The Stumble That Just Won't Die

Lets not forget, he said it runs fine when cold.

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Chet

You say you can feel the stumble with the car in neutral. With the car running, with the air cleaner off, try placing your hand on the primary side of the carb (choke it off) and work the accelerator and see what happens. If it tends to smooth out- you have a vacuum leak.

Next thing to try. Put a jumper between the bat. and distb./coil (i.e. to feed ignition external of normal circuit.

I have not forgotten that, but there is something that has not been tried yet to cure it.

Since it runs great cold, with the choke part way on, in a richer running state, than when it heats up, and the choke is [B]open?

One would think it leans out when warm. He has stated making changes to the jetting, and everything else ignition wise, and nothing has helped it.

A warped intake manifold is a common problem. Until it is pulled and checked with a straight edge, we will not know if it is bad or not.

As far as his 1.6 Ratio rocker arms, I am of the opinion they need to be changed back to the 1.5 ratio. I have only used 4 sets(IIRC) of 1.6 ratio rocker arms on sbc's in my life. One time it was a full set, and the other 3 times, it was half, and half.(intake, or exhaust depending on the camshaft grind......etc)

AS I recall That carb. does not have a choke........

Yea, no choke on this carb. I will try holding my hand over the primaries and see how it acts. I got a few more things to try. I want to get a wideband controller to see what my A/F actually is also. Taking the intake off and seeing if it is warped will be my last check as it takes the most time to do.

Thanks for all of the great input guys,
Jason

If and when you do pull the intake, this wiki article may help.....
http://www.crankshaftcoalition.com/...eak%2C_internal

It has no choke !!

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Chet

I read that already. I just missed it in his earlier post.

Something just doesn't add up. It makes sense it is running too rich, but he claims the plugs are brown. (which says it's on the lean side)

Something just occurred to me while typing this.

Gamester, what type of distributor drive gear do you have?

If it is anything other than the hardened cast metal type, such as the bronze performance drive gear. Pull it and look for excessive wear.

It makes sense, in a newly built, tight toleranced engine, when cold it will be tight, when warm the clearances will loosen up, this also includes a new distributor. Once it warms up, the timing would be fluctuating more until the 'advances' kick in.

At higher rpms, the camshaft will be being pushed forward, thus wiping out the 'softer' distributor drive gears. To run the type of drive gear, you must use a 'camshaft button' between the cam gear, and timing cover.

Savvy- Don't think that would be a problem with non roller lifters since the flat lifter tends to restrict cam walk.

I understand what you are saying. Right after I made that post, a '68 C10 with a 383 stroker pulled up to my shop. The engine builder met him here seconds later. The complaint was a stutter taking off from a stop. Coincidence, or not, it only happened when warm.

We had a 15 minute discussion about the engine build, and the modifications to the truck. The owner of the truck said it was worse before he raised the idle speed by 200 RPM. (It was right around 1000 RPM)

So to see exactly what was going on, I had the engine builder lower the idle RPM to 750, then to 650. The timing was jumping around a little bit at 750, and a lot at 650. (I was setup to check the base, and total timing)

After hearing the distributor was purchased off of ebay, I made the decision to pull it, and try out a known good spare I have here.

It was a bronze, or some other type of soft sintered metal drive gear. It was really worn for only having 300 miles on it. I'll get my spare back tomorrow evening.

In my discussion with the engine builder, after his customer left, he agreed with me, and we both do not understand why many of the performance parts manufacturers sell this type of gear, and both of us have seen more failures in roller cam blocks. Jeez roller camshaft blocks use a camshaft thrust plate!

2 hours before they showed up, I was asked if the engine builder was still building engines, and if I had heard from him, by someone else. (he has been building performance engines in his spare time for years, and is one of the only good guys left in these parts)

I have to put a fuel pump in his mom's car tomorrow.

Maybe I should think of some numbers, and play the lotto. LOL

P.S. It was a flat hydraulic tappet cam.

I'm not sure after reading your post, but are you stating the gear on the distributor is no good and that is causing the stumble???? As far as the RPM goes. When he raised the RPM by 200, what that did was create more vacuum at idle. When you bring it down to 650, the vacuum drops. Kinda what we talked about before with having a low vacuum pressure at idle. The vacuum will effect the Holley, especially off the line. That is why people with low vacuum replace the power valve to compensate.

Using a steel billet roller with a iron dist. gear will destroy both the cam and dist. gear. The common solution was a either a bronze or composite dist. gear. The newer billet cams have an iron gear pressed on which eliminates the prob.

Yes, the distributor gear looked to be worn down by 50%. At the lower RPM the timing was bouncing around, and by his raising the rpm the camshaft must have walked forward enough to steady the timing more.

I did not check to see what the timing was at this point, because a question was brought up about a loose timing chain. It was not loose.

It has a (out of the box)1407 Edlebrock on it right now, and it will be switched to a (modified)1406 by Tuesday.

I did not check the vacuum, but what you said has merit.

Just curious, did you find out for sure what the stumble was on that engine carsavvycook?

BTW, as an update to my situation, tried a thicker carb gasket and even a carb spacer today and there was no change to the warm engine stumble. Also, my inline fuel regulator quit working and my alternator burned up. Not a very productive day to say the least

Later,
Jason

The 1406 carb cured it's stumble, with just a slight change to the accelerator pump. (center to the lower hole, in the pump arm).

This coupled, with the steady ignition timing, fixed it.