Performance Issues

[JSMoore]

Some of you may remember a post I did on here a while back, entitled Worse Than I Thought. I'm thankful to say that all of that turned out just fine. And, after 11-months of work, on too many things to name, today was "SplashDown" day. I'm happy to report that she seems to run out just fine..... except for one thing.

I'm only getting about 3100 RPM, trimmed out for best RPM/Speed, and about 30 knots on the GPS, with 2 on board. The book says I should be getting 4200 - 4600 RPM out of a 4.3l V6.

Here's some info on the setup:
1987 Mark Twain MTC-215, 3800lb dry weight
OMC Cobra 4.3L 2bbl
3 bladed aluminum 15 x 17P prop
All cylinders check 169 to 175 compression, (101.9 hrs on motor)
Timing set at 1* BTDC for 87 octane gas
Points set at 0.019 (I don't have a dwell meter)
Exhaust manifolds and "flappers" in good shape
Brand new reman marine carb
New fuel/water sep filter
Temp stabilizes at 160* - 165* at all throttle settings

Here's where I plan to go with this:
1. Recheck timing and set at 6* BTDC
2. Run all the really old gas out of it, (3/4 of a 52gal tank); change fuel/water sep and refuel with marine gas.
3. If that don't fix it.... look at lower pitch props....... 15 x 15? Maybe?

Now, I'm throwing this out there to see if I'm on the right track, or if anybody has other suggestions for me to look at.
Thanks!
JSMoore

 

[HT32BSX115]

Re: Performance Issues

Howdy,


It sounds like you're on the road to finding the problem....the following might have a few additional items to check.

List of possible causes of low WOT (Wide Open Throttle) rpm.

I know this might be expensive but you should also probably DRAIN instead of RUN the "really old" gas out. If you want to run it, mix it 1:10 in your car or something instead of just throwing it away....


Regards,


Rick

 

[JSMoore]

Re: Performance Issues

It sounds like you're on the road to finding the problem....the following might have a few additional items to check.

List of possible causes of low WOT (Wide Open Throttle) rpm.

I know this might be expensive but you should also probably DRAIN instead of RUN the "really old" gas out. If you want to run it, mix it 1:10 in your car or something instead of just throwing it away....

Thanks, Rick. I'll go back over the list of things in the link you provided, just to double check.

I had, in fact, thought about draining the tank. However, both the fill and vent lines have a couple of 90* turns in them that I can't snake a siphon line down through. And, the anti-siphon valve does it's job by not letting me siphon through the feed line. I may consider spending the $50 - $60 it takes to buy an electric fuel pump from the auto parts store to pump her dry; but will have to give it some thought.

I appreciate the reply and knowledge you share!
JSMoore

 

[HT32BSX115]

Re: Performance Issues

If the tank has never been drained and you have access to the gage sender, get yourself a new gasket and pull it.

Then you can use a long piece of clear vinyl hose to siphon out the fuel, water and other debris...

Just do it outside with a helper to switch tanks for you and have a fire extinguisher handy!

 

[JSMoore]

Re: Performance Issues

I found a problem - maybe not the only problem - but something that may contribute to not getting full RPM out of this V6.

I re-checked the timing, that I was sure I had set correctly, and found that it was WAY off. It was actually running at about 4* - 5* ATDC. The only thing I can figure out is that I must have turned the distributor a little when I tightened it down. Anyway, I got it set at a good 6* BTDC at a steady 500 RPM Idle. I thought the motor ran smooth before. But, this single change REALLY smoothed it out. I can't wait to get it back on the water to see if it's going to make any difference at WOT.

Lesson learned? Just like "measure twice - cut once"...... Set - check - check again!
Cheers,
JSMoore

 

[HT32BSX115]

Re: Performance Issues

Timing (retarded..........can we still say that?:facepalm will definitely limit your WOT power and RPM!

 

[JSMoore]

Re: Performance Issues

OK.... Back in the water after correcting the timing issue. The motor has a good bit more power than before. Top speed at WOT before was 30.7 KTs on the GPS. Now, I'm up to a solid 37 KTs. Speedwise, I'm a happy boater! 37 KTs in this big 'ole, fat, 4000 pound, V6 powered boat, ain't too bad. However, the tach still shows a max of only 3100 RPM. And, I can actually HEAR the RPM increasing beyond that! The sound of the motor tells me that it's "wound up tight".

Bad tachometer, maybe?

Is there a simple way to test that before dropping $75+ for a new tachometer?
Thanks!
JSMoore

 

[HT32BSX115]

Re: Performance Issues

I'm only getting about 3100 RPM, trimmed out for best RPM/Speed, and about 30 knots on the GPS, with 2 on board. The book says I should be getting 4200 - 4600 RPM out of a 4.3l V6.

I thought about your boat a little.... That's a Mark Twain "Cruiser" isn't it? 22' or so?

3800lbs is a fairly heavy boat for a V-6 engine. So if you're REALLY getting 37kts (42 mph!) That's really good!

I would say replace your tach or check your current one with a known tach. They're frequently off at the higher RPMs!

My dad bought a 1966 18' V-Sonic Marktwain new! I was a kid then and literally grew up in it! I sold it in 2005 and it had the origina engine in it and still ran good!


Have you checked out http://marktwainboats.org/ ??


Cheers,


Rick

 

[JSMoore]

Re: Performance Issues

I thought about your boat a little.... That's a Mark Twain "Cruiser" isn't it? 22' or so?

3800lbs is a fairly heavy boat for a V-6 engine. So if you're REALLY getting 37kts (42 mph!) That's really good!

Have you checked out http://marktwainboats.org/ ??

Thanks, Rick. You're correct about the size & weight of the boat. Measures 21 1/2 feet & weighs 3800 lbs dry. So, yes, it's a lot of boat for a V6. But, according to Garmin, the highest speed recorded on the trip was 37.3 Kts. I'm really happy about the top speed, as well as nice relaxed 27 - 28 Kt cruise at an Indicated 2700 RPM. I just don't like something that doesn't work right. And, what I see on the tach just doesn't make any sense to me. So, I'll be trying to run that down.

And, yes, I am on the Mark Twain web site, (same ID as here). Check the "Gallery" over there for a few pics.
Cheers!
JSMoore

 

[91rr]

Re: Performance Issues

Is there a simple way to test that before dropping $75+ for a new tachometer?
Thanks!
JSMoore

See if a buddy has a timing light that has a built in tach. Or if nothing else try this one http://www.autozone.com/autozone/ac...25x9?itemIdentifier=268886&_requestid=1089749

I had to pick one up a couple of weeks ago to verify if my tach was accurate or not.

 

[JSMoore]

Re: Performance Issues

See if a buddy has a timing light that has a built in tach. Or if nothing else try this one http://www.autozone.com/autozone/ac...25x9?itemIdentifier=268886&_requestid=1089749

I had to pick one up a couple of weeks ago to verify if my tach was accurate or not.

Good idea! I may do just that. Or, I may just spring for the marine tach. I'm "watching" one on eBay right now that would work for me and will sell pretty cheap. We'll see.

Thanks for the suggestion!
JSMoore