CB500 runs lean and doesn't want to pull through

[RBoss]

Hi All,

Recently I have rebuilt my engine by putting in a new primary and cam chain. After reinstalling it into my frame I went for a test drive. All went pretty smooth, the engine is nice and quiet, but it doesn't want to "pull through" at higher rpm. It holds power in when I want go above around 6/7000rpm. Spark plugs are also very white so it appears to be running lean. How I don't know... Before the engine rebuild I rebuilt the carbs and had them cleaned.

My setup:
- jet size 100
- stock airfilter
- airscrews 1 full turn outward
- electronic ignition
- new rubber inlets
- new spark plugs NGK D7EA
- 4 into 1 exhaust

Anyone have an idea what could be it?

[K2-K6]

If you need to keep running it like that initially,  then change the plugs to #8 as it'll give you more safety margin. This won't fix anything but with the plug tips getting very hot (white colour on #7) then it risks making pre-ignition which will lead to melted pistons if you're not careful.

Do you know the needle position and what size the idle jets are ?

[Oddjob]

Did you remove the inlet manifolds? always replace the o-rings in them if you did as they grow hard and won't reseal after removal. It's the heat cycle which does for them. I'd imagine that the engine is sucking air in from somewhere causing the weak mixture.

[RBoss]

If you need to keep running it like that initially,  then change the plugs to #8 as it'll give you more safety margin. This won't fix anything but with the plug tips getting very hot (white colour on #7) then it risks making pre-ignition which will lead to melted pistons if you're not careful.

Do you know the needle position and what size the idle jets are ?

Yes needle position is in the middle. Idle screw I don’t know for sure but I figure that is stock either.

Did you remove the inlet manifolds? always replace the o-rings in them if you did as they grow hard and won't reseal after removal. It's the heat cycle which does for them. I'd imagine that the engine is sucking air in from somewhere causing the weak mixture.

I did replace all those o-rings… really can’t think of a place where it sucks extra air from… the air mixture screws should not be twisted in any further right?

[Oddjob]

Check the carb sync screws on the inlet mainfolds to make sure they aren't loose. You could try the carb cleaner trick , run the engine and spray around the carbs to see if the revs increase, if it does, you've got an air leak. Any splits in the manifold rubbers?. Did you remove the cylinder head?

[murdock]

which carburettors do you have ?

are the carburettors statically synchronised?

 do not screw in the throttle any further if the engine has not been softened.

[Johnwebley]

Check the ignition timing, it could be over advanced.
If you can, use a strobe at 4,500rpm

Sent from my SM-A750FN using Tapatalk

[deltarider]

When you rev and snap close the throttle, does RPM quickly return to idle* or is it with some hesitation**? Are your jets etc. genuine Keihin or aftermarket?
*   If it returns quickly, which is good, I'd verify fuel supply is sufficient riding. Any 'extra' inline fuel filters that can impede fuel flow? You may want to read this: http://forums.sohc4.net/index.php/topic,187633.msg2176352.html#msg2176352
** Verify the operation of advancer is smooth. If operation is smooth (good) I'd check for air leaks.
When you are in doubt of airscrews position, I may be able to help; where about in A'dam are you?
Can you make it to my garage near the Weteringcircuit?

[RBoss]

Thanks for all the replies!

I have the feeling it is the ignition timing. I rebuilt the engine without adjusting the timing again and I think that should be done, I did put the plate in the same position but maybe something is off.  I have the feeling that it can’t be the carbs as I had them rebuilt before I rebuilt the engine and I tested them on the bike before that and all went well.

I may be able to help; where about in A'dam are you?
Can you make it to my garage near the Weteringcircuit?

@deltarider, very nice offer! The bike is currently in Driebergen and I’ll try to fix the ignition there. Might that not work, I live close to the Oosterpark so the Weteringscircuit (schans?) is very close, I will keep you posted on the ignition timing!

Ruben

[Trigger]

All the symptoms you have pointed out, point to timing but, i would check the cam chain is not one tooth out first before, you go on a wild goose chase with electrics   

[Sesman]

I’m a bit confused by this one and I’m watching and learning. I thought Erny began this thread stating he had a very obvious and extremely lean burn? This may have precipitated his subsequent query about appropriate air boxes/filters. Purely out of interest, would a single tooth cam timing error create this lean mixture situation?

Sounds like poor Erny will have to start a sequential diagnostic programme…..cam timing….ignition timing and advance…valve settings….fuel supply. This will then only leave the carbs and air system?

Just wondering…

[deltarider]

Ignition always comes first in diagnosing. No matter what ignition you have, concentrate on a correct timing at full advance. I like mine to be just a tad more advanced than the outer mark. Then just accept the timing @ idle it will result in and live with it. Although I recommend using a strobe timing light, I myself don't do the actual adjusting dynamically. I set and then check using the timing light. If I need to readust, I first kill the engine, do an extra adjustment and check again. If necessary, I repeat this. This works best.
The position of the airscrew you have now, is a good starting point. A genuine Keihin one will look like shown below.
You're welcome anytime. I'm interested to see yours, as a 'K(0) 1975' is a rarity.

[Oddjob]

I've been talking to Mark Paris (Hondaman) a lot lately. Did you know BTW he's writing a book on the 500/550, the same type as he did for the 750. Anyway, he recommends removing the advance/retard springs and cutting a half turn off the end of them both, reshape to match original curl and refit. The stronger spring evidently changes the engine from its characteristic "switch ON, Switch OFF" torque sensation at 5000+ RPM to one of more linear response with more torque on hand all the time.

Off topic I know but very useful information which is sort of on topic in a way.

I'd second checking the valve timing before anything else, if it's one tooth out there is a great potential there for major damage. Might be a waste of time checking BUT it's worth it for peace of mind, if of course the engine ran just fine then you've clearly got it right but that's not the case here.

[Sesman]

I’d love to see a physical representation of the spring modification. Any volunteers?

[deltarider]

I've been talking to Mark Paris (Hondaman) a lot lately. Did you know BTW he's writing a book on the 500/550, the same type as he did for the 750. Anyway, he recommends removing the advance/retard springs and cutting a half turn off the end of them both, reshape to match original curl and refit. The stronger spring evidently changes the engine from its characteristic "switch ON, Switch OFF" torque sensation at 5000+ RPM to one of more linear response with more torque on hand all the time.

What makes you think this will be benificial? So far I haven't read one positive result, not one. One was frank enough to say he has experienced a negative effect... And I have followed it closely. Don't you think Honda was able - in a highly competive market - to design the best overall advance? It was never even clarified what advancer this was about: the Hitachi or the TEC? When you have sloppy springs, it may affect the idle somewhat. Then you can either tweak (no need to cut) the end of the spring or order new ones, about € 12,- a pair.