Surging on deceleration/coasting

[65Fastback]

Hello,

is there a quick diagnose for engine surging on deceleration/coasting in gear below 1200 rpm?
Observed for instance while in town driving when approaching a stop light or so in 2nd gear.
The surging stops immediately when hitting slightly the throttle or when I step on the clutch pedal.
If I do none of the two in low gear, the surge builds up quickly until the whole car kangaroos.

The engine runs excellent in all other scenarios after an intense timing and carb tuning cycle last year.

Holley 7448, 59 mains, 10.5 PV, IMS 1.5 turns, orange cam, 1st hole.
MSD 6, DSII curved by WSA111, 20 degr. initial adv., 37 degr. total adv., ported vacuum, 14 degr. max.
14.5 in hg manifold vacuum at idle.
Idle quality is not perfect but fair, idle speed is a bit unsteady around +/- 10 rpm.

I just don't understand what the engine is doing when surging. And what to try against.

Cheers
Joerg

[chad]

only when 'in gear' or also when clutch is out & @ that RPM?
*If 'in gear' is it just cuz the car's weight is a lill much for the system (gearing, carb, transmis, etc).

Has this just developed or always been there?

EDIT:
just reread
"...on deceleration/coasting in gear...
this would support my answ at the (*) above. Let's rule that out 1st (alternately to 'carb only' or 'timing only' ideas).

[pmuller9]

Sounds like it is related to engine torque twisting the engine and the throttle linkage.

[65Fastback]

That's a good point.
But I have in fact enough free play/looseness in the trottle cable at idle position not to stretch the cable by engine torque movement.

When I was using full manifold vacuum some time ago instead of ported, the problem was bigger, if I remember correctly.
Can my high 20 degr. initial advance be a reason for this behaviour? At 1200 rpm - where surging starts - I do have additional rotational advance, assumed 25 degr. adv. in total.

Cheers
Joerg

[bubba22349]

I think your problem is likely the tune up settings on your engine in relation to the local areas condistions. Ie the weather altitude ect. 37 degrees total advance is probably a little bit too much max should be about 34 or 35 degrees. You should try changing the base timing some going backward some, try 1 or 2 degrees at a time, if I had to guess I think you will end up at around 15 to 18 degrees of base timing where you will then find the engines sweet spot.

I assume that you have the carb's wet fuel level set correctly, but if not you need to check that it is. Next the carb mixture settings are probably off it will also need to be retuned to the lean best idle setting and last the curb idle RPM setting this depends on your camshaft and compression, ball park without knowing about all the details of your combo could be from 650 to 850 RPM. Good luck

[pmuller9]

First try running without the vacuum advance line connected (don't forget to plug the vacuum line) and see how it responds with just the mechanical advance in action.

[65Fastback]

Thank you all.
I will play around with ignition timing next weekend and see how the effects are.

Cheers
Joerg

[chad]

Sounds like it is related to engine torque twisting the engine and the throttle linkage.

If there's any interest in this simply put in gear while holding break & observe how motor tqs on mounts in engine bay (reverse too).
(a 2 ppl job of course)
Look all over in there, do it several times.

How long has it been happening? Was it there B4 this "intense tune" was done earlier?

[65Fastback]

Sounds like it is related to engine torque twisting the engine and the throttle linkage.

If there's any interest in this simply put in gear while holding break & observe how motor tqs on mounts in engine bay (reverse too).
(a 2 ppl job of course)
Look all over in there, do it several times.

How long has it been happening? Was it there B4 this "intense tune" was done earlier?

I am sure that there is no effect on the trottle cable from torque twisting of the engine.
Before tuning the engine overall wasn't running well, so I do not remember, sorry.

Cheers
Joerg

[65Fastback]

Hello,

I did some carb/ignition changes last week to see the effects on the problem.

When I go back from full manifold to ported vacuum, the problem degrades significantly.
When I reduce initial timing from 20 to 15 in addition the surging nearly disappears.
IMS screw changes had no effect.

The reason why I tuned ignition so aggressive before, was that it had cured the bad cold run quality completely.
Now - of course - cold run is bad again, meaning engine dies on first stop, no good trottle response behavior etc.
I think I tried everything with choke settings to get rid of that, - without success. Engine has OZ250 heads + intake and a Holley 7448. Cam is Comp Cam 260 with 110 degr. lobe sep. angle.

Today I went back to full manifold vacuum and 17 degr. initial timing. This seems to be the best compromise on cold run and coasting surge.

Cheers
Joerg

[bubba22349]

Hello,

I did some carb/ignition changes last week to see the effects on the problem.

When I go back from full manifold to ported vacuum, the problem degrades significantly.
When I reduce initial timing from 20 to 15 in addition the surging nearly disappears.
IMS screw changes had no effect.

The reason why I tuned ignition so aggressive before, was that it had cured the bad cold run quality completely.
Now - of course - cold run is bad again, meaning engine dies on first stop, no good trottle response behavior etc.
I think I tried everything with choke settings to get rid of that, - without success. Engine has OZ250 heads + intake and a Holley 7448. Cam is Comp Cam 260 with 110 degr. lobe sep. angle.

Today I went back to full manifold vacuum and 17 degr. initial timing. This seems to be the best compromise on cold run and coasting surge.

Cheers
Joerg

The idle mixture screw not having any effect means the carb is completely out of the idle circuit. What kind of vacuum readings do you have at idle? What is the engines idle RPM? Do you know what power valve (rating) you have installed? I usally run the Distribitor vacuum advance on the ported rather than manifold' vacuum in this case though with the carb out of the idle circuit it won't work right. So I think that since your carb's compeatly out of the idle circuit this is were the main problem is when that gets fixed everthing else will fall into place. Good luck

[65Fastback]

Thanks for answering.

I'm not sure what "completely out of the idle circuit" means. I can only adjust idle mixture screws and idle speed.

Vacuum reading is 14,5 in.hg. at idle.
Engines idle RPM is 820 rpm.
Power valve is 10.5, which is far of the "idle vacuum devided by 2" equation but it completely solved my former lean problem when accelerating from cruise. Which may come - as I just learned - from a completely out of the idle circuit situation(?).

I am very interested to learn, what I can do to achieve a normal idle circuit. I have been through the standard procedure several times. IMS between 1 1/4 and 1 3/4 does not show any difference in vacuum reading or rpm. I am on 1 1/2 now. Float level is on the bottom of the sight glas.

Cheers
Joerg

[bubba22349]

There are a few things that can cause the idle mixture screws to have little or no effect in leaning out the mixture or by what I mean as completely out of the idle circuit. To name a few the idle circuit is jetted too rich or the air bleed is plugged up or incorrect, idle RPM set too high, wrong power valve rating. So is there any change when adjusting the idle RPM screw at the throdle arm? If so you need to get the RPM down as low as possable to were it just bairly will run 450 to 500 RPM. Then see if the idle mixture screws will have any effect. Good luck