Unsure if it helps the general brains trust, I found a ripper of a DCD manual from the mazda guys,
DCD Weber problem
- Thread starter rubyalpine
- Start date
H'mm. There may be a gasket on all four sides, but the fuel bowl needs to be vented - somehow. There may be a passage / cavity / gallery on the underside of the top cover. From memory most carbys are vented to above the venturi area, under the top "horn" of the carby.
Been busy the last few days and haven't had much time to think about this issue. Just read through this thread again, we need to get this solved for every ones sanity.
I have thought of another test that can be tried.
Remove the carby top and put it aside, remove the fuse that supplies power to that electric fuel pump. Fill the float bowl to about the half way mark and then start the car and see how it runs. It should idle and be able to be revved up until the float bowl runs dry. You should still be able to look down the primary barrel and see what is happening with the auxiliary ventury (if fuel is pouring out into the primary barrel).
The thinking is to eliminate the bottom part of the carby with all the jets etc. You could repeat the process raising the fuel level until you can simulate the problem you are having.
I have thought of another test that can be tried.
Remove the carby top and put it aside, remove the fuse that supplies power to that electric fuel pump. Fill the float bowl to about the half way mark and then start the car and see how it runs. It should idle and be able to be revved up until the float bowl runs dry. You should still be able to look down the primary barrel and see what is happening with the auxiliary ventury (if fuel is pouring out into the primary barrel).
The thinking is to eliminate the bottom part of the carby with all the jets etc. You could repeat the process raising the fuel level until you can simulate the problem you are having.
Sorry, I've been busy on un-related projects, and now going away for one week. I will get back to this problem when I get back home and will keep you all informed;
Henry
Henry
Again, looking at the diagrams, it is difficult to imagine why the main jet would leak.
Follow the fuel passage through the main jet to the nozzle and you'll see the fuel has to climb above the float level to get to the nozzle. This doesn't happen, no matter how badly set the float is. The only way it can happen is if the air above the fuel in the bowl is under pressure or something really sucks down the carby throat. I would suggest again the test suggested by Col earlier with one modification. Take the needle valve off the carby cover completely, take the float out, disconnect the fuel pump, fill the bowl with fuel roughly to where it should be in normal operation, put the carby cover back on and try to start the engine. Taking the needle valve and seat off should allow the bowl to be at atmospheric pressure. Even if the engine doesn't start (it should start) you should see if the fuel leak happens again. If it does happen again you're back to square one, if it doesn't, you found your problem.
I can't see in the manual if the bowl is vented. It has to be, but I need to read the manual to find how.
Follow the fuel passage through the main jet to the nozzle and you'll see the fuel has to climb above the float level to get to the nozzle. This doesn't happen, no matter how badly set the float is. The only way it can happen is if the air above the fuel in the bowl is under pressure or something really sucks down the carby throat. I would suggest again the test suggested by Col earlier with one modification. Take the needle valve off the carby cover completely, take the float out, disconnect the fuel pump, fill the bowl with fuel roughly to where it should be in normal operation, put the carby cover back on and try to start the engine. Taking the needle valve and seat off should allow the bowl to be at atmospheric pressure. Even if the engine doesn't start (it should start) you should see if the fuel leak happens again. If it does happen again you're back to square one, if it doesn't, you found your problem.
I can't see in the manual if the bowl is vented. It has to be, but I need to read the manual to find how.
I tried COL's idea as above. The motor won't run on the fuel in the float chamber with the top cover off. It will run for about 2 or 3 seconds on starter fluid, but as soon as this is used up, it cuts out.
I put the top back and replaced the main jet with a grub screw ( now no supply from main jet) and motor will not start. I opened idle mixture screw a bit and increased idle speed, ie. I allowed more fuel through idle system. No difference. fuel still pumps out of main jet. It seems to me that the fuel is being sucked through the main jet because it cannot flow through idle system, but I have checked idle jet and holder and blown through idle tracts with compessed air. Did this on 2 of the carbs but no difference. I will try again next week.
Thanks all for your input.
Henry
I put the top back and replaced the main jet with a grub screw ( now no supply from main jet) and motor will not start. I opened idle mixture screw a bit and increased idle speed, ie. I allowed more fuel through idle system. No difference. fuel still pumps out of main jet. It seems to me that the fuel is being sucked through the main jet because it cannot flow through idle system, but I have checked idle jet and holder and blown through idle tracts with compessed air. Did this on 2 of the carbs but no difference. I will try again next week.
