Engine Engine stumbles. Distributor?

[b3uhbp]

My AMC 258 i6 / 4.2l stumbles on acceleration and begins to knock when under a load. The carb (a Weber 32/36) has been rebuilt, new spark plugs, new fuel filter, new fuel pressure regulator, new vacuum advance. Any idea what it might be? I’m thinking it may be the distributor, a DUI HEI. HELP!

 

[Scout1067]

I have the same motor and carb setup on my 81. There is no fuel pressure regulator with that set up. The fuel return line from the carb naturally regulates fuel pressure at least it does on mine with the stock manual fuel pump. Are you running an electric fuel pump?

The only time mine stutters is when it's cold and disengage the manual choke too soon.

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[caveman]

An HEI is a lot better than a stock distributor but the fact that yours spark knocks under load means that an adjustment is in order and will help.

 

[TSB8C]

An HEI is a lot better than a stock distributor but the fact that yours spark knocks under load means that an adjustment is in order and will help.

What he said While there could be other issues (lean mixture, plugs/wires, etc), knocking under load usually means too much advance.

 

[b3uhbp]

An HEI is a lot better than a stock distributor but the fact that yours spark knocks under load means that an adjustment is in order and will help.

The timing was just checked and set but didn’t help. That’s why I think it’s the distributor.

 

[caveman]

I really believe that who ever did the adjusting must have messed up. If it was adjusted right and there was something wrong in the distributor, It wouldn't spark knock. It would just be lacking in power. It sounds like you have your heart on getting a new distributor any way, And I don't blame you. The OE was junk when new. So go with an HEI that's all set up for a jeep. Their cheap now days and work great.

 

[Bent]

Make sure the vacuum advance is on the correct vacuum port. The correct port draws from above the throttle plate so it should have no vacuum with the throttle closed at idle and increase as the throttle opens. if it is drawing straight manifold vacuum it will have too much advance. Also if your vacuum advance has a hex shape on the front of it then it is an adjustable vacuum advance. Find a hex wrench to fit down the nipple into the adjustment screw. Count the turns to go from stop to stop on the adjustment, then set it in the middle and tune from there. Base timing should be 8 degrees before TDC. With the HEI you can also use a weight and spring kit to slow the mechanical advance if the distributor uses gm parts. My junkyard chevy 4.1 big cap HEI with AMC V8 distributor gear has worked great for many years. The DUI should run well with that carb.

 

[NP435]

There is a lot to be said for resetting timing until it wont ping under a heavy load on a hot day. specs be dammed. Saw that in 4wheeler in the 80s talking about IHC 392 V8 engine's ( I had one in a 3/4 ton 4wd truck) . That beast of a gas engine would pull like a 460 ford. MPG was laughable at best towing.

 

[b3uhbp]

Make sure the vacuum advance is on the correct vacuum port. The correct port draws from above the throttle plate so it should have no vacuum with the throttle closed at idle and increase as the throttle opens. if it is drawing straight manifold vacuum it will have too much advance. Also if your vacuum advance has a hex shape on the front of it then it is an adjustable vacuum advance. Find a hex wrench to fit down the nipple into the adjustment screw. Count the turns to go from stop to stop on the adjustment, then set it in the middle and tune from there. Base timing should be 8 degrees before TDC. With the HEI you can also use a weight and spring kit to slow the mechanical advance if the distributor uses gm parts. My junkyard chevy 4.1 big cap HEI with AMC V8 distributor gear has worked great for many years. The DUI should run well with that carb.

Great info. I’ll check it all out. Thanks

 

[Lumpy Grits]

Find another mechanic or learn to rebuild the carb yourownself

 

[b3uhbp]

Find another mechanic or learn to rebuild the carb yourownself

Very helpful.

 

[Scout1067]

Very helpful.

Actually rebuilding a Weber 32/36 is not super hard. You can get the complete rebuild kit from Redline and as long as you have a workbench and a standard toolbox it's easily doable. The instructions with the kit are pretty clear. The super fun part I found was adjusting it for good idle after the rebuild, rebuilding itself was straightforward

I rebuilt mine and hadn't touched a carb other than a lawnmower carb in over 20 years.

Redline is the only authorized Weber dealer in the US. If you are not sure what you need just contact them their techs are pretty helpful. http://www.redlineweber.com/

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[petemcl]

My 258 stumbles on acceleration and begins to knock when under a load. The carb (a Weber 32/36) has been rebuilt, new spark plugs, new fuel filter, new fuel pressure regulator, new vacuum advance. Any idea what it might be? I’m thinking it may be the distributor, a DUI HEI. HELP!

Often this is caused by dirt in the fuel system. Make sure that your fuel cap is vented. Pull the top of your cab and look inside. If it looks like this you will need to pull your gas tank and thoroughly clean it out, clean all your fuel lines and the fuel pump and finally go through your carb making sure that all your passages are clean particularly the accelerator pump circuit. Make sure that your rebuild kit contains o-rings, seals, gaskets and diaphragms that are alcohol resistant. Might be hard to find for a Weber. NOS kits will not do.

Also check your fuel cap as many parts suppliers have misinterpreted EPA regulations and are supplying non-vented fuel caps.

 

[generalmoto]

My 258 stumbles on acceleration and begins to knock when under a load. The carb (a Weber 32/36) has been rebuilt, new spark plugs, new fuel filter, new fuel pressure regulator, new vacuum advance. Any idea what it might be? I’m thinking it may be the distributor, a DUI HEI. HELP!

I have upgraded my stock distributor on my AMC 304 to HEI coil with MSD 6AL ignition box. I had tried to counter a low throttle stumble with advancing the distributor but then got the load knock. here's what I figured out,

-set timing and adjust distributor ahead until you hear a knock at cruising speed when you mash the pedal. then back it off until knock gone.

-with the stock ford style ignition the vacuum advance was adjustable with an allen key. yours may be adjustable as well. it is the final tune. I used this post to tune it and it is a wealth of information. on the right below the picture of the vac advance canister is the how to which is gold.

https://www.fordmuscle.com/archives/2000/03/timing/

If you bought an MSD generic distributor you will most likely have to set up the mechanical advance (the weights in the distributor)

if you suspect the fuel system check your sparkplugs at idle, part throttle etc to see if they are burning rich or lean or just right.