle.

The other issue...which someone else explained in detail here: http://forums.sohc4.net/index.php?topic=75136.25    is that the idle will hang at about 3k RPMs if I downshift to a stop.  I have to use the idle adjustment to bring the idle back down, but after some time, it just ends up hanging again.  If I adjust it down to the point that it doesn't hang at all, the bike doesn't want to idle.  Also, if I allow the idle to hang at 2k-3k, the bike still will struggle to take off from a stop...the RPMs dip down and I have to slip the clutch to keep it going.  Like I said...pretty much the same symptoms as the ones outlined in that old thread.

So...after reading that thread and considering my bike's symptoms, I've been trying to connect the dots and figure out if the solution in that thread would make sense for what my bike's been doing.  For those that didn't click the link...that OP's issue ended up being floats that were set too high.  I'm pretty convinced that my idle hanging issues are due to the fact that my float levels are also too high...I'll admit that I "roughly" measured the float height, was confused by the instructions, then just eyeballed them and figured they were OK.  I doubt they were OK though...since the PO had hidden a leaking overflow tube by plugging it with a screw    I didn't adjust the tangs at all when I cleaned the carbs (hangs head in shame).  I assumed that the lack of a leak from the overflow tube and/or the carbs meant that my floats were fine...and just left them how they were.  Probably a mistake on my part.

So, before I go back and bang my head against the wall trying to figure out how and where to measure the float height, does it even make sense that too much fuel in the bowls would cause the bike to run rich at idle now?  I'm assuming that the excess fuel in the bowls is now being "sucked up" by the main jet with the increased vacuum pressure at idle...which was absent before I fixed the vacuum leaks.  Rolling on the throttle is actuating my accelerator pump, and that extra shot of fuel into the carbs is causing the bike to die almost instantly.  At least that's my theory. 

The mixture screws are all set at 1.5 turns out.  My plan is to synch the carbs first, then try to solve the problem by playing with the mixture screws and adjusting them according to the engine speed (as detailed in the carb FAQ).  If that doesn't work...then I guess I'll be pulling the airbox to prove that the accel pump works...and if that checks out, I'll be pulling the carbs off to assess the float height again.

Am I on the right track?  Or is there something obvious that I'm missing here.

[jahmic]

Actually...I guess the FIRST thing I'll be doing when I get home is pull the plugs.  Last weekend, after fixing the leak, I was able to ride the bike about 2.5 miles to a friend's house, then back home.  Things seemed to get worse along the way...and today I wasn't able to get the bike moving at all really...so I'm thinking the plugs are fouled at this point from running rich.  So, I'll be pulling them to confirm that rich conditions are my issue...but I'm almost certain that's the case.

[Spanner 1]

Can improper float height cause a rich condition @ idle.... for sure. !!
Do you know the correct float height for your carbs ?, are you sure ?, as there are 2 different float heights for PD carbs. Yours should be 14.5 mm ( not 12.5mm as the 78 K bikes are ).... if set at 12.5mm then waaay rich with too high fuel level in the bowls.....

[jahmic]

Well, you guessed it Spanner1...I had the incorrect float height.  But actually, the float height that I had written down was 26mm...which was pretty much impossible to measure from the edge of the carb body.  So, in my confusion, I was trying to measure from inside the carb body...who knows what measurement that ended up giving me for float height when I confirmed they were all about even and "good enough". 

I'll get that sorted and report back.  As I said, I didn't adjust the tangs, just left them where the PO set them.  I'm starting to understand why there was rust from the gas tank gunked up underneath the float bowl gaskets when I first cleaned them...seems like they've all been well over-filled for quite some time.

Hopefully adjusting the float height knocks out both of the issues that I've been having.  The weather didn't agree yesterday, so I only had time to pull the plugs.  They didn't look TOO bad as far as being fouled, which I expected since I wasn't blowing smoke out the exhaust.  But, they definitely looked like they were running rich, and I was able to smell the gas when I pulled them...a couple were even still wet.

Looks like I need to get a computer back at home again; not being able to search for info easily while I'm at home has led me to do some jobs more than a couple times.  Lesson learned.

[Duanob]

At the top of the Tech page is the downloanmanuals page. go to "Motorcycle Carburetor Service Manual"

All your carb questions shall be answered.

