4x4 Action Group

Still need to check the cam timing which could also be the problem.....

Tell me, what is your manifold vacuum at idle?

Anthony Forgey wrote: ↑

Sun Jul 26, 2020 9:51 am

Tell me, what is your manifold vacuum at idle?

The highest I can get it to with checking the timing and idle mixture is 18 inHg (sorry only have an old gauge).
Signal is constant, no drop outs
Seems somewhat low to me. I know that is within the 18 to 21 inHg and that we are at altitude but I have always got 19 to 20 on other engines before in JHB.

I didn't check it again when I made some changes in the carb - see my next post

So, after nearly gassing the jeep behind me on Saturday I dug into the carb again yesterday an appears I might have made some headway.

I borrowed a different carb slide spring from a mate. This carb we think came off a 1200 Vulcan and the spring is a lot higher rate than the spring I had made up.The spring I had made was +25% over stock. This one must be ~+50% (if not more).
I have now managed to drop down two pilot jet sizes (could possibly go one more), leaned out the fuelling on the transition ports to a point of no misfire (but feel the idle has to be a tad on the lean side to do this) and still maintained good drive-ability.

I have ordered another size down for the pilot to try and will take the borrowed spring in to have it measured and a new one made.

So some progress it seems but will need to be trail tested.....

15 in.hg or more is good. It's been my experience that you need a lean mixture, good idle vacuum and a good amount of advance timing for crisp off-idle response.

Anthony Forgey wrote: ↑

Mon Jul 27, 2020 7:45 am

15 in.hg or more is good. It's been my experience that you need a lean mixture, good idle vacuum and a good amount of advance timing for crisp off-idle response.

As low as 15? Ok so my 18 is good to go then

After yesterday I am now back to thinking that it is all a mixture problem. What sort of lean mixture are we looking at?
I have always set the idle by finding the mid point between the lean and rich drop off points. And if I am battling to find it use a vac gauge to get peak vac.

At the moment I am about 1/4 turn lean of this point on the idle mix screw.

Come borrow my Vacuum gauge. Im in Jukskei Park in Randburg

Thanks Ant.... fairly sure my gauge is OK.

Sorry, I misread.

No worries....

Wooky wrote: ↑

Mon Jul 27, 2020 7:25 am

Anthony Forgey wrote: ↑

Sun Jul 26, 2020 9:51 am

Tell me, what is your manifold vacuum at idle?

The highest I can get it to with checking the timing and idle mixture is 18 inHg (sorry only have an old gauge).
Signal is constant, no drop outs
Seems somewhat low to me. I know that is within the 18 to 21 inHg and that we are at altitude but I have always got 19 to 20 on other engines before in JHB.

I didn't check it again when I made some changes in the carb - see my next post

With vacuum that high you definitely do not have a hot cam in there. On all our V8`s with hot cams we run around 11 to 13. The first indication of a hot cam is low manifold vacuum.

grips wrote: ↑

Wed Jul 29, 2020 5:30 am

With vacuum that high you definitely does not have a hot cam in there. On all our V8`s with hot cams we run around 11 to 13. The first indication of a hot cam is low manifold vacuum.

Thanks Grips....

Don't know all that much about cams, so a novice question.
Why would a hot cam drop the manifold vacuum?

Hotter cams have more overlap and duration to take atvantage of scavaging to get more fuel and air into the chambers at higher rpm. This hurts vacuum at low rpm.

The reason why many high performance conversions run vacuum pumps for the brakes.





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grips wrote: ↑

Wed Jul 29, 2020 9:47 am

Hotter cams have more overlap and duration to take atvantage of scavaging to get more fuel and air into the chambers at higher rpm. This hurts vacuum at low rpm.

The reason why many high performance conversions run vacuum pumps for the brakes.

Makes sense... thanks Grips

So, in that case, you don't want a hot cam in a crawler type toy.
Do you get cams specifically to boost the low end ,not necessarily for this motor but in general?

I run split duration cams to try and get the best of both worlds. Low down torque and horse power higher up in the range.
My experience is that stock cams for bakkie engines is the best bet for low down torque if you does not need the extra hp higher up.
I will not go higher than 218 deg duration @ 0.050" on a normal cam for a 4x4. Would rather add lift.
Appoc is the expert with the cam stuff maybe he can explain better.