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David Brown Cropmaster Carby?

cobbadog

Active member
As you may know I've been having issues with getting the David Brown Cropmaster running properly. It would appear that the silly little spring on the needle and seat has been a fair amount of the problem and with this removed it no longer starves for fuel, yippee!
Next problem is air mixture. The engine runs rich and at idle sends up black smoke from the chimney. No matter where the air mixture screw is, all the way in or almost out of the carby it makes no difference to the way it runs. On full revs it runs ok but has flat spots when it revs up from idle. Not coughing just hesitating a few times then it revs fine.
I have had the carby totally apart, made new seals for the throttle shaft but the old ones I fitted a while back, "O" rings were in perfect shape but the latest thing they suggest to fit is nylon washers so I did that as well to get the best seal possible. All galleries are perfectly clear as you can clearly see through them all. Main jet tube and jet are clean as and so is the bowl.

Any clues to solving this problem please?
 

windmillman

New member
Re: Carby ??

If the air adjustment makes no change, I would look for somewhere some air is finding a way in, intake manifold gasket leak, or perhaps carburetor mating surfaces warped, or a leaking seal washer under a jet. Applying a little oil around a suspect area with the engine running will sometimes help , if there is a leak you would see the smoke. Just somethings to consider.
 

ulgydog56

Member
Re: Carby ??

make sure your hitting on all cylinders, check your plugs and see if one cylinder is running cold, it will run rich and hesitate if not running on all and check for week spark mite have to check timing too...:brows:
 
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cobbadog

Active member
Thanks for the replies. Dobo, the wall actually fell down and nothing left to bang me head against.

Leading up to this I had the manifolds surfaced planned and a new gasket fitted when I did the rebuild of the engine a few years back but when looking at it all while the tinware was off there was no sign of anything untoward BUT I will check with some Lanox. New carby to manifold gasket done and this was also surfaced so is level.

SO this is what I have found so far;
The silly spring on the needle is now gone and no more starving for fuel.
Removed the main throttle shaft and made new seals for each end. Originals were a felt washer and if you import a kit for this carby they are now nylon washers, but washers alone will not work on this carby as there is a recess for the felt seals and the washers are made to sit against the carby body. So I re-fitted the "O" rings I used a while back along with nylon washers. Re-packed the 14 tiny needle rollers that the shaft sits in.

Started on the electrics;
Removed the distributor cap and tested the plug leads and found an issue with one HT lead and the coil lead, so I will make all new ones again with new fittings.
Pulled the spark plugs, they are gapped correctly but are black as can be so I run them on the wire brush and used another brush to clean deep up inside and a lot of carbon fell out. Used the meter on these and one plug is not happy so Monday will be 4 new ones to go with the HT leads.
Cleaned and gapped the points. These are not pitted at all and were set at the right gap of 0.012" as advised from the manual.
Timing is perfect as it fires up before the starter motor stops running.

To be continued soon.
 

Darryl

New member
Take the mixture needle right out and blast 120 psi into it with an air blower gun because that carbi circuit is the smallest in the carb and if it doesn’t adjust then its blocked.
 
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cobbadog

Active member
Hi Darryl,
The gallery for the air mixture screw on these Solex FV-30 carbies are unusual to say the least.
The needle is screwed in from one side and a blanking off plug screws in from the other side. This so the carby can be used on different engines and be reversed. The hole is around 3/16" all the way across and you can see clearly right through. Tomorrow I will post a picture of the needle in position from the inside of the top part of the body. You can see the needle and where it is set now you can just see the beginning of the taper of the needle. There is no matching taper for the needle to go into to regulate air flow so it is a mystery to me on how it actually works. Maybe with a picture you can see what I am describing.
 

davob

Member
Cobba, Is the adjustment you are having issues with like the one show in the attached picture? If so there is a whole bunch of info on the site below regarding the F type Solex carb and how to tune. Goes into detail on how to select the correct venturi tube and jets according to engine bore/stroke and max power rpm.

Could be that in the past incorrectly sized jets were fitted or maybe they have become inadvertently enlarged due to being cleaned out with a small drill or tip cleaner

http://www.oldengine.org/members/diesel/Carburettors/solex2.htm
 

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ianoz

Member
Hi Cobba , you say it is a TVO model . Which means it should have a heat box on it to use the exhaust to warm the kero . I am wondering if it has rusted through and adding exhaust gasses to the intake.This could explain why the air screw makes no difference .
Just trying to think outside the box {Carby }
 
Hi Team,
I stand to be corrected, BUT, I seem to recall hearing that kero jets are larger than petrol jets in the same carbie's.

As said above some idle mixtures are done by Air Bleed screws that actually make a mixture leaner as it it is screwed Outwards to let more air in to the mix or Inwards to richen it.

