C2 - '65 300hp AFB - Help Needed
We just finished a body-off on a '65 300hp. We've got one remaining issue that I need help with. The engine had less than 10K miles with no issues when we bought it. so we put it on the stand to only freshen it up and put the correct pan on it, rebuilt the carb, put new front and rear seals in it. We repainted it.....that was the only thing we did to to the engine. The engine had a PCV valve on it before, but we did NOT put one back on as per the assembly manual. Then came the start-up, during which we had numerous issues such as......it would not idle below about 1500 rpm, we had to put the timing about 20 degrees BTDC and the engine shook excessively at that RPM. Idle mixture screws were set at three turns out. A few hours later we got the thing calmed down by blocking off the vacuum line from the rear of the carb to the oil fill tube (no vacuum on the crankcase). The idle slowed to 700 rpm, eliminated the shake, we reset the timing to approx 10 to 12 degrees BTDC and the idle mixture screws wetre set to about 1 1/2 to 2 turns out. Everything is now hunky-dorey except one thing.....it starts up just fine after it's warmed up, but.....It is a bear to startup when cold. You've got to pump the accelarator like a bad thing (4-5 times) before it will start. But once it starts and runs awhile.....it will start just fine! All you 300 hp experts out there......Help!
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Re: C2 - '65 300hp AFB - Help Needed
Sounds like you had a major vacuum leak to start with. I would question why no PCV valve? Better check your AIM and other sources to make sure you have that system connected correctly. On your 65, I believe the PCV mounts in the oil fill tube with a hose to the rear of the carb. Your fresh air to the crankcase should be via a large connection tube between the hole behind the distributor to the air cleaner. Make sure you use the correct PCV valve as well.
As to your hard starting, is your choke connected correctly and operating?
Other than that, make sure you have your distributor in correctly with #1 facing front and the VAC can positioned midway between the plug wire/shielding bracket and the intake manifold. If it is tight to one or the other you may be a tooth off or the gear on the distributor was put on 180 degrees off (if it was disassembled).
Stu Fox -
Re: C2 - '65 300hp AFB - Help Needed
Agree with Stu, if no PCV valve like Holley's there is a restriction at the carburetor so the engine air circulates (in thru the air cleaner out through vacuum restriction)but check AIM because AFB's may use valve in 65.
Does the accellerator pump work after sitting overnight, if not, the check valve under the squiters may be left out and no shot of gas on pedal pump when setting choke.
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Re: C2 - '65 300hp AFB - Help Needed
Actually, I believe the restriction in an AFB is built into the 90 degree elbow that comes off the rear of the carburetor to connect to the PCV line It's shown on the AIM on UPC 6 Sheet 3. If you get the correct fitting it should solve your problem.Bill Clupper #618Comment
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No PCV on a '65
Clup's advice should fix your problem. You probably have a generic elbow fitting with no restriction. No valve in this system.
About the hard start when cold. Do you have the phenolic spacer and metal heat shield plate under the carb? If not, the bowls will boil down dry after a hot shut-down due to modern fuel's lower boiling point.
Even with the correct set-up, cold starts require quite a bit of pumping/cranking on my '65 300hp. My other '65 with a Holley 2818 is no different.
On future teardowns, the exhaust gas ports on the manifold mounting surface for the carb are going to get plugged.Comment
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Re: C2 - '65 300hp AFB - Help Needed
I stand corrected. The 65 does use the metered orifice fitting instead of the PCV valve system. Sorry about that. It is probably the same fitting (90 degree) as used on the Holley, just that it is mounted into the back of the base on an AFB with a reducer, but it must be used (don't block off or use an open fitting w/o an orifice).
That may or may not account for the major vacuum leak. Be certain you are using the correct base gasket arrangement; on a 63 L-75, (? 65) a phenolic spacer is used along with a SS plate on top of the spacer to seal off the exhaust heat chamber. If you use any other base gasket arrangement, check the fit of the gasket to the carb base and the manifold to ensure it seals off all vacuum bleed chambers properly. Some aftermarket spacers, made up of plates and gaskets in a sandwich, do not seal well and have vacuum leaks.
It is sometimes difficult for posters, myself included, to give good troubleshooting advice with limited descriptive input. One has to make a lot of assumptions as to the correctness of the carb assembly and installation, as well as with the ignition system. The best we can hope for is to say something that rings a bell.
Stu FoxComment
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Re: C2 - '65 300hp AFB - Help Needed
Thanks to you all for pointing out that the fitting out of the rear base of the carb is a special restrictive fitting. I'll add this piece of knowledge to the database in my head! I also noticed that this type of vacuum control system is titled "closed engine positive ventilation" and not a PCV system. Apparantly GM called it this on the base and 300 hp models before PCV. Apparantly when this fitting is installed, the crankcase vacuum will be regulated to something between a "whole bunch" and "zero"
We've ordered the correct, restrictive fitting from Volunteer Corvette and we should recieve it in a few days. I will be in touch with you when we set it up. Thanks again......Brad
ps: We had already installed the proper carb gaskets in the proper order, ie: 1) manifold 2) paper 3) phenolic 4) SS 5) carbComment
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Re: C2 - '65 300hp AFB - Help Needed
PCV stands for "positive crankcase ventilation", but there are variations on the name. In the early days some systems were called "closed" and some "positive" depending on the design architecture, but the difference has blurred with the passing of time.
