hey, i need some serious help here... i don't know what the heck is up with this engine.. this is the deal, i broke a piston in my '86 IROC so i pulled the motor and put a shortblock from an '89 IROC into it, both motors are 305s..

anyways, i ended up getting the '86 non-centerbolt heads rebuilt and putting them on the '89 roller block so that i wouldn't need to change my intake and MEMCal..

i started it, and found that it runs absolutely HORRIBLE... it backfires out the exhaust and has NO power, not wanting to rev over 3000rpm and will not hold an idle...

i'm thinking tha ti adjusted the valvelash too tight, as I went 1.5 turns past 0 lash.. that is what my Chiltons said, was that too far?

i think the problem might be valve lash, as if they are tightened down too much the exhaust valve will be opening too early in the combustion stroke and some flametravel is going into the exhaust... am i right?

also, do roller lifters require LESS valve lash than flat tappet..

thanks, i appreciate your help!
Colin Opseth

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Webmaster: www.IROC-Z.org

 

also, the engine is throwing no codes except EST (which i had disconnected to set base timing)

thanks again,
Colin

 

I'm not sure about the exact cause, but all GM hydraulic lifters should be set at 1/2 to 3/4 turn past zero lash. I would guess that 1 and 1/2 turns past zero lash is enough to keep the valves open too long and make it run like poop. Readjust the valves and go from there. (Where the hell did that book come from????? )

My $.02

hope that helps.........



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Matt

1987 GTA L98 MD8 GW6

"Stop Lights timed for 35Mph are also timed for 70Mph"

 

taht was my fear all along..

i know, when i was setting them on the engine stand i remember thinking "holy s h i t, that's alot of lash... but that's what the book says..."

i yanked off the valve covers and backed them off 1/2 a turn all the way around, i hope that fixes the problem..

i woulda started ti but it was 12:30am and i'm sure my neighbours would appreciate that

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Webmaster: www.IROC-Z.org

 

Damn, Your Nicer than Me.

WE Pulled in here last night at 2:00 AM with a Vette On open Headers...



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60 Ranchero - Project ( Money Hole )
85 Sport Coupe LG4 - Daily Driver
Reader's ride -> My Ride

Just another Hot Rod kid, or thats what they all tell me.
Livin' the Stereotype

 

question still isn't answered ... i think the valve lash is too tight, are roller lifters more sensitive to valve lash (ie. they should be set with less lash than a flat tappet)

thanks

 

You may try looking at the pushrod length. I'm not certain, but there may have been a pushrod length change from the early non-roller blocks to the '87-up roller types. A pushrod too long or too short would definitely screw things up.

As for valve takeup, I always set mine at 1/2 to 3/4 turn, never as much as 1 full turn.

And a third thing to consider: are you certain the distributor timing is set correctly?

 

thanks for the reply

distributor is set at 0 BTDC for sure, no doubt about it and we double checked once the car was running...

i am using the '89 pushrods with the '89 rocker arms in the '86 heads...

the '89 rocker arms are self-aligning but the '86 rockers are not... from what i was told it wouldn't matter either way as both rocker arm styles are the same ratio..

i think the valves are just too tight, i'll try starting it in 5 minutes and see how it runs.. bbiaf

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Webmaster: www.IROC-Z.org

 

runs like a pile of dog s h i t...

i think the pushrods are binding in the heads, don't know why but that's my guess...

i'm so frustrated, both of my motor swaps in my IROCs have been like this, yet i can do a big block with no trouble.. obviously the way these cars are designed really SUCKS.

lame.

i'll put the roller heads on so i can use the correct rocker arms.. gotta look around for an '87 intake now, i guess...

thanks for the help guys...
Colin

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Webmaster: www.IROC-Z.org

 

One last suggestion, and try not to get offended if it sounds to simple...

did you make sure the plug wires are routed to the correct spark plugs?

I've done this myself, and it's an easy mistake to do if you're in a hurry.

 

yup, double checked..

thanks man..

now that i think about it i remember seeing some chaffing on the pushrods right where they go through the heads...

i knew this job would be too easy to be true

i can't wait until this IROC is running and sold so i can finish my big block!!!!

thanks man,
Colin

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Webmaster: www.IROC-Z.org

 

Just some ideas here:

When you had the valve lash set waayyy too tight you could possibly have bent a push rod or two and now they are binding and/or throwing off your valvetrain geometry. Keep in mind that the push rods will naturally show some signs of wear where they pass through the heads. If you had the lash set at 1.5 turns past zero lash and only backed it off a half turn, than you are still too tight but you probably should have noticed some improvement in the running quality.

As far as parts interchangeability you may also want to check the rocker to valve retainer clearance. I had a friend that could not figure out why his engine was running like crap and finally noticed that the exhaust valve retainers had rotators on the tops of them and the rocker arms were hitting them causing valvetrain bind that ended up bending ALL of his exhaust push rods (hardened Crane ones! ). Boooo hissssss.

I can tell you that the factory push rod length for your roller cam motor is 7.200 inches. I'm not sure about a non-roller motor but my guess would be that they are a bit longer and the lifters themselves are a bit shorter on the non-roller setup.

And, don't take this the wrong way, but I'm sure you know that you should always check your valvetrain geometry prior to assembling an engine when you are straying from a 'stock' setup so I won't even mention it .

Anywayyyyyyssss, I hope you figure it all out .

Laterzzzzzzzzz

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Matt

1987 GTA L98 MD8 GW6

"Stop Lights timed for 35Mph are also timed for 140Mph"