Topic: cb500, diagnosing a rich condition.

[paul_D]

Hello,

I have recently started up my rebuild project cb500 this week. I have been playing with the carburetors all week among other things. The bike starts up easily enough but when I try to ride it, it is very hesitant and burbles/pops when i start to accelerate, with very little acceleration once this starts.  I have pulled the plugs and photographed them as seen below along with the exhaust. All are very sooty and along with all that I have read on this forum leads me to beleive I am running extremely rich.

Now the bike is a 1972 CB500 with 649A carburetors.
125 main jet
40 slow jet
1-1/8 turns out air screw
needle clip at 3rd position
floats set at 22mm

4-1 jardine exhaust
Unifilter foam original replacement oiled (dripping a little oil)
tool tray with metal screen covering filter
engined timed statically,   prestolite iginition system
D7EA plugs, strong spark when they were fresh.
often run on reserve from clean tank and rebuilt petcock due to limited fuel in tank.
carbs sync with homebuilt manometer

if anything is missing its because I forgot, please ask.

I have run my own gauntlet of research here online and tests to try and improve the engines responsiveness by troubleshooting the system.  Please any input would be appreciated.  I will run the bike without the filter tomorrow to see if that is the culprit. But anything else is much appreciated.

[Sable]

We share the same problem it sounds. If I figure mine out I will be sure to share with you. I'm going to change my main jet to a 38. See if that helps.

[Kemp]

I think you are quite rich for so few mods to intake and exhaust. I think a 105 main, needle at 2nd clip from the top, 38 pilot and air screw at 1.5 turns out. Because you are running the original airbox you really just need a bit more fuel. A 105 will give you more fuel not just at wide open throttle but everywhere.

[flybox1]

the filter is not the culprit, but it dies add to the situation.
an over oiled uni filter, and those jet sizes will certainly keep you on the rich side.
dry the filter as best you can, and decrease your main and pilot jet sizes to 38/110 and leave the needle clip where it is.

[TwoTired]

I thought the 649a carbs were only sent to places in Europe?
Anyway, these carbs also had the air filter restrictor as shown below, and required #78 mains for the stock bike.  I believe forum member Delta tried running those carbs without the restrictor and using larger jets as the 627b had, and found the plugs sooting up pretty badly and generally poor operation.  It was never identified what ELSE was different in the 649A carb besides the #78 mains, that made them run so rich.  That restrictor HAD to have required internal changes beyond just the main jet to allow the engine to run properly.  But, the specifics are unknown.
So, you have a unique situation here, as no one on this forum has gotten those 649a carb to work without the inlet restrictor.  To find out what the difference is, you will have to obtain a set of 627b carbs and then do a detailed measurement and comparison to find what the differences are.
Of course, once you have the 627b carbs you could just put those on the bike do the traditional jetting/tuning adjustments that the forum members here are more familiar with regarding the further induction and exhaust mods made to your bike.

[paul_D]

I thought the 649a carbs were only sent to places in Europe?
Anyway, these carbs also had the air filter restrictor as shown below, and required #78 mains for the stock bike.  I believe forum member Delta tried running those carbs without the restrictor and using larger jets as the 627b had, and found the plugs sooting up pretty badly and generally poor operation.  It was never identified what ELSE was different in the 649A carb besides the #78 mains, that made them run so rich.  That restrictor HAD to have required internal changes beyond just the main jet to allow the engine to run properly.  But, the specifics are unknown.
So, you have a unique situation here, as no one on this forum has gotten those 649a carb to work without the inlet restrictor.  To find out what the difference is, you will have to obtain a set of 627b carbs and then do a detailed measurement and comparison to find what the differences are.
Of course, once you have the 627b carbs you could just put those on the bike do the traditional jetting/tuning adjustments that the forum members here are more familiar with regarding the further induction and exhaust mods made to your bike.

That seems interesting.. What about swapping(ordering) all the internals for standard parts?  Now i don't have the hump either, just a tool tray.


As for the work I tried this evening, i cleaned the air filter and reoiled with a light spray of air filter oil to coat the outside. And then I tried to clean the plugs abit as I am having a difficult time tracking down some D7EA's instock in Niagara Falls Ontario. (no one seems to carry them)   Timed the bike dynamically with a strobe light. The 1-4 timing is perfectly steady. The 2-3 timing marks seem to stutter + - 1mm.

