'66 Monza's carburetor - help

[Chuck]

I am working on my '66 Monza's carburetor. The bike came with Dellorto UBF24BS that the book says is the correct instrument. The slide "70," needle "E11", and all the jets are also correct as per the book. The carb body was badly bent from being over tightened and leaked a lot of air making the bike hard to ride because the motor would not slow down after the throttle was turned off and the motor would not idle without racing. I machined the carb body mount flat and got a new rubber manifold but no improvement. I sent the carb to Domiracer to be rebuilt because they had a new carb body and slide. The carb came back with different slide, "60" and needle is a "E17". The result is a good running bike but I think it runs lean, it has a whitish looking plug and I hear small poops from the exhaust on deceleration.

Starting isn't easy but the technique I have developed - tickle the carb and then kick the motor over several times with the throttle open and with the ignition off - find the spot just after compression - turn the bike on and kick with the throttle being open about 1/2 way and the motor starts up running nice and fast. After it starts I have to keep the revs up or it will die. The choke doesn't seem to do anything open or closed. After the motor warm up it runs nicely and will ideal and I turn it off with the on/off switch. I haven't tried starting the bike hot yet. Should I raise the needle or change the jets or is there something else I am not thinking of? I also have the original slide and needle.

I have also thought of changing the carb to something newer and hopefully better. Because my legs aren't any good I need a bike that starts easily. Some people have recommended changing the carb to a new Amal or a new Dellorto. The Amal would be easier, I was told that I would just have to make the manifold lug wholes little larger to fit the lugs on the cylinder. Someone would have to tell me which Amal to get and what the jetting should be. I think an Amal for a British 650 with two carbs should be close. The Dellorto would be more difficult because I need to find a manifold and I would also have to again rely on advice as to which carb to get and the jetting. What do you think is my best choice?

Chuck

[JimF]

Real quick: I have a Dellorto SSI with a choke and have never found the choke to have any affect.

The tickler serves the same purpose, that is to enrichen the mixture for starting.

I think the DellOrto SSI is one of the simplest of carbs to understand, having less jets and adjustments than a Mikuni that feeds my second 250. The theory and application of the carb is so basic and primitive that it is easy to tune. That being said I struggle to get the SSI to work with any air filter while the Mikuni is all perfectly dialed in with an air filter.

As you know, changing the entire carb can set you back as you try to figure out all the jetting across the entire throttle range. You have to decide if you should take a reasonably well setup carb and set it aside for a new adventure in carb tuning.

My Mikuni just attaches with a tight fitting fuel-proof hose to a generic Ducati intake manifold. A call to Sudco International got me a customized kit of jets and needles recommended for my Duc 250 and the recommendations were spot-on perfect.

Perhaps the fastest and cheapest way to get where you want to go is to work on the carb you have. As you say, it runs well. You seemingly just want it to start off a bit easier. Your bike has a battery and a regulator, is that right? Keep in mind that poor starting can sometimes be attributed to a worn down battery. I don't mean an old or bad battery, just one that is weakened by undercharging (low RPMs) and overdraining (headlight use.) I would hate to see you go back to starting over with a new carb ($$$) multiple jets and needles for tuning ($$) and then have that all bring you back to where you are now.


Whatever you choose to do should be interesting for all of us to follow. I hope you get the bike to be a real easy start so you can easily and reliably start, ride and enjoy your bike.

Jim

[DewCatTea-Bob]

" The carb body was badly bent from being over tightened "

____ That's a fairly common issue with those type carbs (with body molded manifold-flange).


" The result is a good running bike but I think it runs lean, it has a whitish looking plug and I hear small poops from the exhaust on deceleration. "

____ Then it's quite likely that you have the fuel/air-mix.screw adjusted too lean. _ So simply try a richer setting.


" Starting isn't easy but the technique I have developed - tickle the carb and then kick the motor over several times with the throttle open and with the ignition off - find the spot just after compression - turn the bike on and kick with the throttle being open about 1/2 way and the motor starts up running nice and fast. "

____ Never heard of that one before ! _ And I'd never resort to such a routine, either !
__ I recommend turning-on a petcock, then 2 or 3 quick tickles, then turn-on the key, then kick-over the engine while next slightly cracking-open the throttle at the same time... (I believe I'd not 'crack the throttle' until just-after I've felt the engine go-past comp.TDC.)
That process always worked well for me (on a cold motor) ! _ For warmed-up engines, the tickling step is not needed. _ And the 'choke' (which is what it actually IS, with THIS carb), is never really needed unless it's well below 50-degrees.


" I turn it off with the on/off switch. "

____ You should get into the habit of shutting-off the petcocks soon before you expect to shut-down the engine.


" Should I raise the needle or change the jets or is there something else I am not thinking of? "

____ It seems your idle fuel/air-mix simply needs to be richened-up (and the idle-screw then adjusted to compensate, for better idle).



" I have also thought of changing the carb to something newer and hopefully better. "

____ Your best choice would be a 27mm 'square-slide' type Del.carb from a 1969 or newer wide-case 250-Scrambler.


" The Amal would be easier, I was told that I would just have to make the manifold lug wholes little larger to fit the lugs on the cylinder. "

____ That seems to be nonsense to me, so perhaps whoever was actually referring to some new Amal-model which I'm unaware-of.


