Stalling when hard braking

[91r/t]

Ok tuning on my stroker big block and I got it close to perfect. Car wants to stall out when your hard on the brakes. Braking normal its fine, but put the car into a hard brake when it is diving forward and it wants to stall. I can catch it with throttle and keep it running. I have seen all kinds of opinions on float level and such for my quick fuel 850, but it was half way in sight glass so I lowered them. Didn't get a chance to take it for a spin, but have seen many opinions saying rear float too high and I said F it and made them both right above the bottom of the sight glass. Any other ideas? Shouldn't be a vacuum leak in the booster because it was fine with the old engine same booster. Fairly large cam and makes about 9"/mg at idle. Power brakes actually work good so I am leaving them. Shoot if ya got any other suggestions.

 

[NOSTRT980]

jet extensions front bowl. my 2 cents https://www.manciniracing.com/mohojetex.html

 

[91r/t]

jet extensions front bowl. my 2 cents https://www.manciniracing.com/mohojetex.html

Wierd never needed these before. I figured I was flooding out not starving for fuel.......never thought of that.

 

[NOSTRT980]

been using them for years, if the carb doesn't have them I install them. cant hurt

 

[91r/t]

Need a float with the provisions for the extensions.

 

[69Bee]

….Car wants to stall out when your hard on the brakes.... Fairly large cam and makes about 9"/mg at idle.

You answered you own question at the end of your post. When you lightly use the brakes, the engine can still produce enough vacuum to keep the engine running, but when you romp on the brakes, the booster sucks too much vacuum from the engine and kills it. I had a 360 in a Charger that had a cam that was too large (8-10" vac at idle), and would do the exact thing that you are experiencing. I am willing to bet that if you applied the brakes lightly a few times before stopping, you would have a hard brake pedal due to a lack of vacuum. Try a vacuum can and am sure your problems will go away. You could also test by removing the hose to the booster, and plug the intake side. Then try the jumping on the binder test, and am sure it won't stall.

 

[91r/t]

You answered you own question at the end of your post. When you lightly use the brakes, the engine can still produce enough vacuum to keep the engine running, but when you romp on the brakes, the booster sucks too much vacuum from the engine and kills it. I had a 360 in a Charger that had a cam that was too large (8-10" vac at idle), and would do the exact thing that you are experiencing. I am willing to bet that if you applied the brakes lightly a few times before stopping, you would have a hard brake pedal due to a lack of vacuum. Try a vacuum can and am sure your problems will go away. You could also test by removing the hose to the booster, and plug the intake side. Then try the jumping on the binder test, and am sure it won't stall.

That makes sense. I just figured because the pedal wasn’t hard I had sufficient vacuum for the whole thing to work but it must be robbing enough where it’s pissing off the engine. Looks like it’s time to convert over to manual discs. I’ve got the parts to do it now so I’m done dicking around.

 

[kiwigtx]

Big engine + big cam + small vacuum = very little power brakes.

Been there, done that.

Options include (but not limited to) - change the cam for one which makes more vacuum, install vacuum storage tank, or go hydro-boost brakes.

 

[steve340]

That can be fuel sloshing out of the bowl vents. Fuel will come out the rear bowl vent and dump on to the butterflies causing a stall.

 

[lewtot184]

probably primary jets getting uncovered. fuel level is usually the cause. too high of a level in the secondaries can cause fuel slosh thru the boosters during hard breaking. maybe a combo of both? the tune-up in general may need to be re-tweeked.

 

[91r/t]

Yeah I lowered both floats last night but didn’t get a chance to take it for a ride. Now the weather is supposed to be **** for the next week, so I guess I wait until I can take it for a spin and see if that helped.

 

[hunt2elk]

I have the same issues on a Quick Fuel 750 dp. Manual brakes. Already has jet extensions. Tried vent tube extensions also. Kind of gave up trying to figure it out, so will be keeping tabs on this thread to see if you learn anything.

 

[91r/t]

I’m going to try running the front float bowl a little higher than halfway up the site glass and then I’m gonna leave the rear float bowl at the bottom edge of the site glass. At that point I don’t think there’s anyway the fuel is sloshing away from the jets on the primary side because it’s pretty high up in the bowl and I’m not really braking that hard where the car is in a serious dive. Also, I never had this issue with the last motor with a small cam where my floats were set before, so if this doesn’t work I am leaning towards the vacuum problem with the brakes on my particular application.

 

[lewtot184]

this is an issue that I thought was reserved for 6paks only,..lol. another thing that will cause this is a lean idle condition. idle air bleed and idle jet may need some tweeking. with 9" of vacuum, and i'm assuming in neutral, timing may be a concern. you may need to be more aggressive on timing. bet there's several factors going on.

 

[91r/t]

this is an issue that I thought was reserved for 6paks only,..lol. another thing that will cause this is a lean idle condition. idle air bleed and idle jet may need some tweeking. with 9" of vacuum, and i'm assuming in neutral, timing may be a concern. you may need to be more aggressive on timing. bet there's several factors going on.

Timing is at 28 degrees initial and it’s a 4 speed. Another thing I am going to try if the floats don’t work is bump my idle air jets back to 70’s instead of 74’s just to see if that helps/cures the problem. The floats weren’t an issue on the other engine, so I have my doubts on those fixing this issue. I am going to try it, but I am leaning towards the lean idle/vacuum issue more.

 

[lewtot184]

74's are 11.5% larger in area than the 70's. that's a noticeable step to the lean side. from what info I have most of those QF850's had .070" idle air bleed. you might want to check the idle jet. i'd think the idle jet should be at least .035" with the 70.

i'd also look at the timing curve in the distributor. it may need to be brought in quicker.

 

[91r/t]

Yea for sure. I am going to put the 70's in. If it fixes it and is a bit rich, I will pick some 72's up and try that. I have the 70's so that will prove it out for me without buying jets. It came with 70's last year but I was trying to tune out an issue on my old motor that ended up being more major and hence the new engine. I will throw the 70's back in and start from scratch. I still tend to lean towards the vacuum problem as normal braking is fine just hard pedal application is killing the engine. With the stick in neutral there's no load on the engine while I am doing this unlike the auto with the converter and such.

 

[lewtot184]

what ignition are you using?

 

[Dennis H]

Characteristic. Very hard braking.

 

[91r/t]

what ignition are you using?

Rev N Nator ......all of this is only hard braking and the car runs like a scalded dog so I think we are getting away from whats really going on. The jetting on the plugs looks almost spot on as well. It’s gotta be an idle circuit or vacuum issue. The car also as instant throttle response so the accel pump is tuned correctly as well.

Characteristic. Very hard braking.

Of? Lean idle or vacuum loss killing the engine.