The Ford Barn

Killerbee · 07-27-2016, 08:06 AM

I recently put a downdraft with a freshly rebuilt Holley 94 on my banger engine. I'm having issues with it idling and also loading down and backfiring. It backfires out of the carb and the exhaust. This all points to an vacuum leak to me. I've checked everywhere it could be leaking sprayed started fluid over the base and where it meets up with the block with no difference in idle. If i half choke it it will idle and will not pop or backfire. Once again pointing to a vacuum leak. Not sure what I'm missing here. All vacuum ports are plugged on the carb. Is it possible the intake is warped and not meeting up to the block flush.

CA Victoria · 07-27-2016, 08:18 AM

What size jetting was the carb set up with?

Killerbee · 07-27-2016, 08:28 AM

51 jets

CarlG · 07-27-2016, 10:50 AM

If you are not using a fuel pump, that is most likely your problem. I have run both a Weber and a Stromberg 97 with and without a fuel pump. They both ran much better with a fuel pump. When running without a fuel pump, symptoms were pretty much like you described, but only some of the time.

PC/SR · 07-27-2016, 11:07 AM

Probably not an air leak, just running lean, but you have to eliminate the possibility first. The 51 jet should be OK, but try a 52 just to see. Carefully adjust the idle screws because they run on the idle circuit longer than stock because they were designed for bigger engines. Idle adjustment is very sensitive. The idle hole size in the throttle body can make a difference at idle and running at low speed, there are two different sizes. Mine worked better with the smaller size. Try the accellerator pump link on the longest throw, the winter setting, and see what happens on acceleration. They do tend to run lean at low speeds. Tuning the Holleys is much more art than science, but they do work nice when set up. What's your cam?

Killerbee · 07-27-2016, 11:26 AM

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Stock cam. I'm getting plenty of fuel to the carb as it sits lower than the tank. There's no difference where the accelerator is set it was about the same in all three holes. I am running the stock distributor. Could it be a timing issue.? Maybe I'm not getting enough advance?

Killerbee · 07-27-2016, 11:30 AM

https://youtu.be/qgAcCQcdjv8

emf · 07-27-2016, 01:24 PM

I also thought that the carb sitting lower than the tank would be OK. Granted, I'm running two carbs, but the pump made a great deal of difference. I see in your video that you did not spray around the junction of the block and manifolds... Just a thought.

Frank

midgetracer · 07-27-2016, 01:48 PM

I agree with Carl and EMF. I fought with one on a 31 tudor that belonged to a friend of mine. He insisted a fuel pump was not necessary. He had a single Stromberg 97 (new). I tried everything and finally convinced him to put on a fuel pump regulated to 2 psi and it is the best running A I have ever worked with. Try a fuel pump.

1931 flamingo · 07-27-2016, 02:25 PM

If you've backfired thru the carb the PV may be bad. Don't most run an 81 carb??
Paul in CT

Dave in MN · 07-27-2016, 04:19 PM

I recently set up two overhead valve engines with down-draft single carb and dual carbs. We were looking for the maximum power we could get from the combinations. Not until I added the rotary fuel pump and a regulator did I have success jetting them to idle and perform at full load on the dyno. So, I agree with the gents above who suspect you need a fuel pump. One of them was a new Stromberg 97, I will go back and check my records as to the best main jet for the 97.
Good Day!

CarlG · 07-27-2016, 04:59 PM

Quote:

Originally Posted by Killerbee

...I'm getting plenty of fuel to the carb as it sits lower than the tank...

That's what I thought as well. I am using a single 97 carb and had a little over 6" drop from the bottom of the tank. However, even with more than half a tank of gas, it wasn't enough head pressure to keep things percolating like it should. Added the fuel pump and a regulator set at 2.5# and problem solved.

