Slanty, 904 issue

[TwistedPenta]

Ive searched the forum and cant find anything. I have a 71 Duster. Original 225 and i believe original 904 and 7 1/4. I'm not sure on the rear end gearing. But i can barely get to 70mph on the highway. The engine is running hard as it can. No tach so i don't know the rpms. Ive used a gear calculator to check different rear-end combos from factory and none seem to be fitting that kind of highway speed. Im guessing the slant won turn over 4000-4500. Tires are 215/70 14.
I need better highway speed without setting off a grenade in the engine. Does that speed soud normal? Any input or suggestions?

 

[player1up]

Well with 2.76 gears you should be at 70 at about 2500.
I think optional gears were 3.23, that would put you at 71 at 3000.

Grab a sunpro mini tach and see what it's really turning.


I've had a 1bbl slant pegged at 120 and it sounded like the engine was seconds away from letting go.
The slant has enough torque to keep the shift points really low most of the time so when you get it up to +3k it's a little disturbing.
You could swap an 833-OD in there and it would help.

 

[fishy68]

Slant's are real long stroke lower rpm torque engines but unless someone has changed the gears to something real low something is wrong and I highly doubt it's a transmission issue. Most likely running out of fuel at high speed (weak fuel pump or plugged fuel filter, dirty carb, etc..). Or it's possibly misfiring at high speeds. The engine may sound like it's running hard but that doesn't mean it's actually spinning a lot of rpm. That's where you need a tach to see what's actually happening

 

[TwistedPenta]

I believe thats what im gonna do. Just try to find a used tach and prolly yank the diff cover off and count teeth. Also i do know that my factory speedometer is way off.