hot restart issue 327/340 engine L76

[Edward J.]

I am having a issue after driving the car for 30 minutes it tends to crank longer after a hot soak, I believe its a fuel vapor lock or percolation problem. The car runs great and starts great cold. no other driveabilty problems

I am wondering if anyone has blocked of the heat cross over passage on the cylinder heads on the L76 car or maybe used a metal heat shield between carb. base and gasket from a 300hp engine, keep in mind its a AFB carb. As I see it the heat cross over passage is not needed because the choke uses heat a heat tube on the exhaust manifold. Am I correct or missing something?

Thanks for your input. the car will be heading in a few weeks to it first chapter judging and would like to at least try something, maybe some of you 63 guys have done a fix for this. if not maybe I will be the first to try.

[John D.]

Re: hot restart issue 327/340 engine L76

Wiring the heat riser valve in the open position worked wonders for the same issue on my 67 350 hp....

[Mike M.]

Re: hot restart issue 327/340 engine L76

I had this problem on a '71 Olds 442. The gas was actually percolating after the car was killed and had sat for awhile (it would crank and start fine if it sat for just a little whie-it was that heatsoak that was causing the problem). The gas came shooting out the top of the carb (Qjet in this case) and actually flooded the two front bbls on the carb. The fix was to block off the heat riser port (in my case I blocked off both of them and bought a new carb with an electric choke). I think you could block off one port and wire open the heat riser and see how that works. Also, using non ethanol gas will likely help this condition. After i did this I have driven the car in all sorts of hot conditions with no more problem. My 2 cents.
Mike

[Daniel Y.]

Re: hot restart issue 327/340 engine L76

be advised that wiring the heat riser open will result in a deduction during operations at the meet. During the operations you will not be judged for re-starting your car. if you do, it shouldn't get that hot during this short time for a small block... after the meet - try to get it fixed

[Tom R.]

Re: hot restart issue 327/340 engine L76

I am having a issue after driving the car for 30 minutes it tends to crank longer after a hot soak, I believe its a fuel vapor lock or percolation problem. The car runs great and starts great cold. no other driveabilty problems

I am wondering if anyone has blocked of the heat cross over passage on the cylinder heads on the L76 car or maybe used a metal heat shield between carb. base and gasket from a 300hp engine, keep in mind its a AFB carb. As I see it the heat cross over passage is not needed because the choke uses heat a heat tube on the exhaust manifold. Am I correct or missing something?

Thanks for your input. the car will be heading in a few weeks to it first chapter judging and would like to at least try something, maybe some of you 63 guys have done a fix for this. if not maybe I will be the first to try.

If you do block off the heat passage holes at the front of the carb mounting area on the manifold, what is the drill size and tap you would use to install threads so you could insert those small hex drive plugs I've seen in other places? Thanks.

[Edward J.]

Re: hot restart issue 327/340 engine L76

Guys thanks for all your inputs,

John, the heat riser is not the issue as the runs fine in start, its only after a 20/30 minute drive and the car sits for about a hr. or less cranks for about 15-20 seconds before start and after start runs alittle rough til it clear's out.

Dan the car is acting as your Olds was, but no fuel dumping after shut down.

Tom, on the L76 car it has the aluminum intake there is no passage under the carb. area. It just crosses from one cylinder head to the other head and heats the bottom of intake area for those extra cold mornings, what I am thinking of doing is - some of the intake gasket kits come with stainless block off plates that install on the gaskets to block the cross over. since the I don't drive the car in the winter, The heated crossover I believe can be done away with. The AFB carb uses the exhaust manifold for the heating of the choke.

[Tom R.]

Re: hot restart issue 327/340 engine L76

Guys thanks for all your inputs,

John, the heat riser is not the issue as the runs fine in start, its only after a 20/30 minute drive and the car sits for about a hr. or less cranks for about 15-20 seconds before start and after start runs alittle rough til it clear's out.

Dan the car is acting as your Olds was, but no fuel dumping after shut down.

Tom, on the L76 car it has the aluminum intake there is no passage under the carb. area. It just crosses from one cylinder head to the other head and heats the bottom of intake area for those extra cold mornings, what I am thinking of doing is - some of the intake gasket kits come with stainless block off plates that install on the gaskets to block the cross over. since the I don't drive the car in the winter, The heated crossover I believe can be done away with. The AFB carb uses the exhaust manifold for the heating of the choke.

Edward, thanks for getting back to me. My base engine 67 w/ powerglide does the exact same thing. Wait 45 min or so and it starts right up but with a little stumbling until it clears. I also have the very thin stainless gasket that is supposed to be the last gasket that is in contact with the carb base. I suspect I could install that gasket on the intake manifold first to block the two ports for the exhaust to heat up the carb as a temp solution for reducing heat. I was looking for the technique someone has used to drill and tap the holes for plugs that could placed there to essentially eliminate hot gases from getting to the carb. Has anyone done that and what are the specifics of the parts/processes they used. Thanks all.

[Edward J.]

Re: hot restart issue 327/340 engine L76

Tom, This has been on the discussion more than a few times for those engines that have the heated cross over under the carb base , they can be blocked off as you said drill and tapping is the easiest way, But I don't like the idea of the metal chip dropping into the cross over and if not careful they drop in to the intake. One other pitfall is the choke has lost its heat from the crossover so that means a retro fit of a electric choke coil.

