What's causing loss of power?

[jerry atric]

My GTS has a 1" Amal Carbas specified in Mario Colombos's book. However, he also specifies 27mm  (1 1/16)Del Orto after 1935.

My engine bogs down when opening throttle and only manages 45mph flat out.Cuttout is number 3 which is correct. Timing and valve timing are correct. Any ideas please? I'm a bitt stumped apart from fitting a bigger carb.

[Dave Knaack]

The bigger carb wouldn’t make a significant difference, the bike should manage a much better speed with the carb you have.  There’s another problem somewhere, I’d be looking at the mag or timing.  Either ignition timing or valve timing 

Sent from my iPhone

On Dec 17, 2025, at 4:54 PM, jerry atric <baconsl...@gmail.com> wrote:

My GTS has a 1" Amal Carbas specified in Mario Colombos's book. However, he also specifies 27mm  (1 1/16)Del Orto after 1935.

My engine bogs down when opening throttle and only manages 45mph flat out.Cuttout is number 3 which is correct. Timing and valve timing are correct. Any ideas please? I'm a bitt stumped apart from fitting a bigger carb.

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[John O Regan]

Hi Jerry,

The brass jets oxidise from the ethanol in the fuel and that reduces the flow 

normal cleaning will have little effect on the oxides, I resorted to using the cleaning reamers for the oxy torch on one carb

John

On Wed, Dec 17, 2025 at 10:54 PM jerry atric <baconsl...@gmail.com> wrote:

My GTS has a 1" Amal Carbas specified in Mario Colombos's book. However, he also specifies 27mm  (1 1/16)Del Orto after 1935.

My engine bogs down when opening throttle and only manages 45mph flat out.Cuttout is number 3 which is correct. Timing and valve timing are correct. Any ideas please? I'm a bitt stumped apart from fitting a bigger carb.

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[PATRICK HAYES]

are you sure that the spark advance curve is working to full range?  Failure to make full advance is classic for bogging performance. 

 

Patrick Hayes

Fremont CA

 

On 12/17/2025 2:54 PM PST jerry atric <baconsl...@gmail.com> wrote:

 

 

My GTS has a 1" Amal Carbas specified in Mario Colombos's book. However, he also specifies 27mm  (1 1/16)Del Orto after 1935.

My engine bogs down when opening throttle and only manages 45mph flat out.Cuttout is number 3 which is correct. Timing and valve timing are correct. Any ideas please? I'm a bitt stumped apart from fitting a bigger carb.

 

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[RICHARD YAMANE]

What Amal carb do you have?

Rick Yamane
Importer of Vape/Powerdynamo ignition systems and Polisher of bikes at Motion Pro

On 12/17/2025 2:54 PM PST jerry atric <baconsl...@gmail.com> wrote:

 

 

My GTS has a 1" Amal Carbas specified in Mario Colombos's book. However, he also specifies 27mm  (1 1/16)Del Orto after 1935.

My engine bogs down when opening throttle and only manages 45mph flat out.Cuttout is number 3 which is correct. Timing and valve timing are correct. Any ideas please? I'm a bitt stumped apart from fitting a bigger carb.

 

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[Alan Comfort]

So many things to cause a loss of power. Could be a single item or a combination of things that cause our old bikes to slow down. A compression test can tell you something. If it is low, say less than 75 psi, then a leak down test could point out the culprit(s). A worn cam and/or cam followers can cause a loss of power. The valves might be opening at the right time, but are they opening fully and closing at the right time? If there is low power or erratic running at all throttle positions and the spark plug is neither black and sooty nor burnt , then it is probably not a carburetor issue. The idle jet controls the mixture when the throttle is closed, the cut away controls the transition from closed to partially open, the needle controls the mid range mixture and the main jet controls  the mixture at wide open throttle.  An incorrect float level will also have an impact on the fuel/air ratio. and power delivery. If you are using an air filter, that will rob some power if the element is inappropriate as in a K&N with too much oil or the intake duct is too long. Try running without the air filter. If the bike goes faster at 3/4 throttle than it does when wide open, try a smaller main jet.Inspect the muffler to ensure that there is not a big carbon build up. Try running without the silencer. Baffles have been known to break loose and impede the exhaust.