Some PCV systems have valves, some don't, but any system that recirculates crankcase blowby gas, which is mostly unburned fuel-air mixture into the inlet system to be consumed by combustion qualifies as a PCV system, which includes the system on your '65 327/300. These systems were required on all cars sold in California beginning in the '61 model year and nationwide beginning in '63.
A system with a valve provides better flow control than one with a simple restrictor, but the latter is typically cheaper. GM started with valve-based PCV systems in '61-'63, then went to a valveless simple restrictor design for 64-'65, but back to a valve-based system in 1966. Guess why?
DukeLast edited by Duke W.; May 12, 2009, 08:45 PM.Comment
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Re: C2 - '65 300hp AFB - Help Needed
Duke;
Ok, I for one will bite. Enlighten us. My guess would be it is nice to have a sort of check valve between the crankcase and the fuel vapors present in the carb and air cleaner. From my own experience with a stuck PCV valve on my 63 which caused a crankcase explosion, I guess I'm kinda stuck on this concern.
Learning of the use of the restrictor fitting is somewhat new to me as I have little experience with later C-2's, other than having investigating them for ideas on how to improve the operation of my AFB equipped 63 L-76. I think I'll pass on the restrictor and just keep my PCV valve clean.
Stu FoxComment
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Re: C2 - '65 300hp AFB - Help Needed
GM must have concluded that the simple restrictor design did not provide satisfactory performance. A valve provides must greater latitude in designing the flow versus manifold vacuum curve.
With the simple restrictor design, sometimes flow may reverse - at WOT especially if the engine has higher than normal blowby. That's why the restictor systems have such a large tube between the opening at the back of the block and the air cleaner. Under most vacuum conditions flow is from the air cleaner to the back of the block, through the engine and then into the carb base via the restrictor, but it likely reverses at WOT.
In this case the block is not actually "positively" vented, which means having a constant vacuum source that pulls in fresh air. (Even at WOT there is a net depression between the air cleaner interior and the inlet manifold.) It is just vented to the air cleaner.
DukeComment
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Re: C2 - '65 300hp AFB - Help Needed
The engineers must have changed their minds again. My 2003 5.3L truck has a restrictor. I had an oil change a couple of years ago where the service guy told me I needed a new PCV valve because it wouldn't rattle. I loved the look on his face when I told him to look inside and he discovered that there was no ball.
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Re: C2 - '65 300hp AFB - Help Needed
PCV systems went through a rethink with the advent of port fuel injection engines. For one thing, they don't need an anti-backfire device (the anti-backflow feature built into PCV valves and flame arrestor screens). Because the flow above the injectors is not "wet", a backfire won't propagate to the crankcase and cause and explosion.
My '88 MBZ 190E 2.6 has a simple restrictor design - essentially the same architecture as the '64-'65 Corvette engines.
My 2.2L '91 MR2 merely has the crankcase vented (via a cam cover nipple with an liquid-vapor separator build into the cam cover) to the throttle body with one restricted passage to below the throttle plate and one above, but there is no fresh air inlet. Essentially the crankcase is just vented to the inlet system.
For '92 they went to a valve based system, but I don't know if it has a fresh air inlet.
My Cosworth Vega had an OE valve-based system with a fresh air inlet from the air cleaner to the bottom of the block with the out line and valve routed from the cam cover nipple to a full manifold vacuum source. This design was constantly pushing oil from the crankcase into the air filter housing, especially during race track hot lap events.
Inspired by the MR2 design, I capped the crankcase and manifold nipples and merely vented the cam cover to the air cleaner nipple that was formerly used for the fresh air inlet. The line includes a tee and "downtube" that acts as an oil trap.
This design eliminated the oil ingestion and appears to ventilate the crankcase adequately.
Also of note is that the LS6 engines for the C5 Z06 had an oil ingestion problem through the PCV system, so PCV design remains as much art as science and requires testing to insure adequate ventilation without causing oil ingestion, and this is especially critical on a vehicle that will see high dynamic loading. A system that works okay for a truck in normal service may not be adequate for a Corvette run in track events.
DukeComment
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Re: C2 - '65 300hp AFB - Help Needed
Brad...same 1965 L75 setup, same problem on fuel starved cold start after hot shut down. I have been carrying some fuel in a small squirt container to restart. Seems like an obvious boil out or evap issue. Also have manifold, paper, phenolic, SS sequence. I did not block manifold heat crossover when installing the I-manifold. Don't know if this is a fix or not. Hmmm........
JimVComment
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Re: C2 - '65 300hp AFB - Help Needed
Sounds like the same problem we are all having, even with Aluminum Manifold L-76's which are worse. You could plug the exhaust passages, but you may then have drivability problems. Best to start with wiring open the heat riser and work from there. I use two base gaskets (sandwich) and the L-75 spacer on my L-76 and it helps a lot. I typically see a reduction in temps at the carb of 35 to 40 degrees with my IFR gun. I also use a hose to the filter and an electric choke to isolate as much heat as possible from the carb.
Stu FoxComment
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