Motor RPM drop

[RattleCanned79]

I have a 79 Bronco im strictly mud racing, at the start line in nuetral and holding 3500-4000 RPM but as soon as I put into drive or into 1st the motor will dog done to 1500-2000 and will not build power? Put a 4 barrel Edelbrock Performer intake with a Edelbrock 650 carb on it but also acted the same way with the original 2 barrel carb....? Any ideas, tips, info?

 

[miesk5]

yo,

Edelbrock.com - Tech Center - Tech Videos

http://www.edelbrock.com/automotive_new/misc/tech_center/install/tech_vids.shtmlWhether this is your first carb installation or you've been doing it for years, this video shows you how to install the carb on a variety of ... Hesitation and Stumbling ...

Stall when put into Gear; Hesitation, stalling, stumbling, flat-spot, or dead-spot during acceleration: Backfiring or spit-back up through carb; Hesitation, dead-spot or stalling after first mile of warm-up; Periodic backfiring with black exhaust smoke; Dead-spot, flat-spot, hesitation, stumbling, backfiring (carburetor) Source: by Recarbco Fuel Systems recarbco.com

Check the float level, timing

GL!

 

[Rons beast]

Great carb info Miesk5, Thanks.

You haven't said if you are running stock gears, and sounds like your using an auto trans. This isn't the best combo for mud racing. I'm guessing you have big "mudder" tires as well. Dropping an auto trans from 4K to drive is not the best launch technique, and my thought is the gearing cannot get the tires moving. The result is an engine that bogs. Like starting with the parking brake on.

If you want to get a heavy vehicle moving you need to have low gears, (high ratio)

First do some simple system checks:

Check your base timing, and your advance.

In a safe area hold the brakes on and in drive give the eng. about 2000 rpm. If the engine starts to miss there is a breakdown in the ignition system. If not, all there should be ok.

If you're running the rpm at 4k and not starving the eng. of fuel , that system should be ok.

You said you had the same problem with the 2bbl.......My guess is the gearing just won't allow the truck to get moving fast, especially with the bigger tires.

Check all vacuum lines anyway.

What does the truck do if you have it in Dr. and just stomp the gas? On pavement?

Let us know, and good luck.

maybe ask some other racers what gears they are running.

 

[RattleCanned79]

Yes stock 3.50 gears. When I put it in DR. And mash it, it takes off better but still not like it should. Im running 35 buck shots id like to change gears for sure up to 4.80 4.90 area. Im running the stock motor with 14 advance with vacum unhooked.

It almost seemed like the tranny shifted half way through the pit pulling up to the line and taking off without bringing up R's just stomping it from the line

Last race for me this year so I've got a while to get it going right for next season

 

[Rons beast]

OK trying to get big 35s moving in mud, with 3.50 gears is an issue.

There may be an issue with your tranny, but no way to tell on here.

do you have the vac advance line plugged, so no vacuum leak? And why disconnect it anyway? when you accellerate, vac drops and the advance is not there anyway. I'm not a mudracer, maybe that is something that you all found is beneficial. If so let me know why.

Do you know what others are running, gear wise?

You may get quicker just changing the tires to something narrower that can be pushed through the mud easier, with the engine in it's power range. And you could sell the old tires to some other mudder.

Just a thought.

Good Luck

 

[RattleCanned79]

Ive got 4.88 gears on there way for the truck i know for a fact it needs them! depending on the truck and horse power the faster guys in my class run from 4.10 to 5.14 which is the highest I can go in the front dana 44 reverse cut gear in that truck I believe. No vacum leaks that I found al seems well, is 14 advance about right in a stock 400? Tires are narrow outside to outside 11in at the most. When ive always set the timing ive unhooked it...its base timing then the motor can go from there in any mtor ive built or worked on always have done this course this is with the motor running/

 

[Rons beast]

Ok, my bad....I thought you were running the motor with the vac advance off. Yes 14 sounds good for a base. You can increase that and change the springs and counter weights in the dist. if yours will allow. Since you are racing you can dial it in and back off combos till you get the best power without the "ping".

The biggest help is gonna be your new gears. They will allow the motor to climb to it's power range faster, and increase the available torque for a good launch.

Keep us informed has to how it goes.

Good Luck

 

[swampthing]

another thing i would do is invest in a higher stall converter maybe around a 11 to 1500 stall