Bucking - a theory to be shot at

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yeadon_m
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Joined: Sat Jan 02, 2010 7:10 am
Location: Canterbury Kent

Bucking - a theory to be shot at

Post by yeadon_m »

Folks,

I've been recommssioning my GT750B the past two weekends, and have an
observation, which over coffee this morning just became a theory to be shot at
by you, the experts. Bucking is a hardy perennial so hoped this might be worth
sharing.

Its a team sport question, but I'd love to hear from any
Kettle carb specialists in particular!

OK - surging and bucking. My B is a bugger for this. I have done everything
thats advised here, repeatedly - ultra clean slow circuits, validated again
yesterday with very careful carb to carb comparisons and with the carbs on my A
model (that bike doesn't buck). Plus everything else from ignition, chain
tension, airfilter fit to airbox, idle speed, mixture screws, pilot air
restriction jets, float height, the works. Still bucks like a wonky donkey :-)

So, what did I notice yesterday (had carbs out to fit a fuel filter....)
-the B runs beautifully, will idle independently at 1000rpm on each cylinder,
responds to mixture screws, carbs are sync'd.
-but, the throttle appears not to return totally to idle after I blip the
throttle. Idling nicely at 1000rpm; a blip, and it hangs up at 1200rpm, but goes
to 1000 immediately if I reached in and press the throttle shaft closed again).
I think now, recalling when I had them apart last autumn, one of the butterflies
is slightly sticky, perhaps is slightly askew on its shaft? Its definitely a
physical stickiness - its not that mixture screws are too lean.

I only noticed this when playing with carb sync, and didn't realise possible
significance of it until this morning…...read on only if you can be
bothered....thinking alound here...

My understaning of how these carbs work is that as throttle is closed but motor
is turning at high speed, there is huge intake vacuum…..the throttle butterflies
that are then fully closed serve to mount a huge vacuum between butterflies and
motor, such that idle mixture is pulled in via pilot air circuit which keeps it
rich enough to prevent bucking.

Presumably, partly open butterflies `defeat' the vacuum effect on slow circuit,
when going to closed throttle with engine turning fast, causing lean mixture and
bucking.

If true, it would explain why pilot air restriction jets don't work in this
case, and also why Tim Harwood (I remember saying something like..)
advises `if you have poor idle and have to set tickover to >1200rpm it WILL
buck'. (as here, the butterflies are not sticky, but SET to be more open than
they should be).

IF CORRECT theory, then no amount of fiddling with mixture screws or pilot air
restriction jets will address it, only fixing the butterflies…. (but, a bit of a
strip down to get to these...)

Before I bother to take it all apart again, it'd be great if someone says 'yep,
I've had this before, and when I fixed it, bucking reduced' or at least 'that
theory makes sense, let us know what happens'.

If I get told 'nope, thats irrelevant' then I'll leave if for now. Plenty other
bits of fettling to be getting on with!

Apologies to those who like short posts. I did say, read on only if you can be
bothered!

Thanks for any advice,
Mike
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BAZ
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Re: Bucking - a theory to be shot at

Post by BAZ »

You seem to have been very through in your rebuilding and setting up the carbs. So looks like we are looking at something unusual! Anyway I have had a leak in the float bowel between the pilot and main jet cursing all types of weird symptoms so fit the float bowls off your other carbs and sees what happens. Also check that the slides are not sticking and that you have the plastic plate on top of the needle jet not the bottom.
The screw you refer to is not a mixture screw it affects air and fuel. Just set at 1 turn out. Make sure the carb tops have no air leak from were the clutch cable passes. Sometimes I have had Carb bodies with internal problems and just would not work properly. I may have a spare body if you need to test Let us know how you get on BAZ
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Rijko

Re: Bucking - a theory to be shot at

Post by Rijko »

i love reviving ancient threads .. did you ever find a solution,
or are you still bucking around :lol:
bull-riding-animation-1.gif
bull-riding-animation-1.gif (66.91 KiB) Viewed 987 times
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PaulD738
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Location: Warrington Cheshire

Re: Bucking - a theory to be shot at

Post by PaulD738 »

Simple solution, get a J/K model with VM carbs (none CV) they don't buck no matter how badly the carbs are set up ;)
They're rubbish them Jap bikes lad they won't last five minutes! you want to get yourself a nice Royal Enfield!
A quote from my old dad 8-)

I started out with nothing and I’ve got most of it left!
wardy
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Location: north lincolnshire

Re: Bucking - a theory to be shot at

Post by wardy »

or a z650 like mine... no bucking at all!
Andy;
1976 gt750A; 86 FZ750
1977gt380A
1982 z400j
1981 z650f.....1988 Sufolk punch 14"
"look;it's my bike; when you own it ,you can paint it any colour you like!"
(I once painted my Z650 PINK and had the pink panther logo on the tailpiece)
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Waterbuffalo
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Re: Bucking - a theory to be shot at

Post by Waterbuffalo »

I've said this before and have proved it to be a source of the surging situation even after all the rest has been addressed to the carbs etc. Ignition timing directly effects the surging and it can be tuned out.
Two strokes, its just that simple.
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Rijko

Re: Bucking - a theory to be shot at

Post by Rijko »

waterbuffalo, i'm not familiar with the quirks of the GT750.
Fortunately mine does not surge.
Can you explain please ? Maybe comment on electronic ignition too ?
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Rijko

Re: Bucking - a theory to be shot at

Post by Rijko »

let me tell you where i'm coming from, i'm sure people will smile and remenisce.

