I am close to giving up

[mikestp]

After numerous attempts to clean and rebuild my carbs on a 1981 XS400H, I am close to throwing in the towel. I can only get one of my bikes to run on one cyl (the right side), and only start it when full enrichment is on. The other bike (identical year and model) is a similar story. I cleaned them ultrasonically 4-5 times, I even sent them to a chap in Florida and incurred a cost of 300 to get 4 carbs rebuilt with shipping. They are still doing exactly the same as they were before they went to shoot crocs in Florida swamps.

I found a rubber boot from an unknown bike that has about a 1/2 inch offset. I think it might be a CX500 Honda... If I had another to match I think I could mount a pair of Honda CM400 Keihin carbs and just chuck these Mikunis in the swamp.The Honda carbs like the bike itself is soo much better designed...

Also they are easier available off Flea bay and much cheaper to rebuild, and they have a "REAL choke", Also no need to adjust float levels... No diaphragms to fix/replace...

Has anyone else given up on Mikuni and gone over to the VB series of Keihun carbs? If so what did you use to mount them? Also will I need a pushme pullme throttle for the Keihins?

Mike...

I will try a bit more with the Mikunis as I have 300 bucks into them, but I have no problems like this with my Honda bikes....

 

[bcware]

What size jets, main and pilot?
Float height?
Idle mixture screw turns out?
Did you inspect the diaphragms closely?
Did you synch?

 

[Drewcifer]

at this point, after ultrasonic cleaning, you do start to suspect its NOT the carbs, right? Gotta be another set of issues to pick apart. Get manual, multimeter, and calmly get in the zen of bike maintenance. The bike surrenders to your patience as you master it. It's weird, because the issues ALWAYS make you wanna give up, hence the need for the zen of it....

Good luck...

Drewcifer...

PS, you can seal up old boots with thin layers of rtv...

 

[mikestp]

When I sent the carbs to Florida I asked about jet size due to my use of pod filters and low/no restriction mufflers. The reply was that the stock jets would be ok. So whatever the stock sizes are.... I suppose thats what they are... Float height checked out at 25 and 26 MM I did not change it from the Florida settings. This last cleaning I changed the float seats and put new o rings on them. After installing them the floats still checked the same.

Idle mix screws have been tried at 1.5 2.0 2.5 turns out. I dont think the idle screws are important until ll I get it idling at any speed and on both cyls.
Also the boots look fine. The diaphragms check out 100%

When I do a compression test both bikes in both cyl are all 150

When I cup my palm over the boot without the carbs installed and spin it on the starter it holds a low pressure for more than 5 seconds... Indicating no leaks in the intake valve or the boots. Spark seen when shorting to the engine case on all cyls.

As for Zen... Ill Zen the fracking carbs out to the swamp soon...

I have cleaned then again since getting them back from Florida, as the smell of the Florida cleaner reeked.... In hindsight the trip to Florida was a tremendous waste of time and money. I could have bought new Keihun carbs for 100 each.

Anyone ever try different carbs?

 

[jmd_forest]

Lets assume your carbs and compression are good and you are generating spark. That leaves fuel flow and timing. Try giving the bike a few shots of starter fluid and see if it will continue running for a minute or so. If so, you probably still have some type of fuel flow problem, most likely petcock or float valves. Make sure the coils are hooked up correctly. If there is any doubt, simply reverse the connection and try running the bike. Lastly, try statically timeing the bike.

 

[DanMach]

Just keep plugging away. I think you can assume that your carbs are ok at the moment. Being cleaned and rebuilt you should be able to get it to run so-so at least on both cylinders. Assuming compression is decent, you should be getting something again, but what is a work or doesn't work system?

Spark! I know you have 'good spark' when you test it outside the cylinder, but thats not enough. You gotta dig deeper into that and see what its doing.

 

[1979XS4002-F]

With the ignition coils leads swapped will the dropped out cylinder follow?

 

[drewpy]

When I sent the carbs to Florida I asked about jet size due to my use of pod filters and low/no restriction mufflers. The reply was that the stock jets would be ok. So whatever the stock sizes are.... I suppose thats what they are...

trust me, you need bigger jets!

 

[Drewcifer]

Yeah, I second the opinion of the other guys. Every time my electrical shorted I was ready to sell the bike. You have to get zen or the bike will kill you.

First prove you have spark. Prove you get fuel. Prove your valves are correctly spaced, prove you're getting air.

Your compression is fine, so it's got to be coil/plugs/wires maybe?

if you're not patient the bike will make you so.

We feel your pain but you have to slow down and think man, or the bike wins. A dude this week had a rebuilt engine that someone had flipped the timing out of sync by 180 degrees and reversed the plug leads on his coils. Imagine his shock when he checked the timing seeing it dead on but 180 degrees out of phase and the surprise after switching his wires and the engine starting!

his thread here: http://xs400.com/forum/showthread.php?p=83415#post83415

Sometimes it takes eliminating stuff day by day. Post each step, we'll help as you go. So long as you don't give up, we're here to help. We've ALL been there.

