DOHC 12E (probably restricted) lack of power - carbs modified?

overpl

overpl

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Hi,
I bought a 1987 DOHC 12E XS400. It is probably restricted (from germany - they have stupid laws about HP's for youths).

The main problem is that my XS starts poorly, and never starts with choke - only without it.

At 7000RPM XS chokes a bit, and in general I feel like riding a 125ccm scooter bike.

It should have 45HP (about 6s to 100kph) or when restricted - 27HP (about 9s) - mine has about 10s to 80!kph and have a real hard time to make it to 100.

I've noticed in manual that regular 12E had carbs Mikuni BS34 16N10 (restricted i guess) or 12F00 (not restricted, but in manual they just use (G) stands for germany - although this is super weird because i've heard that they restricted them IN germany).

I have 12E frame no. but they said that germans had 12F frame no. - although in 12F00 carbs jets are 132.5(left) & 130 (right) - SO AS MINE...

So i'm now a little bit confused... first of all - what jets should I have in 45HP version, and 2nd of all what is wrong with my motorcycle:/ what should I do now? I cannot even use a repair kit since I've found only kits for 1 carb and they have different jet sizes..

Also I really would like to know what is a diameter of intake manifolds in 45HP version? They say that XS used to be restricted also there.

Kind regards,
Over.
 
Hi there and welcome.

First off, have you cleaned your carbs? Starting poorly is an issue when you have dirty carbs, and most XS's do. Cleaning your carbs might also bring some improvement in performance. My bike didn't go any faster than 110 kph, and after I cleaned the carbs it went at least 130. Still have to try top speed.
 
Yeah, they are clean..
It is important for me to know diameters of intakes. And sizes of main jets. I have 132,5 and 130, in manual they are marked as german version.

I have an idea, that maybe previous owner tired to unblock it and opened intakes, but didn't use SMALLER jets (127,5;117,5). Is it possible that XS with opened intakes needs smaller jets becouse of regular flow, when restricted intakes (i dont know is this a proper name-this thing between carbs and engine) needs bigger jets to work properly? And becouse of that having good intakes and bigger jets my engine is getting overfload?
 
Didnt certain overseas versions have restrictor plates which kept them from making as much horsepower as some of the other models without them?
 
if your engine is getting too much fuel, your plugs should come out sooty black. Is that the case?

If I had my carbs off I could measure the intake boots for you but I wasn't really planning on it to be honest :)

Either way, if your jetting is wrong your bike will start and run poorly, and this should show on the spark plugs. I'd start there if I were you
 
But nobody knows what jets are used in 44HP? Also boots? I Will check the plugs. But for now i have 2 unknowns - jets and intakes;).

What about my theory - that bigger jets where instaled due to restrictions in intakes? And when i have normal intakes (legs assume) bigger jets can be the problem? Becouse in manual there are 2 sets of main jets - 132.5;130 for Germany, and 127.5;117.5 for E(which is described as UK)
 
What is a restrictor plate? Im sorry my tech. English is rather poor:/
Ive heard about 2 ways of restrictions in XS dohc:
1) intakes are smaller and sth is changed with jets (that is what im trying to figure- are they bigger or smaller - i thought obv that restricted are smaller but since i've heard that someone claimed that he had smaller jets in 44HP i came up with this theory that restriction is mainy becouse of boots, and then they put bigger jets to make it work, as smaller jets were too small for that downsized intakes)
2) above + module + camshafts(??)
 
And where i could find it? But i dont think there is such a thing at my XS. I've read a ton of threads all over the internet and no one claimed that.
Is there a sport who could check at his DOHC 44HP if there is any?:) i mean the jets and intakes sizes. I will put volonteer photo at my wall and sing a sonet;-)


Edit: i've checked manualy that carbs can open in full way - if you know what i mean- so i dont think there is a restriction plate
 
Last edited:
If they put in bigger jets, the mixture would have been richer. A restriction in the intakes will make less mixture go into the cylinders. So bigger jets wouldn't have any positive effect on the performance of your bike, unless you make the restriction disappear as well.

If they restricted it by making these intake boots smaller, they must have also put in smaller jets to make the mixture less rich. Basically you can see the jets as the determining parts of how much fuel goes out of your carbs. The amount of mixture is determined by the ease of flow through your system, so by the exhaust, the intake boots, and the air filter. So when you restrict the flow through the intake boots, and keep the amount of fuel going out of your carbs towards the engine the same, you'll end up with a mixture which is too rich.

