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Datsun 720 idles but no power

963 Views 5 Replies 3 Participants Last post by  zanegrey
Hi all,
I have a 1908/81 Datsun 720 4x4 single cab that I have restored. Recently I started experiencing starting problems which has led to now other problems.
So let me explain: Initially vehicle would start easily when cold and run fine but once hot very difficult to restart and would take several repeated cranks to finally start.
I then changed the ignition coil but no real change. Then I took off the Carb , stripped and cleaned it, bought a carb kit and replaced all the jets etc refitted carb. Vehicle ran but not well and spark plugs were sooty, tried various adjustments no real change, reset valves and points but still no power could barely pull off. Replaced spark plugs and leads no change.
I am now at my wits end!!!
Could a valve be broken or stuck?
Is there still a blockage or something wrong with the carb?

What I have noticed is if I pull the spark plug leads off while the vehicle is idling there doesn't seem to be any change in the engine on number 4 cylinder or very slight.
Could this indicate the problem?
Should I pull the carb off again and strip it and clean it again or pull off the head to check the valves.
Any advice would be appreciated
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Run a compression test on all cylinders. If #4 cylinder is very very low, then there may be a blown head gasket in the area of #4. Follow up with a "leak down" test on #4 to determine where the pressure is going. The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn.

If compression on #4 cylinder is within spec, then check to see if there is a loss of ignition on #4 or the spark is very weak.
Hi Rogoman
Thanks for reply. I will try compression test and leak test, I suppose I can just use compressed air for that. I've also been wondering if the distributor cap is not cracked, can't see one but still.
I fitted a new coil so not sure if it would be weak spark..
A leak tester does require a compressed air supply. First you need to get a leakdown tester which consists of two hoses, one or two gauges, and a hand valve. To do a leakdown test, fully warm up the motor. Remove the spark plugs. On the cylinder that you're testing, bring the piston up to TDC on the compression stroke. Install one of the hoses into the spark plug hole; The other hose to a high pressure air supply. Apply a certain amount of air pressure by adjusting the hand valve. Watch the gauge which will indicate the percentage of leakage. An engine in great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble.

Thanks for the info.
I'm going to see if I can borrow one from a friend if they to expensive.
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