85' 750 maxim-x poor fuel economy

When I say poor fuel I mean bad. Took the 85' out for a drive, had a friend take my 86' along. Fueled up before leaving, fueled up after returning from 160 Km. trip. No hard riding, both bikes driven the same. Both bikes are almost entirely identical. When fueled up at end of trip, 86' required 7.7 litres, the 85' took 12.6 litres. Yes, it was almost dry. 'why ?' The 85' runs nice after 3000 RPM, a little stumbly in the lower revs, but still runs and doesn't stall. It has to be carbs, but what? If you drove the bike you'd say it performs well, and it 'feels' as though all four cylinders are pulling equally well. I say 'feels' since I am no expert. I've had my 86' for over 4 years now, just got the 85' and have been doing some little things that have been neglected or done 'not just right' by the previous owner. No slander to previous owner, he did what he could with what he had, just I want it as close to original as possible, which is difficult since these and parts for them are hard to come by. Yes, I know, there's some on E-bay and other sites right now, but it hasn't always been that way. Anyway, any suggestions ? I don't want to mess with the carbs in any serious manner if I don't need to.

 

What do your plugs look like? I bet black and sooty.
If so, then, using a properly fitting screwdriver, turn each Pilot screw in until seated, counting the turns to record your "start" position. Adjust all 4 an equal amount leaner, maybe a quarter turn at first, which is alot, see how it drives and check the plugs. Compare plugs to your other bike.

Things to do without pulling carbs,
Check float level using drains and clear tube. You have to pull them to adjust.
Check to see that the enrichment cable has a little slack when closed.
Check for brake drag,
swap air filters with other bike?? An air filter won't get you down to 25 MPG.

Did the PO ever try to re-jet or clean the carbs?
Valve adjustment? How's the compression? Really thick oil? (lol).

After that, you're pulling the carbs.

 

You may want to give the petcock a quick check, if it is faulty there is the possibility that gas is getting sucked into the vacuum line or even just leaking out. (I forget who but there was a post on here a few days back where a guy had that problem).

 

Thanks for the replies. I did check the plugs, yes black and sooty and the 86' looks good in comparison. The enrichment cable does have some slack when all the way in. I haven't tried the float level/clear tube yet, was in process of making one up. Don't have an air filter in at this time. Don't know the compression yet. No brake drag, all smooth and free. Re-jetting is a possibility as the PO was into that sort of thing. Didn't want to try any adjusting until I was confident I hadn't missed any simple things first, but may do that sometime soon anyway. Don't know the original mileage as gauge has been changed, and don't know if valves have been shimmed previously, and not sure yet how to check them. Saw an article somewhere about that, will look it up again and check. Petcock is a possibility, I notice is will revolve around 360 degrees instead of 180 like my 86' but thought it might be a different style. Also, did not notice gas anywhere or smell of gas anytime.

 

that was my problem!! to check I put a piece of clear vac line in place of the factory black line after running about a minute I could see the fuel in the line (you will not be able to suck the fuel through but you could use a vac pump) This dropped me to 15mpg so its not likely to be you problem but if it's leaking less than mine was it could be! you can also check your plug on that cylinder as it will be very black from running rich.
Shaun

 

Obviously running rich; that, coupled with the stumbling at low RPMs, tells me that something in the carbs is tweaked. I just had an issue where I had the idle screw set too high on mine; it didn't idle higher, but it did stumble off the line. Backed it off, and it settled down.

Another possibility would be the idle mixtures set too rich, if the jets haven't been changed. Colortune plug might help here.

As for the petcock, that definitely sounds like it's missing a piece -- it should turn 180 degrees like just about every other petcock out there.

Checking the shims/valve clearances shouldn't bee too much different from the aircooled engines, the procedure for which can be found by searching the forums. If you actually have to change any of them out, that's going to be an adventure (which I haven't had to suffer through yet, praise the deity of your choice). Reason being is that the shims sit *under* the cap, which means removing the camshafts.

I don't blame you for not wanting to tear into the carbs if it isn't necessary, as removing them isn't exactly the easiest task, but I did take an existing writeup on how to clean them and added purty pictures. Lemme know if you want the link.

I'm sure you've heard the name by now, but parts CAN be had for the X, from Chacal. Once you figure out what you need/want, send him a PM and he'll get back to you with price and availability.

 

Get a new petcock.
To check shims/valve clearances, search for bigFitz's pictorial instruction post.
Without an air filter? Get one, now.
Try turning the fuel/air mixture screws in 4 or 5 minutes (pretend the hole is a clock). Clean off the plugs, then run take it out for a 10 mile cruise and then look at the plugs again.

 

I'm have "poor" fuel economy also.
Not nearly that bad though (getting about 34 mpg)
My other X was giving me about 42 last year.

The odd thing is. I pulled the plugs and three of them are WHITE WHITE WHITE and #4 has a nice tan color.
Where the heck is my gas going?

It's synced perfect and will sit and idle til the tank runs dry.
I don't have as much power as X2 but I'm still tweaking it and it's getting there.
Float levels are dead on.
Petcock doesn't leak a drop
choke works perfect but will kill it once it warms up.
No brake drag.
Compression is 145 across the board

How can I be so lean and still get crappy mileage?
Granted most of my miles have been high-speed highway miles with a windshield but the other X was set up the same way last year.

 

To check the valves on these bikes is an absolute headache. I waited until the winter, tore mine down, pulled the engine and did them again. I think it was easier (no bs) to pull the engine than check the clearances with the engine in the bike.

Mike, don't know what is going on with your bike. I get about 40 ish on mine. Could it be the gas? do you get the gas from the same station? I've gotten some horrible gas from the same station on two different days. I had gas for my tractor and it had so much water in it it wasn't funny. I've had this with my bikes too. Runs, but misses and spits like crazy.

I know you are perfectly competent. I'll think about it and shoot you a pm. Did you get my pm on that xj700 air head in W philly??