68 BB hot fuel issue

[Dale C.]

Hate to be so vague on this old beat to death topic of gas boiling, (and I have read most of old blogs including Dick Wittington's great explanations of the why, which are above my pay grade) but I'm stumped. My ole BB starts and runs OK but when it gets warmed up (about 15 to 20 minutes of freeway driving in this Texas heat) and I come to a stop it stalls out. Sometimes it starts back up and sometime it doesn't, until it cools off a bit. I have plugged, replaced and rebuilt carbs, even tried a spare Holly I had. I have Replaced fuel pumps, found no vacuum leaks found no ignition or timing problems. My question is about just going the insulation route. I Don't see a problem insulating the fuel line exposed in the engine compartment, but what about insulating the engine compartment from the exhaust manifold? Your thoughts about using a heat shield or wrap on the exhaust manifold and damaging the cast iron, warps or cracks from overheating? Header and Manifold Heat Shield - Heatshield Products.
Dale

[Duke W.]

Re: 68 BB hot fuel issue

Need more information: Which 427 engine option? Has anything been modified, and if so, how? You say no ignition or timing problems. How did you determine that? Is it TI or single point? OE spec or modified?

Duke

[Mike T.]

Re: 68 BB hot fuel issue

Dale - I ran across this on a recently purchased 66 NOM L72 Roadster. Drivability was good but after getting home after a short drive, I saw gas from the rear fuel bowl dripping into the secondaries. In my case, it was due to a high secondary float level and the heat riser was not opening up causing hot exhaust gas to back up through the head and intake and over to the drivers side, overheating the fuel in the carb.

[Dale C.]

Re: 68 BB hot fuel issue

Thanks for reply. Duke it's a 390 HP Q-Jet with Air-conditioning and no other power. It was judged top Flight 2018 so it's correct - all OE. The only difference is the vacuum pickup is different for an AC car. See pictures. I don't think that would be a problem??? Not TI but did go breakerless years ago. I have accumulated three of those very rare Carbs 7028209FG if anyone is interested. Have some correct fuel pumps also. Anyway, I was just concerned about insulation consequences. Car runs so rough when hot I don't even want to take it out. Any help would be appreciated.
Dale

[Joseph S.]

Re: 68 BB hot fuel issue

Dale,
Not sure if this is going to help you or not. I have been contending with what I think is a fuel percolation problem on my 63 L75. All is fine unless temps are in excess of 90+degree (as they are in TX) and I make a few stops so the carb gets a good 'heat soak'.
First I notice a hesitation account the accelerator pump will not pump the boiling fuel- then there are idle problems as the fuel boils out of the main jets.

I plugged the heat riser ports under the carb, changed the phenolic carb spacer to a wooden one. - some improvement. Bent fuel pump to filter fuel line about 3/8" away from block - a little more improvement. Now I have insulated the pump to filter line (velcro stuff easily removed) - yet to try but I assume will help.

To do if line insulation does not help. Block exhaust gasses from heat riser at head. (heat riser has long since been wired open)

Car cooling system does not run hot but does run a little warmer than another 63 L75 I have that has no fuel trouble.

I could use aviation fuel,( as you know higher boiling point) but I enjoy using the car in road rallies.

I know this car is nothing like yours but just my experience, with help from guys on the forum, in my ongoing issue.


Joe

[Bruce W.]

Re: 68 BB hot fuel issue

Dale, are you sure the coil is good? I had a similar problem. the repo coils made in China are prone to leak when they get hot.
Bruce

[Dale C.]

Re: 68 BB hot fuel issue

Great advice, I'll give them a try.

[Duke W.]

Re: 68 BB hot fuel issue

There are a number of ways to decrease heat, and several have been mentioned. I don't know if your manifold has that "heat slot" right under The front of the carb like some 327s, but it it does block the openings with welsh or pipe plugs. Wire open the heat riser with stainless steel safety wire.

Convert what is likely ported vacuum advance to full time. You many have to change the VAC for it to pass the Two-Inch Rule.

I'm not a big fan of those electronic switches. IMO they are less reliable more prone to sudden failure than mechanical points.

Duke

[Dale C.]

Re: 68 BB hot fuel issue

Thanks Duke. I'll get on some these fixes and get off the originality mind set. Start driving it more and looking at it less.

[Dale C.]

Re: 68 BB hot fuel issue

I think a good start would be, actually measuring and recording some IR heat rises on some of the suspect components.
Dale

[Mark E.]

Re: 68 BB hot fuel issue

Have you confirmed that when you "come to a stop it stalls out" it's caused by fuel percolation in the carburetor? As one post suggests, the problem may have a different cause such as an overheated coil.

If it is percolation check what Duke suggests first, especially ignition timing. If the problem persists, before wrapping the exhaust (which may have little effect for this problem) I would plug the intake heat crossover at the base of the manifold by replacing the intake to head gasket. 1968 does have the "heat slot" along the carburetor flange and plugging its holes will help. But plugging the crossover at the cylinder head keeps exhaust gases out of the manifold altogether. Also consider replacing the heat riser with a spacer to eliminate that restriction.

One trick I used on my 1970 454 (which is the stroked version of your engine) is to add a heat shield to the base of the carburetor. For 1970-72 (which does NOT have the exposed "heat slot" under the carb), Chevy made a shield as part of their California emissions package (NA9 in 1970; you can make out the shield's PN in the photo). I don't know if Chevy made a shield compatible for manifolds with the "heat slot" or if the heat shield shown will work for your application but you might look into this.

My '70 454 runs great in Texas heat and your '68 L36 should too.

20210906_093924.jpg

[Duke W.]

Re: 68 BB hot fuel issue

I think a good start would be, actually measuring and recording some IR heat rises on some of the suspect components.
Dale

Excellent idea. As an example emission controlled engines with ported vacuum advance, exhaust manifolds will measure up to 900F. That on a fully warmed up engine after idling for a few minutes. If converted to full time vacuum advance with a VAC that passes the Two-Inch Rule the 16 degrees added advance will reduce exhaust manifold temperature to no more than 500F. That proper total idle advance is low twenties to low thirties and is usually achieved by the sum of initial and full vacuum advance. Low overlap cams are okay with the low to middle of this range and high overlap cams like the upper end.

Keep in mind that these commercial IR guns measure a conical area, so keep the gun as close to what you are measuring as possible so the reading doesn't get contaminated by background temperatures.

Duke