Bogging/Choking at high rpm under load

[Trihulled]

Hey everyone, this is my first post so go easy on me. I've been reading and testing until my eyes bleed but haven't come up with a solution to this problem.

My friend and I are restoring a 1976 Thunderbird that's running a Mercruiser 165 Inline-6 engine (I/O). The current problem is this: when I put it under load out on the water I can hold it at about 2500 rpm for a few minutes but then it will start to choke, sputter, and die unless I throttle back. After the first time it will happen whenever I try to hold anything over 2000 rpm for more than 30 seconds, but it always 'resets' when I throttle down. Off-load, whether it's in neutral in the water or in-gear with a hose-bib on land, it will run for days at any rpm.

Here's some things I've done related to this:

carb rebuilt
new coil
new fuel filter
installed a fuel pressure gauge between filter and carb (after pump)

From what I've read I have a possible solution that I will try out tomorrow. The fuel pressure fluctuates slightly but it doesn't seem to be congruent with the loss of rpm and power, however I've read about dirty tanks, watery fuel, and possible debris clogging the pickup so I'm going to try a clean tank with fresh fuel.

Another issue that I'm not sure is affecting it: The wiring diagram calls for a resistance wire to power the ignition coil; we removed that (smart, I know) when we were stripping everything and haven't replaced it (we just did a standard wire). From what I've read that means that the coil is now seeing 12V rather than the potentially lower (9V seems common) voltage that it ought to. On that note, 1) how can I see how many volts I should be routing to the coil? 2) other than damage to the points and reduced life-span, could this higher voltage be causing or contributing to my high-rpm/high-load problems?

Thanks, any thoughts would be much appreciated and apologies if there is an answer somewhere believe me I've looked just point me there!

 

[alldodge]

Hey everyone, this is my first post so go easy on me. I've been reading and testing until my eyes bleed but haven't come up with a solution to this problem.

My friend and I are restoring a 1976 Thunderbird that's running a Mercruiser 165 Inline-6 engine (I/O). The current problem is this: when I put it under load out on the water I can hold it at about 2500 rpm for a few minutes but then it will start to choke, sputter, and die unless I throttle back. After the first time it will happen whenever I try to hold anything over 2000 rpm for more than 30 seconds, but it always 'resets' when I throttle down. Off-load, whether it's in neutral in the water or in-gear with a hose-bib on land, it will run for days at any rpm.

Here's some things I've done related to this:

carb rebuilt
new coil
new fuel filter
installed a fuel pressure gauge between filter and carb (after pump)

From what I've read I have a possible solution that I will try out tomorrow. The fuel pressure fluctuates slightly but it doesn't seem to be congruent with the loss of rpm and power, however I've read about dirty tanks, watery fuel, and possible debris clogging the pickup so I'm going to try a clean tank with fresh fuel.

Another issue that I'm not sure is affecting it: The wiring diagram calls for a resistance wire to power the ignition coil; we removed that (smart, I know) when we were stripping everything and haven't replaced it (we just did a standard wire). From what I've read that means that the coil is now seeing 12V rather than the potentially lower (9V seems common) voltage that it ought to. On that note, 1) how can I see how many volts I should be routing to the coil? 2) other than damage to the points and reduced life-span, could this higher voltage be causing or contributing to my high-rpm/high-load problems?

Thanks, any thoughts would be much appreciated and apologies if there is an answer somewhere believe me I've looked just point me there!

:welcome: to iboats

If your fuel pressure is staying above 4 psi then you shouldn't have a clog somewhere. You should be getting fuel, now if your float is set to low this can cause an issue with insufficient power.

Running a points system without the ballast resistor in place will cause the coil to heat up.

 

[Trihulled]

Thanks AllDodge,

Bittersweet as a first post. I try to be a passive researcher as much as possible before I jump into asking, but here I am!

I didn't notice anything below 4 psi even when it starts dying. I have a clean can that I'm going to try just for kicks. What I have noticed is inconsistency in the fill level of the glass-tube filter we have in the fuel line. Sometimes it is full up and other times it's barely full, but the pressure seems OK. That makes sense based on whether the float is allowing fuel into the bowl or not, but I can't not any pattern when it's choking out... I'll have to check to see if there are float adjustments on the carb (I hadn't noticed any). It really does seem like a fuel issue since I can get it up in rpm and even WOT at times and it'll be fine for 5-15s before it starts to sputter.

