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Jetting some new carburetor

Posted: Tue Oct 02, 2018 2:20 pm
by ejseibert
Hello All,

There are multiple aspects to this question.

I picked up a low mileage 1980 Honda CM400A (Hondamatic). I had to rebuild to carbs and clean out the varnish, however I could not get the accelerator pump jet on the right side clear. So I picked up another set of carbs for parts but they came from a CM400T (Tach.) They used the same engine but the "T" model used a Keihin CV 30mm carbs where as the "A" model used Kehin CV 28mm carbs. And there jetting was different.

Model Carb Jets
CM400A 28mm 112 & 68
CM400T 30mm 118 & 72

There are no rebuild kits for the 28mm carb that I could find online. Luckily the carbs are virtually identical in every way except the venturi bore size and the jetting. If there are any other differences I have not been able to find them. So I ordered a rebuild kit for the CM400 T 30mm Carb and simply reused the old jets to rebuild the original carbs. However, it was running lean, so I rejected them using the 118 and 72 jets and it ran much better.

I was happy until I found a leak coming from the accelerator pump on the left carb. This pump also supplies fuel to the accelerator jet on the right carb through a tube. I removed the carbs and replaced the diaphragm on the pump only to discover that the right side jet was clogged. No matter what I did, I could not unclog it. I did not like the idea of only having only one accelerator pump jet working and the possible imbalance this could cause.

That is when I ordered the new carbs only to discover they came from a CM400T model (30mm). I decided to give it a go as the only additional parts needed was new intake manifold to accommodate the larger engine side of the new carbs. The rebuild is done and the CM400T 30mm carbs are on the bike jetted with the 118 and 72. The accelerator pump now works on both carbs. However it bike now seems to be running RICH.....

1) Why would going a nearly identical but slightly larger carburetor with a larger venturi cause a RICH condition? If my understanding is correct the larger size of the venturi causes a decrease in air speed through the venturi and therefore should cause a decrease in the vacuum and decrease the fuel supplied causing a lean condition.. If anything I expected to stay the same with the now functional accelerator jets or possibly need to go up a size on the jets.

2) With the accelerator pump on the left carb connected via a tube to the right carb should I expect less fuel to the accelerator Jet on the right carb? Is this accelerator pump setup going to guarantee problems?

3) I also have access to a set of 1979 CM400A 28mm carbs which are also virtually identical except that they don't have the accelerator pump. Would it be better to rebuild and install these and forgo the accelerator pump all together?

4) For that matter would it be better to dump the Keihin carbs all together and buy a set of new Mikuni carbs compatible with my bike.

Thank You in advance.