proper spark plug gap

barrelback · Dec 4, 2011

Carb 350 small block running Thunderbolt lV ignition. Can't find proper spark plug gap in service manual. What coil (make/model) is Ok with the Thunderbolt lV Thx.

Bondo · Dec 4, 2011

Re: proper spark plug gap

Ayuh,... Welcome Aboard,...

AC Delco, MR43T, at .035"....

Don S · Dec 4, 2011

Re: proper spark plug gap

Can't find proper spark plug gap in service manual.

It's always found in section 1B (Tune up specs) in the OEM manual.

What coil (make/model) is Ok with the Thunderbolt lV

One that meets the specs in the OEM manual in the ignition section.

Without knowing anything about your 350 chevy with thunderbolt IV ignition, no year or what your engine serial number is, I sure wouldn't want to give you the wrong part number for the coil.

barrelback · Dec 5, 2011

Re: proper spark plug gap

Don,
followup data-
1978 350 (truck) 4bolt with 1992 heads fed by Holley 650 marine carb with Thunderbolt lV ignition
fresh rebuilt/stock driving a 1987 202 Chris Craft Stinger (21').
Thanks,
Jt

Don S · Dec 5, 2011

Re: proper spark plug gap

http://www.iboats.com/HEI-Ignition-...5733093--**********.379954382--view_id.172023

barrelback · Dec 6, 2011

Re: proper spark plug gap

problem- no fire-in-the-hole
Looking for a suggestion
Newly overhauled 350 small block 4 bolt with 1992 heads fed by new Holley 650 w/new fuel pump, w/spark by Thunderbolt lV w/new no resister coil w/new AC Delco TR43T plugs gapped at .035"
During engine assembly noted cam timing marks, set hydraulic valve tappet clerance per manual. Rotated newly assembled engine to 8* BTDC, set dizzy rotor at 12:00 & routed wires to plugs IAW cap. Pressurized fuel system w/vac pump.
Confirmed ignition with spark tester at dizzy&#1 cyl.

Cranking, engine won't fire. Shot one squirt of raw gas in each venturi. nothing!
Help.

Plan to crank with timing light- a double check on timing.
Need a suggestion.
What did I forget?!
Running out of ideas.
Thx

achris · Dec 6, 2011

Re: proper spark plug gap

barrelback said:
...Plan to crank with timing light- a double check on timing....

That and make sure the dissy isn't 180? out. It will still show as 'correct timing' on the marks, but it would be firing on valve overlap, not compression...

mark1905 · Dec 7, 2011

Re: proper spark plug gap

- Compression

- Timing

Check both.

Betting dollars to donuts that it's one of the two..

barrelback · Dec 7, 2011

Re: proper spark plug gap

Ok. Dark cloud turned black. @#*!
confirmed dizzy/plug are wired correct.
flipped dizzy 180*, nothing
put compressioin gage on #1- zero compression WTF!!!!!!!!!!!!!!!
Suspect assembly issue-@#*!
review-
during assembly, determined #1TDC using #6 valves "rocking" then
adjusted valve lash using "rocking" of opposites to establish TDC of firing order.
set all rockers at 3/4 a turn from onset of cam contact.
pressurized engine through all 16 rocker arms (+/-3 min with Makita)
moved crank to 8*BTDC
installed dizzy with #1 at 12:00 oclock.

Not a clue why #1 has zero compression. Plan to check all 8 when blood pressure returns to normal. #@*!

Any suggestions gratefully accepted.
@#*@

Don S · Dec 7, 2011

Re: proper spark plug gap

Did you adjust the valves properly? If you did, did you have it on #1 TDC on the compression stroke when you did the adjustment? If not, the valve adjustment is all wrong. Have you ever adjusted valves on a Chevy engine before? Trust me, it's a serious question.

barrelback · Dec 7, 2011

Re: proper spark plug gap

This is my first Chevy. Lycoming, yes. Hercules, yes. That said, no.
I followed the manual step-by-step but obviously I messed up.

So, when adjusting the valves, i relied on 6 ex closing/intake opening "rocking"=#1 near TDC. Following the manual's order, all 16 valves were adjusted with the same procedure.

What to do?
pull the valve covers/ back the rocker arms off/ bring #1 up on compression/ re-adjust 16 valves ?!

