My latest little mini-project has been fixing up an old Coleman Powermate Maxa 4000 generator.  My dad bought it new in 1991 and we used it to power powertools while we built the house.  From 1991-1998 it was run 8 hours a day for most weekends.  Since than, it's been sitting in storage.  I decided to pull it out and get back into running condition.

Here's some quick specifications:
- 8 HP Tecumseh engine with a cast-iron sleeve
- 4000 Watts
- 120 V 20 Amp split duplex receptacle
- 240 V 15 Amp duplex receptacle

Though it ran well when it was retired, it had a few minor issues.  The recoil starter would not catch.  It had been repaired once, but had stopped catching.  The other issue is that the 240 volt outlet had developed a power fluctuation issue.  The muffler maybe burned out and need to be replaced.  It is also in need of an oil change, and a throttle adjustment.

Took the recoil starter off.  When the rope was pulled, the dogs would not extend.  I browsed through my brother-in-law's collection of old small engine parts.  Was mostly end covers for old B&S motors, but I did find one Tecumseh starter.  Took it home, but unfortunately it wouldn't work.  It bolted up fine, but was a shallower design, and the dogs were about an inch outside of the cup, meaning a longer cup would be needed to use it.  I took both starters to the local small engine shop.  The owner looked at the original one, and 10 minutes and 10 dollars later, it was working again.

I pulled off the endbell and did some probing.  Found out the fluctuation was from a broken ground terminal in the 240 V outlet.  Simple fix, replace the outlet.  Decided to replace the 120 V outlet too while I had it apart.  It was getting a little loose anyway.  Interesting thing is that the 120 V outlet was a Leviton spec grade, while the 240 V outlet was an Eagle.  I went to my favorite hardware store and picked up a new Leviton Prefered 120 V 20 A brown outlet.  Couldn't find a replacement for the 240 V, so I ordered one online.  Since I had to order it, I went ahead and got a Cooper (Eagle).  Below are photos of the new and old outlets.

New outlets are installed in the endbell.  The wiring layout is kind of interesting.  Each leg from the alternator passes through a 20 amp circuit breaker, down to 240 V outlet (one leg on one side, and one leg on the other), than up to the 120 V outlet.  The outlet is split, so the "bottom" outlet is fed from one leg and the "top" outlet is fed from the other.  That allows each half of the 120 V outlet to supply up to 20 Amps, and up to 33 Amps total.  The 240 V outlet can supply up to 15 Amps total.

Located in the center of the endbell is the end support bearing for the rotor, and the brushes for the rotor's electromagnet.

You said it is loosing the power when loaded:
First, check on, if the engine is really able to give the full rated shaft power (if the rpm does not start to drop; look on the throttle - it it reaches "full open", it means the engine isn't able to give more power).
Other item would be the excitation capacitor. Many of these small generators use the "synchronous reluctance" concept, where the rotor excitation is extracted from the stator by the means of a powewr factor over compensating on the output. That is the use of the capacitor. Normally the excitation is limited by the rotor saturation flux. But if the compensation is insufficient (= worn out capacitor, but too low rpm as well), the output voltage drops.
So for this, check the capacitor value and the rpm governor adjustment.

Because of the genset 20 years sitting in the shelf, I qould guess for the governor spring fatigue. The centrifugal force pulls the weights apart, when it overcomes the spring force, the governor reduces the throttle and so maintain the rpm, where the centrifugal force is equal to the spring tension. SO when the spring has lower tension, that force balance is reached at lower rpm, so the genset operates slower than it should (1800 or 3600rmp for 120Hz, depend if the generator is 2 or 4 pole design).

It wasn't dependent on load, (could be just a 60w bulb) and was only on the one outlet.  It actually was a broken contact causing any split phase load connect to it to lose power.  The contact with the ground screw was loose and would freely wiggle around, and with a continuity meter, would make or break the circuit by just wiggling it.  The contact would be randomly made or broken as the thing bounces around while running.

The throttle my fault.  I dug the thing out about 7 years ago, put a little gas in it and tried to start it.  Since the recoil starter didn't work, I used a cordless drill with a socket on the nut on the end of the crank.  It worked, but for some reason I felt the need to mess with the throttle screw.  I got it tuned close to where it needed to be with a cheap volt meter looking for 120 V from the outlet, but now I can actually look at the frequency to ensure it's set properly.

