Topic: CB500 1976, original main jet size?

[FoxDK]

Hallo,

According to this thread: http://forums.sohc4.net/index.php?topic=77884.0 Is main jet size for a CB500 Four with Keihin 649A Carbs, size 78.

But in my Haynes book, is the size, size 100.

I'm confused. Which size is the right one?

[goldarrow]

Stock main jets are 100, slow jets are 40

[dave500]

in Europe it might've had a sort of silencing snorkel over the air intake?these I think had a smaller main jet?

[Deltarider]

Some Germans and I have experimented with #100 main jets and the removal of the airbox cover. It brings nothing but extra intake noise. Since your bike is original I recommend the #78 main jet, slow jet #40 and the airscrew 1 turn out ⁺/_- ¹/₈. I myself returned to this stock setting soon after the tests and my bike performs as it should. I have dyno results to prove this. There's no extra power gained by using #100 main jet or by removing the airbox cover, not in the original state with stock induction, 649A carbs and Honda's 4-4 exhausts. My bike on the dyno at over 180 KM/H was still on the rich side. I could post or mail a link of a thorough test of the CB500K2 and CB750K6 combined in the German magazine Motorrad that shows the normal power output and a top speed of 179 km/h.
Haynes is not a very good manual nor is Clymer. These manuals do not deal with all types and certainly not with ours, the CB500K2 (ED,F,G). Best manual is the one made in close cooperation with Honda France but that one is in French. The Honda Shop Manual although not flawless, isn't bad either. And you can always ask me.
So be aware that the American 500K2 (1973) is different from the 500K2 in Europe (1976). BTW here is mine.

[scottly]

Some Germans and I have experimented with #100 main jets and the removal of the airbox cover. It brings nothing but extra intake noise. Since your bike is original I recommend the #78 main jet, slow jet #40 and the airscrew 1 turn out +/- 1/8. I myself returned to this stock setting soon after the tests and my bike performs as it should. I have dyno results to proove it.

Delta, I would like to see the dyno graphs for the 78 main, both with and without the cover, and the 100 main without the cover, including air-fuel data. Thank you.

[Deltarider]

Dyno of a 100% stock CB500K2 (ED,F,G) with #78 main jets and cover in place is here.
http://forums.sohc4.net/index.php?topic=82538.msg928648#msg928648
Testing with #100 main jets and without airbox cover I did on the road. When I experienced it didn't bring anything but extra intake noise I returned to #78 main jets and airbox cover. On the German site I read later others experienced the same.

[Deltarider]

And here is the A/F. I remember it was difficult for me to have that sniffer (?) in place.

[FoxDK]

Thank you for Your replies.

I have tried with both size jets, 78 and 100. It run noticeably better with size 100. However, the spark plugs are a bit sooty. They did not soot with size 78, but it runs much better now with size 100.

The Air filter is new and the spark plugs are new NGK DR7ES.

[Deltarider]

Sooting of the spark plugs with #100 has been reported by German owners.
Interesting yours ran better. Mine didn't. On the dyno it ran over 180 km/h with #78. With me (90 kg, 196 cm) on the road it does 170 km/h @ 8.500 rpm (nose between the gauges). Did you change airfilter, spark plugs and jets at the same time?

[flatlander]

And here is the A/F. I remember it was difficult for me to have that sniffer (?) in place.

i did a dyno earlier this year with my '76 550F, stock setup and brand new air filter.
result is here: http://forums.sohc4.net/index.php?topic=134863.msg1522043#msg1522043

i find the comparison interesting looking at the figures for torque, HP and A/F mix.

[Deltarider]

What happened to your 5^th gear?

[flatlander]

5th is alive and kicking. we used only 4th on the dyno.

[Deltarider]

From all the tests I've seen the 550F(1) proved the most powerfull of the 500/550 series.
Your graph is a bit difficult to read. Is A/F around 13?

[flatlander]

sorry the picture is a bit small. A/F is 14.6 max when the guy threw the throttle wide open, between 12 and 13 rest of the time.
550F1 seems to have more HP but less torque than 500K2. at least when you combine mine with yours. don't know if you can generalise from that but that's the only direct comparison i've seen.

[Deltarider]

I really don't know if Dyno's are all the same.

[flatlander]

probably not.

[FoxDK]

Sooting of the spark plugs with #100 has been reported by German owners.
Interesting yours ran better. Mine didn't. On the dyno it ran over 180 km/h with #78. With me (90 kg, 196 cm) on the road it does 170 km/h @ 8.500 rpm (nose between the gauges). Did you change airfilter, spark plugs and jets at the same time?