That's also what happens the other way around, when you put freeflow air filters (pods) and a different exhaust on, more mixture will be passed through your engine. If you don't adjust the jetting accordingly, you will have a lean running bike.

Again, just look at your plugs. They will tell you exactly what's going on, whether your bike is running rich or lean.
 
The plugs are black.

I've reached to some repair kits for carbs, and for 44HP version they have 127.5;117.5 jets, and for restricted 132.5;130 - so thats kind of odd - because you said they should put smaller jets than original - that's why I need so badly to check the jets sizes and intakes with someone who have 44HP DOHC.

So plugs are black, so engine is drowning in fuel - so assuming I have full diameter of intakes, I should put smaller jets I think.

Ideas?:)

Thanks,
over.
 
Well I think you're right. But yea that is weird, maybe my story isn't 100% correct then.

Try adjusting your idle screws first, see if that helps any. Maybe you can post pics of your intakes, that way we can see if there's anything out of the ordinary. Also it could be possible there is a different type of restriction, something to do with the camshafts but I don't know how that would work.

If I have my carbs out any time soon, I'll let you know the sizes.
 
After swithing to stock jets (127.5;117.5) XS runs much better. Maybe its not a speed demon, but f.ex. I had 120kph with 7000rpm at 6th gear. How is that compared to yours?:) Everything looks quite good, but still sometimes I have a feeling that engine stops for a nanosecs;) from time to time, and sometimes when I change gear to lower with gas (I don't know the name in english for that;)) there is no reaction for my adding gas, and then I try another time and everything is normal .. strange.
 
So updating the thread:

After changing the jets XS runs better but still I feel like she would like to fly but sth holds her down. So far I think that maybe I have restrictions also at camshafts - but honestly I don't know how to check that. Sb knows the dimensions of proper camshaft?

Also I have a gas consumption problems - I've noticed that I made 100km with about 6,5 L. Its quite a bit, right??

I have also strange feeling that my XS is choking at 1 and 2 gear sometimes. But she works gr8 on idle.

What should I do? What can I check without spending extra large amount of money?
 
My DOHC XS400 from '89 tested at 135 kph last week, and it still had some more to go. I'm thinking about 140-145 kph should be doable. Although my speedometer goes nuts at those speeds, it wobbles like crazy. I don't have the original speedo and tachometer (no tach at all actually) so I don't really know how much rpm I'm producing. But I'm guessing a bit more than 7000.

The top speed of your XS won't be reached in 6th gear though, 5th is quicker.

If your engine seems to stop for a nanosec, it's probably having some misfires. Also if your throttle doesn't work when shifting down gears, there's definitely something up.

I don't know about the camshaft and restrictions. But what I can recommend is that you adjust your valve clearance and check if your valves are doing okay. I know a guy here in the Netherlands who had a bent intake valve which didn't fully close anymore. Sometimes the bike would push exhaust fumes back into the carbs, which of course doesn't really help.
 
I love my XS but sometimes I just want to drive her in the ocean - gr8 for her that Poland don't have access to ocean;)

But seriously - I read somewhere that XS were restricted also at camshafts and modules - that is just sick.. I read also that restricted version have 31Nm at 3000RPM instead of 36Nm at 8000 rpm... I literally just dunno what to do next...
I'll check the valves but honestly if its restricted - than its restricted and drives good as a 27HP... Where I could find the sizes of camshafts ?;/ does anyone can recognize from photo if I will take one?

In Poland XS dohc is not a popular bike - maybe someone could look at their local sites for parts to 44HP version and tell me the prices? I'd buy you a beer cuz I don't have even money for 2 beers;]
 
ermm i got my 44hp version here in holland for 200 euros. Then I got 75 back cuz it didnt run well, while the seller said it did. So for 125 euros I got one with very dirty carbs, which I then cleaned. Needed pretty much a new everything though, tires, brakes, clutch cable, flasher relay, mirror, battery, sprockets, chain, etc. etc. and it looked like crap.

Edit: eh oops I misread the question. What parts do you need exactly? If I were you I'd look on ebay in poland to see if there's anything out there you can use.
 

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