WOT, Prop, and Sluggy Embarassment

[Noobguy]

Hey All,

So, with the wonderful help of archived iboats posts, I took a beat to hell 78 renken bowrider with an mcm 120/mr drive and got her running. PO must have been an idiot (it must have been contagious when I decided to buy it...).

Anyway, I'm experiencing a few of odd things that I couldn't find specific answers for in the archives.

1) WOT has pretty low rpms. Comparing audibly to no load RPM I can get on muffs, it's nowhere near the ~4500 I'm looking for. Don't have a tach on the water other than the gauge, and it's pretty terrible with a 1000 rpm bounce, but averaging the tach readings I'm sitting at about 3200 (recommendations on a handheld tach?). I can hit 35 mph on glass by myself (according to GPS) but acceleration 60 seconds at least to get up on plane. If I add 3 kids and my lady (approx ~700 pounds total above my 220# fat ass) I can't plane at all. I added trim tabs, but those things don't seem to have any effect at all, positive or negative. I don't think I'm getting the up front torque to have them matter either way.

2) If I cup my hand over the carburetor air intake, I can actually get RPMs to increase, so long as I don't cover the entire thing. That was weird to me. Maybe a vacuum leak, but I don't know where to check other than the line from the block to the carb choke. Also, I can eek out a little more RPM when my shift box is at 90% of WOT, but it drops 50 maybe 250 rpm at full WOT. No noticeable change in GPS MPH though.

3) I've actually had my little bro up on a wakeboard (170# guy) but he had a really hard time until it planed out (maybe 75 yards). Poor kit had bruises on his triceps.

4) I have no idea what pitch prop I have. I know it's a 3 blade prop, but I'm looking to get back to basics. Does anybody have good experience with a towing prop and a cruising prop for this engine/drive? I'm not too concerned with top speed, as we only fish and wakeboard. Utah reservoirs don't really need top end mph.

5) I tricked myself by towing it to 9000 feet. Damn thing sputtered and hissed and ran like it was anchored to the dock. Don't intend on going that high often; looking between 4500 and 6200 feet. Wish it was fuel injection...

What I've done so far: compression test was a little low at ~100 psi, but none of the 4 cylinders were outliers (100 to 104). Rebuilt carb, replaced plugs, gapped dissy with a new head, aligned engine, found and fixed a leak in my outdrive (PO forgot the seal between lower and upper......:facepalm, new impeller, battery, starter solenoid, rewired dash, checked for a spun prop, experimented with various trim settings (once on plane it's pretty easy to groove into top speed). I also checked that the shift box is opening the carb all the way by removing the cable connector and opening the throttle by hand at speed (girl was driving). No audible change in RPM, no change in speed, seemed like cable is doing it's job.

Sorry for the long post, but want to provide all the details. The engine just feels like she's not turning like she should. Any ideas?

 

[alldodge]

:welcome: to iboats

With only 100psi on the cylinders, my guess is your going to need a rebuild. That engine appears to be tired. Cupping the carb (which adds more fuel) helps, and you may need a carb rebuild, but the engine is just tired from my perspective

 

[Noobguy]

Well, it's almost 40 years old. I'm thinking my best bet is to eek out what life is left. Took my prop off and found it's a 19" pitch. Seems high for 120 hp (and they're tired ponies, at that).

Picked up a 17" aluminum prop to test, and will rebuild the carb again, but I think I'll sell her as a fishing boat and upgrade. I'll know what to look for this time.

Anything else you can recommend to increase RPM short of a 12" prop?

 

[alldodge]

Anything else you can recommend to increase RPM short of a 12" prop?

If you do a leak down test it will determine if its the rings or valves, but if its just a tired engine there isn't much so long as the ignition components are in good shape.

 

[Bondo]

Anything else you can recommend to increase RPM short of a 12" prop?

Ayuh,.... Welcome Aboard,...... I think you'll find the smallest available prop for yer drive is a 15" pitch,......

Yer motor would fall into the "Rebuildable Core Motor" class, at 100 psi per cylinder,....

 

[Noobguy]

Think it'd be worth doing on a small engine? I can pull the engine and take it to a machine shop, but I'm scared to spend the money for marginal gains.

