What are biggest tyres that will fit 1978 XS400, and how to get faster??

[butterbeats]

Hi xs400ers,

What are biggest tyres that will fit onto 1978 XS400? Without any mods.

and how to get faster?? Currently flat out getting it up over 45mph. Tuned and running smooth but lacks power? What suggestions do you guys have? Without a rebore.

I have cleaned the carbs. Cleaned fuel tank. New spark plugs. New air filters. New pipes. Taken off rear pegs.

what next??

thanks

 

[Wolfe_11B]

Well bigger tires will make you slower, so I wouldn't go that route.

If you are only getting 45ish, it's not tuned.

Checked valves? Checked compression? Checked plugs? Points right?

If all that is good, then the carbs are not right.

 

[kshansen]

Somethings for sure not right if 45 MPH is maxed out! That is a 1978 in my picture to the left and it has a sidecar as you can see and the little bit I have run it on the highway, at least on the level it has no real problem hitting 60MPH. Just doing lap in the back yard I have done 30 MPH and the limit there is stopping on the grass before hitting house or trees!

If problems around the house slow up for a day or so might get time to put a few short road runs on it and maybe get it in for it's first annual inspection in over five years.

As for tire sizes what I put on my 1978 XS400 were Shinko 712 100/90-18 front and 110/90-18 rear. Don't think in that style/brand anything bigger would fit. I chose those as I had a 1978 given to me that had those sizes in Dunlops on it. The Shikos are actually a little bit of a tighter fit than the Dunlops.

 

[lemonhead]

I put Bridgestone s11 spitfires on my '79. Same size as kshansen. Front tire is a tight fit under the fender, but it has sufficient clearance and doesn't rub once it's in place.

As for the speed, I concur that something needs tuning if you are only hitting 45. After almost 30 years of being stuck in a barn, when I freshened up my bike, I only visually set the butterflies in my carbs after boiling them (didn't manometer balance them), new carb boots and vacuum and fuel lines and my bike pulls to 60 like crazy and creeps to about 85 flat out...

 

[butterbeats]

had the compression checked and its around 140. I had the carbs cleaned and one float had a broken float holder.... ie the float pin fits in the top has cracked off... the float still sits in place but this may effect the float itself.

i have not replaced any fuel lines etc.

 

[butterbeats]

previously points had a small fuel leak, leaking on them from the gas tank, leak has since been repaired....

 

[Wolfe_11B]

140 psi would explain it. Have you pulled off the breather tube from the case? I bet you'll see lots smoke.
Find out if its rings/bore or valves/seats causing your compression issue. I know the book says 150 is ok at the lowest with a bit of leeway, but 140 sure would feel sluggish compared to 175.

 

[butterbeats]

My mechanic said 140 was good for its age.....???????

 

[butterbeats]

My mechanic said 140 was good for its age.....??????? So I need to do the rings?? how do I tell??

thanks Wolfe aways a good help!!

 

[butterbeats]

My bike with the new exhaust front pipes... back pipes turns out were off a harley and I picked them up for $15 at a swap meet. When I picked them up they were unknown quantities. The pipes are a little quiet... but I had hoped to pick up some power once I had replaced the mismatched honda pipes I had on the bike when I got it... the headers did not match the engine and had air gaps.

 

[Liamthedevastator]

My mechanic said 140 was good for its age.....???????

Probably means that it's impressive for it not to have lost much more than that, but I trust the forum member's here more than any motorcycle mechanic I've met (the last one I met told my buddy there was no way to diagnose the charging system on a '81 CB750 and he could do the work for $400 parts included). Besides, if the manual says 150psi is the bottom of the acceptable range then I'd start trying to figure out what's causing that compression drop

 

[Wolfe_11B]

Pull the plugs and see if they are oil coated or full of shiny carbon. While they are off, pour a tiny bit of oil or trans fluid in there, attach compression gauge and give it a few kicks. If the compression goes up to 160/170 you have a ring or piston clearance problem. If it stays low that means you have a valve problem, and either the valves are out of adjustment or the seats need to be recut and lapped.

 

[butterbeats]

the plugs were fouling recently from a small fuel tank leak dropping fuel onto the points and throwing one of the plugs out. But they were always quickly getting covered in dark black soot. I will try the test suggested above first chance I get.

 

[xschris]

These bikes will run down to 120 with a cold dry compression test. A lot of guys on here have done it. 150 is very good. I would look at the carbs and that broken float post. Any amount of extra movement with the float will make for poor running.

 

[butterbeats]

Thanks for another perspective. I am keen to try all simpler remedies before a rebore.

I will start with the carb because I know it is broken/not 100% for sure. It will be semi hard/expensive to find a replacement carb here in Australia. What is the best way to repair the post? What is the easist way? Can I simply fashion a top half to the hole the float fits into. Weld a top half u shape onto the existing post. Allowing the float to sit evenly in place.

thanks.

 

[lemonhead]

Jbweld makes a petrol friendly epoxy that is sandable and drillable...

Got a pic of the broken part?