Some carb balancing questions

[Fastmongrel]

Just checked carbs were clean and fitted a new pair of Venhill throttle cables I adjusted cable adjusters all the way in (front cylinder adjuster had been further out than rear) then wound them out 4 turns until the cable slack felt about right. Warmed up the engine removed carb balance pipe and fitted the Morgan carb tune. As soon as I started the engine again tickover went from a lumpy 1200 rpm to almost 3000 rpm thought I had pulled a cable out of carb so checked everything and restarted 2500rpm. Removed carb tune refitted balance pipe and revs dropped back down.

I didn't touch the cable adjusters or idle screw when fitting carb tune nor have I got any where near adjusting tick over.

What have I done wrong
I haven't touched the mixture screw yet
Is there any other adjustments I need to do
Why does fitting carb tune send revs up

Please help a thicko

Ps I have read the blue book

[bigred]

My first thought is that you have an air leak in the morgan carbtune assembly.
however Its amazing what can happen when you pull old machines fuel systems around. I had sticking acellerator cables that kept the throttle slide up. Then the rubber manifolds split due to old age. The choke mechanisims can let in too rich a mixture. There is a tube that is connected between the carbs that might help slightly balance the difference between the carbs initial settings.
All of the above happened after a few miles on a recommisioned 500.
If there is a split or air leak in the carbtune equipment you will notice a much lower vacuum on that side. In my case it was a zero reading !
A spray of easy start around the inlet manifolds and or the carbtune ( not the carb input) will show a change in revs
Im sure others will have some suggestions.

[Fastmongrel]

Sprayed the carb rubbers and it stalls when I spray rear cylinder airbox to carb rubber.

I swapped carb tune pipes about to check if there was a leak one side and the front cylinder pulls more vacuum however I connect it.

Would it help to run the carbs without the airbox connected.

[morini_tom]

Consensus is to keep a Morini on its stock airbox. Doing otherwise generally causes more trouble than gain.

An airleak which causes the revs to rise at idle will normally be between the carb and the engine (essentially bypassing the slide’s function as a throttle)

I wouldn’t worry about chasing a leak in the carb to airbox rubber- its function is to efficiently get clean air from the air filter to the carb. It won’t effect engine running even if it’s leaking a bit, and there isn’t a strong vacuum upstream of the carb so it doesn’t have much ability to pull air in that side (and if it does, any air still travels through the carb, so still gets metered by the carb.) you should balance the carb with whatever air filter and airbox connection you intend to ride with though as these will affect the required carb setting.

If it was running ok before you connected the carbtune it sounds more like a leak woth that- either a poorly fitting adaptor tube into the manifold, a split in one do the carbtune’s pipes or internal damage. If you have a mittyvac you can hook that up and check your carbtune can hold a vacuum and that it reads the same on both gauges for a given vacuum.

[Fastmongrel]

I haven't got a mityvac I was wondering if a 30ml syringe could produce a vacuum enough to move the carb tune rods. If I pull the syringe the same distance on both tubes I should get roughly the same movement on the guages.

If it turns out a carb rubber is leaking is there any way to fix the leak. New carb rubbers seem to be rarer than rocking horse poo

[3potjohn]

I managed to make some old and cracked carb rubbers to work by slathering them with threebond. They never failed but were replaced as soon as possible.

[Fastmongrel]

I have had the rubbers off the rear carb and they look a bit perished where the jubilee clamps sit. They also felt a bit flabby. I have tightly wound on a couple of layers of plumbers self amalgamating silicone rubber tape where the clamps go and that seems to have tightened things up. I might try winding the tape all round the rubber.

My knees and back meant I had to give up but I will have a go at front carb rubber tomorrow.

I tested the Morgan carb tune by connecting a new 30ml syringe to each pipe in turn. Pulling 20ml of air moved the rods to the 30cc hg so I think it's okay. Not much to go wrong with them no mercury to spill.

