Engine idles high, then sputters when throttle opens

Hi everyone!

First off I'd like to say that I've been lurking on these forums for many moons, and finally went around and made an account. I've spent the last 2 months gathering parts slowly for a complex build, and now she's finally all put together. I've had to improvise and customize a lot of stuff on this build, and everything has (for the most part) come out better than expected, but now I seem to have hit a wall

I've got a 4-stroke HS 142f engine, purchased in a kit off eBay from an outfit called PetrolScooter. They had the best price on what seemed to be an identical kit to the dozen or so other sites I looked at. It's a 49cc, looks identical to the other HS 4-stroke engines in the various YouTube videos and posts here on these forums. Kit was purchased at the end of 2013. Engine had a very faint smell of fuel and/or oil when I inspected it, but did not appear to ever have been fired up. I have not made any adjustments to the valves.

Carb is stock. It DOES have the brass mixture adjustment screw, which some models made in 2012 seemed to either not have or have a blob of metal obstructing the screw.

The major issue I ran into was with the air filter - the bike frame I had didn't have enough room for the stock air filter - it bumped into the seat post, and because of the position of the down tube and the size of my mounting plate (with the SBP jackshaft kit), the engine wouldn't fit in the frame unless the filter came off. I bought an aftermarket air filter and used a couple of manifolds, a copper elbow joint, a couple of lengths of heavy duty rubber tubing and some hose clamps to build a 90-degree angle adapter to mount the air filter off to the side, so it didn't hit the seat post.

To further save space, I also removed the plastic "insert" plate that was clamped in between the carb and the engine and replaced it with a thicker gasket.

Here's where I'm having trouble:

• Engine starts up, but revs high - idle adjust screw is barely touching the throttle mechanism. Test performed with and without throttle cable connected, same results.
• Carb adjustment screw is 3 turns out (that is, I tightened it until it bottomed out slightly, then cranked it back out counter-clock 3 turns).
• When the engine starts, it seems to sit right around the 2k RPM mark (as per the tach on the handlebars).
• When I twist the throttle, the RPMs go down and the engine sounds like it's going to stall - this is the behavior through most of the throttle range - opening up all the way doesn't get me past 2500 RPM, and the engine doesn't seem to like it. Only left it there for a few seconds.
• Engine will only run with the choke ON. If I even so much as nudge the choke lever, the engine will sputter and stall. This is mystifying - when I had the carb disassembled, I could see what the choke was doing and the fact that so little movement is having such a large impact makes no sense.

What gives? Am I totally blowing it here, or could this be a simple fix? Do think my engine is running too rich/lean? Any advice would be greatly appreciated!

Thanks for the tips! Currently the air filter manifold is off the carb and I'm seeing the same performance. Definitely going to clean out the slow jet. I ordered some replacement stock gaskets from bicycle-engines dot com and they should be here in a few days... I haven't specifically checked for air leaks yet around the thicker gasket, but if cleaning out the slow jet doesn't improve things that'll be the next step.

I sort of suspect that gasket... as soon as the replacement stock gaskets get here, it's coming off and those are going on. If I have to re-build the air filter manifold, so be it - ain't much use anyways if the engine won't run

That did it! Cleaned out the jet and took a look at the gasket while everything was disassembled... I don't know what I was thinking putting that gasket in there, it was terrible - didn't seat properly against the engine and the carb, massive air leak. Swapped back to the stock gaskets and the spacer plate, and now my engine runs great - purring idle around 1200RPM, and fantastic throttle response on up. I discovered a tear in the inner gasket, which will be replaced with the new ones that shipped out today.

Once everything is re-assembled, I will post pictures and videos. Lesson learned here: aggressive customization and enthusiasm is no match for bad/wrong parts

Thanks for the pointers guys!

Well, I've got everything pretty well dialed in - or about as well as I seem to be able to.

May have jumped the gun a little - been doing some test riding around the neighborhood and the bike seems to have okay power, but I can't seem to get it up above about 3100 RPM. Even at WOT it tops out around the 3k mark. I'm confident the tach is reporting it correctly (and the engine doesn't sound anywhere near what redline apparently sounds like, judging from YouTube videos). I've been testing with gearing roughly equivalent to a 58T rear sprocket on mostly flat streets.

