Stock 79FJ40 …..starts and run with choke on but….

[crewzerdog]

Have a stock fully smogged 2F that starts and drives fine and even idles as long as the choke is about 3/4 way out……however this is the weird part…..once the engine gets fully warmed up (hot) and while it is idling…..the engine will suddenly begin to rev up at very high rpm and if I push the choke back in (off) it idles back at normal and will do so without any choke. Once it cools back down the problem returns. I’ve seen other threads about a blown fuse affecting the fuel cutoff solenoid (ics) but fuses are all fine. It seems that maybe one of the vacuum switching valves (vsv) that plug into the head/water housing are suddenly ‘opening up’ or closing off’ causing the carb to dump more fuel …..has anyone else run into this problem? Any suggestions on what to test? Fuel pump is fine and is an in-line fuel filter. Thanks in advance.

 

[crewzerdog]

The choke is dumping extra fuel into the engine to assist with cold starts. The stock carburetor also has an idle up feature that is adjustable (see FSM) which increases idle while the choke is engaged (on). As suggested by @Skreddy above, the choke should be off as soon as the engine warms, the fast idle with the choke engaged should be a non-issue, as it should race with the choke on.

When I need to choke my motor at startup, which is only when its cold outside or the truck as sat for an extended period of time, I choke it to start it and have it disengaged by the time I pull away. If its below freezing, I find I may need to have the choke feathered open to assist in pulling away from stops until the needle on my temp gauge has started to move off of C

Thanks. Again the fast idle with the choke engaged (on) only happens 15-20 minutes after the truck has been running and is instantaneous…..like someone just shot starting fluid down the carb. It’s a quick very high rev of rpm’s that is solved by disengaging the choke. Prior to the high rev the engine actually idles fairly well with the choke engaged….

 

[77mustard40]

Thanks. Again the fast idle with the choke engaged (on) only happens 15-20 minutes after the truck has been running and is instantaneous…..like someone just shot starting fluid down the carb. It’s a quick very high rev of rpm’s that is solved by disengaging the choke. Prior to the high rev the engine actually idles fairly well with the choke engaged….

I'd suggest you take a good look at the 'idle up' adjustment on the carb. Do you have the FSM for your 79, if not I'd start there so that you can make the proper adjustment. You should have a fast idle when starting the truck with the choke engaged, I'm sorry but I'm not good enough to remember what the fast idle specs are but there is an RPM range in the FSM

 

[crewzerdog]

I'd suggest you take a good look at the 'idle up' adjustment on the carb. Do you have the FSM for your 79, if not I'd start there so that you can make the proper adjustment. You should have a fast idle when starting the truck with the choke engaged, I'm sorry but I'm not good enough to remember what the fast idle specs are but there is an RPM range in the FSM

Yes I do. I’ll add that to the growing checklist. Thanks again.

 

[crewzerdog]

Here’s a picture of the hot idle compensator (found on the top right side of the carb) with the cover off……yep there is a piece of bi-metal that must ‘flex’ when hot….doing so opens up a plunger on the far end (right side) of the picture. One of the vacuum lines comes from the carb to manifold insulator and the other goes into the breather/air cleaner. The plunger does move easily. However I have no idea on how to adjust this gizmo…..I may keep the cover off and fiddle with it….lifting up the valve and see what impact it has on the idle.

 

[crewzerdog]

Quick update……got the cruiser out today and it did run a bit better but still requires 1/4 pull on the choke, however it idled pretty well. Maybe messing with the hot idle compensator had some effect? I did check around the carb insulator and the intake manifold for leaks…..sprayed carb cleaner all around and had no rise in rpm. Next time out I’m going to wrench on the carb adjustments and recheck timing as well.