Max Comp Ratio

[Mike Lilja]

Hi All,

Great site and very helpful. I am looking for some specific information. I have a 1980 GS550E with just under 16,000 miles. It has not been running well and after a number of checks have found my comps to be 95, 120, 95 and 80 respectively. I have not done a leak down check to know where it is leaking, but am going to have to remove the head regardless. Since I am going to be in there anyway, I thought I would bump the compression. What I want to know is:
1.) How high a CR can I go without distorting the head?
2.) Besides decking the head, can the block be decked as well?
3.) What piston manufacturer would you suggest? Based on some initial checking, they may have to be custom pistons, unless you know a supplier.
4.) The spline on the output shaft is worn, are replacements shafts available?

Thanks for the help.

Mike

[Guest]

Did the bike sit for awhile?
With sixteen thousand miles there really shouldn't be a problem unless you have stuck rings.
Are you checking with the engine warm and the throttle open full?
Put a little oil in one cylinder and do a CR test. If this bumps up the numbers your rings aren't seating for some reason. If it doesnt' rise your valves aren't seating.
This also all depends on the fact that you don't have a blown head gasket.

[Nessism]

Yea, as Chef mentioned, make sure the valves are adjusted properly. For some reason people really neglect valve adjustment which is a shame considering how easy it is to swap a few shims.

Considering the mileage on the engine I suspect rings are about as far as you would need to go unless water/rust got into the cylinders and pitted the metal.

As to the subject at hand, the stock compression ratio is low because that’s about all that the engine can handle without detonation. The combustion chamber shape is very old school; before the engineers started to figure out this sort of stuff and switched over to four valves per cylinder. Some 550 owners have installed 650 top ends on their engines so this may be a more viable way to go if you are looking for more power.

Good luck.

[BassCliff]

Greetings and Salutations!

Hi Mr. Mike Lilja,

I'm the virtual welcome wagon and here is your mega-welcome!

Let it be known that on this day you are cordially and formally welcomed to the GSR Forum as a Junior Member in good standing with all the rights and privileges thereof. Further let it be known that your good standing can be improved with pictures (not you, your bike)!

Perhaps you've already seen these, but I like to remind all the new members. In addition to the carb rebuild series, I recommend visiting the In The Garage section via the GSR Homepage and check out the Stator Papers. There's also a lot of great information in the Old Q&A section. I have some documentation on my little BikeCliff website to help get you familiar with doing routine maintenance tasks (note that it is 850G-specific but many tasks are common to all GS bikes). Other "user contributed" informational sites include those of Mr. bwringer, Mr. tfb and Mr. robertbarr. And if your bike uses shims for valve adjustments, send an email to Mr. Steve requesting a copy of his Excel spreadsheet that helps you keep track of clearances, shim sizes and other service work.

These are some edited quotes from one of our dear beloved gurus, Mr. bwringer, with ideas on basic needs (depending on initial condition), parts, and accessories.

***********Quoted from Mr. bwringer************
Every GS850 [and most other models] has (or had) a set of well-known issues that MUST be addressed before you have a solid baseline for further troubleshooting. It's a vintage bike, and it's quite common (as in, every single GS850 I have had contact with) that there are multiple problems that have crept up and slowly gotten worse over the years. It's not like a newer vehicle, where there's generally one problem at a time.

These common issues are:

1. Intake O-rings (install NEW OEM or Viton only - common nitrile O-rings will quickly deteriorate from heat)
2. Intake Boots (install NEW -- these cannot be repaired)
3. Valve clearances (more important than most people think)
4. Carb/airbox boots
5. Airbox sealing
6. Air filter sealing
7. Petcock (install a NEW one)
8. On '79 models, install new points or Dyna electronic ignition (or at least verify that the old points are working correctly)
9. On all models, it's fairly common to have problems with the spark plug caps. These are $3 or $4 each, and often worth replacing if you're keeping the stock coils/wires.
10. Stock exhaust with NO leaks or holes -- good seals at the head and at the junctions underneath.

