Only Getting 4k RPM
 

I have a 88' 311 with 420's and TRS drives. I bought the boat with 25p mirage 3-blade prop and today with throttles mashed and boat trimmed I only got up to 4000 rpm's. Do I have a motor issue or to much prop. A couple of guys at the lake seem to think I need to drop down to a 23p. I wasn't paying attention to speed cause I didn't have my gps on. There is a buddy of mine that has a 292 with 454 mags and he ran pass me.

IMO the 25's are to much for top speed. I am running 25's on my boat now (same as yours) and I can get higher then 4000 rpm. What are you trimming it to on the indicators? They like a LOT of trim. I think that a 23 is a better all around prop.

I would think you should be around 5000-5200rpm, you'll gain 3-400 rpm with the 23's. Do you have fresh fuel? I take it that both engines are running at 4000? Tabs up, trimmed, you should be doing close to 5000rpm easy if things are right.
BC

I filled the tank in mid July full of 93 octane gas, it was at a meijer gas station:eek: oops. Yesterday I put 40 gallons of 90 octane from the marina ethanol free. My tabs were up and my trim indicators were around 5. Both throttles were wot and both running even at 4k

Are they Mirage Plus or the old style props? How many hours on the 420's ? Are you sure someone didn't replace the engines with 330's ? I hope not, but I'm sure it's been done. The old 420 was a good engine. You should be running close to 70, give or take a few. You are way low on power for sure, unless the tachs are both reading wrong.

There mirage plus and I hope they weren't replaced. I guess I'd better take a look at the serial numbers on the blocks to be sure. Although I did have two mechanics work on the boat and I told them both that they were 420's. I know valve covers can be removed but they say 425.

Try a set of 23 non plus, the plus has more cup and needs another 150rpm in the same size pitch. Worth a try before you get into anything major. My 357 came with 21's, so 23's might be the ticket on the 311.

fixx
 

the mirage plus like lots of trim..get the boat to a fast cruise,trim up the drives until you hear the props cavatate then bump them down just until they bite again then hammer down and see what rpm's you get out of it..

I had an 88 with 420's and I think it came with 23 in 3 blade cleavers. I replaced them with 24 in 4 blade Hydromotive props and it easily turned up 5 grand with lots of trim. Ran 70 or so. It should be easy to tell if you really have 420's.

I think your boat is right on. your in fresh water in a 311 SR1. The 25 will give you a better mid speed and less top end. 23's will give you more RPM and speed. A 4 blade (24) will give you even better mid but is hard on the drive if your jumping waves. My boat with dinged up 23's and 330hp 4400 and 62MPH But 65 in rough water. Small point -- salt water is faster then Fresh

By the way 365hp and the 420hp - has square port heads with Alium. Manifolds. If someone had switched motors and put the Alium. manifold on the peanut heads, your boat wouldn't run for ***

Salt faster than fresh? Guess you don't have tides then...I lose sometimes 5 miles an hour in tidal waters even in slack tide...the post's a year old...wonder how he made out?

"365hp and the 420hp" peanut heads???????

That setup will never turn a 25 plus thats basically a 27 p prop. Try a set of 25 mirage non plus and start from there.

Salt more bouyant than fresh. It is faster

My 311 with mags will turn a 25 pitch Mirage (not Plus) 4900 rpm and 68 Gps. Motors are stock except for Gil manifolds. Speeds are in salt water with slight chop.

exactly what mine turned with the mags with intakes and emi thunder...