Author Topic: extra hundred cc causing my throttle to hang??? -SOLVED  (Read 910 times)

Offline lazlokovacs

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extra hundred cc causing my throttle to hang??? -SOLVED
« on: December 23, 2022, 03:49:23 PM »
I've got an 1100 cali vintage motor re-converted to carbs and running a jetting recipe from one of the gurus in germany.

The bike runs better than any guzzi I've ever ridden, by about a million miles!! 

except, I'm just getting the throttle hanging at around 1/4 throttle. annoying.

The weird thing is,they are the exact same carbs, same springs, same throttle cables and throttle, same everything transplanted from my lemans 1000 and that never ever hung up.

(while troubleshooting this, I have obviously changed everything out, cables, carbs,throttle etc)

I can cure this by putting in some heavy springs, but I find that heavy throttle thing really unpleasant, and like I said, I had the exact same set up on my lemans and it worked great with the lighter springs.

I really don't want to touch the jetting or anything as the bike really does go perfectly, like an absolute dream.

So.... I dunno, is it possible the extra vacuum of the 1100 engine is causing the throttle to stick??? I really can't think of anything else.

Oh, and it's running the same sachse ignition unit as was on the lemans as well...

any thoughts welcome
« Last Edit: December 25, 2022, 01:39:46 PM by lazlokovacs »

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Re: extra hundred cc causing my throttle to hang???
« Reply #1 on: December 23, 2022, 03:55:20 PM »
My first guess is a bit of varnsih or some wear on the slides. With no idea of what type of carbs is is really a guess.
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Offline lazlokovacs

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Re: extra hundred cc causing my throttle to hang???
« Reply #2 on: December 23, 2022, 03:57:39 PM »
PHM 40s

I've swapped the slides out and even swapped the carbs out.

Online chuck peterson

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Re: extra hundred cc causing my throttle to hang???
« Reply #3 on: December 24, 2022, 07:09:05 AM »
I’m thinking of the difference in cable routing and length between a LeMan and a Cali Vintage…wag
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Re: extra hundred cc causing my throttle to hang???
« Reply #3 on: December 24, 2022, 07:09:05 AM »

Online Huzo

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Re: extra hundred cc causing my throttle to hang???
« Reply #4 on: December 24, 2022, 12:08:25 PM »
From a purely mechanical viewpoint I’ll ask this…
You say that the throttle “hangs” at about 1/4.

Is it on both slides or only one and with the engine not running, if you close the throttle very slowly with the nice light springs, does the slide movement progress totally smoothly down to the stop…I would take the air filters off and view down the carburettor bore to witness this.
I am trying to determine. If the “hanging” that you describe, is a mechanical sticking of the slide/s, or a mixture related issue.
Is the needle nice and firmly held in place with the circlip held down against the slide body. That is to say, the needle is not “floating” upwards with vibration, or failing to follow the slide downwards smoothly as you close the throttle.

I think it’s highly significant that the heavier springs cure the issue.
That suggests it’s not a tuning or mixture issue, but purely a mechanical binding (if you will), of the slide in the bore.
Again, both sides ?
What would happen if you take one plug out and run the motor, on one cylinder at a time ? Will the problem occur on both sides or is it restricted to one side or the other ?
I would love to be able to look down the bore of the carb while the engine is running and watch the behaviour of the slide during the anomaly.

Offline lazlokovacs

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Re: extra hundred cc causing my throttle to hang???
« Reply #5 on: December 24, 2022, 01:47:35 PM »
I’m thinking of the difference in cable routing and length between a LeMan and a Cali Vintage…wag

good point chuck....

Huzo - really interesting stuff, thanks for chiming in... my thought processes have gone in that same direction. I have a go pro, maybe I could film the carb from air filter side and see whats happening. I can make it happen on the centre stand

a better way of describing it might be that it sticks open, imagine being in town 1st gear and when I'm about to change up to second I close the throttle but the engine stays at 4-5k without dropping

Offline moto-uno

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Re: extra hundred cc causing my throttle to hang???
« Reply #6 on: December 24, 2022, 01:55:06 PM »
  It may be any of the above , but , my personal experience with running 40mm Dellortos on my Le Mans 2
after increasing the displacement to 992cc was that the light springs started hanging up very frequently . There
was no change in any thing ( cable routing , replacement , etc ) . Going larger simply overwhelmed the light
springs ability to close the throttle with haste . Get yourself a twist assist and problem solved . They were a
plastic paddle of sorts that mounted to the throttle and allowed you to rest the palm of your hand on the
throttle and ease the load on your wrist ! Peter
« Last Edit: December 24, 2022, 07:03:43 PM by moto-uno »

Online Tkelly

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Re: extra hundred cc causing my throttle to hang???
« Reply #7 on: December 24, 2022, 01:56:01 PM »
I had a similar problem on my SP,it turned out that the throttle cable was starting to fray and was hanging up in the sheath when I wanted to slow down.A new cable solved the problem.

Offline lazlokovacs

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Re: extra hundred cc causing my throttle to hang???
« Reply #8 on: December 24, 2022, 04:42:40 PM »
  It may be any of the above , but , my personal experience with running 40mm Dellortos on my Le Mans 2
after increasing the displacement to 992cc was that the light springs stated hanging up very frequently . There
was no change in any thing ( cable routing , replacement , etc ) . Going larger simply overwhelmed the light
springs ability to close the throttle with haste . Get yourself a twist assist and problem solved . They were a
plastic paddle of sorts that mounted to the throttle and allowed you to rest the palm of your hand on the
throttle and ease the load on your wrist ! Peter

thanks peter, looks like we're in a club of 2 then!!

