good compression on all 8 cylinders, only runs on 4?

hallgren87idi

Registered User
Joined
Jul 10, 2013
Posts
95
Reaction score
1
Location
gold bar, washington
So this Is bugging the hell out of me. My buddy and I have been putting together a 1988 5.0 roller engine. Stock engine, HEI distributor, put a b303 grind camshaft in from a local cam grinding shop, and a set of better valve springs. Here is where it gets weird, everything is in the car it runs, has oil pressure, but only runs on 4 of 8 cylinders. Compression ranges from 175-185 around all 8 cylinders. Intake manifold was sealed, took that off once to verify, after spraying everything that could have been a vacuum leak with ether, and finding no leaks. Thought maybe we needed longer pushrods so we got longer ones after checking with an adjustable pushrod. No change after putting the intake back on with different pushrods, checked for leaks again none. Got pissed and loostened the intake and pulled the driver side cylinder head to check for a head gasket leak. Couldn't see anything to say for sure it was leaking, but didn't pull the other head and check that one. Pulled the valves out of the two cylinders that didn't fire they were all carboned up, so we cleaned the seats with sandpaper and the valves were pitted. One thought is a combination of water leaking into cylinders could kill them, and maybe the pits on the valves are causing them to not seal? But then why such good psi?? This is really ticking me off :mad::mad: Auto swap meet is coming up this weekend, I'm gonna try find a different set of heads with screw in studs so I can have adjustable rockers. The guy helping me is a chevy guy and this is my first build we are pretty stuck. It should run on all 8, we tried a new set of plugs no change. Maybe the wires are bad, nope put a plug in and ground the plug and it sparks. So we really don't know other than the heads must be junk, or head gaskets. But I don't wanna try new head gaskets and bolt on a set of heads I am unsure if they are even good.
 

IDIoit

MachinistFabricator
Supporting Member
Joined
Aug 26, 2014
Posts
13,318
Reaction score
3,879
Location
commiefornia
your post is pretty damn confusing.

how does carbon build up on a cylinder that does not fire?cookoo

why would you pull the heads off a engine that compression tested fine?

id bet that your problem lies in your distributor.
 

hallgren87idi

Registered User
Joined
Jul 10, 2013
Posts
95
Reaction score
1
Location
gold bar, washington
It might not be carbon but thats what it appears to be. It could be carbon from when the engine was ran before, I don't know. I got the engine from a guy and put this camshaft in it. We pulled the heads to see what the valves looked like. I don't see how it could be ignition, it has spark on all 8 cylinders we checked. Why would the distributor decide to send good spark to 4 cylinders and not the other 4? Doesen't make sense. It's a hei chevy style distributor brand new, had no issues with the other one I purchased before this. This whole damn thing is confusing me, its not rocket science but It's trying to be.
 

stuborn nut

Registered User
Joined
Mar 8, 2012
Posts
40
Reaction score
1
Location
Salyer,California
so what kind of intake carb set up do you have? If it is a dual plane intake could be a carb issue. Which 4 cylinders does it run on? 2 on each side?
 

hallgren87idi

Registered User
Joined
Jul 10, 2013
Posts
95
Reaction score
1
Location
gold bar, washington
Edelbrock rpm air gap with holley 770cfm vacuum secondaries also tried running with a holley 600cfm vacuum secondaries.
Edit: yes 2 on each side I know #5 and #7 not sure other 2.
 

hallgren87idi

Registered User
Joined
Jul 10, 2013
Posts
95
Reaction score
1
Location
gold bar, washington
#5 and #7 on driver side, I'm not at the car I couldn't tell you the other 2.
Edit: Driver side lol, little bit tired.
 
Last edited:

hce

So can i....
Supporting Member
Joined
Nov 10, 2009
Posts
1,072
Reaction score
329
Location
Glasgow MT (Official middle of nowhere)
check your firing order. new cam could be a different then the stock firing order.
Non h.o. firing order 1-5-4-2-6-3-7-8
h.o. fiiring order 1-3-7-2-6-5-4-8
Notice that 5 and 7 have changed order. my bet is 3 and 4 are not firing on the passenger.
also if it is a carburetor with a dual plane intake check to see if both sides of the carb is fueling
 

laserjock

Almost there...
Supporting Member
Joined
Nov 20, 2012
Posts
8,841
Reaction score
3,129
Location
Maryland
I vote ignition and or cam timing. I ran into this years ago. My uncle tuned up his truck with new plugs and wires. His was an OBS truck. They pulled my dads in (a 91) to check the wire routing after it wouldn't run. Turns out there is a model year change in there somewhere and the firing orders were different. I figured it out by reading a Haynes Manual or owners manual or something. He knew the old order and that's how he put it back and then confirmed it with the older truck. Wrong.
 

IDIoit

MachinistFabricator
Supporting Member
Joined
Aug 26, 2014
Posts
13,318
Reaction score
3,879
Location
commiefornia
stupid question. does the runners on the affected cylinders have an EGR port?
is the intake plentum open or closed?
 

BDCarrillo

Supporting Member
Supporting Member
Joined
Jul 19, 2014
Posts
1,245
Reaction score
10
Location
Abilene TX
stupid question. does the runners on the affected cylinders have an EGR port?
is the intake plentum open or closed?

If they're E7TE heads then there should be EGR ports in each exhaust runner, with a crosslink at the back of the heads.

Also, chucking longer pushrods into a B-cammed 88 is a seriously bad idea... 88 was a unique year for 302 pistons, and they were extremely flat with very minimal valve reliefs. It's easy to smack the valve/piston on that year.

The B-cam is definitely HO firing order... all Ford letter cams are HO order. Make sure you're not using "chevy" cylinder numbering as well...
 

hallgren87idi

Registered User
Joined
Jul 10, 2013
Posts
95
Reaction score
1
Location
gold bar, washington
The camshaft is a b303 grind not an actual frpp grind. I called delta to verify they do not change the firing order. Went from 6.272 pushrods to 6.300 not that big of a difference. ..they were needed, the rocker arms wouldnt preload the lifter correctly.
 
Last edited:

hallgren87idi

Registered User
Joined
Jul 10, 2013
Posts
95
Reaction score
1
Location
gold bar, washington
Just got back from working on it. Installed new aluminum heads with adjustable roller rockers. Turns out the idiot that wired the dizzy had it for a ho engine 1-3 etc and I wired it non ho 1-5 etc ran great. Yes im the Idiot that wired it wrong but proud to admit it though, Because I found it.
 
Last edited:

Latest posts

Forum statistics

Threads
91,217
Posts
1,128,502
Members
24,045
Latest member
Ramtough01
Top