HELP! Carb Jetting Contradictions!

In the Shed
1

Hi everyone!

I need some sage advice on my Non-California 1984 VF1000F carb overhaul.

I am getting contradictory information from two reputable sources. I was hoping the experts on this site could tell me what’s up, and provide a little explanation.

Per the Honda manual and Clymer, the front carburetor main jets are larger.

So I get a jet kit from CarbJetKits.com. They have labelled the larger jets as “rear cylinder.” I contacted them on this, and they maintained that “the Honda parts fiche has been known to be in error in this regard”, and that larger jets DO belong on the rear cylinders, due to the rear cylinders running hotter and leaner.

This is where a little bit of knowledge is turning out to be a dangerous thing for me, and leading to heaps of confusion. I’ll explain my thinking.

  1. I don’t see that a liquid cooled V-4 engine is going to have the problems of hotter rear cylinders. I always figured this was an air cooled V-twin problem, due to obstructed airflow to the rear cylinder. I had assumed the disparate jetting of the VF1000F was a side draft vs. downdraft thing, not a hotter cylinder thing.

  2. The front main jet holders have 12 holes, vs 8 for the rear. This makes me think the front IS intended to have the larger jets, consistent with higher fuel flow.

Adding to my confusion is that my rear diaphragms are torn, which I know can happen due to backfiring under lean running conditions. I can’t consult the stock main jets, because they got heavily damaged in the removal process.

For what it’s worth, my bike is modded with a 4 into 2, no collector muffler setup.

Oh, wise sages of the VF1000 world, how to jet my baby’s carbs? And please do provide an explanation, as I am seeking to understand why I am going to be jetting my carbs per your recommendations.

Thanks a thousand! (Get it?)

  • E
2

I’m not too sure if these comments are going to be ‘sage like’ but I’ll have a crack at it!

I ‘think’ that because the two rear carbys are sidedraft and the two front carbys are downdraft the rears produce slightly more power (more efficient)than the fronts, so the fronts are jetted higher(and more holes to better atomise fuel)to compensate and equalise the power output from the front cyls.

The setup of each bank of carbys is different due to them having to deal with differing conditions/gravity/cylinder positions/sidedraft/downdraft etc, also the reason for the different length springs.

The reasoning for having the 150 in the rear seems odd as all the parts lists say 145 for the rear, I would guess that all the bikes rolling off the production line had 145 in the rear as well, surely a mistake this big would have been picked up long ago by mr Honda![:)]

I would go with tradition and have the rears 145 and fronts 150, you can check the plugs later on down the line and see how the colour looks, you may find the jetting can be changed later on to suit your local conditions. The bikes do get a bit hot in the rear if they are sitting a long time, maybe the cause of both rear diaphragms being torn althought they are pretty old and may just be worn out.

Don’t take the above as 100% gospel as it’s just a (deluded?[:)])theory I’ve come up with while trying to work out how these buggers work!

Cheers