Chrysler 75hp main jet question .066 to a .074

[fcross69]

I was recently informed from a old mechanic that the only difference between the 75hp, and 90hp was the carb jetting. I researched the bores and strokes the are all the same. I then looked at timing and advancement along with reeds all the same. The only difference I found was the main jet size is this true? If I go from a .066 main jet to a .074 main jet I then can say I now have a 90hp instead of a 75hp. the year I am asking about is 1975. Has any one done this I know I would have to set fuel mixtures and idles different is there anything else? The carb #'s on that year.

 

[fcross69]

Re: Chrysler 75hp main jet question .066 to a .074

The carb numbers are them same no difference in venturi size and butterflys also and the crank and port is the same also just got a extra 75hp for a spare and was going to tweak it out to see if I could get any more out of it than my original 75hp I have now with a milled head and some porting work that one is running 185lbs of compression on all 3 cyclinders. The one I just picked up is stock with 155lbs of compression but it has a tilt trim unit which my other does not. Any help on 75hp tweaks would be appreciated thanks.

 

[Frank Acampora]

Re: Chrysler 75hp main jet question .066 to a .074



Wrong--Wrong--Wrong! -----Talk to a different old timer.

Yes, bore and stroke is the same with both engines---BUT:

The 75 HP has circular bypass and exhaust ports. The lower unit is a 1.76 to 1 ratio.

The 90 has elongated exhaust ports with about 30% more area and a 2 to1 ratio lower unit. They (the engineers) let the engine rev 500 RPM higher and rate the horsepower at a higher RPM.

Changing jets or carbs will yeild about (0) increase in horsepower. The 75 block is ported to deliver 75! Neither engine delivers a true 75 or 90 Horsepower.

If you mill the head, you will increase compression. You may increase it to the point where you need premium fuel. It is safe to mill the head (or deck the block) .060 inch without causing piston interference.

Porting the 75 block can effectively make it a real 90 BUT: You must elongate the ports downward so the piston does not travel below the bottom of the port opening. THEN, you can either square the ports and streamline the inter-port webs or mill out two webs from each bypass and exhaust.

I currently have an experimental engine waiting for summer. This one was additionally ported therough the bypass side wall and piston skirt. I also added a machined 1/2 inch thick plate tio the exhaust chest to increase volume and decrease backpressure.

In the last photo notice the shape of the original exhaust ports. In the third photo, notice that the exhaust ports are below the top of the piston at bottom dead center. This block started out life as an 85.

And, someone may have switched carbs before you in an effort to gain horsepowe. The 75 usually had three WB carbs with 1 3/16 venturis while the 85 and 90 usually had three WB carbs with 1 5/16 venturis.

And, while you are going whole hog, buy a later junker (after 1879) and swap the block onto the midleg and lower unit from the one piece. The one piece lower unit is by itself about 2-3 MPH faster than the two piece.

 

[Frank Acampora]

Re: Chrysler 75hp main jet question .066 to a .074



Here are a couple of more porting photos.

And: The Mercury non distributor ignition is far superior to anything Chrysler ever had. The block can be modified to use a late 90s Force (Mercury) ignition

 

[fcross69]

Re: Chrysler 75hp main jet question .066 to a .074

Why do the parts number refference as the same when I dbl checked them I run a 115 lower unit on my milled and port 75 now which did pick me 4mphs on the gps or as my buddy calls it the lie detector lol.
Here are numbers block and exhaust numbers all the same for 1975 only
Chrysler 90 HP (1975) POWER HEAD Parts

Chrysler 75 HP (1975) POWER HEAD Parts

Definatley considering the ignition swap I have fun at the sandbars watching everyone asking what is that big white and gold motor you have I enjoy my Chryslers and they always start a conversation on the water.

 

[Frank Acampora]

Re: Chrysler 75hp main jet question .066 to a .074



Pat numbers? Why? Who knows? Example: The 120 and 140 are the exact same block with different porting and cylinder liners (the 140 ports are about 1 1/6 higher than the 120 to account for the longer stroke. The bore is 3.375 vs. 3.3125.). Different part numbers? NO! the only difference you can see is a patch of green or red paint on the bottom of the bottom cylinder. The 120 has a 2.80 stroke. The 140 (and 125) has a 2.875 stroke. Different part numbers? NOPE! Part numbers, casting numbers etc are all the same. The only way to tell is by measuring the crankpin throw. I always wondered how the factory kept track.