Thanks all for your input.
Henry
I am not sure I follow. How can fuel "pump out of the main jet" if the main jet is replaced by a grubscrew (i.e. blocked)?
I think we need you to tell us what you identify as the main jet.
I had a look tonight in the carby book and it seems there are three circuits in this carby. The cold starting circuit (an entirely separate carburetor within the carburetor), the idling/progression circuit and the main.
These three circuits only share the float chamber.
It is not clear in my book, but it appears to me the idling/progression and main circuit share part of the emulsion tubes' well. If that is true, then the inlet is also shared, and that is the main jets. This would preclude the engine starting/running at all if the choke is closed and as you say, the main jet passage is blocked with a grubscrew.
There is also the accelerator pump which is similar to that on DCOEs, piston and spring arrangement (rather than membrane and spring) with a separate inlet, delivery channel and outlet nozzle. Only the primary venturi nozzle is open in this carby.
Now given the way the cold start circuit is designed with the channel going up and through the carby cover and then down again through the enrichment valve, it is little wonder the car didn't start and run with the cover off.
This is one thing we didn't know about the DCD.
The idle circuit is then the only fuel delivery option available in this experiment and with a normal setting, the idle circuit is not going to deliver enough fuel. Remember, it is only supposed to keep the engine running when it's warm.
I think you need to open the idle mixture screw a heck of a lot if you want to try again the cover off experiment and even then I am not sure the air corrector won't bleed enough air in to offset that.
Or warm up the engine whichever way you can and then run the cover off experiment.
I would choose the latter as it is more relevant to the problem we're chasing.
PS. The carby bowl IS VENTED to the atmosphere but indirectly through an air corrector passage in the cold start circuit that goes all the way to the bottom of the starting jet well. It is in fact situated just opposite the starting jet well, in the centre of the carby, right next to the main circuit emulsion tubes. See picture here:
See the little hole right in the centre of the carby with a little brass ring in it. To the right of that there is a shiny brass plug with a tiny hole in it. That is the air corrector for the starting circuit. This picture is of a slightly different carby but that is where the vent to atmosphere is on yours, where you don't have the brass plug, just a big diameter hole. The starting jet is on the opposite side of that hole with the little brass ring in it.
Which gives one possible solution to the conundrum.
If this passage is blocked, the float chamber is no longer vented hence pressure can build inside when the air above it is squeezed by the fuel coming in and fuel is going to pour out of wherever it can, the path of least resistance my guess would be the main circuit outlet nozzle. Level in the float chamber will not rise so floats won't even try to close the inlet.
Can you confirm your gasket looks like this one:
See coutout of weird shape in the center, right between the venturis? Kinda looks like there's four holes but they are connected and go around a separate central hole. If you don't see that shape in your gasket, you found your culprit. If not, I would definitely have a look down these holes to do with the cold start circuit.
I think we need you to tell us what you identify as the main jet.
I had a look tonight in the carby book and it seems there are three circuits in this carby. The cold starting circuit (an entirely separate carburetor within the carburetor), the idling/progression circuit and the main.
These three circuits only share the float chamber.
It is not clear in my book, but it appears to me the idling/progression and main circuit share part of the emulsion tubes' well. If that is true, then the inlet is also shared, and that is the main jets. This would preclude the engine starting/running at all if the choke is closed and as you say, the main jet passage is blocked with a grubscrew.
There is also the accelerator pump which is similar to that on DCOEs, piston and spring arrangement (rather than membrane and spring) with a separate inlet, delivery channel and outlet nozzle. Only the primary venturi nozzle is open in this carby.
Now given the way the cold start circuit is designed with the channel going up and through the carby cover and then down again through the enrichment valve, it is little wonder the car didn't start and run with the cover off.
This is one thing we didn't know about the DCD.
The idle circuit is then the only fuel delivery option available in this experiment and with a normal setting, the idle circuit is not going to deliver enough fuel. Remember, it is only supposed to keep the engine running when it's warm.
I think you need to open the idle mixture screw a heck of a lot if you want to try again the cover off experiment and even then I am not sure the air corrector won't bleed enough air in to offset that.
