Author Topic: Carburetor problems  (Read 4536 times)

Offline F-22

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Carburetor problems
« on: April 24, 2018, 07:40:37 AM »
Hello, I believe I am having an issue with the Dellorto carbs. This is actually not on a Moto Guzzi, but on a Motor Morini (although the carbs are basically the same, 25mm square slides, similar to what the V35 had). I think I have to replace the slow/pilot jet, just not sure if I need a bigger or a smaller one. Currently, it is the stock "50", but there were some mods done to the engine during a rebuild (different cam, bigger bore), and while the bike had similar problems before, it seems to just be worse now.

The bike starts cold fine without any choke at all, I just give a little throttle. When it warms up, it runs rough at the first quarter of the throttle (up to around 4000rpm). Pulls really nice if I open it up, and without a problem at higher revs. If I want it to accelerate just slightly (pull the throttle a little) at low revs, the engine kind of hesitates before it accelerates. It has the most rpm if I screw the mixture adjustment screw all the way in (the rpm do not change much if I change the adjustment screw, but they rise at the last quarter of a turn and until it is completely in). The spark plug does not seem too bad to me (probably gets cleaned when the engine goes to higher rpms), but I have never tried to rejet any carbs and am no expert on this kind of stuff.



Should I go for a 45 or a 55 idle jet? I'd guess 45, but I just want to be sure before ordering the wrong jets...
« Last Edit: April 24, 2018, 07:43:21 AM by F-22 »

Offline Zoom Zoom

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Re: Carburetor problems
« Reply #1 on: April 24, 2018, 07:52:01 AM »
Based on your description, it sounds like it is running rich on the idle circuit, since you mentioned it starts easily without using the choke and bogs a bit off idle when you open the throttle easy.

If it were me, I would start with a smaller idle jet.

Is there anyplace you might be able to stop and have someone put an exhaust gas analyzer  in the tail pipe and get a reading. That would help you  make an educated choice.

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Re: Carburetor problems
« Reply #2 on: April 24, 2018, 08:20:11 AM »
Better forum to ask this question: http://www.morini-riders-club.com/forum/index.php
Charlie

Offline F-22

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Re: Carburetor problems
« Reply #3 on: April 24, 2018, 08:34:53 AM »
Thanks, but the Morini forum is not as active, and I think the question and answer would be the same for a small Guzzi or a Morini, they are very similar anyway (Italian Heron head V twins...).

If I screw in the mixture adjustment screw, does it make it leaner or more rich (in this case, I'd guess more lean, but I've read that on japanese carbs the mixture gets richer)?

I thought a smaller jet would be better too. I think I'll order a couple of 45's and try them out. Good thing Dellorto used the same 5mm jets on nearly all their carbs, so I sourced them relatively cheaply from a moped tuning online shop.
« Last Edit: April 24, 2018, 08:37:02 AM by F-22 »

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Re: Carburetor problems
« Reply #3 on: April 24, 2018, 08:34:53 AM »

Offline acguzzi

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Re: Carburetor problems
« Reply #4 on: April 24, 2018, 08:38:59 AM »
I'd suggest a problem with the cold start circuit, its probably leaky or corroded and not shutting off properly

Offline Chuck in Indiana

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Re: Carburetor problems
« Reply #5 on: April 24, 2018, 09:05:57 AM »
Quote
has the most rpm if I screw the mixture adjustment screw all the way in (the rpm do not change much if I change the adjustment screw, but they rise at the last quarter of a turn and until it is completely in).
I'm with ZZ. You are removing fuel when you turn the needle in.
This is instructive..
carb_jet_circuits by Charles Stottlemyer, on Flickr
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Offline Groover

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Re: Carburetor problems
« Reply #6 on: April 24, 2018, 09:08:15 AM »
Does the response change when you turn the low end needle in? You mention the rpm's go up, which confirms it's leaning out (when turned in), buy how does that affect the throttle response? Does it lighten up the hesitation, making it somewhat better? How is the response if you ease into the throttle?

