1980 XJ650 Carb Tuning

Hello All!

First post here and hoping to get some help with these monstrous Mikuni carbs.

I got the bike a couple months ago in fairly rough condition, airbox not connected properly, bike would not run well without choke at all. I have returned most of it back to stock except the exhaust pipes which are aftermarket and more open. (I tried to buy donor stock exhaust and it was full of dead mice... neato.) I got brand new rubber manifolds between the air box/carbs since the old ones were stiff and shortened. Also new manifolds between the carbs/engine. I recently put new coils/plug wires as well to make sure the spark was good. Today I got compression readings from the 4 cylinders with good results: Cylinders 1, 2, 3, 4 read 115, 120, 120, 120 respectfully. The fuel petcock works as intended and doesn't flow freely unless set to PRI. The gas cap is also in good condition. So aside from carbs I cannot fathom why else this thing has destroyed my towns stock of NKG BPR7ES spark plugs. The plugs from cylinders 1-4 are rich, lean, lean, and mega lean. The 4 cylinder plug is completely white. Also why I'm thinking it has to be come kind of carb balance. These carbs were bought refurbished from eBay since the originals were impossible to get apart with on-hand tools without ruining the screws. The refurbished carbs look good in person and everything moves smoothly but I've never seen them apart before. The seller (whom I don't doubt their expertise) claims to have rebuilt Mikuni carbs hundreds of times and has gone through these thoroughly. Still, the bike doesn't run well, it idles steady, but hangs every now and then. Its hesitant from a stop, then hits a flat spot at 6k rpm and will lose all power on wide open throttle. No backfires, or cracking just runs like crap and I'm frying spark plugs in less than a mile ridden. I rode it around the block to get it registered last weekend and the brand new plugs are already ruined. Not sure what else to do or how to do it at this point being its my first project bike.

Thank you for your time!

 

What do you mean by frying plugs, what do they look like, can you send a photo of them ?

 

Haven’t seen Mikuni carbs on ANY xj650. It should have Hitachi carbs on it.

 

Yes...Hitachi, I am an idiot. And yes here is a picture of the 4 plugs, as they sat in the engine from left to right. Fried may be an overstatement, but I have never driven a vehicle that has fouled them up this way before.

 

Fuel mixture is off alright one and three rich two lean four very lean, verify that you don't have any air intake leaks, check the intake boots and gaskets between carbs and cylinder heads and around all sides of carburetors etc
You should also check the valve clearance, watch the video on carburetor wet sync of floats on this site and check the volume of the float bowls to verify that your getting the correct amount of fuel to the bowls, you can due the check of the floats with the carbs on the bike but will need to remove them if they are off and that is were the fun starts.
These are the basic diagnostic checks you much do, there is no silver bullet... at this level.

 

Will do, carbs are off right now and float bowls are open. I will watch the video and see about getting the floats squared away.

 

Lets do this right, OK? VALVE CLEARANCES first.

Accurate wet-set of float levels and a careful bench sync.

Then a running vacuum sync.

Based on symptoms and plugs, float levels are likely the primary culprit here, but you won't get a good vac sync (or a good running motor) if the valves are out of spec. So do those first to save yourself some head-scratching in the near future.

 

I have to ask, do we know who was the carb rebuilding expert ?

 

@bigfitz52 I'll need some tools for valve clearances, and a vacuum sync is that the 4 gauges thing that I see people using trying to get the needles all pointing the same way?
@Toomanybikes and my carb building expert was a seller "sbmisajw" aka Steve. Despite the name he actually communicated allot with me and even took the carbs back for another look when I said I still had issues. So no issues with him at all.

 

Alright I did a little research and have a carb vacuum sync gauge set and valve gap tool on order. Will report back once I've checked valve clearances, wet float bench synced, and vacuum synced.

 

Here you are - not factory spec...

 

From a maxim X ?

 

Oops, misread the question - the bike is a Jap import 650 maxim, with gold wheels, trim around the tank, and some bits obviously painted over. The carbs - and airbox are from an xj600, fj600 in the us.
It came to me missing the bulhorn bars, carbs, footpegs and control switches...
I never saw the original title/registration documenr.

 

Good time to spend some time reading about the procedures your about to get into.
A lot of us have had to due them more then once, hint...read and try to learn from the mistakes so many have made here on this site.

