CHEVY AUTO PROBLEM ?

[crockett]

Can anyone give me some advice, i've just rebuilt my 1972 Chevelle 350ss.

its got KB flatties, performer heads and performer air gap manifold and a comp cams extreme energy 262 cam.

runs nicely but going up the road she will not pull into top gear so i'm doing like 30mph at 3000rpm, dont want to risk going much higher as its running in, but cant drive around like that.

is it something needs renewing in the auto box or is it something to do with vacuum or lack of from the cam ???

any pointers guys

[Crafty_]

What box is it ? TH350 ?

[crockett]

yes th350, checked levels and theyre ok, is there any adjustment underneath the car ???

[Crafty_]

No I don't think so.

TH350 works on vacuum to change gear, that vacuum comes from the intake. If you have none at all it won't shift out of 1st, so clearly you do have some there, but IIRC its possible its not strong enough to shift 2nd - 3rd whilst still allowing 1st - 2nd. Check the line and any fittings (t-pieces etc) for splits/breaks.

I don't think it should make a difference, but is the kickdown connected ok ?

Other than that you might need the help of an expert.

[odyssey2200]

3000rpm at 30mph ?

Not an engine problem if it genuinely is in top gear.

There seems to be a lot of slipping going on there.

Have got changed the transmission oil?
What condition was it in?

[shovelheadrob]

It sounds like you have not connected the vacuum hose for the vacuum modulator http://www.google.co.uk/imgres?imgurl=http://www.d... or you may have it hooked up wrong. It needs to see the manifold vacuum, so needs to be on the engine side of the carb butterflies. I'm assuming that the shifter linkage is adjusted correctly as this could also be the problem although you should have noticed if it is going into park where reverse would be!

[buick350]

did the TH350 work before engine build? ie: have you known it to work recently? I built a th350 and it had a similar problem, hose had fell off modulator

[crockett]

Thanks guys - yes the hose had come off the modulator somehow ???? must have come off when i jacked the gearbox up when lowering in the engine somehow, thanks all you guys for the headsup, i found the problem straight away.

[crockett]

as mentioned above the gearbox goes into top now. the only thing is she keeps changing up and down, any ideas anyone, is there some adjustment or failing that, can anyone suggest a reputable yank auto garage in Essex/London ?

[shovelheadrob]

Detent cable adjustment? The vacuum modulator controls the shift points, the detent cable is for kickdown only & if set wrong will cause downshifts on part throttle. You can disconnect it to narrow the problem down, if the trans still hunts between 2 & 3 you could have a vacuum leak on the circuit for the modulator or even a faulty modulator. You can get adjustable modulators, they can help if you have a performance cam which makes less vacuum.

[crockett]

Hey Rob/guys, thanks for any help, it has got a mild cam in, Comp cams extreme energy 262, i was told this would not affect the vacuum too much ??

The cars not running right anyway (when cold, wont idle and i have to keep feathering the throttle) and there is a hissing sound inside the car but i cant find any vacuum hoses hanging off either the engine or behind the wiper switch area (the cars a 72 Chevelle ss350)

on another note, once warmed up she runs pretty well but when i switch her off and then try to restart it sounds like a flat battery, i'm guessing this is in part due to me setting the ign timing at about 18btdc on the strobe. or is cos the engines nice and tight ? (rebored/flat tops/etc)

[shovelheadrob]

That's not a really radical cam so should be fine, reckon you need to find the source of the hissing. As to the starting, 18 could be a bit much, what does that give you total (with vac advance disconnected)? You should run around 35 total.

[crockett]

18/20ish BTDC - strobe light timing is what i set it at (engine warmed up and vacuum disconnected)

think i will set it at 15 now

[shovelheadrob]

With your tuning mods you need to make sure the total is correct, you can get parts to recurve the distributor, i.e. lighter springs so the advance comes in quicker.

[crockett]

Rob - you said about adjusting the detent cables/rods ? how do i do this ? i'd like to try it before i buy a new modulator. Cheers.

Dave

[shovelheadrob]

With the carb in the full throttle position the detent (also known as kickdown or tv) cable should also be at it's full travel, some will have a tab on the cable which has to be released as you are setting it, others may have a pinch clamp on the cable.
Some images here https://www.google.co.uk/search?q=th350+detent+cab...

[crockett]

Adjusted the DT cable but still surges up and down the gears at part throttle, so i bought a new modulator. I assume its a press fit or interference fit into the side of the box as the new one has no threaded insert or hexagon part to put a spanner on.

So do i just lever the old one out ??? can it be done without removing the gearbox ??

[1000foot]

th 350 modulator is easy to change.......one bolt holds a retaining strap.remove bolt , slide off strap , prise out old modulator.

reverse this to fit new one. make sure the modulator pin doesnt come out with the old vacuum unit.

reconnect vac hose , drive it !!