1981 Elco 305 - 2 cylinders not firing

RustBucket

RustBucket

Greasemonkey
Feb 8, 2018
202
28
Houston, TX
Been having this elcamino for about 2 years but i never went in depth with it why i ran like crap. Popping out the carb at high speeds. So recently i made time to look at it (blame it on the covid19)

Went around pulling plugs 1 by 1 to see if they would all make the engine bog down. #1 and #7 seem to have no change when pulling the wires off. Swapped wires and plugs around to see if the problem shifted to a different cylinder but it remained at #1 and #7.

Did a dry and wet co pression test on those two cylinders plus a good cylinder (#2) to see what the differences would be. Here are the results.

#1 dry:50psi wet:60psi
#7 dry:90psi wet:105psi
#2 dry 135psi wet:150psi

Is there something i can try before removing the head to see exactly what's going on with it? Here's what I've done in the past week with it;

Started off by changing intake gasket because it was suspect (i previously had used the plastic strips the gasket kit comes with)

Adjusted valves while intake was off (each cylinder at tdc, tighten until 0 lash, then 1/2 turn more)

After doing the dry/wet co pression tests i took a hammer and a block of wood and tapped on the valves (removed rocker arm for this, then adjusted valve with engine running)

Exhaust does not smoke at all. Engine oil does not look like chocolate milk.

Thanks guys, hopefully you guys can point me to what to do next.
 
TURNA

TURNA

Rocket Powered Basset Hound
Jul 24, 2009
11,010
113
Socialist NY
Check your valve springs you may have a crack one or two.

If not, may be a head gasket or get ready for a rebuild
 
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Clone TIE Pilot

Clone TIE Pilot

Royal Smart Person
Aug 14, 2011
2,233
113
Galaxy far far away
Could simply be sticky valves. Valves that are slow to close behave similar to late cam timing and can cause lack of compression. Run some MMO in the crank case to see if it improves.
 
RustBucket

RustBucket

Greasemonkey
Feb 8, 2018
202
28
Houston, TX
Could simply be sticky valves. Valves that are slow to close behave similar to late cam timing and can cause lack of compression. Run some MMO in the crank case to see if it improves.
Definitely will do that. Will i need an oil change after?
 
MrSony

MrSony

Geezer
Nov 15, 2014
5,559
113
Des Moines, Iowa
Definitely will do that. Will i need an oil change after?
Yes. Add Marvel mystery oil.
Let idle. Blip throttle a few times. Let cool. Drain and cut open filter. Glitter is bad. If no shiny bits put good conventional oil with a zddp additive and see if it's better. Do this after inspecting for any obvious broken things of course.
 
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airboatgreg

airboatgreg

Royal Smart Person
Oct 2, 2016
1,916
113
Does it pop through the carb when you rev the engine in park or neutral?
 
O

oldsmobile joe

Royal Smart Person
Nov 12, 2015
1,319
113
mpls
after performing a compression test, and finding low compression. the next step in diagnosing your concern is to perform a cylinder leak down test to determine where the compression is going.
popping back through the carb usually indicates valve train issues, burned valves, sticky valve stems or lifters. while flat cam would give low compression, it would not cause popping back.
mmo or gm crank case cleaner may clean up the rings and give better compression, it would not address the popping back. it may help with sticking lifters depending on where the lifters are sticking.
adding zinc at this point would be a waste of money.
 
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