Briggs 3.75hp flathead runs great!

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chazdaddy

New Member
#1
#1
I have 3.75hp flathead, got off small tiller. Wasn't running, so took it apart, cleaned up carb, lashed valves, etc and now runs very well. Actually, this thing runs really well, will pull almost 40mph, gets up to approx 30mph quick.

When I had it apart, I noticed the cam lobes (plastic cam) are flat with a lot of duration. Engine doesn't rev out like I thought it would, but has great middle range torque/ hp. I want to get a 3.5hp and/ or 4hp to compare. I bet this engine is way better than 3.5hp.

There doesn't appear to be a lot of these cheap. Most are on edgers, and sellers on CL are asking a lot for edgers in my area (seems like I can get a 5hp tiller as cheap or cheaper).

Does anyone else have a 3.75hp Briggs that runs like this? :thumbsup:


 
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bowtiebutler

Member
#2
#2
I don't have any 3.75 flat heads. All mine are 5hp, but I had a 5 that ran really well, and when I tore it apart to modify, I found a plastic cam in it as well. I was surprised to see this, but apparently they work well. It now has a Dyno 98-3 cam in it, but that plastic cam did run well.

Matt
 
Outlaw Ace

Outlaw Ace

#3
#3
A 3.75hp is a 3.5hp with an honest HP rating, all my 3.5hp engines run way harder that the 3hp engines. Unless you bypassed the governor you are still spinning only 3600 revs but the torque it provides it does get to speed quicker. The 3.5hp briggs run much harder than a 3hp, I have one with a 3 and one with a 3.5 neither one governed and they will run 35 or so with the 11/70 gearing.
 
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chazdaddy

New Member
#4
#4
Hello Outlaw Ace,

I am looking for 3.5hp so I can compare. I was really impressed with the 3.75hp, so if the 3.5hp is the same, very cool. Runs pretty close to the stock 5hp and smaller/ lighter. :thumbsup:

I would have to take the engine apart to take a pic of the cam, but is definitely flat at the top of the cam lob. Never seen a cam flat like it before. Will be interesting to compare 3.5hp cam to it. :shrug:
 
Outlaw Ace

Outlaw Ace

#5
#5
chazdaddy said:
Hello Outlaw Ace,

I am looking for 3.5hp so I can compare. I was really impressed with the 3.75hp, so if the 3.5hp is the same, very cool. Runs pretty close to the stock 5hp and smaller/ lighter. :thumbsup:

I would have to take the engine apart to take a pic of the cam, but is definitely flat at the top of the cam lob. Never seen a cam flat like it before. Will be interesting to compare 3.5hp cam to it. :shrug:
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I would sure be curious to know if there is a difference, a 5hp will not fit all frames and any additional edge with a smaller engine would be a plus. I have never torn one down but I do remember a lawsuit against B&S concerning actual HP ratings and on Craftsman stuff I saw 3.75hp and 5.5hp engines and thought it was weird but attributed it to the lawsuit. If there is an fact a difference in the cam then that and the aluminum flywheel from a 3.5hp briggs generator would possibly give me that edge without port work or shaving the head. I had a 5hp on one of my DBs but had to run it without the rear fender, sure would be nice to have the extra power from a smaller package.:thumbsup:
 
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chazdaddy

New Member
#6
#6
I have noticed, not all 3.5hp Briggs are 148cc, some of the 3.5hp edger engines are 127cc. That's the key, make sure to get one with the same bore as 5hp. I do remember measuring the piston on my 3.75hp and it's same as 5hp, so should be 148cc. :thumbsup:
 
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BCOWANWHEELS

New Member
#9
#9
never in my days did i ever even think i,d see a plastic lobed cam...... i bet hi-revs will hurt it.. also nice bike, clean and simple. i have 2 - 3.5 briggs engines from 1959, both need carbs........ because gas went bad in them
 
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