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Can't get RPM above 3200

G

gardnersf

Member
Hi all,

I finally got my boat out on the water for my first test run. Only one issue so far.

I warmed up the engines fully, idled out of my cove to the end of the no wake zone and broght the boat on plane slowly (1500 RPM which is hull speed, then up to 3000). At around 3200 RPM, the stbd engine started faltering, RPM's would vary from3200 to 3000. If I applied more throttle , the rpm's on stbd would drop further. Port was fine.

Dropped RPMs around 3000 and they would hold. (a minor fluctuation here or there)

Bringing boat back to idle or 1500 RPM and the engines ran just fine.


I Put the boat back in the slip, then pulled the flame arrestor off and inspected the carb. My theory is that the secondaries aren't engaging. I verified that the throttle plates on the secondary open as the throttle is advanced. I sparayed carb cleaner on all the linkages and also the nozzle that runs just under the choke plate for the secondaries. I'm no carb expert and I'm not sure what opens the choke plate on the secondaries.

I ran the boat again with the same results. I had a few passengers so I didn't want to run with the hatch removed and the flame arrestor off. The more throttel I apply after 3200 RPM, the more inconsistent and bogged the engine gets. It still ran flawlessly under 3000 RPM

My thought is that the carb needs to be rebuilt. Not sure what other diagnostics I need to do or other ares to look for. I had recently changed the spin on fuel water separator. I changed the carb inlet fuel filter about a year ago. I did not check it again yesterday as I was running out of time.
 
I have twin 1984 454 CIDs so we are close. This problem sounds like it is caused by insufficient fuel flow. I would be careful about advancing the throttles beyond the point where the engine RPM begins to fall. At this point it is quite possible that you have a very lean mixture and that is not good for the engines.

In the search for the root cause you could consider the following:
1) Check the filter on the end of the fuel dip tube
2) As round4 suggested check the anti syphon valve
3) Check all rubber hoses to assure that they are not kinked
4) Change the fuel filters (don't forget the strainer at the inlet of the carb)
5) Check your fuel pumps (check the pressure in the discharge line - it should be 4-7 psi.

If this doesn't clear the problem you can start looking at other problems like timing and distributor issues. I would eliminate the fuel problems first.

Let us know if this solved the problem.

Chuck Hanson
 
I Happen to have a spare anti siphon valve from my last boat so I will check that. On my old boat the bad anti siphon valve gave me vapor lock after running the engines for a while, then I couldn't run at any rpm until it cleared. The engines run fien after dropping the RPM.

I meant to check the carb inlet filter yesterday but forgot. i will go down tomorrow and check it out. The fuel water filters are brand new but they coudl be clogged.

I already checked the hoses. They are in good shape (They were replaced as part of the purchase a couple of years ago)

So far it loosk like the carb may not be the culprit?

Thanks, for the advice. I will keep you posted.
 
Definitely fuel starvation. Do not run it like that or you may burn a piston.

Jeff

PS: I hope you're not running a fine filter BEFORE the fuel pump? Fine filters should only be located AFTER the pump to prevent what you're experiencing.
 
Sounds to me like the air valve is messing up. That's what you are calling the "choke plate" on the secondary side. Two items to check are the choke pull-off and the tension spring (which you can't see until the carb is disassembled.

The choke pull-off can be checked by inspection with the engine running and then cut off. The spring can be checked by checking the tension on the air valve - it keeps the air valve shut until the engine requires additional air. Compare it to the carb on the other side.
 
Good. Now replace that tiny filter at the carb--they're famous for plugging. And keep your fingers crossed!

Jeff
 
Mark, good comment. I always think of a lean mixture resulting from fuel sarvation. I never thought of too much air for a throttle setting. This makes me want to check mine also. Could you explain the mechanism of how when he advances the throttle he drops RPM if this is the cause?

Thanks,

Chuck Hanson
 
Chuck:

Welcome back to the States!

The lean condition is described around the "tracks" as a "lean bog". If you snap the throttle open on your Q-jet, you'll see the air valve is pretty much stationary. what will happen is somebody notes that "restriction" is limiting the HP the engine can make. So the enterprising individual who owns the carb finds there is an "adjustment" for the air valve. the normal path is to remove most of the tension.

When the throttle gets snapped open, the air valve will now jump. off to the track/highway. Throttle gets buried, engine revs up and bogs (coughs, dies, etc). Owner very disappointed, can't "undo" the fix, and winds up buying a Holley.

When the throttle is buried, the engine now wants more air. An ill-adjusted air valve will supply it and much faster than the fuel, hence the bog (lean out). One can use the spring tension as well as modify the choke pull-off characteristics to "tune" the carb to the particular operating traits of the engine and driver.

Sometimes, especially on the older stuff that has seen harsh operation, the springs will fatigue. sometimes, the locking screw backs out. The diaphragm in the pull-offs fail, too.

Now, on to those inlet filters. They aren't made of sintered bronze anymore - I haven't seen a new one in over twenty years. Most marine ones are a rather coarse nylon-like, open weave. They don't clog very often. The auto aftermarket ones are usually the folded paper type. If they do clog, they have a spring that the resulting pressure differential easily compresses letting fuel pass into the bowl.
 
Forgot to add this: When I suspect an engine is running lean condition I add a little choke to see if that helps. If you can do this while going along at that speed, fine. Just shove on the choke linkage (flame arrestor ON) to add some choke. (You'll have to practice this beforehand, to see where to push.) If the motor picks right up, there's your answer.

Jeff
 
Jeff,

You called it, the carb filter was clogged.

Now I cause another minor issue. The last time I changed this filter I got the tubing fitting a little to tight. WHen I was trying to loosen today, my wrench on the filter nut slipped and the fuel line got a little twisted (basically both pieces moved.) teh twist is about a 1/4 twist which may introduce a minor flow disreption

I was abel to run the boat up to about 4000 rpms where it exhibited the same behavior that it did before, rpms started dropping a bit. I quickly throttled down. Port got to about 4300 maybe 4400. Could the twist be casuing the 4000 rpm drop?

I will order a new fuel line and get it installed just in case.

I also set the timing (I installed Delco EST type dsitributor, coil and wires), set the idle mix (it was too rich)

The engine now starts quickly and runs beautifully except around 4000 rpm. i never run it that way except to make sure I can a couple of times a year.

Port engine is later this week or next. i need to get another set of wires. The wires that came with the kits were too short on cylinders 4 and 8 so I had to steal wires from the other kit

Thansk for all the advice and help.
 
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