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  Topic Review (Newest First)
06-24-2021 11:56 PM
ianorman So I think I found the problem. There is a vacuum leak around the brake booster hence it stalls when decelerating downhill or coming to an intersection… I plugged the hose going from manifold to the brake booster and instantly idle speed rose. Question around the heat riser valve on the manifold will this increase brake booster vacuum as it opens when engine warms up
06-18-2021 04:21 AM
ianorman When I pull the manual choke on it doesn’t do it when warm. I think this tells me there is possibly a vacuum leak or manifold leak somewhere
06-16-2021 04:35 PM
T Hoff Since you had Carb issues, I'd look harder there. Is float level set too high? Downhill causes rich condition especially on deceleration.
06-14-2021 07:38 PM
gutthans The static idle setting (no vacuum) is the position of the distributor, usually reading 8/10/12 degrees depending upon other factors. That setting has nothing to do with setting idle speed. Idle speed is further determined with additional adjustments to the mixture and the idle speed screw(s) depending upon carb make. 1600 RPM MIGHT refer to a fast idle speed during initial warm-up where a carb closes its choke and steps-up an idle to ensure proper air flow and mixture when cold.

Now, that doesn't mean that you can't change idle speed by changing the static setting, but you DON'T set the idle speed by changing the static setting...that's a backwards way to do it.

Warm the motor up...set the distributor to read timing at 8* BTDC with a timing light and have the vacuum line disconnected from the distributor and plugged (do NOT disconnect from the carb end as that lets air into the system and screws you up) and leave it the hell alone. Reconnect the vacuum line and move on to other things to get a proper idle speed.
06-14-2021 07:11 PM
ianorman I’ve teed the vac advance into the line you said but still no luck. I might go back and check the timing and carb idle mixture as well since ive changed where the vacuum advance is picking up vacuum…
But what rpm does one set the timing at. 1600rpm is where I heard is a good point at 8 degrees. But the original 79 year old guy I bought it off had it set at 12 degrees in his personal maintenance log book with the truck
06-14-2021 11:17 AM
devildog80 Had a similar situation on my '81-5 with 258 recently. I switched the distributor vacuum over to manifold, and put vacuum for the backfire device shown to carb vacuum. Every time I would shift gears, truck would backfire. Put a "T" in the line for distributor and this, so both would draw from manifold, and solved the problem.
06-13-2021 07:59 PM
Originally Posted by ianorman View Post
there is minimal vacuum coming off the ported vacuum at the front of the carb.
Ported does not produce advance at near idle. It is designed that way to pass emission tests for air quality. Full manifold vacuum will add advance to the idle somewhat, depending upon the specs of the cannister attached to the distributor.

You should take the distributor cap off. Unhook the vacuum tube from the engine and suck on the end to produce a vacuum to the unit. If the can is working, the arm will pull against a small lever that turns a plate below where points or pickup reside, increasing advance. If the arm doesn't move or the plate doesn't turn, or if it sticks and doesn't release easily then it's causing the advance to be off, and that's the backfire.

If it doesn't move get a new vac can as the diaphragm is shot; if it doesn't release, clean the area off with WD-40, a toothbrush, and a soft cloth. Let dry.
06-13-2021 07:45 PM
JEEPFELLER See post 8 in here

06-13-2021 12:51 PM
ianorman I timed it 8 degrees btdc and it made no difference. But for some reason when I unplug the vacuum advance it doesn’t make any difference. Ive got the vacuum advance t connected to the TAC vac at the back as there is minimal vacuum coming off the ported vacuum at the front of the carb.
06-13-2021 10:47 AM
gutthans Is your distributor functioning correctly? Is it possibly gunked-up and not moving smoothly internally with the vacuum changes?
06-13-2021 10:29 AM
JEEPFELLER Are you using a portable tach (to get RPMs right during timing) and a plain Jane timing gun?

Do you know what the book requirements are for timing and timing RPM?

What year do you have?

Mine's a '77, 258, when timing s finished, I'm sitting at 8 degrees at 700 RPM---distributor vacuum hose temporarily removed and capped during he procedure.

honestly, have you been using meters, or have you just been winging your tuning with no clue of where it should be?

What about MIX Screws? Do you know how to correctly adjust them? all of this works hand in hand to get it all correct. I'll help you, JUST ASK!

06-13-2021 01:14 AM
pedal2themetai HI,

800 is a little fast.. It should be around 650 with a manuel transmission and 750 with a automatic in N. Have you adjusted the air/fuel mixture screw. It should be like 1 1/2 turns out from lightly seated, then adjusted from there to best idle at idle rpm..

Good luck
06-13-2021 12:01 AM
ianorman Ive got the idle around 800rpm. It seems like the timing varies. Sometimes you Rev the engine it dies when it comes back down it dies, other times it keeps running… if I unplug the vacuum advance it doesn’t effect idle? Or should it only effect timing at 1600rpm
06-12-2021 10:25 PM
JEEPFELLER What is your present idle speed?

06-12-2021 09:24 PM
Originally Posted by pedal2themetai View Post
I might have missed it but what carb are you running? depending on orientation of the float bowl maybe your having fuel starvation when going down hill ,, Float to low?? How steep of a hill are you going down?

good luck
Originally Posted by ianorman View Post
I’ve been chasing a backfire when decelerating down hill then engine dies. I originally had blocked idle jets then a collapsed hose block the fuel return line causing the too much pressure at the Carter bbd carb.

It only does it after it’s warmed up. I’ve replaced the vacuum advance and now just changed the oil (due to the overflowing carb float problem) thinking it could’ve been thin oil causing a sticky valve.

Any ideas
Carter BBD
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