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David Brown Cropmaster Carby?

Norm Johnston

Subscriber
Last Subscription Date
08/13/2019
G'day Cobba Just a suggestion but have you tried the air bleed screw in from the opposite side where the gallery is just plugged off. this might put the taper on the screw in a different port. Just a thought. Norm
 

cobbadog

Registered
G'day norm and thanks for the suggestion. I really am starting to look at anything listed to fix this bugger. to swap the needle around and bring it in from the engine block side will need to remove the carby again but I am telling you that I am an expert at removing and installing this carby now and really doesn't take long. The only thing that would be trick is having enough room to get to the air mixture screw without toasting the fingers but is certainly worth a try.
I also thought of making a new needle only a lot shorter so that it only reaches through to the first small opening in the carby body. This is where it appears to make a difference. Tomorrow after work, I am going to play around with this idea by inserting the screw in by hand and see at what stage it makes the difference. From memory it makes a difference straight away, as soon as the needle blocks the hole so this may not be the issue either. Time will tell.
I dont mind a challenge but boy this is getting to the top of the list of all the bad ones.
 

cobbadog

Registered
I have not tried that as yet to be able to answer you but that is yet another lead to follow.


Episode 1,234, way too many.
Once again a night of ideas on what to do to resolve this mess. Fuel level in the bowl was measured by connecting a clear plastic tube to the drain cock on the bowl and holding the tube agaist the bowl. Level was about 3/4 full. I have an aluminium washer under the needle n seat and this raises the level too high, to the top of the bowl and floods so went back to having a fibre washer with the aluminium one and back to being 3/4 full. At the moment it does not starve on full throttle standing still but might when on the road, will find this out tomorrow on a quick run up to the forrest and back.
Next was to once again strip the czrby down to being completely apart and double checking cleanliness and re-assembly ensuring that the 3 parts of the main jet are assembled correctly and fitted firmly in place.
Then with yet another read of the Solex carby link something hit me hard in the face, bloody air cleaner. This runs an oil bath cleaner and when I dropped the bowl it was not only over full but the centre ring where the air breaths in was also full of oil. So I lowered the oil level and removed the oil from the middle. Hopefully this was the right thing to do.
Still with the oil bowl off I started the tractor and ran it for a while so it was warm. Then I dropped the idle screw right back so it was not touching the stop and all the time changing the mixture although it did not seem to do much. So I kept winding the mixture screw out until it was in my hand. You can hear the air rushing in through the hole and when you put your finger over the hole the engine revs drop and runs a bit rough. Threw the oil bowl back on and nothing changed there so that is done. There is a remote chance that there is still oil inside the air cleaner where the steel wool looking stuf is and may still be choking the engine. Maybe after the run tomorrow this too will improve.
With a couple of runs up and down the back driveway it certainly pulls well with no hesitation at all where in the past it did. SO if nothing else this has improved.
All I can do now is take it for a run up hill and down dale and along the road and give it a good gallop and re access it from there.

Also when I shut it down just now I always run the carby dry and this time the engine revs did lift ever so slightly as it ran leaner while running out of fuel
 

cobbadog

Registered
Thought I may have had this beat but no unfortunately. The further we drove the worse it got but at least we got there and back for a picture shoot so here are a couple of him in the local State Forrest just a few kilometers from home.
Since getting home I have removed the plugs and cleaned them again as they are black as can be, checked the HT leads with the multimeter, removed the coil and condensor and hooked them up to my testing machine which rates the condensor, checks it for leakage and then tests it. The coil then is connected and a voltage is sent through it and you adjust the gap in the round window to produce a good constant spark over the widest gap on the lowest voltage. Once this is set you then select 'heat' and this heats the coil as if it is on the engine and running at temp. After minutes of heating then switch to test again. I got the same result after the heating which shows it is in good working order. I then removed the points and dressed them up and also found there was a small amount of oil on the base plate so I cleaned this all up with carby cleaner and a soft rag. Put it all back together and still the same. Back on the other side the oil bath bowl is off again to check that oil has not got back into the centre section again but it is clear now.
 

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Darryl

Registered
Cobba, park it in the garden and put ferns around it with some rocks. Nah, not really.
Rig up a fuel pressure gauge or put another gasket under the fuel pump to reduce the stroke. It might be blowing the needle off the seat. Especially if the plugs are black after a bit of a drive .
 

cobbadog

Registered
Its gravity feed this one, no pump and the needle now appears to work properly as its not flooding when the tap is turned on and not running but fuel does run into the bowl.
Thanks for the suggestion anyway but it just may well end up as a garden ornament the way its going.
When is bon fire night? :O
 

windmillman

Registered
Could it be possible the seal between the seat and carb body be seeping a little fuel and slowly raise fuel level while engine is running? You mentioned this is a gravity system, but with an air pressure regulator set very low pressurizing a small container of fuel connected to the inlet perhaps it could be verified. Sometimes you need to get away from something like this for a while and then later when you figure it out everybody will say why didn't I think of that?
 

cobbadog

Registered
Darryl, gravity has a good hold on my tummy and doesn't let go.

