'87 Spree (NQ50) won't stay at full throttle

[toplessFC3Sman]

I just picked up an '87 Spree for my gf to ride to class and locally, and have gotten it to start and idle reliably, but there's still an odd issue. Warm or cold, I can go to full throttle and it responds pretty cleanly and gets to the point where the engine speed sounds like its stopped climbing, will sit there for a second or so, and then die as if it ran out of fuel. If I back off the throttle a lot it will catch again, but if I go WOT immediately, it will seem to run out of gas before I get there. If I wait for about 5-10 sec, it'll reach the RPM limiter and sit there for a second or two again until dying. It acts as if the carb bowl is temporarily emptying and then re-filling once you let off the gas and the fuel flow rate out of the carb drops. A bit more testing showed that it did a similar thing at lesser throttle openings too, it just took more time "for the bowl to empty", if thats whats actually happening.

I tested the float level valve by pulling it apart and trying to blow through the fuel inlet while moving the float, and it goes from fully sealed with the float up to pretty easy to blow through with the float down. Seems to be working right, and doesn't look to have any adjustment.

I tested the vac-actuated petcock (what's its real name?) by idling the engine and pulling the fuel hose off of the carb and draining it into a gas tank. With no restriction at the end of the hose, the hose seemed to be flowing as much as it could (being gravity-fed anyway). Unplugging the vacuum line from the intake runner would choke off the fuel flow until it was just barely dripping into the tank, and that may have just been residual from the inside of the hose. The petcock seemed to be working properly (altho if you go full-throttle on the bike, wouldnt you expect the manifold to hit atmospheric pressure and choke off fuel flow from the tank?). An interesting sub-note; with the fuel line completely disconnected from the carb, the bike continued to idle for about 8 minutes... i was a bit surprised!

I've torn apart the carb for cleaning and all the passages seem clean. I've removed the main jet and needle and cleaned them, the pilot sprays a nice strong spray of WD40 when its pressurized (as well as leaking out of the upstream air hole). I've played with the pilot fuel mixture screw, going from the nominal stock adjustment of 1 3/8 turns out to about 2 1/2 turns out without much of a change.

Where else should I be looking for a solution to this? Any specific carb passages/limiters/odd petcock behavior/something with the bystarter valve? I haven't checked that this is working yet, but it had no problem warming up and maintaining an idle.

[odinxxix]

i wonder if the fuel strainer is partially plugged and wont allow for the extra fuel needed at wot., is there an inline fuel filter? if so it maybe to big and starving the motor. whats the condition of the air filter?

[toplessFC3Sman]

The air filter seems like its in good shape; still foam-y and pliable, and it didnt seem very dirty.

I didn't see any sort of fuel filter, and haven't checked the strainer since I havent drained or removed the tank (but it looks remarkably rust-free, in contrast to my '73 CL350).

In retrospect, I think I adjusted the pilot mixture screw, not the air idle speed screw, to 1 3/8" turns out. Is there a setting for the pilot mixture screw? It didn't seem to make much of a difference (at least with this issue) whether it was almost fully in, or up to 4 turns out.

[fightingplankton]

try adjusting the jet screw.
when i first got mine, i had it too rich and it was basically flooding the engine at WOT.
i found a previous thread about it on here when it was happening to me that said the same thing.

[odinxxix]

i was told by another member when i started and it also says in the manual, the idle screw should be 1 3/8, i used 1 1/2 to 2 turns out. adjust the mix screw til it achieves the highest idle speed then adjust the lg "idle screw" til it comes down to 1800 +/- 100 RPM.

[toplessFC3Sman]

try adjusting the jet screw.
when i first got mine, i had it too rich and it was basically flooding the engine at WOT.
i found a previous thread about it on here when it was happening to me that said the same thing.

What do you mean by the jet screw? The small flat-head screw right above the bowl, that seems to be tied to the fuel mixture for the pilot jet? That one didnt seem to do much.

I've also tried clipping the needle in all 3 positions; it had been in the middle when I got it, but moving it up or down (richer and leaner, respectively, right?) didn't seem to make a difference. I'll play with it more tonight, and take a look at the fuel strainer too.

[Wheelman-111]

Greetings:

Quoth FC3S:

I tested the vac-actuated petcock (what's its real name?)

That's as good a name as I've ever heard. "Fuel Valve" adn Petcock (tee-hee!) are commonly heard around here.

by idling the engine and pulling the fuel hose off of the carb and draining it into a gas tank. With no restriction at the end of the hose, the hose seemed to be flowing as much as it could (being gravity-fed anyway). Unplugging the vacuum line from the intake runner would choke off the fuel flow until it was just barely dripping into the tank, and that may have just been residual from the inside of the hose.

That sounds about right.

The petcock seemed to be working properly (altho if you go full-throttle on the bike, wouldnt you expect the manifold to hit atmospheric pressure and choke off fuel flow from the tank?).

The engine keeps making enough vacuum to keep the fuel flowing unless there's a dodgy fit at one of the barbs or a crack in the line. That happened to me early on.

Adjusting the low-speed mixture probably won't help if your issue is exclusively WOT. I'd repeat the tank, petcock and carb cleaning, only throw on a "Cone O' Stone" inline filter when you reassemble. There may be a steady stream of particles large enough to obstruct the float needle coming from the tank. There's a test for the bystarter in the Factory Service Manual. If it's staying "on the choke", that could partially explain its behavior. Nice work on your checklist so far, and Welcome.

[toplessFC3Sman]

Dumb, dumb, dumb reason for it... It wasn't running correctly because I had left the airbox cover off. The filter was still there and I left the cover off to remind myself to clean the filter, but it was the cover causing the problem apparently. I dont know whether it was because the manifold pressure was getting slightly too high and shut off fuel, or because the slight increase in pressure made the whole mixture too lean (altho judging by the smoke coming out of it, it is definitely not too lean), but putting that cover back on allows it to stay at full throttle for as long as you want. Odd... and quite picky it seems.

On a somewhat related note... how much smoke are these supposed to put out at full throttle? I know they're two-stroke and burning oil, but it smelled quite rich and was pretty smoky.

[fightingplankton]

mine is in stock running order(no mods)
and i have 0 smoke.
i still use the original oil tank instead of premix w/ no signs of problem.
a few of the other guys on here(possibly most) use premix, if you are doing so, this may attribute to your smoke issue.

[toplessFC3Sman]

I still have the stock oil tank and pump, and have no reason to believe that the previous owner used premix as well as the oil pump since one of his selling points was that you didn't need to premix. I'm still on the same tank of gas tho, since I am in Michigan, and its not exactly scooter-friendly weather. I'll try cleaning out the pump and everything too, but it may be running a bit rich (the main needle is in the full-down position, so as lean as possible).

[Wheelman-111]

Greetings:

Leaving a cover off wouldn't make the Top Ten list of dumb things I've done. Without a cover, it was too lean to run well. With it and a dirty filter, it may be a tad rich. The pickiness of Sprees is legendary. However smoke early on could be the result of sitting for a long time. The pump drips sometimes, just a drop a day but the days add up. See if it clears after 25 miles or so.

Nice job of troubleshooting!