Topic: Runaway Revs, Low mpg, air pods....connected issues?

[hman0217]

So here's the scenario...I put air pods on my cb750 late last fall with 115 mains. I have a new appreciation for stock setup now, after all these problems, and have ordered new boots for my airbox and 110 mains (because I now have 4-into-2 mac exhausts and a K&N filter in my airbox). So my next step is to re-install the airbox once these parts come in.

But here are my symptoms...

I'm getting anywhere between 12-20 mpg. yes. 12 mpg on the last tank. I also have been having issues with the idle setting. If I set it so it holds cold, it runs away on me when hot. If I set it to the hot running, i have to feather the throttle for a good ten minuted.

As an aside, I did change the boots from the carbs to the engine, eliminating the possibility of an air leak at that point.

Today, after I changed the spark plug caps, the bike started right up cold without me having to feather the throttle (my number 1 spark plug cap was loose as hell and they needed a change out). But then my issues began... after some riding, the revs started to run away like crazy. What I discovered is this though...the regular fuel (not reserve) was low when the revs started climbing on me. They got so high I shut the bike off. (this was with the clutch engaged. when I was back in gear and moving the issue subsided). Then I turned the bike on again and, sure enough, revs went crazy high. Turned the bike off and put the fuel on reserve. Turned the bike on and the revs went down. So then I'm thinking, there's some connection between the lean mixture and the runaway revs.

Then I fill up my tank, discovering that 3.3 gallons got me 38 measly miles. (damn.) And I ride home. I turn the petcock off about three tenths of a mile from home so that the carbs are empty when I park. Well, right as I am parking, the revs went through the roof. Again - low fuel/lean mixture and revs shoot up.

So what's the connection here, if anything? Is changing back to my airbox gonna help matters? Or do I have deeper problems here? I could always turn the idle screw down and feather the throttle forever each time I ride. But that's not a great fix.

Are my carbs junk altogether? (the floats are set right and they were just rebuilt but maybe they're just crap)

Thanks.

[Don R]

 float levels high, float valves sticking, vacuum leak. I know, you already checked that. Did you use the clear tube method to check the float levels? I had drilled jets on my 76F when I got it. The number could be misleading.

[hman0217]

float levels high, float valves sticking, vacuum leak. I know, you already checked that. Did you use the clear tube method to check the float levels? I had drilled jets on my 76F when I got it. The number could be misleading.

No I set them to 26 mm on the bench. I'll check out this clear tube method you mention. Feel free to recommend a reference.

gosh, I hope it's not the vacuum because the only place left that it could be would be within the engine itself and that's a whole other can of worms.

[calj737]

The runaway revs are hard pointer to either a major vacuum leak, or an ignition advancer problem. I struggle to believe the fuel switch is causing/resolving the issue unless you've also got a wonky inline fuel filter as part of your non-stock setup?

To set your idle, warm up the bike, set the idle, then leave it be. If your bike always starts without choke, its likely you're rich on the idle circuit. If you perform an Idle Plug Chop, what do the insulators look like?

Lastly, are you sure you wish to chase down these problems as you have the airbox going back on? Any settings/tuning changes you make, may well need to be redone once the airbox is back on (except of course ignition/timing issues).

[hman0217]

The runaway revs are hard pointer to either a major vacuum leak, or an ignition advancer problem. I struggle to believe the fuel switch is causing/resolving the issue unless you've also got a wonky inline fuel filter as part of your non-stock setup?

To set your idle, warm up the bike, set the idle, then leave it be. If your bike always starts without choke, its likely you're rich on the idle circuit. If you perform an Idle Plug Chop, what do the insulators look like?

Lastly, are you sure you wish to chase down these problems as you have the airbox going back on? Any settings/tuning changes you make, may well need to be redone once the airbox is back on (except of course ignition/timing issues).

Thanks. Are you suggesting that I don't chase down a "Major vacuum leak" until the airbox is back on? My concern was that, between the ultra low mpg and the runaway revs, that my problem might be much bigger than the airbox. There's nowhere between the air intake and the engine that I'd have a major vacuum leak, as I replaced the boots going from the carbs to the engine already.

I'll try the idle plug chop. And check the timing/ignition and report back.

[hman0217]

The runaway revs are hard pointer to either a major vacuum leak, or an ignition advancer problem. I struggle to believe the fuel switch is causing/resolving the issue unless you've also got a wonky inline fuel filter as part of your non-stock setup?

and no inline fuel filter.

And also, since the runaway revs happen after the engine has been running for some time, does this rule out the ignition? Wouldn't the ignition issue cause a steady problem? I'll check it myself later this week, but I am curious as to whether the nature of the problem could point to this.

[calj737]

The runaway revs are hard pointer to either a major vacuum leak, or an ignition advancer problem. I struggle to believe the fuel switch is causing/resolving the issue unless you've also got a wonky inline fuel filter as part of your non-stock setup?

and no inline fuel filter.

