Top end speed

[Brandon5778]

I had already winterized my boat this year, but yesterday turned out to be such a beautiful day that I had to take it out one last time. Well anyway I just got this engine running real good, I went through the works on it. When I had it out last (before yesterday) it wasn't running too good, at higher rpm I felt it missing and generally bogging down, so while I was waiting for my transom seal and shift cable to come in I put in new spark plugs. I found that the old plugs were all randomly gapped from .030 to .045. I correctly set gaps on new plugs and yesterday's water test I felt no hesitation in the engine and it runs great doesn't miss anymore.

But I feel like I'm not getting all I can WOT. Gps says about 39mph @ 3500 rpms. Boat is a 75 cobalt tri hull with a 5.0 mercruiser. The most experienced boat mechanic we have in our city told me she'd run close to 50, but no more no matter how much you tune up the engine due to hull design. He also said he owned the same boat but slighly different years ago which was why he knew. Well that makes sense, because my dad's boat (identical but with a 351engine) runs 48mph. Not sure what rpms. Should I be getting more top end speed? Is 3500 rpms to be expected out of a 302? If not how would I increase RPMs at WOT? Should I just be happy with how it is now?

 

[Scott Danforth]

Read the stickies at the top on low WOT. You may simply be over-propped

 

[stonyloam]

Yeah, need to know your prop pitch and diameter to calculate top end speed.

 

[gm280]

3500 RPMs for WOT just doesn't sound correct. I have to agree with being over-propped. But without other info, that is merely a wild guess. JMHO

 

[bman440440]

also check to see if you are getting true WOT at the carb... it mat be as simple as a stretched out/improper adjusted cable

 

[bspeth]

I think you should be getting more RPMs. A prop change may be in order.

 

[Brandon5778]

Ok, well props are expensive. bman, I don't know exactly how to check at the carb, but I can tell you we installed a new remanufactured carb, and a new shift cable. I really thought the prop that I have on the boat looked like the original, but I couldn't be sure. My question now is can I can simply check this prop thing by taking the prop off my dad's boat and trying it on mine to see if it increases RPMs? I know his boat has the right prop, and it's a stainless steel prop. We have the same boat, but he's got a 351. Worth a try?

Also what RPMs should a 302 run WOT?

 

[gm280]

Ok, well props are expensive. bman, I don't know exactly how to check at the carb, but I can tell you we installed a new remanufactured carb, and a new shift cable. I really thought the prop that I have on the boat looked like the original, but I couldn't be sure. My question now is can I can simply check this prop thing by taking the prop off my dad's boat and trying it on mine to see if it increases RPMs? I know his boat has the right prop, and it's a stainless steel prop. We have the same boat, but he's got a 351. Worth a try?

Also what RPMs should a 302 run WOT?

The fact that one prop is the "right prop" for a certain engine does not make it the "right prop" for any other engine. To answer your question though, YES, if his prop fits your setup you can most certainly try it to see. But boat design, load weight, engine capabilities, prop pitch, diameter and cupping all have a say so in how the RPMs will react. Before getting too involved with buying anything yet, make certain your tach is accurate and reading the RPMs right. And use a GPS for the speed and not a mere pitot tube reading. Once you have a base line to go by, then start changing things to see. Otherwise, you have no idea what is an improvement or not. JMHO

 

[QBhoy]

Sounds like you are way, way over propped.
If it's a carb 5.0, Rpm range should be 4400-4800 and if it's an mpi, 4600-5000.
Have a look at your prop. Get the number off it and google it.
If you are way down at those revs, then the engine is not getting to its optimum rpm and power curve best.
Good luck.

 

[QBhoy]

Or your speedo and rev counter are buggered. Haha.

 

[harleyman1975]

I had an 888 on a 21 ft Cruisers cuddy. I could coax 43 mph out of that set up at around 4600 rpm's but I did not do it very often as it isn't good to pound on something so old. I would check that your firing on all 8 cyl. (you may be only running on 7) that 302 is not a huge powerhouse (only 180 hp when it was new)

 

[Brandon5778]

The fact that one prop is the "right prop" for a certain engine does not make it the "right prop" for any other engine. To answer your question though, YES, if his prop fits your setup you can most certainly try it to see. But boat design, load weight, engine capabilities, prop pitch, diameter and cupping all have a say so in how the RPMs will react. Before getting too involved with buying anything yet, make certain your tach is accurate and reading the RPMs right. And use a GPS for the speed and not a mere pitot tube reading. Once you have a base line to go by, then start changing things to see. Otherwise, you have no idea what is an improvement or not. JMHO

Or your speedo and rev counter are buggered. Haha.

I see what you're saying. And I agree I must check tach first. It reads what it should when only idling though, somewhere around 750 rpms, I think. I was using a gps already, I've learned never to really trust the speedo on old boats. I really never have even glanced at it to see its accuracy lol

 

[77GlastronMT]

well since you replaced the throttle cable I would start there. If you didn't set up the correct length then you may not be getting 100% of your carb. I'm sure there is a procedure but what I do is with the engine OFF push the throttle lever to the full forward position, all the way to the stops. Then go back to your carb and after seeing where the throttle lever is, disconnect the cable and see if you can open the carb throttle any more. Also if you make adjustments make sure you are not hitting the stops on the carb, leave a little play. Good luck.

 

[Leardriver]

I have never thought of props as expensive. Look on Craigslist or ebay, they are way under $100. No other single thing make a boat more enjoyable to operate that a motor that performs as it should. With boats costing from 5K to 20 million, I wouldn't let the price of a prop keep me from enjoying the boat.

