Topic: 78 tuning question

[ClewisEMFD]

Alright, so I have gotten much done here with the wisdom you have all provided me. I am here to summons the greatness the is the SOHC forum.

So I have a 550 motor with PD46c carbs. I have a 98 main and a stock 42 pilot. I have velocity stacks and a mac 4 into 1 exhaust....Still with me here? all the rubber bits have been replaced as far as I can tell

I get this kind of hesitation around 5000 to 6500 RPM. Its not like a misfire but it definitely sounds as if the motor is running rough. Then I WOT and it goes away and runs fine again. I understand that tuning is about throttle position so I would venture a guess and say it is at a lil over 1/4 to about 2/3 throttle. Carbs have been vacuum syncd and the valves an timing are set too. I pulled my plugs and 1/3 are dark brown and 2/4 are a lil bit more sooty. (I know there is a chart that gives a picture of where in the throttle adjustments work but I cant find it...)

So am I in needle adjustment area? or do I need to size my main jet down? I am thinking  am too rich in the middle of the throttle which is causing these problems and a needle adjustment may cure these problems. Any other suggestions before I tear into these carbs AGAIN or am I on the right track?

[TwoTired]

There is no normal reason for the plug to show different deposits among the spark plugs.

Before you fine tune, find out why the spark plug deposits are different.  Hint, each carb delivers air/fuel to one and only one cylinder.  What is different among each carb's set up?  Equally clean?  Any orifice damage?


Do the velocity stacks you have provide a smooth inner wall transition between stack and carb throat?

[ClewisEMFD]

Thats an interesting thought....Although it involves way too much work so SUGGESTION DENIED. lol jusssst kidding. I have had them all apart and I didnt see anything that was very obvious...Although I did find a little sctrch around one of the big tubes that encases the needle. would that lead to a problem? any suggestions what to look for....Could it be my ignition possibly? carbs are synced. Or am I looking for a lill hole that is stealing some extra fuel while I am not looking....

[Vinhead1957]

Sounds like the accelerator pump is not working for one.  The differences in plugs point to rich 2/3  but if it is a weak spark look to the yellow wire connected point set.

[ClewisEMFD]

these particular carbs are non accelerator pumped

[TwoTired]

I have had them all apart and I didnt see anything that was very obvious...

I think you answered your own question right there. 

Could it be my ignition possibly?

Ignition drives 2.3 and 1.4 respectively.  Since you claim signs are on 1.3 and 2.4 boundaries it seems unlikely unless there is some sort of polarity issue...even then...    Um. Are you using iridium spark plugs?

...any suggestions what to look for....  Or am I looking for a lill hole that is stealing some extra fuel while I am not looking....

Have we covered equal compression, cam, and valve action among cylinders?

Carbs have fuel metering orifices and passages as well as air metering orifices and passages.  Seems a safe bet that some thing isn't equal among the 4 carbs.

I suppose I should mention that, many SOHC4s have carbs that see a common pressure from the air box.  V stacks allow each carb to experience a different pressure at each carb mouth, which might occur as the air swirls around the engine.  If the carbs get consistently different pressures among them, the could explain the different plug deposits.   I've never been mean enough to my SOHC4s to subject them to V stacks.  But, I've read reports of leg position and side winds effecting engine operation, when the inlet air plenum was removed.