timh Posted July 14, 2003 Share Posted July 14, 2003 MY CAR: 1975 280Z, 355ci sbc, mild cam 272 adv. dur., new brake booster, good M/C, new 750dp (float level checked over and over, twice by trusted mechanic). Stock style chrome oil pan with stock baffles, new high volume oilpump. Carter fuel pump, holley regulator, JTR recommended oil pressure/fuel pump shut off. Any ideas why it stumbles and wants to die under moderate to hard braking and cornering? It did the same thing with a 600 edelbrock. Quote Link to comment Share on other sites More sharing options...
Heavy Z Posted July 14, 2003 Share Posted July 14, 2003 This post may help: http://www.hybridz.org/phpBB2/viewtopic.php?t=22491 Quote Link to comment Share on other sites More sharing options...
timh Posted July 14, 2003 Author Share Posted July 14, 2003 Thanks. My 750dp already has the fuel bowl vent baffles. Think it'd be worth trying the custom application mentioned? Quote Link to comment Share on other sites More sharing options...
grumpyvette Posted July 14, 2003 Share Posted July 14, 2003 well the first thing ID check is that pressure regulator sensor by disconnecting it and having a friend in the pass seat for one hard run and having that friend watching your oil pressure gauge carefully durring a hard run,. to see if the problem remains,my guess is that the oil pump pick-up is getting uncovered, by the oil running away from it due to inertia durring those hard (G) acceleration/turns/brakeing ETC, the pressure sensors cutting off the ignition when it sucks air! OR theres a loose connection in the wireing shorting out,because, carb problems normally cause an engine to miss due to overly lean or rich mixtures, NOT shut OFF! the second thing Id check is that you have BOTH 6-7psi of fuel pressure AT THE CARB INLET PORT durring a full acceleration run thru a full 1/4 and all gears,AND a fuel pump that pumps a MINIMUM of 1 quart of fuel volume in 30 seconds into a test bottle when the engine idles with zero back pressure.( 2 qts would be about NORMAL) notice you said "Any ideas why it stumbles and wants to die under moderate to hard braking and cornering?" but not hard accelleration, where the oil tends to run TOO the oil pump pick-up Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted July 15, 2003 Share Posted July 15, 2003 DELETE Quote Link to comment Share on other sites More sharing options...
Guest butlersZ Posted July 15, 2003 Share Posted July 15, 2003 I am no expert but my 74 260 did that same thing to me once and it was because my distributor shaft was wobbling around under cornering or braking and causing it to miss or even die because the gap between tohe pickup and reluctor was too large. Just a suggestion. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted July 15, 2003 Share Posted July 15, 2003 DELETE Quote Link to comment Share on other sites More sharing options...
timh Posted July 15, 2003 Author Share Posted July 15, 2003 I'm still running the stock oil pressure gauge. Is it sensitive enough to show a dip in pressure if I just brake hard enough to make it stumble? Thanks for all the input! Quote Link to comment Share on other sites More sharing options...
timh Posted July 17, 2003 Author Share Posted July 17, 2003 Tuesday night I bypassed the pressure switch for the fuel pump, still wants to die. There isn't a change in oil pressure, at least according to my stock oil pressure guage, which may not be resposive enough to give an indication(?). Put with the pressure switch out of the loop, it shouldn't be dieing from low oil pressure. Last night I did the vent tube to vent tube trick on the holley. It's supposed to prevent fuel from sloshing out of the vent tubes when braking hard. No change. Then I lowered the fuel/float on the secodary by 1/4 turn of the adjustment nut. No change. Then I lowered the fuel pressure from 8 to 6. No change. Then I hung a rope from the rafter of my garage and choked the s.o.b. The car was dyno-tuned 3 weeks ago with A/F monitor, so if the new carter fuel pump was loosing pressure, it should have shown up. Correct? Any suggestions? Quote Link to comment Share on other sites More sharing options...
blueovalz Posted July 17, 2003 Share Posted July 17, 2003 If it's any consolation, I've got the same thing happening on mine as well. I've changed carbs, (but they were all center pivot floats and I don't know if this may be a factor or not), float levels, baffled the vents, and pressure with no help. I have finally given up. My engine is perfectly horizontal from front to back, which makes me wonder about this being most engines are several degrees lower in the rear, which is for the most part, designed into the manifold so that the carb is level. With my horizontal engine, the front of the carb is lower than the back part. I've attempted higher primary float levels (with a size smaller front jets), but may not have gone high enough (I'm thinking the fuel was pulling away from the jets). Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted July 17, 2003 Share Posted July 17, 2003 DELETE Quote Link to comment Share on other sites More sharing options...
