1984 250r Performance head, jetting etc for 300ft drag

[bebeav]

hi guys,

i just finished rebuilding my engine and everythins is done now and ready for the drag this week end.

engine modif : Full DG exhaust, Uni filter, 34mm flatside carb (87 trx250r) with 155 main jet and that's all for now.

i rode the thing approx. 3 hours. ok it seems to run on the rich side since it's bogging a little bit on high rpm and the plug is black. so maybe a 145 main jet should be ok.

but my problem is can i run a high compression head (approx 200,210psi) on a freshly done engine with only 3hours on it and if the 155 main jet should be ok or will i need a 165 or higher.

oh and i use honda 10w40 for tranny and klotz super techni plate 32:1 for gaz mixture.

ALP
Beav

[TeamGeek6]

Yep, go back to stock jetting (145-ish) and tweak it till that flat spot goes away. I found 140 - anything to be too rich, ended up with a clean plug in the low 130-s. I found some upper cylinder noise making that much of a drop at 40:1 premix, increased it to 30:1 and most of the noise went away.

The bigger question on a high compresion head is whether the ignition system can handle it.
Spark voltage is proportional to cylinder pressure and has to go up as cylinder pressure increases. Jetting might change a little, but thats easy to tell. Ignition isnt so easy to sort out.

Those Ive spoken with that have gone high comp (Ive not done it yet) have gotten much better results with aftermarket ignition systems.

[Mr. Sandman]

Yep, go back to stock jetting (145-ish) and tweak it till that flat spot goes away. I found 140 - anything to be too rich, ended up with a clean plug in the low 130-s. I found some upper cylinder noise making that much of a drop at 40:1 premix, increased it to 30:1 and most of the noise went away.

The bigger question on a high compresion head is whether the ignition system can handle it.
Spark voltage is proportional to cylinder pressure and has to go up as cylinder pressure increases. Jetting might change a little, but thats easy to tell. Ignition isnt so easy to sort out.

Those Ive spoken with that have gone high comp (Ive not done it yet) have gotten much better results with aftermarket ignition systems.

Jetting via the internet, BAD ADVICE there TeamGeek! Jetting by the seat of your pants is not the way to do things, and going all the way back to stock could cost him a new top end. Now, maybe you know Bebeav and where he rides, the elevation and the temperature, all of which affect jetting and can advise on those parameters, but my guess is that you don't and you are going off what has worked well for you where or when you ride.

To start Bebeav, jetting is not considered a modification, it's a procedure that you do to tune your bike so it runs smoothly. But it does sound like your running rich with the 155, so go down one or two jet sizes at a time until it smoothes out, doing plug chops to along the way to confirm everything.

[bebeav]

ok dudes, i finally decide to not use the high compression head so i can be sure it will be tuned right for saturday drag event.

i start it the first time with the 155 main jet and it was runing badly rich so i took it down to a 138 main jet and now what a blast, my R is finally running how i always wanted, it's pulling great on all gear.

i'm running a 14 tooths front and 42 rear and was wondering if a 13 tooths would give me the best of acceleration when starting in second?? or will it hurt my top speed on the 300ft drag??

ALP
Beav

[bebeav]

i went to the 300ft drag saturday and only got a chance to ride 2 times loll. i finally decided to put my high comp. head to be competitive but that wasn't enough. first run was againt an ATC 500r highly moded approx. 87whp. and the second run was versus a piped, filter, carbed Shee but i was really near him maybe i could finish flush with him if i had a good start. but btw that was a great race day. only 4 trike btw, me with my R, my bro with his 350x (400ex engine), the insane 500r and the fatest machine of the day (no snowmobile engine was enough powerfull to beat him) a highly modified Tri-z (Tri-Z machined bottom end and crank to accept a rotax 800 rod and cylinder). that thing was incredibly fast.

Going to some drag in 3 weeks and i need some good reed valve, anyone got ones? i someone can host me i'll post the vid of me VS the modified shee and a vids of the 500R vs the Tri-Z.

ALP
Beav

[Troll 2]

someone can host me i'll post the vid of me VS the modified shee and a vids of the 500R vs the Tri-Z.

ALP
Beav

Sounds loike a great time! How big is your video? I know you can host it here. I did http://www.3wheelerworldforums.com/attachment.php?attachmentid=16176 and it's 4.42 MB. Sounds like I should make a donation $$ :beer

[bebeav]

thx i'll try this tonight.

ALP
Beav