Topic: Stupid question, but have to ask (regarding carb jet size numbers)

[chrima]

Quick question: The number on the jets (specifically the main jet); is it the higher the number the bigger (more fuel) the jet? I just want to make sure. I assume that is what the numbers mean, but I want to make sure before I order some. I did a quick search and couldn't find the answer.

Reason: I have a 77 550k with a 76 550k engine and carbs. Carbs have 100 mains 38 slow jets. Stock 77 airbox and 4 into 4 exhaust.  Engine is running rich. Kinda makes sense do to the fact that the 77 4 into 4 is more restrictive.

So, I thought I would drop a jet size. I see 98 mains out there and thought I would give those a try. Seems like the next smaller size is 80. That seems like a big jump from 100.

I tried dropping the needle down one slot (ie move the "eclip" up one slot which drops the needle down further). Seemed to have negligible impact.

I do need to get a new air filter. That probably also contributes to the running rich.

Anyway, just wanted to verify those numbers.

[scrapvalue]

You are correct on the smaller number, less fuel on the jets.
But i would recommend installing a new air filter before you change the jets.

[lucky]

You are correct on the smaller number, less fuel on the jets.
But i would recommend installing a new air filter before you change the jets.

The smaller the number the smaller the hole.
For instance a #38 jet is .38mm = .014 thousandths hole.

Better change that air filter FIRST.


What evidence do you have that the engine is running rich??

Is it the spark plugs?
Is the choke off?

The real problem is this:

QUOTE:  " Stock 77 airbox "

The stock air box is VERY restrictive.
You need to get the 1976 air box. Now we know why it rich.

The 1977 and 1978's were TYPE II EPA compliant carbs! 1976 were pre-EPA compliant=still adjustable.
They did everything they could to make it non adjustable and stingy to the max.
Elevation above or below sea level was not a consideration.

The 1978 CB750 air box only had a hole 3.7 inches x 1 inch for to come into the engine. Thats it.

[chrima]

I can tell it is running rich by the carbon build up on the plugs after a short run (5 miles).

I also don't typically need the choke. I might need choke for a cold start, but I can shut the choke off with 20 seconds of startup.

I had a Uni filter on order, but found out they were out of stock and cancelled my order. So need to put another order in. I am still thinking that won't be enough due to the 77 filter box and pipes.

Bike seems to run fine, does bog a little bit. I am thinking it is due to the rich state.

Thanks for the confirmation on the jet numbers. I'll start with the air filter and go from there.

[lucky]

I can tell it is running rich by the carbon build up on the plugs after a short run (5 miles).

I also don't typically need the choke. I might need choke for a cold start, but I can shut the choke off with 20 seconds of startup.

I had a Uni filter on order, but found out they were out of stock and cancelled my order. So need to put another order in. I am still thinking that won't be enough due to the 77 filter box and pipes.

Bike seems to run fine, does bog a little bit. I am thinking it is due to the rich state.

Thanks for the confirmation on the jet numbers. I'll start with the air filter and go from there.

If you have it idling and give it sudden throttle and it bogs or quits it is LEAN not rich.  READ THAT AGAIN. Try the mix adjustment to richen it. forget about the plugs for now. Listen to how it runs.

That AIR BOX is  a PROBLEM. You need to measure the intake area of the 1976 air box and compare it to the 1977 air box.

Whatever the 1976 box has as far as sq inches of intake area you need that same amount on the 1977 air box.


But why cut up the 1977 air box? Just trade it for the one you need for those 1976 carbs.

[TwoTired]

The real problem is this:

QUOTE:  " Stock 77 airbox "

The stock air box is VERY restrictive.
You need to get the 1976 air box. Now we know why it rich.

Perhaps a remedial course in reading comprehension is needed.
This thread is about the CB550.
For the 550, the air box is the same for all models.  And, it is NOT "very restrictive".    It most certainly has design elements that are not fully understood.  And, that goes for the CB750 air box design, as well.


For the OP,
Yes, do use smaller main jets with that higher pressure exhaust.  In fact, the pilot system will need leaning, as well as the slide needle adjustment.

I'll ask why you can't use the 77-78 PD46 carbs?  Although they have a different venturi shape, they used a #90 main jet.   But, they were tailored for just the configuration you currently have.

 I agree that for your current carbs, it's a big jump between #80 and #98.
If you continue on current path, you may have to buy #80 jets and ream them larger to the final size your set up requires.

Cheers,

P.S. Be careful about some "advice" being offered in this forum.  The quality/accuracy has taken a dip of late, imo.

[lucky]

Two Tired....The OP said, "I have a 77 550k with a 76 550k engine and carbs."