I'm also leaning towards the low end being too rich at this time. You may also need to tinker with the atomizer. Those, I believe when you go larger in number, will lean the mixture if I remember correctly.

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Offline F-22

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Re: Carburetor problems
« Reply #7 on: April 24, 2018, 09:18:32 AM »
Does it lighten up the hesitation, making it somewhat better? How is the response if you ease into the throttle?
I had the screws 1/2 turn out before, and it was very similar. Then tried it all the way in (and left it like that for now).
Thank you all, I have ordered a pair of 45 size jets, and hope they come by the end of the week so I can test it out.

I have a bunch of various Dellorto carburetors, but of course all I've checked have a 50 or significantly larger jet. If the 45 jets do not fix it, perhaps I'll test out some atomizers (I definitely have a few different ones). I thought those only affect the main jet.

I also have a transparent float bowl, and it's probably worth checking out the float height when I'm replacing the jets.
« Last Edit: April 24, 2018, 09:19:31 AM by F-22 »

Offline Wayne Orwig

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Re: Carburetor problems
« Reply #8 on: April 24, 2018, 09:21:23 AM »
Not all DelOrto's adjust the fuel with the mixture screw. Some adjust the air.
What model is it? If the screw is on the engine side of the carb, it is likely adjusting fuel, as most do.
As mentioned, first make sure the enrichener is sealing. You may have a high float level.
Don't tweak the jets until you know it is right.

 
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Offline Groover

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Re: Carburetor problems
« Reply #9 on: April 24, 2018, 09:31:37 AM »
I think I was wrong on the atomizer. There is more to it, height etc. Check this Dellorto Manual for more details. I think around page 14 is where that is explained.

http://danskkartingcenter.dk/dellorto_motorcycle_carburetor_tuning_guide.pdf

Good luck.




 
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Offline F-22

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Re: Carburetor problems
« Reply #10 on: April 24, 2018, 09:34:33 AM »
It's a VHBZ 25 HS, and the screw is on the engine intake side (not at the air filter side). If I'd flip the choke levers, the engine would die (I doubt I could even start it cold with the choke on, starts on the first kick if I just slightly pull the throttle).


Thanks for the guide, I did not think there were such publications about Dellorto carburetors in English.
« Last Edit: April 24, 2018, 09:35:52 AM by F-22 »

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Re: Carburetor problems
« Reply #11 on: April 24, 2018, 06:32:47 PM »
Do you have the hose that connects the two carburetors in place? This can effect mixture. The VHBs on Guzzis do not have this port and there are other differences as well. Guzzi and Morini are different enough that what applies to one does not necessarily apply to the other, even with similarities in the design.

If you have an original "blue cover" manual, the jetting specs. for each model are on page 26.2. 

Thanks, but the Morini forum is not as active, and I think the question and answer would be the same for a small Guzzi or a Morini, they are very similar anyway (Italian Heron head V twins...).
Charlie

Offline johnk

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Re: Carburetor problems
« Reply #12 on: April 25, 2018, 07:45:54 AM »
It certainly sounds rich to me but before going into the pilot circuit I suspect the choke plungers might need replacing. It's a brass cylinder with a rubber sealing surface at the base. When new the rubber is flat. After some time it gets deformed and hardens so it can't fully shut off the choke.
I'd recommend starting there.
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Offline F-22

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Re: Carburetor problems
« Reply #13 on: April 25, 2018, 08:26:55 AM »
Do you have the hose that connects the two carburetors in place?


If you have an original "blue cover" manual, the jetting specs. for each model are on page 26.2.
Yes, I do have that hose. But I also use those ports for synching the carbs, and it still runs rough as I seperate and connect them to the gauges. I do have the blue book, but the specifications are for the models up to around 1980. I use a different camshaft from a very late model (90's). I suspect they had smaller jets, but there is even less literature about the late models (in the book, there is the 350K US model that is specified for a 44 pilot jet). I even have an Engilsh A4 version of the blue Morini book, with larger photos (can send in PM if anyone needs it).


