A slightly slow DOHC

[willem]

As I'm working on my first bike ever and have limited experience, I have to ask.

My '89 DOHC (6 speed) is currently running quite well, starts and idles nicely since I cleaned the carbs properly and put in a new battery and spark plugs. It's mostly stock, no pods or different exhausts. Air filter is clean.

My problem now is that my top speed is about 110 km/h (which is 68 mph) which I read is a bit on the slow side. I should be able to get to about 88 mph. So my thought is, there has to be something that is not well adjusted.

What is the most likely culprit? I've read about our bikes running slightly lean in stock settings, is this true? And how would I enrichen it? I haven't gotten around to checking my valve clearance yet, would that help much?

Also, I've experienced some instability at certain speeds. It kind of sways left to right when going around 20 mph. This has improved since I've change my front fork oil, but it's still noticable. At higher speeds it's not really there, so I'm not too bothered. I've checked my steer bearing (what do you call this, the bearing between the front fork and the frame) and it's tight, there's no wiggle room. Also my tires have been replaced, and this problem stayed. Where would I start looking for a cause?

Hope you can help!

 

[16VGTIDave]

Check the valves before trying to re-jet the carbs.

Have you checked the swingarm pivot to see if it has excess movement? The needle roller bearings can fall apart and cause strange handling. Also check the runout on both wheels and the brake disk.

 

[Lou Ranger]

Is your bike a 400 or a 250?
We never got the 250 over here, but I understand that many were sold in Europe up until the late 1980s and they look identical.
Another crazy notion: is it possible that your bike was originally a restricted bike for learners? At least one of our members from Europe told us that learners could only ride larger displacement bikes if they were equipped with limiters - I think from the factory.

http://www.xs400.com/forum/showthread.php?t=8193&highlight=restricted
​

When the DOHC 400s first came out over here - reviewers talked about them being legitimate 100 mph bikes (161). Certainly they should do 150 kph.

 

[bentwrench]

What happens at high speeds? Are you redlining in 5th?

 

[ArtursF]

Well I'm having the same problem with top speed. Mind you I have restricted version (12E with 27Hp). One day it can reach 140kph the other one it barely can touch 100kph.

Here is a nice article about restriction variants on DOHC bikes
http://pilling-sonneberg.de/xs400/45ps.html

I'm planning to do full de-restriction this year
(Will post progress report on forum)

 

[willem]

Ah sorry I knew those were out there, but I have the non-restricted 34 kW/48 HP version. Also it is the 400cc model.

At those speeds I don't actually know if I'm redlining in 5th, my tachometer has been removed by a previous owner to get rid of the bulky dash. But I don't think it is redlining, I am able to get it up to higher revs at lower speeds.

 

[Scorpio1963]

DOHC 398cc should be getting to at least 95mph.I agree with the swing arm bushing seems like you have some play.Have you tryed greasing them.Also check and make sure either the front or rear brakes arent dragging.

 

[willem]

I'll try inspecting the swing arm next then. My brakes aren't dragging, so that should be fine. Any other thoughts on my top speed? How much would checking the valves and syncing the carbs help, could that just be it?

Or is it possible that my cylinders need to be honed out or something?

 

[16VGTIDave]

So the easy things first (valve adjustment and carb sync) before you start thinking about the hard things (engine rebuild). If you can't hear the valves clattering at idle you really should check them now!

 

[Scorpio1963]

Have you changed plugs or wires and check the coils for how much voltage?

 

[Unkle Crusty]

My 83 DOHC 400 was running crappy but still managed 92mph according to the speedo. My goal is to hit 100. The swing arm bearings and the bushing shaft on mine are screwed. That made for some interesting handling.
It is the steering head bearings up front. They can get pitted and cause funny handling as well. Likewise with the wheel bearings. Also make sure the back wheel is following the front wheel, as in straight behind.

Unkle Crusty

 

[16VGTIDave]

Hrmm. Thread necro.

Crusty, check your speed with GPS. The stock speedo's are less than accurate. With 92 indicated, you were probably doing 75, maybe 80. Good luck with your goal, hope you have deep pockets. Our little bikes cost a lot to make the power to go that fast.

Be sure to get your bike in good shape before you try another speed run. If you get into a speed wobble, you won't have the power to get out...

 

[Unkle Crusty]

Will do Dave.
At the moment I have a 5 speed DOHC motor in it. When I get time I may work on my 6 speed motor. Front and rear wheel bearings are new, so will the swing arm after I get a bushing shaft. And I will check the steering head bearings. Tyres seem okay.
Machine shop could not fix the old bushing shaft, so I will likely buy the new one in England. Could also look for used, but might not find a decent one.

Unkle Crusty