Jumpy ride at low revs

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Higgins56

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Don C

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With the cam you're running now you're idle speed must be over 1,000 RPM.

 

c9zx

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Optimize the mechanical and vacuum advance and carb calibration and live with what you get. Big cam and driving at 1500 RPM don't play well together. Chuck

 

Higgins56

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1971 Mach 1 Fastback now with 503 CID 4 bolt block with AFR Bullit heads , Edelbrock Performer RPM AirGap intake and ProSystems carburettor 1050.
4 sp Toploader with 9" Wavetrac rear.
Caltracs and shocks at the rear.
Optimize the mechanical and vacuum advance and carb calibration and live with what you get. Big cam and driving at 1500 RPM don't play well together. Chuck
Hi Chuck, thanks.

The carb is Prosystems 4150 type  1050 cfm thing.Distributor is MSD.At the moment the  vacuum advance is not in use.

 

Higgins56

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4 sp Toploader with 9" Wavetrac rear.
Caltracs and shocks at the rear.
What are you running for a clutch? What is lift on the cam? Often times the jumpyness at low RPMs is due to very high lift cam and an aggressive clutch. I have the same issue and find I have to keep the revs a little higher a lower speeds or feather the clutch a little more to avoid this. What is your idle set at? With higher lift cams your typically see less torque and power down low which coupled with an aggressive clutch will cause the jumping.
Hi Jason, I checked the  lift of the  Comps cam and it is  .667/.664

Maybe  I want to choose the  smaller diameter tyre from Nitto to  get close  to the 26" original size.Also a higher idle cloys to 1000 rpm might help - as I have understood...now the idle is ca 700 rpm.

Clutch is  McLeod RST twin disc and flywheel is  Mcleod aluminium...The cylinder heads are AFR  Bullit 270.

Th intake manifold is  Edelbrock RPM  AirGap.

 

Don C

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What is your initial timing? I don't know how you were able to get the engine to idle that low with that cam. No wonder you're jumping, with the idle that low and having a light-weight flywheel.

 

donkost

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Wow, this car sounds like a beast! Do you have any official quarter mile times and trap speeds?

 

Don C

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Yes, it must haul, shouldn't be used for a grocery getter.

 

c9zx

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Optimize the mechanical and vacuum advance and carb calibration and live with what you get. Big cam and driving at 1500 RPM don't play well together. Chuck
Hi Chuck, thanks.

The carb is Prosystems 4150 type  1050 cfm thing.Distributor is MSD.At the moment the  vacuum advance is not in use.
Connect the vacuum advance to manifold vacuum to see if it improves the low engine speed behavior. Chuck

 

Higgins56

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4 sp Toploader with 9" Wavetrac rear.
Caltracs and shocks at the rear.
Nature of the beast.

Guessing that it happens when you're trying to idle through a parking lot with the clutch engaged in first and the cam starts lopeing at low RPMs. Yeah, it's not fun, but you didn't build a stock engine, so it won't behave like one. I usually get the car rolling, then slip it out of gear and let it roll.

You have a large tire and what I'd consider borderline rear gears for the setup.  The 2.78 first helps with the takeoff and the torque of the big block helps quite a bit, but you effectively have a 3.25 gear when corrected to a stock 26" tire.

You can try messing with idle RPMs, mixture and timing to make it a little "softer", but primarily it's the cam. You have to learn to live with it or change it out.
HI there,

 well....I started now to  get up the idling revs from 700 to 1000 rpm.

Actually it smoothens the running  at the idle speed which is ca 10 mph now. I noticed that the gears changed well except the reverse. It was impossible to  engage it when motor was running.It reminded me  of an incidence when  the  spring clips  in my Toploader rods got tired and the rods started to waggle and  jammed. I have not  yet checked if this happened again. Last time it was the 3rd gear that jammed. I have also purchased 17" wheels  where I can fit  modern  and cheaper 245/40 tires  with  26" running diameter. The tires I get next week.

 

Don C

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When you increase the idle speed to accommodate a "lumpy" cam the throttle plates may be open into the transfer slots, which makes it impossible to adjust the idle mixture. This requires drilling small holes in the throttle plates, which will increase the idle speed, allowing the throttle plates to be lowered below the transfer slots.

