Pilot jet sizing

[TurboRestorer]

OK, I have been informed that the bike will run sooty black plugs using modern fuels & its best to reduce the size of the pilot jet from a 50 to either 45 or 43.
So which gives the best results on a 1981 350 Strada? Aiming for a pilot screw turn-out of 1 1/2 turns.

[davel182]

My 1982 Sport came with pilot jets 50, main jets 115, and atomizers 260BD. The plugs got sooty and pulling away was far from smooth. I changed to pilot 43, main 112, and atomisers 260K. Clean plugs and pulling away much smoother. However I tend to ride on the gentle side, so I couldn't comment on the acceleration / high rev range performance.

[TurboRestorer]

Strada has pilot:50 & main:112 so I might go for 45's as I prefer things a little on the richer side. It does stumble at pull off a little so smoothing that out would be good.
Thanks

Added: I have now sorted most of the jetting out & have a good stable idle using std needle position & atomiser, pilot jet #42 & 3/4 turn out on the screw & setup using colourtune, main is being adjusted & currently using #95 but need to go lower.
I am using Keihan silencers which could be heavily baffled! Current MPG is 68.

[EVguru]

The idle circuit has a fixed orifice (the idle jet), followed by an adjustable orifice (the idle mixture screw).

For a wide range of idle jet sizes, you can achieve any idle mixture you like from far too lean to far too rich.

The progression circuit has a fixed orifice (the idle jet) and a fixed demand source (the vacuum created by the slide cutaway).

The leaner idle jet is to correct for the over-rich progression mixture that results from modern fuels with their very high aromatic content.

I've gone down as small as 40 idle jets to smooth out the stumble.

[penman]

A little sequel to this thread, and many thanks to Paul who has, without knowing helped me with an Amal - related problem! Anyone who rides classic bikes will be familiar with hesitation or stumble coming off idle - as you start to open the throttle, the transition from the idle part of the pilot circuit to the progression part. Received wisdom (not always reliable) and my assumption (definitely not reliable!) had it that this was due to the mixture going lean as the slide starts to rise. The cure was to set the pilot mixture a quarter turn on the rich side of the "sweet spot". Wrong. With nothing to lose, I re-set the pilot mixture on my T140 Bonnie to a quarter turn on the weak side of the sweet spot. OK, cold idling was a bit weaker but as soon as the engine started to warm up, no hesitation at all coming off idle and in fact there is slightly improved running well into the mid-range. So I'm well pleased. Just waiting for the registration to come through for the Morini and I'll be ready to jump on this problem if it occurs!

Oh, I should say that there is a down side. It seems to lead to hooligan mode being accidentally selected. Drat.

[3potjohn]

I had size 50 pilot jets as standard and I could feel the bike stutter as it came over the pilot to main jet point. i have tried 43's and that got rid of the stutter but it fellt a bit strangled and much slower to warm up, which was not good in traffic at least in the first mile or two,so am going to try 45's next and expect this to be about right.At least they are simple to change out.
John

[nick16]

I totally agree with EVguru and maybe I can give a little contribute . Using an application of mine based on Arduino board and excell I graph the data (speed and time) obtained in an acceleration with third gear from about 2000 RPM to about 7000 RPM with standard setting (50,E17,260BD,115) on my 1976 3 1/2 Sport. In the photo this curve is the black line and I used it to compare with alternative settings. My goal was not to obtain the best maximum performance but to obtain a more regular erogation of power in the range mentioned , even if , at the end, better orogation means better performance.My best result , represented by red line , has been obtained with this setting :
38 idle
E24 (middle position)
260 Q
112
The choice of atomizer is not so critical as the needle . I tried almost all the possible combinations of position of 12 holes 0,8 mm of diameter on the atomizer but differences are very little.
With this setting erogation is more quick and regular (red curve is almost a straight line with a greater gradient) and the acceleration from 30 km/h to 90 km/h is about 0,8 second faster.
I wrote a post in italian forum you can see , even if clearly in italian at http://www.morinispecial.it/joomla/foru ... da-te.html
Excuse me for my bad english.