 

[mikestp]

I do have spark at both cyl. I can get both bikes to run on one lung. Both bikes have good compression. 150 in all 4 cyls (two bikes) I again stripped down both carbs after getting them back from Florida. The only "new" items in one of the carbs was a couple of float bowl gaskets, but they were of the materiel i never use so I replaced them with the rubberized versions rather than the cheapo cardboard. Also I installed the two finger float seat retainers, the chap in Florida forgot to install them. At the same time I removed the float needle assembly and installed new o rings in the one set of carbs, the second set of carbs were so badly decayed I replaced the brass seat as well as the o rings. One of the carbs did have the two finger hold downs installed.. I also found in the pilot screws... three rubber o rings and one metal o ring. Luck has it I have 10 carbsets here for parts, so I replaced the metal o ring. I reset the float levels as I replaced a weird set of float needles with a better quality set (the needles with "viton" tips (sorta rubber) rather than the metal tips that had been installed. 25 mm on all 4 carbs.
If the float levels were out in my opinion it would not cause the bikes to "one lung... "

I will look at the ignition stuff later today... But they are electronic, and I did not disturb them. Both bikes used to run reasonably well a couple years ago....

 

[Drewcifer]

Just stay chill, have a beer, smoke a bowl, whatever you need to do to let the bike talk to you. Usually you start at the last place you worked on something. It's usually where a mistake was made...

What was the last thing done to the bikes when you had them running two years ago?

I heard last night about a guys bike that had clogged fuel LINES, but they kept trying to unclog the carbs. So make sure your petcock, tank, lines and filters for fuel are ship shape...

PS: Are you batteries fully amped up? Not just volts but amps?

 

[mikestp]

Batteries are new and fully charged.. Tanks are not being used as well as petcocks. I have a bottle and gas line to do initial tuning.

 

[226dark]

Batteries are new and fully charged.. Tanks are not being used as well as petcocks. I have a bottle and gas line to do initial tuning.

Spark on outside but does that guarantee spark in combustion chamber. Would have said possible plug gap off, replace plug or being fouled by too much fuel but you mentioned you have to have choke all the way out just to get to run.

That last post reminds me of a recent thread where the problem was the plastic bottle he was using he did not allow to breath (like the gas cap would) by poking a couple holes in the bottle. Just a thought.

 

[mikestp]

I usually just fill up the 4-5 foot clear gas line, tie it off to the handlebars and watch the gas burn down.. So no vacuum in the "bottle"... no bottle

 

[danisawesome]

When you went through your carbs, did you make sure all of the small passages are clear? I recently rebuilt my carbs and, even after ultrasonic cleaning and soaking them in carb dip, they were still clogged up. I shot carb cleaner and compressed air through them and that did the trick. It seems like you've gone through most other things, but I didn't see anything about the passages being checked.

Here's a link to the guide I used as a reference for my rebuild http://www.mtsac.edu/~cliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdf. It's for a Suzuki I believe, so disregard the float levels and anything else that would seem to be bike specific, but they are the same mechanically and there are a lot of useful pictures, including where the passages are.

Hope this helps! No guarantees it will solve your problems, but while you have them apart, you might as well check.

 

[mikestp]

Yes i always use air to see if all passages are clear. I also rewashed and did it again since they came back from the Florida guy. Rewashed mostly to get rid of the reek of the Florida cleaner...

 

[226dark]

I didn't use any compressed air. I used about 5 bottles of carb cleaner and safety glasses.

Here's a link to the guide I used as a reference for my rebuild http://www.mtsac.edu/~cliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdf.

THAT should be required reading for everyone here. I didn't even spray the short passage in the bowl itself! My method of bench sync was going by the 3 tiny holes, bypass holes, above the butterfly so the 1st one was just barely visible in both the same amount.

Larger jets as mentioned above (with pods).
I would Fill it with gas as you do, then without even running it drain the float bowls into a small container to verify both have the sufficient fuel - that would also make sure if there was any water in left side bowl is gone. Only the right on both bikes fires?
If you slowly throttle up at some point do the rpm's suddenly jump and the left cyl kicks in ever?
Is one pod soaked with filter oil and just not getting enough air?
As suggested did you try reversing the coil wires?
Did you check the spark plugs and reverse those also, or new ones?
Recheck timing and these bikes seem to prefer slightly more advance than recommended setting.

That link from Dan is good reading, yes, required reading one quote:
Pay attention to the choke pickup tube,
passage in the float bowl, and pilot jets. There are very small passages in
these parts that need to be verified as clean.

 

[Drewcifer]

See? With the global time difference you'll get a lot of good leads here once the globe has slept and checked in. Drewpy is a forum Jedi who has seen it all too, so watch for his input as well.

Hope these leads are helpful. once we get you running (and we will) I know others as well as myself are looking forward to your customization, how to, and ride pics..

Stay zen

Drewcifer

 

[mikestp]

I have a question regarding the ignition on the Yamaha 400. With the coils for the two cylinder bike... Do both spark plugs fire at the same time? In other words do both spark plugs fire each time the left and right cylinders need a spark? Is one of them obviously a "dead spark?" Or are they sufficiently isolated to only fire each seperatly when they are supposed to? On many motors there is a dead spark because it was simpler and easier to do so rather than differentiate between the two... I am supposing however that they do spark distictivly as there are two coils... Can anyone confirm this?

And of course the real reason for such an off beat question... The sparks seen on my bikes are there, but they seem to be firing far to quickly for the speed while spinning under the starter. Is this perhaps an indication that something in the ignition control is a bit wonky?

How many sparks per second should I expect to see while spinning under the starter?

 

[JARichmond]

Have you checked the resistances throughout the sparking assembly. Specifically your primary and secondary coils, the resistance across your plug leads, and also I should ask if your spark plugs are hot enough. What plugs are you running? If you don't know, I would suggest getting a pair of new NGK BP7ES. If you don't have a manual for the specs (which are correct as far as I know) you can download one for free on the site.