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

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.

https://thumbs2.imgbox.com/6d/c7/xnU77VIj_t.jpg
https://thumbs2.imgbox.com/25/ad/QYgRa6b4_t.jpg https://thumbs2.imgbox.com/fb/58/l7U3N77Z_t.jpg https://thumbs2.imgbox.com/b9/fa/KyxSnfT3_t.jpg

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.

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

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.

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.

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.

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.

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.

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?

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

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.

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?

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

This is the brand new spark plug I installed yesterday..

Running conditions:
From Idle to half throttle never going above 5000rpm.
Around 40miles in it.

https://thumbs2.imgbox.com/d6/a1/eMboLj6i_t.jpg https://thumbs2.imgbox.com/cb/9a/brBbr3aP_t.jpg https://thumbs2.imgbox.com/69/9a/oBz5xit9_t.jpg https://thumbs2.imgbox.com/b7/73/txuigKUH_t.jpg

definitely not running rich or flooded.

nutmeg brown indicates the jetting is pretty close under current rpm ranges.

Yeah the bike runs great..

My only issue right now is the inconsistent idle when coming to a stop, I cant seem to tune in the idle at all. Either is too low and stalls or its too high that rev hangs on 2000rpms. The idle screw is almost fully out to the point that it stalls if I go any lower but wont keep the engine running if I try to dial it in.

Perhaps the A/F screw is lacking some tune?

I agree that the plug looks perfect, don't touch nothing! It will just continue to get that toasty look that we all know and love.

As for the idle problem, think about what you have, that carb is as simple as dropping a rock. The idle adjustment is a screw that impinges directly on the angled flat on the slide inside the tube. The slide pulls the needle out of the jet tube and releases fuel. That is a very mechanical issue, not a fueling problem. Slide in, needle up, faster/ slide out, needle down, slower.

Unscrew the top and pull that slide out and check it for burrs. Make sure the slide runs freely and that the spring is not binding the throttle cable. Make sure the cable is in its notch and the cable end is in its recess. Lots of times the cable gets tangled in the spring as you replace the slide and screw down the cap. Ask me how I know.

Something is probably blocking the slide from a full return consistently. That is where you get into burrs, sticky throttle cable, weak return spring....

You have this nearly worked out. It is now some little something that just needs a scrape with a pocket knife.

Ill try to look into a possible binding issue but I seriously doubt it..

When I think I have the idle dialed in I go for a ride and bamm again at 2000-2100RPM, I lower it on the fly (Seems to settle back to ~1700RPM) and then it suddenly dips into the low 1000s which either stalls it or seems like its going to but never goes back to 1700. I start to ride and stop again and repeat the same issue.

Right now I have to settle for the bike to hang the revs high for a while and if the light last long enough it will eventually (But taking its sweet time) go back to 1700rpm.. It also does this weird thing that once it has passed enough time it will idle high again completely on its own..

Ex:
>I stop for a light put on Neutral or Pull the clutch bike revs high on the low 2000s RPMs.
20 seconds pass
>Bike idles back down on its own to 1700rpms (My desired idle speed)
10-15 seconds pass
>Bike starts to idle high again to 2000s RPMs without me touching anything.
Rinse and repeat for as long as I am stopped.

This is the really the only issue I have at the moment.

Also I am running NGK DR8EA (7162) Plug which is what the manufacturer recommends, should I switch plug now that I have the mods?

just keep riding it for now until things settle in.

Make sure your clutch cable adjustment has a a little bit of slack same goes for throttle cable..

the idle knob should only be a faction open from a fully shut/seated throttle slide before making the necessary a/f adjustment.

plug type seems fine.

Forget about your plug, and forget about your "mods", they are not relevant in this situation. You have changed your life by 1.22 cubic inches. That is not enough to factor in and everyone here has done the exhaust and cone filter thing. That is why we have a base line of jet sizes and adjustments to apply to the CG platform.

That carb is a tube with a slide to block the air and two holes to control the fuel flow. Those holes don't change. They certainly don't change randomly while you sit on the bike. The idle screw does not move in and out on its own either.

I do know one thing that will cause your symptoms.

Your air/fuel mixture screw has a fine point that closes the pilot jet when it is fully screwed in. It is a very delicate point and can be damaged if the adjustment screw is turned in too hard. It is easy to damage the point if you turn the screw in and out several times, going to full in, then turning out the desired number of turns from closed. I think you have made that trip several time now.

It also has a tiny spring to keep it from vibrating and changing the setting constantly, It can be mashed and become useless if tightened too hard,,,,, and that is the problem you seem to have.

I would check that A/F screw tip and spring.

Past that I am clean out of ideas.

Are you running a good quality plug like NGK or Denso, or are you running a Chinese plug?

Tom, post #20 last sentence

In my line of work (small engine mechanic) an erratic idle is usually a vaccuum leak... i would double check the intake and carb are tight, no loose vac lines, and all gaskets on the intake side of things are in good shape. After that i would double check cam timing, and double check valve lash.

Update:
Bike still runs great when hot.. Idle is still erratic (gonna redo the manifold and carb gaskets to check for any leaks as suggested by bogieboy).