82 xj maxim 650 rough running

Hello all.

I just bought an 82 xj 650 for cheap as a winter project. It's a bike I'm giving away when it's done.

The bike was in the middle of a Cafe build so it has a 4 into1 exhaust handle bars were from a dirt bike.

Bike has not run in years says the previous owner, I got it home and started checking things out and discovered dry rot on the intake boots. Using quick start I verified an air leak.

Ordered new boots and bike started with help from the quick start. The bike would stay running with full choke but bog and die without choke. Tried adjusting idle speed screws blah blah no difference so I ordered a carb rebuild kit replaced the jets with the same size ones which as far as I can tell are stock.

After rebuilding carbs the bike would not start had to close the air fuel mix screws back a turn so they are now at 2 full turns.

The bike will start but turning the throttle bogs the bike down even feathered and now I got back firing thru the carbs and exhaust.

Bench synced carbs but have no sync gages yet waiting on the mail for them but if I can't get bike to operating Temps I can't synch anyways. I've checked for air leaks again don't think there is any.

Could timing be off? Top end was supposedly done it looks as if the gaskets are new. Getting some feeler gages tomorrow to check valves.

Also I thought maybe a restricted pet cock so I installed a new one.
Also did compression check kand all cylinders are at 150psi ,with new plugs new gas.

I'm at a loss to where else to look after the valve Check.

Any ideas where to look next would be very appreciated

 

1982 XJ750 Keeferheydude from Alberta. Hi. I have the same problem. I have a Haynes manual. Big help. If you don't have one. They are invaluable. Did a thorough carb rebuild/bench synch , new plugs, l tested pickup coils tested ignition coils resistance. (Marginally Passable) Has some spark as well(weak). Snipped plug wire ends, Replaced fuse centre. Checked and greased all wiring connections on the bike. Tested all safety shut offs and many more small things. Mixture screws out 2.5 .No vacuum leaks. New air filter. New oil. Has compression. I got her running , over ran too fast And kept stalling and now just backfires. and I think there is a crack somewhere in the casing of the coil or a resistor cap. I heard a rythmic snapping noise possibly indicating an arc. The coils passed the resistance test marginally. Did not test resistor caps. They're 40 years old so I ordered new ones , coils, wires and resistor caps. Plug and play coils, through xj4ever. I did many hours of research to find the source of backfiring,. Much of the source points towards coils going bad if I did the rest of the work. So I'm hoping coils might help. I'm also sourcing a TCI, just to have one on hand as mine was looking pretty corroded inside. I resweated the proper connections on the board just to be sure and put it back on the bike. There could be a miriad of other causes of this problem. Timing. Always the elusive vaccum leak etc. I have the new coil set now . But I'm Getting a bionic hip at months end. Won't be able to give you any updates till I recoup a bit and put it together. These are only things I tried, I'm not giving instruction,. Don't wanna get in trouble lol.. Hopefully this helps you bud. Good day eh!

 

I also performed tests found on the forum on the TCI .. No results, but I may have been doing things wrong. That's when I re-sweat the plug terminations and a few other soldering points on the board. (I only pulled the cover and left the board in place. It did not want to come out so I didn't force it.). There was some internal corrosion though. The bike did start a few times. But like I said I'm buying another TCI for good measure.

 

Bog on throttle/backfire = lean/no fuel. Check the vacuum plungers/needles and confirm they move up and down smoothly. Depending on where you got the rebuild kit it could be a hodgepodge of mismatched parts - the first kit I got off ebay was and the needle/jet were the wrong sizes and would bind. Check float level, the crappy ebay carb kit also had rubber gaskets that weren't quite the right size and actually interfered with the floats.

 

First time I put my carb back together, I tested the vacuum plungers, lifted them up and let them drop. #3 was noticably different, pulled top cover off and I had creased/folded a small portion of it causing a leak across it.
I purchased a 2nd carb set cheap and tested it before I pulled it apart, sure enough, when they put it together they creased the diaphragm.j


Question: What rebuild kit did you get and what parts from the kit did you use?
For the first carb I did the fuel jets were larger than spec and the idle air jet was spec for an 81 not 83 so I went back to spec to start with and ordered more jets from xj4ever.

