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General Tech SU HS4 Carb Pistons Do Not Rise When Cranking

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Ranger68

Ranger68

Freshman Member
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My 69 MGB was running great, then shut off for 6 hours.

Will not restart!!

Fuel - check!
Spark and static timing - check!

Air?? I notice that the HS4 carb pistons do not rise when cranking.
Correct oil in dampers. Proper clank when dome pistons have been raised and released. Needles do not appear to be binding.

Ideas?

Thanks for any advice!!
 
They rise when it's running, not when cranking.
 
It turns over? and quickly enough?

what do the plugs look like?
 
Checked for spark at the plugs?
If you lift a carb piston up while cranking do you see fuel spray at the bridge?
Take the valve cover off and confirm that the valves are opening and closing. And/or do a compression test.
 
While cranking it over spray a little bit of starting fluid in the carburetors. If it fires, even just briefly, then you know you have a fueling problem. If there’s no change, then you know you have an ignition or compression problem.
 
Compression is at 150 - 155 psi
Static timing ✅
All valves operating normally - checked and set valve clearance at .015”
Spark at all plugs - used starting fluid and backfired through back carb.
Both fuel bowls full - gas flowing when cranking.

??
 
Sounds like the timings off. Are you sure the distributor isn't 180 degrees out?
 
is it trying to start or just spinning?

Also, as a wild card just check and see if the distributer bolt is loose? Mine was having all kinds of issues (it ran but badly) and my mechanic spotted the loose distributer - which I would never have thought of.
 
Static timing check shows rotor pointing at #1 spark plug lead when #1 is at tdc. And it was running fine when last shut down - refused to start after 6 hour shutdown. Distributor is snug. Spraying starting fluid when cranking gives sporadic burps. Fuel pump works, fuel bowls are full and gas accumulates at butterfly when cranking.
 
is your gas good? (I know that this shouldn't impact the strter fluid scenario)

wild card - could it be an ethanol varnish issue?
 
Filled tank with non ethanol gas 3 weeks ago. Have been using non ethanol for 3 years.

The mystery is that it ran fine and would not start after sitting for 6 hours. During that restart attempt it sputtered for about one second and then nothing. That would seem to indicate an electrical fault, however it turns over without dragging and shows good spark at all plugs. On the positive side, I discovered and replaced a fuel line that had cracked and was at risk of failure.
 
Timing chain jumped a tooth or 2 (would cause valves to still open and close but not at the right time)? Compression check might be worth doing, also turn the engine over by hand and when #1 comes up to TDC confirm both valves are fully closed.
 
Timing ✅. Fuel in carb bowl ✅ blew air though jets and verified that there are no obstructions. Fuel flow from tank to carb bowls ✅.
Poured gas into carb venturi and it now fires intermittently when cranking, but does not catch and run.
??
 
I may be wrong (as usual!) but I don't think you have a fuel delivery problem. I think you have an ignition issue. When you introduce fuel or starting fluid to the intake you're getting a weak attempt at firing. It should fire normally for a revolution or two if your had a normal spark.

If you pull the plugs after attempting to crank are they wet with fuel? If so that indicates an ignition problem.
While the plugs are out, get them thoroughly dry and be sure they are clean and the gaps are correct. If it still doesn't start I'd guess a weak spark due to a bad coil or (more likely) condenser, assuming you have stock type ignition.

Also check that the LT connections at the coil and distributor are in excellent condition.
 
Success!! Thanks for all of the tips.

The root cause of the problem was that the distributor was not clamped tight. When static timed it would start, idle nicely but would quit under medium acceleration. There is apparently enough pressure from the distributor worm drive on the camshaft to push it up and off the distributor drive lug.

And for a bonus I found during the troubleshooting:
- A cracked flex fuel line coming off the main fuel line into the cab lines. Replaced!
- The positive wire connection from the coil to the distributor was attached by only one wire strand at the spade end going into the distributor (it could have failed at a most inopportune time!!
- The fuel bowls each had some sediment in them; after cleaning and blowing air to clear the lines from the fuel bowl to the jets, it idles and runs smoother!
 
Yay! Thanks for the update.
 
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