Underseat fuel tank

fjman · September 29, 2019, 06:10:58 AM

I have a 84 fj1100. I was thinking of fabricating an underseat fuel tank. Think of it as a long term fix because our fuel tanks are not getting any younger. Sure I can use a fuel tank liner but I see this more as a "permanent fix". relocating the wiring and other stuff wouldn't be much of a problem. Now the question is what kind of fuel pump should I use? What should I install to stop the continuous fuel pump? Any help would be much appreciated as I don't have in depth knowledge as to converting our fj1100.

Pat Conlon · September 29, 2019, 10:11:15 AM

Change your float needle seats: http://rpmracingca.com/proddetail.asp?prod=3CF-14107-15
Get a Yamaha fuel pump: http://rpmracingca.com/proddetail.asp?prod=YamFPump
Get a fuel pump controller: https://www.ebay.com/itm/12v-Electric-Fuel-Pump-Relay-On-Off-Controller-Safety-Switch-3-Sec-Key-On-Primer-/332246190246

Have you done a volume calc? 231 cubic inches per gallon so if you want 5 gallons you will need 1155 cubic inches of tank volume.

I'm not sure you have that kind of space down there.

I would love to see what you come up with.

Motofun · September 29, 2019, 10:21:34 AM

If worried about the fuel tank integrity I recommend Caswell tank liner. That stuff is by far the best stuff on the market. I dumped the excess out a CB400F tank I lined onto my gravel driveway....

ZOA NOM · September 29, 2019, 11:32:29 AM

The Midget (Mike Ramos) has significant experience with an auxiliary tank. I believe it was a soft temporary tank, however, but I bet he can tell you some things.

fjman · September 29, 2019, 10:53:08 PM

Quote from: Pat Conlon on September 29, 2019, 10:11:15 AM
Change your float needle seats: http://rpmracingca.com/proddetail.asp?prod=3CF-14107-15
Get a Yamaha fuel pump: http://rpmracingca.com/proddetail.asp?prod=YamFPump
Get a fuel pump controller: https://www.ebay.com/itm/12v-Electric-Fuel-Pump-Relay-On-Off-Controller-Safety-Switch-3-Sec-Key-On-Primer-/332246190246

Have you done a volume calc? 231 cubic inches per gallon so if you want 5 gallons you will need 1155 cubic inches of tank volume.

I'm not sure you have that kind of space down there.

I would love to see what you come up with.

Thank you for the reply. I was thinking of fabricating something similar to the picture I do have the space as I am currently not using the stock airbox
I'll also have to fabricate abattery box after the carbs. I will do the measurements to the rear subframe. Hopefully something around the 15L range.

Pat Conlon · September 30, 2019, 01:08:40 AM

Can a lithium battery work on it's side?

If so, put the battery on its side in your rear cowl...all that room you will now have

T Legg · September 30, 2019, 01:24:26 AM

Maybe you could cut the bottom of the old tank out mount your new tank where the original tank was and use the top of the old tank as a cover for it.

aviationfred · September 30, 2019, 03:34:34 AM

Quote from: Pat Conlon on September 30, 2019, 01:08:40 AM
Can a lithium battery work on it's side?

If so, put the battery on its side in your rear cowl...all that room you will now have

I have the Lithium Batteries that Are sold through RPM. Their add does say that the battery can be used in any position.

https://www.firepowerparts.com/product/featherweight-lithium-battery/50983

Fred

ribbert · September 30, 2019, 08:00:23 AM

Quote from: T Legg on September 30, 2019, 01:24:26 AM
Maybe you could cut the bottom of the old tank out mount your new tank where the original tank was and use the top of the old tank as a cover for it.

I have had the same idea but take it one step further, a solution for everyone as the need for tank replacements is only going to increase over time.

Get a plastic tank made to the FJ shape (we need someone in the business) and then, as you say, cut the bottom out of the original and use the top as a decorative cover / shell.

Noel

giantkiller · September 30, 2019, 08:42:38 AM

Quote from: T Legg on September 30, 2019, 01:24:26 AM
Maybe you could cut the bottom of the old tank out mount your new tank where the original tank was and use the top of the old tank as a cover for it.

That's what I was going to do. Maybe make a carbon fiber cover from the old tank. One of the many projects that got put on hold. When I started building my shop/house

fjman · September 30, 2019, 09:32:55 PM

anyone here who has an experience using mechanical or vacuum fuel pumps? I think it simplifies the process .

FJmonkey · September 30, 2019, 10:05:52 PM

I wonder if a plastic tank will survive siting atop of an air cooled engine at the top end of temps sitting in traffic. Not all plastic like products are thermo-formed so I guess it can be done.

Tuned forks · September 30, 2019, 10:13:38 PM

Mark, wouldn't it have to be a thermo-set plastic?
I like the carbon fiber idea but instead of a fake tank inside the original shell, make a carbon fiber duplicate of the original tank. Now that would be cool.

Joe

T Legg · September 30, 2019, 10:34:35 PM

My XL 250 has a plastic tank as do many dirt bikes with giant desert tanks.they may not get as hot as an fj though but having a tank full of gas would keep the plastic cool.you could have a metal pan support the plastic tank so no plastic mounting tabs would deform from the heat.

FJmonkey · September 30, 2019, 11:14:04 PM

Joe, yes, it would not need to be a thermo-formed plastic. But many fuel tanks are rotation formed (Roto-mold) and are a thermo-formed plastic. Other methods will just cost more due to the added labor and materials. Not saying it can't be done, just that the price might be above what one is willing to pay. The concept is perfect for replacing the rusting steel tanks on our aging loved ones.

Travis, the fuel thank with a volume of fuel would have a cooling effect till it is no longer filled. And you might have to agree that some FJ owners love to ride them till the last (maybe only) gas stop on the route.

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Underseat fuel tank