Some discussion here relating to the viscosity of the respective fuels-----

https://www.yesterdaystractors.com/cgi-bin/viewit.cgi?bd=ferg&th=15522

As an aside, an Uncle of mine was an A Grade Mechanic and renowned for his engine tuning ability.
He told me about having too large a bore jets in a carby and how to prove it by inserting small pieces of copper wire strands stripped from a cable, suitably bent over at each end to stop them being dawn thru the works.
A number more or larger diameter strands were used until the required result was obtained.
These strands had the effect of reducing the fuel flow rate thru the jet and so lean off the mixture.
If this proved to make a better mixture then the jet bore would be soldered closed and re-reamed with jet reamers until suitable mixtures were obtained or a correct value jet obtained and inserted.
Most jet sizes are determined by the number stamped on them from a makers chart that we would not have now.
The number usually indicated a flow rate or diameter by obscure calculation known only to each maker.
Not easy is it.


Cheers,
Eddie B.
 

cobbadog

Active member
Thanks gents for all the interest and ideas.
Firstly Darryl your pic is very much like my carby. The plug shown as "u" is on the engine side of the body and the jet "g" is under the metering needle but I can't see the needle in your pic.
However Fig 14 from Dobo shows the needle in question and above the jet as in my carby.
Ianoz, the heat box on these engines is usually above the exhaust/inlet manifold at the front end of the manifold. It only draws warm air from there down to the air intake of the air filter which has a sliding tube to open or close the warm air. At the moment I have no heat box and the air intake is near the manifold but not very close. It only just fits past the side cover so isn't drawing much warm air from that position.

In my pics you can see 2 needle and seat set ups. The larger one in place is an old original one and the other is mine with an extra washer on it to space it down to lower the fuel level. If you look closely you can just see the little spring that used to hold the needle in place. It is now living in the tool box and so far the needle and seat appear to be working fine without the spring. I was also asked to check the fuel level which will be a simple thing to do as this carby has a drain cock at the bottom of the bowl so a clear plastic tube fitted to the drain cock , turn it on and hold the tube upright and this will give me the fuel level. I have not done this as yet. I will now have a look through the links that have been kindly posted and see if the penny drops for me. New plugs and HT and coil leads will also be a big help in getting this to run right again.
 

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cobbadog

Active member
Still playing with this and thought of a few things last night after receiving a link to Solex Carbies cleaning and tuning them.
First thought was, is the blanking plug opposite the bleed screw (air mixture screw) suppose to be open to allow air in. NO. It is a blanking plug and although it looks like a jet from the outside it is definitely a blanking plug.
Then were all the jets the correct size. YES.
Again are the galleries clear from blockages. YES.
You can see in one of the pics all the way through where the blanking plug and air bleed screw go.
 

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cobbadog

Active member
Next was again making sure everything else was where it was suppose to be and work out where does the air come from and go to. SO I have put up pics of the top and bottom side by side and then marked with an arrow where the air comes from and goes to. Although it is tricky to see, you should try taking a pic of it, there are no blockages but at least I now know roughly where the air bleed screw should be to start metering the air flow. The link posted has been a great help and I have confirmed the jet sizes with the chart and only the main jet is slightly different being a little bit larger than middle of the road on the charts. But this should not effect the running rich at idle and the idle jet (g) on the chartI think is the right size as I cannot read it properly.
 

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cobbadog

Active member
Something I have just now noticed is what I thought was the blanking plug marked as "W" is not and it is the screw to hold the choke tube in place. In the picture of the Solex FV carby there is no sign of the air bleed screw or the plug, yet it is there on the carby and is mentioned in the section on 'slow running pages 14 -16 on how to adjust it.

It takes me a little while to understand the terminology they use as to what I normally call parts. If I keep this up I will become a Pommy.
 

cobbadog

Active member
Latest update !
Well I made all new HT leads and one for the coil. All tested perfect once made then again fitted into the cap. All new plugs fitted and the needle and seats that I bought home do not fit so they can be returned to John, thanks John for the loan. I even made a new gasket for between the manifold and carby.

So now I know exactly what it is NOT. Non of the above or I am confident it's not the carby.

From start up today I got a whole new set of issues and the main one is backfiring. So next attempt is to check the timing and fix that if required, then try a new coil and see where that takes me.

:eek:
 

Glenn Ayers

Subscriber
If I had a brandy new backfiring issue .... I would be checking the sparky pluggy wiries & maggy cappy for correcty firing order.

.
 

cobbadog

Active member
That also will go onto the list of things to attack next. I appreciate all responses because as we all know how complacent we can becaome and sometimes it is the ridiculously silly things that can cause these issues.
 

cobbadog

Active member
Not running kero at all,but thanks for the input.

Well guess who swapped #1 n #2 plugs leads over causing the back firing? ME !!!!!!
That sorted that issue and then I did the start up and it jumped straight to life but still running rich. So back to the bleed screw (air mixture screw for us Aussies). I slowly a bit at a time wound it out a 1/4 of a turn at a time and not making any difference until I actually pulled the screw all the way out. The immediate result was the engine sounded smoother like it use to sound, so this points me back to the carby yet again.
Ever feel like you are going around in circles?
Anyway, in the pic below that I posted before it shows you both top and bottom of the carby and where the screw goes through. So the screw makes no difference until it is removed in total so that points me to the last of the 3 openings, the one closest to the head of the screw is somehow different to the one on the opposite side but they look identical when you poke around in there and shine a torch. It actually waits until the screw is all the way out, not just at the point where the taper would be above the 3 hole.

Any clues please?
 

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