Topic: Main jets #78 CB500 Europe (areacode ED, G)

[Deltarider]

I ride a 1976 built CB500 Four. Here is something I have never understood. For some countries on the European continent the carbs [649A] had main jets #78 and not #100 as in France, UK, America and Australia [627B]. The model with #78 main jets also had a lid covering the airfilterbox. Now, the difference between a 78 and a 100 jet is considerable and one would think it must have had something to do with emission regulations. But… in those days, there weren’t any!  So my question is: what made Honda decide to have #78 jets on the models for Germany, Holland and some other European countries (areacode ED, G)

[bryanj]

It was down to local regs, like red rear indicators and funny shaped lights

[Deltarider]

There were no local regulations, not on emission/power output, not in those years. Question remains: why 78 main jets on the 500K1/K2 [ED/G]?

[bryanj]

There may have been no written down regulations but there could hace well have been a "gentlemans" agreement between the governments

[TwoTired]

That is a curious thing, for sure.  And, while I don't have a direct answer to your question, I have few for you.

1 - Are you certain the jet number isn't #98, instead of #78?

2-  Have you actually measured the orifice size?  A 78 stamped Keihin jet should be  0.78 millimeter in diameter.

3 - Do you have the PD style carbs or the earlier Keihin type.

4 - Is your 500 a K3, K1 or K2?

5- Can you post a picture of your bike?  Carbs?

6- Do you have intake and or exhaust restrictors?  What diameter outlets/ inlets do you have on your bike?  An inlet air restrictor would certainly increase the "pull" of fuel on the jet, necessitating a smaller orifice size.

7- Were those countries trying to limit top speeds of these bikes as a "beginner" bike?  What is your top speed?

8- I can't imagine how the bike can run anywhere near correctly on 627B style carbs simply rejetted with #78 mains.  There would also have to be other changes inside the carbs like air jets, throttle valve needles, emulsion tubes, etc.  Have you checked these components for disparity, too?

Cheers,

[Deltarider]

Hi Twotired,
Here are my answers. Believe me, I did my homework and I have never found the answer.

1.  Positive, #78. The #98 jets were on some 550 models.
2.  One can clearly see the difference between the 78 and the 100 orifice (I have them both). You don’t even need a magnifying-glass.
3.  The 500K2 has the earlier Keihin type. The PD type carbs were on the 500K3/550K3 only.
4.  500K2. The 500K3 was later imported in some other European countries. (btw they had #90 main jets in their PDcarbs).
5.  Our carbs have stamped number 649A. The models that have the #100 jets have stamped number 627B. By the way, the European 500K2 is unlike the American 500K2. Ours had the bigger gauges and the newer striping on the tank. I will send a photo soon.

6.  Pages 32 and 33 of the Parts list show the carburetor parts. The only difference you’ll find is the main jet: 78 for areacode ED,G and 100 for all other countries. On p 57 and 58 you’ll find that corresponding cover that restricts the airfilter. Again, only the ED and G models have it, not the French, UK, American and Australian models. There are no other restrictors. I have checked that thoroughly by comparing all parts numbers.

7.  No. In those years there were no speed limits and Germany is the only country in the world that still has no speed limit. Germany has insurance classes though, but they have no meaning in this (27HP, 50HP). Ofcourse the model with #78 jets delivers more than 27HP. On a flat road, I once did 8500 rpm in 5th gear, 170 km/h precise. Nose between the gauges.
8.  Again, the only difference is the size of the main jets and that lid over the airfilterbox.

Now, ofcourse that lid and the #78 jet are related. But why on the ED and G types? It doesn’t make sense. There were NO regulations, not on emission, not on poweroutput. Mind you, already in 1973 the 500Four for our markets was fitted with the #78 jets. But why?

[Deltarider]

Hello TwoTired. Here some pics of carbs, aircleaner cover CB500K2-ED. The question remains: why #78 main jets and restricted aircleaner.

[crazypj]

I'm not certain but I think certain areas (parts of Germany etc) had a special low power learner version?
I know they later introduced a 27~ 28bhp limit for learners, we sold grey import 600 and 800 BMW's which were totally gutless until rejetted
PJ

[TwoTired]

The engine is an air pump.  If the inlet air is restricted, the power output is restricted, particularly at high RPMs where it consumes the most air.  The added inlet restriction at high RPM will make the carb throat vacuum much higher as the engine gulps for more air.  This increases the pull of fuel on the main jet.  To keep the mixture RATIO correct the jet must be made smaller in size to keep the carbs from delivering too rich.   It wouldn't surprise me if your bike didn't have a different throttle valve needle profile, the emulsion tubes had different sized bleed holes, and that the air bleed jets near the mouth of the carbs were also drilled to a different size.  All these can effect the mixtures delivered to cylinders.

I'm currious what your drive chain sprocket tooth counts are.  The standard 500 has a 10,500 red line.  If you truely achieved 105MPH (170KPH) at 8500 RPM, I suspect your drive gearing has been altered as well.  Most of the magazine test reports had the 500/550 topping out at 98MPH.  How certain are you that the speedometer is accurate?  What size tires do you have?

What is your fuel economy like?  With the air restriction, jetting, and possibly gearing, it could be the economy king of the SOHC4s!

If I had to guess, I'd say that you could get an easy power boost by swapping in 627B carbs, removing the intake restrictor, and using 17t front and 34T rear drive sprockets.  That is, assuming there aren't other performance limiting changes made to the bike.

Cheers,

[Dave L]

Hi
Could it be down to noise regulations, these may have been in force look at the changes in the two strokes of
the time the kawasaki triples differed greatly in power if i recall.
The cb750 also changed a lot over the years  both in top speed and noise ,some countrys had anti tamper carbs and
anti tamper exaust bafles on there bikes.

Dave L

[jensk]

I live in Denmark which normally due to EU have the same restrictions on motorvehicles as Germany - unfortunately not the same free speed limit though. 

I have asked around about this and even the old Honda dealer i usually buy part at haven't heard about restricted CB500 og 550's with this air intake restriction. I guess the areacode doesn't cover Denmark.

Jens

[Deltarider]

Crazypj, TwoTired, Dave L and jensk, thanks for the replies.

I cannot tell whether there are more differences between the 649A and the 627Bcarbs than just the main jets. What I do know is that in the parts list the only partnumbers that differ are those for the main jets AND for the complete “carburetor assy”. So maybe  emulsion tubes, air bleed jets and needle profile are different too. I just don’t know.

My transmission is standard: 17/34. I like to keep it that way. My speedometer may exagerate a little, the tachometer is accurate, so in 5th gear 5000 rpm equals 100 km/h, 6000 is 120 km/h, 7000 is 140 km/h, 8000 is 160 km/h. Very convenient.

Now to add confusion, on p. 135 in the original 500/550 workshop manual, you will find the CB 500 rated for 50 BHP (SAE)/9,000 rpm and the CB 500 K1, K2 for 44 BHP.  If these figures are correct and if the Honda people haven’t mixed up SAE and DIN horsepower, this arises a new question. How can journalists in German and Dutch motorcycle magazines, who tested the 500K1 (1971) and K2 (1976), have measured 48 DIN HP (50 SAE) and done top speeds of 179 km/h where in the same articles under the technical specifications you will find those #78 main jets!?  And in the photo’s you see that restrictive aircleanercover. It just doesn’t make sense. I was hoping some old Honda mechanic would have remembered WHY the ED, G, models had these modifications. 

Anyway, under favourable conditions on a flat road, I can do 8500 rpm in 5th gear, 170 km/h. I use Continental tires, 3.25 19 TK 22 front and 4.00 18 TK 44 rear.