For the governor setting use some medium load, as the governors usually sag the rpm a bit with the load, so to have the range centered, the load has to be at about the half power.
The main purpose of this sag "programmed" into the governor, is to have stable rpm regulation without oscillations or irregular operation.

Screwed some 1x4 strips down on the porch.  They fit inside the bottom frame and keep the generator from vibrating it's way over the edge.

Also changed the oil.  Was some pretty black stuff that came out of it, that I suspect was SAE30 put in back in 1997 or 1998.  Being around 32 degrees, it drained really slowly.  The new oil is Mobil Super Synthetic 5W30.  I also poured a half-ounce of oil down the spark plug hole and turned the engine over several times to lubricate the top of the cylinder.  According to the manual, ya hold a shop rag over the spark plug hole, and in the process, I discovered the ignition system works fine.  While holding the rag, I forgot to ground and anchor the plug wire, so it was resting on my hand when I pulled the rope 

Poor engine, the oil should be normally exchanged once a year (reason is the corrosion protection additives degrading over time)...

Normally would be, but that was when it was last used.  It's been sitting in the back corner of a shed and only run once since 1998.  Luckily the fuel was run dry out of it.

Speaking of fuel, just filled up a gas can mixed with Sta-Bil and IsoHEET to fuel it up.  Would like to find a new spark plug for it today but that can wait.  May fire it up this evening.

Tracked down a new spark plug, put it in and fueled it up, than fired it up!  Took several pulls to get it going, and it was very smokey while it burned off the oil in the top of the cylinder.  Once the oil burned off it stabilized.  Discovered a small problem though.  The spring that holds the choke lever in it's set position is worn out and bent, and when the engine was running, was not able to hold the lever in place, so it would flop around and try to choke the motor.  I had to hold it in the open position to keep it running.  Still a little more work to do, but it does start.  Forgot how loud the thing is!

Living off-grid I find projects like this one of interest.  Is this a backup for your house?
How is the hertz/cycles? (do lights flicker slightly, constantly?)
I saw a 3500w one of these once that had awful regulation, a skillsaw being run on it would dim lights down to less than half brightness! 
I run my house off a little 2000w Honda (those things are excellent in terms of reliability/fuel economy/clean power/noise).  The 3000w version is also pretty good, though less portable).  Also run a 10Kw Isuzu diesel for larger loads.

It's regulation is fairly simple, though not terribly precise.  It' uses a centrifugal governor driven off the timing gear.  An idle screw on the carb sets the minimum throttle.  As the engine speeds up, the weights fly out, pushing the throttle closed until it hits the idle screw.  When a load is put on, like pulling the trigger on a skill saw, the motor lugs down, which causes the weights to fall, pushing the throttle open, until it starts to overspeed, when the governor tries to close the throttle again.  It's output is 60Hz sine wave, but as the motor speed rises and falls while compensating for changing loads, the voltage and frequency rise and fall too.

At one point, I had a mercury vapor wallpack hooked up to it, and whenever something large, like the saw would start, it would cause the light to drop out.

It was originally purchased to run power tools, but now I plan to use it as backup power.  I also have a 2000w Honda that I mostly use to power the camper.  Amazingly quiet and runs almost all night on less than a gallon of gas. 

The original muffler was quite small, and it's internal baffles were pretty well burned up.  Really more of a spark arrestor than a muffler.

I put a new muffler on it.  Helped with the noise.  It's still quite noisy, but it is a little quieter.

For the choke issue, I have a new detent spring on order.  I bent the spring where it would hold it from closing (forward) but would still allow it to swing backwards and close.  To prevent that, I stuck a stick-on rubber cord clip to the air cleaner to block the choke lever from turning backwards.

Fired it up.  Plugged in a skill saw with a zip tie on the trigger as a load and plugged in my kill-a-watt as an output indicator.  With the saw running, it would run about 56Hz and put out 115 volts.  With no load, I adjusted it to about 61Hz and 134 volts.  I couldn't get the throttle under load to go any higher.  There maybe other adjustments, but it was getting dark and they are all under the hot muffler.

Yeah those 2000w hondas are excellent generators.  My whole house runs on one...