Try one thing at a time, starting with the simple and easy. No joy? then it might be time to open up the cam chest and inspect the cam lobes and followers The compression test and leak down test might point toward an overhaul of the piston/rings/valves/seats/guides and head gasket.

An easy test of the ignition timing is to use a strobe light to ensure that the magneto is advancing to spec.

It is a process of elimination but do not expect your GTV to shred its tires or go 80 mph uphill. it should easily cruise at 65 on the flat, but expect to slow down on an uphill grade. Remember that it makes about 18 hp on a good day.

My Falcone had a gradual loss of power this year and it turned out to be a worn out cam follower. A new follower transformed the bike.

Alan in Roberts Creek

On Wed, Dec 17, 2025 at 2:54 PM jerry atric <baconsl...@gmail.com> wrote:

My GTS has a 1" Amal Carbas specified in Mario Colombos's book. However, he also specifies 27mm  (1 1/16)Del Orto after 1935.

My engine bogs down when opening throttle and only manages 45mph flat out.Cuttout is number 3 which is correct. Timing and valve timing are correct. Any ideas please? I'm a bitt stumped apart from fitting a bigger carb.

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[jerry atric]

Many thanks for the replies, Guys. The carb is an  Amal 276DO/1DV (1 inch)

Some interesting advice, Hopefully I'll be back with an answer.

Alan, its a GTS so less poke than a V but my previous S would certainly do a bit more than this one!

[RICHARD YAMANE]

Those are pretty simple carbs. I don't think there are any "secret" passages that like to plug up.The idle mixture screw controls air so if you turn it in or clockwise, it richens the mixture. If it runs better closed or excessively open that will be a clue as to a direction to go with jetting or needle position. Also keep in mind, just as the older Delorto's the choke is pulled open so once warm the choke lever should go full to the tension side.Did the bike ever run well in this configuration?

It does sound to me like the mixture is going weak as the carb opens, since you haven't complained of any roughness or misfiring (unless I missed it).

 

Rick Yamane
Importer of Vape/Powerdynamo ignition systems and Polisher of bikes at Motion Pro

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[Andrew Nahum]

‘Bogs down’ when opening throttle implies leaning out.  Before changing anything major  check the fuel flow.  Undo feed pipe at carburettor end and convince yourself that you have a free flow down to it. Are your fuel lines rubber or metal pipe?  Modern fuel can make old flexible fuel lines crumble and expand inside.  I’ve had to replace several with modern fuel resistant piping. Rust and debris can also accumulate around the tank feed exit or in metal pipes at the bends. As a temporary check, blow back up the fuel line with compressed air (I’ve used CO2 from those pocket size bicycle tyre ‘pumps’) and loosen filler cap.  Don’t blow up a fuel line with your lips like we used to do!  Modern gasoline is toxic. 

Check float chamber - is there any debris at the bottom?  If so it is probably a result of tank rust and breakdown of the tinning.  It will travel further into the carb body. 

Check the feed passage from the float chamber into the carb body.  It can be blocked with chalky metal oxides or debris.  Agree with other listers that jets can be blocked and may need cleaning and eyeballing.  But so can the passage that feeds the main jet.

This is often overlooked. You can’t eyeball it sadly but can poke with bent wire and partly verify by putting fluid into float chamber and watching how it emerges in carb body at main jet.  (Has to be off the machine).    And all the passages and drillings should be checked because the body metal is poor stuff and easily makes oxides or salts. 

You could put the whole partly dismantled carb in an ultrasonic bath.  Then blow through .

 How is float height?When float chamber is full you can just see the fuel meniscus in the needle jet, but it can’t be eyeballed directly when installed on a Guzzi ‘slicer’.!  Where is the main needle set? There should be 3 or 4 grooves where the clip on the slide holds it with top one being leanest. 

On 17 Dec 2025, at 22:54, jerry atric <baconsl...@gmail.com> wrote:

My GTS has a 1" Amal Carbas specified in Mario Colombos's book. However, he also specifies 27mm  (1 1/16)Del Orto after 1935.

My engine bogs down when opening throttle and only manages 45mph flat out.Cuttout is number 3 which is correct. Timing and valve timing are correct. Any ideas please? I'm a bitt stumped apart from fitting a bigger carb.