 

[NHGuy]

That cupped over the carb gain has me thinking you could check for a vacuum leak. While it's running spray some carb cleaner at the base of the carb, at the intake manifold gasket, and at the carb shaft entries. By shaft entries I mean the ends of the throttle shaft and the ends of the choke shaft.
The carb base and intake seals are easy fixes.
The carb shafts are less common on boats than cars because boats aren't constantly changing speed like a car.
Shaft seals are a pain in the behind to repair but can be done by making a bushing with super thin sheets of nylon or delrin. You'd have to mic the shaft and mic the hole and wrap it with sheet stock that's a hair under half the thickness, because it surrounds the shaft on both sides of the diameter.
There are kits for popular carburetors, but I don't think that one qualifies as a common rebuilder item. OTOH if its just that and a carb isn't too much $ you could buy one and be ready quicker, just watch out for the same issue if it's rebuilt.

 

[NHGuy]

100 is pretty low compression. I agree. When is it time to hang it up? I suppose you could get a few psi by running 20 w 50 motor oil.

 

[Noobguy]

Carb is the original Rochester 2 barrel. Pretty simple carb, but have thought about replacing it anyway. It doesn't come back to idle properly unless I manually pull the linkage closed. Almost like the fast idle screw plate is bent.

I can run 20w50 easily enough, and I'll check the carb manifold gaskets. I thought vacuum leak, as well. I'm not sure I'm prepared to spend that much time on rebuilding the carb, though. Probably easier to get a rebuilt one, or possibly take it to a pro.

 

[Noobguy]

So I spoke with a local machine shop. They have pretty reasonable prices for a rebore, but I've never done this before. If I haul the engine out and drive it over to em, can I expect a typical engine machine shop to bore, replace valves/rings/pistons as required, or am I looking for multiple vendors to help me with this? The shop said they'd get me sorted, but I want to hear others' experience before I get the cherry picker out.

 

[Scott Danforth]

I myself would look for a boat with a good running 3.0 from 1990 or earlier and swap that in place of the 2.5 liter you have there now. a used low hour motor from a neglected hull would be less money than a rebuild.

however to answer your question, a good machine shop will give you the option of you pulling the motor apart, and re-assembling or them doing the labor. However most like to do the tear down to investigate what needs to be done and what may have gone awry.

most machine shops have a similar charge for your 4-cylinder, however check yours.
I would assume that the breakdown would be similar to the following:
tear down and inspection ~$100-$150
Hot tank/oven bake ~$75-$100
Valve job and head surface ~$100-$150
Block bore and hone ~$250
New pistons, rings, bearings, gaskets ~$500-$600
Cleaning and assembly ~$100-$200
incidentals/shop supplies ~$50

On top of the list, there are the options such as painting, running the engine, etc.


you definatly have a fuel system issue if cupping your hand (basically choking the engine) brought up your RPM's I would suspect a vacuum leak as well. The low compression could be from a variety of items, and I agree that a leak down test is required.

 

[ttankmoran]

At what elevation are you testing compression? If between 4500-6200 ft your 100-104 Psi is equivalent to about 115-120 PSI at sea level. Considering you can get 35 mph out of a 2.5 at 4500-6200 ft, your engine is NOT tired in my opinion.

I would start by having you carburetor gone through, rebuilt, and rejetted for the elevation you typically run your boat. A 19pitch is a pretty big wheel for a 2.5 to turn at sea level, you will wake this thing up considerably by propping it correctly for your elevation. If you have the 1.94 gear ratio, and you can run 35 mph GPS at 10 percent slip thats about 4200 RPM

Also, a 2.5 is not a power house at sea level, let alone 4500-6200 ft.

 

[Noobguy]

Sorry for the late reply; been busy with work and just bought a 3/4 ton pickup, so haven't been boatthinking much. Appreciate the response. I'm at about 4525 feet in my shop according to my Android - nifty altimeter app.

Dropped it in with my new 17" pitch prop (larger diameter from 14" to 15" but also bigger ears) today and holy ****e did it jump out of the hole by comparison. Maybe 3 seconds to get up, 15 to max speed. Even kept it planed going through hard turns; it's NEVER done that. Girlfriend felt that lovely drift over the turn instead of plowing the bow through it, and asked if that was normal lol. Was shocked at the improvement, even in 20mph gusty chop. We were less a 200lb guy than usual, but even still, it's never performed like that even with just me in it.

