Desmogged Tuning (1 Viewer)

Joined
Jan 9, 2016
Messages
285
Location
Florida
For the life of me, I can't get my '87 tuned so doesn't ping, backfire, or be really hard to start when hot.

It has a carb and distributor recurved/rebuilt by @FJ40Jim . All the hoses are hooked up correctly per the desmog guide. When everything is set to spec, I get a ping under loads like merging on to the interstate or going over a bridge. It is also very hard to start when hot (the carb fan is working correctly). I can retard the timing a bit to eliminate the ping, but then I get a backfire on coast down. I can add a little more advance and it starts right up, but also pings more. I've tried moving the vacuum advance hose to the secondary pot and that helps a little. The best I can get right now is adding a little more static advance and disconnecting & plugging the vacuum advance altogether.

The engine idles nice and smooth, vacuum at idle is nice and steady, everything works. Get's decent mileage (10-11 mpg). When I pulled the plugs a couple of the core insulators were burned & blistered so I'm not imagining the ping.

I've tried running higher octane but that just seems to move things around a little (ping starts a little later, etc.)

What am I missing?
 

OSS

Joined
Jun 30, 2017
Messages
2,909
Location
Oblivion
Your description falls in line with pre-ignition (as opposed to detonation)—the fuel mixture may be igniting before the spark plug sparks. One cause of pre-ignition is overheated intake air.

Since the engine is hard to start when it is still hot and the carb & distributor were just rebuilt, (and the carb fan is working) the monster under the bed might be the Heat Control Valve inside the manifold. It can stick in the "cold" position, overheating the intake.

Also be sure that the cool air intake duct is installed on the air cleaner neck, and if not already done so, disconnect the vacuum hose to the Hot Air Intake valve if it's connected.

That's one place to look

(PS - don't use 87 octane fuel )
image.jpeg
 
Last edited:

1911

chupacabra
Joined
Aug 11, 2006
Messages
6,184
Location
Parker County, Texas
 
 
Another source is carbon build-up in the head and on the valves and pistons, if it's been a long time/many miles since a valve job. It can glow red-hot and pre-ignite the fuel-air mixture. This used to be a lot more common in the old days, but improved head design and improved gasoline has made it fairly uncommon now - but still a possibility if everything else is ruled out.
 
Joined
Jan 9, 2016
Messages
285
Location
Florida
Also be sure that the cool air intake duct is installed on the air cleaner neck, and if not already done so, disconnect the vacuum hose to the Hot Air Intake valve if it's connected.
I'll check the valve. No duct installed because of second battery. Is there a way to run one?
 

OSS

Joined
Jun 30, 2017
Messages
2,909
Location
Oblivion
I had dual batteries too. I ran a duct like shown below. Got rid of the headlight washers since they were useless and it's reservoir took up too much space. The installed duct ran inside the fender to the hole cutout in the radiator support like the original did.
Got the duct at O'reilly auto parts. Good quality.

image.jpeg
 

klinetime574

SILVER Star
Joined
Jul 23, 2006
Messages
5,702
Location
Chicagoland, IL
 
 
OSS and 1911 definitely have some solid advice. I’d check into both of those things, especially the heat control valve.

@OSS Interesting you say that about not running 87. I’ve always run that besides some ethanol free 91 when I get it, and on drive the other day I topped off with about 16 gallons of 93 and I swear the 2F ran smoother and just sounded better. I didn’t know if this could just be the “full tank” effect or maybe there is some tech behind this? Desmogged 2F with a header, distributor has not been recurved (yet) either
 
Joined
Jan 9, 2016
Messages
285
Location
Florida
@OSS , alrighty, I've made quite a bit of progress.

The heat control valve was fixed in the "cold" position. I fixed that, then, through MUCH trial and error, have the static timing set just a bit advanced from spec. Vacuum advance on the secondary pot. I can't get it to not ping or not backfire using the primary pot. BUT, this configuration seems to be working. It may ping when the altitude compensator kicks in, but I'll burn that bridge when I get to it.

I also pulled the headlight reservoir and used the old duct to connect to the hole in the fender well...

Seems like all is copacetic. Thanks! I never would've thought that silly heat control valve would make that much difference.
 

Seth S

SILVER Star
Joined
Jan 4, 2011
Messages
3,674
Location
Vermont
 
what octane fuel are you running? could you have your vacuum advance hoses swapped? Have you checked your plugs recently?
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top Bottom