GX160 Max Rev issue

[captainandie]

Hello,

Checked other postings looking for someone else with this issue but couldn't find it. I have a GX160, 18 lb valve springs, governor removed, #90 jet in stock carb, and 8 degree advance timing key. Measuring my free load RPM I hit about 6400, and it acts like it still has a governor; hits peak revs, revs down, then peaks again, pulsing up and down (I'm not sure how big an RPM swing it is, but you can hear it). My buddy who has a built up clone 200 revs at 6800 and hold steady, no pulsing. Any ideas?

 

[TT]

You need some billet parts like a flywheel and a rod to rev that high. There is a thread you need to read and someone else can direct you to it. Be careful your motor could explode and hurt you bad.

TT

 

[maddcarson]

I Free Reved my 3 hp briggs with just a carb on it for a good 2 minutes and it didnt explode.

 

[captainandie]

I am planning to upgrade those parts, but my question is specifically what would make it rev and cut like that? Is that a symptom of valve float, or the timing is too advanced, etc?

 

[SAFETYpin]

I was only able to hit 5700rpm under load with a similar set up + billet parts... My upper rpms the engine sounds like its breaking up a bit.. A bit of popping/sputter, I was blaming valve springs and cam profile for the engine just laying down. And not wanting to make power past that RPM.

 

[Caseyslowgto]

I have a 160 with all the billet goodies and stock cam with 1.3 and 26lb springs(way too much) and from the speed calculator found out that I was spinning close to 7200rpm and it half acted like it still wanted to keep going but probably not a good idea with stock valvetrain.

 

[texasfabguy]

what have you done to the head? what are you running for fuel?

 

[Caseyslowgto]

what have you done to the head? what are you running for fuel?

Not sure who your asking but I have the old style 14cc with stock valves and a full port. Running a mix of 93 pump gas and 110 octane but only cause I thought I had some detonation issues.

 

[captainandie]

I'm running the original GX160 head and 89 octane. I don't think it's a pinging issue (could be wrong, never experienced it before). If it was too advanced would it rev high then reduce, back and forth like that?

I should also mention that it only does this when I pick up the back end. It does not do it when I'm actually riding it.

 

[Caseyslowgto]

I'm no expert by any means but doesn't the 160 already have 4* of timing. If so by putting an 8* key would make a total of 12*. Just wondering but is all that possibly too much?

 

[captainandie]

I've read that also. My plan is to put the stock key back in and see if the problem goes away. If it does then maybe it's time for a 4* key for a total of 8.

 

[Caseyslowgto]

I've read that also. My plan is to put the stock key back in and see if the problem goes away. If it does then maybe it's time for a 4* key for a total of 8.

That's what I'm running and it seems to do fine.

 

[SAFETYpin]

I am running the 4 degree key also, but now I am concerned why my machine doesn't rev up any higher.... Maybe my gearing is just to tall.....? I think I am at 4:1, ill have to go count some teeth when I get home...

I have the engine apart right now, and the head is off at the machine shop, for a -.030 mill job. The plan is 55# springs, stainless valves, split keepers/retainers, Champion shaft-mount rockers and an Isky BlackMamba cam... I have the parts... Now I am paranoid that I have some other issue to find first....

 

[Caseyslowgto]

I am running the 4 degree key also, but now I am concerned why my machine doesn't rev up any higher.... Maybe my gearing is just to tall.....? I think I am at 4:1, ill have to go count some teeth when I get home...

I have the engine apart right now, and the head is off at the machine shop, for a -.030 mill job. The plan is 55# springs, stainless valves, split keepers/retainers, Champion shaft-mount rockers and an Isky BlackMamba cam... I have the parts... Now I am paranoid that I have some other issue to find first....

That's what I'm talkin bout. I am looking at doing something very close to that(cam and valvetrain) but why the 55lb springs. Doesn't he sell them with 26lb?
I am at 4.5 to 1. Clutch-12t, rear sprocket-54t with 11" wheels. I had a budy run his car next to me and i was doin 55mph on the kart.

 

[SAFETYpin]

That's what I'm talkin bout. I am looking at doing something very close to that(cam and valvetrain) but why the 55lb springs. Doesn't he sell them with 26lb?

He does, but I left the spring decision up to him, with my goal of trying to reliably run 8k. All in all its a very affordable package, just shoot him an email with your goals.

I just finished counting teeth, I am 12 on the clutch and 60 on the rear, jackshaft is 18 in and 18 out, so that puts me at 5:1. I just mearsured the circumference of my rear tire at 44.5in, which gives me a top speed of 48.04mph at 5700rpm. Here is my faviorite calculator


What are other people spinning with 18# springs and OEM cams?

 

[texasfabguy]

I'm running the original GX160 head and 89 octane. I don't think it's a pinging issue (could be wrong, never experienced it before). If it was too advanced would it rev high then reduce, back and forth like that?

I should also mention that it only does this when I pick up the back end. It does not do it when I'm actually riding it.

So no port work?no milling?

 

[Caseyslowgto]

Sorry for the thread jack. Just a thought but you said when you pick up the rear of the bike that this happens, could it quiet possibly be starving for fuel?

 

[captainandie]

Starving for fuel is my next guess. It's just a gravity feed tank, do you think it's possible with a #90 main jet in a GX160 stock carb to be pulling more fuel than I can supply?

 

[captainandie]

So last night I checked out the fuel system, seemed to be getting plenty of fuel. So I swapped out the 8* advanced timing key for the stock key and the problem went away. I guess it was just too far advanced for the rest of the system.