Need 421 setup help.

[b-nasty]

I decided not to part out my bike because I realized why I started to build it when I went out to a jamboree last week. Nothing like banshee power!!

 

I have a 421 cub drag ported with blaster pistons, cases are ported, RDZ head with 20cc domes. 39mm mikuni carbs setup for alky with powerjets.

 

What should my compression be at? Squish? Timming?. Help on carb setup.

 

Just wanting some good advice on what everything should be out so nothing gets damaged since it will be the first time the motor has been started. Let me know if I need anything or you guys need some more info. Thanks

[badassbanshee479]

who ported the engine? different builders like to set there engines up different.... there is not like a conman setup that everyone uses......

[RadarRacing]

Just double check everything when you put it together. Check squish and make sure its in the .043 range give or take a few thous. Also have someone who knows how to tune alky carbs help you if its the first time for ya. might call Rob at RDZ and buy the base gasket from him and tell him you bought one of his setups so you'd like the gasket thickness he recommends.

[J-Madd]

I decided not to part out my bike because I realized why I started to build it when I went out to a jamboree last week. Nothing like banshee power!!

 

I have a 421 cub drag ported with blaster pistons, cases are ported, RDZ head with 20cc domes. 39mm mikuni carbs setup for alky with powerjets.

 

What should my compression be at? Squish? Timming?. Help on carb setup.

 

Just wanting some good advice on what everything should be out so nothing gets damaged since it will be the first time the motor has been started. Let me know if I need anything or you guys need some more info. Thanks

 

 

I can tell you how one of my 421 Cubs was set up. 0.045" squish, 185 psi (20 cc domes), dumps drilled to 0.125", 60 pilots, egn needles. I also ran a different set of carbs that were drilled to 0.120" with some different needles (don't remember what they are.) Its funny that one my stock chassis, the 0.125" dumps worked fine, but when I went to a 50 lb lighter chassis - I had to turn the pjs all the way in and still no heat on the plugs.

 

Another 421: 0.052" squish, 170 psi; 38 mm a/s PWKs, 0.118" dumps/mains.

Another 421: 0.050" squish, 180 psi, 39 PWKs listed above.

 

All 3 of these motors run about the same, even set up that different. I'm convinced that you can setup a 421 any way you want, and it'll perform pretty good. All of these run 4.0s with an average rider and 3.8s to 3.9s with a jockey. It's in the fine tuning/perfect setup that you get it to really fly (e.g. 3.7s).

[b-nasty]

thanks for the advice guys. But what do you mean by drilling the dumps? also what plugs do you guys run. thanks

[RadarRacing]

He means drill the dumps if the carbs aren't setup for alky yet, sounds like they are. But not all alky carb setups are right for every motor, make sure the needle flows enough to keep it from being lean low-mid. Run either a normal banshee plug or most a 9 heat plug B9ES or the irridium equivalent of that plug. Also gap your plug tight like .018 or even .016.

[J-Madd]

To clarify - the dump tubes are the tubes that the needle rests in. You have to drill those and the main jet. If yours are drilled, findout the largest index drill bit that will fit in the tube and main, then measure it with calipers to see what size it is. Note that sometimes the main is drilled smaller than the tube. That is ok; but if the tube is smaller than the main, you'll only flow as much as the tube allows.