This will be a bigger topic as more ethanol is replaced at stations that currently are not regulated buy some states to have.

In my state ethanol has been around the last few years. and my 72 base engine corvette has no issues. it seems that alot of the higher compression and SHP engines are efected and those engines that have a heated cross over. My take is the gas is purcolating in the float when shut down. Ed

[John H.]

Re: hot restart issue 327/340 engine L76

Edward, thanks for getting back to me. My base engine 67 w/ powerglide does the exact same thing. Wait 45 min or so and it starts right up but with a little stumbling until it clears. I also have the very thin stainless gasket that is supposed to be the last gasket that is in contact with the carb base. I suspect I could install that gasket on the intake manifold first to block the two ports for the exhaust to heat up the carb as a temp solution for reducing heat. I was looking for the technique someone has used to drill and tap the holes for plugs that could placed there to essentially eliminate hot gases from getting to the carb. Has anyone done that and what are the specifics of the parts/processes they used. Thanks all.

Tom -

For the carb pad "hot-slot" fix on a '67 base engine (iron 783 intake manifold), those two holes in some castings will accept a 7/16" steel cup plug, while others won't and need to be tapped for a 1/4" NPT allen head pipe plug.


HotSlotFix.jpgHotSlotPipePlug.jpg

[Michael G.]

Re: hot restart issue 327/340 engine L76

Ed, I've had great results this Summer with my 63 L84 by insulating the entire fuel line, from under the battery, all the way to the fuel unit, including insulating the fuel filter from its bracket. Fuel bowl temps have dropped dramatically. As a result, my car has run perfectly all Summer, no matter what the outside temperature. I have run about every gas combination, including straight pump gas with 10% ethanol, with no negative effects, no stalling, no idle variation, no problems with hot re-starts. I'd say you ought to try it, its cheap and easy and the insulation is easy to remove for judging.

[Duke W.]

Re: hot restart issue 327/340 engine L76

If you search "percolation" and "vapor lock" you will find a lot of threads including a recent one started by me asking for temperature readings. Several were provided including before and after adding various insulation.

The distillation curve of E10 is the culprit due to lower midrange boiling points, so percolation after a hot shut down and possibly vapor lock in hot weather, low speed driving are issues for many.

Percolation causes an excessively rich mixture after a few minutes following a hot shut down. All restarts within about 30 minutes should use the "flooded engine" starting procedure described in your owner's manual.

Duke

[Stuart F.]

Re: hot restart issue 327/340 engine L76

My exploits with this problem with my 63 L-76 have been stated in a number of threads over the past year or so. First off, my changes would, needless to say,not pass judging as I use the L-75 phenolic spacer in a sandwich with two standard L-76 gaskets. I also use an electric choke and a rubber type fuel hose from the filter to the carb. The hose was necessary to allow for the 3/8" + increase in carb height, and serves to isolate the carb from any metal heat conduction to the carb. I run my heat riser normal and no crossover blocks. I see about 35 to 40 degrees reduction in temp at the carb, which does the trick for me. I now use unadulterated 90 octane (no ethanol).

As for hot starts; don't use the flooded start method! Instead, allow the engine to crank for 2 to 4 seconds, then apply some throttle and it should start right up. Sure, it will stumble some so you will have to blip it a few times to clear it out. This method allows the accumulated rich/raw fuel in the manifold to clear out through the cylinders before you introduce a proper fuel/air mixture. This works very well for me every time. I used to floor it before which only exasperates the problem. It's hard to train onesself to do it this way, but just try it and you will be amazed. BTW, I live in hot humid Florida and deal with this problem for at least 9 months a year.

[Edward J.]

Re: hot restart issue 327/340 engine L76

I am thinking of gettting some 100LL avgas about 6 gallons to try just to get by the judging next weekend and then try some of the fixes. I am still leaning toward blocking off the cross over ports on the cylinder heads,This will shut of the heat to the underside of the carb. which I believe is the culprit, For me taking of the intake one more time is not a problem. a simple weekend or less .

[Tom R.]

Re: hot restart issue 327/340 engine L76

Tom -

For the carb pad "hot-slot" fix on a '67 base engine (iron 783 intake manifold), those two holes in some castings will accept a 7/16" steel cup plug, while others won't and need to be tapped for a 1/4" NPT allen head pipe plug.


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John, that's what I was looking for. I'll give the 7/16" cup a whirl first. Thank you Sir.

[Stuart F.]

Re: hot restart issue 327/340 engine L76

Ed;

One item you can do to help for the interim is to drop the float levels down at least a 1/32" or 1/16". Also, if you can get your hands on some Tomco inlet valves to replace the needle/seats you'll get improved fuel control to reduce the percolation spill over. There are some other tricks to do with the primary venturi clusters, i.e. units from the late series 3721SB AFB's have taller anti-percolator vent tubes (main well vents) which also helps control the percolation.

Just remember the starting procedure I outlined in my previous post too.

Blocking off the crossovers in the manifold can get you a big hesitation when the engine is cold, and you will also notice a different exhaust note - probably not so much with a standard street exhaust system, but it will be there. There are several other intake manifold gasket sets available that already have blocks in them. I know I have several sets, but I don't recall there original application. I believe one may be for some trucks. There are also some that have partial block offs that use a metal plate with a smaller oval passage in them.

Stu Fox