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Getting car ready for Ohio trip (backfires through carb)

Green67Alpine

Green67Alpine

Former SAOCA Membership Director
Gold Level Sponsor
Hi Folks, I'll try to keep this short and to the point. The set up '74 Mustang II 2.8, Offy dual plane, Holley 390 cfm, Dan R cam, lighty ported. Accuspark distributor. Timing now set to 14° advance. Running mostly regular unleaded fuel. I do not know what main jets are in the carb, the power valve is probably 6.5 the one that comes in the rebuild kit. Vacuum is 12-13 at about a 700-750 idle at temp.
Symptoms: runs well at highway speed until you mash the gas, hesitates and often backfires through the carb, this symptom also happens after "gliding " and then accelerating. Also dies at idle when coming to a stop (sometimes). The hesitation and backfire through the carb has been a problem for a while, advancing the timing to 14 helped some.
I've researched around and found conflicting opinions on the cause and cure, as of now the confidence level to head to the Invasion is not at its highest.
Any and all ideas and suggestions are welcome and appreciated.
Tom j
 
14 degrees advanced shouldn't cause pre ignition. Now the stumbling/hesitation when mashing the gas sounds like you are getting a very lean mixture when demanding full throttle. Checking the accelerator pump to confirm its delivering the extra bump of fuel is where I would start and then move to the idle jets.

Has this problem just started or something you have be dealing with for awhile?
 
Carbs have many circuits, so it easy to have more than one problem. Sounds rich idling if its dying at the stop light.

It sounds lean accelerating. Like the power valve is to low. Might try a 7.5 or 8.5., main jets could be small. Accelerator pump etc.

Its very common for accelerator pump check valve to get stuck.. You open the throttle and the spring collapses but the accel diaphragm arm does not move. If you pull the shooter off and take a pick and wiggle the check valve the pump will work.

Its a carb so its easy. Go back to square one. Make sure carb is clean, air bleeds transfer slots, crossover passages. Make sure the throttle blades arent open too far and the transfer slots are fully exposed.


Check the jets I think stock 52 primary. 52 orifice in the metering plate.
 
Basics ...

Check filter, no "crud" coming from tank, fuel pump output and pressure should be about 6-7 PSI for a Holley, and the float levels.

Hope this helps,
 
Thanks for the replies, I've got a lot to through and will report back.
The problem has pretty much been around since we got this car running.
Fuel filter is clean as it can be, fuel level just runs out of the port, and the pressure is about 6 psi.
It seems to be functioning properly, just not set up and adjusted correctly.
Anyone with this basic setup, what jets, power valve, pump cam and accerator spring are you using?
Tom j
 
Try to set the floats to where no fuel runs out unless you shake the car. Retorquing the intake should be done a couple of times. New engines seem to need it.
You can try 55's in the front and leave the rear plate alone. To get mine to fuel even front and rear, we ended up going to 50's in the front and drilling the rear to 57.
Pump and cam were left alone for the dyno.
 
Thank you, I'll give that a try in the next day or two. This engine is much less powerful than the one you built(your thread) and I'm sufficient in building and assembly, it's the final fine tuning that gets off track, I've tuned my stock Alpine into a non runner a couple of times:),
Tom j
 
OK come to find out the carb has 48 jets. And like I thought the 6.5 power valve. Going to clean it up (lots of carbon from the backfiring) and try again over the weekend.
Tom j
 
Check plug wires, maybe new plugs. Backfires, misfires etc.. New fuel if the car has been sitting a while.
 
Have you checked them P.V."s? My brother had a lot of Holley's on his Fords...changing 'em was regular for some reason? I had a trigonometry teacher tell me once .. your not inventing the wheel .. improvements ..'course. ... keep it K.I.S.S.

.... David
 
Can't tell if the power valve is good or not going to change(to a 7.5 ) to be safe, the plugs have less than 1000 miles on them, the wires (10 mm Ford racing if that makes any difference) are about 2 years old and in good condition. The regular fuel I just put in last week.
Thank you to all for your thoughts and ideas, I'm running out of time to test drive before the trip.
Tom j
 
Good one ! Hopefully it'll not just be one more time.
Got the parts together and the carb kinda cleaned up, put a 31 squirter, 50 jets, and a 7.5 power valve, don't really like to change that many parts at once but to change any one thing I have to remove the carb from the car ( not as steady or accurate as I was at one time) since it's easier than taking the intake off to get the part I dropped, so we'll see how it goes.
Tom j
 
Hoping ya' the best outcome Tom! ... I feel ya" {( not as steady or accurate as I was at one time)}... changing to many things at one time can hamper diagnosing the problem if it fixes it .. though you could back track it ... should be easy since it's just the carb ?

.... David
 
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Tomorrow will be the moment of truth, or a hectic scramble to get it right. Worse case scenario, switch the hardtop and head off in the stock Alpine that has very little prep for a trip in this heat.
Tom j
 
Success ! After much help from Steve and Greggers, we're ready to head to Dayton. The 50 jets, 7.5 power valve and 31 squirter with the timing at 13 the acceleration hesitation and backfiring are gone. Cruises nicely between 2800-3200 rpm which gives us interstate travel speeds. Running at 195° on a 90° day can't ask for much more.
Leaving out early Thursday morning and hope to pull in early to mid afternoon. See ya'll there !
Tom j
 
Happy to check in after a great time was had in Fairborn, Ohio. We got off to an early start said goodbye and safe travels to Jim (Alpine 1789) and hit the road by 5am. The car with its recent adjustments ran the best since getting it together and on the road (3+years) . We had no problems other than the passengers were overheating once in the typical Atlanta traffic, the car handled it better than us.
Lots of old faces and new friends in the sunbeam community made the weekend memorable, hope to see everyone in the future.
Tom j
 
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