I've played with the adjustment screw on the carb a bunch, and no setting (from half a turn up from zero on up to 5 turns out) seems to let the engine break 3k revs. It's back at 2.25 turns from zero now.

Could it perhaps be a problem with the exhaust? Stock exhaust pipe is on there currently.

After I found and plugged the air leak from a few days back, I haven't done any cleaning on the spark plug (or anywhere else, for that matter) - is that recommended? Would a dirty spark plug produce symptoms like this?

This thread seems to come close to describing what I'm seeing: http://motorbicycling.com/showthread.php?t=50092
Engine would buck and sputter and cut out cruising, but would idle fine. The root cause in that thread seemed to be a combination of a weird flywheel and CDI. OP couldn't get his past 6k RPM... I wonder if I'm having a similar issue? I purchased this engine at the end of Dec 2013, so I wonder if it's from the same batch as OP's engine?

Right now the bike runs alright, and I'm confident I fixed the air leak problem from before, but I still feel like there's something borked here. Any thoughts or ideas?

Checked the throttle cable this evening while out for another test/tuning ride, and full twist on the grip opens the carb all the way. I even oiled the throttle cable a little bit and it snaps back nicely. The "sweet spot" seems to be at about half throttle - that's where the engine seems to produce the most power, and I tested it on a fairly substantial hill this evening. I'm tempted to pull the transmission off it tomorrow and try revving it with nothing attached to the shaft, just to see if I can get it past 3k.

Maybe it's the transmission that's screwing things up? It's the stock trans, I don't think it's the infamous Hoot (doesn't have that characteristic figure-8 shape), but it's not great - I'll probably bite the bullet and go with a Q-matic, as the only drawback about that seems to be the price (you get what you pay for). The transmission on there came with the kit, it has no identifiable markings other than the letters "XHD" embossed in the metal. Haven't been able to dig up much info on it by googling...

I just don't know here - maybe it's the carb itself? Could be that somewhere else in it is dirty, but to me it doesn't seem like that would cause the engine to level out around 3k RPM. I do not believe my tach is mis-reporting the RPM, as my engine sounds nothing like the screaming/wailing that redlining engines produce in YouTube videos.

Maybe just not enough gas is getting to the engine? It doesn't sound bad or like it's bogging out or sputtering, just that it reaches a plateau in RPM and even at WOT won't get above that. It's a mystery... tomorrow maybe I'll pull the carb and do a full thorough cleaning.

Well, the plot thickens.

Tonight I pulled out the spark plug and it's a coffee brown color - not too black and not all white, so I think my fuel mix is pretty good? I ordered the NGK plug from SBP anyway, and it'll be here in a few days.

I also removed the CDI while I was in there, but I can't really tell if it looks like the one that appeared to be limiting RPM in the other thread: http://motorbicycling.com/showpost.php?p=516095&postcount=45

Doesn't really look similar... mine's a weird kind of cylindrical thing. Is that, well, what a CDI normally looks like?

I also removed my transmission to inspect the centrifugal clutch, just to see if anything looked wonky. After riding not very many miles at all (maybe 20? since the bike's been roadworthy), I'm seeing what, to me, looks like a lot of dust build-up. I'm a computer tech by trade so any amount of dust is a big amount, so it could be nothing. Going to keep an eye on the transmission - it's the "stock" one that came with my kit. I don't think it's the infamous Hoot (doesn't have that iconic figure-8 shape), but the only markings on it are the letters "XHD" embossed across the side. Google reveals very little on this transmission's origins, but BikeBerry appears to sell them. Still, all the research I've done indicates that the vast majority of stock HS 142f transmissions are total crap and will fail soon after being put into service. Since it seems that 4G T-belt drives are backordered most places, I'll probably bite the bullet and get a Q-matic.

Anyways, I revved the engine a little without the transmission on, and apart from it being really cool watching the centrifugal clutch expand out as the RPMs went up I could definitely see and hear the "plateau" that my engine seems to get to. I was real careful in running it without any load, as I know these engines like to over-rev, and still it only got to ~3k RPM - reached that at about 1/4 throttle and did not increase as I increased the throttle about another 25%. Maybe it really is a funky CDI rev-limiting? Maybe HS has made even more conservative modifications to the rev-limiting that the CDI does?

I made a couple of adjustments to the carb, but the rain is rolling back in here and I don't want to go performance testing in the dark in the rain haha. Tomorrow's ride to work will tell!

