[SOLVED] Having to restrict the intake airflow to start the bike

[245luigi]

Hello all!

I have a like50 2t. Yesterday I removed the air filter restrictor and began jetting trials. I have a 38 slow jet and 98 main jet in at the moment, chop shows lean so ill be sure to upjet the main. (With restriction I was at 35 sj, 82 mj)

Thing is since I removed the airbox restrictor I have been having to restrict the airflow going into the airbox on cold start up. All I do is squeeze the rubber tube/snorkle going into the inlet side of the box. Once the bike has started, I remove my hand and there is not issues with idle and riding in general.

What can I do to make the bike start like it used to!? You know sans hand restrictor... Im enjoying the bike with the more open air system because i got 2 - 5 mph out of the top end.

[wheelsoffreedom76]

   Make sure the auto bystarter and air/fuel passages for it are functional.
http://hondaspree.net/wiki/index.php5?title=Bystarter:_What_It_Is,_What_It_Does    This is a little different, but the operation is the same as yours. If it is working right, it should start even if you remove the air box entirely.

  I suggest reading this article about jetting as well. http://scootrs.com/tech.cfm?tip=tunecarb

[245luigi]

Thanks so much for the links. Im pretty comfortable with jetting. Love me some plug chop.

Admittedly though, I dont know much about auto bystarters/chokes etc. Why should it start even with no airbox? Im legitimatley curious.

Lets say the bystarter works b/c at the moment I have no reason to believe its gone bad. Next diagnostic step?

[wheelsoffreedom76]

  Assuming that your scooter started & performed normally before you did the air box and jetting changes, and assuming after  making the changes you adjusted the air screw and idle properly and the scooter performed normally other than at cold startup.

  In the process of making the changes, you have caused a lean starting condition (we know this because you pinching the intake hose restricts the airflow making for a richer starting condition)

  Increasing the idle jet from a #35 to a #38 would actually have a slight benefit to cold starting given proper air screw setting.

  Increasing the main jet size would have no effect on cold starting, as the main jet plays no significant role below half throttle.

  Modifying the air box for increased air flow would make the starting condition leaner, but the amount it makes it leaner would not by itself make manual air choking necessary for cold start, as a properly functioning setup will start cold with no air box.

Again I am assuming that aside from cold starting, and your lean plug chops, the scooter performs normally. That being said we are back to the auto enrichment circuit. While it is unlikely that the auto bystarter failed at the same time you performed other changes, it is possible that it had failed before that and did not become obvious until you made it worse by leaning the start condition further. An otherwise normally functioning scooter will start cold with the auto bystarter in the closed position in mild climates, though it would be barely rich enough to start and idle right away.

   Another possibility is during your replacing of the main and pilot jets, a particle of brass from the jet heads or threads, or any other small material may have blocked the tiny orifice to the auto enrichment circuit, which would result in the auto enrichment supplying extra air at cold start, but no extra fuel (too lean to start without manual choking).

  Some things have been assumed in this diagnosis, if they are different let us know. It would also be helpful to know all of the mods that have been done. 

[245luigi]

Wow B-E-A-U-TIFUL write up! Thanks.

So I was thinking about this all day and as soon I got home from work I decided to start the bike.

VOILA. Started right up! You were absolutely right. Now my theory as to why I needed to pinch the air hose/snorkel. As soon as I would replace the carburetor and connected all the hoses after a jet change, I would attempt to start the bike right away. I know...I know quick on the draw, not waiting for the bowl to fill. I think that pinching the air hose would create a larger vacuum over an empty bowl; thus "forcing" fuel through the carb faster. Since my bike rested 12+ hrs, the bowl certainly filled fully and thus easy start up. How that sound??

