What to try next?

[bob654]

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 also have an inline fuel filter.

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

Anything you guys can add would be a great help.

 

Edited July 5, 2022 by bob654

[wraith]

Dose it rev through the rev range in all gears?

If so can't see it being ATU, probably main jet.

Plug chop is the way to go.

[Fazz711]

Plug chop is a good idea. 

Try and get an idea where your fueling is at? 

Is it un restricted vance n hines? 

May need bigger mains if it is. 

 

[coombehouse]

Run it without the airbox lid to see if that changes anything. Then run it with the lid but without the filter, then without the lid & filter. If nothing changes, then at least you know that side of things is ok.

[Mike711]

Have you checked the coils are 3 ohm?

[Captain Chaos]

11 hours ago, bob654 said:

Anything you guys can add would be a great help.

oilcooled 1100 engine, turbo and EFI.

[/end_thread]

[Tony Nitrous]

Might not be relevant to yours, but I bought a big bore ET with mish mash of bits and a lots of mods and mileage on it…. 

I had an in-line filter, it was plenty big enough, located properly, checked and double checked but the bike was never at its best. I binned it, and brought a brand new replacement tap and filter that improved it.

The ET also came with a few airbox lids. Running the stock filter it seemed better with the lid that had several large holes in it.

[bob654]

11 hours ago, wraith said:

Dose it rev through the rev range in all gears?

If so can't see it being ATU, probably main jet.

Plug chop is the way to go.

Yes it revs through.

ATU?

[bob654]

11 hours ago, Fazz711 said:

Plug chop is a good idea. 

Try and get an idea where your fueling is at? 

Is it un restricted vance n hines? 

May need bigger mains if it is. 

 

I have a one size up main set 115. which I will try today. 

[bob654]

10 hours ago, coombehouse said:

Run it without the airbox lid to see if that changes anything. Then run it with the lid but without the filter, then without the lid & filter. If nothing changes, then at least you know that side of things is ok.

Will do thanks.

[bob654]

2 hours ago, Mike711 said:

Have you checked the coils are 3 ohm?

I will.

1 hour ago, Captain Chaos said:

oilcooled 1100 engine, turbo and EFI.

[/end_thread]

Chaos Captain hehe

[wraith]

1 hour ago, bob654 said:

Yes it revs through.

ATU?

If it's reving through in all gears, I wouldn't think it's ATU. As the ATU would not advance in all gears not just top.

[Reinhoud]

15 hours ago, bob654 said:

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 also have an inline fuel filter.

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

Anything you guys can add would be a great help.

 

What brand sparkplugs?

Mine doesn't want to run properly on Denso. Took me a while to figure that out

[gs7_11]

Have you done a compression test?

Ignition timing and advance checked with strobe?

Cam timing; 100% sure its correct?

Jetting will be near enough at standard, with standard airbox and V&H.

Fuel flow through filter is worth checking. 

If it won't go past 70 in top, that's a major issue, not something minor. Even a rough GS750 will go over 100 without too much trouble.

[bob654]

5 hours ago, Reinhoud said:

What brand sparkplugs?

Mine doesn't want to run properly on Denso. Took me a while to figure that out

NGK.

[bob654]

1 hour ago, gs7_11 said:

Have you done a compression test?

Ignition timing and advance checked with strobe?

Cam timing; 100% sure its correct?

Jetting will be near enough at standard, with standard airbox and V&H.

Fuel flow through filter is worth checking. 

If it won't go past 70 in top, that's a major issue, not something minor. Even a rough GS750 will go over 100 without too much trouble.

 

Cheers.

[bob654]

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?

[Fazz711]

4 minutes ago, bob654 said:

NGK.

Have heard that some plugs are dodgy take knock offs so may be worth trying another set

[bob654]

Yes, will keep that in mind, thank you.

 

[gs7_11]

Don't drill the airbox.

A standard bike with all standard bits will do 125mph.

I used a stock 750 airbox on my 7/11 with a 1085 engine, 28mm carbs and V&H pipes......140mph!

Fix the problem, don't bypass it.

Possibly a mixture problem, not airflow. Float heights? Leaky float valves? Blocked air jets in carbs?

Edit: assuming nothing daft wrong with airbox.

Edited July 6, 2022 by gs7_11

[gs7_11]

GS7/11 1085cc, VM28ss, V&H pipe, stock 750 airbox, stock jetting,  90bhp rear wheel.

[bob654]

GS750 top speed is 105mph. So Ive read. Having increased jetting and put on a V and H exhaust, and found out the bike wants more air and when it gets it it does what its supposed to, then why not give it more?

 

[bob654]

37 minutes ago, gs7_11 said:

GS7/11 1085cc, VM28ss, V&H pipe, stock 750 airbox, stock jetting,  90bhp rear wheel.

I will check the Float height , cheers.

[Reinhoud]

8 hours ago, bob654 said:

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?

Hmm, if the bike is stock it should be alright, what condition is the air filter?

The air filter box should have a hole in it to suck the air through, is that open?

[Reinhoud]

How close to stock is your bike? If it's very close to stock, you shouldn't start changing jet sizes.

 

With problems like this, always make sure the basics are ok, valve clearence, spark plugs, carbs working properly, and synchronisation, that kind of stuff.