Lawrie · 07-27-2016, 05:14 PM

you can make the 97 run great on the gravity feed,you just have to modify the needle valve,mine has been trouble free for thousands of miles

glenn in camino · 07-27-2016, 07:45 PM

I'm running a Webber on one of my As. works really good.

colin1928 · 07-28-2016, 04:46 AM

Bad holley power valve will do as described

1929 · 07-28-2016, 06:28 AM

Quote:

Originally Posted by CarlG

That's what I thought as well. I am using a single 97 carb and had a little over 6" drop from the bottom of the tank. However, even with more than half a tank of gas, it wasn't enough head pressure to keep things percolating like it should. Added the fuel pump and a regulator set at 2.5# and problem solved.

I had a similar problem with the 97 Stromberg. I bought a fuel pump and a regulator, but I didn't install it yet because I think the jet size change from 45 to 43 has it running much better.

1929 · 07-28-2016, 06:30 AM

Quote:

Originally Posted by Killerbee

51 jets

how do the spark plugs look? did you check them yet?

cw253748 · 07-28-2016, 07:16 AM

Carb is too lean. For one check to see if you have fuel squirting out with the engine off in the carb from the accelerator pump. I think you need to tear the carb apart again.

Bob Bidonde · 07-28-2016, 08:55 AM

I had this problem and it was caused by vacuum leaks inside of the carburetor. In my case, the gaskets were not properly in place.

Killerbee · 07-28-2016, 09:11 AM

I'm getting plenty of fuel out of the discharge nozel. I haven't pulled the plugs. I haven't been able to drive it with this carb. I'm thinking power valve or just tired old carb. Might just buy a new carb with what I'm seeing about old tired Holleys they can be finicky.

07-27-2016, 08:06 AM	#1
Killerbee BANNED Join Date: Mar 2014 Location: South jersey Posts: 40	Down draft problems I recently put a downdraft with a freshly rebuilt Holley 94 on my banger engine. I'm having issues with it idling and also loading down and backfiring. It backfires out of the carb and the exhaust. This all points to an vacuum leak to me. I've checked everywhere it could be leaking sprayed started fluid over the base and where it meets up with the block with no difference in idle. If i half choke it it will idle and will not pop or backfire. Once again pointing to a vacuum leak. Not sure what I'm missing here. All vacuum ports are plugged on the carb. Is it possible the intake is warped and not meeting up to the block flush. Last edited by Killerbee; 07-27-2016 at 08:30 AM.

07-27-2016, 08:18 AM	#2
CA Victoria Senior Member Join Date: Feb 2011 Posts: 2,113	Re: Down draft problems What size jetting was the carb set up with? __________________ Tim Downtown, Ca

07-27-2016, 08:28 AM	#3
Killerbee BANNED Join Date: Mar 2014 Location: South jersey Posts: 40	Re: Down draft problems 51 jets

07-27-2016, 10:50 AM	#4
CarlG Senior Member Join Date: Nov 2010 Location: Anchorage, Alaska Posts: 9,115	Re: Down draft problems If you are not using a fuel pump, that is most likely your problem. I have run both a Weber and a Stromberg 97 with and without a fuel pump. They both ran much better with a fuel pump. When running without a fuel pump, symptoms were pretty much like you described, but only some of the time. __________________ Alaskan A's Antique Auto Mushers of Alaska Model A Ford Club of America Model A Restorers Club Antique Automobile Club of America Mullins Owner's Club

07-27-2016, 11:07 AM	#5
PC/SR Senior Member Join Date: May 2010 Location: Santa Rosa, CA Posts: 1,279	Re: Down draft problems Probably not an air leak, just running lean, but you have to eliminate the possibility first. The 51 jet should be OK, but try a 52 just to see. Carefully adjust the idle screws because they run on the idle circuit longer than stock because they were designed for bigger engines. Idle adjustment is very sensitive. The idle hole size in the throttle body can make a difference at idle and running at low speed, there are two different sizes. Mine worked better with the smaller size. Try the accellerator pump link on the longest throw, the winter setting, and see what happens on acceleration. They do tend to run lean at low speeds. Tuning the Holleys is much more art than science, but they do work nice when set up. What's your cam?

