Carter carbureator

[Tim G.]

The Carter carburetor on my '65 300 horse 4 speed has a stumble that begins just as soon as I'm preparing to take my foot off the clutch in first gear. I can get around the problem by over-revving the engine, but it's getting to be a nuisance. I don't know much about these carburetors, but if it needs a rebuild, I'd rather let a professional tackle it. Does anyone have a recommendation for a rebuilder or possibily a quick fix for this stumble?
Thanks.

[Edward J.]

Re: Carter carbureator

Tim, sounds like you may have a accelarater pump problem. which its not a bad fix, you might beable to find someone local to fix. I know these days its hard to find anyone who knows carburators.

[Duke W.]

Re: Carter carbureator

Is it the correct model carburetor for the application?

Are all the adjustments discussed in the '65 Shop Manual Supplement manual correct?

Where are the idle mixture screws set?

It should idle very steady and smooth @ 500, 18-19". Does it?

Can't diagnose anything without knowing exactly what it is and some basic data...

Duke

[Stuart F.]

Re: Carter carbureator

Tim;

I have done everything imaginable to overcome the same problem with my 63 L-76, but then I was dealing with a different animal than you are. Besides the normal low speed roughness of an SHP engine, I have a 3.36 rear and a close ratio 4-speed, so it's like starting out in second gear. I ran experiments with two different carbs over the span of both seasons here in central Florida, and I'm happy to say I have the problem licked. But it's like Duke said, to make any kind of guess as to what your problem is will require a lot more input from you. I can tell you one thing, with mine, it had little to do with the accelerator pump. I tried different kinds of pumps, springs and adjustments, and they had very little impact, because the main culprit in the equation is today's gas. You may well have to get into the areas of jetting, float levels, ignition timing (both mechanical and vacuum advance) and your exhaust heat riser arrangement. I had to and now mine runs almost as good as new. It should be easier with your L-75.

Stu Fox

[Clem Z.]

Re: Carter carbureator

http://www.corvetteactioncenter.com/kb/questions/716/1964%3A+Service+Bulletin%3A+Recalibration+of+Carter+AFB+Carburetors

[Timothy B.]

Re: Carter carbureator

Tim, Double check that the vacuum advance hose is not egg shaped and small leak where it's plugged into the V/A diaphram. Also, double check the accelerator pump plunger height as it may be too high. The book say's 1/2" stem height but people lower the floats some because of gas quality and that effects level in pump cavity.

The floats set the gas level in the carburetor and accelerator pump cavity but if the plunger is a little high air between the cup and gas will cause a momentary lack of fuel squirt. That's why float level is important on carburetors as it effects more than just level of fuel.

Look for all the easy things first and don't over tighten anything, the AFB is a great carburetor.

[Stuart F.]

Re: Carter carbureator

If you are going to change your pump stroke setting, be sure you follow instructions regarding the closing of the primary butterflys, i.e. idle adjustment screw backed full out. If set with the idle screw engaged with the throttle lever to set idle speed, the pump will be too high and create the condition of which Tim speaks.

If you remove the top and change out the pump, be aware that most rebuild kits do not provide new springs with the pumps. You must change them over from your old pump. And, be certain to include the spring that sits in the pump chamber under the pump as that is designed to spread the pump plunger in the chamber for a good seal during the pump stroke (lot of people forget it).

Clem;

Thanks for posting that Service Bulletin. Almost everything in it I had to learn the hard way. My main carb is a 3721SB, and my back up is a 3720SA. I am currently using a combination of parts from each and some jet modifications to arrive at an ideal arrangement. The cluster variations are mainly air bleed sizes. The accelerator pump squirters are also sized differently, but I don't see that in this bulletin so that change must have taken place with the SB update.

One more complex in the equation is the relationship between the ignition timing (mechanical or initial, as well as the VAC used) and the transition ports at the primary butterflys. The more initial timing (mechanical and vacuum) used requires you to readjust your idle set screw (back out) to achieve the desired idle speed. This then closes down the primary butterflys and reduces the exposure of the transition ports. Ideally you want them to appear as perfectly square ports with the butterflys set for idle. Anything less, or for that matter more, will affect the off idle performance hesitation or bog.

Then there is the heat riser butterfly situation discussed at some length in recent threads. With the L-75 300hp engine, make sure you have the correct gasket, phenolic spacer and SS plate under the carb. Running that carb too hot will not only give you sluggish low speed performance, but a lot of heat soak after shut down resulting in fuel evaporation. I believe we have all concluded that it is almost a must to wire open the heat riser butterfly during these summer months.

Stu Fox

[Timothy B.]

Re: Carter carbureator

Tim,

I need to make a correction, the fuel level in the accelerator pump cavity is detirmined by the pump piston suction through the check valve in the bottom of the float bowl.

This fuel level can be different than the float level. Double check that the pump squirts with movement of the throttle plates, if not, that's the circuit you need to check. There is also a check needle under the squirters that forces the pump piston to suck gas and not air.