timh Posted July 17, 2003 Author Share Posted July 17, 2003 Terry, Increasing the float level on the primary side is what I was planning to do tonight. After that I'm gonna spend the $20 and get the jet extension kit from Holley (#116-10). I'll let you know how that works. By the by, I bought some ZX calipers from you earlier in the year, finally did the 300/280zx vented disc conversion last month. Seems to be working well. I've been hitting the brakes a lot lately trying to fix this problem. Tim Quote Link to comment Share on other sites More sharing options...
blueovalz Posted July 17, 2003 Share Posted July 17, 2003 To be honest, I never had the problem until I did the brake fix. OH, BTW, I also thought that perhaps the brake booster was depleteing the vacuum reservioir, thus providing an extremely weak vacuum signal and killing the engine that way. So I added a couple of extra bottles to increase the area a bit, but it had no effect in either brake vacuum improvement (which wasn't bad to begin with) or in the engine stalling area. So it was back to the drawing board. Waiting to hear about the jet extensions! Quote Link to comment Share on other sites More sharing options...
80LS1T Posted July 18, 2003 Share Posted July 18, 2003 No offense to anyone but this is the kind of stuff that makes me scared to work with carbs! Its also the reason I went EFI! I just dont have the patience to tune my car all the time and adjust the carb. I dont have that willingness to learn about carbs because I know they are on their way out(well atleast on the way of daily drivers, not dragsters or modified cars.) Guy Quote Link to comment Share on other sites More sharing options...
timh Posted July 18, 2003 Author Share Posted July 18, 2003 Here's an interesting thought on the topic. http://realbig.com/detomaso/1998-04/1138.html I run a 6.5, when I get time I'll check and see how far my vacuum drops when braking. This sounds promising. Quote Link to comment Share on other sites More sharing options...
blueovalz Posted July 18, 2003 Share Posted July 18, 2003 Very promising. I added volume to the vacuum reservoir, but never thougt about the powervalve situation. This will be my next attempt to fix this. Addressing the previous post about the carb vs EFI choice: I chose the carb set-up for several reasons over the EFI set-up. First of all, I knew a lot about carbs (old school racer). This, and the fact that it is a lot cheaper and easier to trouble shoot (for me anyway) a carb set-up than it is an EFI set-up. Carb parts are much cheaper. Once a carb is right, then very little work is needed to maintain it. No electronics are needed unless you get an electronic ignition (my Malory Uni-lite is about as far as electronic as I've gone so far). Driveability is a minus on this, but I love a car that makes me a driver, instead of a passenger. Quote Link to comment Share on other sites More sharing options...
timh Posted July 19, 2003 Author Share Posted July 19, 2003 Well here's what I have now. Under hard braking there's fuel sloshing up through the jets and out of the secondary booster (venturi). According to Holley's tech line, that's the first place fuel will slosh through, then it will come up through the vent tube if there's enough of a problem. I never did find fuel in the crossover tube (vent to vent) that I tried earlier, because it never made it that far. I eliminated the problem during braking by lowering the secondary float 1/4 turn then test driving. I ended up lowering it 1 full turn clockwise before it eliminated the stall. Now I have an intermittant off-the-line stumble, which will be easier to fix. Other than that, I didn't get any indication that the secondary bowl was running out of fuel, so I may be on to the solution to my particular problem. How to determine this is the same problem you're having: Test drive the car around the block with the the air cleaner off. Brake hard forcing the stumble, shut the car off and quickly and look at the secondaries if you're having the same problem fuel will still be wet on the throttle plates and the center of the venturi most likely. If the vent tube above the secondaries is also wet, the fuel is getting that much farther. I'm not sure how low my float is right now but it's one full turn lower that the traditional "trickling out" setting point. It's kind of hot to be working on a carb in Oklahoma, but I hope to find out more asap. Tim Quote Link to comment Share on other sites More sharing options...
blueovalz Posted July 21, 2003 Share Posted July 21, 2003 1/4 turn of the float adjustment to lower the rear bowl fuel level (from the proper "trickle" setting) has pretty much cleared up my problem except for extreme braking. I went another 1/4 turn and all my problems have cleared up. Thanks Tim. Quote Link to comment Share on other sites More sharing options...
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