Top Speed reduced and New Carb.
 

The title says it all. I purchased a brand new carb since it is just as easy than reworking an old one and replaced the 103 main jet that it came with and put in a 120. The air/fuel adjustment screw was turned all the way in, then back out 1 1/2 turns. Low end power is excellent and it pulls strong until about 24 mph, then it takes about another 100 feet to hit 25mph where it tops out on smooth roads. Previously, I was able to hit almost 30mph. There is no popping or anything, do you think I should go up to the 125 main jet? I have 120, 125, 130 and a 135. We never really take it on smooth roads, but I am just aggravated since we have lost a little top end. Any ideas?

Run it at top speed, shut it down and remove spark plug right there. (plug chop is what that's called) Look at plug and see if it's wet, tan/light brown (perfect color) or white. (lean = needs more fuel)

Is it the same size carb?

xlint89, thanks for the reply. Yes, the new carb is the same size as the old one. I also have several new spark plugs that I can put it and test it at WOT. I am at sea level here in houston, so I think I may be running lean. I will check it tonight and repost.

Also, one question about the air/fuel adjustment screw. I saw instructions a while ago somewhere on how to properly adjust it, but cannot seem to find them anywhere. Does anyone have a link to them?

Great advice on reading spark plugs, spoken like someone that has raced some...hmm.

My guess would be if the stock main jet was a 103 and you jumped up to a 120 you may be too rich. Too much fuel can make your engine sluggish. You can only pump so much fuel into a stock combustion chamber before you start to go backwards.

I've seen it a hundred times where guys will put a 750 or 800 cfm carb on a stock small block and brag about their big carb and then wonder why the car turned into a slug.

You may have to tinker with your adjustments a bit too because even the same carb may be slightly different due to manufacturing tolerances. So, use the 1-1/2 turns out as your baseline and work from there.

You'll be sure to find the answers you need here, lots of guys with good knowledge and experience.

If the engine is using the old restricted exhaust and stock air box then you are running too rich. Reduce the main jet to a 110 or 114. What engine are you working with. If a GY6 and an older model the exhaust tail stub will be somewhere around 1/2 " ID, the newer buggies are around 1" ID. Alsothe stock airbox is way restricted. In other words if running a less restricted exhaust with min.1" tail id and a UNI Filter a 125 jet is fine but in the stock setup it will be way too rich and this will be noticed at higher throttle. Leave your exhisting plug in the engine then at full throttle run for about 1200 ft and while foot is on throttle cut the engine, when the engine shuts down remove foot off full throttle then immediatly pull the plug and read the color. What size jet was in the carb you replaced?



TOM