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720 idles but no power under load


MarkR

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Hi all,
I have a 1980/81 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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Before you dismantle, check your compression on all cylinders Then you can pull the valve cover and check your valve clearance if #4 is very low.  Might be as simple as a valve adjustment or a head gasket.

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What engine is in your 720?

 

Is it loosing coolant and needing to be topped up more often now than before?

 

Were the spark plugs sooty before you replaced the jets? The primary jet is stamped with a number near 100 and the secondary is around 150. They can be mistakenly interchanged producing a very rich primary barrel mixture. 

 

If they were already sooty this would indicate a rich mixture. Is the choke shutting off? It should be closed when engine is started cold and open fully in 5-8 minutes.

 

As jagman suggested check all valve clearances. It's free. A compression test on all four might point to something.

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Hi Datzenmike

The engine is an L18 petrol.

Not loosing any coolant and the oil is not milky

The spark plugs were only sooty after I did the carb. The fuel mix was too rich and initially I did have the jets swapped around. The carb kit came with a 100 for primary and the original was 102 which I eventually put back, secondary is a 170.

Even after all this and adjusting fuel mix the engine wants to die when I try to pull off.

The carb has a manual choke and is operating properly

I intend to do a compression test and recheck the valves when I get back to the farm in a few weeks just trying to plan ahead at the moment and get some starting points.

Thank you

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Thanks everyone. I'm going to check all these ideas out and let you know the outcome. I did notice last time I worked on carb that one of the mounting bolts would not tighten up properly may be stripped and that maybe where the air leak is

 

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5 hours ago, banzai510(hainz) said:

if this happen all of a sudeen I think its a valve lash pad or rocker came off

 

starting could be loose intake carb

 

 

Says it idles. lash pad or rocker would really affect idle.

 

 

 

Would you say that the loss of power happened AFTER to took the carb off and rebuilt it?

 

If after the carb came off then the primary jet may have something partly blocking it.

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Thanks everyone. I'm going to check all these ideas out and let you know the outcome. I did notice last time I worked on carb that one of the mounting bolts would not tighten up properly may be stripped and that maybe where the air leak is.

Power loss came after carb rebuild so there might still be a blockage somewhere

 

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