Things I noticed tonight. The bike starts up easily and idles without choke.   The idle is eratic. When I blip the throttle it seems to hang. Other times it will race up on its own.  Try riding the bike and I get nothing but hesitation. When the bike is idling and i touch the throttle it hesitates (less than 1/16th of throttle).  The exhaust is smoking, white mostly with alot of soot.

What affects are the black plugs going to have that I might eliminate by changing for new ones now ?

[TwoTired]

The white portion of the spark plugs is an electrical insulator between the center electrode and the spark plug body.  Soot is an electrical conductor.  Coating the insulator with a conductive material allows spark energy to flow though the soot trail rather than jump the spark gap. This makes igniting the air fuel mixture in the chamber problematic.

[paul_D]

As I thought, going to try some new plugs to see if my symptoms disappear.

[paul_D]

New plugs and they did not help the rich condition. After riding 2 miles they were completely fouled again.

Then I lost spark, checked the coils through and through, all the readings were good. 4.8 and 4.7 Ohm on the input, 14.7kOhm on the outputs.  All the plug caps are new 5kOhm caps, all reading within 5% (dont recall exact readings).  Found the blue wire had somehow disconnected. Also found some bad cable splices which i removed and soldered in new wire. All in all, each plug has a good spark. And there are no more electrical issues I can find.

Although the battery is now depleted, with the aid of a battery booster set at 12A I managed to start the bike.

This morning I tore apart each carburetor. I checked each carburetor to make sure each and every passage was still clean and clear. (they were, or are now)
Though I noticed some sediment in the carb bowls during my recleaning. I checked the fuel strainer and it was dirty. Cleaned it and the tank, refilled the tank with 91 octance w/o ethanol from Shell (V-Power).

So the carbs (model 649A) are currently set at 125 Main, 40 Slow, 3 groove for the needle, the air screw 1 turn (360 degrees) out, floats rechecked for 22mm setting which they are.

Put it all back together, and I still have made no improvements.  Any thoughts ?   Should I find 78 main jets and try them even though its my idle circuit that is troubleing me ?

[paul_D]

I should note, as this was asked in another post for another similar problem.

1 of the posts that holds the main jet in the carbs is putted and slightly corroded.

[HondaMan]

New plugs and they did not help the rich condition. After riding 2 miles they were completely fouled again.

Then I lost spark, checked the coils through and through, all the readings were good. 4.8 and 4.7 Ohm on the input, 14.7kOhm on the outputs.  All the plug caps are new 5kOhm caps, all reading within 5% (dont recall exact readings).  Found the blue wire had somehow disconnected. Also found some bad cable splices which i removed and soldered in new wire. All in all, each plug has a good spark. And there are no more electrical issues I can find.

Although the battery is now depleted, with the aid of a battery booster set at 12A I managed to start the bike.

This morning I tore apart each carburetor. I checked each carburetor to make sure each and every passage was still clean and clear. (they were, or are now)
Though I noticed some sediment in the carb bowls during my recleaning. I checked the fuel strainer and it was dirty. Cleaned it and the tank, refilled the tank with 91 octance w/o ethanol from Shell (V-Power).

So the carbs (model 649A) are currently set at 125 Main, 40 Slow, 3 groove for the needle, the air screw 1 turn (360 degrees) out, floats rechecked for 22mm setting which they are.

Put it all back together, and I still have made no improvements.  Any thoughts ?   Should I find 78 main jets and try them even though its my idle circuit that is troubleing me ?

I'd sure start with smaller mainjets. Those are mighty big for a 500.

Some other things to check: did you replace the O-rings in the castings that attach the carbs to the engine? If they leak vacuum, it causes a large 'flat spot' of overly rich mixture in the lower RPM ranges.

For other sparkplugs: look for the ND (Nippon Denso) brand in X22ES-U heatrange, same as the D7E series.

The "timing jitter" you see on the 2-3 points is caused by a slightly bent spark advancer hold-down shaft. These get bent when the engine is turned backward via the big nut there, with sparkplugs in the engine. You can remove the points plate and an advancer and tap it straight again witha dial indicator and small [plastic] hammer. If they bend more than .010" the jitter appears. They should be true to .003" or less runout.