windmillman, you are right about walking away from it but I did give it the bird before doing so. :brows:
So in the mean time this has been whats been going on. Was given the name of a company on the Gold Coast Qld, Classic Carbs, who keep all of this stuff for old carbies of many different makes. Rang Gavin and he sounds on the ball with this older stuff and he told me he has the genuine needle and seat to suit this carby so it has been paid for and is in the mail today sometime.
I have a compression tester sitting on the counter in at Taree to be picked up tomorrow after work so this should give me an idea of any possible issues inside like valves or rings. I hope this is not the case as the engine only has around 100 - 120 hours on it after a full rebuild and a heap of time and care involved in the assembly. I keep telling myself, "I only want to see this once" when your deep inside the guts of something so I never rush and always double check.
I have not expanded my compilation of bad words but I am starting to think of some new ones that seem to be appropriate.
So this is why the walk away and go play with the caravan lights and see if they can annoy me as much. I do like a challenge.
 

cobbadog

Registered
While waiting for the new needle and seat to arrive I bought a compression tester today and tried it out. I did the test on a cold engine, not sure if that is the right thing to do and got the following readings;
1 - 75 psi
2 - 75 psi
3 - 72 psi
4 - 75 psi
Each held the pressure for a short time before I released the pressure.

So the question now is, what should the pressure be and should I have done it hot? I looked in my workshop manual and it gives you all the specs like bore diameter x stroke, cu in, bhp and a compression ration of 4.7:1 but no numbers on what compression it should have.
 

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davob

Registered
Hi Cobba,
To get a ballpark compression pressure it's basically atmospheric pressure x compression ratio

In your case
14.7psi x 4.7
=69.09psi

I'd say your safe
 

cobbadog

Registered
Must have been the surface grind on the head that has made it jump up from 69.09 to being 72-75.
I have a high performance tractor engine.
 

Scotty 2

Registered
Hello Cob
The main thing is that all the compressions are pretty even.
If I remember, the problem gets worse as David warms up. If so, did you check the valve clearances? Number 3 is worth a look
The thing that has me mystified is that David ran beautifully before repainting.

Cheers Scott
 

cobbadog

Registered
Apparently these are good numbers. Formula offered up to me was air pressure x 4.7 which is the compression ration.
14.7 x 4.7 = 69.09 psi and I have a good average above that at 74.25 psi. This may now be considered to be a high performance engine with a ratio like that!
New needle and seat arrived today from Classic Carbs on the Gold Coast. It is a genuine Zenith part which fits the Solex perfectly. It also came with a choice of 2 thicknesses of fibre gasket to adjust the fuel level. Tomorrow hopefully I will put it back together and see what happens.

Good thinking Scotty, thanks mate.
 

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cobbadog

Registered
I have been using that "doing word" an awful lot but then I always do anyway, so nothing has changed.
So, just for everyone's amusement I got to have a short play with it today.
Fitted the new needle and seat using the thin washer. Then as described in the manual remove the main jet and with the top half of the carby bolted to the manifold and connected to the fuel supply and throttle linkage you can then swing the bowl out towards you and then turn on the fuel and watch where the fuel level comes up to. Right on spec, it stopped about 2-3mm from the top of the jet holder. Beauty one mate, I'm winning and all that stuff. Screwed the bowl back on and turn the fuel on and I waited and all seemed good, no fuel dripping out. Tried a start up and no go. then while switching off the ignition fuel starts dripping out. "LOTS OF DOING WORDS RIGHT AT THIS MOMENT"
Removed the bowl and fitted the thicker washer but still dripped. Then I noticed the float has a small dent in the top and I am now thinking that this might be the cause of it not shutting off properly all the time.
So I am chasing a new float for the Solex 30-FV if anyone has one, it should have 26 gm (26 grams) stamped on top. You can just see the small dent on the top and I dont want to be the one to try and take the top off the float, knock the dent out and try and put it back together. I just know what will happen.
 

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cobbadog

Registered
That may work too thanks. I am a bit hesitant to play around with it as if it goes kaput I'm stuffed. Many years abck I tried a solder repair on a float and it went pear shaped, literally, totally stuffed it because I ahd too much heat on it trying to solder the hole up.
 

AussieIron

Registered
Last Subscription Date
08/07/2018
There's a good article (manual) on fv-30 carbs on oldengine .org You probably have seen it. It talks about different weight floats for different bore carbs. Hope you find answer, I bet it's a simple fix, the puzzling ones usually are. Cheers, Neil.
 

cobbadog

Registered
Thanks for that Neil, already done that and saved it plus printed important parts out and took them out the back to the play room. All the specs in this carby are exactly as listed in those lists, but thanks for the kind thoughts.

In today's episode, we explore the internals of the engine. I pulled the rocker cover off and checked the clearances of the valves. A couple were slightly tight but still had plenty of clearance when running warm. So I went through all of them and checked the gaps and made them as specified. Typical Pommies don't give you a COLD setting only HOT so at the moment they are set cold to the gaps suggested. I lost spark at one stage and went hunting and in the end I looked closely at were the power enters the dizzy. The bolt ran close to ground so I put a short piece of insulation under it and that appears to have solved that problem. I did have a go at the float. I opened the hole at the bottom of the float and used a piece of fencing wire, (the most handiest tool on a farm) to push up the small dent at the top. I did have to bump it a little with a hammer but got it up fairly close. So with the rocker cover off I did a start up and took this video. It still does not run smooth and with out touching anything it occasionally picks up the revs slightly and goes back to running rough again. If I pick the revs up a little the engine still runs rough but not as bad but then spreads the love of oil to everyone nearby.


https://youtu.be/oZI-Ga7_UGQ
 
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