twin mac 84 problems

[Drew St. Amant]

Alright so we thought we had the issue solved, last weekend in ran great we had a little steering trouble so we fixed the steering issue tried to test it this weekend and we couldn't get both motors to stay running and it wasn't the same one cutting out it would change sides tried all kinds of needle settings tried changing to different fuel tried putting water through it from a hose on the bench to see if we had water leaks checked the filters multiple times not a speck of trash we even tried removing a prop from one motor and running the boat with one or the other engine to see if we could find what was going on and both engines ran fine separately we tried pulling all the fuel out of the tank with a pump to see if we had air bubbles tried without the cowling to what that did, nothing we did helped or seemed to give us any indication to what was happening any ideas?

 

[Mike Hughes]

Aaaaaahhhh, the twin thrash. When they are good nothing better. When they are not...........

I was testing for nats once and just could not get it right. My friend suggested to go back to a fat setting and just keep tweaking a little at a time. We did that about 20 times slowly working it and it was getting better one step at a time. The 21st time we lean just a little more and it was like we went right back 10 steps. SInce it was nats prep and I had other things to work on. I gave up. I did not need that trouble at nats. So I parked it. I have not brung that boat back out since. It has been 3 yrs. Still pissed at it.

Good luck. The above technique does help to diagnose problem sometimes.
Mike

 

[timothy johnson]

Sorry to read all your frustrations about your twin project. Sadly Mikes suggestion to put the boat on a shelf may be your best option. Good luck!!

 

[Mike Hughes]

I wasnt suggesting that. It is what I did. Sometimes you have to walk away and go back to it when you excited about working on it. Frustration is not fun. Fun is part of why I do this. I really like a twin when they are running good. Nothing better to get the juices flowing.

I have 2 simple rules when it comes to boat racing. If you follow these rules you will have an awesome day.
The number one and most important rule is SIT MORE.
Number two rule is FINISH HEATS.
People have tried to add to the rules over the years, but it always comes back to the basic two rules. All other rules are a subset of rule one and rule two.

I want to have fun and would rather move on from a boat that is not running well to just enjoy the day.
Mike

 

[Timbo Whalen]

One thing nobody mentioned, especially since both engines run fine individually......weird, roller coaster harmonic waveform. Big Twins need a more judicious approach regarding engine mount isolation. The harmonic anomaly scenario can also take plugs out prematurely in Twins that aren't isolated sufficiently.

 

[Drew St. Amant]

So just to make sure I understand correctly you are suggesting I look at the motor mounts to see if they are getting worn causing a weird harmonic?

 

[FloridaScaleBoater]

So just to make sure I understand correctly you are suggesting I look at the motor mounts to see if they are getting worn causing a weird harmonic?

Drew, do you have a "known good" single hull? Mono would be best.
Put each engine in it, run it, see what happens. If both engines run fine in it, it ain't the engines.
When all of this started, you said the tanks were pretty old?
Only way I know to check feed & flow of the fuel system is this:
1: leave tanks in the boat.
2: set boat level up on a work bench
3: fill them completely.
4: get a long enough piece of fuel tubing to reach a catch container sitting on the floor.
5: blow gently into vent/pressure side of tank until fuel starts to flow into catch container.
6: let it gravity feed (syphon) into catch container.
Watch the fuel tubing for ((ANY)) bubbles in the line.
7: let it flow until it stops on its own.
8: if it doesn't empty completely or you see ((ANY)) bubbles you have a cracked internal fuel tube...
Or trash on your tank.
Most likely a cracked line.

 

[Timbo Whalen]

So just to make sure I understand correctly you are suggesting I look at the motor mounts to see if they are getting worn causing a weird harmonic?

It's not necessarily about weird harmonics....it's about isolation.

The isolators deteriorate with time, exposure, etc.

Single cylinder engines are impossible to balance. The problem is there's no way of phasing the harmonic frequencies, so you nullify them as best as possible through isolation.

Look, you've tried everything else, it seems....you state that the engines run fine independently. We can only go by what you report, none of us are there to observe. We can only assume that what you're reporting is accurate and that you know what you're doing when it comes to tuning. I didn't state what we suggested IS the problem....however, when engine isolation deteriorates things start to happen- including radio issues and engine performance. If vibration is excessive it'll aerate the fuel in the lines causing eratic performance.