And also, since the runaway revs happen after the engine has been running for some time, does this rule out the ignition? Wouldn't the ignition issue cause a steady problem? I'll check it myself later this week, but I am curious as to whether the nature of the problem could point to this.

On the contrary, it would be my first suspect. If the advancer springs are worn, the cam won't return. There are small springs the retrieve the advancer after the throttle is let off and these are prone to causing problems after years and miles.

To diagnose, ride the bike a bit. Come home, hook up your timing light, rev the motor and watch the points plate. If the timing marks don't snap back but instead the ignition stays "advanced" you're on the right track.

[evinrude7]

hman, once i changed out the PO's pod set up to stock air box it was a whole different ball game.  i still have the revving issue you have but it's way better and seems there is light at the end of the tunnel, an achievable solution.  couple of days ago it was getting that rev issue and decided to spray around the 8 boots to see if there was in fact a leak.  i remember tightening all the clamps down when i added the stock air box.  i mean really tightening them in ocd fashion.  fast forward a couple weeks i sprayed the carb cleaner and sure enough there was a change in the sound of the motor.  i found a few loose clamps.  these boots get warm and get cold.  the boots expand and shrink.  we are also at a time of year where temps are fluctuating a bunch.  any how i'll have to check all my boots again but i'm planning on a few other fixes besides the air leak. for one, i believe my advancer springs are worn out.  sometimes when i twist the throttle even though the throttle snaps back the bike slowly returns to idle.  second, according to the PO the jets may be larger than stock.  so i'll be ordering stock size from http://www.jetsrus.com/ and replacing those. 

my 2 cents.  i'm new to this motorcycle trouble shooting business.  good luck. 

[Sigop]

For what it's worth, which likely not much, I had a revving problem as it heated up and it would come and go.  I finally killed my air leak by unmouting the airbox from its top bolts, tightening up on the rubbers from airbox to carbs, then dragged airbox back under the mounts.  I put the stretch into the (new) airbox to carb rubbers after pushing them flush and tightening them down at the carbs. Note the down angle that can cause a gap in the seal. CB750 K3

[evinrude7]

Sigop, good point. +1

Sent from my LG-D850 using Tapatalk

[hman0217]

For what it's worth, which likely not much, I had a revving problem as it heated up and it would come and go.  I finally killed my air leak by unmouting the airbox from its top bolts, tightening up on the rubbers from airbox to carbs, then dragged airbox back under the mounts.  I put the stretch into the (new) airbox to carb rubbers after pushing them flush and tightening them down at the carbs. Note the down angle that can cause a gap in the seal. CB750 K3

Totally. Except in my case i have the airpods on and brand new tight boots from my carbs to the engine so I'm thinking there's no leak external to the engine. (and of course hopefully not in the engine)

I'm gonna try Calj's suggestion later this week soon as I get the chance. I'm hoping that's the ticket.

[hman0217]

Finally got some time... Today I replaced the points and condensers per spec. I also replaced the tach cable. But here's the thing... The seal I got for the tach cable was too fat (diameter ok but too thick) so I left it out. After fifteen miles into my ride, two things happened: 1. The sky high revs kicked back in and 2. I noticed oil leaking out of where the tach cable meets the engine.

Question: is the tach cable insertion point a potential area for a "major vacuum leak", the likes of which have been haunting me in the form of super high revs and Rock bottom fuel economy?

Still going to try the plug chop next, although I already know there's a richness problem as my number one plug is black as night after just a couple hundred miles. On that note, it's also running colder. (I did just replace the plug caps so I believe that's eliminated)

Anyway just wanted to ask the question in case I'm chasing my tail while there's an obvious answer right under my nose

Thanks

[calj737]

The tach plug won't effect your idle. Fresh plugs are required to diagnose running problems.

[mrbreeze]

Vac synch carbs. But first....make sure everything else is in order. All adjustments and no air leaks, clean fuel and fuel supply. My bike wouldn't stop doing the rev up thing until I vac synched. The difference was like night and day. Also....the clear tube method is very important to make sure all bowls are equal and within specs.
Float measurements and bench synch are just a starting point.

[hman0217]

Vac synch carbs. But first....make sure everything else is in order. All adjustments and no air leaks, clean fuel and fuel supply. My bike wouldn't stop doing the rev up thing until I vac synched. The difference was like night and day. Also....the clear tube method is very important to make sure all bowls are equal and within specs.
Float measurements and bench synch are just a starting point.

Roger that. I did, however, come across this debate regarding the clear tube method that suggests such a degree of precision is a bit overboard:

http://forums.sohc4.net/index.php?topic=138594.0

Do you disagree?

