Carb synch hep needed

[tobydmv]

1980 Morini 350 strada w e-start

Hey guys,i'm making slow progress on this recent bike and could use a little help with the carb synch. I'm using a Morgan Carb Tune and cant say i've ever had this much trouble. It has metal bars instead of mercury inside the gauge. After some fiddling with the gauge i've managed to get the idles very even and used the air bleed screw to get the level high as possible. But when I open the throttles the front cylinder seems to loose the vacuum and the meter drops suddenly for that side. If I rev it pretty high the gauge will kind of catch up but seems like only after the slide closes back. The rear cylinder pulls right up to the top like a champ despite the fact its popping through the exhaust. So I pulled the slide, and its a 50, just like the other side. The main jet is 112 and idle is 50. The slide does not appear to be worn and the micrometer backs that up.

How does it run? Pretty good despite the carbs being out of synch. There is a problem with the rear cylinder constantly popping and backfiring a little. I think this is an exhaust leak and trying to get the exhaust collets set up correctly. I did the timing last week with the strobe light and its about spot on around 3k rpm.

[penman]

From what you describe, it sounds as if the front slide is lifting before the rear one. Everyone has their own favourite way to adjust this, I like the "finger in the intake" method. First, use the vacuum gauges to adjust the throttle stops for balance at idle. Now stop the engine and Take the rubber elbows off the intake side of the carbs so you can stick a finger in to feel for the point where the slide starts to rise as you gently open the throttle a tiny bit. Ajust the two cables until both rise absolutely together. If it feels "spot on", it will be very close. Now put the rubber elbows back on and check the balance again on idle and also on small throttle openings. If the throttle is off its stop, only adjust the cable adjusters. If it's closed, only adjust the throttle stop screws.

You might find a 50 pilot jet is too big for modern fuel - you might like to try 44 or 42 for more snappy off-idle response. By the way, remember that the idle mixture screw acts on the fuel, not the air on these carbs, that has confused a few people, me included!

I hope that's helpful and apologies if I'm stating the obvious!

Joe.

[harrymuffin]

If the vacuum disappears on one carb as you gently open the throttle then that carb is opening before the other and so you need to adjust the cable, either increase the slack on that carb or take up the slack on the carb where the column is high and has a high vacuum - slow to open. I tend to only adjust the one cable so that it does not get confusing. If you have the Morini instruction manual there is detailed information on how to set the carbs at idle and when the throttle is open. If you do not get the slides opening at the same rate then each half of the engine won't be doing the same work. Strange that you have asked this as I have just set my carbs up on a 500 after rebuild yesterday. I have known people use ball bearings to set their slides so that they both enter the carb at the same time making sure that the slide does not open so much that they enter the port, a bit fiddly and risky but that was the way they do it! Try setting up thee Webers on an engine without vacuum gauges or air flow meters.

[mgelder]

What Harrymuffin said.

You need to set the carbs at tickover *and* with the throttle slides pulled open slightly by the twistgrip pulling throttle cables; two separate operations compared to most modern multis where you just set the balance at tickover.

The most critical setting is probably the 'just off idle and coming on to the throttle slides' setting, so it pays to do the tickover setting, then the partly open throttle setting, then check the tickover again, then check the pilot screw settings again, and so on.

More here: http://www.realclassic.co.uk/techfiles/ ... gauge.html

[EVguru]

The VHB(Z) carbs don't have an air bleed.

They're really not too hard to adjust; https://youtu.be/TEdK7DNuNfM

[72degrees]

The VHB(Z) carbs don't have an air bleed.

They're really not too hard to adjust; https://youtu.be/TEdK7DNuNfM

Which reminds me that I need to drill a hole in the new rubber elbows and arrange blanking plugs for the 375/2C, having pinched the ones from that for the project. Those have very neat 'waisted' blanking plugs that just pull out and push back in. I suppose I'll go for just a very tight fit set screw on these or does anyone know of a supplier for the neat ones (they came with the original PHBH28s and elbows kit from Contact developments 35 years ago)?

The special Venhill throttle cables with adjusters up by the twistgrip make it a quicker job, as I don't have to remove the tank, but then to be fair they may actually need checking more often because of those. One day I'll get round to acquiring an auxiliary fuel tank to pop on, but with the 125H tank on the project it's fairly easy to get to the carb top adjusters anyway.

