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Discussion Starter #1
Hey.

I've got an '81 honda CB750 with the straight RC04 engine.

I imported it from the states and therefor the reason I'd like to see if you guys can help me figure out what's wrong with it.

It has had some alterations such as free flowing air intake/ filter (essentially just nets covering the intake for to the carbs) and a 4-1 exhaust fitted to it from Vance&Hines.

When I first got it home (Sweden) it wouldn't start at all so I took the carbs completely apart and cleaned them and ran through all jets and everything.

The main jet has btw been changed to 110 (original is 98) and 2nd jets are 68 as original. Nothing too weird considering the 4-1 and the removal of airfilter.

It now has completely new fuel and here's the problem:

It starts fine and runs great on low revs. When I start to rev it it stutters and starts to go real funny and doesn't rev out completely if you know what I mean...

The octane is different over here. I run it on 95 which is the lowest we have here and I just can't seem to figure out what is wrong? Should I go even higher on the jets?

It doesn't take any air... i've done the normal spraying around the carbs.

thx
 

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Try adding some faux fuel, (starter fluid, brake cleaner, any flammable aerosol) while you rev it. If additional fuel "cleans up" the stutter then you know insufficient fuel is your problem and not timing, etc.

Octane should be irrelevant. 95 is pretty high.

Check your spark plug gaps. I'd also rev it up at night, in a dark garage, and look for arcing / breakdown of your ignition wires. you can try spritzing around the wires with a water bottle to "help" view any breakdown. (Something like a windex bottle that makes a fine mist)

After all that I'd be concerned about your valve clearances, and/or a weak or damaged valve spring. Those also get revealed at higher rpm's.
 

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Jets

If you are trying to gain revs with no load on the engine, I suggest the small jets may be plugged. If you are trying to get revs while under load, the main jets be too large. It depends if the engine is choking, fuel starved, or gurgling, too much fuel.
Changes to the exhaust and the air intake, do not always require larger main jets. If the engine sucks better than it did before the changes, then it will suck sufficient fuel with smaller jets.
The above assumes everything else is adjusted and working properly.
There is a guide for tuning Mikuni carbs on the internet. Will work for other brands of carbs.
I went through the same process with my XS400, and my XS11.

Unkle Crusty*
 

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engine stutter.
happens when there is too much fuel in the cylinder, so you need to make sure your floats are CLOSING FULLY.
 

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Drastic changes in intake and exhaust can require a few tries to get the jetting where it needs to be. Personally, my gut feeling is you need to change your slow and main jets a bit more, maybe some other tweaking too. CV type carbs like you have can be very difficult to run like that, too.

I would do some digging on the net to see what someone running a similar setup is running for jets and start there.

This is all assuming you've ruled out other things like a stuck advance, bad coils/plugs, dirty carbs, etc.
 

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Discussion Starter #6
Well. I tried giving it fuel (starter gas) while revving it and when I reached the revs where it would start stuttering and I went a lot cleaner so the assumption is too little fuel I presume. Some conflicting advice. What's next. Order a lot of different jets and go from there?
I have keihin carbs.
 

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Have you checked for air leaks? What shape are your boots in?
 

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Discussion Starter #8
If by boots you mean the rubber part between the cylinders and the carb then that's in good nick and no air leaks.
So larger jets or smaller jets?
 

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Jets

If the main jet is a 130 for instance, go up two sizes. In each gear go WOT. If the splutter is worse you know you are going the wrong way. If it improves you know you are on the right path.
It may run clean in the lower gears, and get worse in the higher gears. You are getting close. Run it up a hill in the lower gears which will put more load on the engine. Start with WOT and the main jets. Assuming the engine idles and will pull out of the drive in first.
Recheck your float levels and study all the jets with a magnifying glass. Recheck all passages in the carbs as Cmon suggested.
A tiny sand like particle can sit in jets and passages, and be a real pain to find.
The particle can move around causing good and bad running.
For my XS400 I have 117.5 120 122.5 125 127.5 130 135 and 140. I went up when I should have gone down. Pod filters and non stock exhaust. Does 10,000 in all gears except top, where I run out of power due to wind resistance.
But 92 indicated, is okay for an 84 400 twin.

Unkle Crusty*
 

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Discussion Starter #11
I'm stoked... after first ordering through a Swedish company to find out if I was on the right way I then ordered a bunch around where the caferacers united form had me going.

So I ended up with 75 for slow jet and 128 for main jet.
It rund like a dream and an added side effect is that it starts immediately! Maybe a quarter turn! I have that on my Africa Twin and never thought I would experience that on another bike ever!

Thanks to all! :)
 

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Well done! I had exactly the same problem on the same bike but I could never get it to run properly with the CV carbs and K&N filters. I even had it up on a dyno but no joy. After this picture was taken, I ended up changing the carbs for Mikuni RS flat slides and the bike now pulls like a train.

 

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Discussion Starter #13
Sorry I'm not in with the lingo yet - what does CV mean?
Wasn't it a whole bit harder to get completely new carbs to work on your bike?

I wanted to show a picture of mine too but I need to post more first! I'll get around to that soon.
 

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CV Constant velocity I think.

Sorry I'm not in with the lingo yet - what does CV mean?
Wasn't it a whole bit harder to get completely new carbs to work on your bike?

I wanted to show a picture of mine too but I need to post more first! I'll get around to that soon.
CV carbs have a diaphragm in them. I think it is to smooth the engine revs going up and down. Because they take air suction to lift the diaphragm and the slide with the needle in it, it makes it difficult to tune them. At least for me and many others.
If a slide is slow to lift, that cylinder will lag behind the others. Some folks say different filters mess up the air flow and cause running issues. The air inlet is easy to find, so find filters that do not block it. Study a stock filter for a guide. The foam filters fit better and restrict the air less around the air inlet, than the paper filters.

Used to be we would study the spark plugs to get the jetting right. But with these things the guide suggests running at WOT ( wide open throttle ) to start. And make your adjustments from what you find out. But I found that what the engine was doing did not always lead me in the correct direction. Also on my XS400 Yamaha it has a fancy pre-combustion chamber that made reading the plugs difficult.

Stock my 400 had different size jets in each carb. Some 4 cylinder bikes also do this, but I found all the jets were the same on my clean XS1100.

Changing to carbs with a direct pull on the slide and needle, should give quicker throttle response, but it will also shut off ( drop revs ) quicker too. When you twist the throttle, you know the slides are going up. They are also easier to tune.
Now if I can find four old style carbs at a good price for my spare XS1100.

Unkle Crusty*
 

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Great looking bike, Johno!
 
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