Carburetor maintenance:

Replace the intake boot o-rings, and possibly the intake boots. Here's the procedure:

http://bwringer.com/gs/intakeorings.html

Here's an overview of what happens with this particular problem:

http://cycleorings.com/intake.html

You'll also want to examine the boots between the carbs and the airbox. There's a good chance these are OK, but check them over.
And finally, if things still aren't exactly right, you'll want to order a set of o-rings for BS carbs from the GS owner's best friend, Robert Barr:

http://cycleorings.com

Once you receive these rare rings of delight, then you'll want to thoroughly clean and rebuild your carburetors. Here are step-by-step instructions that make this simple:
http://thegsresources.com/gs_carbrebuild.htm

OEM Parts/Online Fiches:

I would definitely double and triple the recommendations to use Cycle Recycle II and Z1 Enterprises as much as possible. These guys are priceless resources. Z1 tends to have slightly better prices, CRC2 has a wider range of goodies available. If you're near Indy and can bring in an old part to match, CRC2 has a vast inventory of used parts.
http://denniskirk.com - Put in your bike model and see what they have.
http://oldbikebarn.com - seems to be slowly regaining a decent reputation, but it's still caveat emptor. They don't have anything you can't get elsewhere at a better price anyway.
http://www.babbittsonline.com/ - Decent parts prices. Spendy shipping. Don't give you part numbers at all. Useful cross-reference if you obtain a part number elsewhere. Efficient service.
http://bikebandit.com - Fastest. Middlin' prices. Uses their own parts numbering system to obfuscate price comparisons -- can be very confusing for large orders. Cheapest shipping, so total cost usually isn't too bad.
http://flatoutmotorcycles.com - Slow. Cheapest parts prices, crazy shipping costs. Don't expect progress updates or much communication. Real Suzuki part numbers.
http://alpha-sports.com - Exorbitant parts prices. Different type of fiche interface that's quite useful at times, especially with superceded part numbers. Real parts numbers. Shipping cost and speed unknown due to insane, unholy pricing.

Stainless Bolts, Viton o-rings, metric taps, dies, assorted hard-to-find supplies and materials, etc:

http://mcmaster.com - Fast, cheap shipping, good prices. No order minimum, but many items like bolts come in packs of 25 or 50. Excellent resource.
http://motorcycleseatcovers.com - Great quality, perfect fit (on original seat foam), and available for pretty much every bike ever made. Avoid the textured vinyl -- it's perforated.
http://newenough.com - You DO have riding gear, don't you? Great clearances, always outstanding prices and impeccable service.
***************End Quote**********************

Additional parts/info links:

GSR Forum member Mr. duaneage has great used upgraded Honda regulator/rectifiers for our bikes. Send him a PM.
New electrical parts:
http://stores.ebay.com/RMSTATOR or http://www.rmstator.com/

http://www.ricksmotorsportelectrics.com/index.php

http://www.electrosport.com/

For valve cover and breather gaskets, I recommend Real Gaskets (reusable silicon):

http://www.realgaskets.com

Carolina Cycle
http://www.carolinacycle.com
Ron Ayers Motorsports
http://www.ronayers.com
MR Cycles
http://www.mrcycles.com
Moto Grid

http://www.motogrid.com

Salvage/Used

http://www.ricepaddymotorcycles.com

http://www.ozpowersports.com/

If all else fails, try this:

http://www.used-motorcycle-parts.org/

Used bike buying checklists:

http://www.amadirectlink.com/roadride/Riderresc/checklist.asp

http://www.clarity.net/~adam/buying-bike.html

Lots of good info/pictures here:
http://www.suzukicycles.org

http://www.cyclechaos.com/wiki/Motorcycle_Wiki

http://www.bikepics.com
Basic motorcycle maintenance/repair:
http://www.dansmc.com/mc_repaircourse.htm
Online Clymer manuals:
http://search.ebscohost.com/ Click on "Small Engine Repair" then "Motorcycles". User=library, password=library. Note: This link may not work if you are on a school campus.


Thanks for joining us. Keep us informed of your progress. There's lots of good folk with good experience here.


Thank you for your indulgence,

BassCliff
(The unofficial GSR greeter)

Click here to visit BikeCliff's website.

[Mike Lilja]

A little more Info

First off, thanks for the replies and an admission. DOH! I forgot to hold the throttle open. I should try again.
* The first thing I did was to check the valve clearance and all is good.
* The comp test was done warm (See admission above)
* My next step was to do a leak down check to know where it may be leaking
* As far as max compression is concerned, my goal is not so much power, but economy. Anything from 87 to 105 octane fuel is readily available around here (Minnesota.) And yes, a bit more power is always good too!
* I anticipated cutting some of Singh's grooves in the head to lower the octane requirement and sending the head off for TBC coating.
* The bike has been hard starting lately and the high compression cylinder appears to not be sparking with the plug installed as that cylinder does not warm up (the exhaust pipes were checked with my infrared thermometer.)
* Installed new and correctly gapped plugs.
* Installed new spark plug caps.
* I replaced the coil firing that cylinder and the plug still does not fire in the cylinder. The plug is black from carbon and lots of black smoke from the left exhaust pipe. Methinks the entire ignition system is weak as all sparks are difficult to see.
* My initial guess with the low compression was either a valve not sealing or some issue with the rings.
* I will try the oil addition as well and see what happens.

