new bigger engine! equals less top speed?

[MrC.]

So I removed an old 3.5 Briggs and clutch which runs great. Replaced it with a new 6.5 pred. And new clutch. Same gearing same belt and chain configuration. Now engine revs hard will literally smoke the drive tire but tops out really fast. I cant get my head around this one. I first thought the new clutch had a higher stall but that wouldnt reduce top speed. Then I thought clutch pulley diameter difference but there the same. So then I removed the governor thinking that was the limiter to speed. Nope now it revs way high rpms smokes the tire but no more top speed. I know its a gearing issue but I didnt change the gearing. It should have been the same top speed or more by doubling the hp. But it seems slower or may be the same but at alot higher rpms. This is my daughters kart I dont want the engine to blow with her on it. Aside from regearing it and use the new rpms I dont know what to do.

Thanks
MrC.

 

[mckutzy]

What is the is the kart, the OD of rear tires, and the actual tooth counts of all the sprockets...

Pics as to what is the kart in question....

 

[Hellion]

How much you want for this beastly 3 1/2 horse Briggs?

 

[bob58o]

Maybe the old engine was not governed and the new one is?

 

[Hellion]

Shhh, that's ↑ the elephant in the room.

 

[anickode]

Sounds like the governor on the old engine was screwed up.

With almost double the power, I'd put a smaller axle sprocket on there.

 

[MrC.]

The 3.5 briggs, did NOT have a governor. Now the NEW engine does not have a governor, I have checked for full throttle and its good.
Its not that the New engine is running poorly, its a hemi, and pulls strong to high rpms.

I will post pics tonight.

Its a belt to jack shaft to #35 chain factory setup. I think its a carter cart, two seater. It has 6 inch rims and 165 tires, nothing big or odd. the belt pulley looks like 2 to 1 ( two inch pulley on clutch, 4 inch pulley on jack shaft. then a 10 tooth #35 gear to (guessing) a 70 or 80 tooth drive gear. Again I only changed the engine and clutch everything else is the same.

At this point I am thinking it is just geared low and wasn't that noticeable with the 3.5 briggs, but is really turning up the 6.5. If I ran the numbers correctly than I have a gear ratio of 13.99 to 1 which I know is very low, the easiest thing at this point is to change the output jackshaft sprocket to say a 20 tooth which should net me about 7 to 1. I am just surprised it was ever geared this low from the factory.

Thanks for the help
MrC.

 

[Goat]

You mentioned pulleys. Could it be that the extra power is causing the belt to slip?

 

[Bbqjoe]

Get yourself in the 6-8:1 gear range and you will be happy with the results I believe.

 

[anderkart]

Your top speed difference could be explained because valve float limits rpm of your un-governed 6.5 hp engine to around 5500 rpm. Un-governed flathead Briggs engines like your old 3.5 hp can rev quite a bit higher than 5500 rpm, because they're valve's don't start floating untill a higher rpm.