stumbling and occasional backfire

[lnydam]

I have an 87 chevy 1/2 ton 4x4 that i just did a motor swap on, I took out the old 350 and put in a crate 383 from blue print engines. I cannot get the truck running exactly right. When i take off from a stop, it runs ok until it shifts into the next gear, at which point it stumbles, and backfires once and a while. On the highway, if i put my foot into it to pass, it cuts out for a second. In neutral it revs just fine, only under load does it act this way.

I was messing around with it and unplugged the temp sensor on the water neck and the truck runs way better when it is warmed up than with it plugged in. When its cold however, I get the same stumbling and occasional backfiring. I replaced this temp sensor since it was like 10 bucks and it didn't do anything. Right now i just drive it with the temp sensor unplugged and wait for it to warm up, then it runs pretty good.

My timing is 14 initial and 36 total. The distributor is an MSD pro billet with no vacuum advance, but I ran it in the old motor and didn't have this issue. 

Any help would be much appreciated, it disappointing when you spend thousands of dollars on a sweet motor and can't get it right.

[rich weyand]

Do you mean back fire (out the carb) or after fire (out the exhaust).  I'm guessing after fire.  After fire is one of those weird symptoms that can be from insufficient advance OR too much advance.  I'm thinking retarded, because your stumbles are when the vacuum changes.

A distributor with no vacuum advance is a "performance" distributor only from the point of view of a racing application.  It sucks on the street.  Your previous engine didn't exhibit this issue, but the more you tweak a motor from a low-performance stock configuration, the more picky it will be.

Race distributors do not have vacuum advance because they have their foot in it all the time -- no vacuum advance needed -- and vacuum advance is just one more thing to break.  And if it breaks to advanced with their foot in it, the motor could dynamite.

Get a distributor with the stock HEI setup.  The stock advance curves are pretty good for the street.  You can pick up one for $40 plus $10 shipping here: http://www.skipwhiteperformance.com/detail.aspx?Item=6501-B.  Pop it in there and see if it solves the problem.  With the money you have invested already, that's a cheap enough test.

[lnydam]

OK thanks I will give that a try!

[Captkaos]

I am guessing it no longer has a TBI on it?

[rich weyand]

I am guessing it no longer has a TBI on it?

I second the question.  I assumed you were running a carburetor, but need to know.

Your symptoms could also be from running way lean, which might be the case if you simply transferred the carb from the 350 without re-tuning it, or if you just popped it on there out of the box, particularly if your 383 is drawing significantly less vacuum than the carb was set up for.

If a proper street distributor does not solve the problem, the next step is to get an A/FR meter on there and see what your mixture is doing, but that's $300, so do the $50 distributor first.  We KNOW that's wrong.  (Or I do, anyway.)

[overtime]

I had a similar problem with my 81 c20.  A big part of the problem was the exhaust leaks.  Check to make sure all exhaust bolts are tight.  Check the plugs to see if they're black.  I agree with rich about the distributor.  What carb are you running? 

[roundhouse]

If you still have the TBI then you need a custom tune computer chip to run the larger 383 engine


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[lnydam]

I still have the TBI

[lnydam]

The distributor that you gave me the link to wont work cause im running tbi correct? Also, the truck has never had an 02 sensor in it for as long as i have owned it. No idea why. didn't have one when i bought it and never had an issue

[rich weyand]

The distributor that you gave me the link to wont work cause im running tbi correct? Also, the truck has never had an 02 sensor in it for as long as i have owned it. No idea why. didn't have one when i bought it and never had an issue

So how is the computer supposed to know what the mixture is so it can meter fuel?  Sounds like the default mixture the computer uses when it can't see the sensor worked OK for your 350, but won't work with this engine.  If you have TBI, you really need the O2 sensor to the computer.

[Irish_Alley]

wouldnt this cause a cel?

[78BIG-TEN]

Am I understanding you are running a distributor at 14 base and with a mechanical advance to make it 36.

[lnydam]

yes 78bigten that is correct. Yeah i figured that lack of an 02 sensor probably wasn't helping, but since it wasn't an issue on the old motor i kind of forgot about it even not having one. It has a brand new Holley throttle body. Holley P/N 502-6.

[Captkaos]

Are you running the correct HEI for an ECM controlled motor?  You set it's timing at 0 with the bypass connected and then plug it back in.
Are you sure you don't have an 02?  It would be in the back of the exhaust manifold.
The Holley TBI is the size of the 454 GM TBI, generally you need a chip for it.

[78BIG-TEN]

Never saw a TBI engine running a conventional distributor,not quite sure how it could fire the injectors
without being tied into the ecm,TBI units don't have any mechanical advance in them