I never fussed with a 115 and don't know what the lower unit ratio is. Mark the prop and turn the flywheel 2 turns by hand. If the prop turns about 1 1/4 turns the gear ratio is 1.76 if it only turns one revolution, obviously it is a 2 to 1 ratio.

This boat--the one in the avatar-- did about 35 with a warmed up 75 and a two piece lower unit (first photo). With a ported 90 and a two piece lower it topped at about 42. When I switched to a one piece lower(third photo) top speed was 45. A bone stock 105 with a one piece lower unit (with mermaid, 2nd and last photo)) also drives it to 45.

My experimental engine is slated to go on this boat. If it reaches more than 5500 with the same props, or drives the boat at more than 45, then the porting and exhaust modifications will be a success.

 

[Frank Acampora]

Re: Chrysler 75hp main jet question .066 to a .074



BTW: Here is my 140 and some more cover art.

I also forgot to mention: Carbs are jetted for expected airflow at wide open throttle. The jet LIMITS the amount of fuel delivered ONLT at WOT. Since the limiting factor in horsepower production is the amount of AIR the engine can pump, It is common practice to run slightly rich at WOT. This ensures that all the oxygen available will be used. The power loss from a slightly rich mix is less than from a lean run AND there is less danger of melting pistons. RE-jetting a carb without changing the airflow through the engine will NOT increase power. It will stay the same or drop slightly.

Even though I port my engines, I leave the carb jets alone--however, I use different carbs. My 90 uses carbs off a 100 or 105 four cylinder. These carbs are WB carbs with 1 5/16 venturis, and will bolt to the three cylinder engine. The jets are larger.

I have drilled jets larger with no positive horsepower increases.

Your mechanic was probably confused because if you look at carb charts, as horsepower increases, jet size does also increase. But, a change in jets alone simply will not give more horsepower.

 

[fcross69]

Re: Chrysler 75hp main jet question .066 to a .074

Very nice pics the 115 lower unit is a 2 to 1 and it brought my 75hp to life I can plain a bayliner 1750 in 10secs at 32mph on gps with 4 people with a 13x15 prop and with a 13x17 I can run 37mph on gps with out tilt or trim. But with my head mod and slight porting I am running 5 gallons of 93 octane and 1 gallon of 113turbo blue racing gas with quick silver and every other tank seafoam and she purs like a kitten. Nobody on the Illinois River knows what it is and they all ask. Heres a pic of her on salt water last summer down at gulfshores took my little Bayliner out on the gulf and had a blast. The other is her on the Illinois before Bath Illinois the home of the Redneck fishing tournament. She only weighs 1040lbs though so that helps. I would like to see 40mph and able to turn my 13x19 with 2 people. Maybe after tilt and trim setup can I sqeeze those extra Mphs but ready to keep tweaking for every last bit of horsepower.

 

[Frank Acampora]

Re: Chrysler 75hp main jet question .066 to a .074



YES! In the past I have posted that simply changing from a 1.76 to a 2 to1 ratio will give 5-8 MPH increases. Your 75 is doing quite repectable but it is costing you in fuel. I'm not sure, but I think I was editing when you posted so you might want to re-read my last post.

Here are a couple of photos of my little 15 foot Glastron on Barnegat Bay. The weather turned nasty, it was raining sideways, and the birds were walking. Waves were kicking up but she handled them well.

 

[fcross69]

Re: Chrysler 75hp main jet question .066 to a .074

I am averaging about 4mpg with 17pitch and around 2 with 15 pitch but I dont keep her too much in the fuel saver mode. I run about half way between fuel saver and WOT keeps me at around 26 to 27mph on gps. I never leave shore with out my girl lol.

 

[fcross69]

Re: Chrysler 75hp main jet question .066 to a .074

Like the Glastron I been looking for a old checkmate or a old seaspirit for a 120 I have on the stand for more of a play toy without the wife. I like the new boats but I love the looks of the old stuff better pre 1970.