I am close to giving up

[mikestp]

The plugs are the stock NGK BP7ES I have sandblasted and set Gap at .030

They seem to spark fine at one atmosphere. This is the same for both bikes.

 

[danisawesome]

I didn't even spray the short passage in the bowl itself!

Those passages were extremely clogged on mine, probably from having gas sitting in the bowls for years. Took a bit of effort, but they're real clear now!

 

[bcware]

trust me, you need bigger jets!

This ^^^^

Your jets are way too small.

What kind of pod filters do you have? With just a free flowing exhaust and stock air filters I am at something like 3 sizes up on the mains and 1 size up on the pilots. My mixture screws are at something like 3.5 turns out and I have a bigger pilot than you. You need more fuel!

 

[mikestp]

Well that might be the case.... However that main jet re-sizing only effects the running of the bike after it is off the pilot or starter jets. The fact that you might be right about mid throttle and WOT jetting may be correct, but it does mostly nothing for idle performance and part throttle performance. I should at least be able to get both cyl firing and the bike idling without worry about mid and full throttle mixture settings. I guess I could order different jets. and let my customers wait another week or two... But I really would like to get it running now. If required I can stuff a sock up the exhaust and shove a rag in the intake to block the breathing of the bike so as to simulate the stock exhaust and intake? lol

I had same results with the idle mix screws out 4 turns

It really does make me think that I would be better off installing Honda carbs on all Yamahas.

 

[bcware]

You didn't mention anything about the pilot jet. You need at least one size larger, perhaps two.

You also didn't mention what kind of pod filters you have; it matters.

I was not able to get my bike to run at all with cheap emgo pod filters directly attached to the carbs.

There is nothing wrong with the bs34 carb; it just needs to be clean. After all you've done I'm sure yours are far cleaner than mine. The only option left is that your settings are wrong.

Did you actually measure the float height yourself, or did you rely on someone's report?

Have you played with the idle speed screw? If that is way off you can have problems.

I forget if you mentioned whether you have synched the carbs. Have you at least bench synched them?

Once you've personally verified all settings and you know the carbs to be clean and in good condition (gaskets, o-rings, etc) the carbs can be ruled out.

You've got good compression, so ignition would be the next step once the carbs are done. But, like I've said, they are not done yet.

I have tried restricting pods and exhaust in various ways as well; it did not help with the cheap pods.

 

[mikestp]

The pod filters are currently not installed. There are no filters at this time. The carbs have the correct float level. About 1.5 - 2 mm below the joint. They are all synced manually. I need to get them running to do a better carb sync. The idle set screw is turned a bit to allow it to run, I can crank it up a bit more. Both bikes have a similar problem. When you mention pilot jet.... the air jet? or the small aeration jet next to the main jet?

I have one manifold mount that looks to be able to adapt the Honda carb, if I can find another to match I can test the Honda on the Yammi, but I would prefer to get it running with the Mikuni.

None of this should matter to get the bike to idle with two cylinders running... I can wrap the pod filters with tape to restrict them... perhaps that would allow it to run? The guy in Florida implied that there is no problem with the carbs and although I asked about jet changes he insisted that it was not required. I have since redone the carbs again to fix what he missed... The dealer locally here says he runs bikes up often with no filters for tuning and has no difficulty with the bike at least running with no filters attached. I can find some old original filters and use them to test.....

I would like to try replacing the ignition or at least swapping it out with a spare. I have two CDI units to use as testers.

Mike...

PS I think I can rule out the actual carbs. They are cleaner than new.

 

[Drewcifer]

LOL cleaner than new! YEAH! we were trying to tell you that a problem is usually a chain of issues, and you have to slowly eliminate.

The guys are warning you that in their experience you're not getting enough fuel based on jet size. Drewpy and BCaware both run custom carbs, and have plenty of experience.

If it were me, I'd put the stock carbs back on or find some to see if it runs at factory spec. Then when it runs I'd control each thing you change one at a time. As it is you have CDI, weird jetting, MANUALLY adjusted jets on your carbs that aren't synched, no filters...etc. IN other words, if it's two issues instead of one this will take you a month to figure out. You have too many variables changing. This is the opposite of even basic scientific control.

One cyclinder running and another not running can be a few things. We once found an inlet jet clogged with a slip of Kreem that was poorly laid in a tank and made its way through the fuel line.

 

[mikestp]

WEll I have no tank or petcock in the mix. for now I run a gasline of clear hose and watch as the gas burns or fills the floats. so thre is not gastank, no kreem, no petcock. No filter and the jets are not "weird" they are the stock jets. I will put on the old filters to see if that will allow the bike to run.

 

[mikestp]

I do notice a huge amount of gas being sucked in. and the bike runs extremely rich. It makes my eyes tear there is so much raw fuel being sucked in. The mufflers sometimes "pop" there is so much raw fuel in them...

