Help wanted adjusting engine slow running speed and any intake vacuum leaks


Asked by Jun 15, 2016 at 03:31 PM about the 1963 Ford Thunderbird

Question type: Maintenance & Repair

I have a 1963 ford 390, new good carburettor, no vacuum leaks, choke is free, timing is correct 6 deg idle speed, Idle screw free,
I cant get it to run at around 480rpm it runs at 650 etc.

The fuel mixture is accurate,
Question??? if the barrel butterfly blades are fully closed surely the engine will stop or at least have little fuel??  Ive cheeked these  barrel blades and they are down/closed as on the off position.

If the car has no fuel from these front two barrels how can it run.
help required, the only way seems to be to retard the engine which doesn't seem a good idea  help any one

Is there a way to check for leaks using some sort of spray etc ( vacuum leaks etc)??

9 Answers


TUNE UP SPECIFICATIONS *When using a timing light, disconnect vacuum hose or tube at distributor & plug opening in tube or hose so idle speed will not be affected. Year: 1964 Engine: V8-390 Std. Trans. V8-390 Auto. Trans. Spark Plug Type (Autolite): BF-42 Gap Inch: .034 Distributor Point Gap Inch: .017 Dwell Angle Deg.: 26-31 Firing Order: 1-5-4-2-6-3-7-8 Ignition Timing*: BTDC (Before top dead center.): V8-390 Std. Trans. - 4 deg.; V8-390 Auto. Trans. - 6 deg. Timing Mark: 0-3-(6)-10, TDC-0-10-20-30 Hot Idle Speed: V8-390 Std. Trans. - 500 (If air conditioned, turn A/C switch to "Full On" position) V8-390 Auto. Trans. - 485 in Drive (If air conditioned, turn A/C switch to "Full On" position) Comp. Press. Lbs. (Plus or minus 20 lbs.): 180 Fuel Pump Press. Lbs.: 5-6 So if you can't get the idle mixture screwsw right then ther eis a problem with the carb, 650 is too fast for a good idle. Have to tried a normal reset of the air mixture screws? All the way in then out 2.5 turns. Checking for vacuum I always used water in a squirt bottle, so you can pinpoint the leaks if any. however on these older engines a vacuum leak will usually cause rough running not increased idle speed.

Best Answer Mark helpful

Thanks tennishoes, I've not counted but tennishoes must have solved a dozen things with my car over the last 2 years. 1) First the idea of a water/mist bottle sounds good Ill try that first. Then I find I can slow it down with the carb screws 2) Secondly Ill reset the timing to spot on 6 deg instead of7-8 ( although I understood a higher BTC was more efficient? 3) Excellent (hot idle speed info ill fix that) One other thing I noticed is that when I pull OFF a vacuum hose it doesn't change the speed much but I remember ears ago it used to allow the engine to speed up. Ill try these now ( its Thursday here in London) so should be able to inform tennishoes better. just one quick question in the description above what does ( V8-390 Auto. Trans. - TDC-0-10-20-30 Hot Idle Speed) mean,? Thanks again, notice everyone , this response from tennishoes must have taken a while to type out and research, , so thanks for that Neil Willis


That is a reference to the timing marks on the degree wheel in a fully warmed up engine, as the rpm's come up you should notice the timing increasing past 10 then past 20 and so on, usually tops at about 38 total advance unless the curve has been changed, I have seen some of these handle 42 total advance but the engine must be modded to handle that kind of advance. A water mist bottle will help cleaning the carbon out and will also help with pinging problems that the old 390's had a problem with due to low octane fuels back then and it is worse now, the trick is finding just the right flow, once you find that and run it for a while then you can start with experimentation with advance curves and base timing advance over and above specs. HTH

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Thanks tennishoes First Ive reset the timing at 6 deg as you said, it was about 8 as far as I could tell. That reduced the speed from640 to 580. So that's todays job done, Tomorrow Ill start on the carburettor, Ill screw IN then OUT to 2 1/2 turns each as you suggest and take it from there. The carburettor was new about a year ago but has only done about 25 miles since in the garage most of the time. Ill report tomorrow. thanks again tennishoes

1 people found this helpful.

Thanks again tennishoes. After the timing which lowered the idle speed to around 590 or so, Today I followed your instructions and screwed IN the mixture screws and OUT 2 turn then back IN about 1/2 to get it smooth Ive now got down to 510 or so, still a bit high to stop the transmission banging when I put it in gear. I could slow down the idle speed with reducing the timing but am reluctant to do that. I have now about19 deg vacuum and 510 speed I guess that's going to be that for now unless anyone can suggest yet another way of getting the idle speed down to around 470 as recommended. Thanks everyone.


Try turning idle base screw back just a little to see if it drops some more, I know you said that the butterflies were closed but do that just to make sure, you can also try turning the air bleeds in just a little more to see if you can get by with it. Other than that your done unless you want to tear down the carb and do some re-jetting, but that takes a lot of trial and error.

1 people found this helpful.

Thanks tennishoes Ive even fitted a small spring on the accelerator arm to make sure its seated each time ( the speed screw is screwed well out of the way). , the dashpot is off, so one last question. then Ill let you get on with some better information. If the butterfly blades are actually fully closed, is it true that the engine wont run as there shouldn't be any air flowing to pick up the fuel?? If that is the case Ill not go as far as getting involved in jetting as I don't know enough about that but I could take the carb off and see if one /both of the blades are sticking just before closing and perhaps sand that bit Thanks again Neil Willis


If the carb has been setup correctly during the building process the engine will idle with the butterflies completely closed, it gets the proper amount of fuel and air through the idle air mixture screw ports. I have seen a few carbs that we're supposed to be new have a slightly twisted throttle shaft so one century was closed and the other was cracked just a little, that threw everything off, you my also check the secondaries, to make sure they are completely closed and seated, quality control on stuff is not what it use to be, they just slap them together and ship them out, just look a ignition points today, they use to be set up so all you had to du was put them in and set the dwell and that was it, nowadays you have to make sure they aren't bent, are contact the whole point contact area and not just a part of it.

1 people found this helpful.

Thanks tennishoes Following that comment Ill make a few additional fuel adjustments firs to make sure that that sides ok. Its kind of you to respond so many times I'm afraid I can only tick "Helpful" as the "best answer " ( you gave ) has already gone, Thanks again, Neil

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