I had a lower rebuild kit from xj4ever (don't like the flat fine screens). a rebuild kit from the guy I bought it from and a 3rd rebuild kit from amazon. For the most part, the brass parts from the original carb can be used if cleaned up. I did not trust the brass needles in the amazon rebuild kits. I just cleaned all the other brass parts in ultrasonic and used carb cleaner then polished up.
What all parts did you use other than needles and did they have the hole diameter numbers on them? use the wikidot page on the bike/year you have. Don't forget the Haynes manual has the labeling for the air jets backwards. Center is main,

Btw, while on the bike, I take a tube and then check the bowls. I have been troubleshooting a bowl fill issue and have it solved I think. Was happening after just 8-10 miles. At the end the last few rides, I have been checking the fill level. If there is a clog/restriction somewhere or excessive air in the fuel line, I have found that this can happen. What I was experiancing was a lean condition into my ride. Let it sit 30 minutes and it was fine again. Since the carb is tilted on the bike, you have to look at where it is when filled and compare later. This is mine.

 

I did not check the float levels in the carbs.
I turned the air fuel screws out another half turn so there at 1.5 turns from seat and bike will idle perfect but when I open throttle it's bogging and backfiring into exhaust then will die.

The carb rebuild kit I got was from Amazon and the jets were the right numbers I believe. I replaced everything but the diaphragms as the kit did not cone with any but there moving freely and not creased.

At least now it's idling I can synch the carbs maybe. And I'll double check the diaphragms today when I check the float bowl levels

 

Check the plugs. 1.5 turns is probably not enough and you may be running very lean. 2.5 - 3.5 turns is probably more like what is needed.

 

Plugs are new with correct gap.

I started at 3 turns but bike would not start started after I turned down to 2 turns

 

There is no fuel accelerator pump on this bike like many cars so your idle can't be lean or it will bog.
A simple test I feel is to bump up the choke (fuel enrichment lever) to the point RPMs go up a little, you will know its rich and if it stops bogging, you know you should turn up the idle screw.
My idle screws right now in the cold air are at 3 turns or more. Doing a gauge syncrho after bench synchro did help with the idle being reliable at lower RPMS, but did not solve much more.

 

Indeed, no acceleration pump, but the means to enrich is built in. How I hear you ask?
Simple, it’s done by calibrating the hole in the bottom of the slide. This hole is important. What it does is allow the vacuum generated at the bottom of the slide to be fed to the top - this is what lifts the slides.
So, when you twist the grip, the butterflies open, and the rate at which the piston lifts depends on the size of this hole. How high it lifts depends on the amount of vacuum generated by the flow of air under the slide - not engine vacuum (directly). This is all down to Bernoulli’s theorem, to do with energy in a flowing fluid - I won’t try to go into it here.
So, what happens if the hole in the slide is too small? Well then the vacuum below takes longer to get fed to the top, and the slide is delayed. “Damn, I hear you say, that’ll make it weak”. Well actually no.
Since the vacuum below the slide has increased, the main jet and needle sees this increase, and fuel just gushes out.
This, is acceleration enrichment.
And this is one of the reasons pod filters fail. It fails in acceleration because the upstream pressure is increased, meaning the slide runs slightly higher, hence weakening the mixture. The whole point on these carbs being that the fuel emanating from the main jet is at a controlled pressure - CV being constant Vacuum. And this is all down to what Mr Bernoulli came up with, and the slide, spring, diaphragm and little hole in the bottom of the slide does.

 

Update I've got the bike running good now iy was a small air leak in one of the boot caps.

But while I was running the bike it just shut off and would not start again plus looking over the wiring harness it's been cut up and pieced together by previous owner and the lights sometimes worked same with starter I decided to swap in a newer harness.(what a mistake the harness I got is wrong and now can't seem to find a good one.

Debating on a minimal harness built from scratch.

This bike has been modded so much I don't know what's stock and whats not fml