XJ750 Midnight Maxim dies out

hey everyone,

I’ve got an 1983 Midnight Maxim 750 that I picked up barely running and figured I’d fix it up since it seemed it just needed a carb job.

I cleaned the carbs over the winter and nothing looked like it needed replacing and the float heights looked alright. Bench synched them and threw em back on.

The issue is that when the bike starts running everything is fine, I’m ideally waiting for it to warm up so I can finish tuning it. About 2 or so minutes later it starts sputtering out and dies.

Here’s what I’ve done/noted so far
I’ve set the mixture screws 2.5 turns out and I’ve also tried them at ~2.75 turns out.
I’ve also tested the battery it’s getting a solid 12.7 volts.
When I pull the plugs they’re all black so I’ve cleaned them and checked for spark on all of them. (I plan on buying 4 new plugs since it’s a cheap possible issue to eliminate)
how strong should the spark look?
I checked the fuel levels while the carbs are on the bike and they look to be about 3mm more or less except for 1, the screw is too stripped to let the fuel come out.
It’s got an air box and no after market pipe and I’ve sprayed carb cleaner while it’s running to check for any leaks.

My guess so far is that something is causing the bike to run too rich for the plugs to spark since if i pull the plugs and clean them it usually starts again.

 

Slight update. I had left the petcock on prime and came back about 30-40 minutes later to gas coming down the airbox overflow. Engine flooded and wouldn't turn over.

Am I right to think that it's definitely the fuel levels that are off? It seems consistent with the richness and flooding.

Also, apart from changing the plugs and oil is there anything else to consider after a flooded engine? I've given it several days to evaporate now.

 

Out of curiosity, do you know if it's the original petcock or a later one. I think the original works off vacuum from the engine, If I remember right.

If it's the original, and leaked, maybe it needs to be rebuilt/replaced.

But I defer to the (much) more experienced people on this forum.

 

If the petcock was left on prime, fuel will flow no matter what.

 

Fuel levels are suspect, but stuck floats, bad or stuck float needles, leaky float needle seat gaskets, or float needle seats that need to be lapped/replaced are all suspects here.

 

oh yeah... Prime! Sometimes I (don't) have a keen grasp of the obvious things.

 

You stated you did the carbs ,but replaced nothing , which probably means you did not break the rack....spray carb cleaner will NOT get into the small carb passages. The big item that needs replaced are the butterfly seals 2 per carb you have to break the rack to replace, these are a MAJOR source of vacuum leaks , can cause all sorts of running issues. Xj4ever Carrie's these seals or #11 buna seals can be used. . Another seal that gets over looked is the small mixture screw seal. With the carbs fully tore down you can soak in my favorite Berrymans carb dip from autozone is a 1 gallon container with a dip basket , soak for 30 minutes . Details of this is in the church of clean , an afternoon this can be accomplished . One item your float needles are probably grooved if they are vitron tipped recommend replacing , xj4ever sells these as well as the solid brass tip I recommend these less chance of leaking .Definitely drain your oil and put in fresh oil .

 

Thanks guys.

I'll be pulling the carbs again hopefully this week. I'll double check all the float needles and gaskets again, it's possible I missed something.
One issue I know I'm going to have is checking the fuel level of the right most carb, 4 I believe, since the screw to let fuel out is stripped beyond belief.
Any ideas on how I should go about delicately taking it out and replacing? I can always set the float heights once I have them off but that won't get the levels perfect.
Also, I was going with 3mm from carb body, can someone confirm that that's right for 750 midnight maxims? I'm going by the Haynes manual.

Jetfixer - I sprayed carb cleaner while the bike was running to check for vacuum leaks. I figured if there was a leak, it would get sucked up and RPMS would spike but everything stayed steady. That should work, right?

 

Propane is better , carb cleaner evaporates quickly , leaks at the shaft seal may not show up at idle and when bike is not at full operating temp . Picture this cold contract , hot expansion , and the seals on the inner cylinders are more difficult to check . The stripped drain screw a small #21 drill bit and easy out , if not try Alum (Spice Aisle in grocery store) You have to keep it hot it will dissolve the steel screw but leave aluminum alone.

 

I have removed stuck drain screws wth a combination of heat, and an ez-out. I've only ever had one that required drilling all the way through the screw to remove.
@hogfiddles. You deal with this more often than I do. What's your method again?

 

Drill in, heat cycled w/kroil, ez-out w/ heat. Never had one not come out

 

Heat is your friend.
Interestingly, I found the mikuni drains far harder to get out than the hitachis - the thread on the plug goes beyond the thread in the bowl, so it tightens upas you slacken it out.

 

By lapping the float needle seats, do you mean ; polishing them with a "Q"tip dipped in fine polishing compound and checked into a drill ? or, dipping a float needle tip into fine compound
and spinning it around in the seat with one's fingertips ?

 

The seats can be polished, the rubber tipped needles no , chuck them you do not want to them seeping , upgrade to solid brass type is better upgrade , xj4ever Carrie's them , I'm running these , but float height will have to be set as they are shorter.

 

Hmm, really? Years ago SU used brass needle valves and seats, they went to "rubber" needles because the brass ones kept passing. Maybe modern fuels don't agree with the rubber tips?

 

Ethanol eats the Viton tips eventually ,they get a groove around the there and can lead to leaking ,if your petcock is left in PRI or RES fuel will continue to flow when it leaks around tip even slowly ,after say a few days this can fill a cylinder with fuel which will leak past rings filling crank case with fuel. The solid brass seem at seal better , recently pulled carbs to change jets , checked them while carbs were off looked like new , but as the old adage goes "your results may vary" Cheers

 

You won't change my mind that brass is a better seal than viton, but the effects of ethanol on these seals may well sway it back to brass, as you say the proof of the pudding etc.
Good info.

 

Ethanol does not damage viton. Being in use for 30 years does.
I've got viton tipped float needles, and they work without leaking.

 

Wouldn't one get a tighter fit by first polishing the seat with a "Q" tip dipped in fine polishing compound, checked into a drill ? Then dip the viton tip into the same compound and spin the needle
against the seat with ones fingers ?

 

Viton doesn't polish, and there is some risk of embedding the compound into the viton. If you go that route, use an old needle to polish with.