The cam&cam-gear are mated with a cog+4 bolts which prevent cam gear being installed 180* out of syn?!

The * on the cam-gear mated with the crank gear's* during assembly.

achris · Dec 7, 2011

Re: proper spark plug gap

barrelback said:
...So, when adjusting the valves, i relied on 6 ex closing/intake opening "rocking"=#1 near TDC....

That's to adjust rockers on #1. Then you turn the engine 90 degrees to bring the next cylinder to TDC (can't remember firing order, but I'm sure you know it

) and adjust those rockers. Then turn engine another 90 degrees and adjust the next ones. In all you should have turned the engine 2 full turns to have finished all rockers...

barrelback · Dec 7, 2011

Re: proper spark plug gap

achris said:
turn engine another 90 degrees and adjust the next ones. In all you should have turned the engine 2 full turns to have finished all rockers...

Right. Did that during assembly.
So, zero compression now on #1 makes zero sense.
Next plan- pull the valve cover for #1357 for visual on rocker arm adjustments. Maybe the rocker arm's 3/4 turn (pre-oiled tappets) is too tight not allowing valves to now seat.
#@%*!

Don S · Dec 7, 2011

Re: proper spark plug gap

Read this post I did back in November http://forums.iboats.com/showthread.php?t=522424&p=3533249&viewfull=1#post3533249

barrelback · Dec 8, 2011

Re: proper spark plug gap

"when the push rod stops moving, you are at zero lash. STOP. Do not recheck, as it will be loose again. Do this one time on each valve, and one time only or the valves will be to tight."

This is exactly what happened during assembly when attempting to be extra careful. Me bad. Thanks for this.

The issue now, is how to correctly back off the rocker arms AFTER they are pressurized?! Planning to backoff rocker arms 1/2 turn and confirm compression at this point. Plan to adjust per firing order using "opposite" cyl "rocking" to determine when firing order cyl are on their "heel."

This is going to be a #%&! pain rotating the engine 90* (720*) installed.

Moving at the speed of cold tar. #%&!

barrelback · Dec 8, 2011

Re: proper spark plug gap

you are at zero lash. STOP. Do not recheck,

HUSTON, WE HAVE IGNITION!!!!!!!

Don,
Thanks for all of the back-n-forth. The rocker arms were too tight. Backed them off 1 turn and all cylinders had compression. Engine lit off on first hit.

Another issue-
how much psi should the fuel line to the Holly 650 have? The inline fuel PSI gage shows zero with the system primed all the way the Holly. Engine ran on one squirt of fuel shot down the prt&stbd venturis BUT there's no fuel PSI showing on the pressure gage.

Thx

Don S · Dec 8, 2011

Re: proper spark plug gap

Good for you.
I would suggest you go back and adjust all the valves correctly. You may still have some too tight or too loose and probably won't show up until underload and at wide open throttle (WOT) and you may not hear or feel the problem until too late.
4-7 psi on the fuel pump.

Bondo · Dec 8, 2011

Re: proper spark plug gap

BUT there's no fuel PSI showing on the pressure gage.

Ayuh,... If the motors Runnin',... I'd suspect the Gauge....

barrelback · Dec 8, 2011

Re: proper spark plug gap

Don S said:
Good for you.
I would suggest you go back and adjust all the valves correctly. You may still have some too tight or too loose and probably won't show up until underload and at wide open throttle (WOT) and you may not hear or feel the problem until too late.

Two issues: Need a suggestion re how you do a 90* rotation (720*) when engine is in boat now when adjusting rocker arms.

2nd, how are lifters adjusted once they are pressurized with oil? I would guess with #1 TDC back off rocker arm and "find" the lash? But how much of a turn does the the lifter get when full of oil?

Thx

Don S · Dec 8, 2011

Re: proper spark plug gap

Take the spark plugs out, and if the alternator belt is tight, turn the alternator. Should turn fairly easy with the plugs out.

2nd, how are lifters adjusted once they are pressurized with oil? I would guess with #1 TDC back off rocker arm and "find" the lash? But how much of a turn does the the lifter get when full of oil?

Same way, follow the manual. they just don't change as much with oil in them.

proper spark plug gap

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