Maxim 400 - Mystery No Start

Hey, I have an ‘82 Maxim 400 that I’ve spent the better part of the last year trying to get going, but it just keeps giving me trouble. I managed to get it to start on fuel recently, but it doesn’t seem to like it. It’ll start after a few cranks, idle fine (left cylinder runs hotter) and then start dying, after which it will refuse to start for the rest of the day. It’s hard to tune a carb on a bike that starts once per day. Here’s what I do know:

• Compression is 110 - 120.
• I have spark, brand new plugs and caps.
• Factory airbox and exhaust, with new hose clamps.
• Will start on brake cleaner.
• When started, will rev slowly.
• Left cylinder runs hotter.
• Starter is good.
• Battery is OK.
• Timing is good.
• Valve clearances are within spec, with brand new shims.
• Carb has been cleaned, fuel filter installed.
• Carb has new gaskets.
• CARB SETTINGS: 42.5 pilot, 25 mm float height, 125 main on left cyl, 115 on right cyl, needle jet in clip position 3. New float gasket.

Messing with fuel mixture screw doesn’t seem to help. I’ve had it in and out and seen no noticeable change. It’s possible that I have the float height set incorrectly, but I cannot for the life of me get a clear answer as to where on the float to get your measurement from. I’m taking mine from the furthest point out of the metal bit on the plastic float body. If that makes any sense.

I know the 400 is technically an XS, but not really… It’s half of an 800, or something. If anyone can offer help I would greatly appreciate it. I’ve run into so many weird problems that half of my google searches are coming up with my forum posts now, lol.

 

I'm not super familiar with the 400 but here goes my thoughts.

Compression seems low to me but I could be wrong. The service manual I found online shows compression should be 156. Did you try adding a little oil and rechecking to see if the numbers go up?

What were your shim clearances before and after? Out of spec on the tight end will result in hard starting and inability to start when warm.

Did you bench sync the carbs?

 

Did you wet set the carbs?

 

Thanks for replying; I’ve been praying compression isn’t my issue but yes, adding oil does improve test results, piston rings are likely worn… I just didn’t want that to be something I had to do haha
I haven’t synced carbs; don’t have the tool for it, and my shim clearances are written down somewhere on my old phone, but they’re all within spec on the larger end of things. I’ll recheck them soon when I find the motivation, lol.

 

Wet syncing requires a way to level them on a bench or in a jig, a clear flexible tubing.

 

So I just did another compression test, and well shit. Dry the right cylinder got 105 psi after 8 cranks and the left 90, and with oil the right got 130 and the left 120... am I looking at piston rings at this point? Maybe pistons themselves? Didn't realize my compression was this abysmal.

 

Yeah that’s really low , piston rings probably been in the bike the whole time - and if it sat for a number of years the build up of rust on cylinder walls could do even more ring damage. I would say most of your problems start with the low compression.

You’re into it now my man. Full rebuild loading ….

 

That's exactly what I didn't wanna hear, but about what I expected. So as a newbie who's never rebuilt an engine before, what am I in for here? Just replacing piston rings and putting it back together with new gaskets or is there more I should be doing?

 

It is really up to you , depends how deep you want to go and there is a preferred correct repair theory when it comes down to that kind of job. Whether you want to replace the rings ( as long as the cylinder walls aren’t scratched up too bad , generally if you can run your finger nail and snag it on any scratches that may be there then you’d had to get some cylinder work done ) if not terrible you can generally get away with a ball hone of the cylinder walls. There are plenty of helpful posts on this website that go into great detail about ring and gasket replacement. Check out some of that information and make a decision on what you want to do. Has anyone ever done just rings and gaskets - yes. Does the engine run fine yes. Should that be the only job done. Well yes and no. It depends how mechanically inclined you feel / what you want out of the bike and how much time and money you feel like sinking into it.
Hope that sheds some light.
Let me know what you decide , always willing to help where’s needed

 

I suppose it would be a good first step to take the head off and inspect the cylinder walls; if there's scoring I really don't think it's worth it to do honing and finding oversized pistons etc. As is the only piston rings I can find are $120 and ship from Italy, if they're legit at all. All I want out of this bike is for it to run, ride, stop, and do this reliably while I learn. I'm trying not to sink much money into it, was just supposed to be a cheap first bike... the rabbithole has gone deep enough as is.

 

You should still have a runner on your hands , 100+ compression although not ideal should still run for some time
I just went back and read how you were setting float height - I don’t think that’s correct. So there’s a drain screw on the float bowl itself - you can remove that and replace it with a bleeder type fitting that will allow you to connect a clear tube , you then reinstall your float bowl and fill with gas - you then have the tube ( approx 6-8 inches in length and you can tape it to the side of the carb vertically - the measurement is usually +-3mm from the line where the float bowl meets the carb body. You should be able to see the gas in the tube and clearly see how close to that line it actually is. There should be a post in the technical chat with pictures of someone doing this set up and measurement.

 

If it hasn't run in a long time you could be looking at stuck piston rings. You could try the 50/50 mix of Acetone and Auto Transmission Fluid and let that soak for several days, maybe topping off a few time during the period. Once running that could also free up stuck rings.

If it starts on the compression it has it should run, so I would suspect you have other issues. A low compression engine will refuse to fire cold and won't start.

Carbs are almost always the issue on older bikes. They get gummed up and can be difficult to clean, particularly if you are not familiar with the specific unit. If it is running on brake cleaner then I would be looking closer at the carbs - wet set to get float height - choke circuit, not sure if 400 has the jet that so easily clogs in the float bowl - pilot jets and their passages are tiny and can easily clog - rubber diaphragm can tear, have holes, or not be installed correctly - bench sync to get them close for initial starting

 

I'll look for that post, sounds weird, but so is everything on these DOHC 400s... and I also thought the compression would be fine, but it does seem to have gone down from last year when I last checked it. If I remember right, it was 120 right cyl and 110 or 100 left cyl dry back then; since it has lost nearly 20 psi. But I'll give that trick a go before I take off the head.

 

I would assume there's a grey area of poor performance; but I've gone through these carbs more times than I can count, used at least 4 cans of carb cleaner on them, plus the jets have been run for a total of maybe 20 minutes. Syncing is a good point though, I should try to make one of those diy syncers, although I've never had the carbs seperated.

 

Have you checked with more than one compression gauge?

 

I am not trying to second guess your work but saying I have gone through the carbs doesn't mean much because no details were mentioned. Now most of us are not familiar with the 400, but a quick look on eBay and it looks like the carbs are very similar to the 4 cylinder bikes.

So have you wet set?

https://www.xj4ever.com/setting fuel levels.pdf

Did you verify the enrichment circuit in the fuel bowl is open? Use a pin light and shine it down the well to see if the enrichment jet is open.




Were throttle shaft seals replaced and enrichment screw O-rings replaced?

Are both enrichment screws set to 2.5 turns out from a soft seat? - could be different for 400, but if they are fine threads this is usually a good starting point

Did you verify the transfer ports are flowing fuel?

 

Make sure gauge is accurate, try another one. Make sure rings aren't stuck. Check the jet at the bottom of the well on the side of the float bowl. Wet set the fuel levels. Try to set the mixture, see if you can find a colortune plug to use, otherwise check your spark plug color. Vacuum balance, make a quick/dirty manometer. All good suggestions.