[EVguru]

The shaped plugs were an original Morini part, so NLM may have some.

I prefer to use a piece of Kunifer brake pipe and a fibre optic cable cap.

The Venhill inline adjusters are way too sloppy for my liking. The cables should be a slip fit into the adjusters.

['It must be a .....']

Scottoiler do a neat vacuum fitting stub/pipe for rubber carb inlets; contact them as all part/fittingss are available individually, trust this helps, regards Ian

[norbert]

I´m also tired of the problem adjusting the carb cables under the tank and made me this adjusters myself for the Dulcinea project (there is even less place under the tank).

k-DSCI0091.JPG (66.4 KiB) Viewed 14267 times

k-DSCI0246.JPG (20.09 KiB) Viewed 14266 times

norbert

[robint]

Nice .......any plans for production......??

[tobydmv]

Thanks for the help guys, the front cylinder was lifting much sooner than the rear. I undid the cable adjusters and evened up the throttle stop screws by using a small drift that just fit under the slide. The drift measures the slide opening in a way. I think a friend suggested drill bits would also work. Then I opened the throttle slowly and watched each drift start to fall as a further adjustment. Probably safer than the ball bearing but the same idea I think. I'll use the Morgan carb tune next.

While we are talking about carbs, is there an easy way to set the float levels? I dont understand the blue book. It says to set for 10mm when the float arm is in light contact with the needle. But does not mention if the needle is closed or opened position. The float arm also holds the needle so its almost impossible to measure the float without disturbing the needle. Can this be measure more accurately with the carbs on the bike, float bowls removed, and floats fully down/open? That seems like an easier way to measure.

I took your advice and installed 42 idle jets. The bike runs much better off the line now but seems to have a flat spot around 1/4 throttle opening. Its probably too rich because my float level is too high...? I tried opening the choke plungers and it kills the bike almost instantly which seems to suggest we are still too rich...

Thank you for the help and Merry Christmas.
Toby

[penman]

Hi Toby,

Glad to hear that it's moving in the right direction! Find Fig 29/B in the Blue Book, I think you'll find it clear. You need to stand the carb up on end as shown and then measure with the floats hanging down under their own weight. This is intended for the earlier black floats, but it still seems to work the same with the new white ones, which are a bit lighter. For most purposes I think you'l find this adjustment is fine.

I think this mid-range richness it a bit of a bugbear with modern fuel, and not just with the VHBZs, it affects Amals and Mikunis just the same! The smaller pilot jet will help a bit, but if not enough, you are into tinkering. You can try lowering the needles a notch and if that moves the problem to a wider throttle opening, you at least know your "too rich" theory is correct. If you have access to different atomisers, a leaner atomiser may be the answer. Just in case it helps you, here is a chart I made when at the height of my frustration with these carbs. For what it's worth, I ended up with CM atomisers, which work well for me. With BD, I had this hesitation and flat spot in the mid range - though I know it works well for some people! Good luck with it!

Oh, I should have said - holes lower on the atomiser make it richer, higher make it leaner and it's also dependent on throttle position of course. My experience is that hole position has most impact in the mid-range. As you approach full throttle it doesn't make much difference. Hmm, all this of course assumes that your air filter isn't clogged!

Joe.

[EVguru]

Trying to guess the centre of the float is a pain. Use the official Dellorto method.

https://www.dellorto.co.uk/wp-content/u ... tlevel.pdf

[tobydmv]

Thanks again guys, i'll try that pdf you provided Paul. The bike is coming along nicely and I rode about a mile yesterday on it just to enjoy the warm winter we are having. It does indeed have a sort of flat spot around 1/4 throttle and i'm hoping the proper float height will help but i'm running stock main jets and the atomizers are the sport ones according to blue book.

I have a bit of a dilemma with the stock throttle, I think its a Tomaselli unit and the electrics work but the throttle tube is plastic and the cable end slots are broken. The previous owner removed the throttle tube and left the switch part in place. Fitted a new cheap chrome throttle and went on. I'm wondering if I can make a throttle tube on my friends lathe or should I just scrap the whole thing and fit a new Tomaselli Daytona and a Domino starter switch? I'm currently running without a kill switch which is a little risky if the throttles ever get stuck open...