Need help tuning my Vento GT-250 (Same as Hawk)

[skirmich]

Hi!

I need a little help tuning my GT-250 with these mods:
NIBBI P28FL Carb (I bought this before thinking going big bore)
Big Bore Kit to real 250cc (70mm piston)
Full SS Exhaust.

I did a little porting and polish to the head, I am running stock Camshaft on 91octane at sea level.

My current jetting is the last one I used with the stock piston:
115 Main, 38 Pilot, Needle on the 4th position from the top, Air Mixture at 1.5 turns.

My issues:
-Very hard to start when cold (I must use choke or it wont start, Prior it would start without choke).

-When hot and idling If I give it a quick turn to the throttle it would bog down hard, sometimes completely stall.

I am still breaking in the cylinder so I am not going past 5K RPM but the acceleration feels lazy not at all as before.

Any reccomendations for jetting or hints would be appreciated.. I have never tuned a bike before but I have messed with the jets for a little bit.. I got a full set of jets available for testing so what ever you ask me to test I can do it.

[XLsior]

try 40pilot, 130main, middle clip as a new base line for post modified top end.

Cold starts seem to like a small bumb in idle knob adjustment till warm...(my case) you will learn to adjust on the fly depending on the weather...

it takes a good few minutes before the engine gets up to temp so and throttle snaps can still bog early on...

But its possible current carby setting are at fault.

Though it could be spark plug related...

[skirmich]

Originally Posted by XLsior

try 40pilot, 130main, middle clip as a new base line for post modified top end.

Cold starts seem to like a small bumb in idle knob adjustment till warm...(my case) you will learn to adjust on the fly depending on the weather...

it takes a good few minutes before the engine gets up to temp so and throttle snaps can still bog early on...

But its possible current carby setting are at fault.

Though it could be spark plug related...

Thanks!
When the engine is really up to temperature a quick snap to the throttle will stall it 99% of the time.

What about the air mixture screw? 2 full turns out?

Thank you again.

[XLsior]

Try the jet changes and see of the stall out persists...I can't speculate without a decent baseline.

a/f should sit between 1 and 2.5 turns out.

Check plug condition... lean/rich/gap

[Deckard_Cain]

Originally Posted by skirmich

Thanks!
When the engine is really up to temperature a quick snap to the throttle will stall it 99% of the time.

What about the air mixture screw? 2 full turns out?

Thank you again.

Do as XLSior said to start..BUT..

remember, this is a carbureted engine, no pumper, not EFI.. a fast snap of the throttle will usually result in a momentary bog or hesitation. It should not stall of course.. but don't expect FI like throttle response.

As it is, I'd say your quite lean. Bumping up the pilot and main jets as suggested should get you in the ballpark of where you can adjust from there.

[skirmich]

Originally Posted by Deckard_Cain

Do as XLSior
said to start..BUT..

remember, this is a carbureted engine, no pumper, not EFI.. a fast snap of the throttle will usually result in a momentary bog or hesitation. It should not stall of course.. but don't expect FI like throttle response.

As it is, I'd say your quite lean. Bumping up the pilot and main jets as suggested should get you in the ballpark of where you can adjust from there.

Hi, thanks for the input.

My concern is that when the bike was fully stock it wouldnt do that (Bog or stall) at quick acceleration in fact it would crisply rev up without hesitation. Stock carb and factory jets were 105, 38 and needle not adjustable.. It was so rich? That I will never have to use the choke.

It was until I swapped the Nibbi it started needing the choke.

Some other issues:
Once hot I can un-screw the idle screw all the way out and the bike would idle at about 1200 rpm without stalling. I could only rev up tje engine with the idle screw... If I rev it high it will stall once the engine revs go down.

Also. When hot in traffic it would go down to normal idle at about 1500-1700rpm, and out of the blue it will rev up to 2000-2100rpm for a little while and rev down again to idle.. This can repeat a couple of times while I am waiting for the light to change.

[krat]

You may be running lean but it is not the jets if you are at 115.

Even with a "big bore kit" to a full 250cc you are only 20cc better than before and it will not require a massive jump in jet size.

I have a Honda Shadow 750 that only uses a 110 main jet and a 40 pilot on each carb and it is 375cc per jug.

It may be that you are putting more air into the carb than it can handle or your exhaust is still restricted and is choking the flow.

I see you are using a cone filter, perhaps the stock airbox might be better.

Have you taken the guts out of that CAT yet. It does not matter what silencer you are using if the CAT is still in place.

One of the constant problems that I see is that people using these engines never actually adjust the air/fuel mixture since the screw is difficult to reach. They SET the mixture by guess and by gosh, off the engine, then install it.

Proper A/F adjustment is to turn the A/F screw, with the engine running, until you get the fastest possible idle, then set the idle speed using the idle screw on the side. Any other "setting" is just a lucky hit. A flex shaft screwdriver from Harbor Freight is a cheap fix for that.

[skirmich]

Originally Posted by XLsior

try 40pilot, 130main, middle clip as a new base line for post modified top end.

Cold starts seem to like a small bumb in idle knob adjustment till warm...(my case) you will learn to adjust on the fly depending on the weather...

it takes a good few minutes before the engine gets up to temp so and throttle snaps can still bog early on...

But its possible current carby setting are at fault.

Though it could be spark plug related...

Ok so I tried 130 main and It would backfire? (Exhaust explosions) to hell when engine braking.. I just installed 120 main (Everything else the same) to test tomorrow since I ran out of daylight.. I also installed a new plug.

I also noticed the A/F screw likes it below 2 turns, I settled for 1.5 and it seems to idle pretty good.

[skirmich]

Originally Posted by krat

You may be running lean but it is not the jets if you are at 115.