 

Absolutely! I've been reading many links provided about wet-setting and such. So far, the floats dry-set measurements are 17.5mm from where the gasket would sit, to the seam in the float itself, not exactly the top edge. I will try wet setting them tomorrow since I work late today. I got some good ideas on how to set the carbs upright thanks to fitz mentioning people screwing old manifolds to wood in order to hold them. I just so happen to have a bunch of extra manifolds and a solid work bench. Ill also be using the fuel tank being I have enough room and the petcock works well for shutoff. Looking forward to the potential frustrations but hopefully, I can get them all wet set while I wait for the vacuum sync tool to arrive. The valve clearances will be another thing before I put the bike back together but I'm awaiting that tool as well. I found a good video of a guy going through the motions for that job.

 

Yes indeed. Although they don't all use gauges, some use tubes with sliders in them, like the CarbTune Pro or Motion Pro copy thereof. But yes; that is the final step.

The first tool you need for valve clearances is a "primarily" metric feeler gauge set; in other words one marked in metric (it can have American equivalents) but trying to use an American feeler (and/or the equivalent spec) is not the way to go. Then either the valve bucket hold down tool or an 8" length of insulated #12 house wire (depending on which valve holding method you choose.)

Valve clearances, lots of pics, both methods shown: https://xjbikes.com/forums/threads/bigfitzs-airhead-valve-adjustment-with-pics-parts-i-ii.116006/

Wet-settng float levels will require some sort of bench-top fuel supply. Don't burn your garage down, remember that gasoline fumes, not liquid gas, is what's flammable.

And don't be too hard on whoever did your carbs. Wet-setting float levels, bench sync are often left to the customer to do. The running vac sync, of course, requires the bike so that couldn't have been done regardless. Floats can bounce around enough in shipping that I'd check float levels even on brand new carbs.

 

I agree with the above statement about rechecking the floats after having them done but it would also be nice if they told the person having them done before they reinstall them on the bike.

 

Like i said im new to this and still learning so i was more than happy when the seller offered to take the carbs back and look at them one more time because i mentioned having issues. Fitz brings up a good point about the fine adjustments needing to be done by the customer and on the actual bike to which they will be mounted. Im just glad to have an abundance of videos and instructions on this forum on how to do these tasks!

 

You really should be able to get the floats dry and then wet set on the bench close enough. You should also be able to bench synch them close enough to run and ride once fitted to the bike - then use gauges to sync them running.

 

Yes they are all set very close to 17.5 mm from the gasket seat to the seam of the float on the higher edge, not the top edge. Either way today is the day for wet setting!

 

Alrighty gents, just came in after an interesting start. I've watched videos and read threads on how to do this but im guessing that i dont have the order of operations right. First question is do you leave the petcock open on the fuel supply for the whole time or do you shut it off once the bowls are full? (I also don't know how to tell when the bowls are full aside from every carb leaking fuel) I cant figure out how to get a stable level, when the petcock is open, the level climbs well above what it should. Is my fuel tank set too high? If i shut it off, by the time i get the air bubbles out the fuel level is too low because it all ran out. This is what im doing, I have cycled the drain plug and run fuel through a few times before measuring. I leave the petcock open, close the drain plug to let the fuel run out of the clear tube. Lift the tube up for measuring, open the drain plug and make sure no air bubble come out. The petcock is open this whole time and at some point, all carbs begin to leak fuel everywhere and the level in the hose rises and rises until it is coming out of the top of the hose. So i shut the petcock off, the fuel runs out of the hose, and the bowls finish leaking the excess fuel on the floor. Its a mess, but when i try to do the same procedure with the petcock off the fuel level isnt enough to even measure.

 

Your floats must be sticking open, you need to inspect them, doing it is just like the video. Watch it again...
Petcock stays on all the time until you remove the hose for setting the floats if need be , once you get the air bubbles out of the line lift the hose against the front of the bowl and that's the setting.
Reinspect your needles and seats, they may be installed wrong or something, maybe they are of low quality, you may need to replace them ?
Are you using a level, I don't see one one in the photo.
The correct measurement level of fuel in the hose when it is against the carb base is the most lowest level of fluid in the hose or the bottom of the curve of fluid in the hose.
Be precise as you can take your time.
I spent about 3 hours doing mine and on the last test I found a needle and seat that decided to leak then I replaced them all and did the whole resetting all over again. they leak intermittently. Then I took them off the bike again because I didn't like how it ran and reset them again. Carbs off and on three times this summer !

 

I have been at it for 5 hours, it is the same every time no matter what adjustments i make. The first carb spits fuel from the intake side passages and ruins any chance i have at getting an accurate level. I have checked the float and needle and it depresses the needle closed, and springs back every time. I have spares from a kit i got awhile ago but the ones in it look brand new. So i guess for the past 5 hours ive made no progress. I suppose i can try to replace the needles with the ones i have on hand, but this has been soul crushing.

