low to mid range flat spot

[barney 66]

Hi all, I’m sure this has been done many times, but I have just been out on the first ride of the new to me 1983 31/2 sport,
New NGK plugs, air filters standard with standard air box, standard exhausts carbs cleaned all jets cleaned and passageways carb cleaner and airline, can’t remember jet sizes but same on both carbs, floats fine ( no fuel in them ) float heights set and timing checked all Ok.
Came to first island about 4000rpm checked, clear so opened it up coming off the island and I thought somebody switched it off seems worse between 3-5000rpm but more noticeable when you roll off throttle and on again.

I was surprised at this as my 76 Strada doesn’t do any of this and I thought the sport would go better, forget to mention fresh E5 with sta-bil fuel stabiliser in it, on return removed plugs and there dark chocolate/ a bit of soot, any ideas please, bikes a nice ride with the flat bars I now have on but this puts me off a bit

Thanks for looking
barney

[barney 66]

Forgot to mention, new choke rubber as well

[MickeyMoto]

Like here very recently...

https://morini-riders-club.com/forum/vi ... f=4&t=8239

[George 350]

Hi Barney,
Well known issue on the Sports. My belief is it is caused by the very late inlet cam closing, meaning a significant amount of the inlet charge goes back over the main jet and then back again, hence why it goes so rich. As you say, the Strada with a milder cam doesn't do it.
Several 'solutions' - no, not really all are successful - include smaller pilot jets, larger slide cutaways, lower float level dropping the needles a notch and larger plug gaps. Any (and all?) will help minimise the issue.
Very careful carb balancing ( vacuum gauges, not drill bits under the slide!), idle mixture not too rich can help too.
Me? I often think it is why the 350's were fitted with a 6 speed 'box so you could downshift and get the revs above the issue and into the fun range.
Hope this helps,
George

[barney 66]

Thanks George very informative, carbs were balanced with a meter on the rear as there are no ports in the inlet rubber unlike the strada, so the tick over should be fine but when come off that it was done with wire and carefully adjusting cables so they feel spot on but not the best way I know, will tear them apart tomorrow and check all jet sizes and go through what you have said,
Thanks again
barney

[3potjohn]

My 83 K1 never does this. Had it since 2007.
Main jet 112. Let us know jets, needle type and position when you get the chance. Mine did have dissimilar atomisers initially. Maybe a mod for nipping around the Colosseum?
My atomisers were 260K but I drilled them to make them BD which I think are nla, with a 0.8 mm bit. Pilot jets dropped to 45.
So reliable I have made no other notes.
My PHBH carbs on another Morini have extensive notes.
Deffo no air leaks?

[barney 66]

Update, Ok firstly thanks to all that replied, read through other Threads and took carb off this morning and Jet sizes are as follows
Choke jet 50
Pilot 43
Atomiser 260 BD
Main jet no marking, so if anyone can tell me what it should be I will see if I can find an appropriate drill bit to try
Needle E17 set in mid position

also new intake rubbers so no air leaks

Thanks again
barney

[MickeyMoto]

Have you checked the float heights?

Have you synchronised the carbs? I mean not only getting the manometer levels equal but also rolling on the throttle so both columns track together? ie the throttle cable adjustment. I found this helps a lot.

I presume valve clearances are correct and ignition timing is spot on.

[barney 66]

Hi Micky, valves adjusted spot on rear was slightly tight timing spot on with strobe, no take offs on my inlet rubbers to put vac gauges so I use an Su type on rear of carb, at tick over spot on equal, the cables I have adjusted using wire and feeling the free play and listening to them shutting, I’m certain they are near to spot on as I can get.

Might try dropping needles this morning

Thanks barney

[barney 66]

Sorry float heights were also spot on with new needles will recheck as I reassemble this morning again

[barney 66]

Googling tells me that a 115 main jet should be .0455” I have a .047 drill bit that goes in easy so maybe the main jet a bit worn, interested in comments on this please, would this make enough difference ?

Thanks again for looking

[norbert]

Even New original Dellorto Jets should me measured. They use to be bigger than what is stammen on it

[3potjohn]

Main jets from Eurocarb are not too expensive.

[barney 66]

Thanks for all replies, ordered 112 mains and new pilots as well, I will report back after they arrive got back together and balanced

[MickeyMoto]

Hi Micky, valves adjusted spot on rear was slightly tight timing spot on with strobe, no take offs on my inlet rubbers to put vac gauges so I use an Su type on rear of carb, at tick over spot on equal, the cables I have adjusted using wire and feeling the free play and listening to them shutting, I’m certain they are near to spot on as I can get.

Might try dropping needles this morning

Thanks barney

1983 should have the VHBZ carbs with a balance pipe between the carbs. This is where the carbs are balanced. If not, then I wonder which model bike you have. Does it have a tail piece?