SN95 New 5.0 Car Build Bogs At Hit Of Throttle

Guys I need some help! Just finished a 306 build with big heads, equal length headers, single plane intake, .554 cam and a Holley 750 ultra street carb. Runs great after 2500rpm but bogs at the hit. Bogs with a 1700rpm power brake and a 2300rpm trans brake launch. I adjusted carb and timing several times with no real change. The plugs look perfect also. You guys rock so I know somebody has an answer!
Sam
 
  • Sponsors (?)


This is like playing sharades . I'd give the full cam specs head specs . Rear gear . What TRANS etc to give the guys a better idea to help .


Sent from my iPhone using my fingers while my auto correct makes me seem illiterate
 
  • Like
Reactions: 1 user
And in this case what size tire .


Sent from my iPhone using my fingers while my auto correct makes me seem illiterate
 
A 750 is too much carb for a 306. Period. End of story. There are calculators online to determine how many cfm you need based on c.i.d and max rpms you are going to turn, but anybody that's ever run a carb on a 302/306 on the street will tell you that the calculator is not needed. Buy a good vac secondary 600 and be done. Oh, and I'm with Mike. Unless you are spraying that thing at the track, ditch the single plane and get a good dual plane like a rpm air gap. That little motor needs all the torque it can get.
 
  • Like
Reactions: 1 user
Take the air cleaner off, start the engine, let it idle. Be aware that if it backfires with your face over the carb, you may get burnt.

Stick your head to where you can see the accelerator pump squirt when you quickly press the throttle. Watch to see if the accelerator pump squirts when you begin to goose the engine. You may find that you need to play with the accelerator pump settings so that it starts to squirt when you first hit the throttle.
 
Is this car full race? If you plan to put it on the street 3800 is way too much converter, it will be obnoxious.

My (smallish) cam says no less than 2500 stall converter, get a 2800-3200 stall. If it is just for racing you can give it more converter.

A single plane intake will make poo torque below 2500 rpm. They are for high rpm engines. We use one on our 327 stroker with an 1100 dominator carb BUT we spin it well past 7500rpm and add lots of nitrous..... and occasionally blow it up.:p
 
Take the air cleaner off, start the engine, let it idle. Be aware that if it backfires with your face over the carb, you may get burnt.

Stick your head to where you can see the accelerator pump squirt when you quickly press the throttle. Watch to see if the accelerator pump squirts when you begin to goose the engine. You may find that you need to play with the accelerator pump settings so that it starts to squirt when you first hit the throttle.

I've found that to fix an off idle bog you do follow this advice.. ^^

If your squirters aren't coming in when you crack the throttle, advance the accel pump cam to bring it in sooner. However...my guess (from past experience) is that big honkin 750 that is set up for an engine big enough to handle it.. is over fueling the poo out of your engine.

Have a look at this. Skip down to where it says "what size holley?" I'm not pulling it out of my butt.

Holley Carburetor Basics - Mustang Monthly Magazine

@Daddys girls GT's
 
Last edited:
Thanks for all the opinions guys! I noticed today that my fuel bowls are NOT halfway full even at an idle. I am going to try and correct that first. The car screams above 2500 and pulls well over 6K. Engine builder that did the heads and the cam grinder both agreed that 750 was the right choice for my needs. This is my 20 yr old daughters car that she has raced since 14. She said make it faster and barely streetable is fine. We all spray but wasn't going to hookup until she was used to new motor but will be spraying up to 150 or 200 eventually. Thanks for all the help, you guys rock! Look up my little team at mtndragracing.com sometime.
Sam
 
  • Like
Reactions: 1 users
Thanks for all the opinions guys! I noticed today that my fuel bowls are NOT halfway full even at an idle. I am going to try and correct that first. The car screams above 2500 and pulls well over 6K. Engine builder that did the heads and the cam grinder both agreed that 750 was the right choice for my needs. This is my 20 yr old daughters car that she has raced since 14. She said make it faster and barely streetable is fine. We all spray but wasn't going to hookup until she was used to new motor but will be spraying up to 150 or 200 eventually. Thanks for all the help, you guys rock! Look up my little team at mtndragracing.com sometime.
Sam
What gears are you running to help with that cam? I think you can make that big carb work you're just not getting enough fuel at the bump of the throttle and you may need to play with the converter and gearing as well to get a good launch. What do your plugs look like at idle?
 
Thanks for asking! I have 4:10's, locker rear end, full manual valve body with trans brake and B&M ratchet shifter. I plan to do a converter during the Winter but my problem is carb I feel. The plugs look great but that is with the carb leaned almost all the way out. I did notice yesterday that my bowls are NOT halfway full and need to adjust for that and retest. Not sure how I missed that earlier because I had a real time getting pressure down to 6/7 pound range. I actually had to use a 1/2" return line off the regulator back to the tank! Yes it is a killer fuel pump! Thanks for all the help and look my kids up on the website.
Sam
 
The tag with your post says SN95. Is this a 94-95 Mustang? If not, what year?

If you are still using the EFI in tank pump, you are asking for trouble.

Do not use an EFI in tank fuel pump with a carb. You will never get the pressure/flow regulated properly. If the add on pressure regulator fails, you will flood the engine with gas and wash all the oil off the cylinder walls. That will cost you big time $$$. Either go full EFI or use a tank/fuel pump/fuel lines out of an 84 or earlier Stang. Fabricating your own setup is possible but there are some snags to overcome.
 
Oh no I built a pickup tube with a sock where the EFI pump used to be. I made the old return line into my new vent line and put a 1/2" return line from the regulator into the top of tank fitting that used to be the vent. I have everything correct but I just found out the bowls are very low rather than at the halfway mark so I may be running out of gas at the hit. Will work on that this weekend. Thanks for the advice!
Sam