Intake Riser

Posts: 138

Keep in mind that my engine is not stock. I have a 62mm bore, port, polished, and milled large valve head with a stroker crank. You can read the rest on profile attachment on each post.

I have already increased the diameter of the fuel lines and changed out the paper fuel filter to a screen too.

I started with the stock MRP 32mm jet. It was starved for gas. I worked my way up incrementally with respective plug reads. 132, 136, 137.5, 140 & 145. It was not until I reached the 145 jet that it ran strong with still a slight 'starved for gas' feel on full acceleration.

Attached is my most recent plug read with the 145 jet. I realize a tan (grocery bag brown) color is optimal. However, considering this is an air cooled engine with a hot plug (iridium)and I am in San Antonio, TX (very ot weather) I don't mind it being slightly rich.

Posts: 4208

looks lean to me,

Posts: 138

Interesting. I thought the same thing. But, I sent the same picture to Tom (at SYCPOWERSPORTS) the fella who rebuilt my engine. He said it appeared fine if not slightly rich. Personally, I was thinking of trying a 150 jet after I read the plug. What do you think?

Are you reading the top rim of the threads?

If you think I should try a 150 jet, do you sell one? I have only seen 147.5 available.

Posts: 4208

I have always gone by the white part and the electrode. if there is some material on the threads in the head it will be transfered over to the threads on the plug. try the 147.5. also what size pilot jet is in that carb?

Posts: 138

I don't have an answer for you on the pilot jet. Is the pilot jet the one next to the fuel jet in the bowl. Or, is it the screw on the outside that adjsuts the air/fuel mixture.

Secondly, do you think it is worth the hassle of going from a 145 to a 147.5 given there is such a small increase and it appears I'm running pretty lean? In other words why not bump right up to a 150?

Lastly, do you still have my credit card and mailing address info? If so, will you just bill my credit card and mail me a 147.5 and a 150 jet?

Posts: 4208

the pilot jet is the one next to the main jet and is sunken in a hole. it controls just off idle to about 3/8 to 1/2 throttle.

I will just send you a jet, cost more to charge you for than its worth

Posts: 648

The problems you guys are having with these carbs may not be in the carb its self, but rather in the intake length while using those intake risers.
I was taught many moons ago by a retired tuner for Ray Price [held the Harley Davidson pro modified record for years] and was confirmed by a tuner at Yoshimiro Pops [super bike engine builders] that the calculation for a proper intake tract length was to divide 90 by the desired rpm operating range. I asked where the number 90 comes from and was told it is an average between valve overlap and cam duration. The formula is a starting point and any fine tuning of length was done on a dyno. So... Using this theory I decided to try 6,000 rpm as a start, 90/6 = gives me right at 15 inches. A stock gy6 150 head measures approx. 2.25 inch. at the top of the port and 1.50 at the bottom. This measurement is taken from the top of the valve head to the intake mating surface, Average length of 1.75. The average length of my aluminum intake is 1.90. The length of my stock body 24 MM carb is 4.32. Add another.25 for gasket and rubber space of the carb mounting flange for a total of 8.22. I have an inlet pipe [that’s counted as part of the intake tract also] of 6 inches. That gives a total of 14.22 inches. So according to this formula, I am not too far off. The higher the rpm range the shorter the intake tract needs to be. Your problems may be with velocity and the air pulses of that long intake tract. This is just something to consider and fool with while trying to cure the problems

Posts: 890

Posts: 138

Eric,

I appreciate the jet. Thank you.

I don't understand your comment regarding the pilot jet: "it controls just off idle to about 3/8 to 1/2 throttle".

How do I use this information to determine the pilot jet is adjusted appropriately?

Steel,

Thanks for the chart!

CKAU,

Whoa! That is some good info. Without a dyno it allows you to get a reasonable approximation of the intake tract. Thanks!

Posts: 4208

there is no adjustment to the pilot jet, you just go up or down as needed. I will put a #42 in the bag also