[TurboRestorer]

I am getting good results with a pilot #42 with pilot screws set at 3/4 turns out, set with colourtune, mid range is a little flat with std atomiser & needle position (might try needle in highest position) & using a main of #90 at the moment.
Slowly getting there but I cant use plug ceramic colour as a guide as it has perm. stained dark brown, did speak to NGK about this.

[Paz2112]

While on this subject I decided to look at my own carbs a little closer as I also seem to have a flat spot at around 5000rpm. To date I suspected that it was just in need of being balanced but now I am not so sure. When rebuilding my carbs previously in the year I simply striped them, gave them a sonic bath in 80degree cleaner and reassembled (carefully) using a seal kit from dellorto. (see my 350 sport restoration for details)

Yesterday I took them apart for a closer look and also went to http://www.motomoriniclub.nl/carb.html for the jetting for every model. The first thing I noticed is that my model of carb is not listed! I have a pair of VHBZ 25B S carbs with the following jets;

Front Cyl (idle 50, main 115, atom 260k, choke 50, needle E24, needle position 2nd from top).
Rear Cyl (idle 50, main 115, atom 260k, choke 50, needle E17, needle position 2nd from top)

I don't see this exact model and my jets dont match any other standard listed. Also the front carb needle is different.

Does any members recognise this model of carb and do you think the jets look correct for UK fuel/climate?

[EVguru]

Some of the letters refer to the presence of vacuum ports for a balance pipe.

Different needles definitely isn't right and don't rely on the Dellorto supplied jettings, I've found many errors over the years. The needle chart for VHB26-30 range for example. The dimensions of the needle use in a Motobi/Benelli Tornado 650 are quite obviously wrong. The tip of a needle can't be the same diameter as the stem!

[TurboRestorer]

Going by what you've said about the jet sizes, looks like carbs. from a sport model.
These carbs. are the same as I have fitted on a 350 Strada & original jettings would be idle #50, choke #50, needle E17, atomiser 260K & main #112 (#115 for sport).
In a previous post on this thread I have listed what I'm currently running.

DSCN3909.jpg (206.38 KiB) Viewed 22302 times

DSCN2825.jpg (135.18 KiB) Viewed 22302 times

[Paz2112]

Thank you. I suspected that the E24 needle was wrong. I also noticed that even though these carbs are fitted to my sport using the rubber connections, the carb has the internal sleeve from an older fixed mounting. In your picture you appear to have the plastic insert recommended for use with the rubber mountings. In your experience, does this have an effect?

[SoloFrenos]

I too have discovered at the weekend that my plugs are very sooty, manifesting itself as a misfire (interestingly I had no problems with Shell V power but suffered problems after filling up with BP's 97 petrol!) so am going to be playing with my jets. Has anyone had any experience of KMT jets (www.kmtproducts.co.uk) at £1 each (inc vat) they are quite a lot cheaper than Dellorto ones so was thinking of using them till i get jetting right then swap to the correct size Dellorto ones. But just wondering if any one has used these jets?

Cheers, Tony

[TurboRestorer]

Thank you. I suspected that the E24 needle was wrong. I also noticed that even though these carbs are fitted to my sport using the rubber connections, the carb has the internal sleeve from an older fixed mounting. In your picture you appear to have the plastic insert recommended for use with the rubber mountings. In your experience, does this have an effect?

I thought the inserts were for support for the aluminium casting, the casting has slots cut into them & if you tighten the jubilee clip too much you would damaged the carb. body. I realise the insert also restricts airflow, maybe required for 350 but not 500??

[EVguru]

The metal inlet manifolds go inside the carb body and there is a thin wall plastic insert for this. The rubber manifolds go outside the carb body and there is a thicker plastic insert to match the carb bore to the manifold.