1977 CVX 18 Engine RPM and Recommended Prop

[JBD319]

Hi all! I recently solved my fueling issues with a brand new fuel pump, overall boat runs great now. No stalling or restart failure when hot issues. Took it out for 2+ hours yesterday, great afternoon cruise.

I ran a forum search and wasn't able to find my answer. I'm wondering what props most owners are generally running on these boats, and what kind of RPM/Speed they're getting. For reference it is the Mercruiser 188/888 Ford 302 Pre Alpha drive with the 1.5:1 ratio. I bought the boat with a Quicksilver 14.5" 17p 3 blade stainless steel prop, that is in good condition with minor blade rash. At WOT and trimmed up I only get about 3900 RPM and 41 MPH in good conditions. I see the factory sales brochure said the recommended prop is 15.5" 21p and that should run 48-52 mph. Problem is if I go up in diameter in pitch my rpm's will fall even farther and I'll probably lose speed not gain. Thoughts?

I am planning further tune-ups as well. Going to do new plugs/wires, and a distributor cap/rotor if I can find one and maybe the coil over the weekend and see how she runs on labor day.

I'm not looking to break any speed records, but I'd like to get it in the proper rpm range at least which is supposed to be 4400 rpm from what I've read (please correct me if wrong), and run 45-50 mph.

Caveat, it also has a brand new Holley 2 barrel 300cfm marine carb that the previous owner installed but never tuned, so it's likely set at whatever the defaults are. I'm sure some tuning of that would be in order as well, which I'm not really familiar with. My trim system also has a bit of play in it, so I might try to bleed the air out of it this weekend. It runs pretty slow so I'm wondering if under a load it's struggling to trim up? I only gain about 2-300 rpm and 2-3 mph playing with trim. Seems like maybe it should have a greater affect?

Thanks for any and all help!

[Plugcheck]

You stated a bit of prop rash which even minor can have an effect.    Anyone around you able to loan a prop to test?   Your RPM and speed are low, so before investing in tune up gear, I'd suggest a full compression test, WOT, no plugs in.    You didn't mention if this engine had been in storage before, maybe you did in previous email.   Engines stored for long periods can suffer weak springs.   My CVZ with SBC 305/2V was no stellar performer, but it would do 4600-4800 rpms doing low 50's.  If compression is good on all eight, the next item would be to check timing.   WOT I'd guess 38-42 total timing.   Maybe a Ford IO person could chime in(John?).  Good to hear its running well though, but it could/should do better .      BTW, check speed with GPS, and maybe rpms with a tach other than in dash?

[JBD319]

Thanks for the info!

I don't know specifics of the history, but I was led to believe it was run pretty much every season, although unknown number of hours.

I've already got tune up parts coming because I have zero clue when it was done last and it certainly can't hurt. I may do the compression check when I'm changing the plugs. Do you know what numbers psi wise I should be running there?

Having someone check timing would be good also, as I have not done that and am not really skilled at doing it myself nor do I have a timing light.

The rash on the prop is very minor. I definitely believe it could have some affect but I'd have a hard time believing it would cost 500-600 rpm and 10 mph.

Definitely a good list of things to check though! I know she has more in her. I know with the prop I have I'm not going to get much more speed I but I feel like the RPM's should obviously be higher than they are. The speed I gave is GPS verified, my speedo is actually super accurate compared to my phones GPS app. The tach was reading really low and I realized it was set to a 6 cyl, so I changed it to V8. It's possible it's still reading low but I'd need a timing light for that also.

[Glastronjohn18]

Nice project you have there. I have a 351 in my CVX18 but your 302 is similar. The 302 should run between 3800- 4200 RPM per Mercruiser.  Timing is 6 degrees BTDC, with 31 degrees of dwell with the Prestolite dist. Points at .016-.019in. Plugs are Motorcraft ARF 32 gap is .030.  Here's a link to the 302,351 Industrial and Marine service manual. With an older engine compression is likely to be around 140psi. That number isn't as important as all cylinders being within 10% of each other for optimum performance. The Ford Manual allows 25% variance. Hope this helps


https://1drv.ms/b/s!Ao2yGujFDzKGgx9l2GE912zMjF5k?e=JViZPU

[JBD319]

Nice project you have there. I have a 351 in my CVX18 but your 302 is similar. The 302 should run between 3800- 4200 RPM per Mercruiser.  Timing is 6 degrees BTDC, with 31 degrees of dwell with the Prestolite dist. Points at .016-.019in. Plugs are Motorcraft ARF 32 gap is .030.  Here's a link to the 302,351 Industrial and Marine service manual. With an older engine compression is likely to be around 140psi. That number isn't as important as all cylinders being within 10% of each other for optimum performance. The Ford Manual allows 25% variance. Hope this helps


https://1drv.ms/b/s!Ao2yGujFDzKGgx9l2GE912zMjF5k?e=JViZPU

Thanks for the reply!! That's great info. So it sounds like my RPM's are ok actually? Although on the low side perhaps for a prop that is a lower pitch and diameter than factory recommended. So could be a combination of the prop and maybe just a tired motor? I have all the tune up parts to put on this weekend to test out before labor day, so we'll see if that helps!

[CVX Fever]

At WOT and trimmed up I only get about 3900 RPM and 41 MPH in good conditions.

That sounds really slow.  For comparison my '85 CVX18(original & stock 305) will run 58-59MPH on GPS spinning a 22P Enertia(13.5" dia.) prop turning 4100 RPM.. It has the original bone stock 305 in it. Comes out of the hole nice & strong.

With a 17P prop I would expect your motor to over-rev so I suspect there could be an issue with the motor. As others have stated might want to start there. Unless your using the boat for towing pyramids in ski shows shouldn't be running anything smaller than a 19P. When I run my 19P High Five prop on the same boat for water sports I can easily over-rev the engine as it will turn 4800+ RPMs.

[JBD319]

Welp, I figured out my problem! /facepalm. Was changing the wires and plugs, all the plugs were looking really good so I was just going to leave them. Then I get to 6 and 7 which are COMPLETELY fouled, but the rest look fine. So I double check the firing order (which I haven't touched since I bought it a month ago) and sure enough, 6 and 7 are reversed on the cap. So I flip them, finish the wires and fire it up. Even with the old cap, rotor and coil still on, warm idle had immediately jumped to 1200 rpm (had it set at 850). So I turn it back down to about 850-900 and let the water temp come up. Before even in neutral I couldn't get it to rev past 4200ish, was almost like the distributor had a built in cutoff for a redline, which I didn't think it would, not that low especially. Now it free revs to 5,000 just fine!

I'm shocked how well it ran before. It wasn't really misfiring and it didn't sound like it wasn't running on all 8. I had Jeff from the forums over and even he agreed it didn't sound that off.

So after fixing that and replacing the fouled plugs I can't wait to see how she runs out on Monday! Hopefully I see 45+ and more RPM with the prop I have, and now I have a better baseline to mess around with different props. Woohoo!

[thedeuceman]

It?s always nice when you find a smoking gun like that, and Kip is right on with the prop sizes, Keisha and Jake?s 1900 with the same motor runs into the 40s with a 19? ballistic. With two people on that boat you should be able to run a  21? just fine.


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