I have tried pushing down (in) the choke levers, but it did not change the running. If I flip them up just slightly when the engine is warm, the engine dies, so I think those are working. I really hope 45 jets make a difference (they should come tomorrow).
« Last Edit: April 25, 2018, 08:28:54 AM by F-22 »

Offline Chuck in Indiana

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Re: Carburetor problems
« Reply #14 on: April 25, 2018, 09:20:50 AM »
It certainly sounds rich to me but before going into the pilot circuit I suspect the choke plungers might need replacing. It's a brass cylinder with a rubber sealing surface at the base. When new the rubber is flat. After some time it gets deformed and hardens so it can't fully shut off the choke.
I'd recommend starting there.

A good idea, but in my experience, a leaking choke plunger makes the carb rich *everywhere.* Changing the idle needle doesn't do much, also. Don't ask me how I know..  :smiley:
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Re: Carburetor problems
« Reply #15 on: April 25, 2018, 09:47:09 AM »
A good idea, but in my experience, a leaking choke plunger makes the carb rich *everywhere.* Changing the idle needle doesn't do much, also. Don't ask me how I know..  :smiley:


Yep I learned all about carb jetting before I figured that one out!
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Offline Groover

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Re: Carburetor problems
« Reply #16 on: April 25, 2018, 01:11:33 PM »
You could also try lowering this needle a notch (or raising it if that makes it worst)



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Offline F-22

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Re: Carburetor problems
« Reply #17 on: May 04, 2018, 05:30:52 AM »
The 45 pilot jets finally came yesterday. The engine runs a lot smoother now, and the pilot jet adjustment screw is around where it should be (about 1 - 2 turns out on both carbs). I did not think it would have such an impact. Perhaps I should also try changing the needle position, just to see if it would be a further improvement. I always thought the pilot jet is just for idle, and the main jet takes over as soon as you pull the throttle.

Thanks for the help!
« Last Edit: May 04, 2018, 05:31:46 AM by F-22 »

Offline lazlokovacs

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Re: Carburetor problems
« Reply #18 on: May 04, 2018, 07:42:07 AM »
sounds like you're on the right track   :thumb:

I would have suggested 45s and then progressively try leaner jets until it gets worse and then go a step back,

or just find something that works well enough (like the 45) and ride it!

mixture screw fine tunes your mixture with the slides fully closed, so that's when you're at idle and and also when shutting off the throttle when engine braking.

idle jet will affect mixture for most round town riding, ie small throttle openings.

Eg In London traffic I don't think I'm ever really off the idle circuit except for occasionally giving it the big'un to get past/round/away from an idiot....

YMMV

Offline Chuck in Indiana

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Re: Carburetor problems
« Reply #19 on: May 04, 2018, 07:43:25 AM »
The 45 pilot jets finally came yesterday. The engine runs a lot smoother now, and the pilot jet adjustment screw is around where it should be (about 1 - 2 turns out on both carbs). I did not think it would have such an impact. Perhaps I should also try changing the needle position, just to see if it would be a further improvement. I always thought the pilot jet is just for idle, and the main jet takes over as soon as you pull the throttle.

Thanks for the help!

No no, it's not at all that simple. The picture I posted gives a very good visualization of the forces in play.
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Offline F-22

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Re: Carburetor problems
« Reply #20 on: May 04, 2018, 01:18:20 PM »
idle jet will affect mixture for most round town riding, ie small throttle openings.

Eg In London traffic I don't think I'm ever really off the idle circuit except for occasionally giving it the big'un to get past/round/away from an idiot....
Yes, this is a Morini "Kanguro" (a lot like the V35/65TT Moto Guzzi), so an old enduro/dual-sport. I've bored it for 400cc pistons, and while it accelerates really fast now, the 6 speed gearbox is so overkill. I could start in second and shift to fourth pretty normally.


So, for town riding, I do not need much throttle (has quite a bit of torque for such a small engine), and with a larger pilot jet it was really uncomfortable to ride it.
« Last Edit: May 04, 2018, 01:19:01 PM by F-22 »

 

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