The motor was recently rebuilt. I installed a larger cam (270-ish duration) and the adjustable timing set. The timing set is at 2 negative waiting for the triple one bbl set-up. Right now a YF carb from a 240 truck engine sits on the re-worked 250 head. The car idles great in park/neutral. I balanced the pistons/rings/pins to within 1/2 gram. After I put the car in drive the engine shakes and is rougher than a corn cob. I think due to the longer duration camshaft and later closing intake the manifold vacuum is about 14 in Hg. What are the possible causes of my sweet little running engine to rebel when placed in drive?
Rough idle in gear
- Thread starter mugsy
- Start date
What do you have set at 2 degrees negitive the camshaft or the initial ignition timing.
Depending on the camshaft manufacturer the camshaft is normally set at +2-4 degrees advanced.
The distributor should have at least 12-14 degrees initial advance not exceeding 38 degrees total.
Did you mill the head for more compression??
With a 270 degree camshaft you need at least 9.5 compression ratio.
What do you have & we will go from there, Bill
Depending on the camshaft manufacturer the camshaft is normally set at +2-4 degrees advanced.
The distributor should have at least 12-14 degrees initial advance not exceeding 38 degrees total.
Did you mill the head for more compression??
With a 270 degree camshaft you need at least 9.5 compression ratio.
What do you have & we will go from there, Bill
Mine had a rough idle in gear also. What I did that helped it quite a bit was moved the throttle blade on its shaft so that the gap was on the side with the idle jet and the other side more or less hit the wall. It was a tip from a friend that I didnt think would work but figured I didnt have anything to loose by trying it. I guess what it does is improves the velocity where the fuel is getting introduced so it gets mixed better. My carb was an 1100 so Im not sure if or how well it would work on yours.
mach1 mark
Well-known member
I have the same problem, runs great in park or neutral, but put it in gear and it stumbles. I thought it was the dizzy module. anyone else have any other suggestions?
Well I went digging thru my files. I found the can card. Its a Comp Cam #65-236-4 with advertised 260 duration (212* @ 0.050), 110* lobe separation and .440 lift. And yes I did have the block and head milled. The block was 0.030 to zero out the deck and 0.040 on the head to get the cc's I wanted. I think the compression is 9.3:1 or in that neighborhood.
The cam is set at 2 degress retard/negative with the CI adjustable timing set. The initial timing has been set from 10- 14 at various times with littel change to idle or drivablity. The dizzy is a DS II, I bought the complete set-up from a junkyard, but found that the dizzy was junk (no pun intended) (the mech advanced didn't work) so I replaced that with a new dizzy from Murray's (Detroit's version of Kragen/AutoZoo). The control module was replaced as well (more junk) and the coil got swapped out too (this cured the miss at idle). I also did Marks' lighter spring thing in the sticky. Again with no change to the idle quality.
The cam is set at 2 degress retard/negative with the CI adjustable timing set. The initial timing has been set from 10- 14 at various times with littel change to idle or drivablity. The dizzy is a DS II, I bought the complete set-up from a junkyard, but found that the dizzy was junk (no pun intended) (the mech advanced didn't work) so I replaced that with a new dizzy from Murray's (Detroit's version of Kragen/AutoZoo). The control module was replaced as well (more junk) and the coil got swapped out too (this cured the miss at idle). I also did Marks' lighter spring thing in the sticky. Again with no change to the idle quality.
Fordcovert hit the nail on the head.
I was having similar issues for over a year (some may remember the 200+ something post I had going on the subject). It all comes down to getting the carb to work with that cam's low vacuum. Frankly, I am suprised you are getting a vac signal as high as 14".
Fordconverts trick will help you to get that carb to work with the cam. That is a old hot rodding trick when dealing with a lumpy cam. You will probably always need to run higher than normal idle RPM too.
I was having similar issues for over a year (some may remember the 200+ something post I had going on the subject). It all comes down to getting the carb to work with that cam's low vacuum. Frankly, I am suprised you are getting a vac signal as high as 14".
Fordconverts trick will help you to get that carb to work with the cam. That is a old hot rodding trick when dealing with a lumpy cam. You will probably always need to run higher than normal idle RPM too.
