'81 Maxim 650 - Popping, backfiring with gas + carb cleaner

Hello,

I imagine this is somewhat common, however I'm a new rider and I'm
worried I've done some harm to my 1981 Yamaha Maxim 650.

Background, with all my mishaps included:
When I bought the bike a couple of weeks ago, it appeared to run great
- started up every time, accelerated smoothly, no apparent issues.
Then, I went out of town and didn't ride the bike for 6 days. I don't
know how old the gas was that was in the tank - I'm assuming it was a
couple of weeks old. Well, I left the petcock in the PRI position,
not realizing that I have a different type of petcock than the bikes
we trained on in the MSF class and I now know that the PRI position
lets gas flow freely into the engine, which can cause big problems in
rare circumstances but I'm sure is always a bad idea when you have old
gas in the tank. When I tried to start it up the first time after my
trip out of town, it wouldn't start. I tried for 10-15 minutes to get
it going, but I then gave up. I returned two days later after doing a
little research, sprayed a little bit of starter fluid into the air
box, and the bike started up. I had to rev the engine a bit to keep
it running, it would die if I let it idle with or without the
choke. So, once I was confident that the bike would start, I
drained the gas out of the tank, filled it up with fresh gas, and put
about 4 ounces of B12 Chemtool Carb Cleaner in the tank. I then
switched the petcock to 'On' (at this point I still didn't know what
PRI meant), put a shot of starter fluid into the airbox, started it up
and once it was warmed up it would idle fine. I then went for a ride
around town, which consisted of mostly under-35 mph riding, just to
get the carb cleaner working. It ran great for the first 45 minutes
of the ride, but then suddenly performance went way down. It started
stalling at stop lights unless I revved the engine, and usually when
first accelerating from a stop it would not accelerate smoothly at
all. There were frequent pops and the occasional backfire coming from
the exhaust, and I was putt-putting along with the throttle cranked up
pretty high. After about 50 feet of this putt-putting, something
would catch, the RPMs would jump up to over 3K and it would start
accelerating quickly. So, I put up with this the rest of the way
home, which was about a 10 minute ride, and I then parked the bike and
put the petcock back to the PRI position. The next day I read that
PRI means prime, and it's bad to leave it in that position, so I set
it to ON later that day, as soon as I could. After a few hours, I
tried starting it up again to see if it would idle, but it wouldn't, I
had to rev the engine to keep it running and there were still frequent
pops and the occasional backfire.

One other thing - I have receipts for quite a bit of work that the
previous owner had done just a little over a year ago, including carb
overhauling and new spark plugs.

Are the symptoms I'm describing just indicating that the carb cleaner
is doing what it needs to do or have I done some damage with my
mistakes? I haven't checked if the plugs have been fouled, I plan to
check that this weekend.

I'm comfortable doing routine maintenance on my cars (fluid changes,
spark plugs, brakes), so I think I can handle maintenance on the
motorcycle, however big jobs like carb synchronization scares me. I
have a manufacturer's maintenance manual for this bike.

Thanks in advance for any assistance.
Ken

 

No, you didn't do anything that would totally screw things up--things were
already screwed up. You may have thought that everything was OK, before you
worked on it, but you were wrong--things were not OK. As another poster
pointed out: gunk, dirt, varnish, and your choke circuit were all
compensating and making you think that everything was normal.

If it were me I'd clean the carbs the proper way, not the "spray some stuff
down into the carb throats" cheat method. You'll need to pull the carbs off
the bike and take them apart if you really want to get them clean. All it
takes is one small piece of crap in just the right spot inside to make the
bike run rough. And, after you do this disassembly/reassembly job, you won't
think that synchronizing carbs is such a big deal anymore.

 

First thing is to determine how well it ran originally, and for how
long.

Easy starting? Smooth idle? Free revving? OK to redline? Running fine
for a couple of hours? Had the bike been in regular use before you got
it? If the answers to all these questions are yes, then it's not
gummed carbs.

It all sounds like carb trouble, but if the bike was running fine
before, then the carbs won't magically have gummed up in just six
days. No way. Could be a coil starting to go, for instance: I've had
that happen twice on old Jap bikes, and the first time it fooled me
into thinking it was a fuel problem, because the symptoms were so
similar.

