Gsx 400 twin won?t pass 45 mph

[Daaam88]

Hi there newbie here !

Ive got a suzuki gsx 400 twin I’ve recently built back up into a new frame.Im slowly solving the running issues but I’ve finally been stumped... The bike will start first time absolutely fine, run through all gears fine however will not pass 45 mph(im not a speed freak however I like thing to run properly) The bike feels like it hits a restriction.


A few things I have tried/sorted so far are as follows :
-making sure the rubber boots from carb to head are sealed properly.
-spark plugs are giving a good reading
-New exhaust system with baffled slip on silencers
- fuel supply is efficient
-carbs have been cleaned with new diaphragms
-jet changes have been made to suit pod filters
-speedometer is calibrated properly
-fresh air filters
-tried various sprocket ratios

All all of the above solved issues but did not change the ‘restriction’ feel when riding .

Not sure if anyone may be able to help or point me in the right direction , would be much appreciated if possible as I’m now at my wits end ��.

Thanks all

[Guest]

What size main jets do you have?

[Daaam88]

What size main jets do you have?

Hi ,
it now has 122.5 main jets in which seems to be giving me the best running/spark readings

Thankyou

[Guest]

Original mains on the 450 were 115 - 120 (depending on model/market). You mentioned pod filters. What exhaust system do you have?

I'm no expert on the twins, as I haven't owned one for nearly thirty years. But, assuming your carbs are squeaky clean, you may need larger mains. I know there are members here who have dealt with this issue, so it's probably worth waiting for advice from one of them.

One last question - were the replacement jets genuine Mikuni?

[Daaam88]

Original mains on the 450 were 115 - 120 (depending on model/market). You mentioned pod filters. What exhaust system do you have?

I'm no expert on the twins, as I haven't owned one for nearly thirty years. But, assuming your carbs are squeaky clean, you may need larger mains. I know there are members here who have dealt with this issue, so it's probably worth waiting for advice from one of them.

One last question - were the replacement jets genuine Mikuni?

The bike has an exhaust system which has been made up to suit the new frame. Consists of two open pipes into baffled slip on silencers.

The jets are genuine Mikuni . I?ve also tried larger and smaller jets whilst checking the spark readings and 122.5 is where we fount the best readings.

thanks for your help

[Cipher]

plug chop reading? Exhaust gas analysis? Does it respond to throttle well while sitting? Rev well fer instance?

[Cipher]

plug chop reading? Exhaust gas analysis? Does it respond to throttle well while sitting? Rev well fer instance? Oh and the valves are set and compression is fine?

[Guest]

You're welcome. The reason I queried the mains is that, at 45mph, the main jet circuit hasn't cut in, and doesn't sound like it wants to. How high will it rev in neutral? You probably need to get it around 5k rpm before you can use spark plugs to judge jet efficiency.

It sounds like you know what you're doing, but if you want some more info, search 'plug chop' on here.

[Daaam88]

plug chop reading? Exhaust gas analysis? Does it respond to throttle well while sitting? Rev well fer instance? Oh and the valves are set and compression is fine?

Hi there ,
ive done a plug readings quite a few times.but now after seeing Steven922 reply^^ if the the main jets aren’t even kicking in maybe this is giving me a false reading. As for the valves And compression that has not been checked as of yet.The throttle opens and closes absoultly fine.

Thankyou

[Daaam88]

You're welcome. The reason I queried the mains is that, at 45mph, the main jet circuit hasn't cut in, and doesn't sound like it wants to. How high will it rev in neutral? You probably need to get it around 5k rpm before you can use spark plugs to judge jet efficiency.

It sounds like you know what you're doing, but if you want some more info, search 'plug chop' on here.

i see , that may explain a lot of why I?m not getting very far Thankyou! In neutral the bike won?t rev above 5k I shall try searching plug chop and see what I can find much appreciated!

[Guest]

Parked not riding, will it rev up to the red line or does it his a wall around 4k? I see that has been answered. Your mains are not matching the air flow.

[Daaam88]

Parked not riding, will it rev up to the red line or does it his a wall around 4k?

Hi there ,
parked it will not redline and like you say it hits a wall around 4 k
thankyou

[Guest]

Hi there ,
parked it will not redline and like you say it hits a wall around 4 k
thankyou

4K is where the ignition advance mechanism opens up; if it's pre '83 when the ignition system went to electronic advance, the advance could be rusted closed.

[Daaam88]

4K is where the ignition advance mechanism opens up; if it's pre '83 when the ignition system went to electronic advance, the advance could be rusted closed.

Thankyou for this John . Working on the bike today so will have a look at the advance now! Much appreciated