RPM not going back down

[LeBouc]

Hi all.

I have a problem with my XS400H and I think it's the carbs that are at fault. I just reajust the valves and the carburetors because I had a hard time starting the bike. Now, it starts at the first touch of the start button and idles perfectly. There was a duct at the back of the bottom part of one carburetor (where the little tube protuding from the top enters) that was blocked and the level of fuel in the bowl was really too low. I brought this level to approximatly 4mm (the Service manual specify around 3mm) from the top of the bowl using the clear tube method to have both carbs level at the same height.

The problem is when I try to ride the bike; the RPM does not go back down when I shut down the throttle (which is working perfectly - doesn't stick) while I pull the clutch and I have to leave the clutch in gear for the RPM to finally accept to drop back. It looks like the vaccum slide is not sliding but I have checked both (pushed them up manually and letting them go back down) and they work as expected. The drop back is not sudden, though as if the fuel in the needle jet did not want to move out while the needle is dropping back (not a big amount of fuell to evacuate!).

Is it possible that the fuel level would be too high? It revs to 4500 RPM with the throttle closed as if it was on a BIG enrichner!

Thanks for your help
(Mikuni 34BS CV carb)

 

[CaptChrome]

The problem is when I try to ride the bike; the RPM does not go back down when I shut down the throttle

That is a classic symptom of the pilot circuit of your carburetors being too lean. You need to open the idle mixture screws on each carb. There have been a lot of discussions on this forum of how to do that, and some very recent explanations. Try to search for that info and if you have questions I am sure we can point you in the right direction.

 

[LeBouc]

Thanks. I'll try that but it's already opened 3 1/2 turns! What I have read is that 3 1/4 was what people were happy with!

 

[CaptChrome]

Every bike will be different, especially if there are changes to it other than the stock air box and muffler systems.

But if the screws are already that far out, the lean conditions could also be caused by an air leak between the carbs and the engine, so you should also test your carb holders for leaks as well as the butterfly shaft seals and make sure they vacuum hoses or caps are sealing. Use something like WD-40 to spray one area at a time and then blip the throttle to see if the revs are still hanging.

 

[LeBouc]

carb holders are brand new! Vacuum hose and cap have been verified. No change to the bike; it is all original. I will shoot compressed air at the pivot of the butterfly (which, by the way, is barely opened while the adjustment screw is full in!). I remember one detail: when the engine spins without me opening the throttle, I can hear some sputter; the general sound is a little harsh - could this mean too lean?

 

[tstidham]

Before I replaced my shaft seals, I had a lot of popping in the exhaust on deceleration. The mixture was likely so lean that the plugs weren't igniting it in the cylinders and letting the raw fuel into the mufflers.

 

[LeBouc]

Good to know! Thanks. One thing, though: the bike only had a starting problem; other than that, it was running just fine!

 

[LeBouc]

So the problem was that the idle adjustment screw has been set very high to compensate for the pilot jet which was not set properly. I reajusted the idle adjusment screw to have a minimal idle then adjusted the pilot jet unscrewing it until the rpm stop raising and then brought back the idle adjustment screw so that it would give me an idle sufficient to avoid the stoppage of the engine. When I go higher than that with the latter screw, the rpm does not drop back when shifting or when arriving at a stop. With those new setup, the engine feels smoother than before so I am happy with the result.

I had a little problem when I made my final test: when I unscrewed the idle adjustment screw, the throttle cable became loose and, in the background, the barrel began to come out of its place. Later, on the road, vibration helping, it came out completely leaving me with an engine that would only idle! By chance, I was not too far from my home so I could walk back, get my car and my trailor and reclaim my motorcycle which was patiently waiting for me! I talk about that to warn that if you unscrew the idle adjustment screw enough so that there is no more tension on it, you have to verify if it is still at its place before closing everything and going for a ride.

So thanks again for your help and have a great time!

 

[kebabs]

Hi all.

I have a problem with my XS400H and I think it's the carbs that are at fault. I just reajust the valves and the carburetors because I had a hard time starting the bike. Now, it starts at the first touch of the start button and idles perfectly. There was a duct at the back of the bottom part of one carburetor (where the little tube protuding from the top enters) that was blocked and the level of fuel in the bowl was really too low. I brought this level to approximatly 4mm (the Service manual specify around 3mm) from the top of the bowl using the clear tube method to have both carbs level at the same height.

The problem is when I try to ride the bike; the RPM does not go back down when I shut down the throttle (which is working perfectly - doesn't stick) while I pull the clutch and I have to leave the clutch in gear for the RPM to finally accept to drop back. It looks like the vaccum slide is not sliding but I have checked both (pushed them up manually and letting them go back down) and they work as expected. The drop back is not sudden, though as if the fuel in the needle jet did not want to move out while the needle is dropping back (not a big amount of fuell to evacuate!).

Is it possible that the fuel level would be too high? It revs to 4500 RPM with the throttle closed as if it was on a BIG enrichner!

Thanks for your help
(Mikuni 34BS CV carb)

 

[kebabs]

might be useful.

 

[LeBouc]

Thanks, Kebabs. Those infos are precious as they will help me understand what is going on. I rode my bike again and things didn't go so well so I am back to changing the float height again. I have read this article (https://www.advrider.com/f/threads/float-height-importance-in-carb-tuning.110771/) in which a factory expert says that the float height influence quite strongly (25%) the fuel/air ratio. I think that, in my carbs, the fuel level is too high (set at 4mm from the top), that it is too easy for the fuel to be sucked into the vacuum chamber and that it is why I cannot adjust the idle to anything higher than 500 rpm without having the engine jumping by itself to 4500 rpm as soon as I ride and pull the clutch while the throttle is closed.

