Topic: Bike Running Uber Rich

[DHuffman]

Hey guys,
I finally made some throttle cables and got to take the old girl for a quick spin this evening.


However, she appears to be running very rich, to the point where my friends say it makes their eyes burn if the follow me too close on the highway. She smokes up the garage pretty good (smell anyway - you cant actually see the smoke so much), and guzzles the fuel.

The bike is a '77 CB750F, with PD42A carbs (off a 78K?).  Before installing the carbs, I gave them a good cleaning, did a good inspection of the jets (which are all stock), and replaced the gaskets, float needles, and floats. Float height and idle mixture screws are set per the manual. Im running the stock air box with a new filter. Last time I pulled the plugs they were a light tan color.

IIRC, these models have non-adjustable needle heights? What are my options for leaning up the mixture? I'd seen another post where people said that cheap air filters can restrict air flow - Im using a $13 EMGO filter from ebay https://www.ebay.com/itm/Emgo-Air-Filter-Honda-CB750K-750-Four-CB750A-Hondamatic-CB750F-750-Super-Sport/312565280108?epid=171269169&hash=item48c657ad6c:g:kgUAAOSwM6Vcq3eU.

Anyone have any suggestions of what to try?

[minimo]

Get new friends.

;-)


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[ballinchestr]

Light tan plugs sound pretty good to me, could you be slowly leaking fuel from somewhere?

[DHuffman]

distinct possibility, but the only oil leak ive noticed is around the drain plug under the bike... yea the tan plugs is somewhat counterintuitive for richness, but I guess the plugs only show me what the Air-Fuel ratio is when the bike last ran (idling in my garage). Guess it could be running rich at open throttle with the needle...

[Bankerdanny]

The 550 and 750 PD bikes were set from the factory to run leaner than prior bikes as a reaction to where Honda thought emissions regs were going. Throw in cheap pods and I can't see how the bike would be running rich.

[Bodi]

Try without the air filter. A too-restrictive air filter can cause rich mix, vacuum in the carb mouths will increase the fuel being sucked up..
Did you clean the carb main jet emulsion tubes? Those tiny cross holes mix air with the fuel, if they get blocked then you get all fuel and no air = too rich mixture.

[DHuffman]

Try without the air filter. A too-restrictive air filter can cause rich mix, vacuum in the carb mouths will increase the fuel being sucked up..
Did you clean the carb main jet emulsion tubes? Those tiny cross holes mix air with the fuel, if they get blocked then you get all fuel and no air = too rich mixture.

You mean the pressed in jet with the small holes on the sides? Yes I cleaned them. I may try pods at some point. I guess there really isnt a downside to running rich other than fuel economy right? Better rich than lean with an air cooled engine, right?

[tlbranth]

When I first put my 750 back together after a 25 year sit, I put in an airfilter from PartsNmore. It ran very rich. Tried different things but eventually figured out I had an Emgo air filter. Chucked it for OEM and things improved a lot. My needles are adjustable and I eventually lowered them a notch to further improve things. Try a quick test run without the air cleaner. I bet it'll improve.

[DHuffman]

When I first put my 750 back together after a 25 year sit, I put in an airfilter from PartsNmore. It ran very rich. Tried different things but eventually figured out I had an Emgo air filter. Chucked it for OEM and things improved a lot. My needles are adjustable and I eventually lowered them a notch to further improve things. Try a quick test run without the air cleaner. I bet it'll improve.

Try without the air filter. A too-restrictive air filter can cause rich mix, vacuum in the carb mouths will increase the fuel being sucked up..
Did you clean the carb main jet emulsion tubes? Those tiny cross holes mix air with the fuel, if they get blocked then you get all fuel and no air = too rich mixture.

Could restricted air flow from a cheap filter also result in the bike being more cold-blooded? I have to set the idle screw up a bit when I start it until it gets warmed up or it just goes to die when I give it throttle. Full choke and faster idle for a few minutes, then its good to go.

[Bankerdanny]

Could restricted air flow from a cheap filter also result in the bike being more cold-blooded? I have to set the idle screw up a bit when I start it until it gets warmed up or it just goes to die when I give it throttle. Full choke and faster idle for a few minutes, then its good to go.

No, the bike wants to be rich during a cold start. The PD series carbs already have a cam that advances the idle when the choke is in use, so you shouldn't need to advance it further with the idle speed screw.

When it is cold do you need full choke to start it?

[DHuffman]

Could restricted air flow from a cheap filter also result in the bike being more cold-blooded? I have to set the idle screw up a bit when I start it until it gets warmed up or it just goes to die when I give it throttle. Full choke and faster idle for a few minutes, then its good to go.

No, the bike wants to be rich during a cold start. The PD series carbs already have a cam that advances the idle when the choke is in use, so you shouldn't need to advance it further with the idle speed screw.

When it is cold do you need full choke to start it?

Yes. If I don't choke it I have to mend the throttle to keep it running at a higher RPM. Needing choke suggests lean, right? Maybe play with my idle mixture screws and swap an air filter for richness in higher RPMs? The whole thing is pretty contradictory to me... also I really haven't ran it much, so my impressions may be off.