68 Ambo stalling at higher speeds (50-60)

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   Took a test drive with the new fuel pump and I’m experiencing stalling at higher speeds (50-60). Had to pull over a couple times (luckily the hazards work!), let it sit, and then got myself home going 20-30. Gonna insulate the line to the carb tomorrow and see if this is possibly vapor lock. The hard line from pump to carb seems pretty close to the block/intake, but it also looks factory. Although the car has been driven around town (about a 15 mile round trip at speed) since I got it running without any of these symptoms. Changes since then: replaced fuel sending unit (which fixed the gas leak at the tank), new fuel pump and filter, trans pan gasket (which also leaks a bit), replaced fouled plugs (again, second time in a month), and then cosmetics (new wheels). I’m sure there’s a litany of things needing tuning, like timing, vacuum leaks, compression testing, gauges need connecting to tell me what’s up. Gotta keep plugging away! I really want to make it to Hines Drive this weekend, but can’t risk going far from home as it is.

Would appreciate any feedback on this issue.

Thanks!

Brian

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skynless Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Nut Joined: Aug/29/2013 Location: Michigan Status: Offline Points: 256	Post Options Post Reply Quote skynless Report Post Thanks(0) Quote Reply Topic: 68 Ambo stalling at higher speeds (50-60) Posted: Aug/21/2018 at 7:59pm
	Took a test drive with the new fuel pump and I’m experiencing stalling at higher speeds (50-60). Had to pull over a couple times (luckily the hazards work!), let it sit, and then got myself home going 20-30. Gonna insulate the line to the carb tomorrow and see if this is possibly vapor lock. The hard line from pump to carb seems pretty close to the block/intake, but it also looks factory. Although the car has been driven around town (about a 15 mile round trip at speed) since I got it running without any of these symptoms. Changes since then: replaced fuel sending unit (which fixed the gas leak at the tank), new fuel pump and filter, trans pan gasket (which also leaks a bit), replaced fouled plugs (again, second time in a month), and then cosmetics (new wheels). I’m sure there’s a litany of things needing tuning, like timing, vacuum leaks, compression testing, gauges need connecting to tell me what’s up. Gotta keep plugging away! I really want to make it to Hines Drive this weekend, but can’t risk going far from home as it is. Would appreciate any feedback on this issue. Thanks! Brian
	1968 Rebel SST 343 1968 Ambassador SST 343

PHAT69AMX Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/07/2007 Location: West Virginia Status: Offline Points: 5919	Post Options Post Reply Quote PHAT69AMX Report Post Thanks(0) Quote Reply Posted: Aug/21/2018 at 8:55pm
	I lack real world hands-on now since i sold my car 4 years ago. An outside Cowl mounted Fuel Pressure gauge may give some insight. Have read reports of new (offshore) replacement mechanical pumps having "pressure problems"... Some providing as high as 12 psi ! So... A Fuel Pressure Regulator may be a good idea... Regulator mounted as close to carb as possible. Which brings us to Fuel Pressure setting.... "Standard" is 5-6 psi... There's a fella who rebuilds all kinds of carbs, has a web page and posts YouTube Videos for customers where he runs their carb on a working motor / homemade "dyno" with load... This fella claims modern blended fuels "require" 3-3-1/2 psi Fuel Pressure to function properly... In some videos he even shows what happens when he cranks the fuel pressure up to 5 psi... Things work "best" in his videos with MODERN GAS with only about 3 psi Fuel Pressure... Not sayin' that this is right, just saying that's the demonstrated "info". I'm guessing you may be getting some "inconsistency" in the Fuel Pressure with engine rpm's. Suggest a Holley 1 to 4-1/2 psi Fuel Pressure Regulator mounted close to the Carb inlet and an outside Cowl mounted mechanical Fuel Pressure Gauge to monitor what happens.
	Link to a Short YouTube Burnout Video

Trader Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6881	Post Options Post Reply Quote Trader Report Post Thanks(0) Quote Reply Posted: Aug/21/2018 at 11:38pm
	Seems that your answering your own questions. First the basics on tuning trouble shooting and then get to picking one off at a time. You don't state vacuum, timing, points or ?, last PCV valve change (fouling plugs), fuel your using or how old the fuel is ... Start with the basics and eliminate one problem at a time in the driveway. Could be a bad ignition condenser, coil, carb rebuild, water in the tank, timing, but the basics will cover that. I don't think there is one answer to your problem until you can narrow this down. And it could be fuel pressure, but that can be tested in the driveway.

Boris Badanov Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Dec/14/2013 Location: NH USA Status: Offline Points: 4209	Post Options Post Reply Quote Boris Badanov Report Post Thanks(0) Quote Reply Posted: Aug/24/2018 at 4:37am
	Fuel delivery is the most likely issue. A heat riser valve stuck shut and over heating the intake and carb are another. Ignition issues are also a top contender. Points set to close, a bad coil and other related gremlins can cause this. I would start with checking the points gap. A running fuel pressure test at speed will tell you if the fuel system is up to par. To me this sounds like an intermittent blockage in the fuel system.
	Gremlin Dreams

maximus7001 Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Jun/27/2011 Location: Winnipeg,Canada Status: Offline Points: 1086	Post Options Post Reply Quote maximus7001 Report Post Thanks(0) Quote Reply Posted: Aug/24/2018 at 5:17am
	Check that the fuel filter and its bowl are not filled with gunk. This has happened with my 68 after rebuilt pump with new filter. I checked everything else first including blowing air into the tank through the fuel hose that normally connects to the pump. Never thought it could fill with that much gunk so fast. Anything over 40mph would do what you describe. The carb was running out of gas. Had to drive 200 miles like that. If the filter was not before the pump (68) the pump would not have survived this. Edited by maximus7001 - Aug/24/2018 at 5:21am
	1968 Javelin SST 1997 GMC Safari AWD 2001 Daewoo Nubira SX (Winnipeg only model) 1997 Honda Accord EX (Canadian Model) Winnipeg, home of the Jets.