JohnnyKnowNothing said:
I'm looking to rebuild the engine on my 1979 GS750, and I am curious as to how hard it really is? I don't know that their is anything wrong with it, but it is super nasty, its covered in oil, and it doesn't look like its been cleaned or serviced for a long time. My feeling is that I'd like to do it now when the rest of the bike is apart, and have the piece of mind to know that its in good shape when its finally time to ride it. For those that have/do rebuild engines what are your thoughts on do-it-yourself vs. having a shop rebuild it?
I have a couple of 8v 750 motors for my cafe. One came with the bike, very dirty, patinaed and had some broken fins. The other i picked up off fleabay and will be going back in the bike come a good break in the weather. The 8v 750 design is actually VERY simple to work on if you've any experience working on bike motors. Its a DOHC design, 2 valves per cylinder and is put together pretty simply. I just spend yesterday helping a buddy do a valve adjustment on his GS850, which is the same design, just bored a lil bigger. It actually turned into a complete top end removal, as we found that he had a broken valve sitting on top of a piston
I had never broken the top end down on these bikes, but it took us maybe 2 hours to remove the cams, head, cylinder jugs, clean all the gasket material off, clean the carbon from the pistons, and while we were at it, re-honed the cylinder bores. We will be replacing the rings and valve seals since the head is apart.
Anyway, I have been working on my 77 GS750B all winter, and have become VERY familiar with the design, and with the motor also. Here is a bit of an overview of things to watch for and do on your bike based on my working knowlege of it.
Your 79 750(C/E) has the same 8 valve motor as the 78 and 77 models. In 80 the 750 became a 16V Twin swirl combustion chamber design with four valves per cylinder. The 8v is touted as 'bulletproof" and frankly, while it puts out just a bit less power than the 16v, i have a torched 16v 750 motor in my basement, and I will stick with the stronger 8v design thank you very much
So, if you ever need parts for your motor, anything from 77 to 79 will work. Also certain bottom end parts from the GS850 and GS1000 motors will be compatable.
The most common things to watch for are the following:
The motor uses a DOHC shim over bucket valve train design. Valve clearances should be checked every 3k miles or so. Improperly adjusted valves lead to difficult cold starting/starting, poor idle, rough running, lean running, and poor fuel economy. Adjusting the valves is actually pretty simple. If you dont have a manual for your bike, i suggest picking one up, but if you arent sure when the last time your valves were adjusted, i would suggest checking them ASAP. If you have a manual (any will do for this) it is explained pretty well, if not i can walk you thru it.
The carbs are hooked up to the motor via rubber intake boots. Between the rubber boot and the aluminum head are Viton oring gaskets. After time, these gaskets become dry, crack or break and no longer seal. They then leak air into the the head. These motors are very intollerant of air leaks. Air leaks can cause poor idle, lean running, which is VERY bad if left for too long, and difficult starting, and also HIGH idle. The boots are held on by two screws a piece, and are pretty easily removed with an impact driver. NEW VITON rings can be purchased VERY cheaply here (www.cycleorings.com) To test to see if you have leaks, start the bike, when it warms and is at idle, spray carb cleaner around where the rubber boot and aluminum head meet. If the bikes idle changes (some say it will rise, mine bogged a bit) you are leaking on that boot. I would suggest, while you are picking them up, that you pick up the stainless allen head boot bolts he offers. The rings and bolts ran me about 15 bucks, and ship via standard mail, which means a stamp is all it takes. CHEAP and you'll go nuts trying to balance and tune your carbs if you dont replace them.
The air box is another source for leaks as well. The airbox should seal tight, drawing air in only thru the inlet. A proper filter should always be installed, and replaced as necessary if your bike is STOCK. (pipes and pods filters are a different story)
Your bike has points and condensor type ignition system. I would suggest, if you have the means, to replace the points system with a Dyna S electronic ignition. Set it once when you install, never have to mess with timing again. Points are a pain in the arse. The stock ignition coils will work with the Dyna system, but you will need to install a 2ohm ballast resistor. You can also replace the coils with Dyna coils, and i would suggest replaceing the wires and caps while you are at it. The ignition system runs about $120 USD, the coils about $80USD and you'll need two. The motor firing order, when sitting on the bike, left to right, cylinders are 1 thru 4. Firing order is 1-4, 2-3 (Cyl 1&4 are on the same coil, fire at the same time, two and three on their own coil, and also fire together)
One of the reasons The 8v motor design is considered "bulletproof" because it uses a roller bearing crank shaft. This is very VERY durrable, and more expensive to build. (the 16v model uses a plain bearing shaft.) It also uses a low pressure oiling system, which is helpfull in reducing pump bearing wear and contributes to the durrability of the motor design.
One of the only flaws to the GS design was the charging system. Frankly, its crap. The Reg/Rec is known to burn up pretty regularly. The most common mod there is to replace it with a Honda unit.
If your carbs are due for a cleaning, the orings site i gave you sells complete interior oring kits for the carbs. These will ALL need replaced if you clean them properly, and are also stupidly cheap from that site. Your carb design are Mikuni VM26SS mechanical slide type carbs. Carb number 3 is the master carb on the rack. It supplies the vaccum to the petcock on your tank. Speaking of, your petcock , provided it is stock, is vaccum operated. Meaning, unless it is on the PRIME postion, fuel only flows when there is a vaccum to draw it (when the motor is being cranked, or is running) They generally after 20 yrs or so need replacing. I say REPLACING, not rebuilding, as most have had NO luck with rebuild kits for these petcocks. They run about 40-60 bucks new, so i say replace it, and dont worry about it again for another 20 yrs.
If you have good compression, but the motor seems to run a bit rough, i would check these things FIRST before breaking it down. Its not uncommon for guys to put 100,000 plus miles on these GS motors without a rebuild. Im not exaggerating either. I would advise you start with the valve clearance check/adjust before moving to anything else. If your valves arent right, you're going to chase your tail with everything else.
I have fallen quite in love with this design, so much so i am purchasing a 82 Suzuki GS1100G next month. Same 8v design, bigger motor, and a shaft drive. Wont be a cafe, but she IS very powerfull.
If you need more info on your GS, check out www.thegsresources.com Fantastic site for GS owners, and LOTS and LOTS of info there. The gent who sells the orings is a member there, and there is a member called Duaneage that offers Honda reg/rec's set up to bolt in and hook up to our GSs for $40USD shipped.
If you have any other Q&A feel free to ask
Sources for parts are as follows
www.Z1Enterprises.com They wont likely have motor or frame parts, but the little bits you need to maintain your bike are available cheaply, and VERY good customer service
www.BikeBandit.com Have about 500 or so OEM parts still available for these 750s. The Bike Bandit fiches are great, but be aware, they use their own part number system. They are NOT Suzuki part numbers.
www.babbittsonline.com Very much the same layout as Bike Bandit, but no part numbers listed. They are also midwest based rather than west coast based, so depending on where you live, they may be faster and ship cheaper. Parts are generally cheaper than Bike Bandit as well
www.flat-out.com Same fiches as Bikebandit. Uses ACTUALL zook part numbers. A bit more 'spensive than most however. This address may be wrong. If it is just google Flatout motorsports. I dont use them often enough to remember it. lol
Josh