[paul_D]

I'd sure start with smaller mainjets. Those are mighty big for a 500.

Some other things to check: did you replace the O-rings in the castings that attach the carbs to the engine? If they leak vacuum, it causes a large 'flat spot' of overly rich mixture in the lower RPM ranges.

For other sparkplugs: look for the ND (Nippon Denso) brand in X22ES-U heatrange, same as the D7E series.

The "timing jitter" you see on the 2-3 points is caused by a slightly bent spark advancer hold-down shaft. These get bent when the engine is turned backward via the big nut there, with sparkplugs in the engine. You can remove the points plate and an advancer and tap it straight again witha dial indicator and small [plastic] hammer. If they bend more than .010" the jitter appears. They should be true to .003" or less runout.

Sounds good, I will change the o-rings as I have some extras (I guess I know why now) and I will order some mainjets from sirius. They are keyster correct ? Still okay to use ?

[paul_D]

Ok, tonight I changed the o-rings for the intake runners. I installed the #100 main jets from sirius consolidated. I installed new plugs gapped to .65mm. I turned it on and nothing. The bike will not start.  I get the occassinonal sputter and sign of life. otherwise nothing.


This is with air filter in, 1 turn(2 rotations) of the air screw and no choke. Same as the previous times it fired right up.I then played with the air screw, no filter/with filter and various levels of choke, no luck.

It seems to backfire when i turn the key off from time to time.

Would the head tube vent tied into the plenum have an effect on it ? I understand this isnt how the bike was originally plumbed.

[harisuluv]

No choke? 

It is normal to need some choke to start.

[paul_D]

tried it with choke and it still wouldnt start.

[Randy]

I'm running a 105 main on my 500 with pods and 4to1 pipe.. I could use a 110 main..but runs great on Angeles Crest Highway ..

[TwoTired]

You are still trying to use the 649a carbs?

[Deltarider]

For experimental reasons I have tried #100 main jets on my 649A carbs. On both occasions it didn't bring anything but extra noise and running rich. In the 649A setup the 500s performed good. Testers in Holland and Germany reached a V_max of 179 km/h. I never got mine faster than 170 km/h. I weigh 85 kg, measure 196 cm and don't wear a racing suit. On the dyno my bike reached 185 km/h. My experience is shared by German 500 owners. For some reason ours run best with #78 MJs and the air duct over the airfilter. BTW, I personally believe NO 500 (or 550) needs a MJ over 100, no matter what exhaust or airfilter. That is unless the engine has been strongly modified.
Have you checked the little O-rings around the mainjets well? If there's any opening the engine will happily suck the extra fuel. Are you sure no one widened the slow jets by using metal strings?
Personally I am not a fan of substitute airfilters. Overhere the paper airfilter elements are widely obtainable, are not expensive and go a looooong way. Servicing can be done within 10 seconds.

[paul_D]

You are still trying to use the 649a carbs?

Yes i am still working with 649a carbs.  I have been away from my parts bike that should have 627b carbs on it all  summer.

[paul_D]

For experimental reasons I have tried #100 main jets on my 649A carbs. On both occasions it didn't bring anything but extra noise and running rich. In the 649A setup the 500s performed good. Testers in Holland and Germany reached a V_max of 179 km/h. I never got mine faster than 170 km/h. I weigh 85 kg, measure 196 cm and don't wear a racing suit. On the dyno my bike reached 185 km/h. My experience is shared by German 500 owners. For some reason ours run best with #78 MJs and the air duct over the airfilter. BTW, I personally believe NO 500 (or 550) needs a MJ over 100, no matter what exhaust or airfilter. That is unless the engine has been strongly modified.
Have you checked the little O-rings around the mainjets well? If there's any opening the engine will happily suck the extra fuel. Are you sure no one widened the slow jets by using metal strings?
Personally I am not a fan of substitute airfilters. Overhere the paper airfilter elements are widely obtainable, are not expensive and go a looooong way. Servicing can be done within 10 seconds.

I would like to keep my 649a's and yes the orings are new and fit nicely into the main jet stay. I would try some #78s but I don't have a restrictor. I will look into getting a paper filter.

What are your settings ? Airscrew. Float height. Needle clip.

[Deltarider]

A restrictor? Honda calls it an airduct. It's probably there to shield the paper airfilter element from ingress of water. Overhere bikes were serious means of transportation and not only for sunny rides.

