Looks like I need a carb tutorial

[jim j geezer]

I have a '59ish 7.5 Scott McCulloch as a parts motor. The lower unit is shot, but the top end seems okay. Got to thinking of using it for... something. It was missing the recoil assembly, so ordered one off ebay. Happened to see an ad for a carb off a 12 / 14.1 HP Scott, and rolled the dice. The carb arrived today. Looked it over and compared it with the original. It'll bolt right up, and the butterfly linkages match.

The original is a Carter, model "N".
The 'new' one is a Tillotson.

My current problem is "carb theory".

The original has an inlet to the venturi that's 1-1/16".
The 'new' one has an inlet to the venturi that's 7/8". <--noticeably smaller


!?!?!?!?!?

Both outlets from the venturi are the same size @ 13/16"

I've been under the impression that a carb rated at a higher HP will have a larger inlet - to more compress and accelerate the airflow.

Given that the hi-speed jet might be larger on the higher HP carb, am I missing anything else?
My head hurts...

 

[Vw56German]

Re: Looks like I need a carb tutorial

Fun puzzle you have there. I think some of your answer may lie in the venturi area itself. I cant see completely in the first pic, but it looks like the original carb has a much smaller venturi area than the replacement. so actual air flow will be determined by the smallest part of the carburator's opening. I am thinking the new carb may flow too much on the high end and cause you to run rich, and possibly not flow enough on the low end because the motor can not pull air fast enough in the larger venturi. Buy hey, this whole thing is an experiment. The world will not end if you try it and see what happens. Keep us posted.

 

[mrcrabs]

Re: Looks like I need a carb tutorial

I tend to agree with Vw56, and of course we know many detuned 15hp engines start off as 7.5's or 9hp.
I would be checking out bore diameters on what you have and the 12-14hp Scott just to compare, also the exhaust porting is important and is a major part in a true conversion for the J/E 9.9 to 15hp.

I'm betting it runs good

 

[jim j geezer]

Re: Looks like I need a carb tutorial

Well this's a kick in the pants.
*TRY* to find any pictures of a 12HP or 14.1HP McCulloch. I goofed and said it was a Scott. I know McCulloch made a 14HP "Mule", for the logging trade, I believe.

 

[mrcrabs]

Re: Looks like I need a carb tutorial

Any particular part or view of the 14.1 McCulloch Jim?
https://www.google.com/search?q=14....e&cd=2&sqi=2&ved=0CE8Q_AUoAQ&biw=1088&bih=489

 

[jim j geezer]

Re: Looks like I need a carb tutorial

Showoff!

Now how about specs of the 7.5 -vs- 14.1, bore/stroke/heads/compression/piston types?

I'll check back in, oh.... 6 or 7 minutes or so...

 

[mrcrabs]

Re: Looks like I need a carb tutorial

Oh gee I lost that paper...lol