Also, I just put the airbox back on too, with brand new boots, because I've learned the hard way that stock is optimal...And back to 110 jets with the K&N air filter and Mac 4-into 2 installed. The day ran out but next up is

1. check for air leaks around the boots
2. vac synch carbs
3. plug chop
4. set air screws and idle screw

[evinrude7]

hman, someone with more experience than me should probably help with this question but i'll give it a try. i had a ton of issues that wouldn't smooth out until i checked and adjusted my float heights.  once i did that a lot of my problems were lessened.  i got mine close but my method for setting them probably wasn't the most accurate with a ruler on the side of the float and peering across to measure.  once i order new jets, intake boots and clamps i'm going to go at it again with the carbs and follow all the recommended specs to a T.  i guess what ever method gets the fuel level correct is what's needed.  clear tube method does make sense. 

[hman0217]

hman, someone with more experience than me should probably help with this question but i'll give it a try. i had a ton of issues that wouldn't smooth out until i checked and adjusted my float heights.  once i did that a lot of my problems were lessened.  i got mine close but my method for setting them probably wasn't the most accurate with a ruler on the side of the float and peering across to measure.  once i order new jets, intake boots and clamps i'm going to go at it again with the carbs and follow all the recommended specs to a T.  i guess what ever method gets the fuel level correct is what's needed.  clear tube method does make sense.

I basically set them the same way. I do defer to the masters...if that's too approximate, I can certainly opt for the more precise path. I do want to get it right and don't want to cut the wrong corners thanks.

[evinrude7]

one way for setting the float heights is take an old gift card or something similar and cut it out into a U shape but with square corners obviously.  the depth of the cut should be the recommended height of the float.  then set the card over the float and the bottom legs should touch the edge where the bowl gasket goes and the float should just touch the height you cut it to.  like this.  don't go by the recommended height in the picture this is just an example.

 

[mrbreeze]

Sometimes they all come out in spec by adjusting the float height and drop only but not always. The clear tube method is really the best way to tell if they are spot on where they should be. The last time I rebuilt my carbs was around 2005-2006 although I've cleaned them a time or 2 since then. When I put mine back together after float adjustment #3 was considerably low compared to the others. It was causing starvation when into the throttle. That's the first time I used the clear tube method and it cured the problem.

[mrbreeze]

I forgot to add....the runaway throttle and the loose cam chain sound at idle are classic symptoms of out of synch carbs. Your cylinders are fighting each other when out of synch. Just remember that everything else must be spot on.....the synch comes last.

[hman0217]

Sometimes they all come out in spec by adjusting the float height and drop only but not always. The clear tube method is really the best way to tell if they are spot on where they should be. The last time I rebuilt my carbs was around 2005-2006 although I've cleaned them a time or 2 since then. When I put mine back together after float adjustment #3 was considerably low compared to the others. It was causing starvation when into the throttle. That's the first time I used the clear tube method and it cured the problem.

Alright...let's be thorough about this. So what's the recommended clear tube level for a cb750 k5? 3-4 mm below mating surface? It's not a published spec.

Or is the exact height less critical than if the four all equal?

[Bodi]

When I hear carb problems and "they were just rebuilt" it's a red flag.
Using aftermarket brass parts is often a disaster, maybe explaining your terrible mileage.
Dismantling the rack and reassembling it can bring more grief. Possibly one or more of the slides don't - or can't - close fully. The lift mechanism is tricky and on racks without a fixed "reference" synch adjuster it's easy to have the lift range wrong so they don't close properly. I don't know if yours has one fixed or all four fully adjustable.

[John Eberly]

Hman,

I don't see where you responded to reply #6 from calj regarding the spark advance.

You might be wasting a lot of time on stuff that won't fix your problem.

Another check for the advance that might show if that's the problem - when the engine is warmed up and revving fast at idle, stop the bike, hold the brakes, and let the clutch out enough to slow the engine down. Sometimes this will bring back a normal idle. If it does, your advance springs are shot - they can't pull the centrifugal weights back in at higher rpms.

[hman0217]

Hman,

I don't see where you responded to reply #6 from calj regarding the spark advance.

You might be wasting a lot of time on stuff that won't fix your problem.

Another check for the advance that might show if that's the problem - when the engine is warmed up and revving fast at idle, stop the bike, hold the brakes, and let the clutch out enough to slow the engine down. Sometimes this will bring back a normal idle. If it does, your advance springs are shot - they can't pull the centrifugal weights back in at higher rpms.

In fact, it will idle high at a light and then when I release the clutch, it will drop to normal. Not exactly what you said but I think it confirms the same thing, no?

[calj737]

In fact, it will idle high at a light and then when I release the clutch, it will drop to normal. Not exactly what you said but I think it confirms the same thing, no?

Not exactly. Engaging the clutch puts load onto the engine, so it will naturally reduce the RPMs. However, once in gear and riding, does the idle race up again? OR does that moment of loading the engine restore the idle temporarily?