Expert help needed!

[Guzziman]

After many years of wanting I finally have a Morini, a 79 Strada very original with only 23000 on the clock.

However, it has a problem. When I open the throttle, particularly when pulling away, it stutters very badly almost to the point of stalling sometimes until the revs pick up. Once on the move it goes well and the problem only occurs once it is warmed up, on a cold engine it is fine.
So far it has had carb overhaul kits fitted and the carbs balanced (several times). The ignition coil has been rewound and the flywheel remagnetised. A NLM ignition system and pick ups has been fitted and set up with a strobe, plugs and caps replaced (5K ohms, is that correct?) None of this has had any effect at all on the problem at all.
My next thought is to remove the heads and barrels to check valves, rings etc, but before doing this I wanted to illiminate all other possibilities, any ideas?

Nick.

[smotorboy]

I'd bet your carbs are just too lean coming off idle (low speed circuits)...

OH kits don't adjust the mixture they just replace what is worn and dried out. ...You need a local carb Guy to set the float height, check for dirty bleed circuits and/or air leaks, and re-jet if needed.....

Good Luck Nick,,, Carbs are simple ,,but unforgiving if off the mark.

Robert in California

[72degrees]

Odd. Often slow running and up to 1/4 throttle are a problem when cold and resolve once warm. It sounds as if the mixture may be too rich when hot so blocked jets seem unlikely. Incorrect jets or float height could do it. Are the jets, needles, slides and needle position standard? So it picks up OK when cold but no "choke" (starter jet) on? Does it start when really cold without needing choke and warm up very quickly?

If it revs cleanly at wider throttle openings and higher revs then idle jets and possibly slide cut away might be wrong but you may have checked all that already. I suppose its just possible that something in the ignition system is breaking down once hot, but you have changed everything - except the transnducers by the sound of it. Still odd that it revs OK. In my experience they don't like start or to rev well if the spark isn't tip top.

If you are worried about compression you could always do a compression test comparison hot v. cold, but I have no idea how many PSI you should be looking for.

I have a fair selection of jets (having experimented with many air filter and exhaust systems on tuned motors ) so I would have just bunged some smaller idle jets in and dropped the needles a notch by now just to see if that made it better or worse.

[smotorboy]

When you say it runs well cold I assumed that is with the choke/en-richer being on. If YES, then the mix is too lean when warm...

However, if it runs well with the choke/en-richer OFF. Then 72degrees is correct that the mix is most likely too rich when warm .

Good catch 72degrees....

Robert

[Harry]

Are the throttles balanced with vacuum gauges? Are the idle mixture holes clear and idle mixture set properly?

[72degrees]

I'm assuming the idle mixture was set at the same time as balancing using a u-tube manometer. Really badly adjusted slow running mixture screws certainly wouldn't help.

I rarely ride my Morini with even one 'choke' on Robert so I made the reverse assumption
It runs a little rich at the bottom end anyway. With the PHBH28s and pod filters on the way in and the 2:1 on the way out, carb setting has been very suck it and see.

It wouldn't do any harm to give the carb bodies a soak in an ultrasonic cleaning bath, but Dellortos are pretty simple with not so many tiny drillings as some Keihins. Don't get me started on dual-choke Teikeis.

[Guzziman]

Thanks very much for all the replies chaps, its seems that the majority suspect carburation which has been my own feeling for most of the time.
I have set the float hieghts as per the manual and experimented with mixture screws and needle hieghts, all to no avail. The balance has been set using gauges not manometers which I suppose may not be accurate enough.
A compression test seems a good next step, although there is an Italian bike speciallist not far away (Moto Corsa) so prehaps I should admit defeat and book it in!

Nick.

[brianh0_4]

Hi Nick,
I had a similar problem, also it made an odd noise with a closed throttle at speed. Carbs were correct, so took it to Stockwell Motorcycles (Yes it was a few years ago), the result a burnt exhaust valve.
Generally it ran OK, but not as 'sharp' as normal.
So check the compression, or wip the heads off and put some petrol down the ports and check for seepage.

Brian.

[EVguru]

I'd guess it's too rich coming off idle, which is why it gets worse as the bike warms up.

The idle mixture is set by the mixture screws in combination with the idle jets.
Progression mixture is set by the idle jets in combination with the slide cutaway.
The needle is parallel and has no effect till above about 1/4 throttle.

Modern fuel has a lot more volatiles in it, so can cause bikes with relatively poor atomisation at idle to run rich.

I went from 50 idle jets down to 43 and the stumble off idle pretty much went away. I had to do the same thing on my 500 Sei, but I've gone a little too lean on that (used jets I had to hand). It no longer bogs down, but does hesitate instead.

[Daddy Dom]

The ignition coil has been rewound and the flywheel remagnetised. A NLM ignition system and pick ups has been fitted and set up with a strobe, plugs and caps replaced (5K ohms, is that correct?) None of this has had any effect at all on the problem at all.

Hi, this thread may be of interest.

viewtopic.php?f=4&t=1476&p=6952&hilit=5k+ohms#p6952

Cheers,
Dom

[3potjohn]

I found I had different jets between my carbs-may have been a tuning mod.Didn't spot this at first. Is it possible to have the slides in backwards? Are the o rings OK? I also had a carb top screw not in all the way due to muck on the threds.Should've made an air leak i guess but in fact made no difference.
I've also had a fuel starvaion effect due to water in the fuel tank on another bike, which prevent accelleration on the motorway.