Long story short, I broke a ring 2 years ago and just now managed to get the engine rebuilt and back in. It starts and runs, but when I put it in gear I get about a 400rpm drop and it's running rough like there's a vacuum leak. At least that's what I thought.

I've got a BTO 700r4, and the metal plate that sandwiches between two carb gaskets looked like it had gotten bent while it was off the car, so I called BTO and ordered a new one. Mean while I put a normal gasket under the carb and left the cables disconnected and all the vacuum ports plugged while I tried to straighten out my running problems.

Initial timing is set at 12. Total is 36 and everything combined comes out to 52 when the vacuum is reattached to the canister. Vacuum gauge is showing 15 inches or so of vacuum with the idle air adjusted out. If I rev it up while it's idling, about half the time it'll backfire. Acceleration is pretty rough, and once it goes into overdrive, any gas will cause bucking and backfiring.

All the backfiring makes me think it's running lean, I'm just not sure what to do about it. It's the same Q-jet I was running before the rebuild without any issues (although it has been mostly sitting for 2 years). We moved it over to the Chevelle while the vette was down and it seemed to be running fine there. There was of course old gas in the tank, but I checked the filter and it's not clogged. I did replace the fuel pump during the rebuild. Is it possible the new pump isn't supplying enough pressure?

I'm at a loss here and am just looking for ideas.

 

With your vacuum at idle it doesn't seem to be valves adjusted too tight, blocked exhaust (chipmunk hiding nuts in the exhaust while the motor was out) and since your carb works well on another engine that's ok.

You could pull the output side of the fuel pump and put a rubber hose on it and then put the end into a good sized container. Then crank the engine with the ignition disconnected. You should get a pretty good volume of fuel out of the pump with just a few second of cranking.

It almost sounds like you've got a plug wire or two crossed up or the cam gear and crank aren't matched up properly.

Not sure what year your car is but egr could be hung open.

You could spray some carb cleaner around the intake and carb gaskets while running and if the engine picks up you have a vacuum/gasket leak.

The backfiring is the troubling thing. That can be caused by improper cam timing, plug wires screwed up, or exhaust valves hung open or stuck closed. You can run an engine pretty lean under no load and not experience backfiring.

 

I'll put a beer on your cam timing is off. When you did the rebuild did you have one of the Timing chain sets that has 3 different keyways cut? Might as well get busy pulling the front cover to inspect it.

 

the big drop when going into gear brings into mind the vacuum advance cannister and if you are using direct or ported vacuum. are you sure the vacuum advance canister is working?

can you put your vacuum pump on the vacuum cannister and look to see if it hold vacuum and at what inches of merc it starts pulling in?


i also had to go to a looser converter on my 700r4, but check the timing first.

 

Vacuum canister is on manifold vacuum. With the canister plugged, timing is all in at 36 degrees. With it hooked up, it's all coming in at 52 degrees. Both at around 2800 RPMs.

 

Backfiring through the carb or exhaust? Is it possible a valve was adjusted too tight?
Were the heads done? If not, how about a broken spring?

 

ok the vacuum cannister is pulling vacuum when you are checking it at idle in park........lets see what your timing is when at idle when you put it in gear and it drops down 400 rpm.....if the timing changes you have a vacuum cannister that is not pulling in at the right vacuum


Azmotorhead may be right ....it may not be a bad idea to check the TDC on the harmonic balancer to make sure it is in fact TDC...and then rotate the engine by hand and follow the valves opening and closing through the firing cycle to see if it looks like they are all closed at TDC of the compression stroke. what if the harmonic balance slipped or if the timing chain is off several teeth?

 

firing order anyone?

 

Well, I've managed to get everything straightened out. The moral of this story is "Don't let your carburetor unused for two years and expect it to work the same as when you took it off."

 

Eiother that or a firing order issue.

 

That was my first impression but he sounds like he got it running fine.