CDI angle 1

CDI angle 2

Spark plug, after running for around 5 minutes (and commuting this morning and this evening)

Well that's good news about the gearbox - after reading everyone's comments about Hoot gearboxes, I figured it was due to blow out on me any minute. I'll probably be giving gasbike.net a call to see if they really have 4G-Tbelt units, but reassuring that I probably have a bit of good ride time left before that's a necessity.
Also encouraging to hear that you don't think my CDI is the newer version. I'd even be happy with ~6900 RPM, not looking to win any races here
I wonder if it really does have a mismatched CDI and flywheel? The engine idles just fine and lower end acceleration is pretty good (never expecting it to shoot of like a Ducati or something, but it gets moving promptly). Today on the ride home from work I shifted into low gear and revved up on a flat section, got to the characteristic 3k mark and just wouldn't go higher, but the RPM ceiling it reaches isn't smooth - the engine speed stumbles and fluctuates around the 3k mark, as if someone was playing on the throttle and intentionally varying it to get the RPMs to jerk around. Mismatched flywheel and CDI is starting to sound more plausible - this weekend I'm thinking I'll disassemble the carb and fully clean it out to rule out anything fuel-related. Maybe it could be fuel-related? I'll play around with the mix screw tomorrow too, see if that makes any difference.

worksuxxx, where did you acquire the flywheel that fixed your revving problem? BikeBerry and their various clone websites seem to sell one, but it's impossible to tell if it's the "old" one or the "new" one without having one in person. Did yours just come attached to another HS 142f or did you buy it standalone from somewhere? I'd sort of like to avoid getting a whole other engine just for a flywheel, but then at least I'd have a spare if I blow up this one

After lots of searching, it appears King's Motor Bikes sells flywheels. No way of knowing if it's the "new" or "old" flywheel, but at 16 bucks it's a small gamble. Part ordered, it'll probably be a week or so for delivery.

In other news, engine didn't start this morning
I rolled her out of the garage and spent a good 5 minutes gently but uniformly pulling the starter cord... and nada. It bugged me all day at work, and when I got home it took a few more tries but then she started right up as if nothing had happened! All I did was top off the fuel last night, and this morning trying to start I tried with and without the fuel tank cap sealed... mildly disconcerting...
When I did get her started, the engine behaved identical to how it's always behaved. I made no changes to the engine (spark plug I ordered is the wrong size, ffs) other than filling the tank. I hope she starts tomorrow!

NGK CR7HIX arrived today, $7 from Amazon with free shipping! Engine started right up, and idled great for a few minutes. Flywheel is arriving tomorrow, so that'll be exciting to compare/install.

I'm thinking it's about time to swap out my Hoot chainbox. It's getting a little bit louder during operation and while it doesn't show any signs of failure, the reputation behind the Hoot gearboxes/chainboxes is... troubling. I tried opening mine up (it's the "teardrop" shaped version) to inspect/oil it, but for the life of me I couldn't get the cover off. I pried at it as hard as I dared with a screwdriver, but I didn't want to exert any excessive force in case I broke it. Frustrating to be defeated by the little bugger, but knowing me I'd end up going overboard and actually breaking it haha. Will probably order a 4G next week - my engine is the tapered shaft 142f, which is apparently no good for the Q-Matic (at least without an adapter, iirc the fate of which was uncertain as per a thread here somewhere).

I'm thinking the chainbox might be a factor in my low RPM problems as well. There's a long gentle hill near my house, and in first gear (biggest sprocket in the back) the engine will get up to the 3k RPM mark and then it will waver and stutter a bit, almost as if something was periodically exerting greatly increased resistance on it. I can feel the momentary spikes and lapses in power as I'm riding too. To me it feels like this could just as easily be the flywheel screwing with the timing and limiting the RPMs that way, but I'm still suspicious of the chainbox... replacement carb might be next. Also ordered an inexpensive feeler gauge so I can adjust the valves properly.

My bike runs okay, I just know it could be better! The saga continues...

New flywheel is identical to the old one

I placed the new one and the old one directly on top of each other, and the key slots are in precisely the same spot. Drat. This flywheel came from Kings Motor Bikes/Gasbike - any wisdom in trying the one from BikeBerry? It looks to be the exact same part, but any chance BB has the "old" style versus the "new" style?

Running out of ideas here... going to try a replacement carb next, I think.