The carb actually responds pretty good to adjustments. I can play around with the mixture screw and see changes to idle speed and vacuum levels. So that tells me the idle ciruit is working. The idle speed cna also be adjusted witht he screw. But I'll try the throttle blade shift, sounds like a good mod.
tanx,
Mugsy 8)
tanx,
Mugsy 8)
I like the sounds of moving the throttle blade trick. I wonder if it will work with my Holley 1904/1960? tri-powered 265/274* cam w/ very rough idle
Mugsy's cam is retarded 2 degrees. See if I remember this right, advancing a cam will give you better low-end, right? and retarding a cam, better top-end?
So if you dialed in your cam for better top-end, wouldn't that sacrafice low-end? reason for rough idle? Just for experimental purposes, try setting you cam straight up, then advanced to see if the idle improves.
I'm waiting for 250 adj. timing set before I can dial-in my setup.
Mugsy's cam is retarded 2 degrees. See if I remember this right, advancing a cam will give you better low-end, right? and retarding a cam, better top-end?
So if you dialed in your cam for better top-end, wouldn't that sacrafice low-end? reason for rough idle? Just for experimental purposes, try setting you cam straight up, then advanced to see if the idle improves.
I'm waiting for 250 adj. timing set before I can dial-in my setup.
Are you running your dizzy off a timed port or manifold vacuum? And can you adjust your pump shot to come in quicker?
I have the Comp 260H in my 300...nearly same specs as your cam...and found that idle was rough on a timed port and vacuum rough at 14 hg. Switched to manifold vacuum and it straightened right up to 18 hg at 650 rpm. I also had a bit of a hiccup when cracking the throttle at low speed but with my Holley 4v I changed the pump shot cam to get a quicker, stronger shot which took care of that. Don't know if there is much adjustment in your accelerator pump or not but between the dizzy vac source and adjusting the pump shot curve it took care of the lean off idle/low speed stumble which sounds like what you are dealing with.
I have the Comp 260H in my 300...nearly same specs as your cam...and found that idle was rough on a timed port and vacuum rough at 14 hg. Switched to manifold vacuum and it straightened right up to 18 hg at 650 rpm. I also had a bit of a hiccup when cracking the throttle at low speed but with my Holley 4v I changed the pump shot cam to get a quicker, stronger shot which took care of that. Don't know if there is much adjustment in your accelerator pump or not but between the dizzy vac source and adjusting the pump shot curve it took care of the lean off idle/low speed stumble which sounds like what you are dealing with.
John,
You are right about the relative rpm range and the advance/retard with the cam.
Also, I am using ported vac, so there is zero signal at idle. I know that on this forum there seems to be a great dabate about using ported or manifold vacuum. Persoanlly I'd like to stay with ported, just the way the OEM engineers designed it.
As for the accel pump shot, the problem is stictly idle quality. When I accelerate the engine smooths out as the rpms move north. I have not had any problems with bogs, heisitations, etc. Just the very rough idle.
I do have a general question about this problem: could it be tranny/torque converter related? Agian is idle real smooth out of gear, but when she's put in drive....
tanx,
Mugsy
You are right about the relative rpm range and the advance/retard with the cam.
Also, I am using ported vac, so there is zero signal at idle. I know that on this forum there seems to be a great dabate about using ported or manifold vacuum. Persoanlly I'd like to stay with ported, just the way the OEM engineers designed it.
As for the accel pump shot, the problem is stictly idle quality. When I accelerate the engine smooths out as the rpms move north. I have not had any problems with bogs, heisitations, etc. Just the very rough idle.
I do have a general question about this problem: could it be tranny/torque converter related? Agian is idle real smooth out of gear, but when she's put in drive....
tanx,
Mugsy
I don't think that the ported v manifold vacuum is as much as a debate as it is a difference as to which one works the best on a specific application which is something easily tested by switching between ports. I recently changed carbs and on a timed port it idled low and rough. On the previous carb it made no difference which port I used. Adapt and improvise according to the situation. Factory specs are fine on totally factory equipped vehicles but when modifications are made the factory specs are not always applicable.
mach1 mark
Well-known member
Mugsy, I was running a Holly 500 cfm, I too am having in gear stumble. I had an old 2100 Autolite laying around, put a shot of carb cleaner in it, bolted it on, and the stumble in gear almost went away. The 2100 CFM is smaller, and venture is smaller also, but I'm wondering if it is this ported or manifold vacuum thing? I'm going to try it and I'll post the results. Keep posting your results, I don't believe we are the only two with this problem.
Auto tranny's right? Sounds like maybe a lean condition hiccup? You might set the parking brake hard, chock the wheels, put it in gear and then try tweaking the carb and/or switching between vac ports to see if that helps. With a big enough cam the idle speed itself may have to be a little bit higher than stock to get adequate flow as long as the throttle plates are not open so far that the transition slots are uncovered making the idle circuit not adjustable.