 

[kebabs]

Thanks, Kebabs. Those infos are precious as they will help me understand what is going on. I rode my bike again and things didn't go so well so I am back to changing the float height again. I have read this article (https://www.advrider.com/f/threads/float-height-importance-in-carb-tuning.110771/) in which a factory expert says that the float height influence quite strongly (25%) the fuel/air ratio. I think that, in my carbs, the fuel level is too high (set at 4mm from the top), that it is too easy for the fuel to be sucked into the vacuum chamber and that it is why I cannot adjust the idle to anything higher than 500 rpm without having the engine jumping by itself to 4500 rpm as soon as I ride and pull the clutch while the throttle is closed.

no problem. adjust the float level to 32mm. I've tried the 26mm did not work for me.i have the same carbs as you.

 

[kebabs]

no problem. adjust the float level to 32mm. I've tried the 26mm did not work for me.i have the same carbs as you.

 

[kebabs]

CaptChrome helped me lots when getting my carbs correct.

 

[LeBouc]

I am done with it for this year! Too many other things to do before the snow covers the land (normally, it begins by mid october and stays from mid-novemeber to end of april!). I was working the two carbs together to adjust it and, just to see, I completely screwed in the pilot screw on the left side; the engine wanted to die so I unscrew it 1 turn and the engine idled again. I tried the same thing with the right carb, no effect at all! Screwing in, screwing out, it didn't change a thing! So I dismantle both carbs to compare them and sprayed through all holes and jet of the pilot jet circuit; same result on both side. Retried on the bike and the right carb pilot screw was not affecting the engine at all! So that's enough for methis year. Sad because the bike was working properly other than the cold starting which was a problem (right cylinder not working until 2500 rpm and then would work without a hicup!). The bike is now parked for winter and I'll go kayaking instead!

 

[kebabs]

I am done with it for this year! Too many other things to do before the snow covers the land (normally, it begins by mid october and stays from mid-novemeber to end of april!). I was working the two carbs together to adjust it and, just to see, I completely screwed in the pilot screw on the left side; the engine wanted to die so I unscrew it 1 turn and the engine idled again. I tried the same thing with the right carb, no effect at all! Screwing in, screwing out, it didn't change a thing! So I dismantle both carbs to compare them and sprayed through all holes and jet of the pilot jet circuit; same result on both side. Retried on the bike and the right carb pilot screw was not affecting the engine at all! So that's enough for methis year. Sad because the bike was working properly other than the cold starting which was a problem (right cylinder not working until 2500 rpm and then would work without a hicup!). The bike is now parked for winter and I'll go kayaking instead!

sorry for the smiling moje. been there done it.bike near drove me over the edge. but with help from this forum I powered on .my bike is running sweet for now.been told to not get used to it as these bikes are temperamental maybe your doing the best thing and stepping bk for a while.there must be something gummed up with your right carb if not responding. Happy kayaking

 

[kebabs]

sorry for the smiling moje. been there done it.bike near drove me over the edge. but with help from this forum I powered on .my bike is running sweet for now.been told to not get used to it as these bikes are temperamental maybe your doing the best thing and stepping bk for a while.there must be something gummed up with your right carb if not responding. Happy kayaking

I also found on the cold mornings if difficult starting. take the plugs out and heat electrode with a blow torch my bike started no problem after doing it.

 

[tstidham]

Your pilot jet or passages to the idle mixture screw are clogged. I had mine apart three times with the same unresponsive idle mix screws. It took using a single strand of multi-strand copper wire (10 or 12 gauge I think) to use as a ream and despite spraying brake cleaner through everything, the copper wire pushed additional gunk out of the pilot jets. I also ran the copper wire through all the passages as best I could with liberal amounts of cleaner chasing it.

 

[LeBouc]

tstidham, I checked and rechecked the passages and the jets and they are all free flowing. I can spray brake fluid and see it spaying out at the right place. The problem is not there. I thought that it could be caused by the rubber cap on the pilot jet inlet that would prevent fuel from entering so I removed them but that didn't change anything. I begin to think that it might be something else than the carbs.

ksbabs, your floats are most probably the metal round ones which protude higher than the platic ones which have a partly flat bottom. So your height is good for your type of float only. Also, if you look at the picture, you will see that the bottom part of the carb has a screw that leads to a nipple; when unscrewing it, it lets the fuel go through the nipple. We can use it to either empty the float bowl or plug a transparent tube to measure the height of the fuel in the bowl.

 

[kebabs]

tstidham, I checked and rechecked the passages and the jets and they are all free flowing. I can spray brake fluid and see it spaying out at the right place. The problem is not there. I thought that it could be caused by the rubber cap on the pilot jet inlet that would prevent fuel from entering so I removed them but that didn't change anything. I begin to think that it might be something else than the carbs.

ksbabs, your floats are most probably the metal round ones which protude higher than the platic ones which have a partly flat bottom. So your height is good for your type of float only. Also, if you look at the picture, you will see that the bottom part of the carb has a screw that leads to a nipple; when unscrewing it, it lets the fuel go through the nipple. We can use it to either empty the float bowl or plug a transparent tube to measure the height of the fuel in the bowl.

yes mine are the metal floats have you checked the fuel mixture screws are the tips of them ok
are your coils ok
I had a idle problem when engine was hot it would cut out at traffic lights etc.i always thought it was the carbs .I changed the coils it now idles perfect when hot.the coils on my bike were 43 years old and needed changing.