Or warm up the engine whichever way you can and then run the cover off experiment.
I would choose the latter as it is more relevant to the problem we're chasing.
PS. The carby bowl IS VENTED to the atmosphere but indirectly through an air corrector passage in the cold start circuit that goes all the way to the bottom of the starting jet well. It is in fact situated just opposite the starting jet well, in the centre of the carby, right next to the main circuit emulsion tubes. See picture here:
See the little hole right in the centre of the carby with a little brass ring in it. To the right of that there is a shiny brass plug with a tiny hole in it. That is the air corrector for the starting circuit. This picture is of a slightly different carby but that is where the vent to atmosphere is on yours, where you don't have the brass plug, just a big diameter hole. The starting jet is on the opposite side of that hole with the little brass ring in it.
Which gives one possible solution to the conundrum.
If this passage is blocked, the float chamber is no longer vented hence pressure can build inside when the air above it is squeezed by the fuel coming in and fuel is going to pour out of wherever it can, the path of least resistance my guess would be the main circuit outlet nozzle. Level in the float chamber will not rise so floats won't even try to close the inlet.
Can you confirm your gasket looks like this one:
See coutout of weird shape in the center, right between the venturis? Kinda looks like there's four holes but they are connected and go around a separate central hole. If you don't see that shape in your gasket, you found your culprit. If not, I would definitely have a look down these holes to do with the cold start circuit.
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Hi Schlitz, I think we have a mis-understanding. When the main jet is replaced with a screw, there is no fuel coming out of the nozzle in the auxilliary venturi. Before I opened the idle screw and increased idle speed, I had removed the grub screw and replaced the main jet, so this allowed fuel to pump out of fuel nozzle which I incorrectly refered to as main jet.
The gasket is fine, exactly as the holes in the carb. I think we are both tending to suspect a blockage in the cold start or idle systems. I will try further solvents and compressor blow through efforts. Thanks for your input.
Henry
The gasket is fine, exactly as the holes in the carb. I think we are both tending to suspect a blockage in the cold start or idle systems. I will try further solvents and compressor blow through efforts. Thanks for your input.
Henry
Okay that changes a bit. Still not sure what nozzle is that you mention where the fuel comes in the carby.
I think the fact that you have tried three different carbies and none worked but with the same jets, floats and I suspect top cover gasket would have you believe it is not the main body of the carby at fault but the parts that you moved across from one body to the other.
This is assuming it is impossible for three bodies to have the same problem. I am not sure however how valid this assumption can be.
I would still go through the one body that was on the car and make sure all the passages are squeaky clean, especially the ones to do with the cold start circuit. I also remember from my DGV (very similar carby) days the little calibrated brass orifices at the top of various channels can get blocked quite easily and very difficult to clean and check. I had one of these blocked when the carby was rebuilt professionally by a tiny glass bead because the carby was wet bead blasted. It took a lot of work with a magnifier glass to find the culprit and see the little glass bead stuck in the tiny little hole. I don't remember what the symptoms were but after that the carby was perfect and I never had to touch it again.
I think the fact that you have tried three different carbies and none worked but with the same jets, floats and I suspect top cover gasket would have you believe it is not the main body of the carby at fault but the parts that you moved across from one body to the other.
This is assuming it is impossible for three bodies to have the same problem. I am not sure however how valid this assumption can be.
I would still go through the one body that was on the car and make sure all the passages are squeaky clean, especially the ones to do with the cold start circuit. I also remember from my DGV (very similar carby) days the little calibrated brass orifices at the top of various channels can get blocked quite easily and very difficult to clean and check. I had one of these blocked when the carby was rebuilt professionally by a tiny glass bead because the carby was wet bead blasted. It took a lot of work with a magnifier glass to find the culprit and see the little glass bead stuck in the tiny little hole. I don't remember what the symptoms were but after that the carby was perfect and I never had to touch it again.
And if that's no good, I say we start sending you carbs and see if the problem goes away. Then maybe you can start swapping your jets and crap into it one at a time (assuming our carbs work) to see what brakes it.
Give me a couple of weeks and I'll send you my DGV, (just moving house yaaaay but boooo) It should use the same floats, jets, tubes etc as your DCD's.
