I bought a rebuilt Motorcraft 2100 carb for my 1970 F-250 with a stock 360. Being that it was the first time I've ever installed a carburetor (along with a new mechanical fuel pump because I couldn't get the fuel filter canister to separate) I was a little surprised when it came to life after a few cranks. Unfortunately, that excitement was short-lived as it became rapidly apparent that the idle was insanely high so I shut it down only seconds later.
I understand there's an idle adjustment screw next to the throttle linkage. So my guess is if I screw that down the idle will go down, but honestly I'm working on speculation based on some searching online. Because as fast as it was racing before I shut it down seemed a little hard to explain away by a simple idle adjustment.
So while I believe that experimentation is a great way to learn, I'm a little hesitant to start randomly cranking down on the adjustment screws. So I could really use some advice in troubleshooting this.
High idle is usually a vacuum 'leak'. When you installed the carb did you replace the lower gasket and also check that the carb base was the same as the old one. PVC hose unhooked or something similar?
A vacuum leak was something I thought could cause this, but I was careful to make sure everything was hooked-up and blocked-off the same. The rebuilt carb came with a new gasket and I replaced it being especially careful to remove any trace of the old gasket.
Physically, the two carbs had some outside differences in the castings, but I there wasn't anything unusual outside of that. I chalked it up to the difference between Autolite and Motorcraft. That said, I did take fitment for granted.
If I can't figure out what's going on tomorrow I'll post some additional pictures to show what I'm dealing with.
I'd pull the carb back off and carefully compare any ports or passages on the bottom to the stock heat riser. I've seen it quite a few times where there's a vacuum passage that isn't sealed by the riser because the profiles are different. It happens.
Ranchero50 wrote:I'd pull the carb back off and carefully compare any ports or passages on the bottom to the stock heat riser. I've seen it quite a few times where there's a vacuum passage that isn't sealed by the riser because the profiles are different. It happens.
Definitely - check the gasket too, I’ve seen non factory gaskets not set the passages in the base plate as well.
Occidental wrote:I understand there's an idle adjustment screw next to the throttle linkage. So my guess is if I screw that down the idle will go down,
Screwing it down increases the idle. Backing it off decreases the idle. Make sure your throttle is backing off when you back off the screw. I've had to add extra return springs before.
Randy
1970 F100 Sport Custom Limited LWB, 302cid, 3 on the tree. NO A/C, NO P/S, NO P/B. Currently in 1000 pcs while rebuilding. Project thread: http://www.fordification.com/forum/view ... 22&t=59995 Plan: 351w, C4, LSD, pwr front disc, p/s, a/c, bucket seats, new interior and paint.
1987 F-150 XLT Lariat, 5.0/C6 auto.
Occidental wrote:I understand there's an idle adjustment screw next to the throttle linkage. So my guess is if I screw that down the idle will go down,
Screwing it down increases the idle. Backing it off decreases the idle. Make sure your throttle is backing off when you back off the screw. I've had to add extra return springs before.
That was my first thought too. You may need to adjust the throttle linkage out a little bit too. Try starting it with the throttle linkage off and see if it's still revving so high. You may have to have someone work the throttle by hand when starting to rule out the linkage.
Mark.
The problem with the crazy idle was that I hadn't reconnected the throttle return spring. I must have looked at the shiny, silver mounting boss a dozen times, but for whatever reason it didn't occur to me that it had an important function. So basically the throttle was sitting back nearly at wide open.
So now I've got to go through and make sure everything runs well and those YouTube videos will be a big help.
Of course now the truck won't start at all (no clicking or anything) so that's great.
Gotta love these old trucks! Put your battery on the charger and call it a night.
Randy
1970 F100 Sport Custom Limited LWB, 302cid, 3 on the tree. NO A/C, NO P/S, NO P/B. Currently in 1000 pcs while rebuilding. Project thread: http://www.fordification.com/forum/view ... 22&t=59995 Plan: 351w, C4, LSD, pwr front disc, p/s, a/c, bucket seats, new interior and paint.
1987 F-150 XLT Lariat, 5.0/C6 auto.
sargentrs wrote: Gotta love these old trucks! Put your battery on the charger and call it a night.
I turned out the neutral safety switch had come out of alignment so thankfully the fix was pretty easy in the end.
Now the truck fires right up, but idles like garbage so I need to go through the setup procedure on the carb. Still, a lot better of than I was a day ago.
Although now I'm pretty sure I hear a ticking lifter...
I like to use a vacuum gauge to tune my carbs. Hook it up to manifold vacuum and turn the air/fuel mix screws until to get around 20-21 Hg vacuum at idle. Your idle will most like increase so back it off to around 700-750 rpm and tune again. I've also had a lot of good results with running a quart of Marvel Mystery Oil in the crankcase. Drain off some oil if you have to. Add a quart of MMO to bring back to full. Run the truck for awhile. I've even driven it for a couple of days. Then change the oil. My old '74 Dodge Dart slant six ticked pretty loudly and had weak oil pressure. After treatment it quieted down considerably and oil pressure increase by 5 psi immediately and stayed there. A lot of people will say it's "snake oil" but I'm a believer.
Randy
1970 F100 Sport Custom Limited LWB, 302cid, 3 on the tree. NO A/C, NO P/S, NO P/B. Currently in 1000 pcs while rebuilding. Project thread: http://www.fordification.com/forum/view ... 22&t=59995 Plan: 351w, C4, LSD, pwr front disc, p/s, a/c, bucket seats, new interior and paint.
1987 F-150 XLT Lariat, 5.0/C6 auto.
sargentrs wrote:I like to use a vacuum gauge to tune my carbs. Hook it up to manifold vacuum and turn the air/fuel mix screws until to get around 20-21 Hg vacuum at idle. Your idle will most like increase so back it off to around 700-750 rpm and tune again. I've also had a lot of good results with running a quart of Marvel Mystery Oil in the crankcase. Drain off some oil if you have to. Add a quart of MMO to bring back to full. Run the truck for awhile. I've even driven it for a couple of days. Then change the oil. My old '74 Dodge Dart slant six ticked pretty loudly and had weak oil pressure. After treatment it quieted down considerably and oil pressure increase by 5 psi immediately and stayed there. A lot of people will say it's "snake oil" but I'm a believer.
That's good to know. I received a new vacuum gauge today so that's going to help a lot in setting up the carb. If the MMO trick works on the ticking noise, then I'm all too happy to give it a try. Changing the oil is pretty easy on a truck like this.
Gotta love these old trucks! Put your battery on the charger and call it a night.