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[John O Regan]

Hi All,

On the 276 the pilot jet drilling is integral to the jet block, which must be removed to clean the passageways

heat the carb body with a hot air gun or dip in hot hot water, the block will push out once the big nut is unscrewed from the body

The pilot outlet mates up to a drilling in the carb body that exits on the engine side of the slide

If the jet block .

Old 276 type carbs the mazac bodies can lose their tight fit around the jet block, this results in being non responsive to pilot adjustment and spitting back through the bellmouth

I have seen quite a number of bikes go on fire if the engine kicks back when starting!!!! fit a gauze to the bellmouth it reduces the fire risk

John

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[jerry atric]

Many thanks for all the tips and suggestions. I had the bright idea of opening both fuel taps and the stumble from pilot through cutaway seems to be cured.

However, fuel is spitting from the air intake (I think it's the pilot airway shown) Lowering the float level makes no difference. You would think the cause would be valve timing but thats seems OK but I will check more thoroughly. Any other ideas please? Thanks guys.

[Alan Comfort]

Fuel spitting you say? After chasing down all the usual suspects for this progressively worsening issue with my Falcone, i traced the issue to the pending failure of one of the cam followers. The first symptom was the decompression lever adjusted to the max and not working properly. The next symptom was the exhaust rocker arm adjuster maxed out. A valve opening/closing check indicated that the intake was spot on, but the exhaust was not even close. When I opened the timing cover, all was revealed. Another few hundred miles in this state might have ended in a catastrophic (and expensive) failure. This was an easy fix and not very costly. These parts are considered to be wear items and need to be replaced from time to time. Parts are readily available. I repaced both followers. The old Falcone is now running better than ever and the best part is that I did not have to waste countless hours fettling the carb or messing around with the ignition. It takes less than an hour to open the timing case to have a look.

Alan in Roberts Creek

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[jerry atric]

Thats interesting. I'm no stranger to the timing chest on these, I'll have a look, thanks. The engine has just had a professional rebuild before I bought it butt usual rules apply hey?

[Alan Comfort]

Cam followers are often overlooked when engines are "rebuilt". New piston rings and lapping in the valves are often described as an "engine rebuild". AAARRGGGH!

When pulling the timing case cover, I find it best to have both valves full closed and before fully separating the cover from the case, slip a thin flat bar in to hold the cam gear in place. This will keep the cam. timing gear and followers in place and will ease the process of re-assembly.

Ciao, Alan

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[John O Regan]

Hi Jerry,

Typical behaviour of a 276 with a worn bore/ slide combination.

The hole in the inlet tract is for the pilot and main atomising air 

The pre war 76 carb had the air drawn in through 4 holes around the base of the main body, I think the pilot air went in at the pointed end of the air screw

War dept wanted the engines to have air filters so the redesign placing the atomising air inlet in the main bore...

Does the engine respond to adjustment of the pilot air screw? When the carbs get worn, screwing in the air screw can help with slow running

When I was an impoverished youth I could not afford much so had to put up with crapped out Amals ! , 50+ years later they are much worse LOL. 

No wonder I am a Mikuni fan

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[jerry atric]

Thanks, John.

Like you, after 50 years of various Amals, I accept that they work on a wing and a prayer, at best😂. I found a perfect sealing float needle in my stock and have fitted that but not tested yet. I read a long thread on  War Dept. Nortons where a couple of owners were experiencing the same symptoms as me with NOS carbs.T hey came to the conclusion that you have to live with it but they surely can't be that bad?

[Don West]

There is a guy that can overhaul most old Amal carbs.  Look up Joe the carb on Facebook.   

Don  

https://groups.google.com/d/msgid/guzzi-singles/1a2a5aa0-7133-4a3c-b8d1-f1b1ccc5fa81n%40googlegroups.com

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[jerry atric]

Thanks, Don. I've seen his work before, he's good. I need to do some more investigation after Christmas first.

[guzz...@aol.com]

As an owner of several Brit bikes with Amals even the most worn out will still work. I have also had Amals re-sleeved by a company out west, the name escapes me at the moment. Might be Lund Machine Co. ?? But in the case of my Goldstar GP carb I replaced it with a Mikuni so I can ride the bike more often.

Tim

https://groups.google.com/d/msgid/guzzi-singles/540462910.5874368.1766567068765%40mail.yahoo.com

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