Are you saying that the intake hole for the air box on the 1976 is the same for the 1977?
How large is it?
Can you tell us the difference if there is any?

I may be wrong sometimes and If I am I apologize, but you do not need to say I need a remedial course in reading.
Do you want me to treat you like that?

[lucky]

I also said to the OP,

 "That AIR BOX is  a PROBLEM. You need to measure the intake area of the 1976 air box and compare it to the 1977 air box. "

I am sure if the Op found out they were the same in every way he would know I had made a mistake.

I will be more careful in the future.

[lucky]

I can tell it is running rich by the carbon build up on the plugs after a short run (5 miles).

I also don't typically need the choke. I might need choke for a cold start, but I can shut the choke off with 20 seconds of startup.

I had a Uni filter on order, but found out they were out of stock and cancelled my order. So need to put another order in. I am still thinking that won't be enough due to the 77 filter box and pipes.

Bike seems to run fine, does bog a little bit. I am thinking it is due to the rich state.

Thanks for the confirmation on the jet numbers. I'll start with the air filter and go from there.

Sorry i made a mistake on that airbox information Chrima.

[chrima]

Thanks all for the info.

Yeah, in hind sight, I would have been better off using the 77 carbs (which I have). From the start, both sets of carbs needed rebuilding. At the time, the 76 carbs seemed easier to get started with and were in a little better shape. I knew I would have to do some fiddling with the air/fuel mixtures.

I think it is close and the combination of a good air filter and the slightly smaller jets will get it there. I ride it to work a couple times a week which is about 5 miles round trip. I just have to clean the plugs every couple weeks. I would like to do some longer trips, so I need to get this sorted out.

Here are a couple of pics of my bike (before/after):

[cgswss]

Just an FYI...

Jet sizes are aprox the dia of the hole in MM times 100

I use wire drills to check the size of jets before I start changing them.  Both my '81 and my brotherinlaw's '83 have jets that were drilled out by the PO.  On his bike the jets are labeled "100" but a '57' number bit fits the hole.  a 57 is 1.092 mm.  That would make the jets 110s.

just  a few to remember:

#65  .889mm    so aprox a 90 jet
#63  .940mm    so aprox a 95 jet
#60  1.01mm    so aprox a 100 jet
#59  1.041mm  so aprox a 105 jet
#57  1.092mm  so aprox a 110 jet
#56  1.19mm   so aprox a 120 jet
#55  1.32mm   so aprox a 130 jet

[lucky]

Just an FYI...

Jet sizes are aprox the dia of the hole in MM times 100

I use wire drills to check the size of jets before I start changing them.  Both my '81 and my brotherinlaw's '83 have jets that were drilled out by the PO.  On his bike the jets are labeled "100" but a '57' number bit fits the hole.  a 57 is 1.092 mm.  That would make the jets 110s.

just  a few to remember:

#65  .889mm    so aprox a 90 jet
#63  .940mm    so aprox a 95 jet
#60  1.01mm    so aprox a 100 jet
#59  1.041mm  so aprox a 105 jet
#57  1.092mm  so aprox a 110 jet
#56  1.19mm   so aprox a 120 jet
#55  1.32mm   so aprox a 130 jet

This list makes no sense.
.889mm=.035 thousandths.

#65 JET would be .025 thousandths of an inch.

 #90 JET is .037 thousandths of an inch.

[dave500]

aprox is no good,,its either one or the other,,you can get away with not quite right running holleys or qjets on big engines,,these little motors require more finnesse,the carbs dont have a power circuit or an off idle circuit,a few have accellerator pumps though,you must have precise jetting .

[cgswss]

OK maybe we are not understanding each other

MY #65 drill is .035"  which is .889mm  so .889 X100= 88.9 which is real close to "90" or a #90 jet

My point is not to "make" jets a different size by drilling them out (god knows there are a very few on here that could drill a .035") but to get aprox jet sizes by checking them with a number drill bit (blunt end please).  Example: my brother in law was running a little lean with his #100 main jet.  So he replaced it with a #110 main jet and had no change.  He didn't want to buy more jets until he figured out why there was no change.  I took the old #100 jet he pulled out and tried different number drill bits to see what size the jet real was.  a #57 drill fit, the #56 did not.  A #57 drill is .043", very close to 1.10 mm or a 110 jet.  The PO had drilled out the 100 jet to make a 110 jet but we thought we were dealing with a 100 jet.  End result was us going up to a 120 jet that made a real difference.

[lucky]

OK...numbered drills. I will look at that chart.

I thought when you wrote #65 it meant #65mm jet.