427/390 th 400 shifts fine at low speed, under load...will not!!!

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

No problem. We want you to get your car running right!

Patrick

Thanks, Patrick....Appreciate it!

Well, I got a call from the mechanic...He left a message that he'd call me today. I'm guessing we'll get this straightened out...

You know, I'm not asking for much....The car is a very, VERY nice '68...well cared for over the years, lots of work done...etc...Am I wrong if, while cruising, or better yet...TAKING SOMEONE FOR A SPIN....that I put it to the floor and get slammed back in the seat!!!??? I mean...just now and then.....Is that so bad? That's all I want.....Who's hurt!!!?? LOL!!!!!

I mean, LOOKS are one thing...and, PERFORMANCE...Well...ANOTHER!

I'll be back,

Bob
(9944)

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

First off...."Thanks" to all for the input on my problem. Got the car back, and all is fine with the shifting. Fuel Pump replaced, and new Sock in the Fuel Tank. Pretty simple fix considering all the "Back & Forth" over the whole thing with my mechanic. OK..Now that THAT bit of drama is out of the way...Let's take a trip back to my ORIGINAL problem of....The "KLUNK" when shifting this TH400 from Park or Drive into any gear...I mean, a HEALTHY Klunk!!!Was told that's the nature of the beast. However, this could be reduced by rebuilding the rear end..which I did....The difference is so minimal, it's tough looking at the invoice and cancelled check. Now, the KICKER...After being told, "This is the best it's ever going to be. They ALL do this!" I showed up to pick up my car. There was a 1970 Corvette, 350/auto sitting there. I asked my guy if it would be OK to start her up, and shift into Drive and Reverse so as to assuage my concerns...Well, I got in, started her up, and shifted....NOTHING!!! N O T H I N G!!!!! Shifted into Drive and Reverse like BUTTER!!! And, of course, the "Corvette Sit" Behaves just like an "AUTOMATIC" should...His answer? "Well, that's a 350, not as much torque. The only other thing you could do is install a Stall Converter, and that would get rid of that 'Klunk' "

Needless to say, gents...(And Ladies) I'm frustrated..Great looking and running '68 427/390 but for that miserable"KLUNK!!!" I'm giving the mechanic who is in Chatsworth, Ca a call, (Forgot his name, but it's in an earlier post) and bring my car in to see what can be done.

Again, thanks for all the input..sure glad there's an NCRS!

Bob
(9944)

[William C.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

What is the idle speed in neutral? Too high an idle speed will give you the problem you describe....

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

What is the idle speed in neutral? Too high an idle speed will give you the problem you describe....

Hi Bill,

Idle is at 600 (In Gear) around 700 in Neutral. I've had the idle as low as 500, same thing. Forgot to mention, new Snubber, as well. Other than that Klunk, car drives & Shifts fine.


Thanks,

Bob
9944

[William C.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

600 in Drive with A/C off is the correct spec for a 390 auto trans configuration, Did you replace the bracket on the front of the differential? That is the usual source of this problem.

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

600 in Drive with A/C off is the correct spec for a 390 auto trans configuration, Did you replace the bracket on the front of the differential? That is the usual source of this problem.

Wow! This is the first I've heard of the "Bracket!" This gives me hope, although the rear end was totally rebuilt, I can't see how the mechanic could miss this...Are you saying the bracket can bend, warp? or.......?

Thanks,

Bob

[Edward J.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

[QUOTE=Bob De Simone (9944);628815]Hi Bill,

Idle is at 600 (In Gear) around 700 in Neutral. I've had the idle as low as 500, same thing. Forgot to mention, new Snubber, as well. Other than that Klunk, car drives & Shifts fine.

Bob, I know of one thing that you can check.Is the vacuum modulator valve the correct one. sometime over the years these fail and get replaced and there where many different ones. the factory ones are a bigger round canister - either black or very dark greenish color. the aftermarket ones are gold diachromate and generally are color coded with rings around the canister this is how they tell whats correct for the applications. Ed

[Gene M.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob,
With you having limited mechanical experience it is difficult to get to the real problem with your car. I would suggest that you make sure the engine is NOT breaking up at 4 grand. Is heat riser open? Original camshaft? We do not have details of what is original or modified. It seems your guy should be testing out the car before handing it over to you. Spending $4 grand could have resulted in everything new: cap, correct rotor, GOOD points, solid core wires, new vacuum advance, condenser, coil, plugs, Sun machine set up distributor, carb rebuild and inspection, check cam lobe lifts, engine vacuum, and FUNCTIONAL tune up with a lot of money left over. I suggest contact the chapter members in your club, there has to be some good "wrench monkeys" (good hands on knowledgeable car guys) to help ya out without a major bank loan. Makes for a good tech session.

