Top speed problems

Matt400

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Hey folks, I'm brand new to the forum as well as motorcycles and was hoping to get some help with my 400. I've done some research and can't seem to find a problem similar to mine.

Basically, everyone I've heard of is saying they're getting around 90 mph conservatively but I am only getting it up to around 50. I talked to the previous owner and he said he hasn't had this problem. It's got new fuel, petcock is working properly, spark plugs are good, oil is good. Is there a good starting point to finding the solution?

Thanks
 
Is this an SOHC? What year?
90 MPH is doable on one, but it would have to be a motor in 100% top brand spanking new condition, proper tires and gearing, downhill, and a massive tailwind. Not to mention you'd have to get your weight down bellow 150lbs. 80mph is more realistic with proper conditions, and that still is going to take a minute.

50 is low though. How's the compression? What are the carbs doing? Is it gurgling at the point it stops? Choking? Does it feel like it hits a wall? How about the valve lash?

Could be a number of things. If the motor feels ok, and it just feels like it can't go any faster at that point, like it lacks power, I would venture a guess at low compression or valves that aren't seating. Carbs are another usual suspect.
 
My bike an 81 SOHC will top out at about 95 indicated on the GPS, but I only weigh about 110lb so there isn't much load on the bike, and I have to basically be hugging the gas tank.
 
Hey folks. So my buddy and I checked the spark plugs, changed the oil, did a compression test, checked the tires psi, sprayed in some carb cleaner, cleaned out the airbox and intake, and then I took it out again.

It goes a bit faster now (consistently 90 km/h as opposed to 80 before). But now, for example, if I start at 60 km/h in top gear and pull the throttle all the way back, it will stop at 90km/h and about 6500 rpm. However if I keep the revs at 8k+ While getting up to speed I am able to get it to 120 sometimes. When this happens it is kind of jerky power where it will accelerate then stop then accelerate then stop until it gets up to 120.

Any thoughts on where the problem could be?
 
I have same problem....never reaching must part 90km. I have cleaned the fuel tank, nice new fuel lines helped 5%. Next is the carbs rebuild.
 
Apparently i cant make my own post on this page so ill just comment here for a similar question. I have an '82 XS400J Maxim, drove it a bit last year but im not really satisfied with how high the bike is revving on the highway. The only possible fix for this that i know of would be to change the chain and sprockets. I personally don't care about acceleration on this bike because I've got an '83 V65 Magna sitting in my garage if i want acceleration. This bike is to save me money both on fuel and insurance. Any thoughts on what i can to here?
 
Your only option is to change the CS sprocket for a 17T. This will drop the RPM by about 500. It's also likely to reduce economy, as the DOHC likes to rev. Above 7000 is it's happy place. Below 3000 is lugging the engine. I've had no issues running 7000+ for a couple hours straight on my bike, and the engine is still in decent shape at 100k km (62k mi).

Ultimately, fuel economy is up to you and the throttle. There is no saving money on insurance in Ontario, just varying levels of extortion. :mad:
 
Alright I guess I can change to a 17t front sprocket, would a smaller rear one help as well? Also would this bike be good to drive maybe 5 hours or so? Thanks for your reply by the way i have absolutely no idea how to post on here
 
There is no smaller rear sprocket that is a direct fitment. If you want to go custom, that is all on you. And as I said before, reducing the engine RPMs isn't the right thing. This isn't some lumbering 1200cc V-Twin that will chug down the road just above idle, it is a high strung 400cc twin that has to be revved. The hp peak is around 9000 RPM, and you will need to go there if you want pass at highway speeds. You really should come to terms with this fact before you go spending more money on the bike.

Only 5 hours? Sure, the bike will go much longer than you will be able to, if it is in good shape. I do 4hr runs (300km) regularly in the summer without issue. I've done 9hr days when camping. The bike is not the limiting factor.

Click the big red button at the top right of the section marked "Post New Thread".
 
I appreciate the help. What I'll do is change the front sprocket to a 17 tooth which I'm assuming I dont have to change my chain for that, I would just move the rear wheel forward, and I'll see how I feel down the highway. If i feel like I'm still revving too high, ill do a custom ratio on the chain and sprockets. High rpm may get annoying with straight pipe lol assuming you have a maxim 400 like me, what kind of rpm does your bike do at roughly 100-110 kph?
 
Welcome to the Forum Daryll.
Dave is undoubtedly correct about how to maximize the performance of the bike, no one gets more out of this little dohc than he.

Running the 400 will certainly save you gas money over driving any kind of car, however, when I changed my front sprocket from 16T to 17 T, I could not measure any difference in fuel economy overall. But, I never did any long distance cruising, just a mixture of short country road cruises and around town driving.
I changed the sprocket because at my favourite cruising speed of around 80-85 kph (50-55 mph), the note the engine made bugged me and detracted from my enjoyment. I wanted to shift into a taller gear but couldn't.
The new sprocket changed the note and made all the difference to my enjoyment. And I liked that I could cruise through town in a different gear at the speed I wanted to go. Nothing to do with performance, just personal preference.

I did do some subjective performance testing with both the 16 and the 17 to see if anything could be measured.
If you can wade through some of my early problems with the actual mechanics, you might enjoy my old thread:
 
Dont worry about the revs being "too high". These engines have smaller rotating parts so they have no trouble turning that fast. Comparing these engines to a car engine is apples and oranges almost. You ever ride a moped? those things turn at 10000rpm and only do about 25mph. You just cant tell because the parts inside are much smaller and are much lighter and just move that fast with ease.
 
Running straight pipes will affect the performance of the motor greatly and not in a good way. If you had a good stock exhaust on the bike you would not be asking this question. The sound would be far less and the bike would make much more torque and power. If you are running pods also, that would also cause power issues.
 
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