I did lose about 5 mph top end, but it's tough to compare since last time I was on glass. Reservoir is at about 5200 feet, but forgot my damn tachometer. Felt like about ~4000 rpm (super accurate ears, yo), and I found my trim's sweet spot for cruising based on rpm/speed. What would you say to a 15" pitch for towing? I'm gonna trade my ridiculous 19" stainless, and can get a credit on a new aluminum prop.

Also, I found the vacuum leak before trying the new prop. It was a stripped brass fitting under the choke intake. Replaced that and used some thread locker; cupping the carb has no effect either way now.

I also forgot to mention my original leakdown test last year was about a 15% loss, from ~100 psi to ~ 85, but I'm not sure I did it right.

 

[Noobguy]

BTW, you guys are all heroes. I went from knowing a little less than nothing to feeling like I almost know a few things. This forum has hands down the most helpful and friendly folks I've met. Thanks so much.

 

[alldodge]

I found the vacuum leak before trying the new prop.

Before you swap the prop out maybe do a good comparison with old. Good to hear your running

 

[Noobguy]

Oh man, compared to the old 19" the 17" is absolutely amazing. I'm thinking about getting an additional prop though, a 15" strictly for towing. I think my fear would be high rpms and possibly cavitation, yeah?

 

[Noobguy]

Oh, I did. Found the leak, took the boat out, no improvement with 19". 17" was great though. Wondering if a 15" will help even more with towing. Losing top end isn't a big deal to me; we run utah reservoirs, so 50 mph just isn't necessary

 

[fishrdan]

You'll need to get your tach fixed to properly select another prop, as you want the engine to run at the top of it's WOT RPM range (4200-4600???) for best performance. Doesn't need to be expensive, just accurate, I used to run an old (free) Sun automotive tach in my old boat and never had a lick of problems with it, even soaked the thing a bunch of times... LOL

RPM permitting - - - If you liked the 17P 3-balde, try out a 15P 4-blade or a 16P 4-blade. The16P 4-blade should run close to the RPM of your 17P 3-blade and the 15P 4-blade should add another 200 RPM or so... The 4-blade prop should get the stern out of the water and on plane quick, without pointing the bow to the sky. I say that with a grain of salt as at my 18P Alpha4 prop is a hole-shot monster, level and up on plane quickly,,, while my rebuilt 16P 4-blade Michigan prop has a LOT of bow rise. Could be a bad repair, but I wouldn't recommend a Michigan prop.

IF you are going to be boating around 5000' all the time, you can (should?) re-jet your carb leaner for the high elevation, just be aware that if you drop in elevation the jets MUST be put back to the rich jets. I usually boat at 1000', but if going to UT and running at 5000-8800', I'll re-jet and re-prop, about an hour of work. One setup for 5000' and another for 8800'. If lake hopping up there, I jet for 5000' and deal with the lack luster performance at the higher lakes.

 

[Noobguy]

Replaced ALL the gauges with a new Faria set. Tach works like a dream now. I'm at about 4600 rpm WOT with 17" prop at about 30mph. Idles kinda high at 750 rpm, but I think that's just a shift cable tweak.

So now that the project boat is pretty much done, I think I want to upgrade the engine. Think I can fit an inline 6 on the same motor mounts/alpha r drive? It'll be a next year thing, but considering it. Be fun to have a mid 70s hull that can outperform a new Moomba.

Agreed 100% on way high elevation lakes. Gonna keep the carb as is, I think. It's performing well at 5k', and most reservoirs are between 4400 and 5500 out here. The odd trip up to the fishing heights I'll be trolling anyway.

I had considered a 16" 4 blade, but I've gotten mixed opinions for an mcm 120. Now that I have a tach, I suppose as long as I keep under 5k rpm I can test whatever I want, yeah?

 

[Bondo]

So now that the project boat is pretty much done, I think I want to upgrade the engine. Think I can fit an inline 6 on the same motor mounts/alpha r drive? It'll be a next year thing, but considering it. Be fun to have a mid 70s hull that can outperform a new Moomba.

Ayuh,.... I pulled a 140hp, 'n dropped a 4.3LX into my barge,....

Much shorter, only abit wider, 'n Much easier to build bigger power, than the ole I-6s,...