Carb crack!

[ahdoman]

Ref; 88 Paso 750
It's been a struggle to try and get my Paso to idle correctly. Jayh has been a great help. Above 3k and under throttle the bike runs great. It's just at idle and after running cool start that I have issues. In order to get the bike to even run I've had to back off my A/F screws about 4 1/2 to 5 turns which is way too much. I've also tried a couple of different jettings per my discussion with Jayh to see what happens. From cold it starts right up with the choke but after it cools down it's a bitch to start. Well, today I decided to tear the carb apart again thinking that maybe I missed something and I found a crack in the venturi where the A/F screw pokes through! When I measure the difference between the good one and the cracked one the cracked one is about .06mm larger at the exit. It looks like someone either dropped the carb on the screw and the force pushed on it and cracked it or they cranked way too hard on the screw and split it. Any body have any suggestions on how to fix this? I don't have the money for going to dual Dells or anything like that.

Also - It also looks like there is a crack around the venturi but it appears that is a casting mark. The venturi is not out of round.

[angelix]

you could try to braze it by using a low temp alloy specifc for aluminium and aluminium alloys , you can find it online.

check here: http://www.durafix.com/

it works well and it is very strong; I used it to repair a racing front subframe and the repair resulted stronger than the surrounding material


I guess you cannot find anything cheaper and it should work on your carb

The other option is to buy another one from ebay, they pop up often.

[jayh]

I'd give it a good clean ,maybe enlarge the crack slightly with a scribe or scraper and smear some good quality two pack epoxy across it,then wipe the excess off with metho or acetone.A light coating of vasoline or grease on the tip of the screw then seat it,will stop it getting stuck and keep the profile correct.
you would have to be very careful not to get much in the orifice,but it could be carefully removed with a small drill bit by hand after it cured.
There are some metal repair epoxies available that are totally fuel resistant.I used some to repair a dellorto float pivot several years ago and its still going strong. J.

I've been thinking some more about your problem.The screws don't actually control the mixture,that is mixed at the idle jet.The screws control the volume of premixed fuel being delivered,the further open the screw the more fuel mixture.Opening the screw further is supplying more fuel,I would have thought that the crack would already be supplying more fuel past? Have you tried closing the screw back in,maybe you are way rich on that cylinder,that would explain why the problem gets worse when the engine warms up. I would try that first,even try closing it nearly fully as the crack may be letting enough past.If the engine idles good when hot,it doesn't matter where the screw is in relation to the other one.Set your good one first as per my earlier directions,then play with the cracked one,you never know what you'll find. J.

[Mc tool]

Why not use a vacuum gauge to set each cylinder seperarely.With engine at temp, set to a fast idle . Close both bypass screws ( ballance screws ) adjust mixture screws to get max vac on each pot ( may have to reset idle ). Then re ballance carbs and reset idle. If one screw setting is wildly different to the other you could probly blame the crack ....... Assuming you have no other vacuum leaks

[ahdoman]

McTool - I've played with that a little bit. I don't have a mercury guage just the type you set in the mouth of the carb. I've always used that for my Dell's on the car so I'm fairly confident with it. The problem is that the vacuum values seem to change cylinder to cylinder with RPM. If I do the adjustment at 2k and get them the same, it's different and out of balance at 4k. I'm thinking I need to go back to the basics and start with checking the timing and then a valve adjustment so I can eliminate that factor.

[Mc tool]

Dunno about the 750 but the 906 is well known to come from the factory with the cam timing miles out , and usually a different amount for each cylinder . My 906 were ( still are ) out , one by twice as much as the other ( 4 & 8 deg) . That really needs sorted before any fine tuning is done. I would go for a second hand carb and at least check the cam timing , you can buy offset keys to dial in the cams so its not megabucks (there are posts here somewhere re this ).
I find a set of mercury colom vacuum gauges very useful , not just for ballancing but also for checking off idle responce and the differences between each cyl , checking power valves, etc, but there is a knack to it , like readin x-rays

[jayh]

I think that the best way to get to the bottom of this problem whether it be fuel,mechanical or electrical is to isolate the cyclinder that is playing up (if any).
Before you start clean your plugs, older ducatis foul plugs easily and once fouled will not tune.If in doubt replace them.All it takes is to leave it idling too long with the choke on and the problems start.

Get your engine idling at the slowest possible speed that it will idle at by adjusting the throttle stop.
Now pull a plug lead,does the engine die ?,if not replace and pull the other plug.
If you can isolate a bad cylinder,then you can try swapping cdi boxes if you have kokusan ignition.
does the problem then swap cyl ?
was the cylinder that died not the one with the buggered idle screw ?
if the problem doesn't swap then try a new plug,lead, swapping coils,etc.
if both die when the plugs are pulled then wind the revs up a little and see if they both slow the same amount when each plug is pulled (a tacho would be handy here).
if they do then the problem is unlikely to be one side of your carby (the idle screw)
also unlikely to be pick up or air gap
more likely to be a universal problem something like a bad earth or wiring common to both sides of the ignition.The ignition mod is worth doing.
or fuel pump,or filter,something common to both sides of carby.
You may have already tried this,if not post your results and we'll see if we can narrow it down based on your findings. J.

[jayh]

More thoughts on your problem,going back to your earlier posts, 5 turns out on your idle screws and wont idle below 1500 rpm is way out,either too small idle jets,which you've checked,wrong idle screw taper, blockage or damage to carby or vacuum leak.

Are you positive the screws are the correct ones.
We know you have some damage,could this have been caused by someone screwing the wrong screw
in the orifice.

double check for vacuum leaks,like throttle shaft seals. J

[ahdoman]

Jay - I've been back through the carb again and it's about the same. I've ordered new valve cover gaskets and belts. I'm going to make sure that everything is correct at the "core" of the engine; valves & timing. After that I'll revisit the carb. I have to make pretty radical changes to the carb to get it to react in any type of way and because of that it makes me think something isn't right with the mechanical basics.

I wish I had another Weber just to bolt it on and see if it makes a difference.

[Duc906]

ahdoman
The body of your carb is rooted, most likely by some heavy handed fool with no idea what they are doing, they've screwed the idle mixture screw in that hard that its broken the alloy out the other side but I think you already know this. It's no wonder you can't get your bike to idle properly. Don't bother trying to glue it back together the idle screw and seat will be damaged beyond repair and no amount of adjustment will over come this. Better to find a second hand replacement carby and use your carby for parts. For anyone contemplating adjusting their idle screws, you must be gentle and only tighten them finger tight, jm2cw.