Diaphragms - when to replace

[Thasat]

Hello everyone.

At first I'd like to thank you all to contributing to the huge and helpful repository of knowledge gathered on these forums.

To give you some idea about my knowledge of the GS and bikes in general - I own the bike for a little less than two years. Unfortunately have not had time to drive a lot on it and during this time period I made around 4000 km only. This GS is my first motorcycle.

Now more about the GS itself - it's a 1998 with a little over 45000 km. I live in Poland and the GS has been imported by the previous owner from UK couple of years ago. Reg. documents for it say only 20 kW so I've been thinking it's some sort of reduced power version. I've been fine with that considering I just started and have not had the necessary skills to complain about the performance. Basically to make things short - it can't go over 140 km/h. Last year I took it to the shop for valve and carb adjustments. Unfortunately I understood the hard way that local shop is completely incompetent for anything else than Chinese mopeds. After "adjustments" in the shop the bike started backfiring into the carbs (or only one carb, don't know yet) on idle. The top speed has not changed though.

Now winter is ending and I decided to get dirty and try to get it back to shape myself.

At first stage I took the carbs out to take a look inside and clean them.
While draining one of the bowls had over three times more fuel than the other. After taking the carbs apart I found out that a few o-rings inside are badly worn and some of them are cracked or broken. I'm sure this could contribute to the poor performance. On the other hand I did not find any symptoms of reduced power carb versions which I was kind of hoping for.

I used lots of carb cleaner and now I'm waiting for the carb repair sets to come with new o-rings, needle seat, float needle and gaskets.

Now comes the main (and short) part of the question. Do I need to replace the diaphragms in my carbs? There is some perforation on them as you can see on the attached photos. These are quite expensive here and I'm not sure if they qualify for replacement.

As a side note - do you think I should take the opportunity and change the jets? I found inside 115 mains and 40 pilots. GS has stock air filter and Motad exhaust.

After the carbs I will follow with checking the valve clearances. Plugs and oil will be replaced as well of course. Now I don't have the tool to measure compression on each cylinder but considering the mileage and poor maintenance it may not be perfect. The bike however never had any problems with starting, newer puts out smoke or something like that so I'm having my fingers crossed about this one.

It would be great if you guys (and gals of course) could help me get the most I can have from this bike so that I can enjoy it in the upcoming season.

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[robfriedenberger]

The diaphragms kinda act like your lungs so they must me air tight, any slits or slight holes will cause them to fail, also if they are dried out they will not function properly. From your pictures its hard to tell any damage. The GS from stock is factory jetted lean, so if you have an after market exahust than you need to re-jet. If I were in your shoes I would clean and rebuild the carb with new jets and o-rings, adjust float height, and fresh fuel. A little issue in the carb is the difference between a running bike and one stuck in a garage. I cant tell you enough that you need to clean the carbs and once they are clean they need to be cleaned.

PS when you pull your plugs check for any differences in the color of the plugs, if one is white and the other is tan than you know what cylinder is having an issue. The compression test should be done you can pick up a gauge at most auto stores cheap and if you cant than you can usually borrow one for free.

[BockinBboy]

Just wanted to add a bit on bikes which are restricted... maybe your performance issue is this, or something else entirely... but removing restriction is quite easy if its there... bikes that have been restricted to 30bhp for learners have restrictor rings on the engine side of the carb manifolds... If you have those, simply remove them when you take off the carbs. 

A rejet would also help quite a bit too, especially bikes with a 115 main... that's the smallest main jet ever put into these carbs from the factory... and I know most US carbs are lean with 127.5/130 sized main jets (granted that your carbs have adjustable needles and US carbs do not, but still my point remains)

- Bboy

[The Buddha]

A tiny hole - one you barely see will prevent that slide from rising. You have to replace them if you even want to run over 6k in 1st, 5 k in 2nd, 4k in 3rd, 3500 in 4th etc etc ... Its dependent on throttle position, and as the slide needs to life @ about 1/4 throttle ... you will not have 1 cyl doing that.

Cool.
Buddha.

[Thasat]

Thanks for your opinion and hints on the subject. I don't suppose there are any holes or fractures in the diaphragms. I just wanted to know if anything else than smooth diaphragms do say that you have to replace them. If you look closely you can notice that the surface is wavy in some places.


Manifolds are just standard ones, not restricted.
Of course I'll check out the plugs and everything else I could possibly do. Actually I'm looking for some good compression gauge to check out the condition of head and cylinders.

As for the jets I'll look up for something bigger. The needles are adjustable as you mentioned BockinBboy. Clip is currently in the middle groove.

[The Buddha]

Wavy is ok. That is normal.
Cool.
Buddha.

[Thasat]

Finally I had the time to put the bike together and unfortunately I had to dig out the old thread to follow up on the issues.

Shortly what's been done to the bike recently:

Valve adjustment
I couldn't fit my smallest 0,02mm feeler gauge under any of the cams. Bought the suzuki tool and some shims to set the clearances correctly. I finished with both exhaust and one intake at 0,07mm and one intake at 0,04mm (couldn't get any better with shims I have). Valve cover went in with new gaskets.

Carbs
Cleaned as well as I could. Put together with new orings, gaskets and complete float valve. Main jets were changed from 115 to 130. Float height has been set to 14mm. After installation I had to lower a little fuel level in one of the floats based on U tube check. Currently left is perfectly at gasket level, right around 3mm higher than gasket level. Needle clip in the middle position.
Carbs has been synced with a homemade syncing tool. Idle mixture screws 2,5 turns out.

Before putting everything together I made a compression test (yes, yes, you convinced me guys ). It read 140 PSI on both with cold engine. Not good really but not tragic as well. At least the reading was even on both.

The bike is idling quite well, at least much better than previously. I feel like the bike could rev quicker with quick and short throttle blips. It occasionally backfires to the carbs but not to often (around once per 20 blips). I suppose it could be solved by fine tuning idle mixture. I badly wanted to see how it rides and took it out for a spin before coming back to the carbs.
Result - poor performance. The same as previously with badly adjusted valves and possible carb issues - tops at 140 km/h.
Power delivery at WOT aside from the fact it's very weak and flat is quite smooth from around 3 to 6k rpm. Between 6 and 7,5 rpm you can feel some bogging and it can't rev over around 8,5k rpm in 5th and 6th gear.

BTW. there is fresh fuel in the tank.

There is some additional fuel filter before the carb fuel inlet. I wanted to throw it away hoping it's the reason but have to get new fuel hose to do it.

I decided to check the compression again on warmed up engine hoping it will be better than 140 and then ... big surprise!
Left  120 PSI, right 110PSI (yes, with throttle open).
Compression gets back to 140 with cold engine.

Any ideas or hints what could be the faulty in my engine?

I will remove the filter anyway and report back with the results. Considering the hot compression results I don't suspect good results actually.

[The Buddha]

Compression lower when hot ?
Probably a Valve is hanging open.
That usually makes it a bear to warm up upon a cold start ... I guess it can make it bad up high too.

Cool.
Buddha.