XJ900 Carb Question

Hi Guys,
as my tuning attempts are getting better and better (as my knowledge grows) I realized that's almost impossible to tune my engine with those Hitachi carbs and pods. The problem resides in the pressure of the air. In simple way, the problem is that the carb (and the engine) can breath too much when I give her a WOT which means the slides moving too far up in the carb throat resulting way too rich mixture preventing the engine to accelerate properly.

The solution at the moment is to use stronger springs, lower the needles, and use smaller jets. But with those changes I'm not using the full potential which is the whole goal of tuning. So, decision made. I have to have XJ900 carbs.

Finally, I found one on eBay for cheap (cca 80 USD) with all the rubbers. Not just the airbox - carb but the head - carb rubbers. That's what I called cheap.

Finally the carbs arrived but I have a problem. There are a lot of rubbers on the carbs. One is for petrol intake. That's fine. 1 for each carb as an overflow from the fuel bowls. That's fine. But there are two additional rubber lines between carb 1-2 and carb 3-4. MY question is: what's the purpose of those rubber lines? I don't want to disassemble the whole carb to find out what are those lines. If someone can tell the purpose it would be great!

 

I believe they are just air vents.
Putting 900 carbs on a 750 ? Now that's ambitious !
Hope it works well.

 

It is, but if that idea is wrong the next thing what I can do is to install fuel injection which is not the easiest / cheapest thing to do. I have to try this one first.

 

Just a bit update on this. Probably others are interested in this idea as well. It should be worthwhile to share my findings. Yesterday evening I removed the Hitachis' and tried to bolt on the Mikunis'.

First, I tried to use the rubbers from the Mikunis' but it raised more problems than solved. Will explain later. So, I decided to use the rubbers made originally for the Hitachis'. I would say it's not impossible however the distance between carbs 1-2 and 3-4 in the carb rack is a bit larger. You have to be patient and you have to have strong hands to overcome this problem But that's not a good solution so here is the list I have to do in the future to have a proper set-up.

The XJ900 rubbers are 1mm larger in inside diameter. Because of this they don't exactly match with the intake ports. Fortunately the bolt pattern is the same, but to have a proper air flow I have to modify the intake ports a bit. I need to remove a bit of aluminum from the lower half of the intake ports. As it seems, it won't be a problem. Effect on the overall airflow (and on the torque delivery) is unknown at the moment.

Because of the problem mentioned earlier (different carb spacing) I can't really use the two outside rubbers. Unfortunately the rubbers are not straight what I do need to marry the XJ750 head with the XJ900 carb rack. I have to find a shop who can manufacture two straight (or almost straight) rubbers between the carb and the head. That will be the hardest part I think.

The throttle and choke cables are different. I have to go to a machine shop the manufacture a new set of cables.

I can't use the pods (and I don't want to use them on this set up. The velocity stacks are just beautiful) because the intake side of the carbs are 3 or 4 mm larger. I have to go to a shop which deals with fiberglass and I have to order a new airbox.

This means a new filter element as well.

So, because of those issues I decided to put this project on hold. The current set-up is fine for everyday usage. On top of this, my next son is about to born and I will have time in the next few months to spend in the garage. We got 2 months till labor and I want to ride as much as I can during this two months. After that I can spend a lot of time in my garage because of obvious reasons. Also, when I drop the head in the machine shop I want the valves to be changed. The XJ900 valves a bit larger: 2 mm for the intake and 1 mm for the exhaust.

As a conclusion, I would say it's not impossible either way (XJ750 rubbers - cables vs. XJ900 rubbers - cables) but it needs a few things to be a proper solution. By the way, guys, those Mikunis' are state of the art compared to the original Hitachis'. Not just the externals but the internals as well. They are way better carbs than the Hitachis' as it looks after the first check.