Give me a couple of weeks and I'll send you my DGV, (just moving house yaaaay but boooo) It should use the same floats, jets, tubes etc as your DCD's.
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Nope. The DCD is a different animal to everything else.
Did you get that book, Bowie? If you want to give away the DGV I'll have it back, thank you very much.
Did you get that book, Bowie? If you want to give away the DGV I'll have it back, thank you very much.
Hehehehe. In 12 months you should stick that 807 in.
<thread hijack>
Ah. Sad to say the 807 didn't make the move down to Melbourne with me
It's been looked after buy another reno tragic, very very confident it's going to be put to very very good use
I've sacrificed heaps in moving back actually. The Motorbike (it yes a Series 2 Yamaha, Yellow TDM900. Bloody wanted one for ever and got it $$ (made money selling it) and was just getting comfortable with it, Fell in love with it because of the Yamaha TRX that shared the parallel twin, anyway, I'll never see another one that nice in yellow again. booo) that 807, and the Yellow R12 submarine I nearly had back together.
Moving sucks. No wait. Moving without a heap of cash sucks!
Thankfully new house has a good garage, moving in after Easter, shenanigans can continue
</thread hijack>
Ah. Sad to say the 807 didn't make the move down to Melbourne with me
It's been looked after buy another reno tragic, very very confident it's going to be put to very very good use I've sacrificed heaps in moving back actually. The Motorbike (it yes a Series 2 Yamaha, Yellow TDM900. Bloody wanted one for ever and got it $$ (made money selling it) and was just getting comfortable with it, Fell in love with it because of the Yamaha TRX that shared the parallel twin, anyway, I'll never see another one that nice in yellow again. booo) that 807, and the Yellow R12 submarine I nearly had back together.
Moving sucks. No wait. Moving without a heap of cash sucks!
Thankfully new house has a good garage, moving in after Easter, shenanigans can continue
</thread hijack>
Bummer.
If you had told me I would have taken it back.
If you had told me I would have taken it back.
IT'S WORKING!
After being off the road for about a year, my R10 is now running sweetly again. After stripping cleaning, replacing, adjusting, swopping parts and re-assembling the bits and pieces of the 3 carbs that I have, about 50 times, I tried starting the b>?#h again and it just started idling and revving as normal without the clouds of black smoke and fouling of plugs.
All the original jets, choke tubes etc. have been reinstalled and all the settings are as when the problem first started.
I have been playing around with these carbs (and cars) since 1969 and I have never come across any thing like this before, even the guys at WEBER Specialities (N.Z.) could not help me.
The car has just passed a roadworthy test and I'm back, terrorising the streets of Christchurch.
Thanks again for all the help offered by fellow Froggers
Henry
After being off the road for about a year, my R10 is now running sweetly again. After stripping cleaning, replacing, adjusting, swopping parts and re-assembling the bits and pieces of the 3 carbs that I have, about 50 times, I tried starting the b>?#h again and it just started idling and revving as normal without the clouds of black smoke and fouling of plugs.
All the original jets, choke tubes etc. have been reinstalled and all the settings are as when the problem first started.
I have been playing around with these carbs (and cars) since 1969 and I have never come across any thing like this before, even the guys at WEBER Specialities (N.Z.) could not help me.
The car has just passed a roadworthy test and I'm back, terrorising the streets of Christchurch.
Thanks again for all the help offered by fellow Froggers
Henry
At least there's a good garage. Moving is terrible!<thread hijack>
Ah. Sad to say the 807 didn't make the move down to Melbourne with me
I've sacrificed heaps in moving back actually. The Motorbike (it yes a Series 2 Yamaha, Yellow TDM900. Bloody wanted one for ever and got it $$ (made money selling it) and was just getting comfortable with it, Fell in love with it because of the Yamaha TRX that shared the parallel twin, anyway, I'll never see another one that nice in yellow again. booo) that 807, and the Yellow R12 submarine I nearly had back together.
Moving sucks. No wait. Moving without a heap of cash sucks!
Thankfully new house has a good garage, moving in after Easter, shenanigans can continue
</thread hijack>
Glad its going again, the only thing is that we never found the route cause of the problem so it may raise its ugly head again, ol well we will see what the future brings. In the mean time have fun terrorising those streets of Christchurch.