 

TommyK

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30 posts in on a tuning issue and we still don't know anything about the timing. ::devil::

 

Higgins56

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4 sp Toploader with 9" Wavetrac rear.
Caltracs and shocks at the rear.
30 posts in on a tuning issue and we still don't know anything about the timing. ::devil::
Sorry - here you are !



 

71coop

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Higgins56 - I think he is asking about your ignition timing, I don't see that on the spec sheet you posted. Do you know what that is?

IF you find you have a conservative timing curve - advancing the initial timing at idle enough may help with your car's street manners. This is a common thing with long camshafts. For example, in my 351c with a moderately sized cam (272 degrees intake duration) I only run about 16* initial advance, with advance all in about 36* at 3200 RPM (In my view, moderate initial timing). In my corvette's stroker with a 286* duration cam, I run 20* initial advance, with full 35* advance all in at 3000 RPM - it really helps with idle and low-rpm cruising manners. My vette idles around 800 RPM. A little lumpy, but doesn't jerk around. The engine likes it and responds well across the RPM range.

However, seeing the specs above if the can is in the 288*, I would agree you may be able to smooth out the car's manners with more initial timing advance. You will probably have to find a way to limit full mechanical advance so you don't over-advance at higher RPMs, but that is almost certainly faster, cheaper and less work than a cam swap.

From earlier discussion in this post, I assumed a very long duration cam (like a 300+ degree cam),and suggested a cam swap may be in order. That was probably not the best suggestion... I should have asked some questions first. Timing can help with longer duration too, but your cam really isn't that big - it should be able to be improved with some tuning.

If you do adjust timing, you may find some carb adjustment is then in order. I would try one change (advance timing, limit full advance to around 35/36*), see how it responds, then try another (tweak carb), see how it responds, and so on.

 

Higgins56

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1971 Mach 1 Fastback now with 503 CID 4 bolt block with AFR Bullit heads , Edelbrock Performer RPM AirGap intake and ProSystems carburettor 1050.
4 sp Toploader with 9" Wavetrac rear.
Caltracs and shocks at the rear.
Higgins56 - I think he is  asking about your ignition timing, I don't see that on the spec sheet you posted. Do you know what that is?

IF you find you have a conservative  timing curve - advancing the initial timing at idle enough may help with your car's street manners. This is a common thing with long camshafts. For example, in my 351c with a moderately sized cam (272 degrees intake duration) I only run about 16* initial advance, with advance all in about 36* at 3200 RPM (In my view, moderate initial timing). In my corvette's stroker with a 286* duration cam, I run 20* initial advance, with full 35* advance all in at 3000 RPM - it really helps with idle and low-rpm cruising manners. My vette idles around 800 RPM. A little lumpy, but doesn't jerk around. The engine likes it and responds well across the RPM range.

However, seeing the specs above if the can is in the 288*, I would agree you may be able to smooth out the car's manners with more initial timing advance. You will probably have to find a way to limit full mechanical advance so you don't over-advance at higher RPMs, but that is almost certainly faster, cheaper and less work than a cam swap.

From earlier discussion in this post, I assumed a very long duration cam (like a 300+ degree cam),and suggested a cam swap may be in order. That was probably not the best suggestion... I should have asked some questions first. Timing can help with longer duration too, but your cam really isn't that big - it should be able to be improved with some tuning.

If you do adjust timing, you may find some carb adjustment is then in order. I would try one change (advance timing, limit full advance to around 35/36*), see how it responds, then try another (tweak carb), see how it responds, and so on.
71coop,

 thank you of your kind words - I really would not like to change the cam due to the  work involved...I really have to dig into the ignition timing as have been suggested already....

At the end of Sept the car is  going to a dyno test and I will ask to do something for the ignition timing as well.

 

71coop

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I would ask them to try the highest initial timing it will accept w/out detonation or hard starting. They may do that anyway, but doesn't hurt to ask.

 

71coop

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I would ask them to try the highest initial timing it will accept w/out detonation or hard starting. They may do that anyway, but doesn't hurt to ask.

 
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