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Float bowl and needle


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Hi Rolf,

I don't know about anybody else, but I'm confused by the question.

Is it possible that you are talking about the stock '72 Hitachi SU-type carbs rather than Mikunis?

The PHH type sidedraught Mikunis usually fitted to the L-series engines don't have any needles in them, but the stock Hitachis do.

I don't know about the float level differences and needle lengths if it is indeed the Hitachis that you are asking about, but I'm sure that somebody else on here will................

Alan T.

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Alan, you are right.

I always had a problem when accelerating under loud at about 4500 rpm. Some days ago I got a book from Des Hammill on SU carbs and there are a lot of technical advices. He mentions that in both float chambers the level and amount of fuel should be the same. As in the Haynes manual he also mentioned that the distance H between the cover and the float bowl top should be the same for closing the needles valve. So I was surprised that my rear carb had a different fuel level and that the rear needle was about 3mm further in. From my other Z I still have the original SUs and so I opened those , too, and they were quite exactly the same. So I wonder, is that normal?

Another thing is the level of the suction needle in the piston. All manuals say the thicker upper part of the needle should be at the same level as the pistons underside. Now the Z's SU-Type pison's underside has a diagonal "ditch" where the needle seat is in. This level is about 1mm higher than the piston's level. If I take that as the reference level the fuel will be richer or the fuel mixture screw has to be turned out one round less. Which level is right?

I added a pic to this thread but it was returned because it was too big. I add it to the Galery, carbs section.



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Rolf, I'm not too good with carbs.. but here goes what I read in the FSM about setting the needle in the suction piston.

What you want to do is to take a straightedge and lay it horizontally on the bottom of the suction piston and set the needle height when the needle shoulder is even with the bottom of the piston. In other words, set the straightedge across the bottom of the suction piston and when the shoulder of the needle contacts the straightedge, this is when you set the needle adjusting screw. It should be level with the bottom of the suction piston bottom.

Now on to the float setting, for some reason the FSM I have referrences a diagram yet it doesn't give the dimension in the diagram...:stupid:

But, here's is what it says about setting the float level.

With the float chamber on the bench, take off the cover and with it lying on the bench with the float lever side up. Lift up the float lever with the tip of your finger and then slowly lower it. Stop lowering when the float lever seat just comes into contact with the valve stem.

I'm just guessing here, but I take it to mean that the float level is correct when the top of the float is 14 to 15mm from the bottom of the cover. This is the dimension that isn't in the diagram so I'm just guessing at this. The dimension that is specified in the diagram is 23mm from the shoulder of the float to the bottom of the cover. :ermm:

As far as there being a difference in the float levels, I don't see any reason why they should not be the same, but perhaps over time one of the arms has gotten bent slightly(maybe someone else was tinkering with it at one time?). If you have the float levels different, I would believe it would almost be impossible to truly synch the carbs correctly because one would be inherently running leaner than the other.

Hope that helps.....

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I saw your photo, and have to agree that something is amiss. Your are correct in your statment that the fuel level will effect the mixture. The level in the fuel bowl IS the primary mixture adjustment. The nozzle level is the fine mixture adjustment after the bowl level. It is proccedure to set the bowl levels to the same height, as close as possible.

Have you watched ZTherapy's SU video? Scott really covered this area in it, and stressed the importance of setting the fuel bowls the same.

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Hi Victor,

thanks for your help. When you looked at my photo (in my gallery) you can see that the brass rear valve is about an inch shorter and that means that - when parallel - the distance between the cover and the top of the bowl is also 1 inch smaller. By that there is more fuel in the rear.

Unfortunately I cannot use the ZTherapie video because here in Germany and some other European countries we have a different TV system to the US. The one system is called SECAM and the other PAL. I dont' know exactly which one we have, but it's just the other. I would really be happy to have the Video, but if I can't watch it it doesn't make sense and I can't transform it into the other system. Perhaps that would make sense to ZTherapy for selling it to those of us in the countries with the other system.

Yesterday I put the pieces of my carbs together and now at about 4000rpm - not below - the front carb fires back, out of the intake's horn, not the rear one. Is the front carb too lean or is it because of less fuel in the front fuel chamber or what could be the explanation and what to do?


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