Got a set of throttle bodies too but that's a secret

Thanks for your input Mr Stripes. I wont bother with plug chop type runs then, dyno runs it is.

To sum up how the carb's are set up now ( I haven't touched 'em since my last update ) I have :-

117.5 main jets, needle clips in 4th from top slots, 52.5 pilot/choke jets with all 4 mixture screws set 1 turn out.
Any leaner on the mixture screws & it's reluctant to start & idle smoothly !
Between 8,000 & 12,000 it flies so the main jets & needle positions can't be that far out.

As I said earlier it's too rich around 2,500 - 3,500 rpm with the 52.5 pilot/choke jets fitted so will eventually put the stock 47.5's back in & try the mixture screws at 2.5 - 3 turns out & see how it behaves, but only after the main jet & needle position fueling is checked out on the dyno first.

I did say in my last update that I've slotted the ignition pick-up's mounting holes to advance the ignition timing 3 -4 degrees.

Now then, folks. I finally have an update to post. Since my GK76 passed its MOT on March the 10th I haven't been near the poor thing as I've been spending every spare minute I've had, (which hasn't been much lately due to work, bad weather or other distractions), trying to get my 3XV TZR's engine bolted back together after it lunched a piston ring & cylinder late last September! Anyhoo, last weekend I finally had a free Saturday that coincided with excellent weather all day so I was able to get stuck into removing the carb's (yet again) so I could fit the kit of Viton internal seals I bought from Lite Tek / Blair.




When putting the carb's back together I used the now redundant, old mixture screw washers as spacers on the needles, as they are a perfect size & thickness to lower the needles half a clip slot, so lowered the clips from the 4th richest slot to the 3rd/middle slot then used the washers as spacers to lift them to between slot 3 & 4. i.e. slot 3 & 1/2.
I was interested to see if that would lean off the mixture enough to affect performance & therefore give me an idea of how worn the needles & needle jets are. Also, as there had been a really bad stutter around 3,000 RPM I swapped the 52.5 pilot jets I'd bought & fitted with the stock 47.5's to see if that made any difference, again to gauge how worn the needles & needle jets were. I also reset the mixture screws to the stock 1.5 turns out.

Once the bike was bolted back together I took it out for a test ride to see how it now responded.
With the engine warmed up the stutter around 3,000 rpm has been lessened. In fact the cooler the engine is, & also if the air is cooler, more dense & less humid the better it is, so the needles & needle jets have enough wear on them to richen the mixture around 3,000 RPM. I'm gonna buy & try smaller 45 pilot jets to see if they improve the stutter further.
At idle speed, with the engine fully warmed up, I've had to adjust the mixture screws in to 1 & 1/4 turns out to smooth out the idle & stop the engine rev's dipping when the throttle was blipped. I tried one full turn out, but the rev's started to hang so I put them back to 1 & 1/4 turns out. It's the best compromise due to the wear on the parallel portion of the needles with the throttles closed.

Above 4,000 rpm to approx. 9,000 rpm the engine didn't pull as strongly as before with the needles lowered half a clip as mentioned above, so I've since removed the spacer washers & raised the needle clips back to their 4th from top slots.
The engine now pulls harder from 4,000 - 9,000 RPM with the needles clips back at slot 4. From 9,000 upwards it pulled progressively harder to 12,000, where it felt like it peaked, but continued on to 14,000 RPM, so the 117.5 main jets can't be far off being correct (for my engine & mod's).

During the week just gone I used my GK76 to commute to work for the first time. Apart from the glitch around 3,000 rpm, which can be ridden through with careful use of the throttle, (it happens to coincide with bimbling through 30 Mph/50 Kph speed limits ), everywhere else in the rev range above 4,000 feels good.
I had a couple of chances to give it a big handful of throttle on long straights & it just pulled progressively harder to 12,000 rpm where it flattened off a bit. (Peak power rev's?) In fact, I'm happy enough with how it's performing that I'm trailering it down to Cadwell Park for next weekends Classic bike track day.

The two tanks of fuel I've gone through this week disappeared at 41.2 & 43.5 mpg. Is that about right for a GK76 spending most of its time at an indicated 100 - 130 Kph, (60 - 80 mph but occasionally faster. ).

Anyhoo, here's a photo of the old girl taken yesterday afternoon on the way home from work.
She's lookin' not too shabby in the sunshine if I may say so.


I'll post more updates regarding getting rid of the stutter at 3 thou rpm.
And if your going to Cadwell this weekend I'll see you there.

Bye for now.

Wullie.

Good job. Never be afraid to try smaller jets. If they are too small you will know and you also learn how it feels. Rich is vague and lean is obvious. I always go down on the main jet until it falters and then back up one size, of course you can start big if you like. Then it is a screaming banshee that will not foul plugs and destroy your top end with carbon deposits.
cheers

LiteTekBlair wrote:Good job. Never be afraid to try smaller jets. If they are too small you will know and you also learn how it feels. Rich is vague and lean is obvious. I always go down on the main jet until it falters and then back up one size.

Under no circumstances tune a 2 stroke engine.this way.

I know Blair is referring to 4 strokes but others read this forum that may not realise jetting a 2 stroke thus ends in engine destruction.

(No offence intended)

Ahhh yes. What Mr Willandrip said.

Variablevalves suck wrote:You will never get it spot on unless you swap out the thermostat.
The Suzi one opens at too low a temperature, around 75deg, its a known problem with a load of Suzi's from said period, and gives lumpy tick over and a flat spot just as you give it throttle.
The fix id to put a warmer thermostat in, 80 + deg, think I fitted a cbr600f to mine.

Not cheap at £35 but if you pick up a second hand housing with the thermostat still in from ebay its cheap and makes the bike much smoother.

So this is the way to go huh? I too live in a cool climate at 45 degrees south. Is there any particular year/s cbr600 thermostat I need to get?
Excuse my ignorance but how come a CBR600 thermostat fits a GSXR400, are they quite a universal item?

Any other replacement tips gratefully received.

Al.

Hi KiwiBiker.

As mentioned earlier in this thread I currently have an SV 650 thermostat that opens at 82 deg. C, but, as I live in Scotland could do with one that opens around 86 - 88 deg. C as, once my engine is up to normal temp, the temp. gauge is only just into the white zone, ( about 4-5 mm's ), even on warm day's ! ( Around 20 deg. C air temp. ).
It might run a bit warmer once I fit my set of new OEM needle's & Mikuni needle jet's into the carb's tho' ).

Yes, a lot of thermostat's are a standard size so will fit several makes / models of bikes.
It's finding out which model's & opening temp's will fit tho' !

IIRC the CBR F model 'stat you want is from the F3 model. Could someone confirm this, please.

HTH.

Wullie.

Quality job Wullie, I will be in touch, have another one on the way so might have to rummage your spares pile!

Welcome back, Si. Have you collected your "new" GK76 yet? Hope you have less hassle wi' this one.
There's not much in the way of usable spares from your old bike left, but we'll see what we can do.
What's the plan? Service it & check it over or ride it as it is for now?
I haven't ridden your old bike much as it still needs fine tuning. I also have new OEM needle's & needle jets to fit before rebalancing the carbs & then a dyno run to check the fueling out properly.