Three hole vs 4 hole carb?

[fastE]

If been racking my brain last few months trying to make 1800 run right on both engines, long story short one set of carbs has four holes in the throat and the other has three. The one with three runs great when set to speck for my 97 Challenger 1800 787 so I’d say it is what came on the boat. The one with 4 holes keeps running out of fuel it seems. Is there a way to set up the 4 hole carb to run the same?

 

[mikidymac]

Can you post a picture? Not sure what you are referring to.

 

[fastE]

I get in trouble posting pictures with tap a talk but I will try when I get home. But one set of carbs has two little bitty holes and one pencil lead size hole where the butterfly rests closed and the other set has three little bitty‘s and one pencils lead size.
The one with 4 holes seam to want a lower pop off pressure just not sure if the jetting would be the same too

 

[fastE]

After more reading I may have one set that’s BN-40 and one is 38. I had measured the throAt the same but I guess the difference is farther up the throat

 

[snikwad003]

The 40s are the right ones

 

[mikidymac]

Same engines need to have same carbs. If you are running different carbs than the engine came with you are going to have tuning issues and/or blow up the engine.
All 787 use the same carbs with the exception of the newer ones got accelerator pumps.

 

[fastE]

They are both 40’s with pumps but there not the same

 

[fastE]

One set carbs that came on the boat was beyond repair. I have since ordered two set of carbs said to come off 787 out of ski’s. The first set did not have the accelerator pump so I bought the second set

 

[mikidymac]

I wouldn’t run two different sets of carbs on the same boat. The ones with the accelerator pumps will have to be jetted differently if you run them with or without the pump. And the other engine will be jetted even differently.
You are asking for even more problems if they were off a 720.

 

[fastE]

I took carbs back off, and there both the same after all so back to the drawing board. I swapped sides to see if the trouble follows the carbs or stays with the engine

 

[mikidymac]

They are not the same if one has accelerator pumps and one doesn’t.

 

[fastE]

I wouldn’t run two different sets of carbs on the same boat. The ones with the accelerator pumps will have to be jetted differently if you run them with or without the pump. And the other engine will be jetted even differently.
You are asking for even more problems if they were off a 720.

There are no 720’s in the equation? All carbs were off 787 engines. After getting home and examining things closer. Both sets are the same with 3 holes. I think the one with 4 holes came from the set that didn’t have a accelerator pump.

 

[mikidymac]

I was just mentioning that because some places bored the 720 carbs for the 787 to get better fuel economy with minor performance loss. The 720 carbs will bolt up.