 

[Drewcifer]

Okay, that is a good lead. If that much fuel is going in I suspect the valves are out of adjustment. Maybe too tight. While I can't speak from experience because i've never used custom carbs, (my bike is rock stock) that sounds like an overly tight adjustment of your intake valves, causing an excess of fuel to be sucked in. Your exhaust valves might be loose too, allowing too much out.

My bike popped when my points were even slightly off, and my fuel wasn't getting burned.

If you're willing to fix it, find stock carbs, start from zero.

PS, the weird jetting is that you're getting the advice your offset of main to idle is out of whack. IE, you're set up for old carbs in terms of jets but the new carbs aren't working well with the jetting... again, my bike is stock, so I'm talking out my donut hole here.

I also remember people making a liquid carb syncher out of tubing and some water and food coloring....manometer I think it's called??

 

[mikestp]

Ok let me repeat... the carbs are not custom. They are stock. The jets are stock. The valves are adjusted to .005 on the intake and .008 on the exhaust. Both bikes valves are adjusted to that spec. The motors both hold a low pressure when cupped with my palm when the carbs were off. They do not show any signs of leakage or of any lifting off the valve seat. They hold that low pressure for more than 5 seconds. I didn't bother holding it for more.. I assure you it is not valve settings.

 

[mikestp]

I will do a carb sync that will be more accurate than the static adjustments done. but to do so I have to have the bike running... till then I cannot use any apperatus to sync carbs. The bike must be running to do a dynamic sync. Till then both carbs open the throttle plates at the same time. Certainly accurate enough to enable the bike to run.

 

[bcware]

You've got to trust us; we know this bike and these carbs better than some random mechanic who may or may not have worked on an xs400 before.

I have also worked on two honda cb360s, so I am familiar with how those carbs will allow the bike to run no matter what you do to them.

The pod filters are currently not installed.

Air filters matter; the bike probably will not run without any. The XS400 is sensitive to changes in the air intake. Any change must be jetted for.

When you mention pilot jet.... the air jet? or the small aeration jet next to the main jet?

The pilot jet is the small jet covered by a plug; it is right next to the main jet. The pilot jet controls the idle and low rpm fuel mixture. The "air jet" on these carbs is actually on the mouth of the venturi.

None of this should matter to get the bike to idle with two cylinders running... I can wrap the pod filters with tape to restrict them... perhaps that would allow it to run?

All of this matters. Wrapping your pod filters may not work. If your pods have an inner lip, like the emgo ones do, the air passages on the mouth of the venturi cn become blocked (see the photo).

The guy in Florida implied that there is no problem with the carbs and although I asked about jet changes he insisted that it was not required. I have since redone the carbs again to fix what he missed... The dealer locally here says he runs bikes up often with no filters for tuning and has no difficulty with the bike at least running with no filters attached. I can find some old original filters and use them to test.....

Put the original filters on if you have them. Some bikes do run fine without filters, like many old hondas; this bike is not one of the ones that can take that kind of abuse.

 

[Drewcifer]

hmmmm. Post pics. We are getting nowhere.

You can't assure anything because you're not going about it controlling the variables. That isn't sinking in.

post pics of your carbs and your set up.

Upload pics to your album, cut the bb code from below the pics and paste into this thread. It will appear as the photo for all viewers.

Drewcifer

 

[mikestp]

ok.... I will try and get the Keihins on it. I just now replaced the CDI unit with a spare and no changes. As you and others have said I think the Keihins are far better to deal with than these Mikunis. I have installed the original air filters and no change. I can get them to run on one cyl, not two. The Keihins are soooo much better it is worth it to me and I am sure to many others to refit these bikes with carbs that work.
The keihins are cheap on Ebay and in wrecking yards all over. for every single Yamaha XS400 sold there were 10 CB/CM 400 Hondas. The parts are easily available and they do not suffer the same finicky problems the Mikuni have. I have now lost 300 to a guy who guaranteed the carbs will work, to the time I have spent, and I am sure I could have bought two new Keihins for less money.

Thanks for all the advice and help...
Mike,...

 

[bcware]

If the stock air filters made no change I'd look beyond the carbs to ignition.

Does the bike have points?

Have you swapped the connections on the coils to see if it runs on the opposite cylinder?

 

[mikestp]

The bike has no points. It has spark I have changed the CDI unit to another I have spare no change.. both bikes I have swapped out the CDI. no change.

 

[bcware]

Did you flip the coils though?

 

[mikestp]

yes no change

 

[bcware]

Ok, I've got to go back to carbs again if it is running on the same cylinder with the coils flipped.

Riddle me this; is it possible to pull the carbs off and move them over; i.e. run the bike with no carb on one side and plug the "good" carb into the side that won't run. I can't remember if there is room for this, but if you can get it to fit you'll definitively prove it's the carbs if it runs on the "bad" side once you move them over.

This may require extension of the fuel line, etc.