What causes surging? SOLVED

[guido guzzi]

Hello All,
My carbureted 96 Sport 1100 just turned 33,000 miles and has started surging between 3800 and 4000 rpm. I know I should keep the revs over 4K but sometimes it cannot be helped.
Has the stock airbox and not planning to remove it. If it ever did this before it was not nearly as noticeable. Carbs are synched at idle and valves are correctly set to non-US specs (.006 and .008?).
IIRC there's no way to synch carbs at 4000 or so, right? First thought was an air leak creating a lean condition... Any insight appreciated.
John Wendt in Wisconsin

[n3303j]

Sync carbs at idle with manometer and idle stop screws.
Sync carbs at 4K with nanometer and throttle cable tension adjuster.

Check for intake air leaks or slow fuel delivery.

[Rebochi]

  A possible build up of fuel deposits on the emulsion tube and jets, easy to access and clean. Drop the float bowls and have a look.

[guido guzzi]

Sync carbs at idle with manometer and idle stop screws.
Sync carbs at 4K with nanometer and throttle cable tension adjuster.

Check for intake air leaks or slow fuel delivery.

Thank you for the input fellas. 
n3303j , sync at 4K with throttle cable free play adjuster?

[n3303j]

Thank you for the input fellas. 
n3303j , sync at 4K with throttle cable free play adjuster?

Not sure how your linkage is set up. But somewhere in there is something that will let you adjust tension on just one or the other carb cables. Systems aren't perfect so manufacturers install adjustment points so they don't have to be.
Make sure both cables have a bit of slack at idle position.

[cappisj1]

I would look for an air leak first. If you haven’t changed any intake components or taken the carbs off I would not make any mechanical adjustments as nothing should have changed. If you’re 100% sure there are no air leaks and the fuel lines and filters are up to par then you can start making mechanical changes. Put new plugs in it too.

[guido guzzi]

I would look for an air leak first. If you haven’t changed any intake components or taken the carbs off I would not make any mechanical adjustments as nothing should have changed. If you’re 100% sure there are no air leaks and the fuel lines and filters are up to par then you can start making mechanical changes. Put new plugs in it too.

[/b]

It sat all of last year with tank and carbs drained. Normally I would change the plugs before bringing it out again but have not done that yet...

[Wayne Orwig]

[/b]

It sat all of last year with tank and carbs drained. Normally I would change the plugs before bringing it out again but have not done that yet...

New plugs and cleaning the timing sensor may not be a bad idea. Hard to imagine they would cause mild surging though.

[guzzista]

Have you checked the vacuum line from the Digiplex box to the  plastic T and to the carbs ?  I have found both a cracked plastic T and collapsing hoses due to age in the past

[guido guzzi]

Have you checked the vacuum line from the Digiplex box to the  plastic T and to the carbs ?  I have found both a cracked plastic T and collapsing hoses due to age in the past

Vacuum line was replaced a few years ago but it's worth another look.

[guido guzzi]

New plugs and cleaning the timing sensor may not be a bad idea. Hard to imagine they would cause mild surging though.

Yes, will check the timing sensor and the magnet that's supposed to remove the ferrous bits from the bell housing too. It feels like an ignition 'miss' and there's something about the airbox that causes a flat spot at that RPM range. Thank you everyone for your input. Will buy new plugs and look at these other items when time allows... 

[bmc5733946]

The parts book shows cable adjusters at the splitter under the tank. I honestly do not recall that on the carb Sports, but it wouldn't surprise me either. There are many oddities on them so it kinda fits. I am sure that some provision was made for carb synchronization.

Brian

[John A]

I’d be pulling the bowels and making sure the jets are clear and the inlet screens are clear of crud. Even empty they can grow things. Check the float needles for sticking. Some needles have an internal spring that can stick.

[guido guzzi]

I’d be pulling the bowels and making sure the jets are clear and the inlet screens are clear of crud. Even empty they can grow things. Check the float needles for sticking. Some needles have an internal spring that can stick.

Hoping you meant *bowls* John... In that case, it's definitely good advice!

