(LONG) Engine diagnosis and Carb question, GS750 carbs on GS850 overbore motor

[GS750]

Well, I have had the bike for 5 years now and after a few small issues, loads of general updates and repairs, and around 100 thousand Km, I decided to commence a top end rebuild, and install a wiseco 850 kit I bought from USA for AU$160 LESS (including freight and currency conversions) than I could buy it here in AUS without freight.


Since then, I have discovered that it was in fact already bored to 850cc with genuine Suzuki pistons installed at some stage during its previous ownership.

Diagnosis of the stripped engine shows that the blow by that the engine was starting to display on a long trip, was in fact caused by incorrect machining and installation of the overbore.

The engine wasnt actually burning oil, but it was spraying very small droplets out of almost every oil seal it could find to the exterior of the motor.

At first I thought it was just a bad seal, so I replaced everything I could find, and then it was slightly bettter, but not completely stopped, and after an inlet valve suddenly needed re-shimming I then decided to do a compression test.

Compression was actually pretty good, between 155 and 165 on all 4 after I re-shimmed the inlet on #1 and I decided to do a leak down test, which showed that there was issues because compression was only slightly higher on 3 cyls during the wet test.



Well, it seems that someone didnt have the bores correctly machined (or machined by someone who knew what they were doing, or something like that) because 3 pistons had broken top compression rings and after measuring the bores at the bottom of the sleeve where there is no piston/ring wear, they were actually only 68.75mm., and with the rings that were still intact, there was only 18 to 22 thou clearance.


So, my 750 has always been an 850 (at least for the 5yrs duration of my ownership_) and that also explains some of the other issues I have had, such as carby main jet size, exhaust etc.



Now, I am really interested to know from some of the tech guru's here, since my carbys seem to be the original 750's what jet, slide needle and needle and seat sizes it should have (all jets) to comfortably run on the fresh installation of the 850 wiseco, taking in to account the large free flowing 4-1 exhaust, K&N pods, Iridium plugs and Boyer Brandsen electronic ignition.

Thanks in advance!

[tkent02]

The stock 850 with VM carbs had the same 102.5 main jets as the 750.
Wiseco may have some info, but the larger displacement would not effect jetting requirements as much as the addition of the pipe and pods.
You are up from 102.5 to 112.5, probably will need more... My 1000 went from stock 95 main to 112.5 to run well with pipe and pods, didn't work as well with 115 main jet, but it is also tuned for 5,000 feet of elevation here... If that means anything a 122.5 would be in the ballpark for you.
To really know what is going on you will need to do a series of plug chops or dyno tuning to get it right.

[GS750]

The stock 850 with VM carbs had the same 102.5 main jets as the 750.
Wiseco may have some info, but the larger displacement would not effect jetting requirements as much as the addition of the pipe and pods.

To really know what is going on you will need to do a series of plug chops or dyno tuning to get it right.

Ok, wiseco basically replied that they didnt know, and didnt care, and to take it to a specialist tuner........

I dont want to ride my bike 500Km to the nearest dyno tuner. Its running fine at speed but it is a bastard to idle, really fluctuates, and bogs when coming of idle to take off.

Ive checked everything, ignition related stuff (timing etc) and all carby stuff (float levels, checked for air leaks etc etc etc etc etc etc etc etc).

I guess I might just have to take it to Brisbane on the trailer! :O

[hampshirehog]

TKent's advice on the mains is sound - a good starting point. If your problem is mainly at idle and pick up you need to start looking at your pilot circuit and needle.

For your pilot circuit I'd try and wind out your fuel screws maybe 1/2 a turn (the little blighters at the front of the carb and a b*gger to get at). Then tweak using the airscrews - a Colourtune really helps here.

With your set up I'd lift the needles to their highest setting (ie the lowest groove on the needle).

Do the mains first though before you do anything else or you'll be chasing your tail.

[GQROD]

If you are bogging from idle your pilots are too rich, while your mains may be too lean. You can do this your self and save the money you would pay a tuner who will only give up on the bike and give it back to you without accomplishing anything.

Start with plug chops to find out what circuit is rich/lean.

A colortune definetly will help let you know if you are out of the range of adjustability on your mixture. As other have suggested plug chops are the only way to find out where the fuel problem is.

Hope this helps.

[GS750]

Start with plug chops to find out what circuit is rich/lean.

When you talk about plug chops, what exactly do you mean?? Sorry but Im not familiar with the term.

Im running the iridium plugs and at $20 each they may be a bit expensive to cut up (if thats what you mean by "chop")

Looking at the plugs after having ridden the bike 300Km now, as I had expected they were a slightly oily black colour on the earth section of the plug, sort of like soot, and the positive electrode was a slightly grey colour.

[psyguy]

plug chop

is a carb tuning procedure where you kill the engine at a particular throttle opening and take the plugs out to see what they look like

do a search

or, there may be something on the bikecliff website

[Pete Logan]

Google throws up a lot on plug chops.

Basically, the idea is to run the engine at a fixed throttle opening to ensure you are running on each specific carb circuit in turn, starting with the main. Then, simultaneously pull the clutch and hit the kill switch. Halt at the roadside and pull the plugs to see what they look like on that circuit. The point is that even after several minutes' running, stopping in the normal fashion will corrupt the plug reading. You will need several runs to cover each circuit.

So, you need a long, straight, quiet road, some time and a lot of patience.

[GS750]

Ok, I just bought 2 colourtune plugs off an ebay seller. They are new and were only 9.99 GB£ each and £4 postage to Aus.

I probably didnt REALLY need 2 but I figure that since the carbs are individual they would be needing to be checked individually and the Gunsons website lists them at £33 each.

I was planning to stick them into cyls 1 & 2 and then the same in cyls 3 & 4 and check 2 carbs at a time.

It also sounds sh*tloads easier than plug chops.

Thanks for the help, will keep posting if I need help lol

[GS750]

Ps: I also found some pages on the net with circuit diagrams and effects of each circuit at each rev range.

Might save me a few bucks on the dyno after all!

Thanks again for all the helpful advice (and prodding to get things like colortune)

[psyguy]

colortune might help you set your idle mixture for sure
but you dont really want to test a wide open throttle with the bike stationary, do you
also, you want the engine to be under the load when testing

so, my friend, plug chops are still in im afraid

its not that hard, just time consuming

[Pete Logan]

Ps: I also found some pages on the net with circuit diagrams and effects of each circuit at each rev range.

Can you post links ? I'd like to see those.

[GS750]

There is a diagram of the tuning range and effect of each circuit available on www.mikunioz.com/jets.htm

also another good series at www.iwt.com.au/mikunicarb.htm

[Pete Logan]

That's great - thanks.

[GS750]

Got my colortunes, set the idle mixtures....... they are set to the maximum anti clockwise setting (without breaking them)

before and after setting the idles, I noticed that when riding the bike, the engine has a definate hessitation from idle at take off, I have to slip the clutch slightly.

Also around 1/4 throttle it hessitates.

Top end seems to be fine.


I do intend to do "plug chops" but it is really hard since I dont know anything more than I could google, and that bought a wide range of results............

Some said that you cut the plug up some said NO YOU DONT...........