Bigger Carbs For 400 Conversion?

[fastharry]

Do i really need bigger carbs to get the best out of the 400 conversion i am doing at the moment, Baring in mind i will be racing on fairly short 300m speedway tracks in 3rd gear, regards, Harry.

[Bison]

I would like to hear the answer to this question too. I have stock carbs and jetting in my 402 conversion, , and it runs fine, but is there more performance lurking?
Alan.

[SupermotoDave]

Please----- turn the side panels round, I find them quite upsetting like that.

Cheers
David

[SupermotoDave]

Forgot to mention, bigger carbs are a moot point. Morini engines work with quite small ports and carbs to ensure that the flow velocities are relativly high. As you are presumably riding on the throttle and want instant snap then it may be that you will not benefit from larger carbs as much as someone riding WFO around Cadwell. the options if you stick to morini type carbs are 26mm, as used on the 500 or 28s pinched from a guzzi monza. Most people find benefit from using the standard airbox rather than pod filters as the system is quite optimised, but again that is for road riding and not velocitas sinistra (go fast turn left).

Where do you race? do you want some supporters? can you write a story for ATG?

Cheers

David

[smotorboy]

I'd suggest you put the bike on a "Rolling Road" and get an accurate HP-Torque curve so you can gear the bike perfectly to have maximum torque driving off the corners and maximum HP going down the short straights....

Larger carbs may or may not help,,,, without a proper test procedure you will never know for sure just what is what...This means numbers before and after a major change....

These motors are very sensitive to the camshaft being used and the best one for your application dialed in (advanced or retarded a few degrees) will do more than larger carbs I would bet.....

I would think that my supercharged bike would be a Holy Terror on your track with the Torque it is making now ( 39 lb-ft vs 19.5 lb-ft stock)..... That is until the pistons seized from lack of cooling..... I can barely keep the temps in the green above 100 mph,, much less at speedway speeds....

I use to help setup short track race cars, and the cars that pulled hard off the corners always seemed to do better than the ones that made best power later on the straight chute...

Good luck with your adventures,,,,

Robert in California

[MickeyMoto]

In an article on the Dutch site there is a table with carb settings. The German AMEX 400 used PHBH26 carbs.

[pedro]

Just a word of warning here, I recently spent some time and money getting my 501 with pod filters set up on a dyno, when I took it on the road, settings that had been perfect on the dyno were hopelessly weak, due I suppose to the airflow over the pods being very different to the fan on the dyno. Others may have more experience, but that was my findings. Another good reason to keep the standard airbox if you can.

[fastharry]

Thanks for the input guys, I have been told several times by wise old flattrackers that gearing is one of the most important things to get right, acceleration out of the corners is definitely more of a priority than top speed, so i will look for some 26mm's to start with. Im fast running out of time as i have a big meeting coming up at Kings Lynn Speedway on the 15th/16th, There are so many different opinions on filters and airbox's, i will just have to take it down the field and give it a try out for this week, It will be interesting to see how the 400 go's compared to the 80 psi 350 i rode 4 weeks ago, + re-cut seats and a couple of valve guides, hopefully a lot more punch, it seemed to run out of breath half way down the straight last time out, one point i could do with a bit of input on is ignition output, The Danish guy Seron who has several video's on youtube thinks that 180-200 ohms is enough for the output coil, mine starts well but Al Webber who runs a very nice 500 in the states seems to think its not enough and could cause a similar symptons as mine running out of steam, Supporters are always welcome to our races, Its some of the best action you will see on 2 wheels, and its free at most of our meetings,See http://www.dirttrackriders.co.uk for info. The picture is of Guy Martin just pipping me to the flag in the Harley Davidson race at last seasons Dirt Quake, an amazing meeting if you want to see just about any kind of motorcycle ripping round a speedway track, from R1's to a Honda step through.

[Daddy Dom]

an amazing meeting if you want to see just about any kind of motorcycle ripping round a speedway track, from R1's to a Honda step through.

Looks like great fun, I'd love to see a Honda step-through ripping round a speedway track!

[fastharry]

The guy on the R1 is the one to watch, He comes up from the south of france for the event, He is an ex stunt rider and watching him come out of the corners sideways and then wheelie over the line is pretty good!

[mad muller]

nice looking bike fastharry, i cant see bigger carbs making performance any better unless you go with bigger valves , ive got pod filters on my sport and it performs fine in fact i thought it ran worse with the original air box it was very rich and the engine wasnt as crisp off the throttle others will have different views,anyway great bike and all the best. muller.

[fastharry]

I think your right Muller. But i was hoping for more input i have to say, pretty disappointed with the armchair tuners! Went for a run on it yesterday with the 400 pistons in, feels ok, obviously still a bit tight, need to get some miles on it tomorrow, got a big race at the weekend, still feels like its holding back a bit so i might try another ignition tomorrow. Picture of the vw piston.

[SupermotoDave]

Are you racing this Saturday?

David

[fastharry]

Yes, at rye house london. It will be an unusual timetable for this one, Heat races for all except pro's start around 12 i think. then a short break. Then at 6pm the finals start with pro's heat races inbetween them, Then Pro final at the end. Have you ever played with the gearing, im still running standard 14/38 and im thinking of going somewhere in the 40's but not sure where to start.