bigger cam in my lt4 but wont start

[pjtblack96]

we put a bigger cam in my 1996 lt4. upon starting the car backfires and tries to run but it wont. we called a tune shop and they said either the cam or distirbutor is in wrong. we took the car apart again to check that the timing marks were lined up correctly and everythings right. what can we do to get it running?:W;help

 

[1963SS]

I had one pair of plug wires reversed on my LT4 and the car would not run at all. It cranked and backfired.....that's all it did.

 

[vetteboy86]

Check the simplest of things first, as already suggested. I've been bit by plug wires incorrectly routed.

What was your procedure for verifying timing?

 

[ex-x-fire]

Over adjusted valves?

 

[pjtblack96]

Check the simplest of things first, as already suggested. I've been bit by plug wires incorrectly routed.

What was your procedure for verifying timing?

we already took the front timing cover off to make sure the dots on the cam sprocket and crank were aligned and they were...how else could you verify that its right? i know the bigger cam will throw it off a bit

 

[pjtblack96]

Over adjusted valves?

when we bought the cam from comp cams it said that the stock pushrods would work but when we installed the cam there was too much play in the rockers and the valves wouldnt open completely so we ordered longer rods but im pretty sure theyre adjusted correctly

 

[Paddywhack]

I know its a different year and engine, but double check your fuel pressure. After my rebuild, it wouldnt start,only to find out that the pump went bad sitting there. Same symptoms, backfires, wants to run but doesnt.
Good luck!

 

[JAKE]

A stock LT4, when it comes to the valve train, is a different animal. It uses what GM calls NET BUILD, which means there is NO adjustable lifter preload without changing the length of the pushrod. All you do is torque down the adjusting nuts, IIRC 18 lb/ft) and you're done.

For LT1s and earlier engines, it's more complicated, calling for finding Zero lash then setting the lifter reload by turning the adjusting nuts a certain amount. In this procedure, finding Zero lash is particularly critical.

So, after changing the cam, and if you're continuing to use the stock NET BUILD setup, you need to verify that the pushrod length is correct so as to correctly preload the lifters. If you're now using an adjustable valve train, then you need to set each lifter individually using the most often recommended Intake Closing/Exhaust Opening (IC/EO) procedure.

Too little lifter preload can cause a noisy valve train; too much and one (or more) valves can be held open causing the engine not to run well, if at all.

Keep us posted.

Jake

 

[pjtblack96]

A stock LT4, when it comes to the valve train, is a different animal. It uses what GM calls NET BUILD, which means there is NO adjustable lifter preload without changing the length of the pushrod. All you do is torque down the adjusting nuts, IIRC 18 lb/ft) and you're done.

For LT1s and earlier engines, it's more complicated, calling for finding Zero lash then setting the lifter reload by turning the adjusting nuts a certain amount. In this procedure, finding Zero lash is particularly critical.

So, after changing the cam, and if you're continuing to use the stock NET BUILD setup, you need to verify that the pushrod length is correct so as to correctly preload the lifters. If you're now using an adjustable valve train, then you need to set each lifter individually using the most often recommended Intake Closing/Exhaust Opening (IC/EO) procedure.

Too little lifter preload can cause a noisy valve train; too much and one (or more) valves can be held open causing the engine not to run well, if at all.

Keep us posted.

Jake

yea we took the engine apart again only to find that the valves were being kept open by torqing the rockers to given specs so we adjusted them individually like a regular engine and i now have a working hell-raising vette which we started up for the first time tonight...

thanks to everyone who replied

 

[JAKE]

I'm glad to see everything worked out well for you.

Jake

My son, Ryan M. Cameron, graduated from West Point on 22 May 2010! He was commissioned as a 2nd Lieutenant and I pinned on his first pair of "Butter Bars" PROUDEST DAY OF MY LIFE!!