[Fastmongrel]

I have taken the airbox apart how can I go about sealing the airbox to carb rubber pipes they seem to be a loose fit into the bottom of airbox

[Fastmongrel]

Okay got bike ticking over still a bit lumpy but carbs are balanced. Now it's stumbling a bit when throttle is opened iirc this is a problem with modern fuel and needs a different size pilot jet.

https://www.dellorto.co.uk/shop/dellort ... parts/jet/

Is this the type of jet I need and also what size I have read of people fitting no 44, 43 or 42

[70sbikes]

Rubber parts here (not cheap, but good quality):

https://www.startgomma.com/en/magura-mv/moto-morini

I had an ever-so-slight stumble on my Mikuni outfitted 3 1/2 Sport. An Italian bike mechanic on Facebook personally messaged me the following:

All V-twins work better at off idle if not balanced perfectly.
I don’t know Morini but have worked with Ducati, Guzzi & Aprilia at dealer & national level, trained in Italy with Aprilia & Guzzi.
I’m assuming your engine rotates towards the front, if so try making your rear cylinder lead (very) slightly over the front cylinder.
It may cure your off idle stumble. If engine rotation is opposite then the front leads.

I followed his advice and set the rear carburetor idle a little bit higher than the front, then adjusted the cables so the slides lifted together. Stumble eliminated!

I'm expecting some flaming over this (which is why the mechanic DM'd me, rather than putting his advice where naysayers could throw in their $0.02 worth).

Cheers

Trevor

[Fastmongrel]

Very interesting I had noticed that adjusting the rear carb tickover screw seems to have more effect on the revs than doing same to front carb.

So just to make sure I am doing it right if I adjust/balance the carbs so that the rear shows slightly lower vacuum that means that carb is leading the front carb.

[Daddy Dom]

Well, it's a new one on me.
I renewed all my carb internals around 5 years ago and have always set them the same.
Once warmed up, I've had a trustworthy tickover around 1,300rpm but of course, the needle says anywhere between 1,000 and 1,500 rpm.

[Steve Brown]

That balancing tip is a new one to me as well! Not poo pooing it though-I'll try it out!
As for the question about pilot jets-there isn't one size that is right for every bike. It will depend on other things like engine condition and airbox or exhaust systems too. Also altitude and local fuel supplies etc. On my 350 sport I've settled on 43 for the pilot.

On other bikes I found a tip from Paul Compton worked best-This was to fit a throttle slide with a larger cutaway, this allows more air in at that just off idle position. It won't affect the idle itself. Used that now on 2 500s and a 350 with success.
Exhausts do make a difference. On my 500 Sei-v there is a standard set of front pipes and some pattern 'silencers' that are about 6" shorter than the originals. Although they are loud I think they are restrictive to the extent that a standard 120 main jet is too big and a 100 was need to get it to accept anything over half throttle.

[MickeyMoto]

I have just set up a pair of dell ortos from scratch. One tip was to screw out the throttle stop and screw in until touching the slide. This is done by raising and lowering the slide on the cable until the noise changes. The difference between the slide hitting the carb and the adjuster screw is noticeable. Once done, screw both in one turn and always adjust both to the same from now. Adjust the cables until the required slack is available, equal on both sides. Turning the throttle should give a single noise when throttle is released.

Turn the mixture / air screw out 1.5 turns.

Ensure ignition timing and valve clearances are correct.

Start engine and adjust both throttle screws equally to get about 2,000 rpm then adjust idle screws to get the fastest running, as per blue book.

Adjust idle to correct tick over speed adjusting both screws equally.

Bike should run ok.

Attach gauges and fine tune. I didn't bother with this stage!

The Morgan carb tune needs a restrictor tube in the pipe to the gauges.

[Al B]

Setting the carbs to be balanced at idle and then having one open fractionally before the other is how Haynes manuals recommend twin carb MG's, Triumphs, Minis etc should be set up. To my mind that made balancing carefully for idle seem a bit pointless

Twin SU manifolds normally have a balance pipe downstream of the butterflies, so the mismatch in flow could be cancelled out by drawing a bit from the other carb.