 

Old viton tipped float needles are guarenteed to be perished after 10 years, let alone 30.
Also make sure that the spare tank you are using for a fuel source is spotlessly clean. Even a little fleck of rust can prevent a float needle from sealing.
Were the crush washers for the float needleseats replaced? Some are fiber, and those do fail eventually.

 

Here's a pic of the #1 float needle that seems to be not working. Silver with a black rubbery tip. Part if the issue is I don't know what the seller used to rebuild these and when. The rest of the bike is in good shape for its age along with the tank. I've checked for rust many times before and its still good and shiny. So do you gents believe that the float needles are the primary cause of the bowls overfilling? or is there something else I need to check? Ill replace all 4 just in case, but the #1 has been extremely all over the place reading-wise for obvious reason. No possible way to get a static fuel level. This has also probably sent the fuel floats off as well as i had been adjusting to the false readings for a bit before i realized there was no stopping #1 from leaking. (Oh and yes the carbs were leveled front to back, side to side that's why the zip tie was on top to get the front to back level good.)

 

did you do a wet set of the fuel levels in the carbs?
you may just need to adjust floats.
your needle assembly looks new. does the pin on bottom near clip have a springy action?
Setting the fuel levels

have you checked to see if you can stop fuel flow by holding up the floats to cloose the needles?

 

Sounds like you need to get a set from Len at XJ4ever part's
Needles and seats.
It's a huge learning experience and nice to have it working correctly when it's all said and done.
Don't use any ethanol fuel.

 

Just went out to test the #1 again by oversetting the float so it would close very early and it seemed to work for a second, it definitely took longer but alas it still overflowed. The float cant push the needle into the seat any further, its like they're not long enough or something isn't fitting right. I suppose i will try to get new ones. The xj4ever site has many part numbers so i'm not sure which one to get and aren't cheap. Due to budget constraints i will not be able to purchase these for some time.

 

I think that's what needs to happen-e mail Len and he will let you know what you need.

 

Before I email him can any of you identify the ones I'm currently using? Do they resemble the correct ones or no? I cant seem to find images of what the right ones look like, just part numbers that yield no image results.

 

That's interesting -viton perishes? It would explain why they leak with no visible sign of wear.

 

They look alright just likely low quality, doesn't really matter at this stage.

 

Time...Viton is not supposed to wear out, but the combination of age and wear agianst the needle seat adds up to eventual leakage.

 

screw in needle seats?
shouldn't they be slip-in with an o-ring seal?

 

if they were Mikunis, yes. Since it's a 1980 XJ650 I believe we are dealing with Hitachi carbs here.

 

I spoke with Len and for ethanol fuel they recommend the metal tipped valves. Also explained how the valves i currently have don't work because they get cocked at an angle when the float presses on them and they don't seal. This I recreated myself and was able to pass air through it quite easily. Hopefully this is the core of my problem and I can get the carbs working once i have the proper valves. 5 hours of checking incorrect wet levels has at least made me fairly quick on tear-down/adjusting/build-up.

 

Correct. Hitachi carbs.

 

I wouldn't recommend using ethanol fuel unless your bike is going to be a every day driver ,if non ethanol fuel is available in your area and your bike is going to be sitting for long periods of time it absorbs water.
There are some additives you can use with ethanol fuel to reduce the affect of the fuel but I'm not sure how well they work ?

 

Depending on location, NOT using it is not always an easy or readily available option. It is only recently that non-ethanol gas has become more widely available, thankfully.

https://www.pure-gas.org/ The map, while entertaining, isn't that easy to use. I use the list (links by state at bottom of page.) My bike is much happier running real gas now.

 

Just checked that map of gas stations and it seems I'm pretty distant from any of them. If fuel additive works I may do more research into them but I'm unsure an hour drive to VA beach is worth a tank of gas. In other news I busted budget and ordered those valves from Len because I wanna get this thing running in order to surprise my father. He was in the Air Force at Clark Air Base in the Philippines and raced a midnight maxim. Said the bike had to be torn apart many times due to their quality gas with dirt floating in it!

 

They work really well. I've had a carbureted vehicle sit for several years with E-10 in the gas tanks (treated before sitting) and it fired right up once the float bowl filled. I even burned through both fuel tanks without issue before putting new gas in. Granted a Quadrajet isn't a Hitachi, but it is just about as finicky.