Yes, it's a bit strange. 

But, I didn't change the air filter and spark plugs at the same time a changed the jets. Both the new plugs and the new airfilter have been on the bike with size 78 and 100.

Regards Frederik

[Deltarider]

Strange. You don't have the cover still over the airfilterbox, do you?
What are your airscrews set at? I mean how many turns out?

[FoxDK]

Strange. You don't have the cover still over the airfilterbox, do you?
What are your airscrews set at? I mean how many turns out?

In fact, I still have the cover installed, I'll try to drive for a period without it.

The air screws are turned 1 turn out.

Thanks for Your help, by the way 

[Deltarider]

With #100 main jets and the cover still over the airfilter box your sparkplugs definitely will foul.
With #78 main jets and the cover removed you most likely will experience some hesitation around 4000 - 5000 rpm. With the #78 main jets and the cover in place you should have good overall performance when your bike is original and carbs are set as prescribed.
So if you want to continue the use of #100 main jets, the cover has to be removed. Be prepared that whilst riding you'll experience more intake noise. I personally was annoyed by it and since I didn't experience any gain in performance I went back to #78 and the cover over the airbox. The dynotest has shown there's nothing wrong with the power output in that stock configuration.
Why is the airfilterboxcover there? Difficult to say. Maybe Germany had a law for intake noise, but the cover could well have been ment as an improvement to shield the airfilterelement against ingress of (rain)water. As far as I know, it wasn't ment to restrict as there were no laws on that in those years.
Different quality of gasoline in various markets maybe played a role. Who knows.
In an American Honda booklet for the CB500 (that had the 627B carbs with #100 main jets) different carb settings in regard to needle height and airscrew position are prescribed. Curious. Maybe the Americans wanted a leaner idle. Maybe.

[FoxDK]

I have today driven a distance of about 200 km, with size 100 main jets and with out the airfilter-box cover. The bikes runs ok and the spark plugs looks better. So I will continue with this setup 

But yes, the intake noise is a bit annoying. But i can live with that.


I experienced today that one of the carburetors unmotivated started to float (from the overflow tube on the bottom of the carburetor), while the motorcycle was parked with open fuel tap.

After closing the fuel tap, and opened it again a minute later, the float stopped and have not done it since. What can be the problem?

Best regards Frederik

[Deltarider]

I have today driven a distance of about 200 km, with size 100 main jets and with out the airfilter-box cover. The bikes runs ok and the spark plugs looks better. So I will continue with this setup

The proof is in the topspeed.

A tiny particle can be enough to obstruct the float needle from closing. You were lucky it went by itself. Sometimes you can get rid of such a particle by tapping with the stub end of a screwdriver against the floatbowl. If that doesn't help, you'll have to remove the floatbowl to inspect for dirt particles.
Is your petcockfilter still OK?

[FoxDK]

I have not had the opportunity to test the top speed yet. 

I removed the floatbowl today, it seemed clean in there. But I have adjusted the float hight a bit, it was a bit less than 22 mm. I hope this will help.

I think that the petcocofilter is ok, I have not checked it. But i have extra inline filters on the fuel tubes.

[Deltarider]

But i have extra inline filters on the fuel tubes.

Oh, oh and there's your problem. Do a search in this forum and find out how many had issues with extra inline filters. I've abandonned mine decades ago (two weeks after I had installed them). They gave nothing but trouble. If your petcockfilter is OK, knowing the nylon part that protrudes into the tank, you won't need any. Extra filters cannot filter out what the stock filter has already filtered and that was good enough for Honda. Just drain the floatbowls every 3 - 6 months to get rid of any particles collected at the bottom. That's my practice and I have never had a problem since. You may check your fuel tubes for correct lenght and routing. One should be 17 cm of length, the other 28 cm. Ideally the diameter is 5,5 mm.
If you have the correct length there's no risk of kinks. With extra filters you can expect problems with fuel flow.
When you remove the petcock, pay attention to which way to turn. I'm not 100% sure, but I seem to remember it threaded the other way.
If the filter is broken or missing, I strongly recommend the original Honda 16952-341-671. No matter the cost, it is worth it. Dave Silver has them for around 16 pounds. You may have a look in the Parts List CB500K2 [ED,F,G] on page 52 to see what it looks like. Mind you, it's different from filters on the older models CB500 including the American 500K2.

[FoxDK]

Ok. I have heard about that the inline filters can create problems. However, I have run both with and without these extra inline filters without being able to feel any difference.

But I will soon check the filter in the fuel tank and stop using the inline filters.