07-27-2016, 11:26 AM	#6
Killerbee BANNED Join Date: Mar 2014 Location: South jersey Posts: 40	Re: Down draft problems Sponsored Links (Register now to hide all advertisements) Stock cam. I'm getting plenty of fuel to the carb as it sits lower than the tank. There's no difference where the accelerator is set it was about the same in all three holes. I am running the stock distributor. Could it be a timing issue.? Maybe I'm not getting enough advance?

07-27-2016, 11:30 AM	#7
Killerbee BANNED Join Date: Mar 2014 Location: South jersey Posts: 40	Re: Down draft problems https://youtu.be/qgAcCQcdjv8

07-27-2016, 01:24 PM	#8
emf Senior Member Join Date: Mar 2013 Location: Hudson, NY Posts: 511	Re: Down draft problems I also thought that the carb sitting lower than the tank would be OK. Granted, I'm running two carbs, but the pump made a great deal of difference. I see in your video that you did not spray around the junction of the block and manifolds... Just a thought. Frank

07-27-2016, 01:48 PM	#9
midgetracer Senior Member Join Date: May 2010 Location: Bismarck ND Posts: 1,189	Re: Down draft problems I agree with Carl and EMF. I fought with one on a 31 tudor that belonged to a friend of mine. He insisted a fuel pump was not necessary. He had a single Stromberg 97 (new). I tried everything and finally convinced him to put on a fuel pump regulated to 2 psi and it is the best running A I have ever worked with. Try a fuel pump.

07-27-2016, 02:25 PM	#10
1931 flamingo Senior Member Join Date: May 2010 Location: new britain,ct 06052 Posts: 9,390	Re: Down draft problems If you've backfired thru the carb the PV may be bad. Don't most run an 81 carb?? Paul in CT

07-27-2016, 04:19 PM	#11
Dave in MN Senior Member Join Date: May 2010 Location: Jordan, MN Posts: 1,411	Re: Down draft problems I recently set up two overhead valve engines with down-draft single carb and dual carbs. We were looking for the maximum power we could get from the combinations. Not until I added the rotary fuel pump and a regulator did I have success jetting them to idle and perform at full load on the dyno. So, I agree with the gents above who suspect you need a fuel pump. One of them was a new Stromberg 97, I will go back and check my records as to the best main jet for the 97. Good Day!

07-27-2016, 07:45 PM	#14
glenn in camino Senior Member Join Date: May 2010 Location: Camino, CA. Posts: 3,086	Re: Down draft problems I'm running a Webber on one of my As. works really good.

07-28-2016, 04:46 AM	#15
colin1928 Senior Member Join Date: Mar 2013 Location: Australa Melbourne Posts: 878	Re: Down draft problems Bad holley power valve will do as described

07-28-2016, 07:16 AM	#18
cw253748 Member Join Date: Nov 2012 Location: Clinton, WI. Posts: 49	Re: Down draft problems Carb is too lean. For one check to see if you have fuel squirting out with the engine off in the carb from the accelerator pump. I think you need to tear the carb apart again.

07-28-2016, 08:55 AM	#19
Bob Bidonde Senior Member Join Date: May 2010 Location: Long Island, NY Posts: 3,463	Re: Down draft problems I had this problem and it was caused by vacuum leaks inside of the carburetor. In my case, the gaskets were not properly in place. __________________ Bob Bidonde

07-28-2016, 09:11 AM	#20
Killerbee BANNED Join Date: Mar 2014 Location: South jersey Posts: 40	Re: Down draft problems I'm getting plenty of fuel out of the discharge nozel. I haven't pulled the plugs. I haven't been able to drive it with this carb. I'm thinking power valve or just tired old carb. Might just buy a new carb with what I'm seeing about old tired Holleys they can be finicky.