Even with a "big bore kit" to a full 250cc you are only 20cc better than before and it will not require a massive jump in jet size.

I have a Honda Shadow 750 that only uses a 110 main jet and a 40 pilot on each carb and it is 375cc per jug.

It may be that you are putting more air into the carb than it can handle or your exhaust is still restricted and is choking the flow.

I see you are using a cone filter, perhaps the stock airbox might be better.

Have you taken the guts out of that CAT yet. It does not matter what silencer you are using if the CAT is still in place.

One of the constant problems that I see is that people using these engines never actually adjust the air/fuel mixture since the screw is difficult to reach. They SET the mixture by guess and by gosh, off the engine, then install it.

Proper A/F adjustment is to turn the A/F screw, with the engine running, until you get the fastest possible idle, then set the idle speed using the idle screw on the side. Any other "setting" is just a lucky hit. A flex shaft screwdriver from Harbor Freight is a cheap fix for that.

I ran a USB camera inside the bump in the exhaust but it doesnt seem to have anything else than a resonator inside of it (I can throw a nut and it will freely fall through the tube bump).

I have tried messing with the A/F screw but it doesnt behave quite like some youtube videos I saw.. There is no instant REV change but rather a very slow change in RPM, Right now it seems happy with 1.5turns with 40 pilot.. BUT it stills bogs down hard when I pull the throttle quickly to 1/2. I can only fix that if I install a very rich setting (Needle on the 5th position, 45 pilot) but it starts to backfire? (Exhaust explosions) and it smokes the plug.

[XLsior]

exhaust popping on decel might be an exhaust leak letting in fresh air and re-combusting unburnt exhaust gasses in the header pipe...

If the muffler does not have a baffle it could be exacerbating the decel pop issue.

decel pop won't damage the engine...just piss off the neighbors.

did you remember to install the crush ring gasket between the header pipe and cylinder head?

Backfiring out of the carburetor is the more dangerous type of post combustion feedback...

120 main might be fine...its hard to say without a plug inspection directly after a WOT blast and cut out reading.

[krat]

Originally Posted by skirmich

I ran a USB camera inside the bump in the exhaust but it doesnt seem to have anything else than a resonator inside of it (I can throw a nut and it will freely fall through the tube bump).

I have tried messing with the A/F screw but it doesnt behave quite like some youtube videos I saw.. There is no instant REV change but rather a very slow change in RPM, Right now it seems happy with 1.5turns with 40 pilot.. BUT it stills bogs down hard when I pull the throttle quickly to 1/2. I can only fix that if I install a very rich setting (Needle on the 5th position, 45 pilot) but it starts to backfire? (Exhaust explosions) and it smokes the plug.

It sounds like you are running rich beyond all belief. You don't overcome "running rich" by adding fuel.

Standard tuning guidelines are that if you do not run properly between 2-2 1/2 turns on the A/F screw then your pilot jet is too large not too small!

It almost sounds as if you are constantly on the verge of flooding out.

You added a "big bore kit" of 20cc and now think everything should be upscaled, bigger main, bigger pilot, move the clip,,, that is not the case. You should be running fine on a 37 pilot and 115 main with the clip in the middle.

It is not a crime to need the choke to start a cold bike, And let the thing warm up before you blip the throttle to half turn.

BTW, many of the aftermarket carbs have the choke setting reversed from the stock carbs, are you trying to run the bike with the choke set?

[bigdano711]

Originally Posted by krat

BTW, many of the aftermarket carbs have the choke setting reversed from the stock carbs, are you trying to run the bike with the choke set?

That little nugget about the reversed choke....GOLD!

[XLsior]

With his listed mods 37/115 would be lean on a PE28. Might be fine with a PZ30.

Nibbi PE28/FL has an enrichment circuit rather than a butterfly choke.

Without checking the plug its hard to assume whether its a flood out or a lean out. Or just using the "choke" incorrectly.

Maybe the fuel supply is restricted (blocked filter) so the opening the throttle starves the fuel mix...

It's all just guessing without a true baseline.

[skirmich]

Originally Posted by krat

It sounds like you are running rich beyond all belief. You don't overcome "running rich" by adding fuel.

Standard tuning guidelines are that if you do not run properly between 2-2 1/2 turns on the A/F screw then your pilot jet is too large not too small!

It almost sounds as if you are constantly on the verge of flooding out.

You added a "big bore kit" of 20cc and now think everything should be upscaled, bigger main, bigger pilot, move the clip,,, that is not the case. You should be running fine on a 37 pilot and 115 main with the clip in the middle.

It is not a crime to need the choke to start a cold bike, And let the thing warm up before you blip the throttle to half turn.

BTW, many of the aftermarket carbs have the choke setting reversed from the stock carbs, are you trying to run the bike with the choke set?

Thanks for the info.. Yes the choke is properly set down (Off position for the Nibbi).

Tried again with the 120/40 ran better BUT now I have another issue... When I am running at 40mph and come to a stop sign the RPMs hang at around 2000-2100 if I try to lower the RPMs using the idle screw the bike goes from 2000 rpm to basically stalling, nothing in between.

If I try to precisely adjust the idle it goes from running nicely at 1700rpm to jumping to 2100 all of the sudden and staying there until I blip the throttle.. I believe the bike is running too rich at idle?

[skirmich]

Originally Posted by XLsior

With his listed mods 37/115 would be lean on a PE28. Might be fine with a PZ30.

Nibbi PE28/FL has an enrichment circuit rather than a butterfly choke.

Without checking the plug its hard to assume whether its a flood out or a lean out. Or just using the "choke" incorrectly.

Maybe the fuel supply is restricted (blocked filter) so the opening the throttle starves the fuel mix...

It's all just guessing without a true baseline.

I put a brand new plug yesterday so it only has today use, I will pull it and take a picture for you.