 

[Drew St. Amant]

The only single engine hull I have that those motors will bolt into is a hydro and we actually did run that boat last weekend before we tried running the twin and it ran good so we could probably run the engines in that boat to test them and I had never really thought about using gravity so that I could watch the flow. I have been pumping the fuel in ant out with a hand pump so I'll definitely give that a try what Timbo is saying about isolators is starting to fall in line though mainly because the whole boat has been literally shaking its self apart like pieces in the radio box are breaking loose and having to be re-glued and such we bought this boat from another person we didn't build it our selfs some of the other boats we bought from him that are single engine have fallen apart in a similar manner so we didn't necessarily think about it being a motor mount issue we more so thought it a build quality/older boat issue because of other experiences with his boats

 

[ALLAN YUEN]

How old for metal tanks ? If you show any pictures of SG or SGX Twin ?

 

[Drew St. Amant]

Well the tanks are original to when the boat was built I did look and find out I have the updated mac 84s and those are original to the boat as well so I'm not sure when those macs where made but that would date the whole boat what I fo know is it's pretty old and was raced hard for many years it's in need of a redo for sure

 

[ALLAN YUEN]

Well the tanks are original to when the boat was built I did look and find out I have the updated mac 84s and those are original to the boat as well so I'm not sure when those macs where made but that would date the whole boat what I fo know is it's pretty old and was raced hard for many years it's in need of a redo for sure

Need to order new pair tanks because you cant see inside sump with brass tube already cracked line cause air bubbles

 

[Timbo Whalen]

Well the tanks are original to when the boat was built I did look and find out I have the updated mac 84s and those are original to the boat as well so I'm not sure when those macs where made but that would date the whole boat what I fo know is it's pretty old and was raced hard for many years it's in need of a redo for sure

When you were running your tests on one engine at a time, did you run the engine(s) on their respective tanks, in the boat?....think you mentioned they ran fine individually, right?....if that's the case it's probably not the tanks, but both engines running, one acts up.....did you switch left engine to right position and vice versa?....if so, does the same engine act up, or is it either engine in a given mount (rt or lft)?.....see where I'm going?....

John Steltzer uses stainless tubing iin his tanks, Walt uses brass....I was questioning the brass tubing in a conversation with Walt one day and he assured me he'd never cracked one, or had any complaints. Personally, I've never hurt one of Walt's tanks. In the SG/SGX hydros I believe Andy makes the tanks....and I've not hurt the tanks in my 2 SGX's yet, but they haven't been run much, either, on my end.

 

[Drew St. Amant]

The problem never seemed to stick to one engine it would flip back and forth almost equally and they where run on there respective tanks

 

[Timbo Whalen]

The problem never seemed to stick to one engine it would flip back and forth almost equally and they where run on there respective tanks

I guess gravity bubble test both tanks....if that passes, back to the isolation.

Do you have a pair of smaller cheapo Sullivan tanks you could try?......

https://www.amainhobbies.com/sullivan-fuel-tank-slant6oz-sul438/p45077

 

[ids987]

So pressure feed is maybe the only common factor (presumably split with a tee piece; one pressure tap to two tanks).

 

[J.J. Horvath]

Try different servos and Rx pack, one may be bad and under running load it’s causing problems.. you mentioned both engine and steering issues, random issues with both, could be a sign of a bad piece of electronics.

Especially if it sat, maybe corrosion set in and a servo is drawing more load than the system can handle.

I just had a boat that was fine on the bench, but when I started it, the throttle servo would glitch and rev the motor weird. Isolated and replaced the throttle servo, all went back to normal.

 

[Drew St. Amant]

I have thought about using a Sullivan tank to test with I don't have any laying around but I might just get some they are crazy expensive would be worth it if it finds my issue

 

[Timbo Whalen]

I have thought about using a Sullivan tank to test with I don't have any laying around but I might just get some they are crazy expensive would be worth it if it finds my issue

Did you look at that link we posted?....$20 for a pair of Sullivan slanted test tanks seemed like an inexpensive way to validate whether or not your tanks are bad....a new pair of metal tanks will set you back the better part of $150.

I might have ONE new one laying around you could use for testing....would let it go, cheap....also have a NEW Ryan Lipgens HDPE hopper tank for a JAE/JRX hydro I'd like to peddle, too. Just food for thought....still leaning toward vibration issue, though.

 

[Brandon Atwell]

If the vibration is significant enough to break parts loose that need to be glued back down I’d say that’s the first issue to tackle. I mean, vibration that significant would be nearly impossible to ignore when talking about any issue here, vibrating needles and tanks potentially causing aeration in the mixtures down the lines, not to mention the strain that’s putting on each motor. It seems like it would be best to go ahead and start looking at a minor refresh and clean up the important things like the engine compartment and radio box, since those are your problem areas, to get it consistent before you decide to tear it down and do a full renovation if you want to.

Also, great to see you guys back! My dad Mitch was the president of the First Coast Model boaters when you all were still racing, hopefully we can catch you at a district 13 race soon. He’s been having some heart act- ups lately but should be fixed soon!