[guido guzzi]

The parts book shows cable adjusters at the splitter under the tank. I honestly do not recall that on the carb Sports, but it wouldn't surprise me either. There are many oddities on them so it kinda fits. I am sure that some provision was made for carb synchronization.

Brian

Yes, there is some adjustment at the under-tank splitter and also on the carb tops.

[bmc5733946]

Well that then allows synchronization at higher RPM than idle eh? That comment threw me a little knowing that surely some provision was made for synchronization at normal operating speeds as well as at idle! Not necessarily the most convenient spot, though.

Brian

[guido guzzi]

Not trying to disparage anyone's advice but... The way I understand it, adjustment of throttle cable freeplay is only used to ensure both slides move (as close as possible to) simultaneously when the throttle is twisted - from idle. If one has more freeplay than the other, it will move after the one with less, right? 

[n3303j]

Not trying to disparage anyone's advice but... The way I understand it, adjustment of throttle cable freeplay is only used to ensure both slides move (as close as possible to) simultaneously when the throttle is twisted - from idle. If one has more freeplay than the other, it will move after the one with less, right? 

Yup, that's synchronization.

[bmc5733946]

At idle the throttle stop screws control synchronization,  above idle it is controlled by the cable length adjustment, usually right at the carb top or in some cases just below the throttle grip, early bikes with VHBs etc.

Brian

[John A]

Hoping you meant *bowls* John... In that case, it's definitely good advice!

I wonder what spell check was thinking 

[guzzisteve]

Do you have Euro jetting in your PHM 40's ?

[guido guzzi]

Do you have Euro jetting in your PHM 40's ?

No, it's  based on what Ed Milich (sp?) recommends for the big valve motors. Will have to look it up.
It *seems* like the surge/stumble has gotten worse this year but I haven't changed anything. 

[guzzisteve]

Yea, was wondering if it was there before, more like a tuning issue. I had my own set-up for the carbs. Different needle & atomiser but close to Ed's stuff. I must have 200mi on my friends to get this set-up. Open megs for exhaust.
It could be just carbs getting dirty.

[guido guzzi]

Do you have Euro jetting in your PHM 40's ?

Jetting is as follows:
65
50/3
K4 clip in the middle
265
152

[guzzisteve]

If that has been working for you keep using it. I looked for my setting and it uses 60/3 slides and a 268 atomizer, K8 needles but it may not be the final set-up. I wrote stuff down as I went along and this was marked in the shop manual.
This was more than 25 yrs ago when bikes were new. I can't remember everything. I cut & ground the idle mix screws to match VHB ones.
I would leave yours alone unless you know how to read plugs & have your throttle marked in quadrants and bottom one split to 1/8 throttle.

[John A]

If that has been working for you keep using it. I looked for my setting and it uses 60/3 slides and a 268 atomizer, K8 needles but it may not be the final set-up. I wrote stuff down as I went along and this was marked in the shop manual.
This was more than 25 yrs ago when bikes were new. I can't remember everything. I cut & ground the idle mix screws to match VHB ones.
I would leave yours alone unless you know how to read plugs & have your throttle marked in quadrants and bottom one split to 1/8 throttle.

Over the years that’s what I generally ended up with as well. K3 needles but iirc they are about the same as K8’s.

[guzzisteve]

Over the years that’s what I generally ended up with as well. K3 needles but iirc they are about the same as K8’s.

I thought I ended w/K1's till I saw the note in my manual. It also said 160 mains & that's why it may not be final setting

[guido guzzi]

Changed plugs and the stumble/surge seemed worse.
Tightened clamps on carb/manifold sleeves and not just a little - no change.
Tomorrow the tank and carbs come off.

[guido guzzi]

Yesterday morning fresh eyes and ready to dismantle carbs, check for frayed cables, whatever it takes.
Removed tank and carbs come off next. Loosening screws on the left carb top and it hits me: I bet the top of the right carb is loose. BINGO!
It always felt like an air leak because it would happen when revs dropped into 3500-4000 range. Did not occur when accelerating into or through that range because accelerator pumps masked the leak...

[Tkelly]

It’s amazing how things work out if you walk away for awhile.Congratulati ons.