Topic: CB900C with 4 into 1 exhaust needs rejetting

[gl4709]

A friend of mine recently received a CB900C which has 4 into 1 exhaust. No history of the bike is known. It had a fuel leak so we installed 4 new carb kits but left the jets which were installed. There were no numbers on the jets. The bike starts easily and idles fantastic. We can rev the engine with no load and it revs with no issues. The current fuel mixture is 2 full turns. As we take it for a test ride the bike is popping/sputtering some until 4000 rpm which is when it bogs down. I assume the jets and needle are stock size. From what I have read the jets are primary - 68, main - 105, slow - 35 or 38. My guess from reading is that the main jet needs to go to about 125? Can anyone help confirm what jets and needle this bike needs with a 4 into 1 exhaust? The make and model of the exhaust is not known. The air filter is currently stock but my friend is willing to change these to PODs which would make it easier to play with the jets. My thoughts are that the popping/sputtering is lean mixture from the needle and the 4000 big down is the main jet. Help please.

[Don R]

 This should be in the "other bikes" forum but anyway, if you used aftermarket brass and the jets aren't numbered that's where to start. Aftermarket brass and unknown jets will get you lost on your tune-up. Kiehen carbs need correct brass in them and reproductions are rarely correct.

[Flyin900]

Your on the wrong forum for that bike as there is a specific Honda forum for the DOHC models.

That said they don't do PODS and that is the worst thing you can do with the CV carbs. They operate best with the air box intact. A change of just the exhaust to a 4 into 1 doesn't require re jetting normally on these motors. Without knowing what your jet sizes are presently in the carbs is a problem. A good cleaning and the proper sized jets is the place to start IMO.

The carb bank may need a good cleaning as noted since they are much more advanced than the SOHC carbs with a number of different jets and passages, plus an accelerator pump circuit.

Here is the link to the CB750C forum.

https://www.cb750c.com/modules.php?name=Forums

[70CB750]

The CB900C also has what owners call the spawn of the devil - vacuum operated fuel valve.  That was the only time whenthis bike left me stranded - I replaced that POS with mechanical petcock.

[70CB750]

I posted this to the other thread:


My CB900C has 4 into 1 Delkewic and runs perfectly with stock jets. 

Amyway, there is a forum

https://www.cb750c.com/modules.php?name=Forums

The forum is pretty much dead but there is a lot of information available and few guys who know their stuff.

And I have a shop manual in .pdf - I can email it to you if you wish.