I've finally got my GT750 back running after a prolonged fight removing the water pump - thanks Allan for your brilliant insight.
Now I'm back to where I was a year or so ago, with a giant flat spot around 3,000 to 4,500 rpm, and a huge powerband hit around 5,500.
The bike is a '76 GT750 with the stock CV carbs, air box/filter, and Bassani (sp?) expansion chambers, I live about 10 feet above sea level. I would like a baseline setting to measure against the settings that I have.
Definately not looking forward to pulling off the airbox/carbs again.
Normally to determine what's wrong with jetting, the amount that the throttle is open is of more importance than the revs. how far open is teh throttle when this happens, 1/4, 1/2?
The best way to fault find is to mark the throttle grip at closed, 1/4, 1/2 3/4 and full. use a white marker or typex and a single mark on the body.
Then run teh bike in say 3rd gear and try it at a steady 1/4 throttle and let teh revs rise and see what happens. repeat at 1/2 and 3/4 to determine at what throttle opening and revs the problem occurs.
If it's at all throttle opening and always the same revs it is a pipe resonance effect, but it's unusual to be this bad.
How does the motor feel in the flat spot? Is it "starving", as if you've run out of gas, or is it blubbering from being over-rich?
I've just fitted a set of expansions on a 380 (stock airbox, filters, carbs, porting) and found it was running rich and 4-stroking a bit at mid-rpm and 1/3 to 1/2 throttle.
Turning the pilot screws just over a quarter turn out has made it a lot better. The pilot circuit does contribute somewhat to the overall mixture below half throttle.
Might be worth checking the air filter isn't over- or under-oiled, that can also make an impact on mixture.
1976 GT380 - wounded by me, and sold on
2006 SV650S - killed by a patch of diesel and a kerb in Feb 2019
2017 SV650 AL7 - naked and unashamed
I have the same bike with stock carbs, and Bassini expansion chambers.
With the stock jetting it runs very very crisp , actually too crisp according to several plug chops , so I richened up the needles, now it runs too rich in the middle , so I reckon the ideal setup would be stock needles , set to factory settings and go one step up on each main.
sounds to me that your problem is a blocked circuit in the carbs, there are many tiny passages in those carbs, what about a crook coil or plug cap.?
[quote="VintageKiwi"]I have the same bike with stock carbs, and Bassini expansion chambers.
quote]
What is your power delivery like? My bike is somewhat dead under 4K revs (and the revs are slow to build without slipping the clutch) after which it takes off like crazy, it loves to be at 5.5 to 6K revs. I know that the intake manifolds have been ported, I don't know if the actual piston ports have been modified also, but I think that may be why the powerband is so peaky. The plugs are a nice tan color after riding. However the bike does like the choke on to start (just for a second) even when hot, otherwise it will start (weakly) and then cut out.
Motomartin, the choke situation points to carburation being too lean at the bottom end. Try one size larger pilot jets (or screw the mixture screw OUT say 1/4 to 1/2 turn with BS carbs) and/or raise the needles one step.
To add to your problems, the 76 had too much intake timing and it hurts power below 4500. We ran simulations of JKL intake timing and MAB timing with all other parameters the same and the results were interesting.
Below 4500 the JKL made more power and above 5500 the MAB made more. Unfortunately people lower the intake floor even more and shorten the pistons and that hurts the bottom end even more.
The bike needs more revs to get off the line which is fine on a cafe style bike but it's not what everyone wants.
A simple solution would be to slip a 2mm spacer plate under the barrels to get the intakes a little better and that would also add top end power too.
You might also be able to add a little more compression and or ignition timing as long as you are careful. Both of those tend to help bottom end at the expense of the very top end.
But spend some time getting the jetting right at low to mid revs first.
Hi Guys,
The final drive #'s 15 Front and 43 Rear
I raised the clip position on the needles 2 positions and it was much better, not great but faster pick-up, less stumbling around 3 -4 K revs, and more torque, it also smoothed out the 5.5K hit abit. I tried moving the clip up another position but the slow pick up and stumbling returned.
The air mixture screw is 3/4 turn out per manual. I cleaned the pilots they were open but not fully.
The 15-43 is good gearing for most riding on a 76 GT, i use it and it works well for me in the mountains and flats around Alberta and B.C.
The stock 76 tends to go a bit flat under 3,500 in fifth gear on a regular mountain road grade but it starts to pull decently once it hits 3,800 and higher.
The fuel screws (not air screws on these carbs) are VERY sensitive so if you're at sea level the 3/4 out will be fine for starters. With the chambers you will have to fiddle with a 1/16 to 1/8 turn more outwards or a tad richer mixture possibly.
The higher in altitude you go the more you have to turn the fuel screws outwards. Well at least my 76 likes it that way so im basing it off of that engine since its the only one i have with the BS carbs.
In Calgary at around 3,500 ft mine are at 1 1/2 turns out, if i stray in or out from this setting you can instantly hear and feel its not 'right'.
I also use the plain old B8ES plugs and they work well for me, burning a light tan in the centre with a slightly darker edge.
BTW does your carbs have the air jet mod done on them???? i cant remember if we covered this already in any of you other posts.
*Richard id suspect the pipes would also work good with the 43, but you think he might like a 46 instead???. I was typing and posted right after your reply so i didnt see it till then*
Allen/Richard thanks for the info.
Good to know that the fuel screws are that sensitive. They were previously set at 1 turn out.
The gearing does seem tall, and may accentuate the gap between the 1st and 2nd gear ratios. I'll see what the other rear sprocket that I have is....
Darn, the sprocket that was on the bike when I bought it was a 40.
I also have plain ol' B8ES plugs (a new set)
What is this air jet mod of which you speak?
When i first got the 76 it had in a 16-43 and boy was that tall gearing to me. My 'seat of the pants' feel tells me the GTs internal gearing for first gear doesnt want to spool up quickly, so i got rid of the 16 ASAP for a 15 to help cure this issue.
Martin you could also try a 14 and retain the 43 instead of a 15-46, that would shift the spool up time on the front sprocket some more, plus buying a 14 is cheaper than buying a new 46,,,LOL.
Heres the bulletin for the airjet mod, its a lot simpler than it looks, Sudco have the jets you need. Go to bulletin 36 a bit further down this page link.
Allan,
That gearing is OK on a JKL and overall OK on a stock MAB. In fact it should be fine, but with chambers and porting it tends to lose a little at the bottom end, so if cleaning up the carburation doesn't get it to where it should be, I'd suggest say 14:43 or 15:46.
Of course it will rev higher on the highway which ,may be an issue.
MotoMartin
For more info on that air jet mod there's a bulletin that I can never remember the URL for, or check out http://pinkpossum.com/GT750/carb/BS40carbs.htm for a little more information on those carbs.
Cheers guys, I'll check those links out, I'll let you know how I get on.
Update: Re pilot air jet mod.
Yikes, if you get a bit of swarf stuck in the air jet passages the carb has to be replaced.
I've had the carbs off many times, I have not noticed any obvious modifications in that area.
