Streetable Cam??

[Spanish Dave]

hi all
I am wondering if the cam which I have in my 289 is causing slow driving problems. It is a Comp cams Magnum 289h. Since rebuilding the motor using this cam I find the car plucks and jerks (similar to a miss) when I am driving slowly in low gears (4 speed manual) revs less than 2000. I have rebuilt the carb (650 holley) and just fitted MSD pro-billet distributor, 6al box and blaster2 coil .... as my buddy said it was probably ignition. Someone else said airleak so I have pulled the inlet manifold and re-fitted it so that should be OK. I have been told that this cam will produce a rough idle, but should not effect the driving like this. I recall in the past on this forum *streetable* cams being discussed ... is the cam my problem??

 

[LMan]

cam specs?

 

[Spanish Dave]

cam specs?

Advertised duration 280 at both inlet and exhaust ... 230 @ .050 at both ... .512 lift with 110 deg. lobe separation. Comp cams give operating range of 2000 to 6000 rpm. They state needs (or reccommended) 9.5 : 1 comp. ratio ... I have 9.6.

 

[ratio411]

That is a really rough idling cam. I am not sure it should surge as much as you make it sound though. I would look for other issues.
Maybe make sure you are idling high enough. A cam will surge if the idle is too low and you are coasting in gear.
I ran a 228/235 cam that idled rough and it would surge some when coasting in gear, but it went away once I got a little speed up. Again, on that cam, amount of surge was directly related to idle speed. I also had to let the clutch out a little slower.
Good luck
Dave

 

[ForceFed70]

I have a cam with very similar specs.. IMO, it's too much for the street. It'll work... but it's a pain in the butt.

 

[Max Power]

I ran the same cam in a 302 Mustang II years ago. I used the Crane lifters that were similar to Rhoads, so it didn't idle too bad. It was too much cam for the motor, though.

Even being too much cam, you shouldn't have the level of problems you are having. Are you lugging it? Starting off really slow at low rpm? These cams like RPMs, especially starting off. When the Comp Cams notes recommend a high stall converter, it's kinda the same for a manual trans. You have to rev it a bit more to drive it around town.

As you can guess, it's hard to tell if there is something really wrong without hearing it in person.

 

[LMan]

Thats a lot of cam. Your low end is being hurt by the low vacuum signal at/near idle...but I agree with the others, it shouldnt be causing exactly what you are describing. Sounds more like a vac leak. Check every port? All around the intake? Power brake booster (if you have one)?

Have you looked at your advance curve since you got your new ignition?

More adv at the low would help, most likely.

 

[D.Hearne]

I think he probably needs to do a little carb tuning. Work with the accellerator pump timing and possibly changing the power valve. Otherwise that cam while being a little on the large side ( but sounds like a good choice to me) for your compression ratio, is right on for your combo. If all else fails, you might want to look at milling the heads to up the ratio, this would definately help.

 

[Spanish Dave]

Thanx for all the replies

Starting off is no problem .. the plucking (or surging) just occurs when at low revs in each gear .. such as when in traffic etc. To keep the revs up at slow speeds I just run out of gears (to shift down to !!) I have checked for air leaks and can't find any at all. I have also done a basic carb rebuild which included a new power valve etc. I also sawed off (very neatly!) the choke horn. A pal of mine has a 750 holley double pumper just sat on his shelf, I know this is too big for my setup, but is it worth trying it to see if it helps or cures the problem?

 

[D.Hearne]

The 750 will only make things worse. What do you have the initial timing set at? Should be in the 10-14 degree BTDC range.Also what rear gears are you running?

 

[ratio411]

Sure sounds like it is idling too low.
It will surge some, but a bigger cam needs to idle higher than stock.
That cam also needs lower rear gears.
Dave

 

[Max Power]

Removing that air horn might effect the signal thru the booster as well. Try bolting on a different carb and see what happens.

 

[Spanish Dave]

The 750 will only make things worse. What do you have the initial timing set at? Should be in the 10-14 degree BTDC range.Also what rear gears are you running?

I've got the timing set at 16 deg BTDC ... as for rear gears, I've no idea as the plate is missing off the axle. It is an eight inch axle and I think it quite low geared as the car doesn't seem to have much top end speed .... sorry I can't be more helpful. I guess I will have to take the axle cover off and count the teeth or something to find out. If I do this will I be able to make a gasket up myself from a gasket sheet? I am doing the back springs and shocks next and was thinking of removing the axle to clean and paint it up a bit.

 

[D.Hearne]

I've got the timing set at 16 deg BTDC ... as for rear gears, I've no idea as the plate is missing off the axle. It is an eight inch axle and I think it quite low geared as the car doesn't seem to have much top end speed .... sorry I can't be more helpful. I guess I will have to take the axle cover off and count the teeth or something to find out. If I do this will I be able to make a gasket up myself from a gasket sheet? I am doing the back springs and shocks next and was thinking of removing the axle to clean and paint it up a bit.

Chances are your rear gears are 2.79's, that's what most of the 289 cars came with.If so then they're a bit steep for that cam. The chunk or pumpkin gasket is available thru Fel-Pro, if you can get that brand there. Otherwise you can make one from gasket material or do what some do and just use sealant ( I prefer to use a gasket myself) You will also need new copper washers for the studs and nuts . Probably wouldn't hurt to pull the axles to inspect the bearings and install new seals for the axle ends. Some say you can jack up the car and turn the wheels and count the revolutions for the driveshaft, but with an open differential, I've never been successful with that method of determining the ratio. A better way is to use a tach and drive at a steady 60 mph, then compare the revs, & tire size to figure the final drive ratio.

 

[Max Power]

With an open diff and a 4 speed, you can just put it in gear.

Seriously, the booster venturis in a carb with an air horn are meant to get signals from a carb with an air horn. Carbs with no air horns are designed to work that way. While some backyard racers have run without them, they are at WOT all the time and don't care about low speed driveability.

With a 750cfm carb, you have too much flow anyway. Why on earth would you cut off the air horn?

 

[D.Hearne]

I did cut and mill one off a 650 DP Holley, and it didn't affect the low speed driveability at all. The Vic Jr intake and B303 cam and Canfield heads affected the low speed more than the choke horn did.

 

[Spanish Dave]

With a 750cfm carb, you have too much flow anyway. Why on earth would you cut off the air horn?

The carb on the car is a 600 with vac secondaries. I cut the air horn off after reading on the internet several accounts by people who had done this and acheived better performance. .... The car was driving exactly the same before my little piece of *surgery*, so I can't see it being that. It was also the same before and after replacing the stock distributor setup with the MSD one. The carb has been refitted with a new spacer and gaskets (which I assume are OK). I have sprayed WD40 around to check for airleaks again ... but no joy there. I don't think that it is running too rich as the plugs are a nice light colour and evenly matched, if anything it may be a little on the lean side.