[jahmic]

At the top of the Tech page is the downloanmanuals page. go to "Motorcycle Carburetor Service Manual"

All your carb questions shall be answered.

http://www.sohc4.net/?p=65

Thanks for the heads up...I totally missed that link on the tech page.

[BobbyR]

One other consideration. The 77-78 carbs have an actual mixture screw not air screw as in the older models. While 1.5 turns may have been fine off the factory floor, 33 years later that may no longer be true. You may find that a smoother idle will require those screw to be tuned rather than just set. 

[Duanob]

At the top of the Tech page is the downloanmanuals page. go to "Motorcycle Carburetor Service Manual"

All your carb questions shall be answered.

http://www.sohc4.net/?p=65

Thanks for the heads up...I totally missed that link on the tech page.

Funny, me too for about a year.

[jahmic]

One other consideration. The 77-78 carbs have an actual mixture screw not air screw as in the older models. While 1.5 turns may have been fine off the factory floor, 33 years later that may no longer be true. You may find that a smoother idle will require those screw to be tuned rather than just set.

Thanks BobbyR.  Tuning the mixture screws is definitely on my to-do list...according to a few posts that I came across, it should be one of -if not the- last thing that I do once everything else is in order.  Once I have the floats set and the carbs synched again, I'm going to attempt to adjust the mixture screws.

When I pulled apart my carbs, I found that 2 of the mixture screw assemblies were missing washers.  Although all of the o-rings were in good condition, I had to replace the washers with some lockwashers that I found at a hobby shop.  I flattened out the washers, and they are almost identical to the stock ones...they were slightly thicker.

Now that I think about it, it may be worth going back in there and double-checking which carbs have the replacement washers in them, as they may be affecting the mixture screw settings.  In fact...to even things out between the carbs and make adjusting easier, it may also be worthwhile for me to just replace all the washers with the ones I acquired from the hobby shop so that the differences between carbs are minimal.

[jahmic]

Well, I decided to just go through the entire 3k mile maintenance procedure again, rather than focus on what I thought the suspected problem was and hope for a cure.  Besides, while reading through the forums I noticed that there were some nice pointers that weren't included in the shop manual 

I'm not sure what solved the issue since I literally redid EVERYTHING, but...end result...the bike is running much better now.  I found that the floats were set anywhere between 13.5mm - 15mm.  I set them all to 14.5mm with a vernier caliper and the bike doesn't seem to be running rich at idle anymore.  In fact, it's actually running just a tad lean...which I've read is pretty normal with the factory settings as they accounted for the accelerator pump's supply in tuning the bike at idle.

I don't get any bogging at take-off anymore, the response has been sharp throughout the RPM range and at all throttle positions.  Turns out that my accel pump was the likely culprit; when I pulled the carbs the posts were clogged AGAIN.   I gave the carbs another soak and a rigorous cleaning...also got a better inline filter...just in case.  So far so good...only took 3 cleanings, lol.

I did a carb synch as well...of course.  And, it turns out that coating the intake boots in plasti-dip probably helped...I noticed more of a vacuum across the board (about a 25-30% increase) with the new airbox and repaired boots.  Only problem is that I still have a vacuum leak.  I noticed that when I put pressure on the carb body to hold down the adjusting screw while turning the locknut, the reading on my manometer would change.  Sometimes all 4 would return to their previous readings...other times 1 or 2 would hang up a bit.

So...although improved, my vacuum leak issue isn't solved.  The idle still hangs just a tad while riding, but tends to hang at about 1800 - 2000 RPMs now, which I guess is an improvement?  Sadly I probably won't be able to fix the issue until I source some replacement boots for the bike.  So far, I've found 1 of the 4 NOS boots.

Thanks again to everyone for the help...it's great to be able to ride.  Now I just have to rebuild my brake calipers since my rear decided to seize up on me yesterday.    It was on the to-do list anyway...but having to sit on the road and wait for your calipers to cool down in 95 degree weather so you can bleed the line is a PITA 

[BobbyR]

I hate to admit that I did 4 tear downs until I got it pretty close to right. So congratulations!  There was  fellow on here that was making new carb boots, it may be worth a search.

With these old bikes it will be a game of inches until it starts to come together. Once you get them to a certain point they get to be very reliable and you do the normal maintenance and replacing tires and chains as you add up the miles. I call mine a rolling restoration. I try to do one thing a year to improve it. Hang in there, it does get better.