[Tim G.]

Re: Carter carbureator

Thank you for all of the information guys. I appreciate it.

[John F.]

Re: Carter carbureator

Tim, I have the same problem in my 65 300hp car. Have you found a solution to the stumble upon take off?

Thanks,

John DiFede

[Michael J.]

Re: Carter carbureator

Carter carbs are notorious for weak accelerator pump design. They don't last long and are always problems. Most pro rebuilders replace it completely with a newly designed one. Pony Carbs has done all my Holleys and Carters and are excellent. Not cheap though.

[Duke W.]

Re: Carter carbureator

It looks like the OP went AWOL, but this subject deserves more discussion, because it is a common "problem". I used quotes because it is somewhat inherent to the beast.

Carburetors are imperfect devices. In fact, given the A/F requirement over a very wide range of operating conditions, it's a wonder they work at all.

The problem always existed with my 340 HP, CR trans, 3.08 axle SWC, and it's exacerbated by the tall gears. I could either accept some stumble or slip the clutch excessively with more throttle/revs at start-up from a dead stop.

If one uses minimum throttle/revs from a dead stop revs/manifold vacuum drop down to below idle speed. Since idle/off-idle fuel metering is based on manifold vacuum the A/F can go lean to the point of stumble.

Manifold heat is important in this operating regime in order to completely evaporate the fuel. I once tried blocking my heat riser, and it resulted in severe start-up stumble for the first 15-20 minutes of operation, but wiring the heat riser valve open was okay in mild weather.

Even modern cars are subject to this. I have a buddy with two BMWs. His 530i has the problem, but his M3 doesn't. My '91 MR2 with it's digital speed-density EFI system has the problem. My '88 MBZ with its "digilog" Bosche KE-Jetronic system does not.

It seems that modern digital engine management systems get "confused" if revs/manifold vacuum get too low. One experiment I've run on modern manual tranmission cars is to idle in first gear, then apply the brakes to drag down engine revs. When they get much below idle speed they start bucking. In contrast, the analog EFI system in my Cosworth Vega can be dragged down to 200 RPM in first gear, and when I open the throttle it takes off smoothly without protest.

The 3721SB carb. represented the final iteration of the SB AFB, and it only needed to be calibrated for the 300 HP engine, versus both 300 HP and 340 HP in '62/'63. The idle/off idle metering restrictors, which are part of the primary venturi clusters. are slightly larger (richer) - .037 versus .035" on the 3461S, so this should result in better starting from a dead stop performance, but it's still not perfect, just as some modern cars are not.

The first thing to start with is a known baseline, and OE spec is the best place to start, which is why all the settings and adjustments should be checked to be sure they are to OE spec in the applicable service manual. I don't think the accel. pump is a cause of start-up stumble because the throttle is not being moved that much, but make sure it is set per spec, and check that opening the throttle slightly yields a dribble from the nozzles.

The idle/off-idle fuel, air bleed, and emulsion passages are precisely drilled and any deposit build-up can upset metering, so if all the adjustements are per spec, the next step is a thorough cleaning, which means disassembly and soaking the main body and venturi clusters in a powerful solvent that will dissolve any deposits.

After all the above there is not much left except to modify your driving technique, which may result in more clutch wear or just accept a bit of start-up stumble as part of the vintage car experience keeping in mind that many modern cars are not perfect in this area either.

Duke

[David S.]

Re: Carter carbureator

If you do have to send it for a rebuild. Send it to Eric Jackson of Vintage Musclecar Parts. (www.vintagemusclecarparts.com)

-Dave

[Timothy B.]

Re: Carter carbureator

I have a original 3461 and a 3720 SA and 3721 SB. The SB has different primary booster venturies, I can't tell about the SA model. I know the jet sizes changed.

Can't remember the shooter size but that may be something to consider.

[John F.]

Re: Carter carbureator

Thanks to all for your suggestions. Sorry for my delayed response. My carb, a 3721sb has been recently rebuilt and all of the parts are as specified. The car will stumble upon acceleration when fully warmed up. I have my manifold crossover open, but the holes up to the carb base are plugged. The heat riser is wired open. My axle ratio is 3.36 with the 4 speed wide ratio transmission. Another wrinkle is that my 327-300hp was rebuilt by the previous owner with a stock 350 hp cam and larger valves (2.02 & 1.595). The pistons are stock 300hp. I've measured the cam lobe lift @ .299 for both intake and exhaust. The cranking compression is approx 160 lbs for each cylinder.

I've lurked for a long time on this forum. I have changed my vac advance from the stock 238 24 can (300hp 65) to the one recommended for the 350hp - VC 1765. The motor pulls 15" vacuum at idle. For best idle and higest vacuum my idle screws are at 1 3/4 turns. Idle speed is 750 rpm and base timing is 6 degrees with the stock 300hp distributor curve.

Thanks again for your help. If you have any additional suggestions please pass them along.

Thanks,

John DiFede