** And if anyone knows what I can raise the compression to without damaging things, that would be great.

I bought my 550 new in 1980 and has not been used consistently since I bought my first car. The bike was used on and off for years and then used more in the last couple of years. When it runs, it runs great! It rolls off nicely from idle and very smooth. Recently the idle has been inconsistent, meaning at one stop sign it just about dies and at the next is running fine which is what started me checking into things. Then at the end of a weekend ride last year, my buddy behind me said I was blowing smoke and I ran out of gas unexpectedly. It turns out the #2 cylinder quit firing and I was only running on three cylinders with gas still going through all four.

Thanks for your input

[Steve]

You appear to be dead set on raising the compression before making sure that everything else is running right.
That's OK, but it's not the way that I would do it.

One more thing to consider when milling the head or the block to raise compression is that you are changing the distance between the crank and the cams. This will retard the cams a bit, so you might need to slot your cam sprockets to restore proper cam timing.

You also say you are doing this for economy, not power. Yes, a higher compression can yield more power, and that is due to higher efficiency. However, you may introduce other problems that require higher-octane gas. You say it's available, but why pay the extra? Besides your cost per gallon, look at the overall picture to see cost per mile. Also don't forget to factor in the cost of the machine shop work, different pistons, gasket set, whatever else it's going to take to save you a few pennies at a time here and there.

You will probably be better off just doing a good cleaning and adjusting on the carbs, and making sure the ignition system is good.

.

[76FJ55]

I would be supprised if the engine needs to be torn down at all. IF the numbers you provided earlier are with the throttle closed you will most likely see all 4 cylinders with good numbers after you re-check witht the throttles open. I would start by focusing on the carbs. Do the full teardown and rebuild according to the instructions on this site and I would guess it will be back to its original smooth running condition.

I agree that higher compression should give you better economy, but I would venture to guess that the investment in the most likely un-needed rebuild with custom head and piston work would be hard to offset by fuel economy even at todays fuel prices.

Assuming you currently get 45mpg and fuel prices of $4.00 per gal.
and that after you do the mods you get 55mpg (which is an improvement of 22%), now if your original usage is better than 45mpg the scenerio gets worse. And I'll assume $1500 for the rebuild.

$1500/ $4 per gal = 375 galons of fuel

45mpg*(X gal)=55mpg*(X-375 gal)
...X=2062.5 gal used at 45mpg
2062.5 gal * 45 mpg = 92812.5 miles

now to offset the 375 galons going from 45 to 55 mpg you will have to drive a minimum of 92812.5 miles. if you don't think you will put quite that many miles on your GS550 you would be betteroff not doing the rebuild (atleast onot on an economics basis).

[Mike Lilja]

Thanks for the additional information. I am not dead set on just raising the compression. My original thought was, if I had to remove the head anyway, I thought I would go about doing a little more work, because at that point, you might as well have some fun too. I have been trying to track down the issues. I will first re-do the compression test making sure the the throttle is wide open. Then re-check with some additional oil to see if it changes at all. Then, if necessary, do a leak down check. As a side note on the mileage. My original was over 50mpg. It had been closer to the mid forties lately before #2 quit firing. Carb tear down, clean and balance is next. Ignition to follow that.

So, assuming that after all is said and done, the head needs to come off because of rings, valve job, scored bore, etc. and I WANT to play and am WILLING to pay to play and understand the additional complications, does anyone know how high I can go on the compression?

This is not the first engine I have torn into, rebuilt or otherwise performed major surgery. It is one I don't know all the ins and outs and was hoping for some specific information to both not re-invent the wheel and not do something too outrageous.

Your suggestions have truly been helpful and appreciated. I only want to go as far as necessary, but if the opportunity presents itself, I want to be prepared for that possibility. Thanks again.

Mike

[Guest]

The higher compression cylinder is #2. This is the cylinder the petcock gets its vacuum from. If the petcock leaks it could cause problems with this cylinder.