GM V8 dies at idle when warm

LandVF36 · Post by **LandVF36** » Mon Aug 24, 2009 6:04 pm

A fellow boat owner asked for help Sat AM. She has a boat with twin GM 350s (And while not a Trojan, is a vintage boat that she cares for endlessly so I felt compelled to help.) She's spent about $1500 in parts and labor with a local shop and is out of $s for the summer and very frustrated.

Anyway, one engine has not been running well. She's had new ingnition installed (cap, rotor, wires, plugs (electronic ignition / not points). New filters in the carb bases (Rochester Quadrajets), new inline filters, new water seperators. Two weeks ago she had new carbruators installed.

The symptom is this: When cold, you start it up, it runs at around 950 rpm. When the mechanical choke warms up, it idles down to about 650-700 RPM. It will stay running for 5-10 minutes then suddenly die off. If you start it again, it won't run for more than 30 seconds without dieing off again at low idle. If you keep the RPMs up around 1100, it runs fine (but its not a good plan to shift it a lot at that speed). f you put a load on it with closed throttle, it dies. When it dies, if you pull the throttle back, you can hear gas dumping in so I think think its getting plenty of gas, its just dying off.

I didn't have time to dive in Sat AM, but will likely look at it again this coming weekend. Just curious where you guys would start. I was going to check to see where the timing was set and then look for a crack in the intake / vacuum leak. Any other thoughts?

alexander38 · Post by **alexander38** » Mon Aug 24, 2009 7:07 pm

vacuum leak at base of carb. what did they use to seal it ? I've done both of mine this season and used the old lacquer base sealant, but i would check needle adjustment first on it. just because they're fresh doesn't mean they're right.

TC · Post by TC » Tue Aug 25, 2009 7:24 pm

Did she have the problem before all the work, ( distributor work, carbs etc ) or after?
If after, yes agreed with the gasket issue or could have dirt in low speed circuits inside carb. That would be a lean condition.
Need more details. Does it lump itself to a stall ( rich ) or just quits, like running out of fuel ( lean )?

LandVF36 · Post by **LandVF36** » Tue Aug 25, 2009 9:28 pm

It just seems to lean out and stall

TC · Post by TC » Tue Aug 25, 2009 10:07 pm

Well if you are not to keen on tearing down a carb, especially a new one that may have a warranty, first check the obvious a leaking gasket or loose hold down bolt. It can leak thru body halves. If that all looks good then I would swap carbs.
If the problem changes engines then you have found it. Then depending if warranty will kick in or not, you either send it back or find someone to do an exporitory. If the problem goes away, then you fixed it.
If the problem stays with the same engine then back to my original question............did she have the problem before or after the forementioned work?