I'll experiment with some stiffer springs, trying to find the lightest I can get away with

Offline lazlokovacs

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Re: extra hundred cc causing my throttle to hang???
« Reply #9 on: December 24, 2022, 04:43:36 PM »
I had a similar problem on my SP,it turned out that the throttle cable was starting to fray and was hanging up in the sheath when I wanted to slow down.A new cable solved the problem.

Thanks Tkelly, I've swapped out the throttle cables so it's not that

Online Huzo

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Re: extra hundred cc causing my throttle to hang???
« Reply #10 on: December 24, 2022, 06:08:05 PM »
good point chuck....

Huzo - really interesting stuff, thanks for chiming in... my thought processes have gone in that same direction. I have a go pro, maybe I could film the carb from air filter side and see whats happening. I can make it happen on the centre stand

a better way of describing it might be that it sticks open, imagine being in town 1st gear and when I'm about to change up to second I close the throttle but the engine stays at 4-5k without dropping
But does it drop one one or both cylinders ?
Also, here’s another thing.
As your spring extends, it reduces in effectiveness. As it nears maximum extension, the ability to close the slide becomes less.
If you get a lighter but longer spring, your pull will be about the same because you have compressed the spring more as a ratio of it’s (at rest) length.
But.
Because the spring is longer, the closing force will be the same near where the slide is down, as it is when the slide is at full open.
« Last Edit: December 24, 2022, 06:13:47 PM by Huzo »

Offline guzzisteve

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Re: extra hundred cc causing my throttle to hang???
« Reply #11 on: December 24, 2022, 06:46:43 PM »
Hey,  got some info for you.  If you are using the light ones on MG cycle site DelOrto #8550 and 0.7 spring tension@ 70mm there are a few stronger-------- it is the lightest then
#8532 is 1.1 @ 70mm &
#9389 is 1.4 @ 70mm

Just in case you want to go up with the pressure, I didn't look up stock, but I have DelOrto catalogs. Good Luck
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Offline lazlokovacs

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Re: extra hundred cc causing my throttle to hang???
« Reply #12 on: December 25, 2022, 01:39:24 PM »
PROBLEM SOLVED!!!!

thanks everyone, I took the very unscientific method of getting all my old dellorto springs together and squeezing them one by one until I came across a pair that were just a bit longer and stiffer than the ones that were in the bike and.... yes I now have the best running guzzi I've ever owned and the throttle is a hair stiffer than before but totally managable.

maybe I should have started with that before I swapped everything out.

merry xmas all, looking forward to riding the thing up into the mountains tomorrow

 :thumb:

Online Huzo

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #13 on: December 25, 2022, 02:11:00 PM »
Great Laz…..
Feels good doesn’t it…?

Offline moto-uno

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #14 on: December 25, 2022, 04:05:35 PM »
  Yahoo  :bow:  Peter

Online Tom

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #15 on: December 25, 2022, 04:10:56 PM »
Doesn't hurt to use dry slide lube on the slides, slide bodies, cables & cable pivot points.  Do it on a regular maintenance basis.  Oil change/tire change.
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Online Dukedesmo

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #16 on: December 25, 2022, 04:16:43 PM »
Wanting to lighten the pull, I changed the springs in the carbs on my LM2. I bought both the light and the medium flavours and tried the light first - throttle shut fine when engine not running but would hang when running, to the point where it was annoying and possibly even dangerous.


I then fitted the medium and they were fine and not noticeably heavier to the hand than the light so, job done.


It seems they can be (and were on mine) too light, yet others have fitted the light springs with no problem, I guess there are other variables at play?
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Offline lazlokovacs

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #17 on: December 26, 2022, 03:10:46 AM »
I wonder if all light springs are created equal??

Maybe some are genuine dellortos, some are pattern parts, or simply quality of steel is not what it used to be?

Offline Stevex

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #18 on: December 26, 2022, 06:13:35 AM »
I've had light springs fitted to my LM2 carbs since about 2014. I also removed the bell crank lever return springs. So far not had any problems with this set up and the Venhill ptfe lined cables make the action nice and light.

Online Dukedesmo

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #19 on: December 26, 2022, 12:08:29 PM »


Quote from: Stevex on Today at 12:13:35 PM
I've had light springs fitted to my LM2 carbs since about 2014. I also removed the bell crank lever return springs. So far not had any problems with this set up and the Venhill ptfe lined cables make the action nice and light.
___________________ ___________________ _

Just shows that there are several variables here; cables, carb wear etc.? because mine was hanging with the light springs, new cables and the bell crank springs still fitted.
Maybe the carb slides are worn, causing them to 'flutter? but it'd only done 12k miles so I'm not sure on that. It is now a 950cc, would any extra 'suction' be a a likely cause or is it the springs being different? FWIW, I bought the springs from Eurocarb (Dellorto UK) so I assume they are the correct parts...



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Offline lazlokovacs

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #20 on: December 26, 2022, 12:18:27 PM »
also using venhill top of the range cables

Offline guzzisteve

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Re: extra hundred cc causing my throttle to hang??? -SOLVED
« Reply #21 on: December 26, 2022, 12:31:51 PM »
If you look in DelOrto catalog #11& 15 it lists all stock springs & their tension at 70mm and part numbers
Like what I posted above, Then you know they fit. Lightest used on 30mm PHF, stiffer on 36mm & PHM's
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