Any other aspects of the coil/points that might be related to this load issue? otherwise I'll table it as the next thing to look into. We thought it might be a coil/distributor issue, but in my mind high-rpm under load and high-rpm off-load should look the same to the electrical system. The plugs are still requiring spark at the same rate regardless of the loads on the crank I would think, but correct me if I'm wrong.

Thanks again!

 

[Bondo]

but correct me if I'm wrong.

Ayuh,.... Welcome Aboard,..... I think yer Right, it sure sounds like a Fuel delivery issue,......

Carb floats are adjusted by bendin' their tabs,...

 

[Trihulled]

Another thing, before we replaced the coil we were able to reproduce the problem out of water. We ran at high rpm and had it lag again. At one point it died and would not start again until we replaced the coil pack, at which point we couldn't reproduce the problem at ALL and thought it was resolved. But when we took it out on the water it showed up again.

 

[putback]

Choking would be to much fuel. Carb flooding over, gas stink, dark exhaust. Sounds fuel starved. You can discount sludge, water, air leaks, kinked/collapsed hose, tank pickup ect from tank to pump with a length of new fuel line and a clean can of gas attached to the pump to test. I'd look real close at that old carb, 40 yrs old its entitled to some problems. Most likely a good thorough interior cleaning is in order. Striped, dipped and boiled would be ideal but w/o access to the equipment carb cleaner & air pressure can work well. Make sure all interior fuel and air passages are open & free of corrosion. A carb that old has been apart a few times. Often your symptoms are related to the float assembly having been handled w/o care. Bent, out of alignment, hinge and axle rough or pinched, needle valve clip in wrong, float arm rubbing on casting, or float set to low, a good kit comes with a plastic float level gauge. Quick test for float needle & seat. Lift off carb top, blow through fuel inlet [lung power only not air compressor] . The float should be down and you can blow through with little effort. Gently lift float up, it should shut of air flow. Now let float down. Do'es it come down on its own? If yes do'es it allow air through with minimal pressure. If it does'nt operate with lung power it needs adjustment or repair.

 

[Trihulled]

Update! Problem Resolved!

We pulled the engine to fix a small oil leak and dismantled the carb to check it out. The float was only off by a hair but made sure to put it exactly to spec. What we did find was that when we had the carb rebuilt, the guy didn't check the inlet fuel filter, and we found it to be seriously clogged. So, we removed it for the time being, put her all back together, and lo-and-behold she ran like a champ, idled much better, and all the previous issues were SOLVED. I'll buy a new filter soon and throw it in there.

Thanks for everyone's input, and on a side note I am putting together a log of the entire restoration. You can find it here: http://forums.iboats.com/forum/boat-...estoration-log and I will keep adding to it as I have time.

 

[Trihulled]

welp....I need to re-open this thread because it looks like I spoke too soon earlier.

Here's the latest:

Took her out last week for a cruise. I had pulled the filter at the carb inlet and put the fuel float back to spec which I thought had fixed the issue. However, I ran into a few problems. About 15 minutes in I was running at low speed/near idle and she just died out of nowhere and I could get her to start back up. I had to wait a bit and gun the throttle while starting it to finally get her started.

After that, she ran like a champ for a nice long cruise at anywhere from 2500-3200rpm depending on what we were doing, 45 min or so (much much much longer than before the 'fix', if you've been reading this thread). Then I ran into the SAME ISSUE as before. Running about 3200rpm I heard a dip in rpm for a half second, then it would return to speed, then another, then it wouldnt recover quite all the way back up, and so on until it for a few more seconds until it was dead. By this time I was on the way back to the ramp but it happened again probably 10 minutes later.

What could possibly be causing this issue? I have NO working theories and it's supremely frustrating. I've checked and cleared the entire fuel-delivery chain from tank to engine block and there are no blockages, gunk, or kinks. There is no pattern or consistency to when this issue begins only that after the first time it occurs more frequently. Any clues? It'd make sense if I just couldn't get enough power all the time, but to be able to run for a long period and then have some random fuel delivery issue is really grinding my gears.

Just for kicks, here's what I've tried:

Cleaned fuel tank
Cleaned fuel lines
Cleaned in-line glass filter
Greased fuel-pump
removed carb fuel-inlet filter
had carb professionally cleaned and rebuilt and have since inspected for blockages
re-set float to spec

 

[alldodge]

Do you still have spark?

 

[Bondo]

I heard a dip in rpm for a half second, then it would return to speed, then another,

Ayuh,.... Check, 'n Clean all the grounds back to the battery,....