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engine won't rev

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qed

New member
I am restoring an older boat and found it necessary to replace the Mercruiser 260, 5.7L engine. I purchased a long block marine GM 350 from Summit racing. The block came with the front crankcase cover installed, and reportedly the timing was set. The intake manifold and carb are both new Edelbrock products. The coil is new as are the distributor trigger, spark plugs and wires. The thunderbolt ignition is original, however it appears to check out OK. My problem is that the engine will not run up past 1000 RPM. When the throttle is advanced the engine sputters and seems to be on the verge of chocking out. I've checked the flapper valves in the exhaust to be sure that thee is no blockage. Any Ideas?
 
qed said:
.... I purchased a long block marine GM 350 from Summit racing. The block came with the front crankcase cover installed, and reportedly the timing was set.

Any Ideas?
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Timing can't be set until the distributor goes in.....I'd be inclined to start there.....
 
makomark said:
Timing can't be set until the distributor goes in.....I'd be inclined to start there.....
Click to expand...
What I meant to say was that the timing marks on the crank shaft sprocket and the cam shaft sprocket were aligned at when the engine was assembled prior to purchase. Timing has been set to 12deg BTDC. Engine starts and idles fine. just wont rev.
 
OK, I've checked the timing and it seems to be on the mark. Replaced both exhaust headers and risers thinking that the old ones had too much back pressure as the engine would run fine on the trailer with earmuffs but would bog down in the water. Yesterday the engine worked fine and i ran it a various RPMs while up on a plane. Then dropped it down to idle and then back up to speed a few times. Worked great until the last time, about 20 mi. into the trials. Engine would not go above an idle. Normally I think in terms of electrical malfunctions when the problem is intermittent. Could it be the ignition amp? It's the only part left from the old motor.
 
When you strobe the timing marks, are you able to see the system advance as RPM increase?

In other words.... you should see BASE advance at idle RPM (8 to 10 degrees BTDC), and you should see a progressive advance as RPM is increased.
 
You might want to obtain an outboard motor gas tank and hose and hook it directly to the fuel pump inlet. With this in place, depending on how it performs, you can get a better idea of whether the problem is fuel or ignition related. If the problem still exists with this in place, try pumping the fuel bulb to see if the fuel pump could be a problem. A check of the fuel filter or water separator should also be performed.
 
qed said:
does not appear to be advancing. Is this also why the engine tops out at 3800 to 4000 RPM?
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You may have more than one issue going on..... but I will say that if the ignition system is NOT advancing, there is no way you will get any power or performance from this engine.
The ignition system MUST provide a progressive ignition advance as RPM increase.
At approx 3k rpm, you should see an ignition advance of around 28* or so.


As for not reaching 3,800 to 4,000 RPM..... this could be due to;

engine not in full tune.
marine growth on hull bottom.
incorrect propeller.
incorrect final drive reduction.

 
Thunderbolt systems require that a lead (typically purple) be grounded when the engine is off and then started in order to set base timing and then have the engine turned off, the lead ungrounded and the engine restarted as normal.

Also... thunderbolt disti caps have imprints on them for both CW and CCWW rotation engines. Very easy to get firing order mixed up. Engine will run surprisingly well with two wires reversed.
 
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