When i was 16 i had a Zundapp GTS, a 50cc moped with huge tank and seat compared to other Zundapp models.
My uncle owned a car shop and painted the frame and gave the tank, side panels and rear a nice metallic blue paintjob.
With the help of my friends i restored the engine.
Ignition we set using the smoking paper method.

This technique i carried over to the GT750 i bought when i was 18. (over 30 years ago)
No workshop manual ofcourse, no internet ... i thought it was the way to do it.
Had it checked with a stroboscope once and it was spot on.
And i have to say, my GT750 ran great like that.
Way faster than the 2 other GT750's that were around in my town.
Sold that bike, and regretted it ever since. It was my baby.

Nowadays we have internet, workshop manuals, micrometers that are affordable, electronic ignitions ...
A little bit more money to spend, and looking for the best way to treat my GT.
I bought it with a fancy electronic ignition with CDI box, but thinking of going back to points.
Those will not strand you like a broken Dyna S did on my GS1000, leaving me helpless in the middle of the night kilometers away from help.
If this may bring on the surging issue i'd like to know how to prevent that.

So .. your thoughts on ignitions would be appreciated.
Ofcourse anyone else's too !
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PaulD738
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Re: Bucking - a theory to be shot at

Post by PaulD738 »

As you have pointed out electronic ignition is fine until it goes wrong and then your stuffed! Points and condensers can be repaired at the roadside with a few simple tools. I always carry a spare condenser with me while I'm out on the 750J just in case. It starts on the button or kickstart and runs fine without need for fancy electronics. Don't forget these things are lowly tuned dinosaurs that will run and get you home even if the settings aren't perfect.
They're rubbish them Jap bikes lad they won't last five minutes! you want to get yourself a nice Royal Enfield!
A quote from my old dad 8-)

I started out with nothing and I’ve got most of it left!
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Waterbuffalo
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Re: Bucking - a theory to be shot at

Post by Waterbuffalo »

Yes Paul for that same reason I see no benefit putting electronic ignitions on a stock or mildly modified GT750. I have run points with no issues and find them to be stone stock reliable and easy to fix. I simply have a peek in the side cover every three thousand miles or so to check the gaps and lube the felt.
I do ride the GT long distances so I do rack up miles when it is on the road, and while points may seem antiquated on such a bike with a low state of tune, they just last and last.
I even went to the trouble of making new points wire harnesses with custom made flag terminals for my bikes, as the original ones are getting on in age and the quality of the wires and sheathing to the electrical panel were never the best, even when new.
IMG_2471.JPG
The custom made flag termials in brass, with new 18 strand 16 gauge copper wire.

The surging is an issue, the carbs are rubbish due to the stupid design of the BS40s pilot circuit, and the fact they made an error in the air circuit thats directly linked to that same part of the carbs. Adding the air corrector jets to solve it is really the best they could do, replacing them for convential slide carbs should have been done.
I run the BS40s on my 76 bike just to keep it looking stock, so I want the best performance I can get from it without adding convential carbs. The problem is the surging is not only carb related its also timing related, with the way the BS40s work, the overun surging situation actually shows up a mild form of over advanced ignition. I believe its mainly because its impossible to get a crank with all the lower crank pins precisely 120 degess apart, so at the lower rpms a combination of the BS 40 carbs and the mixture issue, the cylinders want to work against each other. Think of it as a mild form of lean burn preignition with a crank not precisely setup due to its design. You only have to listen to the way the exhaust sounds when its hung up at around 2200 rpms, just like the engines running out of fuel, or actually running on by itself such as in a mild preignition situation.

You need to set the timing up statically using the factory marks, but theres also a few issues here, theres play in the points cam and theres play in the propeller, so before you tighten them up with the shaft nut, move them both to one side of the slots and while keeping them pushed in that direction, tighten the nut. Due to discrepancies in manufacturing I often wonder how accurate the points sub shaft slot, the starter clutch case where the pin is and the crank keyway locations are for accuracy. Theres a load of issues up stream of the points cam to make timing vary quite a bit.
Once you have timed the engine statically, then place a fine marker line across the points plate and engine case near to one of the three fixing screws.
Now go for a ride on level ground in high gear at around 40 mph, then slowly let off the throttle, if it surges (most stock bikes will) you need to take corrective measures. Looking at the marks you made, move them apart by one millimeter distance, move the points plate in the retarded direction, thereby retarding the ignition timing. Test it again to see what the verdict is, if it still surges retard to 1.5 millimeters distance apart on the marks and retest. I have never seen the marks need to go more than 2 milimeters apart, remember you are only moving the timing just enough until the surging dissappears.
I know people are going to say moving timing it will make the engine sluggsh etc but with a 24 degree BTDC stock setting, its easy to see Suzuki built a lot of leeway into the timing of the engine so its already in a very low (vague) state of tune. I have proven this timing alteration works time and time again (no pun intended) so if you still get surging after doing the carb mods you need to look into the timing plate position.
Another thing is the stock airboxes internal issues which hamper higher gear roll on performance, but thats for another post.
Two strokes, its just that simple.
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