applying brakes kills engine.

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Nov 20, 2004 | 10:01 PM

007bronco

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From: British Columbia

applying brakes kills engine.

Hi, just threw a new 351M 4 BBL propane into my 80 Bronco. the vacuum tap off for the brake booster was on the #4 intake runner, causing this cylinder to barely fire. I capped that off and the engine ran much much better. I attached the brake booster vac line to the front of the propane carb. Brakes work OK but make hissing noise when applied and engine rpm decreases sharply as well. If I pull the vac line out of the brake booster, engine runs a bit better, if I plug that end (booster end) with my finger the engine will die. Do I have this line connected to the wrong place or is my brake booster toast? When should an engine be timed/tuned/idle set etc, with all vac capped? or some? or none except dist?
007BRONCO

Nov 21, 2004 | 12:03 PM

79_250RangerLariat

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how much vaccum are you making?

Nov 21, 2004 | 04:31 PM

007bronco

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From: British Columbia

dont know, I'll try to check but the only good small vacuum spout is for the propane shutoff/lockoff? so I'll have to make an adapter for the big one on the carb.
007BRONCO

Nov 21, 2004 | 06:28 PM

zwinstead

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From: Hartland, MI

Sounds like your brake booster is toast. Unhook the booster and plug the vacumm port it was hooked to and then adjuster idle speed, mixture etc and get the engine running very well. If the engine idle speed increases or the engine dies when you hook it up you have a shot out booster. The rule is an engine should always idle and run great with no vacumm accesories hooked up. If it starts to run badly when an accessory is hooked up that accesory is most likely failed.

Nov 21, 2004 | 11:15 PM

007bronco

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From: British Columbia

Cool, that makes sense, I'll try that tomorrow, along with a vacuum check. This propane engine is having trouble with idling, I got the timing nice and anywhere above 1500 rpm it runs beautiful but I have to set idle at 1000 rpm to keep it running. I'm just getting this propane thing figured out, it looks dead simple but I'm probably missing something. The brake booster probably is dead though.
P.S. There is no vacuum "tree" on the back of the manifold, it is an aftermarket Edelbrock, if I need to make a new tap off, where should it be?
Thanks.
007BRONCO

Last edited by 007bronco; Nov 21, 2004 at 11:22 PM.

Nov 22, 2004 | 08:42 AM

zwinstead

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From: Hartland, MI

What manifold do you have? I have an Edelbrock Performer Non-EGR on my 400 and it has a vacumm port on the #4 Intake runner. I have my brake booster plumbed in there and it works great. As long as your vacumm powered accessories do not have any leaks it should not matter where you plumb the lines. I suppose if you plumb the brake booster into a cingle cylinder intake runner and pump the brakes non-stop while you drive you may lean that cylinder, but who would do that?? lol Also a good way to check for a lean condition on the cylinder a vacumm accessory is plumbed into is to pull that spark plug and another plug and compare them. If the vacumm accessory plug is very white there may be a problem. But if both plugs are very white you may have a problem with the carb being too lean or a larger intake vacumm leak.

Nov 22, 2004 | 09:36 AM

Sycostang67

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From: Kuna ID

A friend had the same problem with his 80 Bronco, but he had a 302. Everytime he used the brakes the engine would start to bog and die. Thing was you could hear his booster leaking when he stepped on the pedal. I think he replaced it just before he gave it to his son as a graduation gift.