Air Starvation

[ned from PA]

The poor gas mileage of my 1992 GS500 (~38-48 mpg, typically 42) led me to check the valve clearances recently.  The clearances were all fine.  When I got the bike put back together, it had a bad flat spot between 5k and 7k revs.  This flat spot comes and goes with my bike.  I changed the air filter, and the problem only got worse, with the engine seeming to ping (thin, rapping noise) with full throttle between 5k and 7k, and stutter at high rpm.  Thus began my carburetor rebuild odyssey.

The carb sliders didn't rise smoothly, so I thought that deformed diaphragms were my problem.  Delving further in, I found the Dynojet needles were eroded and some of the O-rings had seen better days.  I also discovered that the previous owner had done a Stage 3 DJ re-jet (128 mains) while still using the stock airbox and stock exhaust.

I put all new parts in the carbs and reassembled everything.  The K&N lunchbox I had ordered hadn't arrived, so I put the stock airbox back on to see how she would perform.  The ping-like noise and stuttering were just as bad as before the carb rebuld.

Now I was getting worried.  Could I have accidentally turned the engine backwards while checking the valves, maybe messing up the cam tensioner? 
Then the UPS man arrived with the lunchbox.  Slapped that baby on, zip-tied a piece of paint strainer over the end of the breather hose, and fired her up.  At the first twist of the throttle, the engine gave a throaty roar as it had never done.

The test ride was thrilling, with perhaps a 15% boost in power compared with before, no flat spot, and a satisfying growl from the intake manifold when I rolled on the throttle.  I rode the bike as hard as ever, and my first check of the gas mileage was 52 mpg, better than I ever got before!

Here are some things I learned:

- A set of brass shim stock from the hardware store (0.001", 0.002", 0.003") makes great feeler gauges for checking the valve clearances.  $5 for a whole set.

- Replacement air filters made by HiFloFiltro (and bought from filterking.com for $20) should be avoided.  Based on my experience, a new one is even more restrictive than my used stock filter (causing my rich-jetted engine to gasp for air).

- Berkebile 2+2 Instant Gum  Cutter is an amazing carb cleaner (known to cause cancer by the State of California, so it must be the good stuff).

- Carb rebuild kits from CarbKitCapital.com seem to be good quality.  They include: bowl gasket, three O-rings, and the float valve (kit number 48-1947, $14.95, for early-style GS-twins).  This same website also sells diaphragms for $20 each (steep price, but good quality).

- jc_motorcycle_exhausts (eBay seller) has K&N lunch boxes (part # RU-2970) for $35 with free shipping.

- 7/16-inch-ID fuel hose makes a good replacement breather hose.  I didn't bother with an oil catch bottle, since my engine has very little blow-by.  The piece of paint strainer tied to the end of hose allows for easy breathing and keeps junk out of the hose.

- 128 DJ mains (Stage 3 for lunchbox and stock exhaust), 37.5 pilots, and 3 turns out on the mixture screw seem spot-on for my 1992 bike.  More power, linear throttle response, and boosted gas mileage – all seem to result from the last part I changed – the air filter.  No more air starvation for me.

Thanks for all the posts that gave me the confidence to do the valve check and carb rebuild.  This forum is a great resource.

[007brendan]

I'm not sure what 128 dynojet mains are, I'm guessing that number doesn't correspond to the mikuni sizes?  You might want to try out 40 pilot jets.  Before my exhaust and lunchbox arrived, I switched out the 37.5 for the 40's and it idled much better.

I like the tip about the brass shim stock, I'll check it out next time I find a good hardware store.