View Full Version : 350Q flat spot
05-22-2008, 04:44 PM
I have a Riva Super Florida with a CC 350Q engine. Standard setup except for a Edelbrock 1409 600 cfm carb and pertronix electronic ignition.
The engine is running fine except when I give full throttle quickly. Then the engine is not coming above approx. 3000 revs and it remains there. If the tap is opened slowly the engine will easily come at it's max revs (4400).
When it is stuck at 3000 I have to decrease the throttle and the revs will increase (!).
Carb is clean and quite new. Previous owner has installed this carb and I don't know if the engine did not have this problem in this setup.
05-22-2008, 05:44 PM
"I am having a similar experience my port 350Q will come to 4200 rpm, My Starboard 350Q will only reach 3200 RPM. The symptom has been the same with the original Carter AVS carbs. I installed 2 new Edelbrock 1409's and the RPM issue remains as before.
Both distributers have electronic conversion kits installed in them. They run perfectly except for this issue."
05-23-2008, 02:24 AM
"To clarify. My engine will reach max revs (4400). However to get there I have to increase the throttle very slowly.
Thinking more about it, it could have something to do with vacuum in the intake? Immediate full throttle of course reduces vacuum to zero, where slow increase will maintain a small amount of vacuum all the way?
Following this reasoning this could mean I need weaker springs on the vacuum activated needles in the carb?
...however this does not explain why the engine is stuck continouosly on 3000 when the throttle is opened quickly and not increasing revs after a few seconds?
As you can see: I'm certainly not an expert."
05-23-2008, 06:45 AM
Sounds to me like a fuel delivery issue. Do you have a fuel filter before your fuel pump?
05-23-2008, 09:40 AM
"If you have swapped carbs that takes the carb issue out of the equation.
Check those valves. Tight settings can detract from performance. Check the timing, be sure the PCV system is hooked up and working like it should (this is more important at lower speeds because a stuck PCV valve will lean out your carb mix, you can adjust it to keep running but engine will want to stall. It may affect top end too), as Jeff indicated...check the fuel filter, it could be debris in the tank too...plugging the output tube.
05-23-2008, 09:42 AM
"One other thing I failed to note.....and that would be distributor springs. Next time you're in there, check to see what it looks like down below the plate where the ignition points are affixed.
regards, good luck,
05-23-2008, 10:43 AM
"FYI, The Edelbrock 1409 Marine carb does not have a PCV vacuum connection."
05-24-2008, 11:22 AM
"Mark, you are exactly correct. My 327F on a 1966 20' Fiberglass Chris Craft Sea Skiff has a new 1409 and I had to make an adaptation to take off the fumes, and I am not sure I'm all that pleased with the evacuation from the crankcase now. Oil changes may need to be more frequent, not sure.
The 1409 has a port there that can be tapped for the PCV valve in the event a PCV is desired.
My point in mentioning the PCV issue, is this is very often overlooked when tuning marine motors, and it sure can cause a lot of problems if one is stuck, because it leans out the low speed circuit. At speed this is less or no issue at all.
The 1409 is too much carb for the cubic inches of a small Chevy, but it's the smallest marine carb from Edelbrock. Since Chris Craft equipped the big block 427 with a 625 cvm Carter AFB, the 600 is way big for the small motor. Mine is running okay now, but I still have to resolve some high speed issues, and I think I have a distributor spring issue, not sure.
Looking over the previous postings, linkage and internal friction of the carb butterfly is something that should be checked, along with the function of the choke.
Good luck guys!
05-24-2008, 12:29 PM
"When I see post like this I try to think not what is the cause but how I could create it.
Now with the simple answer usually being the best answer.
Unbalance the fuel to air mix.
Unplug the vacuum.
Will cause the RPM to come up slow to a point with little power at the top end.
Miss meter the jets, (flood) will cause the RPM's to come quick but pass thru the power point and flood the engine ie. loss of power.
Under load, go to full throttle quickly, best guess or measure of speed.
Under load, go to the same RPM slowly, best guess or measure of speed.
Half a knot or better difference, see a carb guy.
No difference check valve lash, compression ?
Keep in mind that it's a process of delivering air, fuel, and spark in the right mix at the right time.
Guessing doesn't help Testing produces results"
05-26-2008, 03:24 AM
"Thanks for all the tips...I have some (test)work to do. Still some questions on the above:
- What is a PCV? I know it from my CAT diesels in another boat, but what is it on the 350Q. On the port valve cover I have an open connection to the flame arrestor, on the starboard cover a small valve controlled connection to the Edelbrock airhorn.
- Which vacuum connection? On my Mallory ignition there is only a weight controlled timing device, no vacuum. The only vacuum actuated devices are the needles in the carb...as far as I can see.
- I'm not experienced with hydraulic adjusted valve rockers, so I'm puzzled how the wrong valve adjustment could cause the symptoms I described?
To be clear: apart from the described symptom the engine is running very well. No strange sounds, no valve ticking, very smooth idle (600 rpm), a little bit rough in just off idle (800-1000 rpm), very responsive, no blow by, no oil consumption, etc.
The carter mechanical fuel pump is reconditioned and giving ample pressure (as far as I can test). The fact that it will come up, and stay up to max revs says to me it delivers enough fuel? There are 2 filters: one big rough filter at the tank (that has been cleaned recently) and a cartridge type water separating one near the engine that has been changed before the winter (when it had the same symptom)."
05-28-2008, 05:39 PM
"Did the work. Reconditioned the timing device in the distributor. Did not help.
Have been doing runs with engine open and no flame arrestor. It became apparent that the symptom is kicking in when the secondary barrels of the carb start opening. I'm pretty sure it is the carb, not the ignition.
The engine reaches it's max revs (approx 4300) when the secondaries just starts opening.
From this I would conclude the Edelbrock 600 cfm 1409 carb is just to big?"
12-04-2010, 11:27 AM
It's possible the fuel level in the carb is a bit too low, sucking air in the imulsion tubes instead of fuel. Try raising the fuel level by 1/32" at a time. Also check the accelerator pump for a full strem of fuel from idle position to WOT (do this in very slow but steady rotation.
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