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LM318 Lacks top end power

D

Dennis B.

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"1986 LM318 with 4bbl. Q-jet c

"1986 LM318 with 4bbl. Q-jet carb.,Mallory electronic dist. Have problems running above 3100rpm. Not starving for fuel as flowscan meter shows good flow, also not popping.Last year changed cap and rotor to correct, this time did not. Timing is set 8deg. BTC and 26deg.BTC advanced.Replaced coil and ballast resistor, replaced fuel/water filter and cleaned carb filter.Plugs are clean,replaced 100hrs ago. Any suggestions, last month was running 3800 WOT. Yes bottom is clean. 1976 Luhrs Classic Glass!"
 
"Vacuum leak somewhere maybe?

"Vacuum leak somewhere maybe? My test for a leak is to spray WD-40 around the carb gaskets and intake manifold when the engine is idling. If you hear a slight change in engine RPM when spraying, you've got a leak."
 
"You need to check your timing

"You need to check your timing at 3,000 rpms with a computing type (advancing) timing light. You should see about 26 degrees of spark advance. Its possible the advance isn't working right.

Jeff

PS: Vacuum leaks are negligable above high idle speeds."
 
"Jeff, already checked with a

"Jeff, already checked with a standard timing light and shows 26deg advance at 3000rpm (no load at dock). Also just removed carb to rebuild,has not been done since last year."
 
Okay--you're narrowing dow

Okay--you're narrowing down the possibilities. Make sure the secondaries are opening--they can stick shut (and it doesn't make much power on those tiny primaries).

Jeff
 
"Finally got back out to test

"Finally got back out to test after carb rebuild.Results are still the same. Guess I will make a pressure gauge to check the fuel pump. Should this be about 3-4psi, any other ideas?"
 
"Be sure those sevondaries are

"Be sure those sevondaries are opening, put a new filter in the carb and add a good,
METAL, in-line filter BEFORE the carb and AFTER the fuel pump, then see if it runs okay.

Checking the fuel pressure at idle doesn't do much--you need to check it with the boat at WOT throttle. Still, if it doesn't make at least 5 psi at idle...


Jeff"
 
"Jeff, I am aready looking for

"Jeff, I am aready looking for a in line filter to put before the carb. Also noticed when backing into my slip at idle the motor almost stalls out. RPMs at no load idle is 900-1000 and in gear drops to 500-600. Thinking this is fuel related, possibly a clogged fuel pump or tank pick up clogged. I did find a lot of white crystal like deposits in the top of the fuel/water sep. when changed last on 8/10/07. Also should I use a in line style filter or a canister style (Fram) before the fuel pump?"
 
"Okay, just came back in from

"Okay, just came back in from a 1 1/2 hour run. I have replaced the fuel pump and installed a inline filter (both Chrysler marine products). Still will not get above 3000 rpm. Also when coming in to my creek at 6knt zone motor stumbles and idles rough. Stalled out unless idle is high. I am thinking of trying a outboard gas can to see if it is in my tanks. Boat is 31years old with original galvanized tanks (65gal ea.) Never had this problem before but with ethonal I think we are stuck with finding problems at the boaters expense"
 
If your potter or sounder has

If your potter or sounder has a readout for alternator voltage output check that it is maintaining required output. 13.2+ is good. It caused my 318 to stumble at idle coming into my slip just as you described at 12.6 to 12.8.
 
Problem with top end power sol

Problem with top end power solved. Had to transfer all fuel out of tanks and refill with new gas. Also low speed stumble seems to be carb out of adjustment ( Q jet ). Will be putting on my spare Holley and testing.
 
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