Side note: I decided I dont have time for all these jetting trials as I use the bike way too much. I decided to go back to my orginal jet settings (35, 82). Once I got the idle back, I seemed to have lost power between 15 - 30 mph. I think I lost power or I could just have been used to the acceleration/speed I was getting during the trials. Now through these trials, I played with the MJ SJ needle (just remove/replacing to position the carb for jet install) as well as fuel and vac lines. Assuming I actually have lost power, what did I do!? I get the same acceleration up to 10 mph thats for sure, I hit the same top speed ~42mph. I just feel like the "umph Im having trouble holding to my handle bars" from 10 - 30mph is gone.....or Im nuts!

FYI set up right now is stock pb16 carb with 35 sj 82 mj, airbox with restriction back in, tecnigas next r exhaust. 1500 rpm contra spring I installed prima tuning weights and have not ordered my proper high quality ones, maybe the tuning ones have gone flat?

[wheelsoffreedom76]

  The best thing to do when you hook everything back up after having the carb empty, is pull the vacuum line off the intake manifold that runs to the vacuum petcock and suck on the line to open the fuel valve and allow the lines and carb bowl to refill.

  I had assumed your cold start issue was on subsequent cold starts and not just the first cold start after mods, thus assuming you still had the issue with the carb bowl full.

  And yes, pinching the hose would help create higher vacuum on the intake manifold to pull the vacuum petcock open, as sometimes just cranking will not produce enough vacuum to open the valve and refill the lines and bowl.

  As for the missing power, I'd go over your vacuum lines and make sure the bowl vent didn't get plugged, and make sure the restriction elbow didn't rotate into the air filter and block it.

  With that technigas pipe and stock jetting you are running pretty lean, and whatever power it makes like that will be when it's hot and at risk of seizing.

   There really is no reason to alter the stock air box setup. The #38 pilot jet can be run, if your needle has adjustable height you should move the clip to a lower notch and if not adjustable you should shim the needle with a .025 washer (thickest that still allows retainer clip to go back in), That will fatten up the throttle midrange. No matter how big you go on the main jet, the throttle midrange will only be made significantly richer by needle height adjustment and/or selecting a different profile needle.

[245luigi]

Considering that Ive been playing with the carb so much these past couple days I figure its time I give it a good cleaning. Hopefully that will catch the problem.

I may have misspoken earlier. The PB 16 stock jetting is 78 MJ, I currently have the 82 MJ installed which was put in place when the tecnigas was installed. Considering Im a plug chop veteran now (I kid, I kid) ill verify my MJ is of proper size.

Regarding the loss of power, from what I understand, the needle is what controls fuel delivery 1/4 to 3/4 throttle, and MJ is for 3/4 to WOT. Could I diagnose the power loss issues by controlling the throttle to either range and seeing how the engine responds?

Or does 1/4 to 3/4 throttle also mean 1/4 to 3/4 of max speed?

Thank you for your infinite wisdom wheelsoffreedom76.

[wheelsoffreedom76]

    The #82 main jet that often comes with a new scooter in the bag with the smooth boss, is considered to be the correct jet for a completely stock scooter. The #78 or any other smaller jet is considered to be part of the restriction placed on the scooter for the 30 mph, or under 3.5 hp rules, and to help meet emissions standards. Some scooters tune out differently than others, I'd certainly watch your plug chops, and check for overheating. I personally like to jet just shy of too rich.

   I feel it is worth noting that many people misunderstand plug chopping procedure, and end up with only a vague impression of the air/fuel ratio at wide open throttle. You mentioned you are doing plug chops, and I assume you are using a new spark plug each time, cutting a portion of the threads off, or examining with a bright light and checking not only at wide open throttle, but at 3/4, 1/2, 1/8 as well. After modifying from a stock setup, it is possible to be jetted for correct air/fuel at WOT, but lean enough at other throttle positions to overheat and seize the piston on an extended run at say 1/2 throttle. While plug chopping is a good final tuning step, I personally like to keep going richer across the throttle range until there is no more performance increase for a given throttle range, then plug chop to verify what the performance has indicated.

    I am by no means a "guru" , or "infinitely wise", and have not been around scooters all that long, but I have been a auto and industrial mechanic for 22 years.