What makes you think the fuel was only a couple of weeks old? If it
was, and the bike had been in regular trouble-free use, then it's not
gummed carbs.

If it had been laid up for some years immediately before you got it,
and you only ride it for five minutes then maybe it is gummed carbs,
or quite possibly crap from the tank in the carbs (though I'd have
thought the fuel filter in the tap would catch it)..... And you've
been given two reasonable answers as to what to do next.

 

It's true there's a lot I don't know about how this bike was treated
in recent years. I do know that the guy I bought it from quit riding
because his knees went out on him. Jan 06 he put about $1000 worth of
work into the bike - including carb reconditioning, tune up, battery,
new fork seals, front brakes, new float bowl gaskets, new air
filter. In April he got the carb needles replaced. I believe he
rode it at least part of last season after that. I'm checking with
the seller now to see what more he can tell me.

I want to burn at least through this tank of gas with the carb cleaner
in it, then if it isn't any better I'll look into the suggestions that
have been made. Problem is, the engine consistently stalls when
idling, and then it won't restart without some starter fluid. I'm
thinking I don't want to ride around with starter fluid strapped to my
bike, so since I've started having trouble I've only ridden the bike
around the neighborhood. It's become quite a pain to go through this
tank of gas. Is there anything I can do to keep the bike from
stalling other than holding on the throttle whenever it's supposed to
be idling?


I think

 

Why not just drain the Gas out of the tank, either thru the petcock,
or take the tank off and empty it? Does sound like it might be Coil
though?
Bg

 

I believe your bike has a pionts and condensor ignition.
The symptoms you desvribe are classic for a failed/weak
condensor. I would start there instead of screwing around
with the carbs since it started the problem in a relatively
short period of time and running.

Lugnut

 

Wrong.

 

Now I feel right at home. My wife must say that at least a
dozen times a day. Then, she usually proceeds to tell me
how stupid I am, why, and exactly how to do it next time!

Cheers,
Lugnut

 

I'm still having some trouble getting my Maxim 650 to run, and I've
gathered some more data that I'd like to share. I hope to be able to
devote a good number of hours to the bike this weekend.

I adjusted the idle screw (my service manual calls it a 'throttle
screw' - item #7 at http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=650201)
so that it would idle without holding on the throttle. That worked
pretty well for a few days, I let it idle for 15-20 minutes a few
times just to see if it could do that without stalling, and I also
went on a very short ride. In my adjustments of the screw, sometimes
the idle would go quickly up to 3000+ RPMS, and I'd cut the engine and
close the throttle screw a bit more.

Then this past Friday I went to fire it up to take it for a ride, and
it started up fine. I held on the throttle a bit to get it warmed up,
around 3000 RPMs. It started sputtering a bit, then there was two
very loud backfires - like gun shots. Then, it stalled. I then tried
to start it again, and it wouldn't start. I sprayed a little starting
fluid on the air filter, tried starting it again, and it backfired
again. This time I saw fire coming from the air inlet, it appeared
that the starting fluid had caught fire. No big deal really, it
burned up and the fire went out. The air filter was not damaged,
much.

So I let it sit until today, and I tinkered with it a bit. I made
sure that the battery was fully charged (I've got a battery charger
that I left attached to the battery while I was cranking the engine)
and adjusted the starter cable so that it was functioning properly.
Figuring that the only thing I really did was fiddle with the throttle
screw, I turned it all the way counter clockwise and then started
turning it back clockwise slowly until it would start. Finally, it
starts now, if I hold the throttle while pushing the start button.
Sometimes, although not regularly, when I'm trying to start it it will
backfire very loudly. Currently, I have to hold the throttle to keep
it from stalling. That makes sense to me, since I don't have the
throttle screw adjusted so that it will idle, only so that it will
start.

One thing that happens consistently is that once it's running and I'm
holding on the throttle at about 2000 RPMs, after running at that
engine speed for 1 to 1-1/2 minutes it will just stall. No backfire
or anything, it just simply dies on me. That has happened regardless
of whether or not I have the choke on.

So tell me, I have to clean the carbs this weekend, don't I? What
else should I check?