Floats are right, so around 22 mm. BTW, floats are always right unless someone has read this forum and started tinkering with them. For experimental reasons I lifted the needles one notch. Next time I'm in there I'll lower them back to 3rd (standard and middle position) again or maybe shim them in between (I like to play sometimes but not always). IMO there's a lot of changing needle positions in this forum. Bike is a bit rich now.
Air screws are around 1 turn (360^o) out. At idle bike is overrich and really stinks at traffic lights. I can improve the idle by turning out the airscrews an extra turn and the engine will run as nice as a sewing machine. Alas at the cost of poor driveability whilst accelerating. So one turn out.
At top speed bike runs pretty rich even with #78s. Removing the airduct brings nothing extra.
I often wondered why in our markets the 500 had the 78s and the airduct. Germany, Holland, Switzerland, Austria and Italy have always been known for superior quality gasoline, but I don't know if that's the reason.

[TwoTired]

A restrictor? Honda calls it an airduct.

Honda is very PC, imo.

What else would you call a device that reduces the inlet cross section area by 1/2 to 1/3?  ...Unless, perhaps, you didn't want to insense the German or Italian speaking buyers?

What else could you call the call the choke butterfly plates?  Airduct turbulators?  Carbutetor enricheners?  Who doesn't want to be rich?

You try running a marathon breathing through a straw, and starving yourself of oxygen. (I'll advise head protection for the experiment.)

rant/
Good lord, this is so obvious.  The 649a was made to compensate for the restrictor.  Outside appearance of the carbs is irrelevant.  And there is something inside besides simple jetting that is different and being ignored.  Until it is known exactly what those differences are, the two parts must remain married together.  ...but not on my bike, thank you very much.

Bike is a bit rich now.

At idle bike is overrich and really stinks at traffic lights.

At top speed bike runs pretty rich even with #78s.

DUH!
Fuel needs oxygen to burn!  Reduce the available oxygen by half and you get unburned hydrocarbons.  Yes, it stinks!

Top speed aside, where aerodynamics can play a very major role in HP requirements, it has always been that the best proof of demonstrating peak HP was a 1/4 mile standing start drag race.  Please show me somewhere in history that 1/4 mile times were improved by restricting the inlet oxygen intake.  Nitrous, superchargers, turbo chargers, all serve to pack MORE oxygen into the combustion chambers.  This has been a constant since the invention of the internal combustion engine.  Suddenly, and without fanfare way back in the 70s, Honda figured a way to do more with less?  But, only on one singular model configuration in the entire SOHC4 family?  Why was this important break though only offered to the "special" people, who are much superior to the those on the North American continent, Central Americas, South America, Great Britain, African continent, Australia, Japan, China, etc.  Perhaps the answer is that all of those "other" people were just TOOO stupid to appreciate the astounding feat Honda accomplished?

When NASCAR wanted to reduce the top speeds of the racers, so they would fly off the track with less frequency.  What was the new rule?   A calibrated restrictor plate limiting inlet air volume. Why, oh why, didn't they call it a NASCAR airduct?   

Less oxygen = less peak power.  ...Except where Earth's laws of physics are different, like the autobahn, perhaps?  Really, I would have expected the Germans to have figured this out!   There are some really smart and rational people there.  But, it seems they may have all moved on from 1974 tech to more "interesting " things.

/rant

***********************************************
To the OP.  I really wish you good fortune with your experiment.  I'd really like to know what is different inside those 649a carbs.  But, most of us stupid Americans have been deprived of these engineering, epitome of operation, marvels.

[Deltarider]

Sorry I annoyed you. Since I have only 34 years experience with the 500 and the 649A carbs, I was only passing on what I know and don't know.  Sorry for that. But facts are facts..
BTW, your suggestion that the opening of the airfilter is critical at idle as you've done before to others, must be a joke. Try covering your airfilter with a lid when your bike is idling. You'll be amazed how far you can cover it, without the A/F being changed. At idle.

[jamesbekman]

TwoTired im pretty sure every question you raised in your last reply has simply already been answered in this thread...

[lucky]

We share the same problem it sounds. If I figure mine out I will be sure to share with you. I'm going to change my main jet to a 38. See if that helps.

That is the idle jet,not the main jet.