" Someone would have to tell me which Amal to get and what the jetting should be. "

____ A 26 or 27mm Amal from a 1960s 500-twin Brit.bike, or a 250-Triumph/BSA single, or 750-tripple, or from a Mototrans 250-Duke,
will bolt straight-on with no changes required.


Hopeful-Cheers,
-Bob

[Chuck]

Thanks for the help.

I bought a small 6 volt battery charger and that eliminated the weak battery problem. I also richened up the fuel/air screw by 1/2 a turn (now 1 full turn out) and that fixed the starting problem. I still haven't ridden the bike very much but I went for a 1/2 hour ride this morning and the bike started on the 1st or 2nd kick. I have been practicing starting and I think the carb is still lean at starting because the motor stops after about 15 seconds unless I race the motor and even then it dies. My new technique, and this I am sure is very weird, is right after the motor start I push the tickler a few little times with the motor running for about the first 30 seconds and then the motor warms up nicely and is ready to ride.

My days of racing around are over and I'm happy cruz around on secondary roads and 40 or 50 miles an hour. The motor is smooth and strong in all the gears with no flat spots. The one problem I still have is getting the motor to return to a slow idle immediately after the throttle is turned off. It can take about 15 to 20 seconds to settle down to a sort of low smooth idle. I have been lowering the idle stop screw but I think I still have a little air leak because the idle still fluctuates a little. Is it OK to use a gasket sealer around the rubber manifold or is that even the source of the air leak? Is this something that can be fixed with such and old carb or should I just learn to live with it? With my British bikes I just bought new Amals when I had these kinds of problems.

If I went with an Amal carb would I use the rubber manifold or mount the carb with a thin paper gasket like on British bikes?

Chuck

[Jordan]

" If I went with an Amal carb would I use the rubber manifold or mount the carb with a thin paper gasket like on British bikes?

Chuck

*************
I had no trouble with just bolting an Amal Concentric onto a 250 Mk3 head (no rubber), which ran very well with it by the way.

Jordan

[Jordan]

I have a Dellorto SSI with a choke and have never found the choke to have any affect.

You mean the air slide? They don't do much, do they.
SSI is a version of Amal TT carb, which was made for the Isle of Man Tourist Trophy race, whence the name.
This road race is on a long course that goes from near sea level and over mountains. The air slide was a refinement that was designed to allow riders to adjust the mixture, according to varying atmospheric pressure due to altitude change. Unlike a Mikuni's enrichener which was designed to help starting, TT & SSI only let in more (or less) air, not extra fuel too. I found it useless as an aid to starting, even if jetted for normal running when fully open.
For use on flat courses, race tuners generally remove the air slide altogether (to allow as much air in as possible), and adjust jetting accordingly.

Jordan

[DewCatTea-Bob]

By: Chuck...
" The one problem I still have is getting the motor to return to a slow idle immediately after the throttle is turned off. It can take about 15 to 20 seconds to settle down to a sort of low smooth idle. "

____ The small carb's rather light throttle-slide return-spring can sometimes take a little extra time (with help from engine-vibration) to close-shut the slide all the way down against the idle stop-screw. _ Thus sometimes the engine will fall from a fast-idle down-to the slower idle (set by the idle-screw).


" Is it OK to use a gasket sealer around the rubber manifold ? "

____ If you think it's really needed, then be sure it's compatible with exposure to fuel,, and don't use too much around there.


" Is this something that can be fixed with such and old carb ? "

____ I thought you mentioned that it's flange/main-body was NEW !?


" If I went with an Amal carb would I use the rubber manifold or mount the carb with a thin paper gasket like on British bikes? "

____ I'd use a stock-type rubber-spacer, to help insulate the carb from the heat of the cyl.head.
However many Amal-carbs have a rubber O-ring built-in to their mounting-flange,
thus-then making an added gasket unnecessary.


Duke-Cheers,
-Bob

[Jordan]

" If I went with an Amal carb would I use the rubber manifold or mount the carb with a thin paper gasket like on British bikes? "

____ I'd use a stock-type rubber-spacer, to help insulate the carb from the heat of the cyl.head.
However many Amal-carbs have a rubber O-ring built-in to their mounting-flange,
thus-then making an added gasket unnecessary.

Rubber mounting is an option, but may not be needed.
It can fix problems with vibration which upsets carb function, and serves as a heat barrier. Also it provides an opportunity to make an adaptor for carbs that don't bolt straight on.
Amals with 2 inch mounting centres line up fine on Ducati singles though.
On my 250, I didn't have any problem with vibes with no rubber tube for the Concentric, and used a 1/16" thick Tufnol "gasket" from a BSA for a heat barrier.
Without a rubber tube, it's a neater installation I think.

Jordan

[Chuck]

Hi Jordan,

Can you tell me which Amal you have on your 250 Mk3? I have a choice of a 26 or 28mm carb. Did you have to do anything to the carb like changing the jets or were you able to use it out of the box?

Thanks,

Chuck

[Jordan]

Hi Chuck,

I had a used 30mm Amal Concentric Mk1 carburettor. I think it was from a Norton or Triumph.
These specs are as it was, fitted to my 1969 250 Mk3.
It worked very well with the head's 29mm inlet port.

Model: R930/300 (right hand, 30mm)
Air filter: not fitted (short horn instead)
Main jet: 250
Needle jet: 106
Pilot jet: 40
Throttle cutaway: 3
Jet needle: 2 ring
Needle clip position: Centre

Jordan