 

[airdvr1227]

That depends. Have you priced stainless props? It can get pricey to start trying out different props without knowing what effect you should expect before buying.

 

[achris]

Hi there,

First question I have to ask, has it gotten to 4200-4600 in the past?
Now, you need to check the engine before changing prop (that would only mask any engine problems)...
Compression test... Are all 8 cylinders in the 140 range and even?
Check the timing AND the advance. Is the advance working properly? If the advance isn't working, you get low revs.
Someone mentioned checking that the carb is opening fully up. Just take the cover off the carb and without the engine running, push the throttle to wide open. Now look down the throttle barrels and check the the throttle plate at the bottom of the carb is fully vertical (you may have to hold the choke plate open to see passed it)....
You also need to check for exhaust restictions. Easiest way is with a vacuum gauge. for that you'll need to hook the guage up and run the engine IN THE WATER... Not in gear, but at least backed down at a launching ramp. NOT ON A GARDEN HOSE!!!! Advance the throttle and watch the guage, shouldn't drop too far as the revs climb. If the vacuum falls away, check the exhaust.

Once you have established all that as good, then look at re-propping. But if the boat ran properly with that prop before the engine work, chances are it's not the prop.

One more question.... What year is the engine (not the boat), and is it a 2 barrel or 4 barrel carb? (yes, that's 2 questions)

Chris.......

 

[Brandon5778]

Hi there,

First question I have to ask, has it gotten to 4200-4600 in the past?
Now, you need to check the engine before changing prop (that would only mask any engine problems)...
Compression test... Are all 8 cylinders in the 140 range and even?
Check the timing AND the advance. Is the advance working properly? If the advance isn't working, you get low revs.
Someone mentioned checking that the carb is opening fully up. Just take the cover off the carb and without the engine running, push the throttle to wide open. Now look down the throttle barrels and check the the throttle plate at the bottom of the carb is fully vertical (you may have to hold the choke plate open to see passed it)....
You also need to check for exhaust restictions. Easiest way is with a vacuum gauge. for that you'll need to hook the guage up and run the engine IN THE WATER... Not in gear, but at least backed down at a launching ramp. NOT ON A GARDEN HOSE!!!! Advance the throttle and watch the guage, shouldn't drop too far as the revs climb. If the vacuum falls away, check the exhaust.

Once you have established all that as good, then look at re-propping. But if the boat ran properly with that prop before the engine work, chances are it's not the prop.

One more question.... What year is the engine (not the boat), and is it a 2 barrel or 4 barrel carb? (yes, that's 2 questions)

Chris.......

Hi Chris, thanks for your reply.

As for your first question, I do not know if it has gotten to 4200-4600 in the past. I suppose it would be helpful to fill you in on some info. I just bought this boat in July not running, PO said it ran previous summer but original starter was toast. So I towed her 3 hours back home. After new starter and points, distributor cap, ect. We got it to fire up but wouldn't stay running long. After a battle with testing fuel system I ended up replacing a fuel pump and the carb with a remanufactured carb, and it finally kept running good but overheated, so we put on a water pump kit in the lower unit and replaced a thermostat and it stayed cool.

Soo, I finally took it out after this and it ran real rough, misfired on the top end real bad and I only got to 3000 rpms WOT. I cruised around at low speed for awhile though until my bellows tore open and I started taking on water. So it was back to sitting till we got the bellows kit and new shift cable put on. While I waited on that I pulled the spark plugs and did a compression test, I don't remember all the exact numbers but they were all within acceptable ranges of each other. I also changed the spark plugs at this time and found the old ones all had different gaps, they also looked pretty old. I gapped new plugs to .035 and when I got it on the water next time no more missing and top end at 3500 rpms, according to my (untested) tach.

As for the advance, I'm not sure. I also don't know much about this so that's something I'll have to learn to do as well.

Testing if the carb opens all the way seems real easy, I will do that next time I'm out at where my boat is being stored for the winter.

I'll also have to look into the vaccum gauge Thanks for pointing out to be in the water, I assume you mention this because you aren't suppose to rev up your engine very high on muffs.

And for the final question(s) I don't know exactly the year of this 302.. I haven't found any markings on the block but I haven't exactly been looking either. It has the Rochester 2 bbl carb. Sorry for being so long winded. I also won't be able to really tackle this until the Spring so I'm just getting ideas and researching into it for now.

 

[achris]

Ok, all good information. The fact that it's a Ford engine means it's no later than 1977. That being the year Merc dropped Ford as a supplier. And a 2 barrel 302 is a Mercruiser '888'.... Max revs for those engines is 3800-4200, so 3500, although low, it's as bad as it could have been... (GMs generally run to 4800)... And yes, you need to use a properly calibrated 'shop' tacho. Dash tachos are notoriously inaccurate.

I have put the timing curve in below. Next time you're on the water, use a timing light and check it's advancing according to the chart.

Sounds like you've had your hands full sorting out 'lack of maintenance' problems.... Let us know how things go when you get around to them...

Cheers,

Chris..........

 

[Brandon5778]

Thank you for all the info Chris, I'm feeling confident in tackling this next spring.

Sounds like you've had your hands full sorting out 'lack of maintenance' problems.... Let us know how things go when you get around to them...

You could certainly say that! But as frustrating as it all was, I kind of liked learning everything I have learned so far. It's an expensive hobby but I doubt I'll ever stop boating.. I will definitely report back when I get started next year