throttle surging

jon powers · 08-18-2002, 01:37 PM

I replaced all 6 rubber motor mounts on my 1967 912 and failed to address a surging taking pace at constant foot throttle position when maintaining speed in first gear. Its like a engine torque feed back controling the butterflys. Has this problem an easy solution that I have overlooked?

Jon Powers

Green 912 · 08-19-2002, 04:18 AM

It is hard to tell without seeing the car or more to go on but it sounds like it could be carbs or even clutch springs.

Do you have stock carbs? Are the linkages, including the bell cranks, in good order? Is it only in 1st? Is it a pulse like someone learning to drive or does the motor start to surge at a given spot?

jon powers · 08-19-2002, 07:38 AM

Stock carbs and stock linkage although an extra spring has been added at the locaation where the throttle cable exits the body and connects to the first bell crank. The problem seems to manifest only in first gear. It is like the motor is given throttle then the throttle is let off but the foot pedel stays in the same location. If you are accelerating no problem only when maintaining say 3000 or so rpm like in a parking lot. It seems to me like the motor torque reaction is controling the throttle in a feed back loop such that to regain control it is necessary to get off the pedle and acellerate again but you can not maintain a constant speed.

Jon Powers

Jones Low · 08-19-2002, 08:27 AM

Jon,

You might check your distributor. 2800-3200 rpm is about where the distributor advanced should be fully advanced and if the weights are stuck or the springs broken it might cause irratic engine surging.

Jones Low
69-912
00-Box

Green 912 · 08-19-2002, 10:09 AM

Timing is a good place to start. It also sounds like you might have a carb issue. When you hit the gas you are adding fuel via the idle jets, the advancement circuit and the accelerator pump. When you try and maintain a constant low load and speed you loose the extra fuel from the accelerator pump.

I would check the idle jets and all that feed them, as well as the advancement circuit.

jon powers · 08-19-2002, 01:07 PM

The motor (which I rebuilt using secrets of the inner circle) runs great so I discount carberator problems and the 009 distributor advance looks good with the strobscope. This problem it seems to me like the the throttle linkage is getting tweaked by the torque reaction of the motor and is the same as if you pumped on the throttle pedel real fast.

Green 912 · 08-20-2002, 07:27 AM

Sounds like you got it pegged then.

Good way to check if the motor/trany is loose is to have a friend put it into 1st with the parking brake on and lug the motor via the clutch in and out. You can look at the motor and see if there is excessive movement

FWIW The 009 is not the best distributor for a stock or nearly stock 912 motor. It is a good unit for full race high RPM (7000+) motors, but those barely idle or make much power under 4500 RPM and have 300+ deg cams. When using a 009 on a 912 motor the advance curve starts too soon, ends too soon, and there is not enough total. You can adjust it to work but it will always be a compromise overall. If you set it up so you get enough final advance then it will be too advanced at low RPMs. If you set it up for correct low RPMs it will not have enough final and you will loose top end power. The 031 ($$$), 022 ($) 0r the inexpensive 050 are better choices. All can be recurved if you have the proper equipment and know how.

Surging and or flat spots are not uncommon in carbureted cars and have common causes.

jon powers · 08-20-2002, 08:34 PM

Green 912:

jon powers · 08-20-2002, 08:53 PM

Green 912: Thanks for the suggestion to climb under the car and look at to see if the motor tweaks alot. I'll lay on a creeper and hold on to the car if it starts moving! :-) In talking to you and reflecting on the problem I think posably the linkage is adjusted so that it enters the motor in a way that allows engine movement to influance the throttle oppening.
As for the advance curve I compaired my curve to one I got out of an orange shop manual i have an it seemed to be close using my strob lighted painted white lines on the pulley - look at the tach through the back window and plot results method. Do you have a curve that would be sutable for my application - stock but with 3/4 race cam? The motor runs wonderful above 2000 rpm to 6000 rpm but falls pretty flat on its face say if a light turns green and I try to accelerate below 2000 rpm in the higher gears. My old 60 horse 1600 normal 356A is way behind the 912 in power and acceleration (but I wish I hadn't given it to my old girl friend when I moved to New Zealand many years ago).

Jon Powers

Green 912 · 08-21-2002, 06:13 AM

I don’t think you need to be UNDER the car to see the motor flopping from side to side, just watch the doghouse!

If the linkage is stock setup, check the rod that goes from the trans bell crank to the doghouse bell crank and runs through a hole in the front tin. It is unusual for there to be enough movement for the rod to pull or be disturbed much by movement but it could be hitting the tin where it passes through the hole.

The cam you have is a point of interest to me. The cam is the “brain” in a 912 motor and it will profoundly influence the behavior of the motor. What cam grind do you have? Do you know the timing duration and lift of your “3/4” cam? If you have a stock to mild 280 to 288 deg (IMHO 288 deg max grind to have in a street driven 912 if you want it to be drivable) cam use the 031, 022 or 050 dis and set about 4 deg initial advance. The final advance should hit around 2800 to 3000 RPM and 32 to 34 total depending on the distributor used. This is average setup, the correct timing also depends on the compression, gas and cam used. If you have 300+ deg cam you can use the 009, but you will have to set in about 10 to 12 initial advance and forget low-end drivability and torque. You will end up with 32 or so final advance. Watch the motor temps! Too much advance will cook a motor the same as if it was running lean. If you set the 009 for a good final advance for top end power it will be a hard starting pig and have drivability problems at the bottom end. Even then, the final will occur too early for street use. Just another opinion but hot cams suck in full weight street cars.

Cams over 300 deg will trash bottom end power and low rpm drivability but give top end and keep the power going past 6000 rpm. (Stock cam 912 motor will go flat at about 5800 RPM) If you have a 290+ deg cam that very well might be the center of your problem. I use a 288 in my semi track motor and it drives well street and track but only after I spend some time setting up the carbs for low rpm use. Mild cam, 9.5 to 1 and 1720 cc but has dyno well over 100 HP to the ground with a nice wide torque band. When I track the car I have to change the jets and the timing but it pulls well to 6800 RPM and the power cams in around 4000 RPM.

There is a good book on 912 tuning for street and track comming out this fall. I have seen some of the stuff in it and it looks great.

jon powers · 08-21-2002, 02:40 PM

Green 912,

jon powers · 08-21-2002, 03:01 PM

Green 912, I am using the beer magnet to draw some help in the power against brake testing, may be tomorow. On the cam question I asked Walt at Competition Engineering in Lake Isabella what cam Ray Litz (now retired) speced for me and he thought it was a Norris 337S at 292 degrees duration and Walt agrees with you that it is to big for the stock compression ratio application (but thats what Ray liked). Walt likes the Norris 3315 at 280 degrees as you say would work good on a street motor. With the 5 speed and downshifting for every stop the big cam is drivable and so sweet at high throttle openings. I have never explored the revs above 6000 red line wondering if the crank has 200000 or 300000 miles on it.

Jon Powers

Green 912 · 08-21-2002, 04:28 PM

290 is a bit much but not too, and it is sweet on the high end if the rest of the motor is set up to it. Recommend a distributor swap in your future. That might help some with drivability by improving low-end timing. 050 is $80 new.

With a stock set up motor the rev limit is more from the valve train rather than the crank. 912s got good cranks! Stronger pushrods and double springs on the valves improve the rev limit. Balancing is important too.

Walt and Ray are as good as they get, you are in good hands.