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!