Not sure what to try.

[bob808]

Hi again,

So recap and latest work done.

Suzuki GS750 USA import 78.

Everything done. Head skimmed, carbs rebuilt, Primary jets standard, main 110 . Vannes and Hines 4 into 1 , standard air box, resealed, new rubbers + clamps airbox and head,no leaks, Dynatek s coils, new leads, and plug caps are NGK, new clutch plates too.

My question or problem is... Bike seems to be working hard to get up to 70 then wont go further.

So far I've checked for clutch slip, wondered if I need to increase main jet size,or if its my advance ignition timing, I have an inline fuel filter too.

Going to look at these tomorrow, and also do some chop tests.

Anything you guys can add would be a great help.

 

[allojohn]

Stock main jet for the '78 is 102.5
Ditch the inline filter, it will slow the flow.
Valves adjusted?
Carbs tuned?
Carbs vacuum balanced?
Who rebuilt the carbs, you/friend/shop? If it wasn't you then you should. Details will follow...

 

[bob808]

I rebuilt them and have been through them many times now. Vacuum balanced.Valves done before and after skim, 102.5 stock increased 4 sizes too 112.5 as per Vannes and Hines instruction.I will remove filter. Thanks.

 

[allojohn]

Yeah, that's what the V&H instructions say.... if you were running pods I'd agree but the exhaust alone does not need such an increase.
Plug chops will give you a better idea of what's going on - (pain in the a$$) the Gunson colourtune tune is more helpful, IMO. Good luck!
And... welcome to the nuthouse.

 

[bob808]

Thank you. To all.

So busy day.

Checked the carbs over, found main jet O rings were perished so changed them.
Put in 115 main jets
Tweaked the Ignition timing, a little.
Removed the inline filter.
Reset the carbs by ear.
Compression is good.

Then started doing trial runs.

1/ Over all not much improvements and still having problems reaching top end.
So
2/ Did a run without baffle, ( wow loud or what), seemed freer no real difference.
3/ Removed air filter box side,********** YES that’s the solution, massive improvement.*********
4/ Put baffle back in, no real difference.
5/ Took out the Air Filter, then put side of box back on, back to poor response.
6/ Baffle in Air box sides off = 100mph. ( Private road.)
The problem seems to be Lack Of Air.

What now?
Drill holes in the Air Box?

 

[Rob S.]

Pods?

BTW, I'm pretty sure your 750 should do a lot more than 100mph. Somewhere north of 120.

PS I see that you're 'across the pond.' I hope you're talking about mph, 'cause 100 kph just doesn't do it for me. A 'buck' is a buck is 100 mph.

Welcome to the loony bin.

 

[bob808]

Motorcycle specs site says 112mph I think I could have done more than100 mph, today but was happy to be up there. These bikes ( I have been lead to believe) dont like pods.
Would you think my float heights need checked?

 

[allojohn]

Not enough air or........too much fuel - same dif.

And if you haven't checked the float levels you absolutely must!! 26mm

 

[bob808]

This morning I checked the float height, which was correct. I checked the plugs which were very sooty, so figured even with the Airbox side opened it was too rich. So I cleaned the plugs, reset the air/fuel screws and replaced the 115, ( V and H recommended main jet size), with standard (?) 102.5 main jets.

Trial run with the Airbox open showed great acceleration and easily exceeded 100mph, on a private road, so I was very pleased. I replaced the Airbox side and set off again fully expecting a poorer performance this time.

Fantastic result even more responsive if anything, revved right through the range quickly and easily, I backed off at 105mph in 4th.

I still have fine tuning to do, no doubt

Thank you all. Although some opinions have been divided, all have been useful, and I am grateful for all the advice and information. Ride on.

 

[rphillips]

I've not fooled with V and H on an old 8 valve, but do remember with the GS1100 16 V, the recommendations from V and H included "remove the air box lid". I don't remember how far up they recommended the mains... Glad you're head'in in the right direction.

 

[Big T]

Reset both air and fuel screws?
to what settings?

 

[bob808]

The ones recommended on here https://gsarchive.bwringer.com/bikecliff/images/vm_carb_rebuild.pdf