Carb diaphragm vacuum

[LeBouc]

Hi. I am still having my problem with my engine (XS400H) which won't accept to go over 6000 rpm. I have checked all the electrical side of the firing and everything is working perfectly. I checked on the internet and it is telleing me that I could have a problem with the vacuum that makes the slide go up. That vacuum should be on top of the diaphragm so that it would suck the slide with it. So i am looking at my carbs and cannot find a hole that could carry that vacuum at the top. I couldn't find any specifics explanations for the BS34 so I am coming here hoping that somebody will explain to me how those CV carbs are working! Underneath the diaphragm, I can see a small hole on the side that seems to be connected to 3 tiny holes just behind the choke one but I cannot get any air through them. Is this somehow related to my RPM problem?

 

[CaptChrome]

So i am looking at my carbs and cannot find a hole that could carry that vacuum at the top. I couldn't find any specifics explanations for the BS34 so I am coming here hoping that somebody will explain to me how those CV carbs are working

There is a small hole in the piston that admits vacuum to the top of the diaphragm:

You can see that when installed in the carb, the hole in the pistons is on the engine side of the venturi which is the point of highest vacuum in the carb.

The bottom side of the diaphragm remains at intake manifold pressure because it is connected through this duct in the carb body:

If you blow air into this duct, you should see the carb piston rise.

 

[LeBouc]

There is a small hole in the piston that admits vacuum to the top of the diaphragm:
View attachment 39611

You can see that when installed in the carb, the hole in the pistons is on the engine side of the venturi which is the point of highest vacuum in the carb.
View attachment 39612

The bottom side of the diaphragm remains at intake manifold pressure because it is connected through this duct in the carb body:
View attachment 39613
If you blow air into this duct, you should see the carb piston rise.

 

[LeBouc]

Thanks for your reply. I was in my shower and was thinking about this little hole in the piston, remembering that in the XS650 forum, somebody specified that that hole had to go on the engine side (no real choice with the tab on the diaphragm!); I was wandering if this was the vacuum transfer channel! Good idea for the pressurised air; it will allow me to see if both springs have the same strength. I read that spring strength can change when left compressed and my bike was sitting for 15 years (I rode a BMW R1100RS instead) and new ones are not available anywhere! I have lowered the clip on the needle to see if it makes the engine gain a couple of hundreds and I will use the enricher as well to see if I gain anything; it could help me diagnose the problem (the main jet is bigger as well). It's weird because the engine pulls really good between 4000 and 5900 and then cough like it's having a big cold! I have noticed that, at 6000, if I slightly close the thorttle, the engine stop coughing and seems to want to give me a tiny little bit more! A puzzle!

Instead of pushing air through the diaphragm chamber intake, I plugged a vacuum cleaner at the engine side; I could see the piston rise somehow and when I masked most of the air cleaner side with my hand (without blocking the air intake of the diaphragm chamber), the piston clearly rose to the top. I made some calculation and, at 6000 RPM, each cylinder sucks a bit more than 80 cubic feet per minute (CFM) and à 9500 RPM, it goes to 130 CFM. I have kitchen fan that is good for 260 CFM (at the max) so i am going to attach it to both carbs to see if there is any discrepancy between the two by changing the speed of the fan and playing with the throttle. With that, I should eliminate any doubt about the piston not rising and choking the engine!

 

[RadlagerMike]

Wehre the gasoline is coming in there is a sieve / strainer inside, if the bike has Not been used for 15 years be sure it has to be cleaned

 

[LeBouc]

Wehre the gasoline is coming in there is a sieve / strainer inside, if the bike has Not been used for 15 years be sure it has to be cleaned

Thanks. It was parked for 15 years but I have put it back on the road since 2021. However, I have lived with the 6000 RPM restriction but I want to have the bike back in perfect order; one never knows when having to reach the top speed this bike is capable of will happen!

 

[CaptChrome]

Great suggestion by @RadlagerMike. That filter is often overlooked. Here is an old post that has the location of that net filter: https://www.xs400.com/threads/fueling-question.21475/post-190503

 

[LeBouc]

Great suggestion by @RadlagerMike. That filter is often overlooked. Here is an old post that has the location of that net filter: https://www.xs400.com/threads/fueling-question.21475/post-190503

Sad that the guy did not come back to tell if the suggestions cleared the problem. I replied to his last post to hear back. I had already verified that but I will check again!.

 

[LeBouc]

Great suggestion by @RadlagerMike. That filter is often overlooked. Here is an old post that has the location of that net filter: https://www.xs400.com/threads/fueling-question.21475/post-190503

As I expected (because I had already taken care of that at the beginning of the season), everything is OK on the fuel delivery side. A very small vacuum at the petcock was enough for the fuel to flow at will (much more than the 2 carbs bowl in 1 second). Then I went to the screen in the float bowl and both are simply in perfect shape. Not a good time to test the raising of the needle because it rains continuously here and I am afraid that the humidity percentage (100%) could affect the result because there is less oxygen in a cubic feet.

I have began to construct my vacuum fan cage to test if the carb's pistons are reacting as expected and the same way on both side. I will be able to report on that sometimes soon.

I have changed the TCI in case the problem would be there but I have the same reaction. It is still possible that both TCI would have the same problem though. I have seen a post here (from 2016) in which a guy had re-soldiered all the contacts in the TCI (just melting the existing solders) and it solved a problem very similar to mine. I am going to try that on the one I had on the bike.