[Domenic T.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bill,
I didn't read thru all the good replys but you are likely dealing with more than one problem.

The Q-jet rochester has 2 inlet needle & seat sizes and I found that to be the 4K problem when I wrenched at chevrolet. If the carb was rebuilt with the standard needle & seat it would empty the float bowl at about 4K.

The vacume modulator shifts with vacume which is gone under full throttle... then the govenor does the shift under full throttle.

If the modulator were bad it wouldn't shift into the next gear very good at normal driving speeds and would only shift when the govenor operated at it's shifting RPM (high).

When under normal full throttle the modulator does not shift into the next gear, the govenor does that.

The difference in the needle & seats (carb problem) will cause the engine to starve at the RPM you describe.

DOM

[Terry M.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Wow! This is the first I've heard of the "Bracket!" This gives me hope, although the rear end was totally rebuilt, I can't see how the mechanic could miss this...Are you saying the bracket can bend, warp? or.......?

Thanks,

Bob

I assUme (but you know how that goes) your mechanic checked the driveshaft universal joints for looseness? That can cause a klunk when load is applied.

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

I assUme (but you know how that goes) your mechanic checked the driveshaft universal joints for looseness? That can cause a klunk when load is applied.

Hi Terry,

No problem assUming....I'll take any advice I can get at this point...The U-Joints are all fairly new and in good order.

Thanks,

Bob
9944

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob,
With you having limited mechanical experience it is difficult to get to the real problem with your car. I would suggest that you make sure the engine is NOT breaking up at 4 grand. Is heat riser open? Original camshaft? We do not have details of what is original or modified. It seems your guy should be testing out the car before handing it over to you. Spending $4 grand could have resulted in everything new: cap, correct rotor, GOOD points, solid core wires, new vacuum advance, condenser, coil, plugs, Sun machine set up distributor, carb rebuild and inspection, check cam lobe lifts, engine vacuum, and FUNCTIONAL tune up with a lot of money left over. I suggest contact the chapter members in your club, there has to be some good "wrench monkeys" (good hands on knowledgeable car guys) to help ya out without a major bank loan. Makes for a good tech session.

The breaking up at 4K has been handled...(Fuel delivery problem..new Pump and tank sock.) New carb, rear end rebuild...etc. The engine is a stock 427/390, with 8K on a rebuild. I really can't say too much about the distributor and all. I do know he had his problems with it, but it's fine now. I asked repeatedly for him to "Map" it. He has the info to do it, but refuses. I'm also told repeatedly that the Clunk when shifting from park to drive is "What these automatics do" Limited mechanic ability or not..I just can't buy that. Off to another mechanic!

Thanks,

Bob
9944

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bill,
I didn't read thru all the good replys but you are likely dealing with more than one problem.

The Q-jet rochester has 2 inlet needle & seat sizes and I found that to be the 4K problem when I wrenched at chevrolet. If the carb was rebuilt with the standard needle & seat it would empty the float bowl at about 4K.

The vacume modulator shifts with vacume which is gone under full throttle... then the govenor does the shift under full throttle.

If the modulator were bad it wouldn't shift into the next gear very good at normal driving speeds and would only shift when the govenor operated at it's shifting RPM (high).

When under normal full throttle the modulator does not shift into the next gear, the govenor does that.

The difference in the needle & seats (carb problem) will cause the engine to starve at the RPM you describe.

DOM

Thanks for the "Education" Dom...I'm learning so much as of late. Mechanic did mention, early on that "Maybe it's the new carb..." then dismissed that idea. When I picket the car up, he had replaced the fuel filter and tank sock. That was it. My only problem now is the rear end CLUNK shifting from Park to D or R...he keeps telling me "that's as good as it's gonna get unless we put a stall converter in".....Really, man..it's exasperating!

Thanks,

Bob
9944