View Full Version : Engine idle goes up while driving and stepping on brake hard
broku518
03-20-2018, 08:59 AM
I double checked and my foot isn't hitting two pedals at the same time. :p
Say I am driving 30mph and step hard on brakes...I would slow down and hear engine idle go up. I have no foot on gas, so I should only hear the idle rpm.
When I am not moving and step on brake, nothing happens (idle stays the same)
Makes me wonder if perhaps the fuel is being rushed in while breaking hard??
Got carburetor and engine mounted mechanical pump.
skidd
03-20-2018, 09:13 AM
Failing power brake booster? Does the same happen when sitting still and mashing the brake pedal? If so, there might be a leak inside your booster. Don't rapid pump the pedal though, that will cause an idle increase even with a good booster as it allows air into the motor as the vacuum re-powers the booster.
Railroad
03-20-2018, 09:33 AM
I would test the check valve first. The valve is usually mounted on the booster can, where the vacuum line attaches. I hope you have power brakes.
broku518
03-20-2018, 09:34 AM
Failing power brake booster? Does the same happen when sitting still and mashing the brake pedal? If so, there might be a leak inside your booster. Don't rapid pump the pedal though, that will cause an idle increase even with a good booster as it allows air into the motor as the vacuum re-powers the booster.
I don't understand this. Brake system should be independent from the engine idle speed. No matter how hard I step on them.
Also, when I am sitting still, and stepping hard on brakes, idle stays the same.
I am not sure what brake power booster is. But I don't think I have one :)
CraigS
03-20-2018, 09:51 AM
This is a vacuum booster w/ a master cylinder installed
https://shop.advanceautoparts.com/p/wearever-loaded-vacuum-power-brake-booster-domestic-50-1098/18690560-p?c3ch=PLA&c3nid=18690560-P&adtype=pla&gclid=EAIaIQobChMIjeK_m5P72QIVmYizCh0lswSFEAYYBSAB EgKu-PD_BwE&gclsrc=aw.ds
broku518
03-20-2018, 09:59 AM
This is a vacuum booster w/ a master cylinder installed
https://shop.advanceautoparts.com/p/wearever-loaded-vacuum-power-brake-booster-domestic-50-1098/18690560-p?c3ch=PLA&c3nid=18690560-P&adtype=pla&gclid=EAIaIQobChMIjeK_m5P72QIVmYizCh0lswSFEAYYBSAB EgKu-PD_BwE&gclsrc=aw.ds
wow, I don't have that for sure! I got these 2 small Wilwood brake cylinders in foot box.
skidd
03-20-2018, 10:30 AM
In that case, totally ignore my post. :)
broku518
03-20-2018, 11:30 AM
I called M. Forte. This is kinda normal for carb. engines. As you brake hard the rear bowel on carb may spill over some gas and increase the idle speed. So no real issue.
boat737
03-20-2018, 12:30 PM
Funny, my Holley 750 is just the opposite. Hard braking and idle goes down. Really really hard braking, and the idle goes down to 0. (Clutch in, or neutral of course.)
skidd
03-20-2018, 12:36 PM
^^ Yep.. me too. my car would die under really hard stops. I fixed mine with a rubber tube extension on the rear float vent. Pretty sure I was just getting fuel slosh out of the vent.
I wonder.. if the OP is getting fuel slosh, and the RPMs are going up.. is it possible the idle circuit is a bit too lean?.. and the slight fuel slosh is putting back the missing fuel?
boat737
03-20-2018, 05:24 PM
^^ Yep.. me too. my car would die under really hard stops. I fixed mine with a rubber tube extension on the rear float vent. Pretty sure I was just getting fuel slosh out of the vent.
I wonder.. if the OP is getting fuel slosh, and the RPMs are going up.. is it possible the idle circuit is a bit too lean?.. and the slight fuel slosh is putting back the missing fuel?
Got any pictures of the tube fix?
Avalanche325
03-20-2018, 05:31 PM
Here is a Holley with a metal one piece tube. I didn't know about these until a few days ago. From the forum, of course.
https://www.holley.com/products/fuel_systems/carburetors/avenger/off-road_truck_avenger/parts/0-95670
rich grsc
03-20-2018, 05:34 PM
Idle would rise on a lean condition.
j.miller
03-20-2018, 05:45 PM
Ha! I was betting leak in break booster but we solved that part.... I can't think of a single thing that would cause it to idle up under breaking other that WFS (wide foot syndrome).
broku518
03-21-2018, 11:36 AM
Ha! I was betting leak in break booster but we solved that part.... I can't think of a single thing that would cause it to idle up under breaking other that WFS (wide foot syndrome).
I thought the same thing. Must be my foot. Not the case, I made sure I was only pressing the brake pedal. It only happened when 'hard' breaking. The idle goes up for a second and comes back to normal.
skidd
03-24-2018, 02:33 PM
Got any pictures of the tube fix?
Sorry.. missed this post. Here's my quick-fix that's been working so far. The top of the hose is just below the top of the air cleaner cover.
https://www.ludicrous-speed.com/zp/cache/Shanes%20Cobra/TheBuild/IMG_20180324_142207_595.jpg
broku518
03-26-2018, 09:10 AM
Sorry.. missed this post. Here's my quick-fix that's been working so far. The top of the hose is just below the top of the air cleaner cover.
https://www.ludicrous-speed.com/zp/cache/Shanes%20Cobra/TheBuild/IMG_20180324_142207_595.jpg
Thanks for the pic.
Check that your return springs are strong enough and that you don't have any binding in the throttle linkage.
broku518
03-27-2018, 09:36 AM
Check that your return springs are strong enough and that you don't have any binding in the throttle linkage.
Thanks Papa. I noticed one rod was rubbing the foot box panel, cleared that. Also installed metal plate behind the firewall to beef it up. It was slightly moving the firewall.
I experimented with multiple springs, easy, medium and hard. No difference, idle still went up.
I am 99% sure the culprit is the bowel over spill. I cut the hose and install it. Will have to wait to test it later....
Thanks,
Martin