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MiltonV

Can't set the 800rpm idle, engine vibrates

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Hello, my neighbor have a 620 I was helping him to adjust the idle but it was impossible, the motor shakes and the more I reduce the rpm the more it shakes

Can't stabilize it at 800rpm because it holds for seconds and then it just shut off, so 1200 rpm seems to hold ok but no less than that. I sprayed some gasoline around the carb to see if there were some air leaks but it was negative for that.

Also removed #4 spark plug and the motor kept running exactly as it was before, so no appreciable change, did the same with the other spark plugs with same result, no visible change in motor.

Any ideas? Suggestions?

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I've got the exact same issue. I just replaced the spark plugs and seemed to help a little. My next plan of attack was to see if adjusting the timing would help and a compression test.

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First, the firing order is 1342 in a counter clockwise direction. If you had the plugs out and some wires off it's common to mix the 2 and 3.

 

Before setting any idle be sure the valve lash is set properly on a fully warmed engine and the ignition timing is set at 120. With this done you can now try to set the idle mixture.

 

First, Milton, I assume this is a stock 521 carburetor and not from a later 620 or anything else??? Does it have the hex shaped thing with the red wire??

101_0100.jpg

 

 

 

If not....

 

Not below 1,200 would seem like the idle circuit is not working. Find the screw with the spring around it on the base of the carburetor. This is the mixture screw for the idle. Sorry crappy picture but it middle bottom above the small left pipe

 

101_0096.jpg

 

Try turning it out to get more gas in and then lower the idle speed screw, the one that opens and closes the throttle.

 

If no results remove the idle mix screw saving the spring and counting the turns so it goes back in the same place. Get a can of carburetor spray that comes with a plastic straw. Stick the straw into the hole and blast away. Don't get that shit in your eyes it stings. Give a good cleaning and put the screw back in and try again.

 

 

 

Lawlis, check the firing order, valve lash (hot engine) and timing first. Has your 620 ever run properly? Is this a recent thing?

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53 minutes ago, datzenmike said:

First, the firing order is 1342 in a counter clockwise direction. If you had the plugs out and some wires off it's common to mix the 2 and 3.

 

Before setting any idle be sure the valve lash is set properly on a fully warmed engine and the ignition timing is set at 120. With this done you can now try to set the idle mixture.

 

First, Milton, I assume this is a stock 521 carburetor and not from a later 620 or anything else??? Does it have the hex shaped thing with the red wire??

101_0100.jpg

 

 

 

If not....

 

Not below 1,200 would seem like the idle circuit is not working. Find the screw with the spring around it on the base of the carburetor. This is the mixture screw for the idle. Sorry crappy picture but it middle bottom above the small left pipe

 

101_0096.jpg

 

Try turning it out to get more gas in and then lower the idle speed screw, the one that opens and closes the throttle.

 

If no results remove the idle mix screw saving the spring and counting the turns so it goes back in the same place. Get a can of carburetor spray that comes with a plastic straw. Stick the straw into the hole and blast away. Don't get that shit in your eyes it stings. Give a good cleaning and put the screw back in and try again.

 

 

 

Lawlis, check the firing order, valve lash (hot engine) and timing first. Has your 620 ever run properly? Is this a recent thing?

Firing order is ok, I've already checked it.

Will try to set the valve lash, I haven't did it yet.

Not a stock 521 carburetor but a stock 620.

The hex thingy is not present on these, the motor is a J15.

I tried playing with the idle mixture needle but after 3 turns it doesn't seem to work, if I close it to the bottom the motor dies.

Will try that with the carb cleaner.

Not my 620 but since my neighbor got it I don't remember it with a smooth idle, I just cleaned the outside of the carburetor and tried to set a proper idle and eliminate the trembling but that was impossible so far.

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The firing order is correct, seeing as it rev's fine and has a good strong running sound when on the throttle.

 

I haven't set the valve lash yet, I didn't know that that would have such a drastic effect on these engines.

 

I was going to check the timing on the engine today but got a little busy and will have to do that tomorrow. I might as well do the valve lash tomorrow as well. 

 

It hasn't ever really ran properly, but in the time I've owned it I think I've only got it up to temp twice. The first time was when I drove it home (which was about a year ago)

 

Whatever the carb was tuned for, it clearly wasn't working right. Here's what the spark plugs looked like.

 

dcc5fcc0dac12e7d04e0ed17e3bdc721-full.jp

 

And this was one of the better plugs...

 

I have since rebuilt the carb, put new plugs in, and "tuned the carb. Though I've never tuned a carb before and I wouldn't be surprised if I royally messed it up, lol.

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EVERYTHING else you do in a tune up affects the carb.  I meant to "yell" EVERYTHING!

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Idle is running with the least air and fuel..... if the timing is off there is a big drop in efficiency or getting the most out of the gas that's running it and it idles poorly. A tight valve may not seal properly and the same thing. May not be noticeable when driving. If the idle mixture is too lean or too rich, again same thing it really affects an engine only running at idle.

 

Your picture didn't show up but a dirty set of plugs will also cause a poor idle as will bad plug wires and worn out points. Though if you say it runs well driving probably not a factor

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2 hours ago, datzenmike said:

Idle is running with the least air and fuel..... if the timing is off there is a big drop in efficiency or getting the most out of the gas that's running it and it idles poorly. A tight valve may not seal properly and the same thing. May not be noticeable when driving. If the idle mixture is too lean or too rich, again same thing it really affects an engine only running at idle.

 

Your picture didn't show up but a dirty set of plugs will also cause a poor idle as will bad plug wires and worn out points. Though if you say it runs well driving probably not a factor

What photo hosting site do you use. I've noticed with the hosting I've used that sometimes it shows up and sometimes it doesnt.

 

I suppose I just figured over time the rockers would loosen rather than tighten. But the previous owner could have not set it right had he tried to do it.

 

Yeah I'm waiting a bit to get the idle mixture dialed in really well until I get the air filter for the weber. The previous owner had none and I've got one coming in the mail this week.

 

It seems to run idle well until the choke is off and it's no longer on the high idle lobe. I did source a 12v key on wire and it is now hooked up correctly to the choke.

 

 

 

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The choke forces a rich mixture and also  opens the throttle slightly to rev it up and warm faster. The rich mixture when cold can mask a lean idle mixture when the choke is off.... makes sense.

 

As long as there is SOME valve clearance the valves will close tightly. If valve clearance is too tight an intake might not close completely and it's basically a vacuum leak for the intake manifold, an exhaust valve will leak fuel and air out making for a weak firing. That cylinder does not now share it's part in keeping the engine at idle. 

 

Timing is important because the cylinder doesn't reach maximum pressure until about 170 after TDC. (give or take) You don't want maximum cylinder pressure near TDC because the piston and rod are vertical or stacked and can't be forced down. Maximum pressure after  about 170 and the expanding gasses are in a rapidly expanding space as the piston accelerates downward. For an L20B 120 before TDC gives maximum push at about 170 after and the idle is strong. A good strong idle needs a properly set timing.

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Hopefully there is No idle cut off selinoid that isn’t hooked up

 

to me it could be a plugged idle jet.there is a brass screw near the accel pump diaphram on the valve cover side.Pull the jet out and separate from the holder and make sure it’s clean.

cut open you fuel filter and one can gauge if you have a lot of dirt going thru the carb

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