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Kingcab

L20b 86mm pistons

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So I have a 79 l20b I just got done installing 86mm pistons in it and I replaced all the crank bearings as well. Everything other than the pistons bearings and rings carb headers and dissy are stock (engine wise) I can get it started just fine I can get it to idle also, but it has a slight stumble wile idling and it stumbles pretty bad in lower rpms. Can anyone help me figure out why it's stumbling and won't run smooth...

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Everything other than the pistons bearings and rings carb headers and dissy are stock (engine wise)

Ok first this line is confusing....

So You bored the engine to 86mm, that has new parts....

 

Carb headers and dizzy are pre-existing on the engine?

 

If that is true....

Have you checked timing?

And have you adjusted the carb at All?

Mainly the air/fuel....

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Yes I got the cylenders bored out 1mm oversize original bore is 85mm and I bored to 86mm and got 86mm pistons. I have timed it a few difrent times advancing the time and reatarting the time, both the timing chain and dizzy. I have adjusted the carb more than several times. And all aftermarket parts I have installed myself. It all ran good before the bore and piston installation.

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What are you setting the timing at?

And approximately where is it idling?

And what type of carb?

 

Also have you checked for a vacuum leak at all?

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Probably the carb. Sediment stirred up removing or mounting it.

 

Firing order is 1342.

 

Set timing to 12 degrees BTDC

 

Check valve lash just in case.

 

Set engine to TDC turning the crank up to the zero timing mark in a clockwise direction only. If you over shoot back well up and re-do as  many times as necessary. Must be clockwise and stop to remove all traces of chain slack and get a good reading. Now take the valve cover off and look through the top hole in the cam sprocket. You should see the V notch in the back of the sprocket just below and slightly to the left of the small horizontal line etched into the cam thrust plate above it. Like this...

 

otMggUE.jpg

 

If you have this..... the cam is properly timed

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Is the carburetor a stock Hitachi or Weber ? 

 

 If it is stock Hitachi and you took it off and turned it upside down at any point to clean or whatever...  They don't like that,, i don't know why if it's built up sediment that gets loose or just something else,, but i seen it do exactly what you're describing a couple of times.  Try taking out the screw in plugs under the float bowl ( you can do this without removing carb from engine) and run some clean gas or carb spray through ,, into,, it. 

 

 Before all the Weber lovers get on here and spew their hate.. A Hitachi is a thousand times better daily driver carb than a Weber will ever be without jetting and all that horseshit.  Hitachi's aren't that hard to learn to rebuild if you are at all not a knothead,  and have a clean,, and very organised work space the first time you try.. 

 

 

I just noticed this is posted in wrong section

 

 

 

 

.

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I have the it all timed to TDC looks just like the pic. Firing order is also correct. And it's a weber carb freshly cleaned out. No vacume leeks that I could find. And it's idling slightly below 1 rpm. Going to chek valv lashing

What is proper valve lash?

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.008 intake

.010 exhaust....

Pretty sure its done cold too...

 

And you said it's timed right...

But How much timing on the dizzy, 12 like mike said?

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Nanner I totally agree. For a much more complex Hitachi carb you get way better drive-ability and mileage. Did you know that the Hitachi was designed with a slight leak back into the float chamber from the accelerator pump? This allows gas to return rather than being dumped into the primary when you slowly step on the gas. The faster you step on it the more gas squirts in. On a Weber no matter how fast or slow you step on the gas the same amount goes in from the pump diaphragm. Little things like this.

 

The Weber is the AK 47 of carbs. It's no more than a pipe that mixes gas and air.

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And is it a real weber....

I ask because someone else here just got a new weber which may not have been a real one and he was all of a sudden having a bitch of a time to get his to run right too....

Lucky for him the company warranteed it and sent him a replacement and all his problems went away for now....

 

Have you opened the carb up at all to clean it?

 

Everything else seems to be correct...

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A new Weber ALWAYS is seen as a vast improvement to an old Hitachi with 130,000 miles and no maintenance on it.

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Yes it a real weber I have an imitation weber I also tried and they are both freshly cleaned. And I'm getting the same results out of both. How do I set dizzy properly?

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Well depending on the type of timing light you have I know 2 ways to do it....

Basically the pointer on the crank is what you need to read....

I believe the l20b has notches on the pulley and a single pointer mounted...

 

Should look like this....

Screenshot_20171207-145251.png

 

Basically top is top dead center....

Verify that location is correct and the pointer is pointing to the notch....

The the light gets hooked to #1 plug wire... the flashing should make the pointer point in between the 10 and 15 mark....

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That's was for a fixed timing light....

If you have one you can input the advance it's even better....

With those I set the advance to 12 for the base timing and read the timing at the zero mark on the pulley...

Same to check full advance except I would set for around 32...

 

If it's not at zero add or subtract from 12 on the gun to make it read closer to zero....

Whatever you end up at... 10 12 14 or more... that is your base timing....

 

Adjust the distributor by rotating....

If you want 12 for your timing, put 12 in your gun and rotate the dizzy till it gives you the best zero reading on the pointer and pulley....

 

If using a regular timing light you would rotate the dizzy to get the pointer to read between the 10 and 15......

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If it runs this shitty if you haven't check the cam timing I mentioned earlier, you should. Then right to a compression check. You should have better vacuum than 10. Maybe one or more low cylinders.

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Okay everything is timed perfectly now and valve lash has been checked and it's still having the same issue

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Compression test #1 cylender 150 #2 #3 #4 all 180 I'm gunna go ahead and take the head off and see what's going on.

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Compression test #1 cylender 150 #2 #3 #4 all 180 I'm gunna go ahead and take the head off and see what's going on.

Well that's unfortunate....

I don't know if you pulled the head yet or not... but isn't there a trick like putting a little oil in the cylinder to see if it's the rings?

 

With the head off you can make sure the the valves are sealing?

 

I know you probably wanna figure out why number 1 is low... but my question to the others here is would that cause the problem you described?

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Don't pull the head off, yet. 150 is a bit low for brand new. The lowest cylinder should be within 10% of the highest. Yours is 20%

 

 A couple of teaspoons of engine oil will seal leaking rings and boost the compression. If it doesn't go up significantly then it will be the valves not sealing.

 

You staggered the ring ends correctly when installing the pistons?

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All this speak and no mention of an ignition miss.

 

 

Plugs good? Wires good? Dizzy installed properly?

 

 

Think gas, fix spark. Vice-versa.

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Yes I staggered the rings I already took off the head though haha. And the valve are properly seated when I did the rebuild I resealed all the valve and replaced the valve stem seals

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My thoughts are.... I put bigger pistons in it does anything else need to be done for it to run perfectly because all other internals are stock it's a w58 head

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All L series pistons are the same pin height so if the bore is right they will fit and work. An 87mm bore only increase the L20B to 1997cc from 1951cc. This isn't going to affect anything.

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Not sure on the pin hight with these pistons but the stock ones didn't come up flush to the top of the block and the ones I installed are just about flush with the top of the block

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