1978 GS750, factory carb setup !!! HELP !!!

[Anonymous]

I own a 1978 GS750. I am trying to find out what the original factory carb setup was. In particular:-

1. how do I know the type of carb that I have, there is nothing marked on them (exept mikuni off course).
2. was the original fitment VM26SS.
3. What are the standard jet sizes.
4. What are the standard float heights.
5. My carbs have two adjustable screws per carb. One on top, inlet side, one underneath by the float bowl. What is the OFFICIAL method for setting these screws.
6. Do these screws only effect the pilot system or do they effect the mixture throughout the entire rev range.

I have played with the carbs a great deal and now have a very good idea of what everything does. The bike runs well, but stone me, it just will not idle smoothly and I am growing tired off it !!! The main problem is that I have two screws to adjust per carb with no idea of factory presets. The yellow paint that they are covered with has long gone ....

Local Suzuki dealers seem unable or unwilling to help.

Finally, are there any other carbs from any other bikes that will fit the GS that are known to work (maybe with some setting up). These carbs are gleaming clean, had them done proffesionally and expensively (ouch) and I am pleased with the end result, however setting them up has become the bane of my life.

All help will be very greatly appreciated.

[Anonymous]

Did you have them done by JRS?

[Anonymous]

I own a relatively stock 78 750, and it has the VM26SS carbs, but I only know of once adjustment screw... the pilot air screw. The one on the float bowl, to the best of my knowledge, is simply the bowl drain... I guess that could be used to lean out the carbs... if more fuel is dripping out than going through... I could be wrong, however, I have not had to play with the carbs YET. waiting for a good set of headers to come my way so I can build a 4 into 4 system, then pod airfilters... :twisted: AS I STARE INTO THE GLEAMING EYES OF THE JETTING DEMON, A GRIN SPREADS MY FACE AND I CANNOT WAIT TO CHALLENGE HIM! :twisted: Anyone have headers? Ben Reilly 78 GS 750

[Anonymous]

Yes, I had them done by JRS. Have you used them ?

The drain screw is located in the centre of the floatbowl. However, offset from centre accessable from outside of it, there is an adjustment screw. Factory set and not meant to be adjusted, EVER. However, god only knows who has tampered with it over the years.

[Anonymous]

I'm thinking of getting a set of carbs done by JRS. Can you reccommend them?
The screw you describe is a further pilot system metering screw. They are factory set, but I have taken them out before to clean carbs (you can't thoroughly clean the pilot system passageways with them in.)
You then just put them to the baseline setting when you put them back in (1 5/8s of a turn out on GS1000, don't know what 750 is, somebody will) and 9 times out of 10 they will be as good as gold.
Both my GS1000s have had this done with no problems.
US models of some bikes had a version of the carburettor with the adjustment here sealed up to comply with anti-tamper legislation, hence one of the previous posts.

Good luck. Remember, with carbs nothing beats good clean passages.

[Anonymous]

I have set this screw to 2 turns out and will probably tweak it slightly after a few good runs after I have checked plug colour. I have set the pilot air screws (on top, inlet side) to 1 1/4 turns out. Does this sound about right to you ? How did you find out yours was 1 5/8s out ?

I used JRS and was very pleased with the result. The internals were spotless, pilot screws (both) were lighly coated with copper slip (or similar), pivot shaft greased, etc. Very good attention to detail. The externals were OK, I thought they would come up cleaner, but this was probably wishfull thinking. They do offer a service for the externals, electoplating probably, but I didn't want the expense. Total cost was ?140, thats ?35 per carb. However, whilst I do reccomend them without hesitation, (the internals really were spotless), I don't know if I really needed to have them done. If you aren't experiancing any problems, ?140 could be better spent elsewhere.

[Anonymous]

BRIR7.11. I noticed your comments in another post regarding oil type and quility, I gather your an engineer. A quick question while I am here. What does the 10 and 40 indicate in "10W40". Is 5W50 not better than 10W40. What oil do you use and do you use an engine flush. Would be interested in your ideas.

[Anonymous]

The numbers are the viscosity index (i.e. the "runniness" of the oil) measured in centistokes per.....something or other. (I forgot and it doesn't really matter..).
A monograde oil would be say, a 50 weight oil or 40 weight. Then multigrade oils were introduced to provide oils which changed viscosity according to temperature, so that in the winter the oil didn't thicken up and become hader to pump.
Hence the first number is the "winter" viscosit of the oil, so "w".
You can see then that a 5W40 has a wider temperature stability range than a 10W40.
The oil acts like an oil of lower viscosity when its temp reduces, i.e. in theory the oil's "thickness" should always be approximately constant.
In the real world, virtually all commercially available engne oils for "general" applications will do for most applications, depending on the ambient temperatures where you live.
The Brit bike crowd often use monograde oils because they have a slightly greater resistance to breakdown in highly stressed roller bearing engines(reputedly) and the lubrication systems in those bikes were marginal.
Jap bikes on the other hand, especially GS's, have massively over-engineered oil systems. You could probably put lawn mower oil in there and be OK. Or lemonade. In the 70's there wasn't all this fancy stuff around, and the designers allowed for a massive amount of neglect.
Many of the improvements in oil technology that have occurred since then have no benefit whatsoever in a GS engine. I know of a GS850 that has done close on 200,000 miles on regularly changed basic 10w40 mineral oil, and shows minimal wear. A well designed engine will run on any old crap. My old VF750 Honda on the other hand needed the best oil and expensive additives because it had major design flaws.
I use Halfords own brand "Basic" 10w40 motorcycle oil, changed at 2000 mile intervals. I would happily use GTX, duckhams, Rock oil, most motorcycle specific oils
, and most BASIC branded car engine oils, (equivalent to GTX). I have used semi-synthetic motorcycle oils, with no appreciable difference except a lighter wallet. I would not use fully synthetic oils, whats the point? There is also a question mark over suitability of these oils for older engines.
So basically, relax about it, don't spend to much, keep it topped up and change it regularly. Remember, the instructions in my owners book (from Suzuki) says little more than "use motor engine oil". There is a specification mentioned, but in reality you won't be able to buy an oil lower than that grade anyway, oil has moved on a long way since 1979.
Hope this helps.

[Anonymous]

I don't use engine flush, but I do drop the sump pan off now and again and clean it and the oil pickup gauze.

[Anonymous]

Excellent, thanks for taking the time to answer. Castrol GTX it is then...