New build, cylinders 3/4 not responding

[TonkaJim]

OK, I got the new engine in the Ghia to idle and even drove it around the block a couple of times, but, when I went to tune the dual carbs, I discovered that cylinders 3/4 weren't responding to any speed or mixture adjustments. Cylinders 1/2 responded appropriately. Also, I'd get an immediate response from 1/2 when opening the throttle, but nothing on 3/4 unless I really opened it up... but then it would backfire up through the carb (almost got hit by a flying synchronizer; had to glue it back together). I swapped the carbs and the issue was till with 3/4, so it's not the carbs. Also, I've set the timing (It's not perfect since only 2-ish cylinders, but close), checked compression (120 PSI all around), adjusted the valves several times and confirmed spark to each cylinder. So, by process of elimination, I'm thinking there's something wrong with the valves... especially with the backfiring through the carburetor.

Tonight, I'm going to pull the rocker on that side and see what I find. Any recommendations on how to diagnose this issue?

Engine is a professional rebuild, but I don't know any specs; carbs are dual EMPI 40 HPMX; distributor is a vintage Bosch.
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'72 Karmann Ghia stock Ravenna Green (1992 - 1999)
'72 Karmann Ghia modified Ravenna Green (2019-)

[Glenn]

Did you swap the 3 and 4 wires?

I suspect you have the wires on wrong. 1-4-3-2 clockwise.

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[TonkaJim]

Glenn - I thought the same thing, but I've confirmed the firing order several times.

j
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'72 Karmann Ghia stock Ravenna Green (1992 - 1999)
'72 Karmann Ghia modified Ravenna Green (2019-)

[Dale M.]

If you have decent compression all around I double its a valve problem... Remember cam only has 4 lobes so if its a cam problem the opposing cylinder(s) will have same problem...

Vacuum leak on 3/4 side intake? ... Usually back fire through crab indicates lean condition...

Dale
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[Paul Windisch]

Check for plugged idle jets.

Also, make sure UNEQUIVOCALLY that the carburetors are synchronized properly. Make sure the progression ports on one side are not exposed at idle.
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[TonkaJim]

Dale - Good point about the cam. I'll check for intake leakage.

j
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'72 Karmann Ghia stock Ravenna Green (1992 - 1999)
'72 Karmann Ghia modified Ravenna Green (2019-)

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[TonkaJim]

Paul,

I've cleaned the carbs several times and verified the jets are clean. I also swapped the carbs and still have the same problem on the same side. I discovered this problem as I was sync'ing the carbs. I can adjust the idle speed screw to sync the air flow, but it does nothing to the engine speed.

j
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'72 Karmann Ghia stock Ravenna Green (1992 - 1999)
'72 Karmann Ghia modified Ravenna Green (2019-)

[jason]

My money is on a vacuum leak at base or not synced.

[TonkaJim]

[/quote]Take the mixture screws completely out to make sure the seats didn't blow out of the carb body.[/quote]

Roy, I swapped the carbs and the problem is still with the 3/4 cylinders, so doesn't seem to be a carb issue.

j
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'72 Karmann Ghia stock Ravenna Green (1992 - 1999)
'72 Karmann Ghia modified Ravenna Green (2019-)

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[TonkaJim]

Roy - Thanks. I'm going to do a leak test tonight. And, per your suggestion, i'm going to bring home the thermal imaging camera from work and take some pics!!

j
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'72 Karmann Ghia stock Ravenna Green (1992 - 1999)
'72 Karmann Ghia modified Ravenna Green (2019-)

[Paul Windisch]

[mikedjames]

On thing that stays the same side when you swap carburettors over is the fuel supply. Maybe the problem is a blocked fuel fitting.

Or maybe its pinched tinware under the manifold letting in air.

Also treble check spark plug lead order...the big backfires sort of seem familiar.
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[TonkaJim]

Mike - thanks. I know I i have fuel, but but I'll double check to make sure it's not blocked and even for both carbs. I have a pressure gauge on the 1/2 carburetor. I'll move it over to the 3/4 to see how it reads.

j
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'72 Karmann Ghia stock Ravenna Green (1992 - 1999)
'72 Karmann Ghia modified Ravenna Green (2019-)

[stanthedog]

as miked also pointed out. make sure the tin isn't in the way. I had to deal with that myself. on mine, it was mostly on the 3-4 side. felt tight but wasn't.

[TonkaJim]

Stan - Ah ha ... by that you mean that the tin could be preventing the intake manifold from seating completely on the head? I'll take a look. thanks!
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'72 Karmann Ghia stock Ravenna Green (1992 - 1999)
'72 Karmann Ghia modified Ravenna Green (2019-)

[Dale M.]

[jeffrey8164]

Do the spray trick!
While it’s running, spray some carb cleaner or WD-40 around the intake manifolds. If the leak is that bad you’ll notice a change right away.
You’ll have a mess to clean up of course
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