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Hi I need info on carb jetting,just fitted a new Dellorto PHBH 38BD 3340 ,Starts very easly now but bangs and misfirst when I shut the throtle,All help is good
Phill

Hi Phill

Deceleration back-fire is caused by fuel getting burnt in the exhaust pipe or silencer instead of in the combustion chamber. Potential causes (if apparently contradictory ...):

1. An air leak in the exhaust system, especially around the exhaust manifold, often combined with a slightly too rich mix. Check for leaks at both the exhaust and inlet manifolds, and also at the joint between the exhaust pipe and the silencer.
(However, if you're running a short silencer as on the 450 SCR, it's virtually impossible to stop some air leakage and the thing will bang back. DAMHIK.)

That done, check the plug. If it shows signs of running rich, reduce the idle jet - but only a little, no big steps. (I've also heard that raising the needle a slot helps. Sounds a bit drastic to me, haven't tried it, no personal experience).

2. Too lean an idle mix. This is why you hear so many modern bikes popping back like crazy on the over-run: pollution control means they're set up very lean. (The science is that a lean mix burns slower: this allows unburnt fuel/air mixture to be expelled down the exhaust pipe where it ignites).

Rule of Thumb to decide which is which:
• If it gives a loud pop or two as the motor is running down, it’s most likely got an air leak (and is a bit rich).
• If it gives a series of sputters/pops, that usually means it’s too lean.

Also:
3. Check the clearance on the exhaust valve isn't too tight. An open valve will facilitate the release of unburnt mixture into the exhaust.

4. From personal experience: some tankfuls of gas will bang back like crazy, others produce not the slightest wimper. My worst was coming into Rome at the end of the Giro with my 450SCR sounding like a a bad night on the Somme while here in France, I frequently get no banging back at all.

HTH

Craig

A 350 SCR with a 38mm carb? For me, that does not make any sense. If the engine is in factory condition, 32 mm are absolutely enough.

A remark adressing deceleration backfire (with closed throttle):

My everyday bike, a Moto Guzzi V7 classic (with Marelli EFI) showed this symptom from the first day after I bought her. She had already the latest available fuel amp stored in the ECU and was always kept in a good service condition.
An advantage of the Marelli injection system is that you can access the ECU by means of a very cheap OBD2-USB converter, made in China for older VW cars. A very smart german programmer adapted this device to the Marelli ECU by re-engineering after sniffing on the data transfer, and offers his code "GuzziDiag" freely to the Guzzi world. Very useful: It contains also auxiliary software for reading and re-writing binary data from and into the ECU.
A second smart US software engineer developped "TunerPro", a fantastic software tool to edit the ECU's control data field, in a digital format that could be re-written to the Marelli ECU by GuzziDiag's writer plugin. This fantastic work of two brilliant people gave me (and many other Guzzi owners of course) the opportunity to completely eliminate the Guzzi's backfiring. I managed this by setting the injected fuel amount in case of closed throttle starting from ~4000/min down to 1600/min to Zero! For me, this was a proof that not a lean but a to rich mixture caused the popping and backfiring.
Imho, this holds also for engines with carburetors.

cheers Hans

ducwiz wrote:A 350 SCR with a 38mm carb? For me, that does not make any sense. If the engine is in factory condition, 32 mm are absolutely enough.

Good point, Hans. Missed that ...

ducwiz wrote: <snip> ... gave me ... the opportunity to completely eliminate the Guzzi's backfiring. I managed this by setting the injected fuel amount in case of closed throttle starting from ~4000/min down to 1600/min to Zero! For me, this was a proof that not a lean but a too rich mixture caused the popping and backfiring.

Good story. Thanks for posting.

Aye Phill,

¿Por qué?

A 350 SCR with a 38mm carb? For me, that does not make any sense. If the engine is in factory condition, 32 mm are absolutely enough.

I have to agree, with Hans. Why? Unless you are planning to enter the machine into the 'Manx Grand Prix' for 2023, or speed records around the Montlhéry circuit.

Hope that you get the wee beastie to run and enjoy riding it.

Good health, Bill

ducwiz wrote:My everyday bike, a Moto Guzzi V7 classic (with Marelli EFI)
...
cheers Hans

Thanks for this info, Hans.

I lament the universal adoption of computers in bikes and cars, because they are ideal for causing undue frustration and expense for owners.

ops it's a 28 not 38
Thanks for all the info
will carry on playing
Phill

Ahhh! 28mm sounds more like it.

As for "will carry on playing"! Being a Philistine, I would bolt an AMAL 2928/300 on it!

Good health, Bill

Jordan wrote:
Thanks for this info, Hans.

I lament the universal adoption of computers in bikes and cars, because they are ideal for causing undue frustration and expense for owners.

I understand deeply , but with modern combustion engines and EFI you have no choice anymore. And we cannot defeat the anti-pollution rules and laws spread all over the world.
Many years ago I owned a Guzzi V65, kind of a predecessor to the V7classic, with carbs instead of an EFI, but still the Heron combustion system. Btw, this was used also in many Moto Morini engines, some Audi cars in the 1960s and early 70s, the famous Ford Kent engine, etc.
I can assure you, the V7 is essentially better in terms of engine response characteristics, fuel consumption, idle stability, starting behaviour. You never will achieve this quality by endless tinkering with carb jetting and setting, ignition timing etc.
Once you have learned the technology and realized that most of the sensors used for the Marelli EFIs stem from italian cars produced in masses, things get quite relaxed. There are many cheap second souce parts available, and the web keeps tons of basic and special information about EFI systems. And there are of couse a lot of comprehensive reference books about the topics available. If you are still willing to learn, EFI technology might loose most of it's scariness. OK, a brand-new Marelli ECU is presently offered for ca. 700-1000€. But, these devices are very robust, I never heard of failures. Used items are frequently offered on ebay and elsewhere. And, you even are not forced to purchase the special ECU labelled for your particular bike model. It's only necessary that the main type code is exactly the same. When I decided to buy a used spare ECU for my v7, it finally was a IAW15.RC from a Guzzi California 1100 i.e. In turn, I used the fantastic GuzziDiag and IAWReader/IAWWriter, and half an hour later I had a clone of my original ECU in my bike. It stayed there running perfectly, while the original item sleeps safely in the shelf.

Prices of some sensors:

genuine Moto Guzzi /Ducati second source retailers

TPS 270€ 35€
RPM/TDP sensor 260€ 45€
Air temp sensor 100€ 8€

etc.

A Marelli EFI is definitely a real advantage. Most of the EFI-operated Ducatis, even some HDs are equipped with Marelli stuff.
Afaik, with a japanese bike you are really lost ...

cheers Hans

If you can open this file you can see what I have been playin with
Phill
ps it's up for sale


https://drive.google.com/drive/folders/ ... sp=sharing