 

Yup... I have to drive about 30 minutes to get non ethanol fuel and I just fill a couple of gas cans.
If you store your bike for periods of time things have changed over the years with ethanol fuel, now they say to drain the fuel tank and garbs then add two stoke oil or something of that nature to the tank to line the inners to prevent rust.

 

That looks like it is in fairly nice condition, how many miles ?
As you are learning it can get really costly fixing up these older bikes and the parts are getting to be a issue to find on the positive side I believe that it is a healthy fun addition.

 

The one i linked was my fathers bike which is long gone by now. He drag raced it in the philipines on a honda race team of all places. He removed the yamaha badges and wore a honda jacket i guess to disguise the bike. Said there was no other bikes at the time that could keep up with the thing. The bike im working on has just over 20k miles on it. And all i need is stock exhaust pipes and silencers in order for it to be back to the oem look.

 

Alrighty gents! I got the new valves and seats from Len yesterday and installed them into the carbs today. No leak from the carbs! I was able to wet set them all within 1mm of each other fairly quickly without spilling all the gas like before. I got the itch to see how they worked on the bike (No I haven't done the valves yet but I need more money next month to get those parts) because before they were awful. The bike took a minute to start this time, after a few tries I opened the throttle a bit on full choke and the bike fired up. I left it to idle in choke until it died once warmed, this was a good sign because before it ran on choke non stop. Turned it on again, slowly shut the choke off and eventually got it to idle without the choke. The revs hang only a little now, it returns to 1000 rpm in a few seconds. I rode it around the block as I usually do because before this was enough to completely ruin the spark plugs. The bike pops on deceleration, depending on how hard you engine brake, this is a new development but the bike no longer has hesitation or flat spots in the rev range. It pulls all the way through, which it never did before. Came back and let it idle again in the driveway because before it would eventually die when I did this. It would dip a little and come back up in rpms but it didn't die. Pulled the plugs to verify they weren't fouled and all of them were still new looking save for a faint haze around the outside ring. So this is a huge improvement from 3 months ago. The carb pilot mixture screws are exposed and no longer plugged so when I get off work I will make sure these are all set the same. Just looking at them they're all sitting in different positions from each other. I am also wondering if the open exhaust pipe is causing the popping on deceleration. This is why I will be trying to get the OEM or close to it exhaust pipes/silencers. Early next month I will take on the valves and see which ones need replacing, the bike doesn't idle rough and there are no ticking noises so hopefully any changes will be minor. Fantastic way to start today, confidence in this machine is slowly being restored.

 

Could be the exhaust, sucking air in causing the backfire, maybe some other members have been here before ? Glad to here that it's doing so much better!

 

Hogfiddles runs a shim pool on an exchange +postage basis. You won't need to replace all of the shims, just a few.

Popping on deceleration is normally indicative of a too-rich fuel mixture.
Check the valve clearances, note the size and location of the current shims, then contact Hogfiddles to get the shims you need. You will not be able to get a good fuel mixture if the valves are tight.

Another thing. It should never take several seconds for the engine to drop to idle (unless it's rolling). You may have a vacuum leak, or it may be hanging because of the carbs not being dialed in yet (you did a bench synch, correct?). You can't dial them in until you know that the valve clearances are in spec.

And another thing. Shim over bucket valves are supposed to tick. If they aren't ticking then the valve clearances are too tight.

I would recommend not riding the machine until you can get to the valves so you can sort the carbs properly.
You risk damaging the engine if you keep riding it as-is.

 

Of course K-Moe, I couldn't help myself honestly I needed some kind of validation that I was making progress! The bike is already taken apart again and back in the garage. Given the bike's past I would not be surprised if the valve clearances were never checked. The carbs were bench synced but I did not bother with the vacuum sync because I knew it was only going to be together for a short time, basically just verifying I had remedied the most serious issue with the carbs. I was planning on getting the tool to hold down the buckets, a new gasket, and some gasket sealer based off of a good guide that was posted by Fitz. I was planning on taking out each shim and trying to record what number it is (if its still legible). So no worries! I absolutely realize the bike wont run right with bad valve clearances and that's the next step for me which is backwards I know but I just had everything for the carbs first. In the meantime ill compile a list of tools/parts/consumables needed for the valve clearance job. That guide will be invaluable! Also I have to say JHC, I've never worked on a vehicle this extensively before, never tinkered with engine parts, and certainly never cracked open an engine but this has been an awesome learning experience.

 

The valve holding tool:

 

^ tool works well makes an easy quick job.
you will need digital calipers to measure the shims that the mark has been worn or faded
also metric feeler guages