Emanuel recommends drilling 3/32 inch holes in the throttle plates the purpose of which is to increase airflow at idle while allowing the throttle plates to remain in the proper position relative to the transfer slot and idle discharge orifice. One would be out a set of throttle plates if it didn't work but no more so than if offsetting the plates didn't work.
Emanuel recommends drilling 3/32 inch holes in the throttle plates the purpose of which is to increase airflow at idle while allowing the throttle plates to remain in the proper position relative to the transfer slot and idle discharge orifice. One would be out a set of throttle plates if it didn't work but no more so than if offsetting the plates didn't work.
Harte3, yes its an auto tranny. The PO installed a C-4 from the 70's. I only know this because when I went to change the fluid and filter, the filter for a '65 C-4 was different. I then took it to a tranny shop and they basically matched the filter. When I picked it up I asked what year the tranny was and the Goober in the tranny shop said, "the filter is for a later/70's tranny". I just left it at that.
mach1 mark
Well-known member
Mugsy, I just checked my Vacuum. I got it up to 15 in park (1200 rpm), runs smooth and ideals in P or N, then I put it in gear, the vacuum drops down to about 5. When you said yours runs at 14 is that in P or N? What is your vacuum when in gear?
My tanny is also a 72.
My tanny is also a 72.
The vac reading was in park. I haven't convinced my wife that she needs to help me with this problem yet 
And as you describe the vac reading going from 15 in park to 5 in gear is why I don't think manifold vacuum is the answer. The dizzy would be way advanced with 15 in. Hg on the canister in park then the timing would retard when the vac drops to 5....
tanx,
Mugsy 8)
And as you describe the vac reading going from 15 in park to 5 in gear is why I don't think manifold vacuum is the answer. The dizzy would be way advanced with 15 in. Hg on the canister in park then the timing would retard when the vac drops to 5....
tanx,
Mugsy 8)
What happens to the idle speed between gear and not? That seems to be a big drop in vac to me. That has to be the range when the power valve kicks in and thats not something you want at idle.
I have a similar problem on one of my cadillacs now. I couldn't figure it out so I brought it to a friends place so he could point out the obvious to me. Looking down the carb without the engine running you can see the top gasket is all wet and looking down the venturi is also wet. We then started it and you could see fuel dribbling out of the main jets at idle. That means I have too much fuel in the bowl. Next thing is to figure out why. Could be the float is not as floaty as it used to be. Could be crud in the needle or seat. Could just be too much fuel pressure ( Q jets are pretty sensitive to this because of the fairly small bowl and floats). Temps too close to 0 around here so its going to have to wait. We tried to pinch off the fuel line to maybe blast out a chunk of crud from the seat but couldnt get it sealed off, it did however lower the fuel pressure and the engine just purred along nice and smooth so I know the motor is still good.
I have a similar problem on one of my cadillacs now. I couldn't figure it out so I brought it to a friends place so he could point out the obvious to me. Looking down the carb without the engine running you can see the top gasket is all wet and looking down the venturi is also wet. We then started it and you could see fuel dribbling out of the main jets at idle. That means I have too much fuel in the bowl. Next thing is to figure out why. Could be the float is not as floaty as it used to be. Could be crud in the needle or seat. Could just be too much fuel pressure ( Q jets are pretty sensitive to this because of the fairly small bowl and floats). Temps too close to 0 around here so its going to have to wait. We tried to pinch off the fuel line to maybe blast out a chunk of crud from the seat but couldnt get it sealed off, it did however lower the fuel pressure and the engine just purred along nice and smooth so I know the motor is still good.
mach1 mark
Well-known member
The vacuum reading is off manifold, this is my second carb I have put on. I don't think its the carb, vacuum from the engine is vacuum from the engine. Now the question I have is Mugsy having the same problem, and if so is it normal for vacuum to drop this low. I believe the second carb that I have put on (Autolite 2100, 300 cfm) just works better with low vacuum.
Is this low vacuum normal, and if so what can I do?
Is it the Dizzy (DUI). If this vacuum is not normal, are valves not closing, head gasket or what? Anyone can chime in to help Mugsy and I, please.
Is this low vacuum normal, and if so what can I do?
Is it the Dizzy (DUI). If this vacuum is not normal, are valves not closing, head gasket or what? Anyone can chime in to help Mugsy and I, please.
