Down draft problems

Killerbee

Killerbee

BANNED
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:
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.
 
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?
 
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?
 
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
 
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.
 
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!
 
Killerbee said:
...I'm getting plenty of fuel to the carb as it sits lower than the tank...
Click to expand...
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.
 
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
 

Attachments

  • IMG_2215.jpg
    IMG_2215.jpg
    31.6 KB · Views: 82
CarlG said:
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.
Click to expand...

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.
 
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.
 
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.
 
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.
 
You must log in or register to reply here.
Back
Top