Epoxy for shorter intake duration?

[MCMAN56]

I have an air cooled Fantic 80 trials bike that had an MX type powerband. Based on Eric's book, I machined the bottom of the cylinder to retard timing and reduce port durations. However, there is no reed valve so this mod actually increased intake duration. Can I use JB Weld on the bottom of the intake port to reduce duration? Can the JB weld be flush with the bore or is there too much heat?

Just the machining helped tremendously but I want a little more at the very bottom. At low rpms the power can sort of just shut off. I'm also planning to try a longer head pipe on the exhaust.

 

[georgieboy]

Imo there is not must heat at that point, JB weld shld work, but my experience is limited.
JB weld is nice to work with although i find it rather difficult to get it settled where i want it, since it is very thin in viscos at first. I did some JB weld myself at the ports but i haven't assembled engine yet. I wld say to lay it back just a fraction that it has some room to expand into the cilinder so it won't hit rings.
It is rather quite at the mods forum lately, won't you say?
Anyway if you done the mod pls refer back, i am interrested.

 

[motometal]

I have an air cooled Fantic 80 trials bike that had an MX type powerband. Based on Eric's book, I machined the bottom of the cylinder to retard timing and reduce port durations. However, there is no reed valve so this mod actually increased intake duration. Can I use JB Weld on the bottom of the intake port to reduce duration? Can the JB weld be flush with the bore or is there too much heat?

Just the machining helped tremendously but I want a little more at the very bottom. At low rpms the power can sort of just shut off. I'm also planning to try a longer head pipe on the exhaust.

please explain more about intake duration vs. reeds vs. no reeds-i'm not following.

 

[MCMAN56]

Reed valves prrevent back flow into the intake system so, in a way, control functional intake duration. (or maybe I should say have an effect on "real" intake duration.) Non reed valve motors have no such control and can have issues with blow back through the carb.

 

[sweden_345]

Would this mean that if I cut the intake port to much it would't be devastating for the performance because no blow back is possible?

Also, when I grind the ports do I want the grain to be linear along the ports or round inside the walls? Or do I simply polish the inside? I'am not certain on either as if the surface was polished the gases would attach to much to the material?

Sry for stupid question just trying to learn :whoa:

 

[adam728]

Would this mean that if I cut the intake port to much it would't be devastating for the performance because no blow back is possible?

Also, when I grind the ports do I want the grain to be linear along the ports or round inside the walls? Or do I simply polish the inside? I'am not certain on either as if the surface was polished the gases would attach to much to the material?

Sry for stupid question just trying to learn :whoa:

Randomly cutting any port, intake, exhaust, or transfer, can have horrible results if you don't know what you are doing.

Cleaning up the ports and getting rid of casting irregularties is a different story. You can do that without changing port timing. Intake port should usually have a somewhat rough finish. Exhaust should be polished to keep carbon from sticking.

Stockholm kicks ass :laugh:
I got to visit there a few months back (spent about a month in Jonkoping / Husqvarna).


To the original poster -
I would not use JB weld on ports. It doesn't handle heat well, and there are chances it could flake off. I can't for the life of me remember the name of the epoxy that guys like Eric Gorr use.

 

[jaguar]

Eric recommended gas tank repair kit putty in his Performance Handbook

 

[mhardee]

Can I use JB Weld on the bottom of the intake port to reduce duration? Can the JB weld be flush with the bore or is there too much heat?

I'd use Devcon.. Have used it successfully in cylinder head intake runner mods/fixes for years..

A little hard to find, pricey but it stays...

 

[Pete Payne]

Devcon does a good job !

 

[hot125mod]

doesnt a longer header on an expansion chamber gain top end. i am confused because when people add a torque spacer it seems to me that it would pull better on the bottom.

 

[cujet]

A longer header (head pipe) on a chamber moves the torque lower in the RPM range.

A change of 1 inch is easy to notice.

Chris

 

[Matt Fisher]

doesnt a longer header on an expansion chamber gain top end. i am confused because when people add a torque spacer it seems to me that it would pull better on the bottom.

General rule:
Longer header before the expansion chamber increases low-end.
Longer stinger after the expansion chamber increases top-end.

 

[hot125mod]

thank you that seems to make the most sense.