Nevertheless, Lynn,s comments safed my day.

 

I was trying to tune the single rotor over the past few weeks. Using a 38mm Mikuni round slide, I got it to idle clean up from 1600rpm. Transition into midrange and up to WOT is no problem. Trying to get the mixture richer at WOT, I did not get the response to enlarging the main jet I expected. The mixture stayed lean, indicated by high EGT and white plug insulators, to some extend supported by the O2 reading. The air flow rate at 6000 rpm is almost double of what Mikuni says is max. I started wondering.

Now Lynn said " carbs get leaner to the top of their RPM ranges" . There may be the answer to my problem: bigger carb!

 

Richard Sohn
N2071U

Years ago, we took our new race car (RX-7) to Sebring Florida to race it for the second time ever. It ran hot, and had poor power up high, then it would miss after a few laps. By the end of the race Sunday we had added an additional oil cooler, bigger main jets, small air correctors, bumped up the fuel pressure, retarded the timing a bit and dumped in aviation gas. The driver dropped out a lap before the end, saying the car was too hot inside to drive, and it was getting real loud.  The muffler had melted inside and was blowing up like a big steel balloon.

 

The next week end at West Palm Beach with no improvement, a local fellow stopped by and wanted to know if we wanted to use his stand pipe because it sounds to lean. So he gets his stand pipe, and its a standard piece of Weber tuning equipment. It screws a banjo bolt into the side of the float bowl and replicates the fuel level in the bowl outside where you can see it. He revved my engine to 9,000 RPM in a long blip and the engine shut off then dropped back to idle. He did it again and after returning to idle it stopped. The bowl couldn't fill fast enough to support an idle. By the time it got to 9,000 RPM, the bowl was empty.

 

The biggest needle seat is too small, so you use a Gross jet. Two balls instead of a needle. Then you find the hole into the needle and seat well is limiting the flow, so you drill that out. Then the bug screen in line with the needle flows only half of what you need, so you throw that out. Then it runs OK but still goes lean at the top. So, you raise the float level until the boosters drip, then lower it just a bit. Then increase the fuel pressure until the needle and seat cannot control it. Then lower it just a bit.

Then you drill into the void that would have been the accelerator pump well for the other handed version of the same carb, to increase bowl volume.

Then for short tracks all is well, and for long straights you can rig a backup pump to come on when 5th is selected. 

 

Then you can use a Weber carb at twice its rated flow.

 

Piece of cake.

 

Lynn E. Hanover