Acceleration problem - Cause Carter AFB?

[Oliver S.]

[Oliver S.]

Re: Acceleration problem - Cause Carter AFB?

After reading a bit I assume the secondary activation is the problem. But an AFB uses counter wights so nothing can be adjusted. What can I do?

Oliver

[Timothy B.]

Re: Acceleration problem - Cause Carter AFB?

Oliver,

If what you are describing is a bog when you accelerate, then it's probably a problem inside the carburetor. The secondary air valve is supposed to stay closed long enough to allow the engine to start fuel flowing through the secondary booster ventures. The air valve opens with air flow requirment as the engine speeds up, it's not opened by engine vacuum. All this is supposed to prevent a bog when the secondary's mechanically open.

Make sure the easy things are working properly first, clean fuel filter, proper ignition timing and centrifugal advance. Many times the carburetor is not at fault and problems are in the distributor or simple tune up.

[Rob M.]

Re: Acceleration problem - Cause Carter AFB?

Make sure your fuel lines and filter are not coughed up by debris. Also check if your fuel cap is venting as required (or when it has a seperate venting exit that it is not blocked). Do you have enough fuel pressure?

[Clem Z.]

Re: Acceleration problem - Cause Carter AFB?

make sure someone has not modified the weights on the secondary air valve. also different engines used different weights on the valve

[Oliver S.]

Re: Acceleration problem - Cause Carter AFB?

I haven't had a look at the fuel pressure. Tomorrow I'll check the obvious things mentioned by you. I'm not sure regarding changes of the weights by previous owners but the last one used a repair kit. He paid much attention at original spec. so I assume this should be correct. Three years ago I wanted to change the fuel filter but the new one leaked and the old one seemed to be ok - I could blow thru it without resistance - so I mounted it again.

Oliver

[Oliver S.]

Re: Acceleration problem - Cause Carter AFB?

Timing is fine, Vacuum Advance Unit works. Fuel filter needs to be checked - I had to drive to the garage due to the rear end problem. I've been told by my garage that the fuel pump rather works or not instead of delivering low pressure when accelerating. Furthermore, the accelerating problem may be caused by gas bubbles in the carb in conjunction with high outside temperature.
I'll further observer this issue.

[Stuart F.]

Re: Acceleration problem - Cause Carter AFB?

Oliver;Is the fuel available in your area prone to excessive evaporation (laced with Ethanol) as that we now have here? If so, to follow up on your last point regarding "gas bubbles", you should tell us more about your engine set up, i.e. do you have an operating heat riser butterfly valve? Does your carburetor have an insulator plate under it? Can you see fuel percolating out of the venturi boosters after a hot shut down? For that matter, is your engine hard to sart when hot?As for the secondary air valve, I have modified some by grinding a portion off the bottom circle of each weight, but I found it didn't make a lot of difference on my SHP engine. Perhaps it might with an L-75 (hydraulic lifter engine). One would have to do a drastic modification or even a removal of the valve all together to make a serious difference such as what you described.It is also possible to install the secondary venturi clusters (boosters) incorrectly with the wrong gaskets. The overhaul kits of today contain a number of different gaskets, some of which can partially block off passages under the clusters. One must carefully look at what gaskets you are taking out in order to determine the right replacements.Stu Fox

[Oliver S.]

Re: Acceleration problem - Cause Carter AFB?

Stu,

I'll check what you mentioned. And yes the engine does start when hot. If I recall correctly fuel pours out of the venturies when the engine is stopped and hot. From the previous owner there is a Hygrade carb repkit 188A for all AFBs from57-71 partially used. Once the carb was off and there was a brown spacer approx. 8/32 thick between manifold and carb.
Maybe somebody can point me to where I can order a correct carb. kit wih respect to correct gasket for 3720 or is the 188A fine?

[William C.]

Re: Acceleration problem - Cause Carter AFB?

On a 300 hp engine, there is not only a brown spacer (insulator) but also a stainless steel (thin) plate between the insulator and the bottom of the carb gasket. Sounds a lot like you may be missinf the stainless plate.

[Timothy B.]

Re: Acceleration problem - Cause Carter AFB?

Oliver,

Here is a picture of the correct gasket, insulator and stainless heat shield arrangement for a 300hp engine. Also, the small freeze plugs to block the exhaust heat from under the carburetor.

[Stuart F.]

Re: Acceleration problem - Cause Carter AFB?

Yes, what Bill says is correct and very crucial on a 300 hp with cast iron intake. The spacer should be of a Phenolic material about 3/8" thick, and the thin Stainlees Steel plate on top of that (between the carb and spacer). The plate is used to block off the exhaust heat passage in the manifold so exhaust does not circulate under the base of the all aluminum AFB. That passage is open on the 250hp engine which uses a Carter WCFB which has a Cast Iron base, but NOT used on the AFB. If left open, the gas in an AFB will get way to hot and boil, as well as eventual damage to the carburetor.

Stu Fox